La **pierre de Courville** (Cv) est un calcaire fin biomicritique du Lutétien avec des teintes beige clair (Figure III-1.a et Tableau III-1). Elle provient d'une des dernière carrière de la Marne et a été très utilisée en construction et en restauration, à la cathédrale de Reims notamment, jusqu'à la fermeture de la carrière en 2005 (Devos et al., 2008).

Elle est principalement composée de coquilles de mollusques Bivalvia et Gastropoda, de foraminifères (miliolides et Orbitolites complanatus) et d'algues vertes (Figure III-1.b ; Fronteau, 2000). Cette pierre de construction contient environ 5% de la silice et 90% de calcite (Tableau III-1). La texture est supportée par une matrice micritique (microsparite) partiellement recristallisée et un ciment spath secondaire dans les macropores ou dans la porosité intragranulaire des fossiles (Figure III-1.b; Fronteau, 2000).

La pierre de Courville est microporeuse et présente une porosité ouverte d'environ 16%. La courbe d'injection de mercure révèle une distribution unimodale de la taille des pores (Figure III-1.c) qui met en évidence un type de porosité localisée principalement dans la matrice intergranulaire. La pierre de Courville possède un réseau poreux avec des rayons d'accès relativement petits, inférieurs à 0,1 µm focalisés autour d'un seul pic modal à 0,1 µm (Figure III-1.c).

<figure><img src="image_48.png" /><figcaption>Figure III-1. Caractéristiques de la pierre de Courville (Cv) : photographie (a), sections fines sous lumière polarisée (b), et distribution des tailles de pores (c).</figcaption></figure>

Le **calcaire du Bajocien** (Bj) étudié est un calcaire oolithique et bioclastique du Bajocien de couleur jaune ocre à roux (Figure III-2.a et Tableau III-1). La pierre de Pas-Bayard a été extraite de «Jaumont» près de Grandcourt pour la reconstruction de l'abbaye d'Orval au début du XXe siècle (Thomachot-Schneider et al., 2011).
La pierre est composée de débris calcitiques comme des fragments de coquillages, de nombreux osselets échinodermiques entourés d'un ciment et de foraminifères. Les échantillons étudiés montrent de nombreux ooïdes entourés de ciments diagénétiques, composés de spath de calcite dans les espaces interstitiels (Figure III-2.b). Les dépôts d'oxyde de fer et de manganèse, bruns fonces a noirs, sont visibles dans la porosité et sur la surface de la pierre (Figure III-2.a et Tableau III-1).

Les calcaires bajociens sont des pierres de construction macroporeuses d'environ 20% de porosité totale. La distribution des tailles de pores présente une bimodalité allouée à une faible porosité intergranulaire et une porosité intragranulaire secondaire. La pierre est constituée à 90% par des pores de rayon supérieur à 0,1 µm avec deux pics à 2 et 0,3 µm.

<figure><img src="image_49.png" /><figcaption>Figure III-2. Caractéristiques du calcaire du Bajocien (Bj) : photographie (a), sections fines sous lumière polarisée (b), et distribution des tailles de pores (c).</figcaption></figure>

**La pierre de Savonnières** (Sv) est un calcaire vacuolaire oolithique gris du Jurassique (Figure III-3.a et Tableau III-1). Ce calcaire de renommée mondiale a été fréquemment observé dans les bâtiments du XIXe au XXe siècle ou proposé pour des travaux de restauration dans toute la partie occidentale de l'Europe (Turmel *et al.*, 2014, Laycock *et al.*, 2008).

Ses principales composantes sont constituées d'oolithes et en moindre quantité de fragments ou de coquilles de bivalves entières ce qui explique sa composition chimique calcitique presque pure (96%; Tableau III-1). Les oolithes en majorité et certaines coquilles de bivalves sont entourés d'un ciment sparitique remplissant partiellement la porosité intergranulaire large et ouverte (Figure III-3.b).
La pierre de Savonnières est une pierre de construction macroporeuse. Ainsi, la porosité ouverte est d'environ 30 à 35%. Son réseau poreux est situé à l'intérieur des noyaux d'oolithes et entre le ciment, et les micropores sont situés dans le cortex des oolithes, ce qui donne une distribution de taille de pore bimodale et confère à la pierre une porosité élevée ouverte avec des valeurs de l'ordre de 30%. Plus de 50% de ses pores ont un rayon d'accès supérieur à 0,1 µm comprenant deux pics à 5 et 0,1 µm et 30% de la gamme intermédiaire (1 à 0,1 µm ; Figure III-3.c).

<figure><img src="image_50.png" /><figcaption>Figure III-3. Caractéristiques de la pierre de Savonnières (Sv) : photographie (a), sections fines sous lumière polarisée (b), et distribution des tailles de pores (c).</figcaption></figure>

**La pierre reconstituée (Rs)** est un matériau artificiel présentant des tons beiges à roux proches des couleurs des calcaires du Bajocien (Figure III-4.a et Tableau III-1). Elle a été utilisée lors de la reconstruction de l'abbaye d'Orval de 1930 à 1948 avec l'objectif de compléter les approvisionnements en pierre provenant de la carrière du Bajocien de Pas-Bayard (Thomachot-Schneider *et al.*, 2011).

La pierre reconstituée est un matériau hétérogène composé principalement de lithoclastes calcaires et siliceux de taille millimétrique à centimétrique, mélangés avec des grains de quartz, du ciment Portland, de la chaux calcique et des pigments minéraux (oxydes de fer), ce qui le rend riche en silice et en soufre (Tableau III-1). Le rapport des éléments à la phase de liaison est très variable, de même que la taille et la nature des lithoclastes de calcaire (Figure III-4.b).

La macroporosité de la pierre reconstituée vient des bulles d'air produites par le dégazage lors de la fabrication (Figure III-4.b). Le réseau poreux est bimodal, avec des micropores situés à l'intérieur des lithoclastes calcaires et des pores de taille intermédiaire dans la matrice (Figure
III-4.c) et une porosité ouverte de 23%. Ce matériau présente une très forte proportion de micropores avec 94% de rayons d'accès des pores inférieurs à 1 µm répartis en deux pics à 0,6 et 0,07 µm.

<figure><img src="image_51.png" /><figcaption>Figure III-4. Caractéristiques de la pierre reconstituée (Rs) : photographie (a), sections fines sous lumière polarisée (b), et distribution des tailles de pores (c).</figcaption></figure>

<table><caption>Tableau III-1. Caractéristiques chimiques, colorimétriques et microstructurales des pierres étudiées (Cv = Courville, Bj = Bajocien, Sv = Savonnières et Rs = Pierre reconstituée ; Sat. Cap. = saturation capillaire).</caption><thead><tr><th rowspan="2"></th><th rowspan="2">CaCO<sub>3</sub><br>(wt.%)</th><th rowspan="2">MgO<br>(wt.%)</th><th rowspan="2">SiO<sub>2</sub><br>(wt.%)</th><th rowspan="2">Fe<sub>2</sub>O<sub>3</sub><br>(wt.%)</th><th rowspan="2">SO<sub>4</sub><sup>2-</sup><br>(mg.g<sup>-1</sup>)</th><th rowspan="2">Sat. cap.<br>(%)</th><th rowspan="2">Porosité</th><th colspan="3">Colorimétrie</th></tr><tr><th>L*</th><th>a*</th><th>b*</th></tr></thead><tbody><tr><td>Cv</td><td>90.3</td><td>0.58</td><td>4.90</td><td>0.39</td><td>0,3</td><td>90</td><td>micro</td><td>75 à 83,4</td><td>0,9 à 2,9</td><td>9,6 à 16,6</td></tr><tr><td>Bj</td><td>94.6</td><td>0.41</td><td>1.09</td><td>1.44</td><td>0,3</td><td>70</td><td>macro</td><td>65,2 à 74,1</td><td>3,8 à 7,5</td><td>16,1 à 29,2</td></tr><tr><td>Sv</td><td>96.3</td><td>0.65</td><td>0.68</td><td>0.25</td><td>2</td><td>48</td><td>macro</td><td>72,2 à 94,1</td><td>-0,8 à 3,6</td><td>3,8 à 18,3</td></tr><tr><td>Rc</td><td>86.9</td><td>0.53</td><td>6.64</td><td>1.94</td><td>3,5</td><td>51</td><td>micro</td><td>65,6 à 74,4</td><td>2,7 à 5,2</td><td>14,6 à 23,7</td></tr></tbody></table>
Pour simuler les effets des dépôts de polluants et de la pluie acide sur les pierres, deux types de protocoles ont été élaborés. Le récapitulatif des caractéristiques des tests se trouvent dans le Tableau III-2.

## METHODOLOGIE

La simulation des pluies acides a consisté en l'immersion des plaquettes (5 x 5 x 1 cm) dans des solutions à pH = 5, d'acide nitrique (HNO₃), d'acide sulfurique (H₂SO₄), d'un mélange des deux acides (2 HNO₃ + 1 H₂SO₄) ainsi que dans une eau de pluie naturelle (pH = 5,9).

Dans le premier test, les échantillons sont placés dans les solutions sans agitation pour simuler l'effet de l'eau en stagnation (immersion passive) tandis que dans le deuxième test les échantillons immergés dans la solution sont placés sur un agitateur à 180 r.p.m. pour simuler l'effet mécanique des précipitations (immersion active).

Les expérimentations ont été menées pendant 32 jours incluant cinq cycles alternant phases d'immersion et phases de séchage. La première immersion a duré 24 heures avant séchage. Les quatre cycles suivants ont impliqué 4 jours d'immersion avant séchage. La phase de séchage était similaire pour tous les cycles et effectuée pendant 64 h dans un four à 45°C. Les triplicatas de chaque type de pierre ont été observés et analysés après chaque phase de séchage puis remis dans la solution dont le pH a été réajusté à 5.

L'évaluation macroscopique des pierres soumises aux immersions acides a été effectuée par photographies des échantillons entiers avec un Nikon Coolpix 4500 avant et à la fin de l'expérience. Des échantillons ont été observés avant chaque phase d'immersion avec une loupe binoculaire de 10x à 60x (Olympus SZH-ILLB) pour détecter et suivre l'altération visuelle que les composants de la pierre ont subie au contact des solutions acides au cours du temps.

Le poids des échantillons de pierre a été mesuré par les balances analytiques Mettler Toledo ou avec un Ohaus Voyager Pro modèle VP410 2C avec une précision de 1 mg. La variation de poids a été calculée par rapport au poids initial à la fin de chaque cycle pour déterminer la perte de poids progressive des pierres pendant l'expérience.

La caractérisation de la porosité a été obtenue à l'aide d'un porosimètre à intrusion Hg (MIP) Micromeritics Autopore IV 9500, atteignant une pression de 230 MPa et mesurant des tailles de pores de 0,005 à 187 µm. Afin d'évaluer l'évolution de la porosité, un échantillon de
chaque type de pierre a été analysé avant l'expérience et un autre échantillon, préparé à partir de la plaquette altérée, a été analysé à la fin de l'essai. Les données et les graphiques obtenus ont permis de calculer le rayon moyen d'accès aux pores (Rm), les pics modaux et le pourcentage de pores dans quatre domaines de rayon d'accès de pore différents: large (> 1 µm), moyen (1 -0,1 µm), petit (0,1 - 0,01 µm) et très petit (<0,01 µm).

La **couleur** a été mesurée et quantifiée avec un colorimètre MINOLTA CR-400 utilisant l'illuminant D65, un faisceau de lumière diffuse de 8 mm de diamètre et une géométrie d'angle de vision de 0° avec un composant spéculaire inclus et une réponse spectrale proche du CEN (2009) courbes d'observateur standard. Les mesures ont été exprimées à la suite des systèmes CIE L * a * b * et CIE L * C * h *. Le paramètre L * est la luminosité et enregistre les valeurs de 0 (noir) à 100 (blanc). Le paramètre C * est le chroma ou la saturation. Plus le chroma est élevé, plus les couleurs sont vives. Les valeurs de chroma sont comprises entre 0 et 60. Le paramètre h * correspond à la teinte (c'est-à-dire le ton de la pierre) avec des valeurs de 0 à 360 (degrés de la sphère chromatique). La valeur ΔE * est définie comme le changement de couleur total pour comparer les variations avant et après les tests comme suit:

$$ \Delta E * = [(\Delta L*)^2 + (\Delta a*)^2 + (\Delta b*)^2]^{1/2} $$

où a * et b * sont des paramètres chromatiques cartésiens. Pour les matériaux hétérogènes comme les pierres, la détermination de la couleur nécessite une étude préalable du nombre de mesures nécessaires, qui est liée à la couleur de chaque minéral, à la taille des grains et à l'hétérogénéité. Le nombre de points de données a été déterminé par le calcul de la moyenne cumulative jusqu'à la stabilisation des valeurs (Vázquez et al., 2013b; Dans cette étude, 40 points de données étaient le minimum requis dans chaque dalle.

La **rugosité** de surface a été déterminée à l'aide du dispositif ZScan 700Cx pour la numérisation 3D. Les données brutes ont été traitées avec le logiciel 3D Reshaper pour le traitement du nuage de points. Le dispositif ZScan est un scanner laser portatif. Il peut rapidement créer un nuage de points sur une surface avec une précision de 0,05 mm et une résolution de 0,1 mm. Son travail rapide et précis a été un critère clé de ce choix, soutenu notamment par Barbero et Ureta (2011), Kuzminsky et Gardiner (2012), Tóth et Živčák (2014) qui ont souligné les limites d'utilisation mais aussi le potentiel significatif de cette technologie. Néanmoins, en raison de la complexité du traitement des données et de l'utilisation novatrice de ce logiciel dans le cadre de l'altération des pierres, seuls les échantillons exposés à l'un des tests d'immersion ont été mesurés avant et après les essais.
Le microscope électronique à balayage (MEB) a servi à observer et analyser les surfaces de chaque échantillon avant et à la fin des expériences. L'équipement était un plateau de table SEM Hitachi TM-1000 avec un spectromètre à rayons X à dispersion d'énergie (Swifted-TM Energy Dispersive X-Ray (EDX)). Les échantillons ont été introduits dans le microscope et maintenus sur le support par une bande de carbone adhésive double face. La tension d'accélération était de 15 kV pour l'imagerie et l'analyse EDX. La distance de travail était de 6 mm. Toutes les images ont été acquises en mode rétrodiffusé.

Les concentrations de Ca, Mg, Si et Fe ont été mesurées dans des solutions acides de chaque cycle par spectrométrie d'émission atomique à plasma inductif (ICP-AES, Iris Advantage, Thermo Fisher Scientific). L'analyse de Ca, Mg, Si et Fe a été réalisée à des longueurs d'onde respectives de 184,006, 279,553, 251,611 et 259,940 nm, à travers trois mesures de chaque solution.

<table><caption>Tableau III-2. Caractéristiques des tests artificiels (pour [H₂SO₄], x = 3 pour les tests en atmosphère et x=4 pour les tests en immersion).</caption><thead><tr><th>Type de test</th><th>Noms des solutions</th><th>[H₂SO₄] (mol l⁻¹)</th><th>[HNO₃] (mol l⁻¹)</th><th>Ratio H₂SO₄/HNO₃</th><th>pH</th><th>Agitation</th><th>Partie du chapitre</th></tr></thead><tbody><tr><td rowspan="7">Immersion</td><td>SAI</td><td>5 10<sup>-6</sup></td><td>-</td><td>-</td><td>5 ± 0.1</td><td>Active</td><td>1</td></tr><tr><td>SPI</td><td>5 10<sup>-6</sup></td><td>-</td><td>-</td><td>5 ± 0.1</td><td>Passive</td><td>1</td></tr><tr><td>NAI</td><td>-</td><td>1 10<sup>-5</sup></td><td>-</td><td>5 ± 0.1</td><td>Active</td><td>1</td></tr><tr><td>NPI</td><td>-</td><td>1 10<sup>-5</sup></td><td>-</td><td>5 ± 0.1</td><td>Passive</td><td>1</td></tr><tr><td>SNAI</td><td>2 10<sup>-6</sup></td><td>6 10<sup>-6</sup></td><td>1/3</td><td>5 ± 0.1</td><td>Active</td><td>3</td></tr><tr><td>SNPI</td><td>2 10<sup>-6</sup></td><td>6 10<sup>-6</sup></td><td>1/3</td><td>5 ± 0.1</td><td>Passive</td><td>3</td></tr><tr><td>NRAI</td><td>-</td><td>-</td><td>-</td><td>5.92 ± 0.1</td><td>Active</td><td>3</td></tr><tr><td>NRPI</td><td>-</td><td>-</td><td>-</td><td>5.92 ± 0.1</td><td>Passive</td><td>3</td></tr><tr><td rowspan="7">Atmosphère</td><td>HC</td><td>5.6 10<sup>-1</sup></td><td>-</td><td>-</td><td>-</td><td>-</td><td>2</td></tr><tr><td>LC</td><td>1.7 10<sup>-1</sup></td><td>-</td><td>-</td><td>-</td><td>-</td><td>2</td></tr><tr><td>S&gt;N</td><td>5.6 10<sup>-1</sup></td><td>1.4 10<sup>-1</sup></td><td>4/1</td><td>-</td><td>-</td><td>2</td></tr><tr><td>S&lt;N</td><td>1.7 10<sup>-1</sup></td><td>4.8 10<sup>-1</sup></td><td>1/3</td><td>-</td><td>-</td><td>2</td></tr><tr><td>HNO₃</td><td>-</td><td>10.1</td><td>-</td><td>-</td><td>-</td><td>4</td></tr><tr><td>HNO₃&gt; H₂SO₃</td><td>1.6 10<sup>-1</sup></td><td>7.1</td><td>1/47</td><td>-</td><td>-</td><td>4</td></tr><tr><td>HNO₃&lt; H₂SO₃</td><td>3.7 10<sup>-1</sup></td><td>3.0</td><td>1/9</td><td>-</td><td>-</td><td>4</td></tr><tr><td>H₂SO₃</td><td>5.3 10<sup>-1</sup></td><td>-</td><td>-</td><td>-</td><td>-</td><td>4</td></tr></tbody></table>

**Immersion** : SAI : sulfuric active immersion ; SPI : sulfuric passive immersion ; NAI : nitric active immersion ; NPI : nitric passive immersion ; SNAI : sulfuric and nitric active immersion ; SNPI : sulfuric and nitric passive immersion ; NRAI : natural rainwater active immersion ; NRPI : natural rainwater passive immersion ;

**Atmosphere** : HC : high concentrated H₂SO₃ ; LC : low concentrated H₂SO₃ ; S>N and HNO₃< H₂SO₃

: H₂SO₃ more concentrated than HNO₃ ; S<N and HNO₃> H₂SO₃ : HNO₃ more concentrated than H₂SO₃.
**La simulation du dépôt de polluants** a été effectuée par exposition des échantillons à des atmosphères acides de compositions et de concentrations différentes basés sur les procédures standard UNE-EN 13919: 2003 (CEN, 2002) qui préconisent l'utilisation d'acide sulfureux comme source de dioxyde de soufre.

Un volume de 65 ml de solution acide est versé dans le fond d'un récipient de 5 l dans lequel est placée une structure en plastique servant de support qui permet aux échantillons être hors de contact avec la solution. Chaque type de pierre et test a été réalisé en un récipient différent. Avant les tests, les échantillons sont immergés dans de l'eau distillée pendant 48h ce qui les sature en eau et pourtant le dépôt se fera sur des surfaces humides.

Plusieurs atmosphères acides avec différentes compositions ont été testées: acide sulfureux $H_2SO_3$ pur, acide nitrique $HNO_3$ seul et de quatres mélanges des deux acides $H_2SO_3$ et $HNO_3$ dont les proportions, mentionnées dans la Tableau III-2, ont été choisies pour simuler des scenarii de pollution représentant les émissions passées et présentes et futures.

Le type, l'intensité et la cinétique d'altération des pierres ont été évalués par des **observations macroscopiques** et **microscopiques**, les variations de **couleur**, de **poids** et de **porosité** (Figure III-5).

Les changements de **rugosité** des surfaces ont également été mesurés à l'aide d'un stéréomicroscope Leica MZ16A et du logiciel associé Leica Stereo Explorer 2.1. La résolution est de 840 Lp/mm, ce qui fournit une précision suffisante pour détecter de petites variations de surface. Le diamètre maximal de vision est de 57,5 mm, de sorte qu'une surface représentative de la pierre peut être mesurée. Cet équipement utilise la lumière visible, qui atténue les réflexions irrégulières et les réfractions des lasers en présence de quartz. Des études antérieures ont déterminé une zone de mesure optimale de 25 x 18 mm. Les paramètres de rugosité de surface ont été calculés à partir d'une moyenne de trois zones dans lesquelles vingt profils 2D espacés de 1,25 mm sont mesuré selon la norme EN 4287 (CEN, 1997). Un filtre gaussien de longueur de coupure a été appliqué pour évaluer uniquement les longueurs d'onde de rugosité. Cette coupure permet de focaliser l'analyse sur la détérioration et non sur la forme de l'échantillon. Les paramètres sélectionnés pour définir la rugosité de la surface sont l'écart moyen arithmétique (Ra), la hauteur maximale du pic (Rp), la profondeur maximale de la vallée (Rv), la courbe de la zone de roulement et la dimension fractale (D).

La production de sels au cours du temps a été quantifiée par la mesure quotidienne des **concentrations en sulfates et nitrates** contenus dans les pierres. Les échantillons ont été
Chapitre III : Suivi à méso-échelle

réduit en poudre, mis en solution dans 1 ml d'eau ultrapure, agités pendant 12 h et centrifugés
à 13400 tr/min pendant 10 min. La phase liquide a été recueillie et le processus a été répété
une seconde fois pour assurer la dissolution de tous les anions. La quantité de nitrates NO₃⁻ et
de sulfates SO₄²⁻ a été mesurée par chromatographie ionique avec un chromatographe liquide
Dionex ICS 2000 équipé d'une pompe, d'un générateur d'éluant (EGC III) avec une cartouche
d'hydroxyde de potassium (KOH), d'un injecteur AS50 et d'une cellule de conductivité DS6.
La séparation des anions a été effectuée dans une colonne Dionex IonPac AS17 de 2 mm
couplée à un suppresseur Dionex ASRS 300 de 2 mm. Le contrôle de l'instrument et
l'acquisition des données ont été effectués à l'aide du logiciel Chromeleon ®.

<figure><img src="image_52.png" /><figcaption>Figure III-5. Types et fréquences d'analyses en fonction de la taille d'échantillons soumis aux tests en atmosphère acide.</figcaption></figure>
Cette étude a fait l'objet de trois articles constituant chacun une sous partie de ce chapitre.

Dans la **première partie**, les résultats de la simulation des effets de la pluie acide ont été
publiés sous la forme de l'article « *Simulation of acid rain weathering effect on natural and artificial carbonate stones* » (Eyssautier-Chuine S., Marin B., Thomachot-Schneider C., Fronteau G., Schneider A., Gibeaux S., Vázquez P.) dans *Environmental Earth Sciences* (2016).

La **deuxième partie** présente l'étude de l'altération des pierres soumises à des tests de vieillissement accéléré dans différentes atmosphères plus ou moins saturées en dioxyde de soufre de façon à reproduire les effets des dépôts de polluants sur les pierres.

La **troisième partie** détaille les résultats des simulations en milieux acides mixtes dans l'article « *Simulation of acid weathering on natural and artificial building stones according to the current atmospheric SO₂/NOₓ rate* » (Gibeaux S., Thomachot-Schneider C., Eyssautier-Chuine S., Marin B., Vázquez P.) en cours de révision à *Environmental Earth Sciences*.

Dans la **quatrième partie**, l'influence de l'état initial de la surface de différents types de pierres sur les processus d'altération et leur réponse à des attaques acides ont fait l'objet de la publication « *Experimental study on the influence of surface finish and composition on the deterioration of building stones exposed to mixed acid atmospheres* » (Vázquez P., Carrizo L., Thomachot-Schneider C., Gibeaux S., Alonso F.J.) dans *Construction and Building Materials* (2016).
# MILIEUX ACIDES PURS

## III.1. Simulation of acid rain weathering effect on natural and artificial carbonate stones

Ce travail a donné lieu à une publication de rang A dans la revue *Environmental Earth Sciences*, volume 75, pages 1-19, 2016.

### Résumé

Cette partie présente les résultats des tests en immersion des échantillons dans une solution d'acide nitrique $HNO_3$ ou d'acide sulfurique $H_2SO_4$ à pH = 5 qui ont été mis en place afin d'évaluer la dissolution des pierres sélectionnées par contact simple avec une solution acide.

Dans le premier test, les échantillons sont placés dans les solutions sans agitation (immersion simple) tandis que dans le deuxième test les échantillons immergés dans la solution sont placés sur un agitateur pour simuler l'effet mécanique des précipitations (immersion active).

Les expérimentations ont été menées pendant 32 jours incluant cinq cycles alternant phases d'immersion et phases de séchage. La première immersion a duré 24 heures avant séchage. Les quatre cycles suivants ont impliqué 4 jours d'immersion avant séchage. La phase de séchage était similaire pour tous les cycles et effectuée pendant 64 h dans un four à 45°C. Les triplicatas de chaque type de pierre ont été observés et analysés après chaque phase de séchage puis remis en solution.

Les paramètres mesurés étaient les variations de poids, de porosité, et de concentrations dissoutes en calcium et magnésium. Les changements de couleur et de rugosité des surfaces ont également été évalués et des observations au MEB ont été réalisées.

L'analyse des résultats montre que les différences entre les effets de l'acide sulfurique et de l'acide nitrique dépendent principalement du type de pierre, de leurs propriétés microstructurales et de leurs compositions. Les immersions dans l'acide sulfurique entraînent la dissolution de la calcite et des composés carbonatés. Après séchage, des cristaux de gypse ont été observé à la surface des pierres. Les immersions dans l'acide nitrique n'ont engendré que de la dissolution, et de la précipitation de gypse uniquement dans le cas des pierres contenant des sulfates avant les tests (pierre de Savonnières et pierre reconstituée).

L'immersion active provoque des modifications plus importantes que l'immersion passive. Pendant l'immersion passive, la dissolution produite est lente. Les réactions ont lieu jusqu'à ce
que l'équilibre soit atteint et l'acide neutralisé. En immersion active, la solution pénètre plus profondément dans la pierre et produit une dissolution plus rapide et plus importante que dans une immersion passive, avec un effet mécanique qui facilite le détachement des grains de la pierre.

La texture et la composition des pierres jouent un rôle important dans la façon dont la dissolution et la précipitation ont lieu. Trois paramètres microstructuraux ont été évalués dans cette étude: la porosité ouverte, la distribution des rayons d'accès aux pores et la connectivité des pores définie par saturation capillaire. Une porosité totale élevée (Savonnières et Bajocien) favorise la dissolution du carbonate, ce qui est plus visible après une immersion passive. L'évolution de la distribution de la porosité due à la dissolution varie en fonction des composants de la pierre. La connectivité des pores joue également un rôle clé dans les processus de dissolution. En effet, une bonne connectivité permet à la solution de pénétrer dans le réseau poreux et la dissolution augmente ainsi que la porosité. Dans le cas d'une faible connectivité, comme pour la pierre reconstituée, la dissolution a lieu principalement en surface et est compensée par la précipitation du gypse.

La composition minéralogique influence également l'effet d'altération des pluies acides. Le processus de dissolution concerne principalement les composés carbonatés. Par conséquent, la dissolution des pierres composées d'un mélange de composés siliceux et carbonatés, pierre de Courville et pierre reconstituée, sera concentrée sur les composés calcitiques, réduisant progressivement la surface réactive.

Une autre conséquence majeure de l'effet des pluies acides sur les pierres est le changement de couleur de la surface. Les changements de couleur sont liés à l'oxydation du fer présent dans la roche. D'après les analyses, le fer est dispersé dans les pierres du Bajocien et de Courville donnant un changement de couleur homogène de toute la surface des pierres, malgré une concentration plus faible en fer dans la pierre de Courville. Le changement de couleur est moins prononcé sur la pierre artificielle car il affecte principalement les clastes de pierres naturelles qui la composent.
# Simulation of acid rain weathering effect on natural and artificial carbonate stones

Eyssautier-Chuine S.ª, Marin B.ª, Thomachot-Schneider C.ª, Fronteau G.ª, Schneider A.ᵇ, Gibeaux S.ª, Vázquez P.ª

Published online in Environmental Earth Sciences on April 25th, 2016.

**Keywords** : Nitric solution; Sulphuric solution; Laboratory tests; Limestone; Reconstituted stone; Microstructural characteristics.

## Abstract

Atmospheric pollutants are changing due to policies aiming at attenuating climate change. Some of the results of these measures are the current inversion on the SO₂/NOₓ proportion and the pH increase of acid rains to 5 in some areas. The aim of this study was to have a better understanding of the relation between the microstructural and chemical characteristics of carbonated stones and their response to acid rain effects. This work also compared the response of natural stones as opposed to an artificial stone used in their replacement in the Orval Abbey (Belgium). Three natural and one reconstituted stone were selected. Nitric and sulphuric acidic solutions at pH = 5 were tested. For each solution, two tests of “passive immersion” and “active immersion”, were conducted. Variations in weight, porosity, dissolved concentrations, colour and surface topography were measured. Stone SEM observations were additionally performed. The results indicate that active immersion is more effective than the passive one. The pore connectivity plays a key role in the stone dissolution and entails a difference between passive and active conditions. The different effect of nitric and sulphuric acids is related to the stone type. In spite of the initial presence of sulphur, the artificial stone appears less sensitive than the natural stone it replaces, due to its low pore connectivity and the concentrated distribution of iron oxides.

<footnote>ª Groupe d'Etudes sur les Géomatériaux et les Environnements Naturels, Anthropiques et Archéologiques (GEGENAA, EA 3795), Université de Reims Champagne-Ardenne, 2, Esplanade Roland Garros, 51100 Reims, France.</footnote>

<footnote>ᵇ Centre de Recherche en Sciences et Techniques de l'Information de la Communication (CReSTIC, EA 3804), Université de Reims Champagne-Ardenne, IFTS, 7, Boulevard Jean Delautre, 08000 Charleville-Mézières, France.</footnote>
### III.1.1. Introduction

Monuments and sculptures are testimonies of history and belong to our cultural heritage. They were built to defy time, but the progressive degradation of stones gives evidence of an irretrievable loss of our past. Stone decay cannot be avoided, but can be limited. Many researches focus on the preservation of monuments through weathering assessment and the understanding of causes to find solutions. Weathering agents are numerous and derived from the climatic conditions (rain, snow, moisture, sunlight, wind; Siegesmund and Snethlage, 2011; Winkler, 2013). Water from rain or ground contributes greatly to the stone deterioration through physical, chemical and biological weathering processes. Rain water chemistry depends on air pollution. Therefore, the stone weathering is different in urban and rural environments (Dolske, 1995; Török et al., 2011; Graue et al., 2013). Due to the industrial revolution, air pollution resulting from the production and consumption of energy like wood and coal led to an increase in the atmospheric concentration of inorganic and organic compounds (Ausset et al., 1998). Sulphur dioxide and COV have been the main pollutants, and the interaction between rain and stone induces the development of soiling and black crusts (Török and Rozgonyi, 2004; Grossi and Brimblecombe, 2007; Monna et al., 2008). Rainwater may degrade the stone by an acid rain effect. This additional acidity comes from the presence of sulphuric and nitric acids (Bonazza et al., 2009).

Urban atmosphere is changing due to a strict regulation of pollutants in industry. Air pollutants decline, and there is a shift in the concentration of the different pollutants and gases. Sulphur dioxide tends to decrease in Europe, while a rise of the traffic causes elevated levels of nitrogen compounds, ozone and particulates (CITEPA, 2015). This creates a new multipollutant situation (Kucera, 2000). The climate change involves an increase in temperature and CO2. This, together with the decline of pollutants, leads to an evolution of the rain pH, which becomes less acid with values around 5 (Bonazza et al., 2009; Franzoni and Sassoni, 2011).

Most of the building stones that appear to be very much affected by polluted environments are made of carbonate such as limestone, marble, calcareous sandstone, with calcite and/or dolomite minerals sensitive to dissolution in acid solutions (Ross et al., 1989; Baedecker and Reddy, 1993). Reddy (1988) defined three dissolution mechanisms leading to material loss: (1) attack by gaseous pollutants such as SO₂ and/or HNO₃; (2) dissolution in “clean” rain; and (3) acceleration of dissolution due to rain acidity.
The study of the effect of acid rain on building stones can be evaluated by “in situ” measurements (e.g. Sabbioni and Zappia, 1992; Reddy, 1988; Grossi and Murray, 1999; Cardell-Fernandez et al., 2002) or by laboratory tests. The latter were developed by the following two main protocols. The first was conducted by immersion of the samples in an acid solution, preferably sulphuric acid and with different pH (Holynska et al., 2004; Franzoni and Sassoni, 2011). The second was carried out in complex chambers that simulated acid rain by spraying the samples and collecting runoff water, or by applying a constant water current on the stone (Johnson et al., 1990; Tecer, 1999; Haneef et al., 1993b; Baedecker and Reddy, 1993; Thornbush and Viles, 2007). The weathering evaluation studies systematically reported a weight loss and the increase in the concentration of $Ca^{2+}$ and other elements in the run-off water due to dissolution (Tecer, 1999; Thornbush and Viles, 2007; Franzoni and Sassoni, 2011). These investigations allowed us to verify the models predicting material loss according to chemical parameters and depending on the environmental conditions (Lipfert, 1989; Livingston, 1992; Webb et al., 1992). The correlations between the weight loss, the carbonate content and the microstructure allowed us to establish a strong influence of intrinsic stone characteristics such as porosity, specific surface area and mineral (or chemical) composition (Grossi and Murray, 1999).

The general aim of this work was to analyse the alteration during different acid rain tests within a specific lithotype such as limestones, which are very sensitive to weathering. Different laboratory tests reported the results of the progressive degradation of stones by simulating the effect of acid rains enriched in sulphuric and nitric acids. Both solutions were slightly acidic (pH = 5) to approach the rain values reached over a few years (Bonazza et al., 2009). A weakly acid pH therefore allowed observing the effects of acid rain in the crystals and within the porous system because of the slower velocity in the reactions that took place. A more acidic pH would have reduced the role of the intrinsic properties of stones and produced a very aggressive effect (Zendri et al., 1996). The tests were carried out in a still solution, i.e. a “passive solution” and in an agitated solution, i.e. an “active” solution. The selected stones were three natural limestones widely used in buildings in the eastern part of France, as well as an artificial reconstituted stone intended to replace one of the natural stones during the rebuilding of Orval Abbey in Belgium (Thomachot-Schneider et al., 2011). All of them have a high carbonate content, but different petrophysical properties. Applying the same experimental conditions to both natural and reconstituted stones allowed comparing decay. The aim was to define the consequences of an acid rain simulation on a reconstituted stone
whose composition includes sulphur (Thomachot-Schneider et al., 2011). Some of the specific objectives were (1) to differentiate the effect of sulphuric acid solutions and nitric acid solutions at pH = 5 and thus have a better understanding of the effect of new pollutants' concentration in rain on building stones; (2) to distinguish and compare the effect of “passive” and “active” immersion on stones, as active immersion reflects more accurately rainfall impact than passive immersion; (3) to carry out an exhaustive study on the decay of calcareous stones in relation to their microstructural properties; and (4) to test the durability of an artificial stone (used in the restoration of Orval Abbey) exposed to acid rains, as opposed to the natural stones.

### III.1.2. Methodology

#### III.1.2.1. Weathering tests

Four laboratory tests were carried out in order to assess the effect of acid rain on carbonate stones. Two acidic solutions were used. The first was a solution of sulphuric acid ($5 \times 10^{-6}$ mol L$^{-1}$) and the second a solution of nitric acid ($10^{-5}$ mol L$^{-1}$). Both solutions were adjusted at pH = 5. For each solution, two tests were carried out. The first was a simple immersion of the samples in the solution (passive immersion), as carried out by some researchers (Franzoni and Sassoni, 2011; La Russa et al., 2015). This test allowed evaluating the dissolution of the selected stones due to a simple contact with an acidic solution. The second test was an immersion of the samples in the solution placed in a mechanical shaker to simulate storm rain (active immersion). For each test, the specimens were three 5 x 5 x 1 cm slabs of each stone type. Three natural stones and an artificial stone were selected (the characteristics are presented in “Materials”). The three stone type samples were placed in a closed container of 4 L volume. Every container was fitted with dots of silicon and plastic foam to prevent contact or collision between the three samples, and rubbing with the bottom of the container.

The tests were defined as follows:

- SPI: sulphuric passive immersion (1 L of sulphuric acid solution, $5 \times 10^{-6}$ mol L$^{-1}$, without agitation).

- SAI: sulphuric active immersion (1 L of sulphuric acid solution, $5 \times 10^{-6}$ mol L$^{-1}$, with agitation).

- NPI: nitric passive immersion (1 L of nitric acid solution, $10^{-5}$ mol L$^{-1}$, without agitation).
- NAI: nitric active immersion (1 L of nitric acid solution, $10^{-5}$ mol L$^{-1}$, with agitation).

The experimentations were conducted for 32 days including five cycles of immersion and following drying phases. The first cycle went on for 24 h of immersion before drying. The following four cycles involved 4 days of immersion before drying. The drift of the pH solution due to the reaction between carbonate stones and the acidic solutions implied a readjustment of pH twice a day until reaching again the initially fixed pH = 5. The drying phase was similar for all the cycles and carried out for 64 h in an oven at 45°C. Preliminary studies revealed that 64 h is time enough for all the stones to be completely dried. The three samples of each stone type were observed (and the various parameters measured) after drying and then put again into the solution. A 10 mL aliquot of all the immersion solutions was sampled and stored at 4°C until further chemical analyses.

## III.1.2.2. Weathering evaluation

The evolution of the acidic weathering impact on the stone decay during five immersion-drying cycles was assessed by macroscopic evaluation, changes in weight, porosity, colour and roughness of the stone samples. The chemical processes were evaluated by scanning electron microscopy, and major element concentrations in acidic solutions were determined.

### a. Macroscopic evaluation

Photographs of whole samples were taken with a Nikon Coolpix 4500 before and at the end of the experiment. Samples were observed before the immersion of every cycle with a 10x to 60x binocular magnifier (Olympus SZH-ILLB) to detect and to follow the visual alteration that the stone components underwent in contact with acidic solutions over time.

### b. Weight changes

The weight of the stone samples was measured by an analytical balance Mettler Toledo with a sensitivity of 1 mg. The percentage of the weight variation was calculated with respect to the initial weight at the end of each cycle to determine the progressive weight loss of stones during the experiment.
### c. Pore access radii

Porosity characterization was obtained using an Hg intrusion porosimeter (MIP) Micromeritics Autopore IV 9500, reaching a 414-MPa pressure and measuring pore radii sizes from 0.005 to 360 µm. In order to assess the porosity evolution, one sample of each stone type was analysed before the experiment and another sample, prepared from the altered tab, was measured at the end of the test.

The data and graphs obtained enabled to calculate mean pore access radius (Rm), modal peaks and the percentage of pores in four different pore access radius ranges: large (>1 µm), medium (1-0.1 µm), small (0.1-0.01 µm) and very small (<0.01 µm).

### d. Colour

Colour was measured and quantified with a MINOLTA CR-400 colorimeter using the D65 illuminant, a beam of diffuse light of 8 mm diameter and a 0° viewing angle geometry, with an included specular component and the spectral response closely matching the CEN (2009) standard observer curves. The measurements were expressed following the CIE L* a* b* and CIE L* C* h* systems. The L* parameter is lightness and registers the values from 0 (black) to 100 (white). The C* parameter is chroma or saturation. The higher the chroma is, the more vivid the colours are. For our device, the values went from 0 to 60. The h* parameter is hue (i.e. the tone of the stone) with the values from 0 to 360 (degrees of the chromatic sphere). The ΔE* value is introduced as the total colour change to compare the variations before and after the tests as follows:

$$ \Delta E^* = [((\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2)^{1/2}] $$

where a* and b* are Cartesian chromatic parameters.

For heterogeneous materials as stones, the determination of the colour requires a previous study of the number of measurements needed, which is related to the colour of each mineral, grain size and heterogeneity. The number of data points was determined by the calculation of the cumulative average until the stabilization of the values (Vázquez et al., 2013b; Vázquez et al., 2015). In this study, 40 data points were the minimum required in each slab.

### e. Roughness

The reverse engineering process used the ZScan 700Cx device for the 3D scanning and the 3D Reshaper software for the processing of the point cloud. The ZScan device is a handheld
laser scanner. It can quickly create a cloud of points on a surface with an accuracy of 0.05 mm and a resolution of 0.1 mm. Its rapid and precise work was a key criterion for this choice, supported in particular by Barbero and Ureta (2011), Kuzminsky and Gardiner (2012), Tóth and Živčák (2014), who highlighted the limits of use but also the significant potential of this technology.

One of the main features of this scanner is its method to be referenced in the space using reflective targets. At any time, if the scanner can "see" at least four positioning targets, the software will be able to calculate its position in the system coordinates and to start collecting data useful for the creation of the point cloud (Hebert, 2001). For a better definition in the images, the scanner parameters (laser power and shutter time) should be adapted to each object that will be scanned. Once the model is scanned, depending on the level of detail required in the point cloud, its density can be adjusted to optimize the amount of memory required. The point density can be significantly reduced without loss of surface quality (Hormann *et al.*, 2001). Once the scanning session is complete, the point cloud can be exported as an STL file and can be modified by a dedicated software (in this case Reshaper 3D). This software offers a wide range of tools to reduce the noise in the point cloud, fill holes, reconstruct the missing areas and modify the original cloud of points to get the necessary numerical model ready to be used for numerical simulation.

Due to the complexity of the data treatment, and the innovative use of this software for stone decay, only the samples exposed to one of the immersion tests were measured before and after the tests. SAI was selected due to the greatest variations after visual observations.

## f. Scanning electron microscopy

The surfaces of each stone sample submitted to the four tests were observed under SEM at the end of the experiments. The apparatus was a SEM Hitachi TM-1000 Table Top with an energy-dispersive X-ray spectrometer [swifted-TM energy-dispersive X-ray (EDX)]. The samples were introduced in the microscope without any coating but maintained on the specimen stage by double-sided adhesive carbon tape. The accelerating voltage was 15 kV for imaging and EDX analysis. The working distance was 6 mm. All images were acquired in the back-scattered electron mode.
### g. Dissolved concentrations

The concentrations of Ca, Mg, Si and Fe were measured in acid solutions of each cycle by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Advantage, Thermo Fisher Scientific). The analysis of Ca, Mg, Si and Fe was conducted at wavelengths of 184.006, 279.553, 251.611 and 259.940 nm, respectively, through three measurements of each solution.

## III.1.3. Results

### III.1.3.1. Materials

The three natural stones come from the north of France and/or the south of Belgium. The textural, microstructural and chemical characteristics of these natural stones and the reconstituted ones are presented in Tableau III-3.

<table><caption>Tableau III-3. Textural, microstructural and chemical characteristics of the four studied stones.</caption><thead><tr><th>Stone</th><th>Classification (Dunham)</th><th>CaCO<sub>3</sub> (wt.%)</th><th>MgO (wt.%)</th><th>SiO<sub>2</sub> (wt.%)</th><th>Fe<sub>2</sub>O<sub>3</sub> (wt.%)</th><th>Capillarity saturation (%)</th><th>Porosity denomination</th></tr></thead><tbody><tr><td>Bj</td><td>grainstone</td><td>94.6</td><td>0.41</td><td>1.09</td><td>1.44</td><td>70</td><td>macroporosity</td></tr><tr><td>Cv</td><td>wack/packstone</td><td>90.3</td><td>0.58</td><td>4.90</td><td>0.39</td><td>90</td><td>microporosity</td></tr><tr><td>Rc</td><td>wackstone</td><td>86.9</td><td>0.53</td><td>6.64</td><td>1.94</td><td>51</td><td>microporosity</td></tr><tr><td>Sv</td><td>grainstone</td><td>96.3</td><td>0.65</td><td>0.68</td><td>0.25</td><td>48</td><td>macroporosity</td></tr></tbody></table>

The Bajocian building stones (termed Bj further in this article) from the French Ardennes and the south of Belgium are oolitic to bioclastic limestones (Fronteau, 2000; Morel et al., 2012; Eyssautier-Chuine et al., 2015). The Pas-Bayard stone was extracted near Grandcourt from “Jaumont oolite” for the Orval Abbey reconstruction at the beginning of the XXth century (Thomachot-Schneider et al., 2011). The Bajocian limestones are yellow ochre to russet macroporous building stones. They can be classified as grainstone (Dunham, 1962) or as oobiosparite (Folk, 1959). Ooids and cortoids are more numerous than true oolites, with bioclasts (Bivalvia or Echinoderms fragments) as nucleus. The texture is grain-supported with early diagenetic spar cements (fibrous and syntaxic) and interstitial drusy blocky calcite. The intergranular porosity is low, but a secondary moulding intragranular porosity (Choquette and Pray, 1970) is observed (Figure III-6a). Iron oxide deposits are in the porosity and spread over the stone surface.
The Lutetian building stones from Reims surroundings are fine micritic limestones extracted in numerous sites in a twenty-kilometre-long quarrying area (Devos *et al.*, 2010; Fronteau *et al.*, 2010b). Within the various quality of building stones coming from these middle to upper Lutetian formations (Eocene), the “Courville stone” is the most famous and was used until a few years ago for monument restorations (Turmel *et al.*, 2014). The Courville stone (termed Cv further in this article) is a russet microporous building stone, which can be classified as a wackestone/packstone (Dunham, 1962) or as biomicrite to biomicrosparite (Folk, 1959). The Cv stone shows an open porosity of around 16 %. Its main components consist of Mollusca shells – Bivalvia and Gastropoda (sometimes with aragonite shell preserved), Foraminifera (miliolids and *Orbitolites complanatus*) and green algae (Dasycladales). The texture is matrix-supported, with partly recrystallized micritic matrix (microsparite) and secondary spar cement in the moulding macropores or in some intragranular fossil porosity (Figure III-6b).

The reconstituted stone is an artificial material used during the rebuilding of the Orval Abbey from 1930 to 1948 with the objective to complete the stone supplies coming from the Bajocian stone quarry of Pas-Bayard (Thomachot-Schneider *et al.*, 2011). The reconstituted stone (termed Rc further in this article) is a heterogeneous material mainly composed of calcareous and siliceous lithoclasts of millimetric to centimetric size, mixed with quartz grains, Portland cement, calcic lime and mineral pigments (iron oxides). The ratio of elements to binding phase is highly variable as are the size and nature of the limestone lithoclasts (Figure III-6c). The texture is matrix-supported, with a microporosity revealed by a diffuse blue colour and sometimes round pores corresponding to bubbles. A macroporosity within the calcareous lithoclasts is also present. Both sizes confer to the stone an open porosity of around 20-25%.

Oolitic vacuolar building stones from the area of Saint-Dizier are frequently called “Savonnières stone”, according to the most famous commercial stone name of this type of limestone (Roels *et al.*, 2003; Dewanckele *et al.*, 2014; Walbert *et al.*, 2015). The Savonnières stone has been quarried since antiquity in the “Oolite vacuolaire” formation from the end of Tithonian (Jurassic) units. This world renowned limestone was frequently observed in buildings from the XIXth to XXth centuries or proposed for restoration works all over the western part of Europe (Turmel *et al.*, 2014; Laycock *et al.*, 2008). The Savonnières stone (termed Sv further in this article) is a grey macroporous building stone. It can be classified as grainstone (Dunham, 1962) or as oosparite (Folk, 1959). Its main components consist of true oolites and to a lesser amount of fragments or whole Bivalvia shells. The texture is clearly
grain-supported, with fibrous to bladed spar cements around oolites and Bivalvia shells (Figure III-6d). As observed in Bajocian limestones (Bj), the closing of the intergranular porosity is incomplete but Sv presents a second intragranular moulding macroporosity too, especially inside the oolites with the partial or complete dissolution of the nucleus. Thus, the open porosity is about 30-35 %.

<figure><img src="image_53.png" /><figcaption>Figure III-6. Blue-coloured thin sections of studied building stones in plane polarized light. (a) Pas Bayard stone from upper Bajocian (Jurassic; Bj) of Orval area; (b) Courville stone (Cv) from middle Lutetian (Eocene) of Reims area; (c) reconstituted stone (Rc) from Orval abbey (Belgium); (d) Savonnières stone (Sv) from Tithonian (Jurassic) of Saint-Dizier area.</figcaption></figure>

### III.1.3.2. Macroscopic observations

Observation to the naked eye showed a clear change in colour between active and passive tests for the two acidic solutions and was ascribed to a higher alteration of stones in active tests (Figure III-7). This colour change was emphasized in Bj, which became orange at the end of the experiment and in Cv which shifted from white to ochre. The alteration of samples seemed similar in $H_2SO_4$ and $HNO_3$ solution. However, at the end of the tests, a salt precipitation was clearly observed by binocular magnifier in samples immersed in $H_2SO_4$ solutions. Gypsum crystals covered the entire surface of all samples, especially in Bj.
<table><thead><tr><th rowspan="3"></th><th rowspan="3">Before immersion</th><th colspan="4">After 17 days of immersion</th></tr><tr><th colspan="2">H<sub>2</sub>SO<sub>4</sub></th><th colspan="2">HNO<sub>3</sub></th></tr><tr><th>SPI</th><th>SAI</th><th>NPI</th><th>NAI</th></tr></thead><tbody><tr><td>Bj</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cv</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Rc</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sv</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Figure III-7. Photos of entire stone samples before (fresh samples) and after the four weathering tests (SPI: Sulphuric Passive Immersion; SAI: Sulphuric Active Immersion; NPI: Nitric Passive Immersion; NAI: Nitric Active Immersion).

Observations with a binocular magnifier after every cycle gave more details and displayed an alteration as reddening after the third immersion-drying cycle. At that moment, Bj and Rc were already redder in the active than in the passive test, and their matrix was completely oxidized at the end of the test (Figure III-8). Meanwhile, the alteration in the Cv specimens was distinguished later, after the fourth cycle.

In Bj samples, russet particles due to oxidation were developed in porous walls and progressively in ooid micritic elements and sparitic cements. At the end of the experiment, iron oxidation intensified the russet colour of stone in a homogeneous way.
In Cv samples, the impact of acidic solutions started with the oxidation of calcitic cements which turned to russet in miliolids and *Orbitolites* (Figure III-8). Other foraminiferae such as *Rotalidae* kept to a translucent original state and dasycladales algae were still bright white calcite, and only the matrix around them was oxidized. At the end of the test, the oxidation spread out over the whole surface of the sample.

As Rc cement contains Bj lithoclasts, the oxidation was similar to Bj samples and those lithoclasts were much more oxidized than the cement (Figure III-8). At the end of the experiment, the cement was russet and lithoclasts became brown with a development of a microporosity (see below). Furthermore, fibrous calcite cement was smoothed in initial porosity.

The Sv stone is the whitest of the four studied stones. Its alteration started with the appearance of a russet colour around the pores that replaced the nucleus of the oolites. The oolitic cortex became yellow at first, then russet by oxidation and the fibrous calcitic cement was smoothed at the end of the test.

<table><caption>Figure III-8. Details of stones before (fresh samples) and after the active sulphuric (SAI) and nitric (NAI) tests.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Before immersion</th><th colspan="2">After 17 days of immersion</th></tr><tr><th>SAI</th><th>NAI</th></tr></thead><tbody><tr><th>Bj</th><td></td><td></td><td></td></tr><tr><th>Cv</th><td></td><td></td><td></td></tr><tr><th>Rc</th><td></td><td></td><td></td></tr><tr><th>Sv</th><td></td><td></td><td></td></tr></tbody></table>
## III.1.3.3. Stone weight loss

The weight variation (ΔW) of the samples during the four tests was expressed in percentage with respect to their initial weight (Figure III-9). All the samples displayed a decrease in weight over time (except two Rc samples immersed in SPI and one in NAI which gained weight from the first cycle until the end of the test, probably due to the insufficient drying). The active tests led to a higher weight loss than the passive ones. Bj and Cv showed a maximal ΔW of -1.5 % in active immersion, whereas the highest values were around 0.3 % in passive tests (Figure III-9.a, b). The difference between values relative to the passive and the ones relative to the active tests was less pronounced in Rc and Sv (Figure III-9.c, d).

In active tests, the immersion in sulphuric acid solution entailed a slightly higher weight loss compared to the nitric solution, with the exception of Cv. In passive tests, the values obtained with both acidic solutions were very similar. Only Bj and Sv showed a slight difference. Bj exhibited a higher loss of weight after passive immersion in sulphuric acid solution, while Sv showed the highest values after passive immersion in the nitric one.

Among the four stones, Sv showed the highest weight loss (1.8 %) and Rc the lowest (1.3 %). Bj and Cv featured intermediate and comparable response.

<figure><img src="image_54.png" /><figcaption>Figure III-9. Weight loss (ΔW, expressed in %) versus time for Bj (a), Cv (b), Rc (c) and Sv (d) during the four weathering tests (SPI: Sulphuric Passive Immersion; SAI: Sulphuric Active Immersion; NPI: Nitric Passive Immersion; NAI: Nitric Active Immersion).</figcaption></figure>
### III.1.3.4. Porosity evolution

The fresh stones studied showed a high Hg porosity, with values ranging from around 16 % in Cv, 20-23 % in Bj and Rc to a maximum of approximately 31 % in Sv (Tableau III-4).

<table><caption>Tableau III-4. Porosity parameters (calculated from Hg porosimetry).</caption><thead><tr><th rowspan="2">Stone</th><th rowspan="2">Test</th><th rowspan="2">Porosity (%)</th><th colspan="4">Pore size distribution (%)</th></tr><tr><th>&gt; 1 µm</th><th>1-0.1 µm</th><th>0.1-0.01 µm</th><th>&lt;0.01 µm</th></tr></thead><tbody><tr><td rowspan="5">Bj</td><td>Fresh</td><td>20.30</td><td>46.3</td><td>43.4</td><td>10.2</td><td>0.1</td></tr><tr><td>SPI</td><td>28.95</td><td>57.0</td><td>35.2</td><td>7.2</td><td>0.5</td></tr><tr><td>SAI</td><td>28.45</td><td>54.3</td><td>38.0</td><td>7.2</td><td>0.5</td></tr><tr><td>NPI</td><td>30.71</td><td>61.0</td><td>32.7</td><td>6.1</td><td>0.2</td></tr><tr><td>NAI</td><td>31.32</td><td>60.2</td><td>32.7</td><td>6.5</td><td>0.5</td></tr><tr><td rowspan="5">Cv</td><td>Fresh</td><td>15.85</td><td>3.9</td><td>18.9</td><td>77.2</td><td>0.0</td></tr><tr><td>SPI</td><td>24.30</td><td>3.6</td><td>42.6</td><td>52.8</td><td>0.9</td></tr><tr><td>SAI</td><td>19.55</td><td>3.3</td><td>23.6</td><td>71.8</td><td>1.4</td></tr><tr><td>NPI</td><td>22.74</td><td>2.9</td><td>35.3</td><td>60.8</td><td>0.9</td></tr><tr><td>NAI</td><td>22.68</td><td>2.3</td><td>29.5</td><td>67.2</td><td>1.0</td></tr><tr><td rowspan="5">Rc</td><td>Fresh</td><td>22.91</td><td>5.9</td><td>36.5</td><td>56.9</td><td>0.7</td></tr><tr><td>SPI</td><td>22.40</td><td>6.2</td><td>25.1</td><td>59.3</td><td>9.4</td></tr><tr><td>SAI</td><td>24.73</td><td>8.2</td><td>26.8</td><td>61.3</td><td>3.7</td></tr><tr><td>NPI</td><td>24.78</td><td>3.9</td><td>21.0</td><td>71.5</td><td>3.7</td></tr><tr><td>NAI</td><td>23.56</td><td>4.8</td><td>12.3</td><td>75.5</td><td>7.4</td></tr><tr><td rowspan="5">Sv</td><td>Fresh</td><td>30.96</td><td>56.5</td><td>26.5</td><td>17.0</td><td>0.0</td></tr><tr><td>SPI</td><td>33.73</td><td>52.8</td><td>36.0</td><td>11.2</td><td>0.0</td></tr><tr><td>SAI</td><td>28.47</td><td>70.0</td><td>22.3</td><td>7.7</td><td>0.0</td></tr><tr><td>NPI</td><td>29.41</td><td>56.2</td><td>30.9</td><td>12.9</td><td>0.0</td></tr><tr><td>NAI</td><td>30.38</td><td>72.5</td><td>20.0</td><td>7.5</td><td>0.0</td></tr></tbody></table>

The Bj stone featured large pore radii access with a bimodal distribution (Figure III-10.a). More than 45 % of the values were higher than 1 µm and almost 45 % were between 0.1 and
1 µm. The Cv stone exhibited a high proportion of small pore access radii with almost 80 % of radii between 0.01 and 0.1 µm, and with a sole modal peak at 0.1 µm (Figure III-10.b). Rc had a bimodal distribution with around 57 % of the pore access radii within the interval for small pores and a high amount of medium pore access radii (36 %; Figure III-10.c). Sv had more than 50 % of large size pore access radii (i.e. >1 µm), and more than 25 % in the medium size range. These two grouped pore access radii are well represented in Figure III-10.d.

The evolution of porosity values obtained by Hg porosimetry is also shown in Tableau III-4 and Figure III-10. Open porosity increased in Bj and Cv with similar values for all the tests. In Rc and Sv, variations in open porosity were not detected. This implies that in the stones in which a notable dissolution occurs, there was no difference between passive and active immersion or between acidic solutions. Regarding the pore radii access evolution, some trends were observed in relation to the immersion type, regardless of the solution composition. The variations were roughly the same for all the stones. The proportion of pore radii access in the large and small ranges grew in the active immersion, while the quantity of medium range radii (0.1-0.01 µm) was reduced. Only Sv behaved a bit differently, with a decrease in active immersion for the small range.

Only Bj and Rc exhibited a different trend between sulphuric and nitric solution, independently of the immersion type. The increase in Bj porosity was higher in the large range when the samples were immersed in the nitric acid solution, to the detriment of the other pore ranges. The immersion in nitric acid solution entailed an increase in the small range of Rc pore access radii, while the immersion in sulphuric acid solution led to higher values of the other ranges.

The Bj and Cv stones exhibited the highest increase in the open porosity, while Rc and Sv showed slight variations of between 0.2 and 3% around the fresh stone values. Measurements after all the tests revealed that the proportion of large pores (>1 µm) increased notably in Bj following all the tests and in Sv after active immersion. The proportion of pore access radii of the medium range increased following all the tests in Cv and the active immersion in Sv. Finally, the data obtained for the small pore access radii (0.1-0.01 µm) denoted a diminution in three of the stones (Bj, Cv and Sv), while Rc showed the opposite behaviour. Regarding the very small pore radii access, only the behaviour of Rc is remarkable, with a change ranging from 0.7 to 4-10 % depending on the test.
<figure><img src="image_55.png" /></figure>

Figure III-10. Incremental mercury intrusion in relation to pore size. Comparison of the pore access radii distribution of the fresh and the weathered stones (SPI: Sulphuric Passive Immersion; SAI: Sulphuric Active Immersion; NPI: Nitric Passive Immersion; NAI: Nitric Active Immersion).

### III.1.3.5. Colour changes

The chromatic parameter values of the fresh stones are shown in Tableau III-5. All the samples showed whitish to ochre colours, according to their L*, C* and h* parameters. Cv and Sv showed the highest variability among samples, while in the case of Bj and Sv the highest was within one sample.

The colour variation during the tests is shown in Figure III-11. The most remarkable difference was observed between the active and the passive immersion tests. The decrease in L* is higher in active than in passive tests and C* increased conversely. The L* parameter behaved similarly for both acidic solutions. The variation in the other parameters depended on the stone type.

All the stones exhibited obvious changes following all the tests (except Cv during SPI and NPI tests and Sv during SPI test). The lightness (L*) decreased in all the measured samples.
Chapitre III : Suivi à méso-échelle

The most noticeable darkening was observed in Bj and to a lesser extent in Cv. The chroma (C*) parameter behaved similar to L*, but with opposite values, with the highest intensity increase for Bj, followed by Cv. Rc and Sv showed the lowest C* values, with an increase in active tests and a decrease in passive ones.

The hue (h*) increased slightly in Cv after the passive immersion cycles. All the tests revealed an h* notable decrease in Rc up to a constant value. The final reddening was thus comparable between the four weathering methods. The h* decreased in Sv for all the tests, but it was more pronounced in nitric acid immersion. The total colour parameter evolution reflected that Bj and Cv suffered the highest changes, mainly in active tests, and slightly higher in HNO₃ than in H₂SO₄ solutions.

<figure><img src="image_56.png" /><figcaption>Figure III-11. Variation of chromatic parameters before and after the weathering tests (SPI: Sulphuric Passive Immersion; SAI: Sulphuric Active Immersion; NPI: Nitric Passive Immersion; NAI: Nitric Active Immersion).</figcaption></figure>
<table><caption>Tableau III-5. Colour values of the fresh stones. Maximum and minimum values reflected the heterogeneity of the samples. The Maximum deviation (Max dev) is the highest value found for one specimen.</caption><thead><tr><th rowspan="2">Stone</th><th colspan="2">L*</th><th colspan="2">C*</th><th colspan="2">h*</th></tr><tr><th>Min and Max</th><th>Max dev</th><th>Min and Max</th><th>Max dev</th><th>Min and Max</th><th>Max dev</th></tr></thead><tbody><tr><td>Bj</td><td>67.74 to 69.25</td><td>2.39</td><td>24.42 to 25.14</td><td>3.35</td><td>78.80 to 79.40</td><td>5.49</td></tr><tr><td>Cv</td><td>73.85 to 81.24</td><td>1.3</td><td>14.79 to 26.60</td><td>2.09</td><td>81.19 to 84.56</td><td>0.79</td></tr><tr><td>Rc</td><td>70.38 to 71.42</td><td>0.42</td><td>24.03 to 24.93</td><td>1.16</td><td>89.07 to 89.84</td><td>1.04</td></tr><tr><td>Sv</td><td>75.5 to 76.92</td><td>0.96</td><td>15.44 to 19.10</td><td>2.15</td><td>83.33 to 93.96</td><td>8.29</td></tr></tbody></table>

### III.1.3.6. Roughness

Some examples of roughness measurements are shown in Figure III-12.a, b. The measured samples were those immersed in SAI since they seemed to exhibit the highest changes in surface topography after naked eye observation and binocular magnifier studies. The 3D Reshaper software and the referenced measurements allow the subtraction of the samples before and after the test as a means to observe in which zones dissolution or precipitation occurred. The location of dissolution and precipitation occurrence was similar in all the stones. The dissolution predominated in the stone edges. The loss of grains in Sv is evident, with areas up to 0.5 mm deeper than in the fresh stone (Figure III-12.b). The gypsum precipitation seemed to concentrate in preexisting pores and defects. Previous large pores visible with the naked eye presented dissolution on their limits and precipitations in their inner part. Precipitation was found in previous defects not detectable in the single images (e.g. sawing marks). In the case of Cv, the largest gypsum crystals were observed in the faces that were dried vertically (Figure III-12.a). Gypsum crystals concentrated in small areas on the Rc surface, corresponding mainly to carbonate clasts.
<figure><img src="image_57.png" /></figure>

Figure III-12. 3D surface topography of Cv and Sv before and after sulphuric active test (SAI). The bottom image corresponds to the subtraction of both results. Values around zero means no change before and after the tests. Positive and negative values indicate the occurrence of precipitation and dissolution, respectively.

### III.1.3.7. SEM observations

The sample surfaces observed under SEM showed combinations of dissolution-crystallization features depending on the immersion type, solutions and stones. The samples submitted to active immersion presented more dissolution or crystallization features than those submitted to passive immersion. The dissolution features were similar in both active tests. When gypsum occurred, the amount of sulphur analysed with EDS was generally twice for samples tested in active immersion.

After immersion in sulphuric acid solution, gypsum crystallization occurred as micro crystals of around 1 µm (Figure III-13.b, c), glaze (Figure III-13.d) or star crystals of around 10 µm length (Figure III-13.a). These crystals did not form a uniform covering of the surface (Figure III-13.b). The gypsum crystallization could be observed in the porous network of Bj as around 10 µm long crystals arranged in a star shape (Figure III-13.a). Sulphur content measured by EDS was around 31 % after SAI and around 16 % after SPI tests. Dissolution occurred in the echinoderm fossils and in the calcite macrocrystals giving them a layered aspect (Figure III-13.f). In Cv, the crystallization of gypsum could be observed in the porous network as microcrystals of around 1 µm (Figure III-13.b). Sulphur content measured by EDS was around 17 % after SAI and around 8 % after SPI tests. Dissolution of the foraminifera tests
occurred as well. Microcrystals of gypsum were mixed in the matrix of Rc (Figure III-13.c). Sulphur content measured by EDS was around 14 % after SAI and around 12% after SPI tests. Dissolution occurred mainly in the aggregate made of Bj limestone, while clinker elements remained untouched. The crystallization of gypsum could be observed in the porous network of Sv as a non-uniform glaze covering (Figure III-13.d, g). Sulphur content measured by EDS was around 33 % after SAI and around 7 % after SPI tests. Dissolution features were observed mainly on the sparitic cement around the macropores and in the cortex of the oolite (Figure III-13.g).

In the case of nitric acid immersion tests (NPI and NAI), only gypsum crystals were observed in Rc that was originally rich in sodium sulphate. In all the stones tested in NPI, dissolution features were hardly observed. For NAI, dissolution was observed in three of the four stones. The dissolution was focused on echinoderm fossils and in the calcite macrocrystals in Bj, in the calcareous elements in Rc and in the macrocrystals like fibrous cements in Sv (Figure III-13.h).
<figure><img src="image_58.png" /><figcaption>Figure III-13. SEM images of gypsum crystal shapes of stones tested in sulphuric active immersion (SAI; a, b, c and d); associated dissolution features (e and f); porous network of Sv after g) sulphuric active test (SAI; covering of gypsum and dissolution features) and h) nitric active test (NAI; dissolution features).</figcaption></figure>
### III.1.3.8. Dissolved concentrations

The concentrations of Ca increased with time for all the stone types with the weathering methods used (Figure III-14). They reached between 600 and 800 mg L⁻¹ at the end of the entire test immersion. Clear differences were observed between active and passive immersions. The final Ca concentrations were higher when the solutions were agitated. The difference was more marked for Cv for which the concentrations were lower than 300 mg L⁻¹ in passive solutions and near 800 mg L⁻¹ in active solutions (Figure III-14.b). The difference was marked for Bj and Sv too (Figure III-14.a, d). It was less pronounced for Rc (Figure III-14.c). The difference between HNO₃ and H₂SO₄ solutions was significant for most of the stones. The H₂SO₄ solution was generally more aggressive (higher Ca concentrations) than the HNO₃ one (Figure III-14). The highest Ca losses were observed for Sv and Bj as they reached 1.5 % in H₂SO₄ active solutions, with respect to the initial content in the samples (Tableau III-6). For Sv, the loss was notable even in passive solutions (around 1.0 % for SPI and NPI). The maximum Ca loss was 1.4 % for Cv and only 1.2 % for Rc.

The difference between the four weathering methods for Mg and Si concentrations was less clear, but trends emerged from one stone to another (data not shown). The losses of Mg stayed lower than 1.5 % for Bj and Sv and 2% for Rc (concentrations lower than 6 mg L⁻¹; Tableau III-6). They reached 3.8 % (20 mg L⁻¹) in Cv. The Si concentrations in acid solutions were lower than 6 mg L⁻¹ for Bj, Cv and Sv and equal to 19 mg L⁻¹ in Rc. The loss of Si, nevertheless, was only 0.43 % for this stone considering the high initial Si content (3.1 wt%; Tableau III-3). For low initial Si contents in Bj and Sv, the losses were close to 0.6 %. The loss was very low in Cv (0.1-0.2 %) as the initial content was near 3 wt%.

The Fe concentrations were systematically lower than the detection limit in the sample solutions. The Fe loss can be considered as negligible whatever the weathering method and the stone type.
<figure><img src="image_59.png" /></figure>

Figure III-14. Ca²⁺ concentrations (expressed in mg L⁻¹) versus time for Bj (a), Cv (b), Rc (c) and Sv (d) stones during the four weathering tests (SPI: Sulphuric Passive Immersion; SAI: Sulphuric Active Immersion; NPI: Nitric Passive Immersion; NAI: Nitric Active Immersion).
<table><caption>Tableau III-6. Maximal CaCO₃, MgO and SiO₂ losses (expressed in % with respect to the initial contents of the stone types; Tableau III-3) by the four stones at the end of the four weathering tests.</caption><thead><tr><th></th><th></th><th>CaCO<sub>3</sub></th><th>MgO</th><th>SiO<sub>2</sub></th></tr></thead><tbody><tr><td rowspan="4">Bj</td><td>SPI</td><td>0.75</td><td>0.87</td><td>0.57</td></tr><tr><td>SAI</td><td>1.59</td><td>1.43</td><td>0.42</td></tr><tr><td>NPI</td><td>0.41</td><td>0.50</td><td>0.32</td></tr><tr><td>NAI</td><td>1.22</td><td>1.16</td><td>0.40</td></tr><tr><td rowspan="4">Cv</td><td>SPI</td><td>0.49</td><td>3.76</td><td>0.15</td></tr><tr><td>SAI</td><td>1.40</td><td>3.72</td><td>0.11</td></tr><tr><td>NPI</td><td>0.33</td><td>2.80</td><td>0.15</td></tr><tr><td>NAI</td><td>1.43</td><td>3.85</td><td>0.12</td></tr><tr><td rowspan="4">Rc</td><td>SPI</td><td>0.48</td><td>0.54</td><td>0.21</td></tr><tr><td>SAI</td><td>1.22</td><td>1.93</td><td>0.38</td></tr><tr><td>NPI</td><td>0.71</td><td>1.53</td><td>0.42</td></tr><tr><td>NAI</td><td>1.03</td><td>1.37</td><td>0.29</td></tr><tr><td rowspan="4">Sv</td><td>SPI</td><td>1.01</td><td>1.10</td><td>0.64</td></tr><tr><td>SAI</td><td>1.53</td><td>1.14</td><td>0.53</td></tr><tr><td>NPI</td><td>0.88</td><td>0.63</td><td>0.63</td></tr><tr><td>NAI</td><td>1.47</td><td>0.94</td><td>0.41</td></tr></tbody></table>

### III.1.4. Discussion

The immersion of calcitic stones in acid solutions of pH 5 leads to a loss of material corroborated by weight measurements, dissolved concentrations in the solutions after immersion, porosity and surface relief variations. The carbonate dissolution is accompanied by other chemical reactions such as salt precipitation and iron oxidation, leading to colour changes (Grossi and Murray, 1999; Vázquez and Alonso, 2015; Vázquez *et al.*, 2015).

#### III.1.4.1. Differences between weathering tests

In general terms, active immersion appears to be more powerful than passive immersion. pH monitoring during the weathering tests indicated a faster increase in active than in passive conditions. Measurements of weight, porosity and dissolved Ca²⁺ concentrations indicate a higher loss of material by dissolution in active immersion. During passive immersion, slow dissolution occurs. The reactions take place on the exposed surface until the equilibrium is reached and the acid neutralized. In active immersion, the solution enters deeper into the stone because of the force of the stirring. This produced a faster and greater stone dissolution than in passive immersion and also a mechanical effect that may contribute to particle detachment (Lipfert, 1989; Webb *et al.*, 1992; Baedecker and Reddy, 1993).
There is no clear trend to differentiate the nitric and sulphuric acid impact. Nevertheless, the highest weight and Ca losses as well as porosity redistribution were predominantly obvious in sulphuric conditions, according to Franzoni and Sassoni (2011) results. The main difference between the tests is the occurrence of gypsum on the stone surface in sulphuric immersion. In nitric immersion, calcium nitrite is expected to crystallize at the end of the tests. Nevertheless, due to the high solubility of this salt, it precipitates mainly in the stone, such observation being unfeasible by the methods used in this study.

### III.1.4.2. Influence of the microstructural characteristics of natural stones

The texture and composition of the stones play an important role in the way dissolution and precipitation take place (Grossi and Murray, 1999). Three microstructural parameters were assessed in this study: open porosity, pore access radii distribution and pore connectivity defined by capillary saturation.

Three natural stones (Bj, Cv and Sv) with different open porosities were evaluated. Sv with the highest porosity (30 %) showed the strongest loss of weight and dissolved Ca concentration for all the tests. Bj with intermediate porosity (20 %) and Cv with the lowest (15 %) behaved similarly as for weight loss and only slightly differently as for dissolved Ca concentrations. This means that the high total porosity favours carbonate dissolution, which is more noticeable after passive immersion. In this case, Rc, with similar porosity in Bj, showed the lowest values in most of the measured properties. These results can be explained by the fact that it is not entirely composed of carbonate minerals exposed to dissolution.

The initial pore size is a relevant factor too. Cv has almost 80 % of the pores between 0.1 and 0.01 µm, which differs enormously from the pore access radii sizes of Bj being around 90 % higher than 1 µm. Nevertheless, both stones display a similar weight loss and a remarkable difference between passive and active tests. The Ca losses and colour changes are also similar in these two stones in a general way. On the contrary, the Ca loss differs substantially in Bj and Sv although both show a great proportion of larger pore access radii.

The third microstructural parameter is the pore connectivity. The Bj and Cv stones of similar behaviour have a high coefficient of capillary saturation (70 and 90 %, respectively), whereas it is only 48 % in Sv. The capillary saturation can be translated as pore connectivity, which allows the acid solutions to enter the stone and dissolve it. A high pore connectivity favours the penetration of the solution and the increase in the stone-solution contact area. Thus, Bj and Cv with a high pore connectivity suffer an evident increase in open porosity after all of
the tests according to their capillary saturation coefficient. The open porosity of weathered Sv is similar to the fresh stone.

The evolution of the porosity distribution due to dissolution varies in relation to the stone components. The pore access radii of Bj increase, keep the same bimodal distribution as the fresh stone, but are relocated in large values (Figure III-10.a). Slightly higher values were obtained in nitric acid solution than in sulphuric immersion. This can be due to a higher dissolution in the nitric solution or to the compensation by gypsum crystallization in big pores observed after the sulphuric immersion. In addition, the dissolution discrepancies between active and passive immersions can be explained by the additional mechanical loss of material focused on the surface that does not affect the pore dissolution into the stone.

The increase in Cv porosity was ascribed to an increase in micropore amount of around 0.1 % except for the SAI test for which there was a clear widening in the pore access radii (Figure III-10.b). The augmentation of micropores was explained by the alteration of microfossils, especially miliolids and *Orbitolites*, which were progressively dissolved as confirmed by SEM. In the case of SAI, the dissolution around bigger pores was observed under SEM, as well as the crystallization of gypsum in these pores as a crown. This explains the increase in the pore access radii size. Cv has a magnesian micrite that entails the highest magnesium loss, as SO₂ reacts preferentially with Mg (Grossi and Murray, 1999). However, no magnesium salts were evidenced during our experiment.

The macroporosity and microporosity of Sv are located in the oolitic nuclei and in the calcitic cortex, respectively (Roels *et al.*, 2003). The initial porosity showed a bimodal distribution, with modal peaks around 0.1 and 7 µm. Acid attacks partially dissolve the fibrous cement (observed by binocular microscope and SEM) and increase the connectivity. In addition, the progressive dissolution of the oolite's cortex increases the microporosity size and improves the access to the macroporous nuclei. Active immersion tests lead to an increase in the size of the pre-existent macropores, even after the crystallization of gypsum.

This heterogeneity in initial texture and dissolution patterns explains why pore access radii distribution does not seem to interfere in the dissolution kinetics, according to the results from Franzoni and Sassoni (2011) for similar stones, and unlike pore connectivity. In Bj and Cv, the open porosity increases similarly for all the tests, while the weight and Ca losses showed wide differences between active and passive tests. This indicates that the active tests produce a more important mechanical effect than a possible higher dissolution. Lipfert (1989)
considered that the mechanical effect of rain involved around 50 % of the material loss. In this study, if we consider mechanical loss as the difference between passive and active immersion, the results go from around 75 % of mechanical loss in Cv and Bj and 30 % (sulphuric solution) and 40 % (nitric solution) in Sv. The latter result agrees with Webb *et al.* (1992) who considered only a mechanical loss of 30 %.

### III.1.4.3. Differences between natural and artificial stones

The mineralogical composition also influences the weathering effect of acid rains. The dissolution process mainly concerns carbonates. While Ca-carbonate dissolves faster than Mg-carbonate, some researchers found that SO₂ firstly attacks magnesium salts (Grossi *et al.*, 1995). Others found that the reaction products (i.e. gypsum crystals) are concentrated only on calcite crystals and avoid the dolomite ones (Malaga-Starzec *et al.*, 2003; Fronteau *et al.*, 2010b). The dissolution in siliceous stones depends on the carbonate content. If stones are composed of a mix of siliceous and carbonaceous compounds, the dissolution will be focused on the calcitic ones. In this study, two stones showed a non-pure calcitic composition, Cv (with magnesium-rich micrite) and Rc. The Rc artificial stone is used for the substitution of weathered Bj. For this reason, it is important to compare the effect of acid rains on both of them. Rc is a reconstituted stone composed of Bj lithoclasts, siliceous sand and non-carbonated cement. This implies that the dissolution reactions will be focused on calcitic elements, progressively reducing the reactive surface. The weight and Ca losses are consequently lower compared to the natural stones. Tecer (1999) also found lower weight loss percentages in gypsum-containing carbonate stones in relation to some pure calcitic ones. Rc showed similar weight and Ca losses in passive and active conditions. The comparison with natural stones evidences that pore connectivity is one of the key factors in stone dissolution. Rc has an open porosity similar to Bj and pore distribution close to Cv. Nevertheless, its behaviour follows the pattern of Sv. This agrees with the fact that Rc shows a weak capillary saturation of the stone (51 %) as occurring in Sv. The alteration thus occurs on the surface rather than inside the stone. The open porosity did not vary during the tests, but the porosity distribution shifted from a bimodal to a unimodal distribution. The modal peak around 0.6 µm disappeared, probably filled with gypsum crystals (Malaga-Starzec *et al.*, 2003; Dewanckele *et al.*, 2014; Vázquez *et al.*, 2015). The proportion of the 0.1 to 0.01 µm modal peak increased after experimental weatherings, mainly after nitric tests. These small pores were found mainly in the calcitic elements, in which sulphur is not present, and thus, only dissolution by nitric acid occurs. As sulphur is a stone component, gypsum should crystallize sooner. The presence
of gypsum on the stone surface favours the cation exchange between acid-gypsum-calcite (Lipfert, 1989). This entails a faster dissolution in calcitic components and could explain the high values found in passive immersion. Nevertheless, as the carbonate content of this stone is lower than in the natural stones, the gypsum content is also lower, as corroborated by EDS analysis and according also to lower weight and Ca losses. The presence of sulphur in the fresh stone involved that gypsum precipitates even during the nitric acid tests. The presence of HNO₃ implied a catalytic reaction and thus a more important carbonate dissolution and gypsum crystallization than in tests with sulphuric acid solutions (Johansson, 1990; Haneef et al., 1993b; Grossi and Murray, 1999; Massey, 1999). However, Rc response to acid rains is smaller than Bj in terms of weight and Ca losses. In spite of its sulphur content, its lower content in carbonate and its poor pore connectivity make Rc an appropriate substitution stone against acid rain.

### III.1.4.4. Variation in the surface properties

One of the main weathering effects of acid rains is the stone colour change. The colour changes are related to iron oxidation as iron is the sole chromophore element in these stones. In active conditions, evident colour changes in nitric solution are slightly higher than in sulphuric solution. The gypsum crystallization over the surface may softly mask the real stone colour change as the transparent crystals may perturb the light reflection. It can also be due to the fact that iron, never detected in solutions, was added to calcium sulphates and then oxidized, giving to the gypsum a yellow-orange colour (Vázquez and Alonso, 2015). Depending on the distribution and oxidation state of Fe, colour changes showed the different patterns (Vázquez and Alonso, 2015; Vázquez et al., 2015). Thus, iron is disseminated in the whole Bj and Cv stone, giving a homogeneous colour change of the entire surface. More specifically, the Fe oxidation in Bj was on first in macropores that assume a migration of iron oxides from the inside over the surface, then in altered ooids and in the entire matrix over the time. The pore connectivity explains also the colour changes since it helps the migration of iron particles to the surface producing a higher oxidation. The colour variation is high in Cv because of its clear initial colour and an oxidation development first in specific microfossils (miliolids), then in the whole matrix. The colour changes in Rc are less evident in terms of colorimetric measures. However, red spots were observed around ochre Bj fragments spread over the cement in Rc. Aesthetic variations after the exposure of acid rain indicated also that Rc behaves better than Bj, with only punctual and not very evident colour changes.
Precipitation and dissolution occurrence on the stone surface is observed by surface techniques. The 3D measurements carried out in this study and the analytical method associated with this technique favour the interpretation of the different process that took place. This technique is widely used in the stone deterioration by pollutant attacks (Tecer, 1999; Vázquez *et al.*, 2015). The studied stones show dissolution marks on the edges and visible loss of grains along with precipitation in preexistent pores and defects. Dissolution stays greater than precipitation, in agreement with the measured weight and Ca losses.

This study highlights that the open porosity and the pore connectivity can be considered as key factors to assess stone weathering by acid rains. These two parameters may allow predicting the carbonate dissolution and even colour change, which are the main visible features of the stone decay.

## III.1.5. Conclusions

The effect of acid rains on carbonate stones was studied with the help of laboratory tests, in which weathering passive and active conditions as well as acid solution composition were the chosen variables. The conclusions of this work carried out on three natural stones and an artificial one are the following:

- The differences between sulphuric and nitric acid effects were related mainly to the stone type including all the microstructural and compositional properties. Immersion in sulphuric acid solution and further drying entailed the dissolution of carbonate and the precipitation of gypsum on the stone surface, while immersion in nitric acid solution generally only produces observable dissolution. Precipitation of gypsum after nitric acid immersion appears if the stones contain sulphur naturally.

- The active experimental conditions are supposed to simulate rain and produce more intense variations than simple stone immersion. Most of them are related to mechanical loss. The proportion of mechanical loss agrees with other researchers' results. This indicates that the simple methodology used in this study to simulate a rain event is comparable to the complex runoff tests carried out by other researchers.

- The stone microstructure influences the dissolution-crystallization patterns through the porous network. A good porosity connectivity is responsible for the porosity increase due to dissolution by acid rain in spite of gypsum precipitation. In the case of low connectivity, even if pore distribution changes, the dissolution is focused on the surface and completely compensated by the gypsum crystallization.
- The acid rain effect on the artificial stone is less intense than on the natural stone it replaces because of lower carbonate content and siliceous components in the cement. The weak pore connectivity causes an alteration limited to the stone surface that does not lead to an important increase in porosity. In addition, the colour change is less evident in the artificial stone as it is limited to red spots around the natural stone clasts included in its cement.

Further works will deal with the impact of dry deposition and karst effect by clean rain to complete the preliminary experimental work on the selected stones. Simulations of the experimental results by commonly used theoretical models will then be applied, in order to predict the stone response to acidic rain impact under evolving environmental conditions.

## Acknowledgements

This research was funded by Reims Metropole with the Project IFEPAR (2014-2017). The authors want to thank Xavier Drothière and Alexandra Conreux for technical and analytical support. Special thanks to Arlette Thomachot for English correction and improvement.
## III.2. Simulation du dépôt acide en atmosphère saturée de dioxyde de soufre sur pierre naturelle et artificielle

Cette partie présente l'étude de l'altération des pierres soumises à des tests de vieillissement accéléré dans différentes atmosphères plus ou moins saturées en dioxyde de soufre de façon à reproduire les effets des dépôts de polluants sur les pierres et à évaluer quantitativement et qualitativement, la réactivité de pierres carbonatées artificielles et naturelles, et riches et pauvres en sulfates.

### III.2.1. Matériel et méthodes

Pour rappel, trois pierres calcaires (Courville, Bajocien et Savonnières) et une pierre reconstituée composent le panel de cette étude et sont communément utilisées comme pierre de construction et de remplacement dans le nord de la France et en Belgique.

Le processus de dépôt des polluants a été évalué par des tests de vieillissement accéléré basés sur les procédures standard UNE-EN 13919: 2003 (CEN, 2002) qui précisent que des atmosphères acides sont produites par la volatilité de l'acide sulfureux. Deux dilutions ont été testées, à différentes concentrations avec de l'eau distillée, une solution fortement concentré, H₂SO₃ HC (High Concentration) et une solution faiblement concentrée, H₂SO₃ LC (Low Concentration). La solution H₂SO₃ HC est concentrée à 5,6 10⁻¹ mol.l⁻¹, et la solution H₂SO₃ LC à 1,4 10⁻¹ mol.l⁻¹. Un volume de 65 ml de solution est versé dans le fond d'un récipient de 5 l dans lequel est placée une structure en plastique servant de support qui permet aux échantillons être hors de contact avec la solution durant les 28 jours de test consécutifs. Chaque type de pierre et test a été réalisé en un récipient différent. Avant les tests, les échantillons sont immergés dans de l'eau distillée pendant 48h ce qui les sature en eau et le dépôt se fera sur des surfaces humides.

Le type, l'intensité et la cinétique d'altération des pierres ont été évalués à la fin des tests par des observations macroscopiques et microscopiques, les variations du poids, de couleur et de porosité. De plus, l'évolution de la réactivité au cours du temps a été quantifiée par la mesure quotidienne des concentrations en sulfates contenus dans les pierres.

Les caractéristiques des pierres testées sont décrites dans l'introduction de ce chapitre et les techniques d'analyse utilisées sont détaillées au début de ce chapitre.
### III.2.2. Résultats

#### III.2.2.1. Observations macroscopiques

Les deux tests de vieillissement accéléré ont mené à une cristallisation de sels visibles en surface des spécimens, ainsi qu'à des changements de couleur visibles à l'œil nu (Figure III-15) confirmés par les observations à la loupe (Figure III-16).

Suite aux 28 jours d'exposition en atmosphère $H_2SO_3$ HC, un jaunissement prononcé apparaît sur les 4 types de pierre, ainsi que des zones noircies sur Bj et une alternance de zones blanchies et jaunies sur Cv (Figure III-15). Après l'exposition en $H_2SO_3$ LC, Bj et Rs ont également tendance à jaunir, avec apparition de zones grisées sur Bj. Sv ne montre qu'un léger éclaircissement. La surface de Cv présente un jaunissement plus prononcé en LC qu'en HC contrairement aux autres pierres.

Après les deux tests, les observations à la loupe révèlent des cristallisations de sels sur toutes les pierres. La forme des cristaux diffère néanmoins selon la pierre et la concentration en $H_2SO_3$ Figure III-16. Les croûtes recouvrent entièrement la surface des échantillons, présentant en particulier des cristaux aciculaires sur Cv, Bj et Sv après exposition à $H_2SO_3$ HC, des spots blancs sans forme définie sur Cv et des cristaux en forme de « chou-fleur » sur Bj seulement après exposition à $H_2SO_3$ LC.
<table><caption>Figure III-15. Plaquettes des pierres étudiées avant et après les expositions à des atmosphères saturées en dioxyde de soufre (H<sub>2</sub>SO<sub>3</sub> HC : [H<sub>2</sub>SO<sub>3</sub>] = 5.7 mol.L<sup>-1</sup>, H<sub>2</sub>SO<sub>3</sub> LC : [H<sub>2</sub>SO<sub>3</sub>] = 1.4 mol.L<sup>-1</sup>).</caption><thead><tr><th rowspan="2"></th><th colspan="2">Cv</th><th colspan="2">Bj</th></tr><tr><th>Avant test</th><th>Après test</th><th>Avant test</th><th>Après test</th></tr></thead><tbody><tr><td>H<sub>2</sub>SO<sub>3</sub> HC</td><td></td><td></td><td></td><td></td></tr><tr><td>H<sub>2</sub>SO<sub>3</sub> LC</td><td></td><td></td><td></td><td></td></tr></tbody><thead><tr><th rowspan="2"></th><th colspan="2">Sv</th><th colspan="2">Rs</th></tr><tr><th>Avant test</th><th>Après test</th><th>Avant test</th><th>Après test</th></tr></thead><tbody><tr><td>H<sub>2</sub>SO<sub>3</sub> HC</td><td></td><td></td><td></td><td></td></tr><tr><td>H<sub>2</sub>SO<sub>3</sub> LC</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<table><caption>Figure III-16. Détails des spécimens avant et après les tests en atmosphères acides à la loupe binoculaire.</caption><thead><tr><th></th><th>Cv</th><th>Bj</th><th>Sv</th><th>Rs</th></tr></thead><tbody><tr><td>Avant test</td><td></td><td></td><td></td><td></td></tr><tr><td>Après H<sub>2</sub>SO<sub>3</sub> HC</td><td></td><td></td><td></td><td></td></tr><tr><td>Après H<sub>2</sub>SO<sub>3</sub> LC</td><td></td><td></td><td></td><td></td></tr></tbody></table>
### III.2.2.2. Changement de couleur

Chacun des tests mène à des changements de couleur perceptibles par l'œil humain, caractérisé par un ΔE supérieur à 3. Les variations les plus importantes ont été obtenues avec la concentration H₂SO₃ HC avec valeurs de 7 à 13, tandis qu'avec H₂SO₃ LC, certains échantillons ne présentent pas de changements perceptibles valeurs de 2 à 11. Cette tendance est inversée sur Cv soit à cause de l'oxydation des composés métalliques contenues dans la pierre qui prédomine en LC par rapport à HC, soit à cause d'une couche de gypse plus fine et donc plus transparente qui laisse apparaître un jaunissement plus intense (Figure III-17).

La tendance générale est une diminution en L* et une augmentation des paramètres chromatiques a* et b*, ce qui donne des couleurs plus vives et orangées. La luminance des surfaces diminue au cours des tests (ΔL*<0) avec au maximum -12 unités pour Cv après H₂SO₃ HC et -9 pour Sv après H₂SO₃ LC. Rs présente les différences de luminance les plus faibles pour les deux tests (> -2). A l'inverse, les paramètres a* et b* augmentent et a* en moyenne quatre fois moins que b* (excepté Bj dans H₂SO₃ LC), avec des différences jusqu'à 3 pour Δa* et 13 pour Δb*.

Après l'exposition à H₂SO₃ HC, Sv présente le plus bas ΔL* alors que Rs comporte les Δa* et Δb* les plus élevés. Après exposition à H₂SO₃ LC, Cv subi les plus importants changements de couleurs avec les plus grandes variations de tous les paramètres.
<figure><img src="image_60.png" /><figcaption>Figure III-17. Variations des paramètres chromatiques suite au vieillissement accéléré en atmosphère acide simple en fonction des valeurs initiales.</figcaption></figure>

### III.2.2.3. Prise de poids

La masse de tous les échantillons augmente au cours des deux tests, atteignant des valeurs systématiquement plus élevées avec $H_2SO_3$ HC (de 0.6 % à 1.9 %) qu'avec $H_2SO_3$ LC (de 0.2 % à 0.5 % ; Figure III-18). De plus, quel que soit la concentration en $H_2SO_3$, Bj présente les plus importants gains de masse et Sv les plus faibles.

<figure><img src="image_61.png" /><figcaption>Figure III-18. Différences de masse $\Delta W$ entre la fin et le début des tests en atmosphères simples, exprimée en pourcentage en fonction de la masse initiale.</figcaption></figure>
### III.2.2.4. Evolution de la porosité

Les modifications de la porosité ouverte dépendent à la fois de la pierre testée et de la concentration en $H_2SO_3$ utilisée (Tableau III-7). Néanmoins, les différences de porosité observées après les tests sont inférieures aux écarts type des valeurs initiales, donc l'effet des atmosphères sur la porosité ouverte est négligeable. Après $H_2SO_3$ LC, la porosité de Cv et Rs diminue respectivement de 2% et 6%, la porosité ouverte de Bj augmente de 2% alors que celle de Sv ne varie pas.

Les transformations au niveau de la distribution des tailles de pores affectent principalement Bj et Rs (Figure III-19). Après $H_2SO_3$ HC, les pores de Bj se resserrent, 11% des pores de rayon supérieur à 1µm passent dans la classe intermédiaire (1-0.1µm) et 3% dans la classe inférieure à 0.1µm. Sv expérimente une redistribution de tailles pour les rayons d'accès supérieures à 1 µm, et pourtant visible seulement dans la Figure III-19, et Cv un rétrécissement des pores intermédiaires (Tableau III-7).

Après $H_2SO_3$ LC, le changement plus évident apparaît dans Rs, les macropores (>1µm) ont disparu et sa quantité de pores intermédiaires (1-0.1µm) a diminué de 15% à la faveur des micropores (+25%). Pour Bj, et Sv les différences entre les deux pics de modalité se réduisent. Les pores de Cv conservent sensiblement la même distribution qu'à leur état initial. (Figure III-19). Les variations dans la distribution des rayons d'accès en Cv sont réparties de façon homogène, mais ce changement n'est évident qu'après analyse des données.

<table><caption>Tableau III-7. Porosité et distribution de la taille des pores de chaque type de pierre avant et après les expositions en atmosphères acides simples. Les valeurs à l'état initial sont suivies de leurs écarts type.</caption><thead><tr><th rowspan="2">Pierre</th><th rowspan="2">Test</th><th rowspan="2">Porosité (%)</th><th colspan="3">Distribution de la taille des pores (%)</th></tr><tr><th>Macropores >1 µm</th><th>Mesopores 1-0.1 µm</th><th>Micropores <0.1 µm</th></tr></thead><tbody><tr><td rowspan="3">Cv</td><td><em>Initial</em></td><td>16 ± 2</td><td>4 ± 1</td><td>22 ± 10</td><td>74 ± 9</td></tr><tr><td>HC</td><td>16</td><td>5</td><td>15</td><td>80</td></tr><tr><td>LC</td><td>18</td><td>5</td><td>27</td><td>68</td></tr><tr><td rowspan="3">Bj</td><td><em>Initial</em></td><td>20 ± 1</td><td>47 ± 2</td><td>42 ± 1</td><td>11 ± 2</td></tr><tr><td>HC</td><td>19</td><td>33</td><td>53</td><td>14</td></tr><tr><td>LC</td><td>22</td><td>47</td><td>42</td><td>11</td></tr><tr><td rowspan="3">Sv</td><td><em>Initial</em></td><td>32 ± 2</td><td>58 ± 3</td><td>29 ± 3</td><td>13 ± 4</td></tr><tr><td>HC</td><td>33</td><td>56</td><td>31</td><td>13</td></tr><tr><td>LC</td><td>32</td><td>53</td><td>33</td><td>14</td></tr><tr><td rowspan="3">Rs</td><td><em>Initial</em></td><td>23 ± 2</td><td>6 ± 1</td><td>41 ± 4</td><td>53 ± 4</td></tr><tr><td>HC</td><td>23</td><td>8</td><td>37</td><td>55</td></tr><tr><td>LC</td><td>17</td><td>0</td><td>22</td><td>78</td></tr></tbody></table>
<figure><img src="image_62.png" /><figcaption>Figure III-19. Incrément de volume de mercure injecté pour chaque rayon d'accès avant et après les expositions aux atmosphères acides simples.</figcaption></figure>

### III.2.2.5. Observations au microscope électronique à balayage (MEB)

Les images obtenues au MEB mettent en évidence la formation d'une couche de cristaux sur la surface de chacun des échantillons dès le premier jour qui n'évolue pas jusqu'à la fin des tests. La forme des cristaux montre la relation entre la cinétique de dépôt et la porosité de la pierre (Figure III-20). Au cours de l'exposition à $H_2SO_3$ HC, Cv Bj et Sv exhibent une cristallisation en surface très similaire associant des cristaux en forme aciculaire et de « choufleur », avec une taille plus petite en Cv qui présente aussi des pores de taille inférieure. Durant le test en $H_2SO_3$ LC, Cv montre une couche des cristaux similaires au test de Haute Concentration tandis que Bj et Sv présentent seulement des cristaux aciculaires et « pointe de flèche » sur Sv. Lors des deux tests, Rs est recouvert d'une croûte craquelée.
<figure><img src="image_63.png" /><figcaption>Figure III-20. Images MEB des échantillons au cours des expositions aux atmosphères acides simples.</figcaption></figure>

### III.2.2.6. Cinétique d'évolution de la composition chimique

Avant le début des expérimentations, Sv et Rs contenaient déjà une concentration en sulfates 10 fois plus élevée que Cv et Bj, à savoir 3,5 mg.g⁻¹ pour Sv et 2 mg.g⁻¹ pour Rs contre 0,3 mg.g⁻¹ pour Cv et Bj.

Au cours des expositions aux deux atmosphères de H₂SO₃, toutes les pierres ont manifesté un comportement très similaire : une production de sulfates pendant les premiers jours puis une stagnation de la concentration jusqu'à la fin de l'expérience (Figure III-21). Les trois pierres Bj, Sv et Rs ont présenté la même évolution dans les deux tests, avec de légères variations des valeurs à la fin du test. Ainsi, Bj est la roche contenant le moins de sulfates avec 3 mg.g⁻¹ tandis que Sv et Rs ont atteint jusqu'à 4-5 mg.g⁻¹. Rs a montré une diminution des concentrations après le jour 10, qui est due à l'hétérogénéité de la pierre. Cv a montré un comportement très différent entre les deux tests. En HC, une constante augmentation tout au long du test atteignant 4,5 mg.g⁻¹ au bout de 5 jours jusqu'à 7,5 mg.g⁻¹ à la fin du test et une formation presque négligeable de SO₄²⁻ en LC, avec valeurs finales de 0,5 mg.g⁻¹.
<figure><img src="image_64.png" /></figure>

Figure III-21. Concentrations en sulfates/sulfites ($[SO_x^{2-}]=[SO_4^{2-}]+[SO_3^{2-}]$) des échantillons au cours des expositions à H₂SO₃ HC (lignes continues) et H₂SO₃ LC (lignes pointillés).

### III.2.3. Discussion et conclusion

L'exposition de pierres carbonatées à des atmosphères riches en dioxyde de soufre, et à une humidité relative proche de 100%, mène à la cristallisation de sulfates comme le confirme, de façon directe, l'observation à la loupe et au MEB, et de façon indirecte, la prise de poids, l'évolution de la porosité la mesure du contenu en anions soufrés et des modifications esthétiques mises en évidence par les changements de couleur.

L'exposition à la plus forte concentration en H₂SO₃ engendre un jaunissement de tous les types de pierres carbonatées alors qu'avec une plus faible concentration, seulement le calcaire du Bajocien (Bj) et la pierre reconstituée (Rs) changent de couleur de façon visible à l'œil nu. Ce changement est dû à l'oxydation des composés métalliques, fer et manganèse, présents en plus grandes quantités dans ces deux pierres que dans les calcaires de Savonnières (Sv) et de Courville (Cv).

Les observations microscopiques et les analyses mettent en évidence la formation du gypse dans les deux tests. La forme des cristaux montre la relation entre la cinétique de dépôt et la porosité de la pierre. Rs est microporeuse, sa surface de réaction en est donc réduite et la croûte reste amorphe car la cristallisation est encore incomplète au moment des observations. Les cristaux de petite taille observés sur Cv ne sont pas ou peu développés du fait de sa microporosité. Les deux pierres macroporeuses, Bj et Sv, présentent des cristaux bien formés et varies. Par conséquent la taille des pores influence la taille et la forme des cristaux. Par ailleurs, les cristaux en « chou-fleur » ou « nénufars » se forment quand le dépôt est important et donc qu'il y a une réaction avec les cristaux de gypse en équilibre. En concentration de
$H_2SO_3$ plus faible, les cristaux sont aciculaires et atteignent leur forme finale en un mois ce qui indique un dépôt plus rapide que l'interaction avec la pierre (Vázquez et Alonso, 2010).

Les résultats de la prise de poids et de la chromatographie ionique sont contradictoires pour la HC et la LC excepte pour Cv. Ainsi, le taux de cristallisation est plus élevé en HC qu’en LC d’après la prise de poids cependant les résultats des analyses chromatographiques montrent un taux de production similaire. La seule différence entre les échantillons mesurés est la taille, 10 fois plus volumineux pour la mesure du poids que ceux ayant servi à l’analyse chromatographique. Ainsi, le processus de cristallisation atteint l’équilibre en quelques jours sur les petits échantillons pour les deux tests, alors que la durée complète du test ne suffit pas sur les gros échantillons, au moins pour le LC test. Dans le cas de Cv, la surface apparaît également plus blanche en HC ce qui indique une croûte plus épaisse donc plus opaque qu’en LC. La concentration en sulfate ainsi que la prise de poids sont également plus importantes en HC qu’en LC. Cv est microporeuse, et la majorité des processus ont lieu en surface et entraine une production et une cristallisation de sulfates directement proportionnelle à la concentration initiale en $H_2SO_3$.

La production d'anions sulfatés a lieu durant les premiers jours d'exposition pour toutes les pierres indépendamment de la concentration en $H_2SO_3$. A la fin des tests, les pierres de Savonnières et reconstituée présentent des teneurs en composés soufrés totales plus élevées que les calcaires de Courville et du Bajocien dues à leur concentration initiale en soufre. Par contre, le gypse de néoformation est plus abondant dans les pierres non contaminées.
# MILIEUX ACIDES MIXTES

## III.3. Simulation of acid weathering on natural and artificial building stones according to the current atmospheric SO₂/NOₓ rate

Ce travail a été soumis dans la revue de rang *A Environmental Earth Sciences*, le 9 Octobre 2017.

### Résumé

Les pierres de construction sont fortement affectées par les effets des pluies acides et des dépôts de polluants. De ce point de vue, le dioxyde de soufre s'avère être un composant clé dans l'altération des matériaux de construction et ses effets ont été largement étudiés. Cependant, depuis les années 1980, les actions mises en place visant à réduire les émissions de gaz à effet de serre pour atténuer les changements climatiques ont fait évoluer la pollution atmosphérique. On observe notamment l'inversion des proportions SO₂/NOₓ dans les émissions atmosphériques.

Ce changement pourrait entraîner des modifications dans les processus d'altération, c'est pourquoi cette étude a pour objectif d'estimer les effets combinés des composés azotés et soufrés sur les pierres en évaluant la cinétique des réactions et le taux d'altération de façon qualitative et quantitative.

Le comportement de pierres fraîchement extraites et de pierres contaminées par le soufre a également été comparé. Trois pierres calcaires (Courville, Bajocien et Savonnières) et une pierre reconstituée ont été soumises à deux types de test de vieillissement accéléré en laboratoire, simulant deux procédés distincts: le dépôt et la pluie.

Pour imiter les effets du dépôt acide, des échantillons de pierre ont été exposés pendant 28 jours à deux atmosphères mixtes composées de H₂SO₃ et HNO₃ dans différentes proportions (rapports [H₂SO₃]/[HNO₃] de 4/1 et 1/3). Pour simuler les effets de la pluie, des plaquettes de 5 cm de côté ont été immergées dans une solution acide mixte (HNO₃ + H₂SO₄) à pH = 5 et d'autres échantillons dans une eau de pluie naturelle à pH = 5,9. Les deux tests ont été effectués avec agitation («immersion active») et sans agitation («immersion passive»).

Les différentes techniques d'évaluation de l'état des pierres ont donné des informations riches et variées sur les mécanismes d'altération. Les observations macroscopiques et
microscopiques ont permis de constater les changements généraux de la surface de la pierre
ainsi que les formes des cristaux et les schémas de dissolution. Les variations de poids ont
permis d'estimer le taux de cristallisation et de perte de matière. Cependant, cette information
doit être prise avec précaution car les deux phases peuvent se compenser en partie. La
chromatographie ionique qui donne la quantité de sulfate SO4<sup>2-</sup> et de nitrate NO3<sup>-</sup> contenue
dans chaque pierre au cours de l'expérience permet de distinguer la part de la précipitation de
celle de la perte de matière. Enfin, la porosimétrie mercure permet d'estimer l'influence de la
porosité sur les processus mis en œuvre et d'obtenir des informations détaillées sur la
localisation de l'altération dans le réseau poreux.

Il apparaît, d'après les résultats obtenus, que la quantité de soufre influe sur les changements
de couleur dus au taux de cristallisation sur la surface de la pierre et à l'oxydation des
composés métalliques. Après exposition aux atmosphères acides mixtes, les nitrates
cristallisent significativement uniquement dans les calcaires non contaminés par le soufre
avant les tests.

Durant les tests d'immersion, la solution acide dissous la calcite plus que l'eau de pluie, en
raison de la combinaison de l'acidité venant des polluants et de l'effet karst venant du dioxyde
de carbone. L'immersion active affecte les pierres plus fortement que l'immersion passive
avec une perte de matière plus rapide du fait du détachement de particule et d'une réactivité
catalysée par l'agitation.

Dans les atmosphères acides mixtes, le comportement du réseau poreux dépend davantage de
la distribution de la taille des pores alors que la porosité ouverte est le paramètre à considérer
dans les tests d'immersion. Une teneur initiale élevée en soufre limite le taux de cristallisation
dans les tests d'atmosphère acide et augmente le taux de dissolution dans les tests
d'immersion.
# Simulation of acid weathering on natural and artificial building stones according to the current atmospheric SO₂/NOₓ rate

Gibeaux S.ª, Thomachot-Schneider C.ª, Eyssautier-Chuine S.ª, Marin B.ª, Vázquez P.ª

Submitted to Environmental Earth Sciences in on October 9th, 2017.

**Keywords:** SO₂/NOₓ, acid rain, acid deposition, aging laboratory tests, building limestone, reconstituted stone.

## Abstract

The building stones are affected by pollution. Since 1980's, the actions to reduce the greenhouse gas emissions led to the inversion of the SO₂/NOₓ proportions in the atmosphere. This study aims at estimating the effects of nitrogen and sulfur compounds on stones by assessing the changes of three building limestones and one reconstituted stone submitted to acid attacks. Two of these stones were already contaminated with sulfates while the two others were fresh quarried. Two different types of accelerated ageing tests were used: i) the exposition to two mixed acid and saturated atmospheres (HNO₃ and H₂SO₃) to simulate the ancient and current pollutants ratio and ii) the immersion in a mixed acid solution (HNO₃ and H₂SO₄) and in rainwater (pH=5 and 5.9), with and without agitation to simulate stagnant water and storm runoff water. Macroscopic, binocular and SEM observations, variations of color, weight, porosity, salt content and dissolved calcium were assessed over time. The sulfur amount influences the aesthetic alterations such as color changes due to the salt precipitation and the oxidation of metallic compounds. During the immersion tests, the dissolution in the acid solution was more efficient than in the rainwater, due to the combination of the acidity and the karst effects. In the mixed acid atmospheres, the behavior of the porous network depends on the pore size distribution while in the immersion tests it is the open porosity. The high initial sulfur content of the contaminated stones increases the dissolution rate and limits the crystallization.

<footnote>ª Groupe d'Etudes sur les Géomatériaux et les Environnements Naturels, Anthropiques et Archéologiques (GEGENAA, EA 3795), Université de Reims Champagne-Ardenne, 2, Esplanade Roland Garros, 51100 Reims, France.</footnote>
### III.3.1. Introduction

Historically, the architecture of the religious edifices, castles and other magnificent buildings is the expression of the high-ranking dignitary power, explaining the willingness of their designers to use materials of excellence such as stone. However, over time and over environmental changes, the stone is altered, losing its original appearance and even its structural stability.

From the first published studies on cultural heritage conservation, the stone decay appeared clearly as depending on the type of stone and on the type of environment (Ansted, 1860 ; Kühlmann, 1863). More precisely, the alteration processes depend on the material characteristics like chemical composition, porosity and texture (Grossi et al., 1995 ; Massey, 1999 ; Vázquez and Alonso, 2015). Thereby, carbonate stones such as limestones, marbles and some sandstones are highly reactive to acid environments caused by pollution (Reddy, 1988 ; Ross, 1989).

The urban environment is particularly aggressive for the building limestones compared to rural environment due to anthropogenic activities such as transports, industrial emissions and heating. That leads to higher concentrations of SO₂ and NOₓ compared to rural environment and entails aesthetical decay as yellowing, soiling or black crusts (Grossi et al., 2007 ; Török et al., 2011) and physical degradation as material loss and cracks (Dolske, 1995 ; Graue et al., 2013). Those alterations are the results of the interaction between the stone components and two decay agents: atmospheric pollutants and acid rain.

In the sheltered zones, the stone is wet but protected from leaching and the dry deposition of gaseous pollutants on the surface is favored. In presence of water, the atmospheric SO₂ is oxidized in SO₃ and formed sulfuric acid in the moisture film. The substitution reaction of carbonates by sulfates results in salt crystallization (Török and Rozgonyi, 2004 ; Charola et al., 2007 ; Grossi et al., 2007 ; Monna et al., 2008). The produced salt, mainly gypsum, tends to remain on the stone surface leading to crusts formation (Bai et al., 2003 ; Charola et al., 2007).

Nitrogen oxides (NOₓ) are known mainly as catalysts for SO₂ oxidation (Massey, 1999 ; Bai et al., 2006) than for nitrate crystallization because the oxidation of SO₂ is faster than that of NO₂ (Haneef et al., 1992). However nitrates are soluble salts and some studies highlighted that when crystallization occurs nitrates may migrate inside the stone (Grossi et al., 1995 ; Camaiti et al., 2007 ; Vázquez et al., 2016). In urban environment, the black carbon particles
(soot and fly ash) and the other pollutants emitted during fuel combustion processes agglomerate in the roughness of the gypsum crusts making particularly anaesthetic decays on the stone surface such as yellowing, blackening and even black crusts (Rodriguez-Navarro and Sebastian, 1996 ; Simao et al., 2006 ; Monna et al., 2008).

On the exposed zones, the wet deposition of acid rainwater affects the stone by leaching that leads to the dissolution of the carbonate components that influence the roughness, the texture and even the chemical composition of its surface (Reddy, 1988 ; Eyssautier-Chuine et al., 2016). The rain is naturally acid, due to the solubility of the atmospheric CO₂ that makes its pH stable around 5.6. Thus the variation of the rain acidity level depends on anthropogenic acid concentration which are mainly represented by the sulfuric and the nitric acids, products of SO₂ and NOₓ oxidation respectively (Lipfert, 1989 ; Bonazza et al., 2009 ; Reddy, 1988). Moreover, when precipitations come in contact with the stone, the run-off catalyzes the reaction between acids and calcite chemically considering the principle of the Collision Theory (Huber and Reid, 1926 ; Shah et al., 2012) and mechanically by causing particles detachment (Camuffo, 1995 ; Eyssautier-Chuine et al., 2016).

Since 1980s, the composition of the urban atmosphere has changed in Europe, particularly because the industrial manufacturers created filters to limit some pollutant emissions to respect the new limitations. Therefore, the industrial SO₂ emissions decreased highly while the nitrogen compounds coming from the traffic emissions increased. The emission ratio between SO₂ and NOₓ reversed from SO₂/NOₓ = 4/1 before 1980 to SO₂/NOₓ = 1/3 nowadays (Massey, 1999 ; CITEPA, 2015).

Some laboratory studies, Haneef et al. (1992 ; 1993b) and Vázquez et al. (2016), have investigated the effects of mixed acid atmosphere on carbonate stones by the use of accelerated ageing test. The chemical reactions during pollutant exposure produce color changes of the stone surface. However, the intensity of the color variation depends mainly on the initial stone color (Vázquez et al., 2016). Both studies also revealed that alterations appear faster with mixed acids than with a single component and that nitrates were systematically detected in depth within stone during a pure NO₂ and mixed HNO₃/H₂SO₃ atmosphere exposures.

Eyssautier-Chuine et al. (2016) have simulated the effects of rain with pure solutions of sulfuric and nitric acids, in agitated and stationary conditions on limestones. The produced dissolution entailed a weight loss and the increase of the Ca²⁺ concentration in the run-off
water such as noticed in previous studies (Tecer, 1999 ; Franzoni and Sassoni, 2011). The
changes of intrinsic stone properties such as color, chemical composition and porosity give
more relevant informations for the decay degree assessment. Indeed, Eyssautier-Chuine et al.
(2016) reported that color changes is related to iron oxidation of stone content and is more
evident in nitric solution than in sulfuric solution, and that the dissolution rate and gypsum
precipitation depend on the initial total porosity, pore size distribution and pores connectivity.

The main aim of this study was to analyze and compare the reactivity of four building stones submitted to mixed acid by exposure to atmospheres and by immersion in an acid solution and in rainwater. The research strategy was to provide textural (visual observations, SEM, colorimetry), petrographic (Hg porosimetry) and chemical (EDS, ion chromatography, ICP-AES) analysis to assess qualitative and quantitative changes. The weathering kinetics were estimated by the weight changes, the measurement of sulfates and nitrates contents throughout the mixed acid exposure tests, and dissolved calcium during the immersion tests. The selected stones were three natural limestones and one artificial stone. Two natural stones were fresh quarried while the remaining limestone and the artificial stone were recovered from discarded blocks during a church restoration.

### III.3.2. Material

This study investigated the behavior of four building stones used in the north of France and in Belgium. Two of them, the Courville limestone (Cv) and the Bajocian limestone (Bj) are extensively used in construction and were fresh quarried for this study. The two other studied stones, the Savonnières limestone (Sv) and the reconstituted stone (Rs) are widely used in Heritage buildings and commonly used as replacement of original stones during restorations (Fronteau, 2000). These two stones were defined as contaminated because the Sv samples stemmed from altered blocks removed from Reims monuments during restoration, and the Rs is man-made and initially S-rich due to its fabrication (Thomachot-Schneider et al., 2011).

#### III.3.2.1. Fresh stones

##### a. Courville limestone (Cv)

The Courville limestone (Cv), exhibited a beige color (Figure III-22.a) and fossil compounds such as Mollusca shells, miliolids and dasycladales (Figure III-22.b), comes from the last
stone quarry of the Marne department and it is the most famous middle Lutetian building stone used for monument construction and restorations such as the Reims Cathedral (Fronteau *et al.*, 2010a). Its texture is matrix-supported, with partly recrystallized micritic matrix (microsparite) and secondary spar cement in the moldic macropores or in some intragranular fossil porosity (Figure III-22.c). This limestone shows an open porosity of around 20 %. The mercury injection curve reveals a unimodal distribution of the pore size (Figure III-22.d) that highlights one type of porosity mainly localised in the intergranular matrix. The Cv stone has a porous network with access radii relatively small, less than 0.1 µm focused around a single modal peak at 0.1µm.

### b. Bajocian limestone (Bj)

The Bajocian building stone (Bj) is an oolitic to bioclastic limestone (Fronteau, 2000) from the French Ardennes and the south of Belgium and which is used in most of the constructions of this area. Its color is mainly russet due to iron oxide deposits visible in the porosity and over the stone surface (Figure III-22.e, f). The stone is composed of calcitic debris as shell fragments, numerous echinoderm ossicles surrounded by a syntaxic cement, and Foraminifera. The studied samples show numerous ooids surrounded by diagenetic cements, composed by calcite spar within the interstitial spaces (Figure III-22.f, g). Its total porosity of around 20 % shows a bimodality of the pore size distribution allocated to a low intergranular porosity and a secondary molding intragranular porosity (Figure III-22.h). Bj is constituted at 90 % by a ranging of pore size higher than 0.1 µm with two peaks at 2 and 0.3 µm.

### III.3.2.2. Contaminated stones

#### a. Savonnières limestone (Sv)

The Savonnières (Sv), is a beige oolitic vacuolar limestone (Figure III-22.i, j) extracted from the eastern part of France, for the construction of buildings from the XIX<sup>th</sup> to XX<sup>th</sup> centuries and is well-known to be used in current restorations in Europe (Dewanckele *et al.*, 2014 ; Laycock *et al.*, 2008). The ooliths in majority and some bivalve shells are surrounded by a sparitic cement filling partially the large and open intergranular porosity (Figure III-22.j, k; Fronteau, 2000 ; Eyssautier-Chuine *et al.*, 2016). Its porous network is located inside the oolith nuclei and in-between the cement, and the micropores are in the oolith cortex making a bimodal pore size distribution and conferring to the stone a high open porosity with values
around 30 % (Figure III-22.1; Dewanckele, 2013 ; Fronteau, 2000). Sv exhibits more than 50 % of pore access radii superior to 0.1 µm comprising two peaks at 5 and 0.1 µm, and 30 % of the intermediate range (1-0.1 µm).

b. Reconstituted stone (Rs)

The reconstituted stone (Rs), crafted during the last restoration of Orval Abbey between 1932 and 1948 (Thomachot *et al.*, 2011 ; 2016), has beige tones close to Bajocian colors (Figure III-22.m). This synthetic material has a varied composition of limestone debris and sand bonded by a Portland cement which contains calcic lime and iron oxides (Figure III-22.n) making it initially rich in silica and sulfur. The macroporosity of the Rs stone corresponds to the air bubbles produced by the degassing during manufacture (Figure III-22.n). The porous network is bimodal, with micropores located within the calcareous lithoclasts and the intermediate size pores in the matrix (Figure III-22.o) and an open porosity of 23 %. Rs has the highest proportion of micropores with 94 % of pore access radii lower than 1 µm distributed in two peaks at 0.3 and 0.07 µm.
<figure><img src="image_65.png" /></figure>

Figure III-22. Characteristics of the Courville (a, b, c, d), the Bajocian (e, f, g, h), the Savonnieres (i, j, k, l) limestones and the Orval reconstituted stone (m, n, o, p) before the tests. Photographs (a, e, i, m) and blue-coloured thin sections in plane polarized light (b, f, j, n), incremental mercury intrusion versus pore size (c, g, k, o) and metals content (d, h, l, p).

### III.3.3. Methodology

#### III.3.3.1. Aging tests

In this study, the stones were submitted to two different enclosure conditions: (i) to reproduce the atmospheric acid deposition, the samples were exposed to the gases produced by two mixed acid and volatile solutions, and (ii) to simulate the acid rain action, the stones were immersed in an acid solution with and without agitation.

##### a. Mixed acid atmosphere exposure

The acid deposition processes were assessed through accelerated ageing tests based on the standard UNE-EN 13919:2003 procedures (CEN, 2002). The acid atmospheres were
produced by the volatility of two solutions mixing nitric and sulfurous acids with different concentrations as detailed in Tableau III-8.

The first solution, designated as S>N, was constituted by mixing 14.4 ml of distilled water with 50 ml of H₂SO₃ (6% w/v) and 0.6 ml of HNO₃ (69% w/w). The final S>N mixture was composed by 5.6 10⁻¹ mol.l⁻¹ of H₂SO₃ and 1.4 10⁻¹ mol.l⁻¹ of HNO₃ to reflect the past SO₂/NOₓ ratio around 4/1.

The second solution, designated as S<N, was constituted by mixing 48 ml of distilled water with 15 ml of H₂SO₃ (6% w/v) and 2 ml of HNO₃ (69% w/w). The final S<N mixture was composed by 1.7 10⁻¹ mol.l⁻¹ of H₂SO₃ and 4.8 10⁻¹ mol.l⁻¹ of HNO₃ to simulate the current pollution ratio around 1/3.

A volume of 65 ml of one solution was poured into the bottom of a container of 5 l to create an acid saturated atmosphere. One container was used for each type of stone with each of the solutions. Before testing, the samples were saturated in distilled water during 48h. The slabs were put on a plastic structure keeping them out of contact with the solution (Figure III-23), and were exposed to the acid atmospheres during 28 days.

b. Immersion in acid solution

The effects of acid rain were determined by means of two solutions, an artificial acid mixture and a natural rainwater. The acid solution at pH = 5 was carried out by mixing 600 ml of nitric acid (10⁻⁵ mol.l⁻¹) with 400 ml of sulfuric acid (5.10⁻⁶ mol.l⁻¹). The rainwater, at an initially pH at 5.92, was collected in a rural area during the winter period without pulverization of fertilizer and biocides. The nomenclature and the composition of the solutions are summarized in Tableau III-8.

For each solution, two tests were performed. The first one was an immersion of the samples in the solution (passive immersion). This test assessed the karst effect by dissolution of the selected stones due to a simple contact with an acidic solution. The second test was an immersion of the samples into the solution placed in a mechanical shaker to simulate storm rain or running off water (active immersion) (Eyssautier-Chuine *et al.*, 2016).

For each test, three slabs of each stone type were used. The triplicates were placed in a closed container of 4 l volume. Every container was fitted with dots of silicon and plastic foam to
# Chapitre III : Suivi à méso-échelle

prevent contact or collision between samples, and rubbing with the bottom of the container (Figure III-23).

The experimentations were conducted for 32 days including 5 cycles of immersion and following drying phases. The first cycle went on for 24 hours of immersion before drying. The following four cycles involved 4 days of continuous immersion before drying. The drift of the pH solution due to the reaction between carbonate stones and the acidic solutions implied a readjustment of pH twice a day until reaching again the initially fixed pH=5. The rainwater was renewed before each cycle. The drying phase was similar for all the cycles and carried out for 64 hours in an oven at 45°C.

<table><caption>Tableau III-8. $H_2SO_4$ and $HNO_3$ concentrations and characteristics of the ageing tests solutions with $x=3$ for acid atmosphere test and $x=4$ for acid immersion test. The stock solution for nitric acid was Panreac® 69% w/w 1.42 kg.L-1, and for sulfurous acid Panreac® 6% w/v.</caption><thead><tr><th>Solution name</th><th>[H<sub>2</sub>SO<sub>4</sub>]<br>(mol l<sup>-1</sup>)</th><th>[HNO<sub>3</sub>]<br>(mol l<sup>-1</sup>)</th><th>Ratio<br>H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub></th><th>pH</th><th>Agitation</th></tr></thead><tbody><tr><td>S>N</td><td>5.6 10<sup>-1</sup></td><td>1.4 10<sup>-1</sup></td><td>4/1</td><td>-</td><td>-</td></tr><tr><td>S&lt;N</td><td>1.7 10<sup>-1</sup></td><td>4.8 10<sup>-1</sup></td><td>1/3</td><td>-</td><td>-</td></tr><tr><td>SNAI</td><td>2 10<sup>-6</sup></td><td>6 10<sup>-6</sup></td><td>1/3</td><td>5 ± 0.1</td><td>Active</td></tr><tr><td>SNPI</td><td>2 10<sup>-6</sup></td><td>6 10<sup>-6</sup></td><td>1/3</td><td>5 ± 0.1</td><td>Passive</td></tr><tr><td>NRAI</td><td>-</td><td>-</td><td>-</td><td>5.92 ± 0.1</td><td>Active</td></tr><tr><td>NRPI</td><td>-</td><td>-</td><td>-</td><td>5.92 ± 0.1</td><td>Passive</td></tr></tbody></table>

<figure><img src="image_66.png" /><figcaption>Figure III-23. Setting up of the acid immersion (a) and atmosphere (c) tests with their schematizing and characteristics (b).</figcaption></figure>
### III.3.3.2. Analytical methods

The type, amplitude and kinetic of the stone weathering were assessed on the samples by macroscopic and microscopic observations, weight evolutions, color and porosity changes. The chemical variations over time were evaluated by the quantification of the calcium content in solutions, and the sulfates and nitrates concentrations in stone samples.

Those analysis were performed on all the sound samples and at different time of the tests as detailed in Tableau III-9.

<table><caption>Tableau III-9. Sample dimensions depending on the performed analysis.</caption><thead><tr><th rowspan="2">Sample dimension</th><th>Analysis method</th><th colspan="2">Analysis frequency</th></tr><tr><th>Acid atmosphere test</th><th>Immersion test</th></tr></thead><tbody><tr><td rowspan="3">5x5x1cm</td><td>Macroscopic and microscopic evaluations</td><td colspan="2">At the end of the experiment</td></tr><tr><td>Colorimetry</td><td></td><td></td></tr><tr><td>Weight</td><td>At the end of the experiment</td><td>After 4, 11, 18, 25 and 32 days</td></tr><tr><td>1x1x2cm</td><td>Hg porosimetry</td><td colspan="2">At the end of the experiment</td></tr><tr><td rowspan="2">1x1x0.4cm</td><td>SEM-EDX</td><td>After 2, 5, 8, 14, 22 and 28 days</td><td rowspan="2">At the end of the experiment</td></tr><tr><td>Ion chromatography</td><td>One per day</td></tr><tr><td>Solution</td><td>ICP-AES</td><td>-</td><td>After 4, 11, 18, 25 and 32 days</td></tr></tbody></table>

<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /></figure>

<figure><img src="image_69.png" /></figure>

#### a. Macroscopic evaluation

The images of the upper face samples were taken with a Nikon Coolpix 4500 and observed with a 60x binocular magnifier on the Olympus SZH-ILLB.

#### b. Colorimetry

Color was measured with a Konica Minolta CR-400 colorimeter using the D65 illuminant and a 8 mm target diameter.
The measurements were expressed following the CIE L* a* b* and CIE L* C*h* systems (EN ISO 105-J03: 1997). The L* parameter is Lightness and registers values from 0 (black) to 100 (white). The a* and b* parameters are Cartesian chromatic parameters. They vary from -60 to 60, a* from green to red, and b* from blue to yellow. The C* parameter is Chroma or saturation. The higher the Chroma is, the more vivid the colors are. For our device, values went from 0 to 60. The h* parameter is the hue (i.e. the tone of the stone) with values from 0 to 360° in the chromatic sphere. Thus, both depend on a* and b*. The ΔE* value is introduced as the total color change to compare the variations before and after the tests as follows:

$$ \Delta E^* = [((\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2)]^{1/2} \quad (1) $$

$$ C^* = \sqrt{(a^{*2} + b^{*2})} \quad (2) $$

$$ h^* = \arctan(b^*/a^*) \quad (3) $$

A ΔE* value higher than 3 is considered as a visual change. The average of each parameter was calculated from 40 points measured on each slab surface, calculated previously as the minimum of measurement necessary to obtain a representative average for all the stones (Vázquez et al., 2013b).

### c. Weight changes

The weight of the stone samples was measured by an analytical balance Mettler Toledo with a sensitivity of 1 mg. The percentage of the weight variation was calculated with respect to the initial weight at the end of the acid atmosphere tests, and regularly at the end of each cycle of the immersion tests.

The surface recession has been estimated for each stone type according to the following equation based on the method used in Grossi et al. (1995):

$$ R = \frac{\Delta W \times 10^4}{A \times d \times [CaCO_3]} $$

With R the recessed surface in µm, ΔW the difference between the weight at the end of the test and the initial weight in g, A the total sample surface equal to 70 cm², d the calcite density equal to 2.71 g cm⁻³ (Reeder, 1983) and [CaCO₃] the calcite content specific to each stone based on the data of Eyssautier et al. (2016).
### d. Hg Porosity

Porosity characterization was obtained using an Hg intrusion porosimeter (MIP) Micromeritics Autopore IV 9500, reaching a 230 MPa pressure and measuring pore radii sizes from 0.002 to 180 µm. The porosity was measured on the fresh stones and on the triplicates of each stone type of size 1x1x2 cm at the end of the atmosphere test. After the immersion test one specimen of each altered stone type was cut in dry condition to obtain smaller sample with optimal dimensions for this measurement. The reason of not having a previous cut adapted sample is the aggressivity of the immersion test that could disintegrate completely some stone types. Parameters calculated from Hg porosity were the mean pore access radius (Rm), the modal peaks and the percentage of pores in four different pore access radius ranges: large (>1 µm), medium (1-0.1 µm), and small (< 0.1 µm).

### e. Scanning electron microscopy

The SEM Hitachi TM-1000 Table Top coupled to a Swifted-TM Energy Dispersive X-Ray (EDX) was used to acquire high quality images of precipitation crystals or dissolution details in the back-scattered electron mode and 15 kV accelerating voltage.

### f. Sulfates and nitrates concentration (Ion chromatography)

To determine the salt content in stones, the samples were powdered, brought into solution in 1 ml of ultrapure water, agitated during 12 h and centrifuged at 13400 rpm during 10 min. The liquid phase was collected and the process was repeated once to dissolve all the anions.

The quantity of nitrates $NO_3^-$ and sulfates $SO_4^{2-}$ was measured by Ion Chromatography with a Dionex ICS 2000 liquid chromatograph fitted out a pump, an eluent generator (EGC III) with potassium hydroxide (KOH) cartridge, an AS50 injector and a DS6 conductivity cell. The separation of anions was performed on a 2 mm Dionex IonPac AS17 column coupled to a 2 mm Dionex ASRS 300 suppressor.

Instrument control and data acquisition were performed using Chromeleon® software. The obtained curve over time, started at time t=0, were smoothed by using the simple exponential operation (Gardner, 2006) following this sequence:
$$ \begin{cases} Y_0 = C_0 \\ Y_t = \alpha C_t + (1-\alpha)C_{t-1} \end{cases} $$

Where C is the measured concentration, Y the predicted concentration and α the smoothing factor with α = 0.25.

RUNSALT simulations were carried out to predict the chemical compositions of the salts produced during the test in function of the relative humidity. RUNSALT is a graphical user interface to the ECOS thermodynamic model for the prediction of the behavior of salt mixtures under changing climate conditions (Price, 2000; Bionda, 2005). The amount of Ca²⁺ present in the building stones was considered in excess and the sulfate and nitrate molar amount were chosen according to the past and current SO₂/NOₓ ratio and the solution concentrations as following:

* For the S>N test, SO₄²⁻ = 3.6 10⁻² mol, NO₃⁻ = 9.1 10⁻³ mol, Ca²⁺ = 4.1 10⁻² mol

* For the S<N test, SO₄²⁻ = 1.3 10⁻² mol, NO₃⁻ = 3.9 10⁻² mol, Ca²⁺ = 3.3 10⁻² mol

### g. Dissolved Ca concentrations (ICP-AES)

The concentrations of Ca were measured in acid solutions of each cycle by Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES, Iris Advantage, Thermo Fisher Scientific). The analysis of Ca was conducted at wavelength of 184.006 through three measurements of 10 ml of each solution.

### III.3.4. Results

#### III.3.4.1. Visual observations

##### a. Atmosphere tests

The both types of weathering tests led to naked-eye detectable changes, notably color differences, salt precipitations and dissolution patterns that depended both on the type of stone and test.

Concerning the acid atmosphere exposure, the macroscopic observations showed a yellowing of every stone with the two concentrations. Specific features were observed for the S < N test such as a located blackening of Bj and Rs and white spots on the Cv sample (Figure III-24). After 28 days, all the stone surfaces were covered by a salt crust independently of the acid proportion. The binocular magnifier observations showed a smooth crust on Cv with the
presence of white spots looking like rhombic gypsum after the S < N test. For the other samples, the intuitive crystals' shape was visible especially inside ooids on Sv and Bj (Figure III-25).

b. Immersion tests

After the immersion tests, all the stones darkened except Cv that lightened in the SNPI and the NRAI. The yellowing was evident to the naked eye on all the stones for the SNAI test (Figure III-24).

The observations with binocular magnifier detailed the aesthetic alterations due to acid immersion affecting the stone components, such as discoloration and erosion (Figure III-25). The rainwater did not produce notable changes on the stones. The stirring did not result in different weathering compared to the motionless solution, though the iron oxidation occurring on each stone as well as the dissolution patterns were more intense in active than in passive condition.

On Cv, the oxidation of the Fe-containing calcitic cements which was identified as russet tones started after the 2nd cycle of immersion/drying in the acid solutions. At the end of the active immersion test, the color changes affected the foraminiferae (Figure III-25). The ooids of the Bj samples were the first concerned by the oxidation. Later, after the 2nd cycle, the russet color of the components and the cement was intensified and spread through the whole surface. The cement of Sv did not seem affected until the end of the test contrary to the ooliths whose cortex was oxidized and partially dissolved producing an increase of the surface roughness. The effects of the acid contact with Rs surface were clearly and progressively visible. The lithoclasts showed preferential dissolution and the russet oxidation affected the components more than the matrix at the end of the tests.
# Chapitre III : Suivi à méso-échelle

<table><thead><tr><th></th><th></th><th>S > N</th><th>S < N</th><th>SNAI</th><th>SNPI</th><th>NRAI</th><th>NRPI</th></tr></thead><tbody><tr><td rowspan="2">Cv</td><td>Before test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>After test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Bj</td><td>Before test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>After test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Sv</td><td>Before test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>After test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Rs</td><td>Before test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>After test</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Figure III-24. Photographs of entire stone samples before and after the mixed acid exposure tests (S>N mixed acid atmosphere with a ratio S:N=4:1, S<N mixed acid atmosphere with a ratio S:N=1:3) and the immersion weathering tests (SNAI sulfuric and nitric active immersion, SNPI sulfuric and nitric passive immersion, NRAI natural rainwater active immersion and NRPI natural rainwater passive immersion).
<table><thead><tr><th></th><th>Before test</th><th>S > N</th><th>S < N</th><th>SNAI</th><th>SNPI</th></tr></thead><tbody><tr><th>Cv</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Bj</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Sv</th><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Rs</th><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Figure III-25. Details of stones before (fresh samples) and after the acid atmosphere (S>N and S<N) and the acid immersion (SNAI and SNPI) tests.

### III.3.4.2. Colorimetry

The initial chromatic parameters of each stone are detailed in Tableau III-10.

#### a. Atmosphere tests

All the acid atmosphere exposure led to naked-eye detectable color changes, confirmed by ΔE* > 3. In general, these variations resulted from a decrease of the Lightness of the stone surface (ΔL* < 0), a variable evolution of the Chroma and an opposite behavior of the hue, with Cv and Sv that changed evidently their tone against Bj and Rs that kept their tone intact (Figure III-26). Regarding the type of test, in general S>N led to higher or similar changes than S<N, with the exception of Chroma parameter for Bj stone, which showed contrary trend. At the end of the exposures, Cv underwent the most important color variations with high delta values for all the parameters.
## b. Immersion tests

In most of the cases, immersion test produced a visible total color change on the studied stones ($\Delta E^* > 3$). In this case, the trends were a slight variation in Lightness and Chroma, more evident in Cv and Rs, and hue that behaved similarly to acid atmosphere exposure, with great changes in Cv and Sv for all the test while Bj and Rs remained unchanged. In general, active immersion led to higher or similar changes than passive, with remarkably high values in Bj and Sv for L* and C*. The differences between the immersion in an acid solution and the natural rainwater were negligible, with sometimes even higher differences measured after natural rainwater test.

<table><caption>Tableau III-10. Ranges of the chromatic values for the fresh samples.</caption><thead><tr><th rowspan="2">Stone</th><th>L*</th><th>C*</th><th>h*</th></tr><tr><th>Min - Max</th><th>Min - Max</th><th>Min - Max</th></tr></thead><tbody><tr><td><b>Cv</b></td><td>75.6 - 81.8</td><td>14.6 - 19.4</td><td>95.2 - 100.8</td></tr><tr><td><b>Bj</b></td><td>66.8 - 73.7</td><td>19.4 - 29.2</td><td>74.8 - 79.6</td></tr><tr><td><b>Sv</b></td><td>72.4 - 78.2</td><td>16.1 - 20.7</td><td>91.8 - 96.8</td></tr><tr><td><b>Rs</b></td><td>66.9 - 74.4</td><td>16.7 - 23.4</td><td>77.3 - 79.9</td></tr></tbody></table>
<figure><img src="image_70.png" /><figcaption>Figure III-26. Variations of chromatic parameters before and after the weathering tests.</figcaption></figure>

### III.3.4.3. Weight changes

After the exposure to acid atmospheres, the gain of weight was evident while, the stones
tended to lost matter after the immersion tests.

#### a. Atmosphere tests

The weight increased for the both tests, but the gain was higher in the S > N test with between 0.6% and 1.2% (Figure III-27.a). For S>N concentration, Sv and Rs showed the highest weight gain (> 1%) while Cv and Bj experimented less changes (≈0.6%). For S<N concentration, the weight gain was less significant, with the lowest values for Rs (0.2%).

#### b. Immersion tests

For all the samples and tests, the weight decreased over time and stabilized slightly thereafter.
The samples immersed in the acid solutions exhibited higher surface recession than in natural
# Chapitre III : Suivi à méso-échelle

rainwater. In the mixed acid solution all the stones showed a material loss more noticeable in active than in passive immersion (Figure 6.b).

After the immersion in natural rainwater, the material losses were negligible for Cv and Bj (< 5 µm) while Sv and Rs showed values up to 40 µm (Figure 6.b). Only Rs revealed differences between the results of active and passive immersion (ΔR = 10 µm).

After the immersion in acid and active immersion, Cv and Bj had the lowest and comparable surface recession values (≈ 30 µm) while Sv and Rs had the highest loss (≈ 75 µm).

<figure><img src="image_71.png" /></figure>

<figure><img src="image_72.png" /></figure>

Figure III-27. Weight variations ΔW, expressed in percentage in respect to the initial weight, at the end of the atmosphere exposure tests (a) and calculated surface recession during the immersion tests (b).
### III.3.4.4. Pore access radii

The open porosity values and the evolution of the pore size distribution obtained by Hg intrusion porosimetry are exposed in the Tableau III-11 and Figure III-28.

#### a. Atmosphere tests

The open porosity did not vary after the exposure of all the stone types to the S> N atmosphere (Tableau III-11). However, for the S < N test, open porosity increased notably in Bj up to 5 % but less in Sv up to 1.9 %, while it decreased in Cv around 1.5 %. The change was insignificant for Rs.

The pore access radii distribution showed different evolutions depending on the stone and the test. Cv and Rs exhibited evident variations for both concentrations (Tableau III-11, Figure III-28). For Cv, the access radii of the existent pores widened for the S>N while for the S<N the proportion of modal pores increased (Figure III-28.a). For Rs, the pore access radii comprised between 0.4 µm and 2µm narrowed after both tests and those between 0.2 and 0.4 µm closed (Figure III-28.d).

For Bj and Sv, the differences in the pore size distribution were more evident, with similar behaviors for pore radii > 1µm (decrease for S>N and increase for S<N) and the contrary for the other intervals. Bj showed an increase of 1-0.1µm range and a decrease of the < 0.1 µm range, with higher difference in S<N test (Figure III-28.b). Sv porosity experimented an augmentation in 1-0.1 µm range for both tests (Figure III-28.c).

#### b. Immersion tests

After the acid immersion tests, the variations of the open porosity were low and evolved in function of the stone and the test (Tableau III-11). After the SNPI test, the porosity increased for all the stones while in the SNAI test, Cv and Rs porosity trended to decrease. Concerning the natural rain immersion, the agitation of the solution did not influence the variation of open porosity that increased for Sv and Rs, and decreased for Cv and Bj.

Concerning the pore access radii evolution, Sv appeared less affected by the immersion tests with maximum variations in the SNPI solution resulting in a decrease of the larger pore access (> 1 µm) and an increase of the intermediate pore access (1-0.1 µm) (Tableau III-11;
Figure III-28.g). Some trends were observed in relation to the solution composition, regardless of the stirring mode. In the natural rainwater, the pore size changes were negligible except for Cv and Rs in NRPI that both showed a loss of intermediate pores and a gain of small pores (Figure III-28.e, h; Tableau III-11). For the same open porosity values, in the mixed acid solutions the amount of the largest pores increased in Bj while the smallest decreased in Cv. The intermediate behaved contrarily, with a decrease in Bj and increase in Cv (Figure III-28.e, f). The smallest range of pore size increased notably in Rs, unlike the intermediate pore radii suffered a diminution (Figure III-28.b; Tableau III-11).

<table><caption>Tableau III-11. Porosity parameters before and after the mixed acid weathering tests (calculated from Hg porosimetry). The initial state values are given with their standard deviation. The values in bold are outside the range of the natural variations of each stone porosity.</caption><thead><tr><th rowspan="2">Stone</th><th rowspan="2">Test</th><th rowspan="2">Porosity (%)</th><th colspan="3">Pore size distribution (%)</th></tr><tr><th>> 1 µm</th><th>1-0.1 µm</th><th>< 0.1 µm</th></tr></thead><tbody><tr><td rowspan="7">Cv</td><td>Fresh</td><td>20.1 ± 3.3</td><td>4 ± 1</td><td>22 ± 10</td><td>74 ± 9</td></tr><tr><td>S>N</td><td>20.4</td><td>5</td><td>49</td><td>46</td></tr><tr><td>S&lt;N</td><td>18.6</td><td>6</td><td>25</td><td>69</td></tr><tr><td>SNAI</td><td>20.1</td><td>5</td><td>35</td><td>60</td></tr><tr><td>SNPI</td><td>20.9</td><td>3</td><td>35</td><td>62</td></tr><tr><td>NRAI</td><td>19.4</td><td>3</td><td>26</td><td>71</td></tr><tr><td>NRPI</td><td>18.1</td><td>3</td><td>16</td><td>81</td></tr><tr><td rowspan="7">Bj</td><td>Fresh</td><td>20.0 ± 1.0</td><td>47 ± 2</td><td>42 ± 1</td><td>11 ± 2</td></tr><tr><td>S>N</td><td>20.4</td><td>45</td><td>47</td><td>8</td></tr><tr><td>S&lt;N</td><td>24.9</td><td>56</td><td>36</td><td>8</td></tr><tr><td>SNAI</td><td>23.1</td><td>55</td><td>36</td><td>9</td></tr><tr><td>SNPI</td><td>22.2</td><td>53</td><td>37</td><td>10</td></tr><tr><td>NRAI</td><td>19.4</td><td>44</td><td>46</td><td>10</td></tr><tr><td>NRPI</td><td>19.9</td><td>43</td><td>45</td><td>12</td></tr><tr><td rowspan="7">Sv</td><td>Fresh</td><td>31.8 ± 0.6</td><td>58 ± 3</td><td>29 ± 3</td><td>13 ± 4</td></tr><tr><td>S>N</td><td>30.9</td><td>55</td><td>30</td><td>15</td></tr><tr><td>S&lt;N</td><td>33.7</td><td>54</td><td>33</td><td>13</td></tr><tr><td>SNAI</td><td>33.5</td><td>55</td><td>30</td><td>15</td></tr><tr><td>SNPI</td><td>32.8</td><td>54</td><td>31</td><td>15</td></tr><tr><td>NRAI</td><td>34.7</td><td>58</td><td>27</td><td>15</td></tr><tr><td>NRPI</td><td>34.5</td><td>54</td><td>31</td><td>15</td></tr><tr><td rowspan="7">Rs</td><td>Fresh</td><td>22.9 ± 1.6</td><td>6 ± 1</td><td>41 ± 4</td><td>53 ± 4</td></tr><tr><td>S>N</td><td>22.6</td><td>5</td><td>36</td><td>59</td></tr><tr><td>S&lt;N</td><td>22.7</td><td>5</td><td>39</td><td>56</td></tr><tr><td>SNAI</td><td>22.6</td><td>3</td><td>30</td><td>67</td></tr><tr><td>SNPI</td><td>23.8</td><td>3</td><td>34</td><td>63</td></tr><tr><td>NRAI</td><td>24.5</td><td>6</td><td>47</td><td>47</td></tr><tr><td>NRPI</td><td>24.2</td><td>6</td><td>31</td><td>63</td></tr></tbody></table>
<figure><img src="image_73.png" /></figure>

Figure III-28. Pore distribution before and at the end of the acid atmosphere exposures (S>N symbolized the ratio [H2SO3]/[HNO3] = 4/1 and S<N symbolized [H2SO3]/[HNO3] = 1/3) for Cv (a), Bj (b), Sv (c) and Rs (d). b. Pore distribution before and at the end of the immersion tests (SNAI meant mixed acid in active immersion, NRAI for natural rain in active immersion, SNPI for mixed acid in passive immersion and NRAI for natural rain in passive immersion) for Cv (e), Bj (f), Sv (g) and Rs (h).
## III.3.4.5. SEM observations

### a. Atmosphere tests

The SEM images obtained during the acid atmosphere tests showed the formation of salt crystals from the very first day to the end of the tests that confirmed and specified the salt habitus depending on the type of stone and test (Figure III-29). No evolution over time in the crystals shape or size was detected for each sample and test. During the S>N test, Cv and Bj surfaces displayed platy crystals, while on Sv and Rs, the crust was composed by a mix of cauliflower and needle-like crystals. For the S<N exposure, Bj and Sv exhibited arrow-like crystals, Rs needle-like crystals and Cv rhombic-shaped crystals.

### b. Immersion tests

After the acid immersions, the samples in contact with rainwater did not display any dissolution feature on the SEM images, while the samples submitted to the acid solution showed different ones depending on the stone. The dissolution patterns in the latter were similar in the active (SNAI) and passive (SNPI) tests and affected mainly the fossils in Cv and Bj, the cortex of the ooliths in Sv and the calcareous components in Rs (Figure III-29). No sulfate neither nitrate crystals were observed on SEM but sulfur was detected by EDX with an amount comprised between 1% and 5%.
<table><caption>Figure III-29. SEM images of stone samples during the acid atmosphere and immersion tests.</caption><thead><tr><th rowspan="3"></th><th rowspan="3">Before test</th><th colspan="4">Atmosphere test</th><th>Immersion test</th></tr><tr><th colspan="2">S > N</th><th colspan="2">S < N</th><th>SNAI</th></tr><tr><th>After 2 days</th><th>At the end</th><th>After 2 days</th><th>At the end</th><th>At the end</th></tr></thead><tbody><tr><th>Cv</th><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Bj</th><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Sv</th><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Rs</th><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

### III.3.4.6. Chemical assessment

#### a. Sulfates and nitrates concentrations in stones

During the atmosphere tests, the amount of sulfates $SO_4^{2-}$ in the stones increased during the first's days for both tests, followed by a weak trend to augmentation or even stabilization except for Sv which showed no variation with time (Figure III-30). Sv and Rs were initially S-rich with concentrations in $SO_4^{2-}$ of 4 and 2 mg.g$^{-1}$, respectively, versus negligible for the fresh stones.

During the S>N exposure, Rs, Cv and Bj showed similar kinetics despite the initial content the $SO_4^{2-}$ concentration with a maximal increase of 2.5 mg.g$^{-1}$ (Figure III-30.a). For the S<N test, Bj and Rs kinetics were similar to those of S>N test and Cv showed lower final values than the other stones (Figure III-30.b). The $SO_4^{2-}$ concentration remained constant for Sv through all the tests.

Nitrate formation differed to the sulfate one. A linear increase was observed only for Cv and Bj in the S>N test. The nitrate concentrations in the other stones and tests could be considered negligible in this study.

At 20°C, Runsalt simulations indicated the possible crystallization of gypsum $CaSO_4 \cdot 2H_2O$ at any humidity, and nitrocalcite $Ca(NO_3)_2$ below 52% RH in the case of the two tests S>N and S<N.
<figure><img src="image_74.png" /><figcaption>Figure III-30. Sulfates [SO42-] (plain lines) and nitrates [NO3-] (dotted lines) content in stones during the S>N (blue lines) and S<N (red lines) exposures for each type of stone.</figcaption></figure>

## b. Dissolved concentrations (ICP)

The concentrations of Ca²⁺ in the solutions due to stone dissolution increased with time for all the stone types during all the used weathering methods (Figure III-31). Sv and Rs showed the highest Ca²⁺ loss whatever the test. Clear differences were likewise observed between acid and rainwater solutions, and between active and passive immersions. The final Ca²⁺ concentrations were maximal in the mixed acid and agitated solution reaching around 600 mg.l⁻¹ for Sv and Rs, and 400 mg.l⁻¹ for Cv and Bj. The passive results were also grouped by contaminated and fresh stones, with 500 mg.l⁻¹ for Sv and Rs, and 150 mg.l⁻¹ for Cv and Bj. In natural rainwater, Cv and Bj were hardly dissolved even in the agitated system. Sv behaved similarly than in the mixed acid solution and Rs showed intermediate dissolution compared to the rest of the stones.
<figure><img src="image_75.png" /><figcaption>Figure III-31. Ca²⁺ concentrations in the immersion test solutions over time.</figcaption></figure>

### III.3.5. Discussion

The contact of limestones with rain and acidic polluted atmosphere led to chemical reactions between the stone components, specially calcite, and the sulfurous and nitric acids. The immersion of the samples in an acid solution and the exposure to an acid environment revealed aesthetic and structural alterations depending on external factors such as the type and conditions of test, and on inherent stone characteristics such as composition and porosity.

#### III.3.5.1. Weathering tests

##### a. Differences between acid tests (atmosphere-immersion)

The main difference between acid tests was measured as weight changes, with a gain for acid atmosphere exposure due to gypsum precipitation, and a loss in acid immersion due to dissolution.
The precipitation of salts in the acid atmosphere exposure occurred from the first day and remained stable until the end of the test as shown by SEM observations while during the immersion test, the salt precipitation occurred only during the four drying stages (Eyssautier-Chuine et al., 2016). That explains that only in the atmosphere exposition test the precipitated salt was visible and up to 10 times higher than in the immersion test after being measured by ion chromatography.

The gypsum is the product of two successive reactions: (i) the dissolution of calcite by acids, called acidity effect (Lipfert, 1989) and (ii) the substitution of carbonate by sulfates as follows:

$$ (i) \quad CaCO_{3(s)} + H_2SO_4(l) = Ca^{2+}_{(aq)} + SO_4^{2-}_{(aq)} + H_2O(l) + CO_2(g) $$

$$ (ii) \quad Ca^{2+}_{(aq)} + SO_4^{2-}_{(aq)} = CaSO_4(s) $$

In the case of the clean rainwater, the dissolution of calcite is due to the inherent acidity of the solution due to atmospheric carbon dioxide presence, called karst effect (Lipfert, 1989; Webb et al., 1992).

The material loss affecting the immersed stones was confirmed by the decrease of weight and the calcium loss. The dissolution stage preceding the crystallization process during the acid atmosphere tests was highlighted by the Hg porosimetry results showing the widening of the pore access radii (Tecer, 1999; Vázquez et al., 2016).

## b. Influence of the fluid chemical composition

To develop the atmospheric tests, the ratios H₂SO₃/HNO₃ were chosen to represent the past and the current environmental pollution ratio, respectively S>N (4/1) and S<N (1/3). From any point of view, S>N appeared more aggressive than S<N.

Macroscopically and regarding the SEM observations, even if salt crystals' shape were different for each stone, the S>N test showed a better coverage of the surfaces. These results were also corroborated by a decrease of the lightness, a higher weight gain and ion chromatography results. The preexistent gypsum content in Sv limited its reactivity to the acid atmosphere. The presence of sodium sulfate in Rs did not prevent the precipitation of calcium sulfate. The nitrogen-based salts appeared only in non-contaminated stones (Cv and Bj) for
the S>N test that indicated the relationship between nitrate precipitation and the sulfur content.

The total recession and the natural recession increased at higher porosities which is especially evident for the natural solution, in the case of very high porosity stones such as the Savonnieres limestone Sv (Tableau III-12; Franzoni and Sassoni 2011). Regarding its low acidity and its origin, the rainwater used during the immersion tests could be considered as “clean rain” and as the karst effect factor in the surface recession models (Lipfert 1989, Livingston 1992, Webb et al. 1992). As suggested by Cardell-Fernandez et al. (2002) and Bonazza et al. (2009) and confirmed by the results of this study, the Lipfert model overestimated the karst effect which appeared as the dominant contributor in the dissolution of carbonate stones surface by the action of the rainwater runoff on buildings. While the Livingston and Webb models underestimated the nitrate fraction implied in the dissolution (Cardell-Fernandez et al. 2002). Those models could arise limited in terms on simulating the current and future atmospheric pollution. Although, Franzoni and Sassoni (2011) showed that laboratory experiments are difficult to extrapolate to on-site recession and generally overestimated due to the complexity of the rain composition.

<table><caption>Tableau III-12. Calculated surface recession in function of the solution in immersion test, karst and acidity fractions and calibrated open porosity in function of calcite content (open porosity (%) multiplied by calcite content (%) from Eyssautier-Chuine et al., 2016).</caption><thead><tr><th></th><th>Natural rainwater (µm)</th><th>Acid solution (µm)</th><th>Karst effect (%)</th><th>Acidity effect (%)</th><th>Calibrated open porosity (%)</th></tr></thead><tbody><tr><th>Cv</th><td>2</td><td>8</td><td>25</td><td>75</td><td>18.1</td></tr><tr><th>Bj</th><td>6</td><td>33</td><td>18</td><td>82</td><td>18.9</td></tr><tr><th><strong>Sv</strong></th><td><strong>35</strong></td><td><strong>56</strong></td><td><strong>63</strong></td><td><strong>37</strong></td><td><strong>28.9</strong></td></tr><tr><th>Rs</th><td>50</td><td>81</td><td>62</td><td>38</td><td>20.0</td></tr></tbody></table>

### c. Differences active-passive

In the acid solution, the samples appeared more affected by the active immersion, while in the rainwater, the differences between active and passive effects were less distinctive. The weight loss and dissolved Ca²⁺ concentrations was two times higher in the active acid immersion. The active immersion catalyzed the reaction between acids and calcite considering the principle of the Collision Theory. This states that to achieve a reaction, molecules in solution
must collide, thus the more collisions the faster the reaction is. The movement of the solution intensifies the motion rate of the reactant molecules increasing the chances of collision and consequently the production rate and the velocity of the reaction (Huber and Reid, 1926 ; Shah *et al.*, 2012). The agitation also generated mechanical effects which led to an erosion by the particles detachment (Baedecker and Reddy, 1993 ; Camuffo, 1995 ; Eyssautier-Chuine *et al.*, 2016).

The rainwater solution was less acid with pH = 5.9 than the acid solution with pH = 5. The dissolution produced by this weak acidity needed more time to allow noticeable reactions for the sound stone and react preferentially on the previous gypsum in the contaminated stones. Indeed, even if the solution penetrated deeper in the porous network in active immersion (Eyssautier-Chuine et al., 2016), the chemical dissolution produced by the rainwater was comparable in active and in passive immersion. The only difference between both tests in the natural rainwater was the mechanical part of the erosion in active immersion that represented only a small fraction compared to the chemical dissolution of calcite as suggested by Bonazza et al. (2009).

## III.3.5.2. Behavior of stones

### a. Comparisons between fresh and initially contaminated stone

The presence of sulfates in the initially contaminated samples of Sv and Rs was confirmed by the ion chromatography results. Throughout the atmosphere tests, the quantity of the sulfur anions in Sv was constant, indicating that its initial calcium sulfate content limited the substitution reaction. The sodium sulfate present in Rs was dissolved due to the high RH of the test, and consequently calcium sulfate could precipitate. In both cases, the preexistent sulfate salts prevent the nitrate formation. Contrarily, the fresh stones, Cv and Bj, experimented an increase in sulfur and nitrogen concentration due to the substitution with $Ca^{2+}$. In addition, both fresh stones showed similar gypsum crystallization shape on their surface.

During the immersion tests, the loss of material was highlighted by the weight and the calcium content in stones decreasing more than twice for Sv and Rs. The presence of gypsum before the first immersion cycle in these two stones explained their higher dissolution due to the elevated solubility of sulfate salts compared to calcite (Camuffo, 1995 ; Charola, 2007).
## b. Effects of mineralogy and texture

For all the tested stones, the first observation was the color change evidenced with naked eye and due to S deposition and metal elements oxidation. The samples showed a darkening after the mixed acid exposure, while the hue decreased similarly for both atmosphere exposition and acid immersion. Iron and Manganese were detected in studied stones, with highest amount in Bj and Rs. The oxidized forms of Fe- and Mn-based compounds are darker than their reduced forms that explains the decrease in Lightness (Völz et al., 2006). In addition, the Fe oxidation entailed a decrease of the hue of Cv and Sv, confirmed by the reddening of their surfaces. The transparent crust of gypsum crystals covering the surface of the atmosphere exposed samples could create interferences in light reflection that modified slightly the chromatic parameters (Vázquez et al., 2016).

In general, the total porosity varied slightly in mixed atmospheres and led to the widening of the pore access radii. The anionic substitution reaction led to a dissolution stage of the calcite preceding the salt crystallization. Both processes entailed variations, i.e. widening and closing, of the pore size distribution. The dissolution/crystallization process were noticed for all the stones within the two tests, independently of the S/N ratio. In Cv, the distribution was unimodal and the modifications affected the same pore size range, i.e. 0.1 µm of access radii. Whereas the bimodality of the pore size distribution induced a variation in the entire pore network distribution, with aperture or closing depending on the different pore access sizes for each stone type.

During the immersion tests, Sv was the most affected regarding the weight and Ca losses that was explained not only by a faster and greater dissolution in high porous stones (Sabbioni and Zappia, 1992 ; Bonazza, 2009) but also by the initial gypsum presence. That explains the lower dissolution in Bj which showed also large pore radii access and confirms that this parameter does not dominate the dissolution process.

## III.3.6. Conclusion

In this study, the behavior of four stones exposed to past and future pollutants ratio, and the effect of acid rain storms was assessed and compared. The stone weathering caused by the effects of gaseous deposition and acid rain was evaluated by the submission of calcareous samples of three natural stones and an artificial one to accelerated ageing tests. In the first
test, the stone specimens were exposed to gaseous phase of two mixed acid solutions with different S/N ratios, and in the second one, the slabs were immersed in one artificial acid solution and in natural rainwater, in still and agitated conditions to simulate water stagnation and storms.

The $SO_4^{2-}/NO_x$ ratio plays a determining role in the alterations induced by the gaseous deposition. Higher the $SO_4^{2-}$ amount is, more aggressive the weathering will be. The current pollution ratio (1:3) produces less intense color variations, mainly by the darkening of the stone surfaces due to compounds oxidation, and similar or lesser salt precipitation than in the past. After simulated rain tests, no salt crystallization was observed even if sulfates and nitrates were detected by ion chromatography. The material loss was mainly caused by the dissolution of calcite for all the stones and of the previous sulfates in the contaminated ones. The artificial acid solution was much more aggressive than the natural rainwater, suggesting that the acidity effect is greater than the karst effect. In the mixed acid atmospheres, the change of the porous network depended further on the pore size distribution. In the immersion tests the dominant parameter was the open porosity.

The differences between active and passive immersion were evident only for the acid solution, in which the samples appeared more affected by the active one. In rainwater immersion, the chemical dissolution was low due to the higher pH. In addition, the mechanical effect of the agitation represents a small part in the erosion process, and in this study it was insufficient to create differences between passive and active immersion. Thus, if natural rain acidity remains stable, the meteorological episodes such as strong storms will not induce a much higher stone deterioration.

In contaminated stones, the initial presence of sulfur compounds limited the crystallization of sulfates and nitrates in the acid atmosphere tests that make them unconcerned by the new and future atmospheric pollution ratio. In addition, these already existent salts will be leached sooner, protecting more the stone from dissolution decay than the fresh quarried ones.
Acknowledgments

The authors want to thank Xavier Drothiere, Alexandra Conreux and Julien Hubert for their technical and analytical support. This study was funded by “Reims Métropole” with the project IFEPAR and the University of Reims-Champagne Ardenne and the “Ville de Reims” with the Project REMITHERM, and “Grand Est” with the project FLUTE.
### III.4. Experimental study on the influence of surface finish and composition on the deterioration of building stones exposed to mixed acid atmospheres

Ce travail a donné lieu à une publication de rang A dans la revue *Construction and Building Materials*, volume 106, pages 392-403.

#### Résumé

Les polluants ont changé au cours des dernières décennies, se manifestant par une forte diminution du SO₂ dans l'atmosphère et, en conséquence, par un rapport NOₓ/SO₂ plus élevé. Les mécanismes d'altération de la pierre par les nitrates, et par la combinaison des nitrates et des sulfates sont encore peu connus. En outre, le comportement de la pierre dépend de sa composition minéralogique et chimique, de sa surface spécifique, de sa porosité et enfin de la rugosité de sa surface.

Dans cette étude, six pierres ont été sélectionnées en fonction de leur composition et de leur texture. Trois d'entre elles étaient des pierres calcaires pures, présentant des différences dans leurs textures: le travertin Albox, le calcaire Fraga et le calcaire Santa Pudia. Les trois autres pierres, qui présentent des différences de composition, étaient la dolomie de Boñar, le grès de Uncastillo et le grès de Villaviciosa.

Deux finitions de surface ont été testées : finition polie et rugueuse. Quatre tests en atmosphères acides concentrées d'une durée de 3 semaines ont été effectués. Ces atmosphères ont été obtenues en ajoutant 50 ml de solution acide dans un récipient avec une capacité de 5 l :

* Deux solutions de composition simple en acide nitrique (HNO₃ = 10,1 mol l⁻¹) et en acide sulfureux (H₂SO₃ = 5,3 10⁻¹ mol l⁻¹);

* Deux solutions mixtes avec des proportions différentes en acides (HNO₃ = 7,1 mol l⁻¹ + H₂SO₃ = 1,6 10⁻¹ mol l⁻¹ ; HNO₃ = 3,0 mol l⁻¹ + H₂SO₃ = 3,7 10⁻¹ mol l⁻¹).

Chaque type de pierre a été placé dans le récipient fermé de façon à ce que les surfaces ne soient pas en contact direct avec la solution.

Les variations de surface ont été mesurées par les changements de poids, de couleur et de rugosité. Le microscope électronique à balayage avec analyse EDX a été utilisé pour observer les dépôts à la surface et leur composition a été confirmée par des simulations RUNSALT.
Les résultats ont montré que les atmosphères acides peuvent causer des dommages à toutes sortes de pierres, même celles dont la teneur en Ca est extrêmement faible. L'altération est moins intense pour les pierres non carbonatées et seuls les dépôts ont lieu. Pour les pierres carbonatées, les modifications sont davantage liées à la taille des pores, à la distribution de la porosité et à la surface spécifique qu'à la quantité de calcium.

Des changements de couleur se produisent dans les atmosphères simple en SO₂ et mixtes, mais leur intensité dépend de la couleur initiale de la pierre plutôt que de sa composition ou de l'atmosphère à laquelle elle est exposée.

La finition de la surface a une influence évidente sur la rugosité finale, avec des variations plus élevées sur les échantillons non polis, bien que les surfaces polies subissent également des changements de rugosité.

Des différences ont été soulignées par rapport au type d'atmosphère, aux propriétés et à la surface des pierres. L'exposition aux NOₓ seuls affecte peu la pierre alors qu'en y ajoutent SO₂, les variations sont évidentes. En fonction du rapport NOₓ/SO₂, les NOₓ peuvent réagir directement avec la pierre et précipiter sous forme de nitrates ou de nitrites, ou alors les NOₓ peuvent agir comme catalyseur de l'oxydation du SO₂ et favoriser la formation de gypse.

Ces résultats prouvent l'importance de la proportion de polluants dans l'atmosphère et leurs différents effets sur les pierres de construction. De plus, la couleur et la rugosité ont été mesurées par des techniques non destructives et efficaces pour évaluer les dommages produits par l'exposition des pierres à des atmosphères acides.
## Experimental study on the influence of surface finish and composition on the deterioration of building stones exposed to mixed acid atmospheres

Vázquez P.<fnref n="1" />, Carrizo L.<fnref n="2" />, Thomachot-Schneider C.<fnref n="1" />, Gibeaux S.<fnref n="1" />, Alonso F.J.<fnref n="2" />

Published online in Construction and Building Materials on December 24th, 2015.

## Highlights

* Acid atmospheres may cause damage to all types of stones.

* In calcitic stones, the effect of acid atmospheres is related to porosity.

* Intensity of colour change depends mainly on the inherent stone colour.

* Artificial finish influences the acid atmosphere's attack.

* $HNO_3/H_2SO_3$ ratio controls whether Nitrogen compounds react with the stone or acts only as a catalyser.

**Keywords:** Acid atmosphere; pollutants; stone; colour; roughness; sulfates, nitrates; artificial finish.

## Abstract

Six stone types with differences in composition and texture were exposed to four strong acid atmospheres formed from a film of different acids: $H_2SO_3$, $HNO_3$, and two mixed solution with different proportions of $H_2SO_3$ and $HNO_3$. The changes on the surface were assessed by weight, colour, roughness and microscopic observation. Exposure to the atmosphere formed by $HNO_3$ hardly affected the stone, whereas the formed from $H_2SO_3$ produced evident alterations. Depending on the $HNO_3/H_2SO_3$ ratio, the nitrogen compounds may react with the stone and precipitate nitrates or nitrites or may only act as a catalyser of $SO_2$ and enhance the formation of gypsum. Colour and roughness are efficient non-destructive approaches to evaluating the damage produced by acid atmospheres.

<footnote n="1">¹ Groupe d'Etudes sur les Géomatériaux et les Environnements Naturels, Anthropiques et Archéologiques (GEGENAA, EA 3795), Université de Reims Champagne-Ardenne, 2, Esplanade Roland Garros, 51100 Reims, France.</footnote>

<footnote n="2">² Dpto de Geologia, Universidad de Oviedo. Jesus Arias de Velasco s/n. 33005 Oviedo, Spain</footnote>
### III.4.1. Introduction

Weathering of natural building stones exposed to the environment is a field of study in constant development. The main reason is that the agents of decay, climate and pollution are changing due to technical progress and political choices. One of the remediation procedures for global warming involves the modification of combustibles, producing a different air pollution that leads to different stone weathering processes (Brimblecombe et Grossi, 2009). In the last century, SO₂ concentrations in the atmosphere have decreased, and thus, the proportion of nitrogen oxides and volatile organic compounds increased in the atmosphere. This also led to an increase in the ozone production in urban air and consequently to oxidation products such as nitrogen dioxide or nitric acid (Ferm et al., 2005). SO₂ has been the main pollutant involved in the decay of building stones, giving rise to crust formation, yellowing and carbonate dissolution (Török et Rozgonyi, 2004; Grossi et al., 2007; Monna et al., 2008; Brimblecombe et Grossi, 2009; Graue et al., 2013; Janvier-Badosa et al., 2014). In recent times, there has been a strong decrease from high levels of sulfate deposition. Currently, the deposits are dominated by diesel soot and nitrogen compounds that may act as catalysts for the SO₂ oxidation (Massey, 1999).

Stone decay generated by mixed gases has been demonstrated to be faster than decay induced by a single gas (Haneef et al., 1992; 1993b). Stones exposed to a mixture of NO₂, SO₂, O₃ and H₂O show higher degradation rates than those exposed to SO₂ and NO₂ individually (Massey, 1999; Bai et al., 2003). The role of metal oxides in the oxidation reactions is, however, under discussion (Haneef et al., 1992; 1993b; Böke et al., 1999; Massey, 1999; Müller, 2008). On a dry surface exposed solely to SO₂, the gas phase oxidation from SO₂ to SO₃ is slow, whereas for a surface exposed only to NOₓ, the conversion from NO₂ to NO₃⁻ is fast. On a dry surface exposed to the mixed gases SO₂+NO₂, the oxidation of SO₂ to SO₄²⁻ is 10 times greater than that for exposure to only SO₂, whereas the oxidation of NO₂ to NO₃⁻ is 0.4 times less than that of exposure to single NO₂. This confirms that the reaction with stone is dominated by the oxidation of SO₂ compared with the oxidation of NO₂. However, NO₂ can enhance SO₂ absorption without producing nitrates, acting as catalysts and increasing the decay produced by SO₂ pollutants (Bai et al., 2006). Nitrogen oxides have been mainly studied from the point of view of their action as SO₂ oxidants. The interaction between NOₓ and the stone surface is less known (Sikiotis et Kirkitsos, 1995; Camaiti et al., 2007).
The effect of pollutants on stone decay is related to its nature and the inherent characteristics of the stones such as composition, texture or porosity (Lipfert, 1989 ; Sweevers et al. van Grieken, 1992 ; Grossi et al., 1995 ; Massey, 1999 ; Müller, 2008 ; Bonazza et al., 2009). Moreover, artificial features such as surface finish lead to different surface topographies and consequently to different decay characteristics (Lipfert, 1989 ; Dolske, 1995 ; Ausset et al., 1999). There are also differences between the SO₂ absorption in calcite and dolomite (Cobourn et al., 1993 ; Malaga-Starzec et al., 2003 ; Camaiti et al., 2007). Calcareous stone reacts with acid pollutants via dissolution. Sulfur and nitrogen salts crystallise on the surface (Lipfert, 1989 ; Cobourn et al., 1993 ; Dolske, 1995). Sandstones with carbonate as fragments, cement or clays are also vulnerable to acid attack. The texture of the stone, especially the grain boundaries, may enhance the crystallisation process (Malaga-Starzec et al., 2003) or the weight loss when exposed to acid rain (Franzoni et Sassoni, 2011). Sandstones are vulnerable to air pollutant attack due to their porosity, even if they have low or negligible carbonate contents. Open porosity allows the airborne particles and gazes (SO₂, O₃, NOₓ) to enter within the stone and interact with its components. Pore distribution and specific surface area influence the uptake of moisture from the air. High specific surface area implies high moisture content, which will favour dry deposition. Dry deposition could be important in the case of NO₂ deposition, as NO₂ is less soluble than SO₂ (Grossi et Murray, 1999).

Acid atmospheres produce chemical and physical changes on the stone surface, potentially reaching depths of a few millimetres. As a result, surface properties such as colour and roughness will provide precious information about the degree of the decay. Colour is one of the most representative features of an ornamental stone. Consequently, the colour variations produced by chemical reactions during pollutant exposure have a priority over the evaluation of the decay (e.g. Vázquez et Alonso, 2010 ; Vázquez et al., 2010 ; Luque et al., 2013). The contaminants studied to date produce yellowing or blackening of the stone. However, the effect of the new proportion of pollutants is less well known, and consequently, so is the colour change. Another parameter closely related to surface decay is roughness. Surface roughness varies due to erosion, dissolution and/or precipitation, providing information about the intensity and the types of decay (Tecer, 1999 ; Bernal et Lopez, 2000 ; Alonso et al., 2008 ; Vázquez et al., 2013b ; Vázquez et al., 2015), and this is sometimes also related to colour (Benavente et al., 2003). In addition, surface roughness increases the exposed surface area, trapping more gas and particles (Ausset et al., 1999), and the deterioration process may be enhanced by the increase in surface roughness (Lipfert, 1989 ; Dolske, 1995).
Ageing tests are used in research to simulate stone decay. These tests do not allow a real comparison between the decay produced in the laboratory and in the real environment. Nevertheless, they are extremely helpful for comparing the durability between different stones and to determine which stone characteristics influence the type and degree of decay, as well as which types of interactions between the stone and the weathering agent take place. In relation to the study of stone decay by pollutants, the standard UNE-EN 13919 (CEN 2002) is an optimal test for research aims. This test produces an intensive acid attack on the stone, allowing observation in the short-term of the precipitation of salts (Vázquez *et al.*, 2010 ; Vázquez *et al.*, 2015) and the evolution of stone properties (Dewanckele *et al.*, 2012).

The general aim of this research is to evaluate the impact of mixed acid atmospheres on the surfaces of different stones by colour and roughness measurements. Six porous stones with different compositions (three limestones, two sandstones and one dolostone), commonly used as ornamental stones, were tested. The samples were prepared with two different finishes, one smooth (polished) and another rough (bush hammered). The strong atmospheres were obtained from single H₂SO₃, single HNO₃, and two mixed atmospheres with different proportions of H₂SO₃ and HNO₃. This study is intended to shed light on i) the impact produced by new mixes of pollutants, ii) the effect of different acid atmospheres on stones with different mineralogies and textures, and iii) the influence of surface finish on the deposition of pollutants on the surface.

### III.4.2. Materials and methods

Six stones were selected in relation to their composition and texture. Three of them were pure calcareous stones, showing differences in their textures: Albox travertine (AT), Fraga limestone (FL) and Santa Pudia limestone (SPL). The other three stones, which exhibit differences in composition, were Boñar dolostone (BD), Uncastillo sandstone (US), and Villaviciosa sandstone (VS). For future explanations in terms of composition, the three limestones were named “pure calcitic stones”, and the group formed by the dolostone and the two sandstones was called “other sedimentary stones”. Two surface finishes were selected in this study: polished and bush hammered finish.
## III.4.2.1. Methods for the characterization

The characterisation consisted of a petrographic description of the materials and a surface evaluation by means of colour and roughness measurements. Mineralogy and texture were studied using polarised optical microscopy (Zeiss Jenapol POM). Pore system parameters were obtained using a Hg intrusion porosimeter Micromeritics AutoPore III 9410, which reaches 414 MPa pressure and can measure pore radius sizes from 0.003 to 360 µm. All of the data were extracted from previous studies (Molina *et al.*, 2011; Vázquez *et al.*, 2013a).

Colour was measured and quantified with a MINOLTA CR-200 colourimeter using the illuminant D65, a beam of diffuse light of 8-mm diameter, and a 0° viewing angle geometry, with a specular component included and the spectral response closely matching the CIE (1997) standard observer curves. Measurements were expressed following the CIE L\* a\* b\* systems (EN ISO 105-J03: 1997; CEN, 2009). L\* is the lightness that goes from black (value 0) to white (value 100), a\* goes from red (positive values) to green (negative values), b\* goes from yellow (positive values) to blue (negative values). ΔE\* is introduced as the total colour change, to compare the variations before and after the tests as follows: $\Delta E^* = [((\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2)^{1/2}$]. The determination of the colour in a heterogeneous material requires a previous study of the number of measurements needed, which is related to the colour of each mineral, grain size and heterogeneity (Vázquez *et al.*, 2013b; Vázquez *et al.*, 2015). The number of data points was determined by calculation of the cumulative average until the stabilisation of the values. In this study, 15 data points were the minimum required in each stone group (i.e., 5 measurements x 3 slabs).

Surface topography and roughness were measured using a stereomicroscope Leica MZ16A and the associated software Leica Stereo Explorer 2.1. The resolution is 840 Lp/mm, which provides enough accuracy to detect small surface variations. The maximum vision diameter is 57.5 mm, so a representative surface of the stone can be measured. This equipment uses visible light, which attenuates the irregular reflections and refractions of lasers in the presence of quartz. Prior studies determined an optimal area of measurement of 25 x 18 mm. Area roughness parameters were computed from an average of three areas in which twenty 2D profiles, with a spacing of 1.25 mm were measured, following the standard EN 4287 (CEN, 1997). A cut-off length Gaussian filter was applied to evaluate only the roughness wavelengths. This cutoff allows focusing the analysis on the deterioration and not on the slab shape. The following parameters were selected to define surface roughness.
- Ra: *Arithmetical mean deviation*. Arithmetical mean of the absolute values of the deviations (Zi) from the mean line.

- Rp: *Maximum peak height*. The maximum value of the deviations (Zi) from the main line.

- Rv: *Maximum valley depth*. The absolute value of the minimum value of the deviations (Zi) from the mean line.

- *Bearing area curve*. Cumulative height distribution curve from the mean profile. The curve is plotted with a normalised abscissa from 0 to 100% of the profile length. The ordinate indicates the percentage of area analysed from the highest peak to the deepest valley.

A parameter that takes into account the whole surface, without the application of the filter, was also obtained:

- D: *Fractal dimension*. A fractal describing a rough geometric shape that can be subdivided into parts, each of which, at least approximately, is a reduced-sized copy of the whole. They are generally self-similar and independent of scale (Mandelbrot, 1983). Fractal dimension results are a good indicator of surface weathering (Bernal et Lopez, 2000).

### III.4.2.2. Material characterization

The selected stones with both finishes are shown in Figure III-32. Mineralogy is compiled in Tableau III-13. For more information, see (Molina et al., 2011; Vázquez et al., 2013a). Tableau III-14 shows the chemical compositions of the six stone types expressed in percentages (Vázquez et al., 2013a). Porous system parameters can be found in Tableau III-15 (Molina et al., 2011; Vázquez et al., 2013a).
<figure><img src="image_76.png" /><figcaption>Figure III-32. Macroscopic aspect of the selected stones with both bush hammered and polished finishes.</figcaption></figure>

Albox Travertine (AT) is a heterogranular limestone, of type travertine, with a yellow-brownish colour. It has a crystalline texture with bands of different colours due to variations in composition or texture. AT has grains of distinct shapes and sizes (micrite, sparite, fibrous), which appear either elongated in bands or filling pores. There are also areas with dark crystals due to plant precursors and Mg and Fe oxides/hydroxides. Very fine stratification surfaces with silt-sized quartz crystals and muscovite can be observed. The insoluble residue is only 1%, and it is composed mainly of quartz and some clay (illite, paragonite). AT has approximately 15% porosity, mainly as elongated and tubular pores following the bedding, and low specific surface area. This stone has pores of large size that cannot be detected by Hg porosimetry with values of Hg porosity about 8%.

Boñar Dolostone (BD) is a crystalline stone with a brownish colour. It has a crystalline texture above a granular relict texture. Its mineral composition is formed by hypidiomorphic dolomite fine crystals that replace the original calcite, and the intergranular spaces may be filled with secondary calcite crystals or may remain empty. The dolomite/calcite ratio is 80/20. The 4% of insoluble residue consists mainly of quartz and clays (illite and chlorite/kaolinite). BD has approximately 10% porosity, with larger pores in the intergranular spaces, many small pores (< 0.1 µm) and notable specific surface area.

Fraga Limestone (FL) is a micrite to biomicrite limestone showing bioturbation and having a grey-yellowish colour. It has a microcrystalline texture due to the transformation of carbonate mud. FL is mainly composed of microcrystalline calcite (1 µm) with bioclasts (10%) such as ostracods, bivalves and charophytes. The insoluble residue is approximately 3%, and it is composed of quartz, clays (illite and chlorite/kaolinite) and organic matter. FL has
approximately 30% porosity with moldic pores, is often dissolution recrystallization, and has many small pores (< 0.1 µm) and high specific surface area.

Santa Pudia (SPL) is a bioclastic limestone that varies from white to yellow colour. This stone has a clast-supported texture with more than 95% fossil fragments (up to 5 mm). Bioclasts are mainly bryozoans, red algae, crinoids (sometimes altered to peloids), and occasional serpulids, echinoderms and mollusc shells. Most of the grains are formed by micrite, and microsparite can be found in the bryozoans and fibrous calcite in the molluscs. SPL shows syntaxial calcite and microsparite cement in low proportions. The insoluble residue is approximately 3%, and it reveals a higher proportion of silt-sized quartz than clays (illite and chlorite/kaolinite). SPL has intragranular and moldic porosity of approximately 35%, fewer small pores (< 0.1 µm) and low specific surface area.

Uncastillo Sandstone (US) is a litharenite rich in carbonate fragments (calcareous sandstone) with brown-yellowish colour. It has a grain-supported texture with medium to fine grains in contact, and syntaxial calcite cement. This stone has similar proportions of silicate (quartz and feldspar) and carbonate (peloids and bioclasts) minerals. Micrite and clays can also be found in the calcite cement. US has the highest clay content (illite and chlorite/kaolinite) of the studied stones and the highest iron content due to the presence of biotite, chlorite and goethite. US has approximately 20% intergranular porosity, and notable specific surface area.

Villaviciosa Sandstone (VS) is a sublitharenite to subarkose stone with a grey to brownish colour. It has a grain-supported texture with medium-sized grains. Their mineralogy consists predominantly of quartz, microcline type feldspars with different degrees of weathering, and rock fragments (silex, lutite). Overgrowth cement is mainly quartzitic with a low proportion of clay (illite). VS has approximately 20% intergranular porosity and a high specific surface area.

<table><caption>Tableau III-13. Mineral compositions of the six types of stone.</caption><thead><tr><th>Stone</th><th>Cal (%)</th><th>Dol (%)</th><th>Q (%)</th><th>Fpt (%)</th><th>RF (%)</th><th>C (%)</th></tr></thead><tbody><tr><td>AT</td><td>99</td><td>-</td><td>0.7</td><td>-</td><td>-</td><td>0.3</td></tr><tr><td>BD</td><td>18</td><td>78</td><td>2</td><td>-</td><td>-</td><td>2</td></tr><tr><td>FL</td><td>97</td><td>-</td><td>1</td><td>-</td><td>-</td><td>2</td></tr><tr><td>SPL</td><td>97</td><td>-</td><td>2</td><td>-</td><td>-</td><td>1</td></tr><tr><td>US</td><td>46</td><td>-</td><td>15</td><td>10</td><td>25</td><td>4</td></tr><tr><td>VS</td><td>-</td><td>-</td><td>68</td><td>15</td><td>15</td><td>2</td></tr></tbody></table>

AT: Albox travertine, BD: Boñar dolostone, FL: Fraga limestone, SPL: Santa Pudia limestone, US: Uncastillo sandstone, VS: Villaviciosa sandstone. Cal = calcite, Dol = dolomite, Q = quartz, Fpt = feldspars, RF = rock fragments, C = clays (Vázquez et al., 2013a).
<table><caption>Tableau III-14. Chemical composition of the six types of stone expressed in percent.</caption><thead><tr><th>Stone</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>MgO</th><th>CaO</th><th>LOI</th></tr></thead><tbody><tr><td>AT</td><td>0.00</td><td>0.00</td><td>0.05</td><td>0.31</td><td>55.33</td><td>43.46</td></tr><tr><td>BD</td><td>1.03</td><td>0.49</td><td>0.38</td><td>17.63</td><td>34.34</td><td>45.48</td></tr><tr><td>FL</td><td>1.80</td><td>0.33</td><td>0.54</td><td>0.57</td><td>53.28</td><td>43.05</td></tr><tr><td>SPL</td><td>2.93</td><td>0.06</td><td>0.37</td><td>0.31</td><td>52.53</td><td>43.20</td></tr><tr><td>US</td><td>47.80</td><td>3.43</td><td>1.80</td><td>0.59</td><td>24.73</td><td>20.40</td></tr><tr><td>VS</td><td>94.74</td><td>2.23</td><td>0.29</td><td>0.08</td><td>0.03</td><td>0.46</td></tr></tbody></table>

AT: Albox travertine, BD: Boñar dolostone, FL: Fraga limestone, SPL: Santa Pudia limestone, US: Uncastillo sandstone, VS: Villaviciosa sandstone LOI: Loss on ignition (Vázquez et al., 2013a).

<table><caption>Tableau III-15. Porous system distribution obtained by mercury intrusion porosimetry. Specific surface area (SAA) was obtained from N2 adsorption.</caption><thead><tr><th>Stone</th><th>p<sub>o</sub> (%)</th><th>&gt;1 µm</th><th>1-0.1 µm</th><th>0.1-0.01 µm</th><th>&lt;0.01 µm</th><th>SAA (m<sup>2</sup>/g)</th></tr></thead><tbody><tr><td>AT</td><td>7.9 ± 1.5</td><td>2.1</td><td>1.7</td><td>2.4</td><td>1.6</td><td>0.68</td></tr><tr><td>BD</td><td>9.1 ± 2.9</td><td>0.6</td><td>5.0</td><td>3.1</td><td>0.4</td><td>2.41</td></tr><tr><td>FL</td><td>29.1 ± 0.2</td><td>4.3</td><td>21.6</td><td>3.0</td><td>0.1</td><td>3.82</td></tr><tr><td>SPL</td><td>33.5 ± 1.0</td><td>18.5</td><td>12.0</td><td>1.8</td><td>1.1</td><td>0.87</td></tr><tr><td>US</td><td>18.0 ± 1.4</td><td>14.0</td><td>2.3</td><td>1.5</td><td>0.2</td><td>2.65</td></tr><tr><td>VS</td><td>19.2 ± 0.6</td><td>15.6</td><td>1.8</td><td>0.9</td><td>0.9</td><td>3.76</td></tr></tbody></table>

AT: Albox travertine, BD: Boñar dolostone, FL: Fraga limestone, SPL: Santa Pudia limestone, US: Uncastillo sandstone, VS: Villaviciosa sandstone. p<sub>o</sub> = open porosity, SAA = specific surface area (Molina et al., 2011).

### a. Colour

Tableau III-16 shows the colour parameters for each stone type and for both surface finishes, expressed as the average and standard deviation of these 15 measurements.

All of the stones were quite homogeneous in terms of colour. Regarding the L\* parameter, all of the stones exhibited high values according to their light colour. Within the bush hammered stones, L\* varied approximately from 65 (US) to 85 (SPL), showing AT, FL and VS values near L\*=70. For polished stones, L\* did not change from the bush hammered surfaces of FL and SPL, whereas the other stones had L\* values of approximately 5 units lower. The measurements of the a\* parameter yielded results from 0 to 1 for both finishes because the selected stones have mainly white, cream and brownish colours. The only exception was AT, which showed a yellow colour and a\* values of approximately 6 in bush hammered and 8 in polished finish specimens. Values of the b\* parameter had the highest
differences between stones, with values between 10 and 20 for both finishes. AT is again the exception, with b* = 28 for bush hammered and 33 for polished finishes. The total colour variation ΔE* between bush hammered and polished finishes is higher than 3 (visible threshold; Vázquez et al., 2013a; 2015) in most of the stones, with maximum values of ΔE*=7.5 in AT.

<table><caption>Tableau III-16. Colour parameters of the sound stones (D65 illuminant).</caption><thead><tr><th rowspan="2">Stone</th><th colspan="3">Bush hammered</th><th colspan="3">Polished</th><th rowspan="2">ΔE*</th></tr><tr><th>L*</th><th>a*</th><th>b*</th><th>L*</th><th>a*</th><th>b*</th></tr></thead><tbody><tr><td>AT</td><td>72.0 ± 2.9</td><td>5.8 ± 1.6</td><td>27.8 ± 3.9</td><td>66.8 ± 0.6</td><td>7.9 ± 0.4</td><td>32.8 ± 2.0</td><td>7.50</td></tr><tr><td>BD</td><td>76.2 ± 1.1</td><td>0.67 ± 0.4</td><td>15.3 ± 0.2</td><td>71.7 ± 1.8</td><td>1.5 ± 0.3</td><td>16.7 ± 0.9</td><td>4.82</td></tr><tr><td>FL</td><td>71.3 ± 6.9</td><td>1.2 ± 0.4</td><td>13.0 ± 3.5</td><td>73.8 ± 1.3</td><td>1.0 ± 0.4</td><td>11.6 ± 0.7</td><td>2.92</td></tr><tr><td>SPL</td><td>86.1 ± 1.2</td><td>0.06 ± 0.4</td><td>12.7 ± 1.0</td><td>87.9 ± 0.5</td><td>0.0 ± 0.3</td><td>12.6 ± 1.2</td><td>1.81</td></tr><tr><td>US</td><td>64.7 ± 0.9</td><td>0.7 ± 0.4</td><td>20.9 ± 1.5</td><td>59.6 ± 0.5</td><td>0.8 ± 0.2</td><td>19.4 ± 0.6</td><td>5.26</td></tr><tr><td>VS</td><td>69.9 ± 0.8</td><td>0.4 ± 0.3</td><td>13.0 ± 1.1</td><td>64.7 ± 1.2</td><td>0.9 ± 0.4</td><td>14.3 ± 0.5</td><td>5.35</td></tr></tbody></table>

b. Roughness

Tableau III-17 shows the averaged roughness parameters and their standard deviations. With the bush hammered finish, most of the stones exhibited similar values, with average roughness (Ra) values between 20 and 25 µm, and maxima peaks heights (Rp) and valleys (Rv) of approximately 60 µm. Some exceptions could be found in the low values of Rv in FL or in the high values in Rp of US. VS had the lowest values among the studied stones. The bush hammered finish produced a regular roughness pattern on the stone surface, which is shown in Figure III-32. This pattern explained the high roughness values found for this finish. In the polished finish, most of the stones showed Ra values of approximately 5-10 µm and Rp and Rv values of approximately 15-25 µm. Because of their coarse grain size, SPL and US exhibited similar values in both polished and bush hammered finishes. Fractal dimension (D) values differed positively from zero. If a value is equal to zero, the surface is completely flat. The higher the values are, the greater the roughness is. Thus, fractal dimension is always