lower for a polished finish due to the flatter surface. In this study, the differences in the fractal dimension between bush hammered and polished stones were more significant than for the other parameters.

<table><caption>Tableau III-17. Surface roughness parameters Ra, Rp and Rv expressed in µm.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Bush hammered</th><th colspan="4">Polished</th></tr><tr><th>Ra</th><th>Rp</th><th>Rv</th><th>D 10<sup>-2</sup></th><th>Ra</th><th>Rp</th><th>Rv</th><th>D 10<sup>-2</sup></th></tr></thead><tbody><tr><td>AT</td><td>24 ± 5</td><td>60 ± 10</td><td>61 ± 17</td><td>2.68 ± 1.30</td><td>9 ± 2</td><td>24 ± 2</td><td>27 ± 2</td><td>0.85 ± 0.36</td></tr><tr><td>BD</td><td>23 ± 4</td><td>56 ± 9</td><td>53 ± 11</td><td>1.70 ± 0.04</td><td>6 ± 2</td><td>19 ± 5</td><td>19 ± 4</td><td>0.49 ± 0.11</td></tr><tr><td>FL</td><td>23 ± 6</td><td>60 ± 22</td><td>29 ± 24</td><td>2.08 ± 0.78</td><td>5 ± 1</td><td>15 ± 3</td><td>19 ± 6</td><td>0.98 ± 0.64</td></tr><tr><td>SPL</td><td>25 ± 10</td><td>63 ± 31</td><td>62 ± 32</td><td>2.01 ± 0.36</td><td>24 ± 13</td><td>58 ± 28</td><td>66 ± 22</td><td>1.59 ± 0.41</td></tr><tr><td>US</td><td>20 ± 5</td><td>78 ± 49</td><td>62 ± 24</td><td>4.56 ± 4.28</td><td>11 ± 6</td><td>52 ± 40</td><td>62 ± 39</td><td>3.64 ± 4.45</td></tr><tr><td>VS</td><td>13 ± 1</td><td>32 ± 3</td><td>42 ± 10</td><td>1.21 ± 0.16</td><td>8 ± 3</td><td>25 ± 11</td><td>27 ± 10</td><td>0.50 ± 0.05</td></tr></tbody></table>

Figure III-33 shows the difference in the bearing area curve between bush hammered and polished stones. These graphs represent the cumulative height distribution curve from the mean profile. Figure III-33 indicates that the polished finish produced flatter surfaces with small differences in height, whereas bush hammered surfaces showed a higher variation in height. No differences were found between finishes in AT and SPL due to the large pores that interfere with the surface data.
<figure><img src="image_77.png" /></figure>

Figure III-33. Bearing ratio curve of bush hammered and polished stones. Rmr(z): Material ratio of profile. Ratio of the material length of the profile elements Ml(z) at a given level z to the evaluation length. That is the percentage of the area analysed starting from the highest point to the lowest. Positive values on the vertical axis represent peaks, and negatives are valleys.

### III.4.2.3. Experimental setup of the ageing tests

Four tests were carried out following the methodology described in the standard UNE-EN 13919:2003 (CEN, 2002), but with some modifications. For each test, the specimens were six 4 × 4 × 2 cm slabs of each stone type (three of them with polished finish and the other three with bush hammered finish). Prior to the tests, the slabs were immersed in water at atmospheric pressure for 48 hours. Each stone type was placed in a closed container on a methacrylate spot over an acid solution film, so that the surfaces were not in direct contact with the solution. The slabs were exposed to the acid environment for three weeks.

These atmospheres were obtained by adding 50 ml of acid solution to a container with 5 l capacity to create a concentration intermediate between those of the two solutions as indicated in the standard (CEN, 2002). The standard was also conceived by using H₂SO₃+H₂O to obtain a SO₂-rich atmosphere. In this study, we chose single acidic (H₂SO₃ and HNO₃) and also mixed atmospheres. The nitric acid was Panreac 65% w/w M=14.4 g·mol⁻¹, and the sulfurous acid was Panreac 6% w/w M=6 g·mol⁻¹. The mixtures were calculated proportionally by volume, as follows:
-Test 1: Acid solution: 350±10 ml HNO₃ + 150±10 ml H₂O. Liquid phase: [HNO₃] = 10.1 mol·l⁻¹. Ratio HNO₃/H₂SO₃ = 100% v/v HNO₃.

-Test 2: Acid solution: 245 ±10 ml HNO₃ + 105 ±10 ml H₂SO₃ + 150±10 ml H₂O. Liquid phase: [HNO₃] = 7.1 mol·l⁻¹, [H₂SO₃] = 1.6 10⁻¹ mol·l⁻¹. Ratio HNO₃/H₂SO₃ = 70% v/v HNO₃ + 30% v/v H₂SO₃.

-Test 3: Acid solution: 105 ±10 ml HNO₃ + 245 ±10 ml H₂SO₃ + 150±10 ml H₂O. Liquid phase: [HNO₃] = 3.0 mol·l⁻¹, [H₂SO₃] = 3.7 10⁻¹ mol·l⁻¹. Ratio HNO₃/H₂SO₃ = 30% v/v HNO₃ + 70% v/v H₂SO₃.

-Test 4: Acid solution: 350 ±10 ml H₂SO₃ + 150±10 ml H₂O. Liquid phase: [H₂SO₃] = 5.3 10⁻¹ mol·l⁻¹. Ratio HNO₃/H₂SO₃ = 100% v/v H₂SO₃.

From now on they will be referred to in the text in terms of acid volume proportion. Thus, Test 1 is referred as HNO₃ test, Test 2 as HNO₃ > H₂SO₃ test, Test 3 as HNO₃ < H₂SO₃test and Test 4 as H₂SO₃ test.

Visual changes were observed with a Leica MZ-16A stereoscopic microscope (SM) and a JEOL-6100 scanning electron microscope (SEM). RUNSALT simulations were carried out to compare with the chemical compositions of the formed deposits (Price, 2000 ; Bionda, 2005). RUNSALT is a graphical user interface to the ECOS thermodynamic model for the prediction of the behaviour of salt mixtures under changing climate conditions. Weight was measured before and after each test with a precision balance (Metler Toledo Excellence with an accuracy 0.01 g) after 48 hours of drying in an oven at 60 °C. Roughness and colour parameters that were used to characterise the materials before the test, were also measured after weight measurement, following the same procedure as for the characterisation.

## III.4.3. Results

### III.4.3.1. Macroscopic evaluation

The evolution of the samples with the tests is shown in Figure III-34. Visually, no evident changes were observed in samples tested only with nitric acid (HNO₃ test). After the HNO₃ > H₂SO₃ test, a yellowing was observed on BD and FL. SPL with the bush hammered finish showed white spots on the surface that may correspond to salt precipitation. In the HNO₃ <
$H_2SO_3$ test, AT exhibited a whitening that could be uniform as in the bush hammered sample or as white spots on the surface in the polished sample. BD with the polished finish also alters to lighter colours compared with the unaltered stone. FL changed to dark brown colours, with black areas on both surface finishes. SPL and the studied sandstones generally exhibited yellowing with a satin finish, and black spots were also observed. In this test, the black areas were better defined than in the $HNO_3 > H_2SO_3$ test (Figure III-34). The $H_2SO_3$ test yielded the highest colour variation. Yellowing was evident in all of the stones, and gave FL a dark colour. Evident white spots that corresponded to gypsum crystallisation were visible in all of the stones.

<table><thead><tr><th></th><th>AT</th><th>BD</th><th>FL</th><th>SPL</th><th>US</th><th>VS</th></tr></thead><tbody><tr><td>$HNO_3$</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>$HNO_3 > H_2SO_3$</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>$HNO_3 < H_2SO_3$</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>$H_2SO_3$</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

4 cm

<caption>Figure III-34. Changes produced in each polished stone after the tests. Black and white spots are clearly observable.</caption>

### III.4.3.2. Weight variation

Figure III-35 shows the weight variation before and after the tests. In general, weight increased slightly. The results showing a loss of weight are not significant statistically and they can be considered as negligible. Maximum weight gains were approximately 0.2% in
the HNO₃ test, 0.38% in the HNO₃ > H₂SO₃ test, 0.2% in the HNO₃ < H₂SO₃ test and 0.45% in the H₂SO₃ test. The results from the HNO₃ > H₂SO₃ test showed the most noticeable increases for all of the stones except for VS (sandstone without carbonates), in which the H₂SO₃ test produced the greatest change, which is well differentiated from the other tests. The behaviour of polished and bush hammered stones was comparable. Only the HNO₃ test showed differences between finishes for almost all of the stones, with slightly higher values in the polished slabs. In this test, SPL got negligible variations for both finishes.

<figure><img src="image_78.png" /><figcaption>Figure III-35. Weight variation in grams before and after the tests for bush hammered finish (BH) and polished finish (P).</figcaption></figure>

### III.4.3.3. Colour change

The stone colour changed depending on the stone and the test. The measured parameters did not show a progressive evolution. However, some trends could be observed. In Figure III-36 is shown the colour parameter variations for HNO₃ > H₂SO₃, HNO₃ < H₂SO₃ and H₂SO₃ tests. HNO₃ test was not represented due to the lack of visible colour variation. (Figure III-36). L* values (Figure III-36.i) indicated a decrease in this parameter that meant darkening stone. In
general, the highest change was observed in the H₂SO₃ test followed by the HNO₃ > H₂SO₃ and HNO₃ < H₂SO₃ tests. Only AT showed its highest darkening for the HNO₃ > H₂SO₃ test. Parameters a* and b* indicated a change in the tone of the stone. Parameter a* increased in a general way for all of the stones and tests, so that stone colours trended to red tones (Figure III-36.ii). AT and VS exhibited a decrease in a* for the H₂SO₃ test for both finishes. Parameter b* increased, meaning that stones changed to yellow colours, with the exceptional decrease in AT for the H₂SO₃ and HNO₃ < H₂SO₃ test with both finishes (Figure III-36.iii). Values over 3 were found in the ΔE* parameter, so the decay can be observed by the human eye. Some trends can be defined with some exceptions, as usual in the assessment of natural materials (Figure III-36. iv). In the HNO₃ test, all of the stones showed values under or close to 3, so no evident change occurred. For the three “other sedimentary stones”, the HNO₃ > H₂SO₃ test yielded higher ΔE* values than the HNO₃ < H₂SO₃ test. In the “pure calcitic stones”, the trend was the opposite, with higher values in the HNO₃ < H₂SO₃ test than in the HNO₃ > H₂SO₃ test. The H₂SO₃ test showed the greatest total colour change for all of the stones except for the travertine (AT).

<figure><img src="image_79.png" /></figure>

Figure III-36. Variations in colour parameters i) L*, ii) a* iii) b*, iv) Total colour change (ΔE*) after each test for the bush hammered finish (B) and the polished finish (P). Values over 3 are considered as visible to the human eye.
## III.4.3.4. Roughness

Figure III-37 shows the differences in roughness parameters before and after the test. Due to the lack of differences found after exposure for the $HNO_3$ test, roughness parameters were presented only for the other tests. In general, the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests showed the highest variations with changes between 100-200% for most of the parameters and stones. As a general trend, it has to be remarked that five of the six stones (AT, BD, FL, US and VS) exhibited a higher increase in the bush hammered finish than in the polished finish for all of the tests. SPL texture and porosity did not allow large differences between the polished and bush hammered finishes. This explains why the results were similar for both finishes. Fractal dimension increased in the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests (Figure III-37.i), and changes in the $H_2SO_3$ test were almost imperceptible. The bush hammered finish showed higher variation in fractal dimension than polished stones for the other tests. Only SPL had the opposite behaviour, which was a higher increase in the relief of polished stones compared with the bush hammered stones.

In general, average roughness (Ra) increased in bush hammered stones for the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests. Ra did not change in the polished samples of AT, BD and FL. In SPL and VS with the polished finish, this parameter increased for the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests. With the polished finish, US showed the highest variation in all the stones, with a decrease of more than 30 µm. Peaks and valleys (Rp and Rv) also followed a similar trend. Bush hammered stones exhibited an increase if submitted to the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests, with approximately 50-100 µm for peaks and 60-200 µm for valleys. In general, polished stones showed lower values than bush hammered samples. Only SPL exhibited higher changes in polished samples. The $H_2SO_3$ test did not cause any variation, or it was featured as a decrease in roughness.

The bearing curves of the $HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$ tests showing the highest variations were graphed (Figure III-38). The trends observed above in the single parameters were clearly identifiable.
<figure><img src="image_80.png" /></figure>

Figure III-37. Variation in roughness parameters before and after the tests for the bush hammered finish (B) and the polished finish (P): i) fractal dimension D*102; ii) average roughness (Ra); iii) peak roughness (Rp); iv) valley roughness (Rv).

<figure><img src="image_81.png" /></figure>

Figure III-38. Bearing area curve before and after the NOₓ > SO₂ and NOₓ<SO₂ tests. Positive values in the vertical axis represent peaks, and negatives are valleys. The abscissa axis indicates the percentage of the area analysed, starting from the highest point to the lowest. Only the bush hammered finish (BH) was represented due to its higher variations compared with the polished finish.
Three behaviours could be defined from these graphs. The first behaviour was that no change could be observed, and distributions of deviations around the mean plane remained similar. This was the case of AT and VS for the $HNO_3 > H_2SO_3$ test and of US for the $HNO_3 < H_2SO_3$ test. The second behaviour was a general amplification of the curve slope. This indicated a proportional augmentation of the deviations above and behind the mean line, as occurred in BD and SPL for both tests, and also in VS for the $HNO_3 > H_2SO_3$ test. The third behaviour was an increase in extreme peaks and valleys while the mean valued remained intact, showing a curve with a sinusoidal shape. This means that a percentage of the area analysed corresponded to new and deeper valleys than in the sound stone. This was the case for AT in the $HNO_3 > H_2SO_3$ test, US in the $HNO_3 < H_2SO_3$ test, and FL in both tests.

### III.4.3.5. SEM analysis and RUNSALT simulations

Examination under SEM of the 6 types of stones showed similarity according to the type of tests, but differences in relation to the composition of stones. EDX analyses were carried out on salt crystallisation. Composition was confirmed by RUNSALT simulations at 20°C and RH=30-98% (Price, 2000 ; Bionda, 2005).

In the $HNO_3$ test, the surface of the samples remained unchanged, almost without any crystallisation. Tiny white spots were observed that corresponded to salt crystallisations. However, due to the small size of the spots, EDX analysis was impossible (Vázquez *et al.*, 2010). For the other tests ($HNO_3 > H_2SO_3$ and $HNO_3 < H_2SO_3$) the amount of crystallisation increased with the proportion of $H_2SO_3$.

For the $HNO_3 > H_2SO_3$ test, RUNSALT simulations found that nitrate should crystallise in AT, BD and SPL, gypsum in FL and US, and no crystals precipitate in VS. SEM observations confirmed these calculations. Crystallisation of hemispherical-shaped nitrate occurred at the surface of the AT, BD and SPL samples (Figure III-39.a). Their sizes reached approximately 40 µm, and their density ranged from 4 (BD) to 30 (AT) crystals per mm². According to RUNSALT, the nitrates found in the dolostone (BD) could be $MgCa(NO_3)_4$, $Mg(NO_3)_2$, $Ca(NO_3)_2$, and $Ca(NO_3)_2$ appeared in higher proportion in SPL and AT. SEM observations confirmed the predominance of gypsum in FL and US. VS did not show salts on the surface.
<figure><img src="image_82.png" /><figcaption>Figure III-39. Characteristic SEM images of the surfaces of the stones subjected to a) HNO₃ < H₂SO₃ atmosphere (hemispherical-shaped nitrate crystal in AT); b) HNO₃ < H₂SO₃ atmosphere (needle-shaped crystals of gypsum, forming rosettesin AT); and H₂SO₃ test atmosphere, exhibiting c) microporous layer of gypsum crystals and needle-shaped crystals in AT, and d) cauliflower-shaped crystals, sometimes truncated in SPL.</figcaption></figure>

For the HNO₃ < H₂SO₃ test, RUNSALT simulations yielded results where no salts could crystallise in VS, and gypsum should be the only salt present in the other stones. This was confirmed by SEM observation, where only gypsum crystals were found. Short rhomboidal-shaped gypsum crystals (SPL, US, FL), sometimes with needle-shaped crystals (AT) and forming small rosettes, covered the surface and filled the pores (Figure III-39.b). Sizes ranged from 30 to 100 µm diameter. The concentration of gypsum increased with the percentage of calcite in the stones, from 0.4% in the siliceous sandstone (VS) to 18% in the micritic limestone (FL).

Compared with the other tests, the H₂SO₃ test showed the largest amount of gypsum crystals (Figure III-39.c and d). Like the HNO₃ < H₂SO₃ test, the concentration was related to the calcite concentration in the stones. On stones with the lowest calcite concentrations (VS, BD), gypsum formed rosettes approximately 50 µm in diameter and composed of needle-shaped prisms of up to a few µm in width and 20 µm in length. The rosettes were dispersed and located in the macropores (VS), or they covered the entire surface, forming a microporous layer (BD, Figure III-39.c). On the other stones, gypsum formed a thick layer of cauliflower-shaped crystals that were sometimes truncated (SPL, US, FL, Figure III-39.d).
## III.4.4. Discussion

The appearance of new types of pollutants and their effect on the stone is still a subject under research. In this study, we exposed six stones to four polluted atmospheres, produced by single acid solution with $HNO_3$ or $H_2SO_3$ and with two different $HNO_3/H_2SO_3$ mixes. The stones had different compositions, three of them being “pure calcitic stone” and the other three being “other sedimentary stones”. All of the stones were tested with two different finishes: polished and bush hammered. This study showed a high variability of the results, due mainly to the difference of material composition and textural properties as well as initial surface roughness given by the artificial finish. However, some trends have been observed.

### III.4.4.1. Influence of acid concentrations

In this subchapter, the effect of each atmosphere is discussed, taking into account all of the stone samples and making no special distinction between stone composition or finish. First, single gas atmospheres are discussed to highlight the extreme behaviours, and then, the analysis of mixed atmospheres is assessed. The hermetic recipient produced a saturated acid atmosphere for 21 days. Stones were already soaked in water, so most of the pores were filled, and a water film layer was developed on the stone surface. Acid atmospheres reacted with the stone surface, and dissolution or mineral precipitation may have taken place.

-*HNO*<sub>3</sub> test: After exposure, no visual changes were observed on the stones. Colour measurement revealed that ΔE* was lower than 3 for all types of stone, confirming that colour variation was not perceptible by the human eye. Nevertheless an increase in weight was measured. That agrees with previous researches from Haneef et al (1992) and Vázquez *et al.*, 2010. This is explained by the fact that $NO_2$ enters into the pores and attacks the stone from the interior (Sikiotis et Kirkitsos, 1995; Camaiti *et al.*, 2007). Statistical analysis revealed that when all of the samples were compared before and after this test (N=36), the weight variation was significant. This indicated that a punctual surface attack leading to salt crystallization occurred even if they were scarce under SEM. No variation in roughness was obtained in this test. These results match those of Camaiti *et al.*, (2007), who did not observe any change in colour or roughness after the exposure to nitrogen-rich atmospheres.

-*H<sub>2</sub>SO<sub>3</sub> test*: SO<sub>2</sub> exposure lead to gypsum crystallization as observed in previous studies (Haneef *et al.*, 1992; Malaga-Starzec *et al.*, 2003; Müller, 2008; Vázquez et Alonso, 2010;
Vázquez et al., 2010 ; Janvier-Badosa et al., 2014 ; Vázquez et al., 2015). In this research, stones turned to yellow as observed with the naked eye. Changes in colour were evident and also statistically significant. ΔE* showed values higher than 3 and up to almost 20. There was a general decrease in lightness (L*), and generally an important increase in yellowing (b*). The brownish-yellow travertine (AT) behaved oppositely maybe due to the initial colour and heterogeneity and the presence of gypsum as white spots that induced an increase in lightness and a decrease in the yellow parameter.

The white crystals observed on the surface of the samples were found mainly at grain boundaries and on and around iron oxides. These crystals formed a crust on the surface, hardly conditioned by the stone texture. A statistically significant increase in weight of all the samples was measured, explained by the large amount of gypsum crystallisation. The roughness showed two behaviours: no variation or a decrease. The decrease in roughness was explained by the filling of the pre-existing pores and irregularities by the gypsum crystals confirmed by SEM observations and also constated by Vázquez and Alonso (2010) and Vázquez et al., (2015).

-Mixed atmosphere tests. They produce more damage than single acidic atmospheres and revealed in general more important changes in most of the properties than in single acidic tests. Yellowing and whitening could be observed in some samples, but without a defined trend. Colour measurements indicated that, in general, L* decreased very slightly and a* increased to red, whereas the behaviour of b* varied randomly. Statistically, in this research the weight variations were significant. The pure sandstone (VS) was the only one that did not show high weight gain, because scarce chemical reactions occurred. The other stones showed the highest increase in the HNO₃ > H₂SO₃ test and yielded salt crystals with a varied composition, including calcium and magnesium nitrates and sulfates such as Ca,Mg(NO₃)₂ that may influence the highest weight gain. In contrast, only gypsum was found in the HNO₃ < H₂SO₃ test. This means that the proportion of HNO₃ and H₂SO₃ may control whether HNO₃ reacts with the stone to form nitrites and nitrates, or acts as catalyser of SO₂ reactions. Both tests yielded high and similar roughness variations on the stone surface. All parameters increased, which may implicate dissolution (Tecer, 1999) and salt precipitation in a non-homogeneous way, indicating stone weathering (Vázquez, 2013b ; Vázquez, 2015).
## III.4.4.2. Influence of composition and stone properties

Stone attack by acid atmospheres occurred on reactive surfaces such as calcite. If no calcium is present in the stone composition, a gypsum crust may be formed by a sufficient contribution of SO₂ or other pollutants and an external calcium source (Graue *et al.*, 2013). If no external calcium source is present, SO₂ reaction can be produced by ion exchange from clays or the hydrolysis of aluminium feldspars or micas in the acid atmosphere (Müller, 2008). VS, the siliceous sandstone with only 0.3% Ca, exhibited the highest gain of weight for the H₂SO₃ test. These values were two to three times higher than in the other tests. (Figure III-35). This indicates that even if HNO₃ accelerates the SO₂ - Ca interaction, the catalytic effect does not influence the mass gain. The highest specific surface area of VS (3.76 m²/g), together with an average porosity among the studied stones and the fact that no dissolution took place, explains the highest weight gain.

When a stone with calcium in its composition is in contact with a SO₂ atmosphere, a gypsum crust is formed, although crystallisation patterns and substrate reactivity depends on the texture of the stones (Malaga-Starzec *et al.*, 2003 ; Dewanckele *et al.*, 2012). To crystallise gypsum, a previous dissolution or a replacement process must take place (Tecer, 1999 ; Dewanckele *et al.*, 2012).

BD, the dolomitic limestone with only 18% calcite, is the stone with the second highest weight gain after the H₂SO₃ test. The dissolution process concentrated more in calcite than in dolomite crystals due to their different solubilities. Nitrogen compounds react preferentially with Mg, resulting in both calcium and/or magnesium compounds (Grossi *et al.*, 1995 ; Cobourn *et al.*, 1993 ; Grossi et Murray, 1999 ; Luque *et al.*, 2013), which may explain the high weight gain of BD after the HNO₃ test. According to RUNSALT, magnesium salts as Ca,Mg(NO₃)₂ could form on the stone, favouring a weight increase similar to the calcitic stones. BD showed the lowest porosity (approximately 10%) but also the highest amount of access to the smallest pore radii, with 45% of the pores below 1 µm. In addition, according to Vázquez *et al.* (2013b), BD has the lowest vapour permeability among the studied stones favouring an accumulation of salts for the HNO₃ > H₂SO₃ test. US, with half of its composition of a carbonate nature, exhibited high weight gain due to surface reaction for the HNO₃ > H₂SO₃ test, and less weight gain due to possible dissolution when SO₂ was the main reactant. FL is a calcareous stone with the highest specific surface area and almost 30%
porosity, which is coherent with the highest weight gain results for the HNO₃ > H₂SO₃ test among all of the stones, and high values in the other tests. SPL has large pores that avoid HNO₃ reaction, whereas the reaction with the HNO₃ > H₂SO₃ test lead to an increase in weight by salt crystallisation. AT has large pores that allowed the water to enter through the stone and smaller pores in which salts precipitate. Thus, the acid dissolution and the salt precipitation is more equilibrated than in other calcareous stones with less porosity or smaller sized pores. Statistically, when comparing weight variation before and after the tests of the two groups (“pure calcitic stone” and “other sedimentary stones”), the changes were always significant.

Colour changes are related to the reactive surface and also to the original colour of the stone and the colour of the deposits (Vázquez et Alonso, 2010). In this research the carbonate content did not influence the final colour change. Parameter a* increased in all of the stones. This reaction may be due to the iron oxidation in the acid atmosphere that may lead to redness of the stone (Haneef et al., 1992 ; 1993a ; Müller, 2008). Regarding the iron content of the studied stones, US (a sandstone with 45% carbonate content) showed the highest content (above 3%). However, the total colour changes were among the lowest. The lack of noticeable colour changes may be due to the fact that the distribution of iron molecules through the minerals was not optimal to induce a significative colour change. Statistically, when comparing “pure calcitic stones” and “other sedimentary stones” (2 groups, N=90) before and after the tests, none of the parameter variations were significant in the HNO₃ test. Parameters a* and b* varied significantly in “other sedimentary stones” in the HNO₃ > H₂SO₃ test, and only b* varied significantly in the H₂SO₃ test. No significant differences in roughness were observed between “pure calcitic stones” and “other sedimentary stones”. Statistically, only D variation was significant for the HNO₃ > H₂SO₃ test for pure calcitic stones. However, an increase in peaks and valleys for these three stones was observed in the bearing ratio curve.
## III.4.4.3. Influence of surface finish

Lipfert (1989) already wondered about the influence of the surface relief on the pollutants' attack. It was determined that carbonate deterioration is enhanced by increased roughness (Dolske, 1995) and that the roughness of already formed black crusts increases the development of the crust, trapping new particles (Ausset *et al.*, 1999). In our research, weight gain in relation to finish varied depending on the acid atmosphere. Thus, polished surfaces yielded higher weight variation than bush hammered surfaces for the $HNO_3$ test. The bush hammered finish may retain the particles on the surface due to the deterioration of the artificial processing. For the other tests, weight gain was similar for both finishes or high for bush hammered. Colour changes did not show any general trend in relation to finish, for any parameter. In general, roughness increased in bush hammered AT, BD, FL and US, whereas polished slabs did not exhibit any change (Figure III-40, BD). This was evident in mixed atmosphere tests more than in the single acidic atmosphere tests. In SPL and VS, no large differences in roughness were found between both finishes. SPL showed pores of millimetric size, even in the polish surface, that explain the higher attack of the acid atmospheres to this polished stone compared with the other calcareous stones (Figure III-40, SPL). In VS, Rp (maximum peak height) was the parameter with the highest variation, which indicates a predominance of crystal formation, slightly higher in bush hammered than in polished specimens. Statistically, only fractal dimension showed a significant change in the $HNO_3 > H_2SO_3$ test for both finishes and in the $HNO_3 < H_2SO_3$ test for bush hammered samples. That means an increase of relief of the stone surface but also that D is a meaningful parameter in stone weathering.
<figure><img src="image_83.png" /></figure>

Figure III-40. Surface differences between BD and SPL with both finishes. ΔRa indicated the variation in average roughness before and after the test, expressed as a percentage. Roughness changes are evident for polished surfaces only in SPL.
## III.4.5. Conclusions

Acid atmospheres may cause damage to all types of stones, even those with extremely low Ca content, as in the case of VS, the siliceous sandstone with only 0.3% Ca. The damage is lower in non-pure calcitic stones, and only scarce reaction occurs. In carbonate-containing stones, the variations are more related to pore size, porosity distribution and specific surface area than to calcium proportion.

Colour changes for mixed atmospheres and for $H_2SO_3$ attack. However, the intensity of the variation depends more on the inherent stone colour than on the composition of the stone or the atmosphere that the stone is exposed to.

The influence of finish is evident in terms of roughness, with higher variations in bush hammered samples than in polished samples. Fractal dimension (D) and bearing ratio area curve were good parameters for evaluating the surface weathering, with a general overview of the increase or decrease in roughness (D) and also of the new distribution of peaks and valleys (bearing).

In mixed atmospheres, the $HNO_3 / H_2SO_3$ ratio plays an important role in the reactions with the stone and salt products. With a low proportion of $H_2SO_3$ in the mixed atmosphere, nitrates and sulfates were found, indicating an interaction of $HNO_3$ with the stone. With a high proportion of $H_2SO_3$ in the mixed atmosphere, only gypsum precipitated on the stone surface. $HNO_3$ acted as a catalyser of the $H_2SO_3$ reaction without interacting with the stone. These results indicate the importance of the proportion of pollutants and their different effects on building stones. Future works will be focused on the deepening on the chemical reactions that took place on the surface and their reaction kinetics.

## Acknowledgements

This research was funded mainly by the project MEC (PN I+D+i) MAT2008-06799-C03-01 and the Project IFEPAR (Reims Metropole). The authors thank the reviewers for their helpful comments.
Ce chapitre est consacré à l'analyse du comportement des pierres carbonatées à l'échelle microscopique par tomographie à rayons X (HRXCT), une technique d'imagerie haute résolution et non destructive. Le but de cette étude à micro-échelle est d'observer et quantifier les changements qui se produisent à l'intérieur des roches soumises aux tests de vieillissement du chapitre 3, et de comprendre les changements induits par la pollution au niveau microstructurale.

Deux des quatre pierres étudiées, la pierre de Savonnières (Sv) naturelle et la pierre reconstituée (Rs) artificielle, ont été soumises à des tests de vieillissement accéléré similaires à ceux du chapitre antérieur. Un des tests a simulé les effets de la pluie acide et l'autre l'exposition à une atmosphère fortement concentrée en acide sulfureux et nitrique, en considérant les ratios d'émission SO₂/NOₓ actuels (1/3).

Après 1, 10 et 28 jours de test, les quatre échantillons ont été séchés dans un four à 40°C pendant 60h, pesés puis analysés en HRXCT.

En laboratoire, la résolution des scans est souvent liée à la taille de l'échantillon. Par conséquent, de petites tailles d'échantillons ont été sélectionnées afin d'avoir une haute résolution. Ceci explique l'utilisation de petits échantillons cylindriques de 4 mm de diamètre et 2 cm de hauteur, fixés sur le dessus d'une tige de carbone pour faciliter l'analyse en HRXCT. La tomographie à haute résolution a été réalisée à l'aide du scanner interne HECTOR (Masschaele et al., 2013) développé par le Ghent University Centre for X-ray tomography (www.ugct.ugent.be) en collaboration avec l'Université de Gand (UGent). Le scanner acquiert de multiples images 2D d'absorption de rayons X ou des projections de l'échantillon placé sur un plateau tournant sur 360° avec un pas de 0,15 (Figure IV-1). Les échantillons sont scannés à 120 kV, avec une puissance de 10 W et un filtre Al de 1 mm pour réduire le durcissement du faisceau. La résolution moyenne de tous les échantillons analysés était de 5 µm.

La Figure IV-1 résume les étapes de reconstruction du HRXCT. Une pile de coupes transversales 2D est assemblée à partir des 2400 projections par échantillon scanné en utilisant le logiciel Octopus Reconstruction (Vlassenbroeck et al., 2007), sous licence XRE. Les volumes reconstruits sont ensuite analysés à l'aide de Morpho +, actuellement connu sous le nom d'Octopus. Les valeurs de l'échelle des gris représentent les différences dans l'atténuation des rayons X. Des valeurs de gris élevées représentent des coefficients
d'atténuation élevés (c'est-à-dire des densités et/ou des nombres atomiques croissants, Cnudde et Boone, 2013). Dans le cadre de cette étude, les composés chimiques susceptible d'être observés peuvent être classés selon leur valeur de gris, du plus clair au plus foncé, $CaSO_4 \cdot 2H_2O > CaCO_3 > Ca(NO_3)_2$ pour les minéraux purs (c'est-à-dire les agglomérations non poreuses).

Les profils de porosité ont été calculés en utilisant le logiciel d'analyse Octopus, représentant la porosité moyenne calculée sur chaque coupe transversale. Par conséquent, un seuillage unique a été effectué, en utilisant les mêmes valeurs de segmentation pour tous les ensembles de données. La porosité a été calculée sur les 100 premières coupes transversales, c'est-à-dire sur environ 500 µm d'épaisseur à partir du sommet des échantillons.

<figure><img src="image_84.png" /><figcaption>Figure IV-1. Principales étapes de l'analyse HRXCT suivies des étapes de reconstruction des images 2D et 3D.</figcaption></figure>
## IV.1. Weathering assessment under X-Ray tomography of building stones exposed to acid atmospheres at current pollution rate

Ce travail est en cours de correction dans la revue de rang *A Construction & Building Materials* après soumission le 8 Septembre 2017.

### Résumé

Les effets de l'environnement sur les pierres du bâti entraînent leur vieillissement. En outre, l'eau et la pollution de l'air représentent deux facteurs majeurs. Le lien entre les polluants atmosphériques tels que le SO₂ et l'altération des pierres est bien connu, et depuis les dernières décennies, les émissions de ce composé ont diminué fortement, contrairement à celles des NOₓ. Afin de rendre les restaurations de monuments historiques aussi durables que possible, les connaissances sur l'altération des pierres doivent être adaptées à ce nouvel environnement. Ainsi, l'objectif de cette étude était d'évaluer l'effet des polluants SO₂ et NOₓ sur la cinétique d'altération de deux pierres de construction.

À cette fin, des tests de vieillissement en laboratoire ont été réalisés simulant d'une part l'exposition à une atmosphère acide et d'autre part l'imbibition d'une pluie acide à pH 5. Le vieillissement a été évalué par tomographie à rayons X haute résolution (HRXCT) sur deux matériaux de construction fréquemment utilisés comme matériaux de restauration dans le nord de la France et en Belgique: le calcaire oolitique de Savonnières et une pierre reconstituée artificielle composée de débris de pierres calcaires pris dans un ciment.

L'utilisation de l'imagerie non destructive par tomographie à rayons X haute résolution (HRXCT) permet d'observer à la fois les altérations de surface et au sein de la porosité de la pierre. Cette technique apparaît comme un outil puissant pour étudier de nombreuses caractéristiques qualitatives et quantitatives telles que la forme et la sphéricité de composants matériels (Cnudde et al., 2011), l'initiation des microfissures et les processus de dissolution (Dewanckele et al. (2012)), la porosité (porosité ouverte, fermée et partielle) et le diamètre équivalent des pores (Lanzón et al., 2012), la distribution de la taille des pores, la localisation des zones de cristallisation préférentielles à l'intérieur de la pierre (Dewanckele et al., 2013), les processus d'adsorption (Derluyn et al., 2013). Dans cette étude, la tomographie à rayons X a également été efficace pour évaluer la cinétique d'altération grâce à des mesures répétées à trois intervalles de temps (1, 10 et 28 jours) au cours des tests.
Les effets du dépôt acide ont été imités en exposant un échantillon de chaque pierre à une atmosphère acide mixte (H₂SO₃ = 1,7 10⁻¹ mol.l⁻¹ et HNO₃ = 4,8 10⁻¹ mol.l⁻¹). La pluie acide a été simulée en immergeant d'autres échantillons de chaque type de pierre dans une solution acide mixte à pH = 5 (HNO₃ = 4 [H₂SO₄] = 6,7 10⁻⁶ mol.l⁻¹). Ces mêmes échantillons ont été scannés avant les expérimentations et après 1, 10 et 28 jours d'essai. La tomographie à rayons X a permis de visualiser les premières étapes de l'attaque des polluants sur la microstructure des pierres. Les deux tests ont conduit à des altérations différentes, principalement la cristallisation des sels après l'exposition à l'atmosphère acide, et des pertes de matière et de l'érosion après immersion. Après seulement un jour de test, les analyses ont montré que l'exposition au mélange de gaz acides a conduit à une cristallisation des sels préférentiellement en surface pour la pierre reconstituée et par substitution de la calcite pour la pierre de Savonnières. L'épaisseur de cette croûte de sel est restée stable jusqu'à la fin de l'essai, après 28 jours. Le traitement des images reconstituées a mis en évidence une phase de dissolution précédant la phase de cristallisation. La dissolution provoquée par le test d'immersion a eu lieu dès le premier jour de test, affectant les surfaces et la porosité ouverte des deux pierres. La pierre de Savonnières s'est dissoute de manière homogène en raison de sa composition calcique presque pure, tandis que la pierre reconstituée a montré une désagrégation granulaire due à son hétérogénéité minéralogique.
# Weathering assessment under X-Ray tomography of building stones submitted to acid exposures with current pollution rate

Gibeaux Soizic<sup>a</sup>, Vázquez Patricia<sup>a</sup>, De Kock Tim<sup>b</sup>, Cnudde Veerle<sup>b</sup>, Thomachot-Schneider Céline<sup>a</sup>

Submitted to Construction & Building Materials on September 8<sup>th</sup>, 2017.

## Highlights

* HRXCT allows to observe the variation in the inner stone microstructure.

* Limestone shows uniform dissolution and artificial stone granular disaggregation.

* HRXCT highlighted the salt crystallization whitin the porous network.

* Salt crystallization on the surface and inside the oolites.

**Keywords:** atmospheric SO₂/NOₓ pollution; acid atmosphere; acid rain; building stones; artificial stone; porosity; high resolution X-ray computed tomography.

## Abstract

Interactions between the environment and the building stones lead to their alteration, besides water and air pollution play an important role. The link between air pollutants such as SO₂ and stone deteriorations are well known. Last decades the SO₂ emissions have decreased much more than those of NOₓ and consequently the SO₂/NOₓ ratio reversed until reaching a current proportion of 1/3. In this new environment, we have to adapt our knowledge on ageing and weathering of building stones, particularly to make restorations of historical monuments as sustainable as possible. The aim of this study was to assess the effect of this new ratio of SO₂ and NOₓ on the weathering kinetics of building stones. For this purpose, laboratory aging tests were performed simulating acid atmosphere exposure and acid rain runoff. Weathering was assessed by high resolution X-ray computed tomography (HRXCT) on two building materials frequently used as restoration material in cultural heritage buildings in a.o. northern France and Belgium: the oolitic limestone Savonnières and an artificial reconstituted stone made of debris of limestone mixed with a cement in variable proportion.

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

<footnote><sup>b</sup> PProGRess - UGCT, Dept. Geology, Ghent University, Gent, Belgium</footnote>
Acid deposition effects were assessed by exposing one sample of each stone to a strong mixed acid atmosphere. Acid rain was simulated by immersing one other sample of each stone type in a mixed acid solution at pH=5. Each sample was scanned before and after 1, 10 and 28 days of testing. HRXCT allowed to visualize the first steps of acid pollutant attack on the stones' microstructure. The two tests led to different alterations, mainly salt crystallization for atmospheric exposure and dissolution for immersion. After only one day of test, HRXCT scans showed that exposure to mixed acid gas led to salt crystallization preferentially on sample surfaces for both types of stone. The thickness of this salt crust remained stable until the end of the test, after 28 days. The treatment of the reconstructed scans highlighted a dissolution stage which occurred before the crystallization stage. The dissolution produced by the immersion test happened immediately during the first day on the surfaces and the open porosity for both stones, although Savonnières dissolved homogeneously due to its pure calcitic composition, while the reconstituted stone showed granular disaggregation because of its mineralogical heterogeneity.

IV.1.1. Introduction

When stones are used outdoors as construction material in buildings or monuments, acid rain and deposition of airborne pollutants are well-known as the major factors in stone alteration processes (Reddy, 1988; Lipfert, 1989; Bell, 1993; Franzoni and Sassoni, 2011). Both of these processes are directly linked to the pollutants concentration, particularly sulfur dioxide SO₂ and nitrogen oxides NOx.

Pure rain shows an inherent pH around 5.5 and is considered as naturally acid (Lipfert, 1989). Thus, rain water induces dissolution and therefore assists in the leaching of the stone surface. The resulting material loss leads to roughness variations and/or granular disintegration depending on the stone texture and composition (Reddy, 1988; Holynska et al., 2004; Eyssautier-Chuine et al., 2016). The intensity of dissolution by rainwater depends on two parameters named karst and acidity effects (Lipfert, 1989). The karst effect is due to the natural acidity of rainwater, depending on the amount of CO₂. The acidity effect is due to anthropological activities and mainly depends on the sulfuric acid H₂SO₄ and the nitric acid HNO₃ products of SO₂ and NOx oxidation respectively (Reddy, 1988; Delalieux et al., 2002). In urban environment, due to fossil fuels used in transports, industrial emissions and heating installations, sulfur dioxide SO₂ has been the main pollutant since industrial revolution and until the 1980s (Figure IV-2; Massey, 1999).
The pollutants emissions are of high importance as it will come in contact and react with the building materials. On the rain sheltered surfaces, water vapor from mists and fogs or condensation can occasionally wet the stone. Thus, the deposition of gaseous pollutants on limestone entails acids production which react with carbonate compounds (Haneef et al., 1992; Delalieux et al., 2002; Bai et al., 2003; Brimblecombe, 2014). The SO₂ also interacts directly with the moisture on the stone surface and reacts with calcite to produce gypsum which then potentially accumulates in crusts (Charola et al., 2007; Bonazza et al., 2009). On sheltered and wet surfaces, crusts thickness reach several millimeters or even centimeters making the underlying rock pulverulent (Brimblecombe, 2014).

Gypsum crusts may acquire a black color in urban environment (Felix and Maravelaki, 1992; Ausset et al., 1996; Grossi et al., 2007) and a spongy or cauliflower shape due to the agglomeration of organic compounds (Török and Rozgonyi, 2004) and atmospheric particles (soot and flyash; Rodriguez-Navarro and Sebastian, 1996; Simao et al., 2006). Thus black crusts pattern are frequently observed in urban areas because of the anthropogenic activities and relict of the last century pollution, leading to higher emissions of SO₂, NOₓ and particles (Graue et al., 2013).

The effects of SO₂ on stones has been extensively studied, from early gypsum crystallization (Ausset et al., 1996) to black crusts formation (Felix and Maravelaki, 1992), the protective effect of these crusts passing by the aesthetic appearance and the necessity or not of removing them (Grossi et al., 2007). In the black crust formation, the NOₓ present in the environment was considered acting mainly as catalyst and not producing its own reaction with calcite leading to salt crystallization.

Last decades, due to the implementation of pollutant emission limitations, the SO₂ emissions decreased highly (Figure IV-2) and entail the NOₓ emissions linked to SO₂ from industrial processes, the NO₂ also appeared during the combustion in diesel motors (Brimblecombe, 2014). The ratio between both pollutants reversed and the NOₓ concentration in the atmosphere is now four times higher than SO₂ (CITEPA, 2015).

Nitrogen oxides (NOₓ) are known mainly as catalysts for SO₂ oxidation (Bai et al., 2006; Charola, 2007), the oxidation in the gas phase being less important than in the moisture film on stone surface. Moreover the oxidation of SO₂ in the moisture film is more significant than that of NO and NO₂ (Haneef et al., 1992). Gypsum tends to remain on the stone surface during this substitution reaction (Bai et al., 2003; Charola, 2007), while nitrates are soluble
salts prone to migrate inside the stone (Camaiti *et al.*, 2007; Vázquez *et al.*, 2016). Indeed when they crystallize on the surface, Morillas *et al.* (2015) suggest that nitrates come from atmospheric aerosols and in a lesser extent from the NOₓ oxidation.

Standard ageing tests exist to simulate, control and accelerate the processes of stone decay such as carbonate dissolution and salt formation. Even if these tests do not simulate the conditions of the real environment, they reflect the weathering processes, the type and degree of weathering and they allow to compare the behaviour between different stones. In this case, with a changing pollution ratio this kind of test would predict the interaction of the new atmospheres with the stones and could be compared to the results of standard SO₂ ageing tests.

During the exposure to NO₂ or to HNO₃ in laboratory, some studies indicate nitrate salts might crystallize on the stone surface (Vázquez *et al.*, 2013b) similarly to the salts found in some natural environments (Sikiotis and Kirkitsos, 1995; Morillas *et al.*, 2015). In addition, some nitrates could be found in depth within the stone during pure NO₂ and mixed HNO₃/H₂SO₃ atmosphere exposures (Franzoni and Sassoni, 2011; Vázquez *et al.*, 2016). Vázquez *et al.* (2016) showed that nitrates were also systematically detected after the exposure to a mixed atmosphere, occurring above a mixed acid HNO₃/H₂SO₃ solution, and Haneef *et al.* (1992) revealed that alterations appear faster with mixed acids than with a single component.

If the presence of black crusts may protect the stones from new pollutant decay and limit the access to weathering agents in the porous network (Marszałek *et al.*, 2014), fresh stones and recently cleaned ones are more prone to pollution weathering because their surfaces are completely exposed to the environment, and consequently their porous system open to new decay. Currently, the treatment of carbonated stones with hydrophobic products is often not a satisfactory protection toward pollutants (Camaiti *et al.*, 2007) that highlighted the crucial need to anticipate their effects on stones.

The aim of this study is to show the effects of the current pollutant ratio (SO₂/NOₓ ratio around 1/3) on the stone's degradation paying special attention in the evolution of its microstructure (dissolution, crystallization and decay distribution).

For that purpose, two stones were tested in laboratory conditions: the Savonnières limestone, and the reconstituted stone, an artificial alternative to the natural limestones. These two building materials have been commonly used over the last years for restoration purposes in
the north of France and Belgium, mainly as substitute of other weathered Savonnières or other limestones with similar aspect. These stones were exposed to ageing tests based on the UNE-EN 13919 standard (CEN, 2002) and other researches (Eyssautier-Chuine et al., 2016; Vázquez et al., 2016) with modifications to adapt current pollutant ratios. Stone changes were assessed over time by visual observation, SEM assessment and High Resolution X-ray Computed Tomography (HRXCT). The HRXCT is a non-destructive technique that allows to visualize both the decay on the surface as well as within the stone at high resolution.

This technique had been proved to be a powerful tool to investigate a lot of various qualitative and quantitative characteristics such as: (i) shape and sphericity of material components (Cnudde et al., 2011), pore infilling, microcrack initiation, dissolution processes (Dewanckele et al., 2012; De Kock et al., 2017), (ii) porosity (open, closed and partial porosity) and the equivalent diameter of the pores (Lanzón et al., 2012), pore size distribution, (iii) location of the crystallization spots inside the stone (Dewanckele et al., 2013), the uptake process of water and salt solutions and the deposition of salts (Derluyn et al., 2013), and fluid flow (Dewanckele et al., 2013). In this study, HRXCT was also efficient to assess the weathering kinetics by repeated measurements at three time intervals (1, 10 and 28 days) during the mixed acid tests.

<figure><img src="image_85.png" /><figcaption>Figure IV-2. Atmospheric emissions of the sulfur dioxide SO₂ and the nitrogen oxides NOₓ in France from 1960 to 2013. Data from SECTEN national reports (CITEPA, 2015).</figcaption></figure>
## IV.1.2. Materials

This study investigated the behavior of two building stones particularly used in the north of France: the Savonnières limestone used during the construction and the restoration of monument such as the Cathedrals of Reims, Saint-Dizier, Beauvais and the prefecture of Nancy (Fronteau, 2000) and as replacement of the Lede stone on Belgian monuments (Dewanckele, 2013; De Kock, 2016), and a reconstituted stone, used in Belgium, during the rebuilding of the Orval Abbey in Belgium (Thomachot-Schneider *et al.*, 2011).

### IV.1.2.1. Savonnières limestone (Sv)

The Savonnières limestone (Sv) is extracted from the quarries located in the area of Savonnière-en-Perthois (North-East of France), at the Tithonian stratigraphical unit (Late Jurassic). This oolitic vacuolar limestone is beige (Figure IV-3.a) with mainly a calcitic composition (Tableau IV-1). A bladed sparitic cement surrounds the oolites and Bivalvia shells (Figure IV-3.b, c) and the cement fills partially the large and open intergranular porosity (Figure IV-3.c) (Fronteau, 2000; Eyssautier-Chuine *et al.*, 2016).

The total porosity obtained by Mercury Intrusion Porosimetry (MIP) shows particularly high values compared to other limestones, with values about 30%. The bimodality of the porous network (Figure IV-3.d) depends on the macropores ($r_p$ > 2µm) located in the oolith nucleus and in-between the cement, and the micropores in the cortex of the ooliths ($r_p$ < 0.1µm; Fronteau, 2000; Dewanckele, 2013).

### IV.1.2.2. Reconstituted stone (Rs)

The reconstituted stone (Rs) was probably crafted during the last restoration of Orval Abbey between 1932 and 1948 (Thomachot-Schneider *et al.*, 2011; 2016). This artificial stone is light coloured with beige tones (Figure IV-3.d) and particularly Si-rich (Tableau IV-1). Its heterogeneous composition includes local limestones debris and sand bonded by a Portland cement, lime and mineral pigments in variable proportions (iron oxides) (Figure IV-3.e).

Its porous network is bimodal, composed of micropores found within the calcareous lithoclasts ($r_p$ > 0.5µm) and the matrix ($r_p$ < 0.1µm) (Figure IV-3.g), displaying a total porosity of around 20%. A noticeable feature is the air bubbles caused by degassing during manufacture, (in blue in Figure IV-3.f), forming closed large macropores.
<figure><img src="image_86.png" /><figcaption>Figure IV-3. Characteristics of the studied building stones, Sv (a, b, c, d) and Rs (e, f, g, h) before the tests: photographs (a, e), binocular microscope images (b, f), blue-coloured thin sections in plane polarized light (c, g) and Incremental mercury intrusion in relation to pore size (d, h).</figcaption></figure>

<table><caption>Tableau IV-1. Chemical composition of the studied stones. Data from Eyssautier-Chuine et al. (2016).</caption><thead><tr><th>In %w/w</th><th>CaCO<sub>3</sub></th><th>SiO<sub>2</sub></th><th>MgO</th><th>Fe<sub>2</sub>O<sub>3</sub></th></tr></thead><tbody><tr><td>Sv</td><td>96.3</td><td>0.68</td><td>0.65</td><td>0.25</td></tr><tr><td>Rs</td><td>86.9</td><td>6.64</td><td>0.53</td><td>1.94</td></tr></tbody></table>

### IV.1.3. Methodology

In this research two tests were carried out to simulate on one hand the dissolution produced by acid rain, by immersing the stones in a mixed acid solution, and on the other hand the wet gaseous deposition, by exposition of the selected stones to a mixed acid atmosphere.

#### IV.1.3.1. Experimental setup

Two cylindrical samples of 4 mm diameter and 2 cm height of each stone type were drilled to be submitted to two types of ageing tests, acid immersion and acid exposure. The tests duration was 28 days and the samples were analyzed after 1, 10 and 28 days (Figure IV-4).

After drying at 40°C in an oven for 60 hours, the samples were fixed on the top of a carbon stem used as a sample holder for HRXCT scanning (Figure IV-4.a, c). Each sample and each carbon stem were weighed separately before the tests, in order to calculate the sample weight evolution over time considering the carbon stem weight constant.
### a. Acid immersion test

Dissolution processes induced by acid rain were simulated by a laboratory experiment consisting of the immersion of the samples in a weakly acid solution and to assess their evolution (Franzoni and Sassoni, 2011; Holynska et al., 2004; Eyssautier-Chuine et al., 2016). In order to lead to the possibility of nitrate precipitation and to assess the effects induced by the new ratio SO₂/NOₓ about 1:4, a mixed-acid solution with a volume of 450 ml and at pH = 5 (150 ml [H₂SO₄] = 5 10⁻⁶ mol.l⁻¹ and 300 ml [HNO₃] = 1 10⁻⁵ mol.l⁻¹) was prepared for each stone based on the research of Franzoni and Sassoni (2011) and Eyssautier-Chuine et al. (2016). One sample of each stone type was immersed individually in the still solution after fixing the carbon stem to a polystyrene plate to retain the rod so that only the whole sample dipped into the solution (Figure IV-4.a).

The pH was controlled every day. It remained constant around 5 during the whole test due to the small sample volume compared to the one of the solution.

### b. Acid atmosphere exposure

Acid deposition was simulated by acid atmosphere tests based on the standard UNE-EN 13919 (CEN, 2002) and adapted to get a mixed acid environment (Vázquez et al., 2016).

The samples were impregnated in water at atmospheric pressure to produce a wet deposition in contact with the polluted atmosphere. After saturation of the samples, one sample of each stone was put in a hermetic box, 10 cm above a film of mixed acid solution ([H₂SO₃] = 1.7 10⁻¹ mol.l⁻¹ and [HNO₃] = 4.8 10⁻¹ mol.l⁻¹), without any contact between the samples and the solution (Figure IV-4.c). The solution had a volume of 65 ml for 6 l of recipient.

Before starting the exposure, the concentration of NO₂ and SO₂ above the solution was measured by Waspmote sensors (GASES PRO BOARD; Libelium Comunicaciones Distribuidas S.L., 2017). It stabilized around 0.6 ppm for NO₂ and 0.2 ppm for SO₂, as the required proportion.
<figure><img src="image_87.png" /><figcaption>Figure IV-4. Setting up of the acid immersion (a) and atmosphere (c) tests with their schematizing and characteristics (b).</figcaption></figure>

#### IV.1.3.2. Analysis

##### a. HRXCT analysis and weight variations

After 1, 10 and 28 days of testing, the four samples were dried in an oven for 60 hours at 40°C, weighed then scanned using HRXCT.

The samples were weighed with a precision balance (Ohaus Voyager Pro Model VP410 2C with an accuracy 0.001 g). The weight differences of the samples (ΔW) were calculated comparing to the initial state of the sample and omitting the carbon stem.

In laboratory based X-ray CT the resolution of the scans is often related to the sample size. Therefore, small sample sizes were selected in order to have a high resolution. This explains the use of small cylindrical samples of 4 mm diameter and 2 cm height, drilled and fixed on the top of a carbon stem to facilitate the HRXCT scanning. High resolution X-ray computed tomography was performed using the in-house scanner HECTOR (Masschaele *et al.*, 2013) developed by the Ghent University Centre for X-ray tomography (www.ugct.ugent.be) in collaboration with the Ghent University (UGent) spin-off company XRE (www.xre.be). The scanner acquires multiple 2D X-ray absorption images or projections of the sample placed on a stage rotating 360° with a step of 0.15° (Figure IV-5). The samples were scanned at 120 kV, with a power of 10 W and 1 mm Al filter to reduce beam hardening. The average resolution of all scanned samples was 5 µm.

The Figure IV-5 summarizes the HRXCT reconstruction steps. A stack of 2D slices was reconstructed from the 2400 projections per scanned sample using the software package Octopus Reconstruction (Vlassenbroeck *et al.*, 2007), licensed by XRE. The reconstructed volumes were subsequently analyzed using Morpho+ a software package for volume analysis,
currently known as Octopus Analysis. The grey scale values represent differences in X-ray
attenuation, with higher grey values representing higher attenuation coefficients (i.e.
increasing density and/or atomic numbers; Cnudde and Boone, 2013). Therefore the grey
value, from the lighter to the darker, is CaSO₄·2H₂O > CaCO₃ > Ca(NO₃)₂ for pure minerals
(i.e. not porous agglomerations).

In the case of the acid immersion test, the examination of the stack of 2D reconstructed cross
sections or 2D slices, allowed to quantify the volume loss. Basically the calculation process
was divided in the four following stages: (i) on the data of the initial scanned sample the best
grayscale threshold of the air phases (including the open and closed porosity), represented by
the black areas on the images (Figure IV-5) was determined as well as the Volume Of Interest
(VOI); (ii) This VOI was reversed to select all the solid components before the tests; (iii) The
VOI was put on the slices of the tested samples after 1, 10 and 28 days of test; (iv) If material
loss occurred during the tests, black areas appeared on images inside the VOI and their
amount were evaluated using Octopus Analysis (Brabant *et al.*, 2011) as a percentage of
material loss.

Besides 3D image analysis, 3D visualization using the software Volume Graphics VGStudio
Max was performed as a final step (Figure IV-5) to assess the surface evolution.

Inward porosity profiles were calculated using Octopus Analysis, represented the average
porosity calculated on each slices. Therefore single thresholding was performed, using the
same segmentation values for all data set. The porosity was calculated on the 100 first slices,
i.e. on about 500 µm of thickness from the top of the samples.
<figure><img src="image_88.png" /><figcaption>Figure IV-5. Schematic overview of the experimental set-up inside the beam line followed by the typical workflow going from image reconstruction and 2D and 3D visualization.</figcaption></figure>

b. Scanning Electron Microscopy

The surfaces of each stone sample were observed and analyzed under SEM before and at the end of the experiments. The equipment 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.

c. RUNSALT

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 behaviour of salt mixtures under changing climate conditions (Price, 2000; Bionda, 2005).

IV.1.4. Results and discussion

The main changes occurred during the first 24 hours for both types of stone and for both types of test. Acid atmosphere tests led mainly to crystallization, while acid immersion induced a
dissolution of the stones. With HRXCT, the changes within the stone microstructure during the early weathering stages were observed in high spatial resolution.

#### IV.1.4.1. Acid Immersion Test

When an acid comes in contact with a limestone surface, it is neutralized via the following anionic replacement reactions (Lipfert, 1989; Webb et al., 1992; Bonazza et al., 2009):

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

$$CaCO_{3(s)} + 2HNO_{3(l)} = Ca^{2+}_{(aq)} + 2NO_{3}^{-}_{(aq)} + H_2O(l) + CO_2(g) \quad (ii)$$

Then the ions are washed away by the leaching due to rain runoff on the stone surface rich in calcite.

The acid immersion test produced a major dissolution phase on the surface and the open porosity of both stones throughout the test due to the fact that the pH was constant at values of 5 during the whole duration of the experiment (Figure IV-6). This effect was stronger during the first 24h then trending to stabilize with time. The acid rain simulation test revealed that dissolution occurred in different ways on the two building materials.

Regarding the external appearance of the sample, the 3D rendering revealed for Sv a homogeneous dissolution of the surface, due to its pure calcitic composition that affected ooids and matrix similarly. Rs exhibited a differential dissolution due to its heterogeneous composition. The carbonate fragments showed an important dissolution while silicate compounds were not affected after 28 days. In addition, the portland cement matrix was more weathered than the rock fragments, leading to granular disaggregation after 28 days of the test (Figure IV-6).
Chapitre IV : Suivi à micro-échelle

<figure><img src="image_89.png" /><figcaption>Figure IV-6. 3D rendering of the top of the Sv (above) and Rs (below) samples before and after 1, 10 and 28 days in a mixed acid solution.</figcaption></figure>

On the SEM images, the roughness increased for both stones due to the dissolution of the ooid cortex and nucleus on Sv (Figure IV-7.a, c) and the matrix on Rs (Figure IV-7.b, d).

<figure><img src="image_90.png" /><figcaption>Figure IV-7. SEM images before the acid immersion (a, b), after 28 days immersed in the mixed acid solution (c,d), and EDS analysis after 28 days of test (e,f).</figcaption></figure>
The differences observed on the sample surface were confirmed by visual analysis of the 3D volumes (Figure IV-6) as well as the 2D slices (Figure IV-8) on which the red areas indicated the material loss during the test. This figure shows that the dissolution starts on the surface affecting mainly the first micrometers, and that the core components were affected but in a weaker manner. In agreement with the mentioned observations, the surface and the open porosity of both stones were mainly affected in the first 24h (Figure IV-8.b).

For the Savonnières, all components were progressively dissolved with similar intensity, because the thickness of material loss was constant with values around 10 µm after 1 day and 100 µm after 28 days. Moreover, some ooids from the sample core appeared partially or completely dissolved. The analysis of the 2D slides revealed that not only dissolution occured during this test and in this stone. Inside of initially vacuolar ooids, some new material appeared (yellow circles in Figure IV-8). This material has a higher gray value than average grey value of the calcite present in the original Sv and indicates precipitation of material with a chemical higher density resulting in a higher average attenuation than the calcite originally present in the Sv.

The reconstituted stone undergoes heterogeneous dissolution on the surface, showing preferential dissolution of the matrix (lower gray values) followed later by the attack of calcitic clasts (higher gray values) after 28 days (Figure IV-8). The thickness of material loss reached up to 100 µm.

As the resolution of the HRXCT scans was 5 µm the ooids in Sv that could measure up to 500 µm were clearly visible on the reconstructed slices, contrarily to the micropores in Rs, which typically had dimensions of 0.1µm. Only degassing bubbles, with a diameter up to 1 mm, could be delimited in Rs.
<figure><img src="image_91.png" /></figure>

Figure IV-8. HRXCT 2D slices at around 1 mm in depth of Sv (above) and Rs (below) samples (above view) before (a, d), after one day (b, e), and after 28 days (c, f) in a mixed acid solution. Red areas represent the initial material that has disappeared, and the yellow circles highlight the crystallization inside ooids.

The porosity profiles represented on the Figure IV-9 were determined by calculating the average of porosity on each slice of 5 µm thickness starting from the top of the samples. As highlighted on the 2D slices (Figure IV-9), the surface recession started from the first day of immersion. For both type of stone, the average porosity increased from the surface top of the samples to the first 150 µm in depth. The porosity did not vary beyond this depth. The surface recession continued over the test with a material loss four times higher for Sv, while six times greater for Rs compared to the first 24h. The intensity of the dissolution decreased over time.
<figure><img src="image_92.png" /><figcaption>Figure IV-9. Sv (a) and Rs (b) partial porosity profiles calculated for each 2D slices with Octopus Analysis (Brabant et al., 2011) before and after 1, 10 and 28 days of the acid immersion.</figcaption></figure>

Dissolution was confirmed and quantified by weight monitoring. The variations in volume were calculated by HRXCT analysis and compared to weight changes in Figure IV-10. Volume decreased more than weight although the trends were similar. Results showed that the weight and volume change did not show a linear trend. The dissolution was more important during the first day of immersion and decreased until 10 days to stabilize finally and reach the equilibrium at the end of the test. After 28 days, Sv samples lost around 8.5% weight and 10% in volume while Rs samples lost 7% of their initial weight and 8.5% of its volume (Figure IV-10). This is in agreement with the differences in initial calcite content and pore size distribution.

EDS analysis showed the presence of sulfur on the surface of the samples that highlighted the crystallization of sulfur compounds (Figure IV-7.e,f). Unfortunately, the size of these new formed minerals was too small to be observed (below 1 µm).
Runsalt was used to simulate the reactions between acid and calcite of stone, and to predict the chemical compositions of the salts produced during the test in function of the relative humidity.

For the acid immersion test, the sulfate ($SO_4^{2-}$ = 7.5 $10^{-7}$ mol) and nitrate ($NO_3^-$ = 3 $10^{-6}$ mol) molar amounts were tested depending on the acid concentrations and sampled volumes as described above. $Ca^{2+}$ of stone can be considered in excess thus the concentration loaded in the software was 2.25 $10^{-6}$ mol to equilibrate the charges.

Runsalt simulations at 20°C (ambient temperature) confirmed the possible crystallization of calcium sulfate at any humidity: in the anhydrite form for a relative humidity (RH) below 65%, and as gypsum $CaSO_4$, $2H_2O$ above 65% RH. Calcium nitrate in the anhydrite form could appear below 40% and in the hydrate form $Ca(NO_3)_2$, $4H_2O$ between 40% and 52% RH.

<figure><img src="image_93.png" /><figcaption>Figure IV-10. Weight variation $\Delta W$ compared to the initial weight (plain lines) and volume variation $\Delta V$ (dotted lines) over time.</figcaption></figure>

## IV.1.4.2. Acid Atmosphere Test

Exposure to mixed-acid gas led to visible salt crystallization on the sample surfaces, for both types of building stone. These crystallizations occurred after the first day and remained stable until the end of the test.

SEM observations brought out for Sv crystals with arrow and needle shapes mostly visible inside ooids (Figure IV-11.a, b), while for Rs crystals appeared solely with needle shape
(Figure IV-11.c, d). The sulfur detected by EDS analysis on the crystallization areas together with crystal shape confirmed the nature of the crystals as gypsum (Figure IV-11.e, f).

<figure><img src="image_94.png" /><figcaption>Figure IV-11. SEM images before the acid exposure (a,b), after 28 days (c,d), and the EDS analysis after 28 days of test (e,f).</figcaption></figure>

The outer surface observed in the 3D renderings showed a crystallization layer on the surface of both stone types although with differences in crystal distribution. Sv showed a homogeneous crust with a preferentially filling inside the ooid voids in an attempt to homogenize the surface. Rs, without macropores present on the surface, showed a homogeneous crust (Figure IV-12).

Differences in the crystal distribution were observable depending on the stone type. Meanwhile, for Rs the salts grew in efflorescence. However, the dissolution phase due to the acidity effect was highlighted by a surface recession on Sv and a new porosity between crust and stone in Rs. On the 2D slices (Figure IV-13), some areas circled in yellow highlighted the presence of minerals different from calcite and with a lower chemical density, probably nitrates.
The 2D slices revealed that for Sv, in addition of the initial crust, the crystallization progressed in-depth penetration and started in the first 24h. The gypsum crystallization is the result of the substitution of carbonates by sulfates (Lipfert, 1989), following the equation (iii).

$$CaCO_{3(s)} + H_2SO_4(l) = Ca^{2+}_{(aq)} + SO_4^{2-}_{(aq)} + H_2O(l) + CO_2(g) = CaSO_4(s) + H_2O(l) + CO_2(g) \quad (iii)$$

This replacement led to a minor recession of the surface (in red in Figure IV-13) and to an in-depth crystallization that remained stable until the end of the test following the pattern described in Fronteau et al. (2010b). The acids reacted with the ooids and the cement without noteworthy distinction. The gypsum crust could be identified by its slightly higher gray value (higher X-ray attenuation) compared to the calcite, shown in Figure IV-12.

On Rs, a partial loss of the initial surface was observed (the red areas on the Figure IV-13), and a needle-shape crystallization was seen after the first day of the test. Due to the heterogeneity of Rs segmentation of salts was rather difficult in the reconstructed grayscale images (Figure IV-13.b, c).

<figure><img src="image_95.png" /><figcaption>Figure IV-12. 3D rendering of the top of the Sv (above) and Rs (below) samples before and after 1, 10 and 28 days of mixed acid exposure.</figcaption></figure>
<figure><img src="image_96.png" /><figcaption>Figure IV-13. HRXCT 2D slices at around 1 mm in depth of Sv (above) and Rs (below) samples (above view) before (a, d), after one day (b, e), and after 28 days (c, f) of acid atmosphere weathering. Red areas represent the initial material that has disappeared, and the yellow circles highlight crystallization inside ooids.</figcaption></figure>

The profiles designed on the Figure IV-14 showed that the samples were affected on the first 420 µm for Sv, and 240 µm for Rs which also displayed porosity variations on 100 µm outside the initial sample volume due to the outward salt crystallization. Deeper, the porosity values were similar to the initial state. The porosity increased over time in Sv while it decreased in Rs that explained the material loss observed on the 2D slices of Sv for which the dissolution stage appeared more aggressive. After the first day, the porosity did not evolve in Sv while it progressively decreased in Rs that confirmed the crystallization growing to the depth in Sv and towards the outside of the Rs sample.

The weight variation over time was similar for both stones with a high increase after one day of experiment and nearly stabilized during the next 28 days of exposure (Figure IV-15). The fact of having a weight gain showed that salt crystallization due to the reaction between gases and the stone was more important than dissolution due to acid corrosion in this test. This can be confirmed by the observations made with HRXCT in which the crust and pore filling
prevailed mass loss. HRXCT highlighted the determinant key role of the porous network in the behavior of stone to acid atmospheres. Both stones are bimodal, but the macroporosity of Sv allowed this limestone to be less sensitive to internal salt crystallization pressure while cracks could be generated in Rs due to tensions in its microporous network.

For the Runsalt simulations of the acid atmosphere exposure, the sulfate ($SO_4^{2-}$ = $1.3 \times 10^{-3}$ mol) and nitrate ($NO_3^-$ = $3.9 \times 10^{-3}$ mol) molar amount were chosen according to the current $SO_2/NOx$ ratio and the solution concentrations.

The amount of $Ca^{2+}$ present in the building stones was considered in excess thus the concentration loaded in the software was $3.25 \times 10^{-3}$ mol to equilibrate the charges.

At 20°C, Runsalt simulation confirmed the crystallization of gypsum $CaSO_4 \cdot 2H_2O$ at any humidity, and nitrocalcite $Ca(NO_3)_2$ below 52% RH.

The substitution of calcite by nitrates follows the same equilibrium than for the sulfates by the equation (iv).

$$CaCO_{3(s)} + 2HNO_{3(l)} = Ca^{2+}_{(aq)} + 2NO_{3}^{-}_{(aq)} + H_{2}O(l) + CO_{2(g)} = Ca(NO_{3})_{2(s)} + H_{2}O(l) + CO_{2(g)} \quad (iv)$$

Nitrates are more soluble than sulfates and crystallized more in-depth. Indeed, in-situ studies (Backbier et al., 1993) analyzed sulfates in surface and nitrates until 10 cm in-depth.
<figure><img src="image_97.png" /></figure>

Figure IV-14. Sv (a) and Rs (b) partial porosity profiles calculated for each 2D slices with Octopus Analysis (Brabant et al., 2011) before and after 1, 10 and 28 days of mixed acid atmosphere exposure.

<figure><img src="image_98.png" /></figure>

Figure IV-15. Weight variation ΔW compared to the initial weight over time.

### IV.1.5. Conclusion

In this study, two building materials commonly used for restoration in the North East of France, the Savonnières limestone and the reconstituted stone, were submitted to two different artificial weathering simulating both acid rain and acid dry deposition effects. Their pore
structure and mineralogical changes were assessed with a non-destructive and high resolution imaging technique, high resolution X-ray computed tomography HRXCT. The HRXCT allowed to perform time-lapse spatial analysis of the experiments on the microscale, showing the importance of early weathering stages in the overall process.

The two tests led to different alterations, the former caused mainly dissolution whilst the latter resulted in salt crystallization.

The acid rain simulation test produced a major dissolution phase on the surface and the open porosity of both stones. This dissolution occurred from the first day on both stones and evolved progressively with a slight decrease until 28 days of testing to lose 5% of their initial surface at the end of the test. In addition, this test revealed different behaviour to dissolution depending on the stones. This process affected both ooids and calcite cement in Savonnières due to its pure calcitic composition. The compounds inside some ooids different from the original stone composition were attributed to salt crystallization, sulfates and nitrates according to the Runsalt simulations. The reconstituted stone exhibited a differential dissolution, focused on the matrix and to a lesser extent on the carbonate grains. Silicate grains seemed less affected. Consequently, the natural limestone exposed a homogeneous material loss thickness while the artificial one exhibited a clear granular disaggregation after 28 days of testing.

Exposure to mixed acid atmosphere led mainly to salt crystallization, preferentially on sample surfaces, for both types of stone. This crystallization occurred during the first day and remained stable until the end of the test. For the pure calcitic Savonnières limestone, the crystallization occurred by substitution of the calcite of the whole surface by gypsum. Thus, the SO₂ and NOₓ reacted completely and no migration was observed. For the reconstituted stone, due to its pore structure and composition, the reactive calcitic surface was of less extent than for the Savonnières, and consequently salts grew on the surface and a migration of the pollutants through the interior of the stone became more important. In the case of the Savonnières limestone, its macroporos network allowed gases to penetrate inside the connected pores and reacted in depth. In the case of the reconstituted stone, the salts precipitated on the core, with micropores that could suffer a great pressure when salt crystallizes and indicated the possible danger of structural changes in situ. The dissolution of calcite due to the substitution reaction was highlighted by a surface recession on the Savonnières stone and a new porosity exhibited between the crust and the stone in the
reconstituted stone. Those results point out a possible physical instability of the formed crust at long-term especially for the artificial stone where salts crystallized on the surface.

In a monument, acid rain and dry deposition processes occur consecutively and so dissolution and calcite substitution by gypsum. The ratio dissolution/precipitation depends on the climatic region where the building is located (rainy, humid or dry) but also on the specific part of the building.

In sheltered areas, the rainwater do not reach and clean the precipitated salts while the exposed areas are leached and without salt crusts. In areas unsheltered from the rain, the salts crystallizing on the surface during the gaseous deposition are more soluble than calcite and will be more easily leached during the next rainwater event. Thus, the amount of material lost could be more important than those obtained in this study by considering a single process. During the 28 days of immersion test (672 hours), the surfaces of both stones recessed of about 100 µm. In Belgium and in the North of France, an average about 700 hours of continuous precipitations occurs per year so in only 10 years the stones could lost around 1 mm of their surface.

## Acknowledgments

This research was funded by Reims Metropole with the Project IFEPAR (2014–2017), in collaboration with the Ghent University expertise Centre for X-ray Tomography. Tim De Kock is a postdoctoral fellow of the Research Foundation – Flanders (FWO) and acknowledges its support.
V. DISCUSSION GENERALE
L'inversion des rapports de concentration en SO₂ et NOₓ dans l'atmosphère observée depuis une trentaine d'années risque de modifier les cinétiques des réactions chimiques avec la pierre et, par conséquent, d'atténuer ou d'aggraver l'altération des monuments. Actuellement, les rapports SO₂/NOₓ sont de l'ordre de 1/3 à l'échelle nationale. Les variations au niveau d'un monument sont plus dépendantes des sources locales, d'où la nécessité d'un monitoring fin prenant en compte les orientations et les expositions à la pollution des façades.

Les objectifs de cette étude étaient de **quantifier finement les contrastes de conditions environnementales au niveau d'un monument** en fonction de son exposition aux sources de pollution (e.g. trafic, résidentiel) et de comparer les cinétiques d'évolution de surfaces restaurées et non restaurées. En parallèle, **l'effet des ratios actuels de polluants atmosphériques a été caractérisé sur des pierres calcaires de construction** typiques des monuments rémois tels que l'église Saint André et la Chapelle Saint Joseph. Il s'agit de **trois pierres naturelles** : le calcaire de Courville, pierre d'origine dont la carrière est fermée depuis 2005 ; le calcaire de Savonnières couramment utilisé en restauration ; le calcaire du Bajocien, pierre de construction et de restauration utilisée dans l'Est de la France et en Belgique ; et **une pierre artificielle** comme alternative aux matériaux naturels.

Dans cette partie, l'influence des micro-variations environnementales d'une façade à l'autre du monument est discutée par rapport aux résultats obtenus en laboratoire. Puis, la durabilité des restaurations est estimée grâce à la comparaison de l'évolution de surfaces altérées par rapport à des surfaces récemment restaurées, par des analyses non destructives effectuées sur site et mises en rapport avec la réactivité des pierres, naturelles et artificielle, testées en laboratoire.
Les données environnementales des stations de l'association ATMO Grand Est, situées en centre-ville (données de pollution) et à l'extérieur de la ville (données climatiques), assurent la collecte de données à intervalles réguliers (une mesure par heure dans le cas de cette étude). Cependant, l'éloignement des stations par rapport au monument diagnostiqué rend le suivi environnemental peu adapté au suivi de l'altération du monument (Baedecker et al., 1992 ; Webb et al., 1992).

Ainsi, afin de garantir un suivi au plus proche du monument, deux types de réseaux de capteurs, **Smartmote** et **Waspmote**, ont été programmés et mis en place à la chapelle Saint Joseph, disposant chacun d'un ensemble de capteurs donnant sur la façade principale, côté rue exposé à un trafic intense au nord-ouest, et sur une façade en retrait, côté cour au sud-est (partie II.2). Ces réseaux ont permis la mesure, l'enregistrement puis l'envoi en temps réel des données environnementales suivantes : température, humidité relative, concentrations en oxydes d'azote ($NO_x$), dioxyde de soufre ($SO_2$) et ozone ($O_3$).

Chacun des deux réseaux de capteurs a présenté l'avantage d'être plus adapté au suivi *in situ* sur monument que d'autres systèmes de mesure plus encombrants et nécessitant une mise en place contraignante et peu respectueuses du monuments (e.g. perçage ; Dolske, 1995). Cependant, des contraintes pratiques (Smartmote) et techniques (Waspmote) sont apparues à leur installation.

La mise en place de la plateforme **Smartmote** s'est avérée compliquée par la nécessité d'une source externe d'énergie et de transfert des données sur le serveur par la base réceptrice. D'un point de vue technique, le système s'est montré fiable avec un taux de transfert des données proche de 100% et une très bonne tenue dans le temps grâce à la faible consommation d'énergie des transmetteurs (Martinez-Garrido et al., 2014 ; Varas-Muriel et al., 2014). D'un point de vue pratique, la plateforme **Waspmote** s'est montrée parfaitement adaptée au suivi *in situ* : discrétion, autonomie en énergie (panneau solaire) et en transfert de données (3G). Cependant, son incompatibilité logicielle a rendu instable son transfert de données.

Les variations saisonnières et annuelles des paramètres climatiques relevées par la station de mesure ATMO et les réseaux de capteurs au niveau de la chapelle Saint Joseph sont très proches. Néanmoins, les amplitudes journalières sont plus grandes avec les réseaux de capteurs.
**La disposition des capteurs, sur deux façades orientées différemment,** a mis en évidence que les variations quotidiennes et saisonnières des conditions environnementales peuvent être très différentes d'une façade à l'autre.

En effet, deux schémas de température et d'humidité relative caractéristiques de ce qu'il se passe respectivement côté rue et côté cour ont été établis à partir des enregistrements qui révèlent un décalage du cycle jour/nuit de la température entre la rue et la cour. Les pics maximaux sont atteints à des moments différents de la journée suivant l'orientation rue ou cour, et sont de durée et d'intensité différentes suivant la saison pour les deux façades.

La température côté cour, orienté sud-est, est maximale en fin de matinée et la température côté rue, orienté nord-ouest, est maximale dans l'après-midi. La trajectoire et l'inclinaison du soleil diffèrent en fonction de l'heure et de la saison (Figure V-1). Ainsi, les pics de température journaliers côté cour sont supérieurs de 5°C à ceux côté rue d'octobre à mars. En hiver, l'inclinaison du soleil est telle que le capteur côté rue reste à l'ombre toute la journée. A l'inverse, les maxima côté rue sont jusqu'à 4°C plus élevés que côté cour d'avril à septembre (Figure V-2). En été, le soleil est plus haut dans le ciel et les capteurs côté rue reçoivent des rayonnements solaires dès le début d'après-midi.

<figure><img src="image_99.png" /><figcaption>Figure V-1. Modèle géométrique de la trajectoire du soleil en fonction de la saison à la latitude 50°N (Aguado et Burt, 2012) illustrant la situation à Reims (49,2583°N).</figcaption></figure>
Du fait de leurs orientations respectives, les deux façades ne subissent pas forcément les mêmes **contraintes thermiques** au quotidien, mais les conséquences sont équivalentes sur l'année. La multiplication des cycles d'expansion/contraction engendre des altérations de type désagrégation granulaire à long terme (Harvey et al., 1966; Hames et al., 1987). Cependant, pour les calcaires, le coefficient de dilatation thermique n'est que de 55 µm m⁻¹ °C⁻¹ (Harvey, 1966), il faudrait donc plusieurs centaines de cycles avec des amplitudes de température supérieures à 20°C, soit plusieurs dizaines d'années à Reims, pour observer des dommages en surface.

De la même façon, bien que chaque façade soit soumise à des amplitudes de température différentes au cours d'une journée en fonction des saisons, le nombre de jours où la température descend en dessous de zéro est le même sur les deux façades. De ce fait, chaque façade subit le même nombre de cycles gel-dégel par an et les mêmes risques de dommages structuraux. D'après les tests de **résistance au gel** effectués par De Kock et al. (2015), sur des calcaires de Caen de porosité en moyenne égale à 20%, trois échantillons sur huit présentent des modifications structurelles, dont un sur lequel s'est formée une fissure de l'ordre du millimètre, après 48 cycles gel/dégel. Sachant qu'à Reims, les pierres subissent environ 35 cycles gel/dégel par an, il est imaginable que ces types d'altération (e.g. fissures) seront visibles avant 10 ans sur une grande partie des pierres de toutes les façades de l'église Saint André et de la Chapelle Saint Joseph.

Hames et al., (1987) ont montré qu'après 17 cycles d'**humidification/séchage**, un calcaire microporeux similaire au calcaire de Courville se dilate de 7 µm cm⁻¹ sans retrouver son volume initial après séchage. Et plus un calcaire possède un réseau macroporeux, moins il aura tendance à se dilater (Hames et al., 1987). A Reims, environ 60 cycles humidification/séchage ont lieu en un an. Par conséquent, les pression exercées au sein du matériau et à la jonction des blocs ne devraient être visibles qu'au bout de plusieurs dizaines d'années, notamment sous forme de désagrégation (Hames et al., 1987; Weiss et al., 2004).
<figure><img src="image_100.png" /></figure>

<figure><img src="image_101.png" /></figure>

Figure V-2. Variations journalières de la température caractéristiques de chaque mois de Mai 2015 à Septembre 2016 (a) et variations de la température et de la concentration en NO₂ les 6 et 7 Mai 2016 (b) au niveau des deux façades de la Chapelle Saint Joseph (playground = côté cour en gris et street = côté rue en noir).

Les parties du monument soumises à la pluie sont lessivées, comme constaté en particulier sur la façade côté cour de la chapelle Saint Joseph et dans de nombreuses études de cas (Baedecker et al., 1992 ; Sweevers et Van Grieken, 1992 ; Webb et al., 1992 ; Grossi et al., 1995 ; Sikiotis et Kirkitsos, 1995).

En laboratoire, après 28 jours de **simulation de pluie acide**, les tests ont révélé **une phase de dissolution majeure** en surface et dans la porosité ouverte des pierres, qui a occasionnée une récession de surface d'environ 30 µm. Les pierres contaminées ont présenté une perte de matière plus élevée que les pierres saines, qui n'est pas assimilée à une plus grande récession de surface, car cette différence vient de la dissolution des sulfates contenus dans les pierres contaminées et non de la calcite. Cette phase de dissolution était suivie d'une **cristallisation de gypse après séchage** pour le test d'immersion en solution d'acide sulfurique pur. En effet, la réaction de sulfatation avec la calcite est favorisée en conditions humides (Haneef et al., 1992 ; Grossi et al., 1995 ; Johnson et al., 1996 ; Grossi et Murray, 1999 ; Malaga-Starzec et al., 2003 ; Luque et al., 2013).

Les comportements des pierres diffèrent néanmoins face à la dissolution. Les analyses en tomographie à rayons X ont montré que la dissolution de la pierre de Savonnières était
homogène avec une épaisseur de récession constante sur toute la surface de l'échantillon. Au contraire, la pierre reconstituée présentait une dissolution différentielle et préférentielle de la matrice et, dans une moindre mesure, les composants carbonatés. Les grains de silicate ne semblaient pas affectés. Par conséquent, la pierre reconstituée présente une tendance à la désagrégation granulaire de sur le long terme, alors que le calcaire de Savonnières exposera des figures d'érosion. La perte de matière, mise en évidence par la diminution de poids des échantillons et l'augmentation du contenu en calcium de la solution s'est avérée deux fois plus élevée pour les pierres contaminées.

Dans les parties abritées de la pluie, la précipitation de sulfates est proportionnelle à la quantité de SO₂ dans l'air, comme l'ont montré les tests de vieillissement accéléré. Jusque dans les années 1980, les émissions de SO₂ étaient en moyenne 10 à 20 fois plus élevés qu'a l'heure actuelle (EC-JRC/PBL, 2011 ; CITEPA, 2015). Les observations de cristallisation de gypse et de formation de croutes noires dans les parties abritées des monuments non restaurés sont très communes (Ansted, 1860 ; Kuhlman, 1863 ; Camuffo et al., 1983 ; Brimblecombe et Grossi, 2009). Ces altérations ne seront plus aussi fréquentes à l'avenir, avec des émissions en SO₂ de plus en plus faibles. Les mesure environnementales ont révélées des ratios SO₂/NOₓ de l'ordre de 1/20 et jusqu'à 1/50 à certaines périodes de pics de pollution à Reims. Ces rapports sont plus faibles que ceux relevés au niveau national ces dernières années. Cela est lié aux variabilités locales et régionales. Les stations de mesures et les réseaux de capteurs utilisés au cours de cette étude sont placés en milieu urbain dans des zones à trafic routier dense. Les sources de SO₂ sont majoritairement industrielles et localement des augmentations de concentrations peuvent être observées à cause des émissions domestiques par le chauffage au fioul, donc préférentiellement en hiver. A l'inverse, les NOₓ sont émis principalement par l'activité automobile et l'agriculture. Ceci explique les concentrations en SO₂ plus faibles et les concentrations en NOₓ plus élevées à Reims qu'au niveau national. Par ailleurs, en moyennant les différentes sources d'émissions à l'échelle nationale (zones industrielles comprises), le ratio SO₂/NOₓ augmente et varie selon les années autour de 1/3.

La station ATMO a enregistré également des concentrations en SO₂ jusqu'à 100 fois plus faibles par rapport aux capteurs Waspmote placés au niveau de la chapelle Saint Joseph. A l'heure actuelle, si les pics de SO₂ enregistrés avec Waspmote restent ponctuels et que la tendance locale au niveau de la chapelle rejoint la tendance de fond, à savoir un taux de NOₓ
jusqu'à 50 fois plus élevé que le taux de SO₂ (données ATMO), l'encrassement devrait être plus lent qu'avant sa restauration (Grossi et al., 1995 ; Grossi et Murray, 1999).

En effet, dans les atmosphères formées à partir d'acide sulfureux pur (partie III.2 et 4), assimilées aux atmosphères fortement concentrées en SO₂ du 20ème siècle, les observations et analyses mettent en évidence la formation rapide de gypse en surface accompagnée de modifications esthétiques soulignées par les changements de couleur. L'exposition engendre le jaunissement de tous les types de pierres de façon visible à l'œil nu. La cristallisation des sulfates entraine une diminution de la taille des pores et affectent principalement les porosités de la pierre reconstituée et le calcaire du Bajocien.

Dans les atmosphères formées à partir d'un mélange d'acide nitrique et d'acide sulfureux (partie III.3 et 4), assimilées aux atmosphères actuelles, les changements sont moins importants que dans une atmosphère pure fortement concentrée, reflétant les émissions passées en SO₂. La cristallisation de sulfates est observée en surface quel que soit le rapport du mélange et elle est, en quantité, proportionnelle à la concentration en H₂SO₃. Autrement dit, moins l'air est concentré en SO₂, plus lente sera la cristallisation de gypse, et cela même si les oxydes d'azote favorisent la réaction de sulfatation. Par conséquent, de faibles concentrations en SO₂ engendrent l'encrassement des surfaces mais de façon moins rapide par rapport aux conditions du 20ème siècle. Dans le cadre de cette étude, il apparaît que la proportion de HNO₃ et H₂SO₃ et le type de pierre contrôlent l'action des NOₓ, en tant que producteur de nitrites et nitrates en réagissant avec la pierre, ou en tant que catalyseur des réactions de SO₂. Au cours des tests, la luminance des surfaces diminue et les valeurs du paramètre colorimétrique b* augmentent, ce qui signifie une tendance au noircissement et au jaunissement, ce dernier étant caractéristique de la production de sulfates. La porosité ouverte varie peu, contrairement à la distribution des tailles de pores et la réaction de substitution anionique implique une étape de dissolution de la calcite précédant la précipitation de sels qui mène à l'ouverture du rayon d'accès aux pores.

Lors de cette étude, il apparaît que les surfaces préalablement encrassées de l'église Saint André et de la chapelle Saint Joseph sont réactives, révélant en particulier une importante dispersion des valeurs de luminance et du paramètre b*. Ces observations mettent en évidence que les croutes sont encore actives et se modifient avec le temps. Néanmoins, ces changements n'ont pas réellement d'impact d'un point de vue esthétique car ils ne font
qu'accentuer légèrement le contraste entre les zones sombres et claires. En effet, l'exposition aux **atmosphères acides pures et mixtes** a principalement conduit à la **cristallisation de gypse** aussi bien pour les pierres altérées ou non. Cependant, la quantité de sulfate dans les pierres contaminées a très peu augmenté après exposition, indiquant que la teneur initiale en sulfate limite les réactions de substitution.

Il apparaît que les **parties restaurées** sont plus stables dans le temps que la partie non restaurée et que la mise en place de l'altération prend plus de temps que sa progression. Cependant, les surfaces restaurées de la chapelle Saint Joseph ont tendance à jaunir, ce qui a été attribué à la production de sulfates suite au dépôt de SO₂, même en petite quantité. A l'inverse, aucun jaunissement significatif n'a été observé pour les surfaces restaurées de l'église Saint André. En effet, l'église Saint André est en retrait de la route principale par rapport à la façade restaurée de la chapelle Saint Joseph qui donne directement dessus et qui est donc plus impactée par la pollution venant du trafic routier. De plus, la façade restaurée de la chapelle Saint Joseph reste à l'ombre une grande partie des journées en hiver à cause de son orientation, ce qui favorise la condensation de l'eau en surface et donc la réaction des polluants avec la pierre après dépôt, ce qui lui donne ainsi des conditions favorables à la cristallisation de sels lors des phases d'évaporation, notamment de sulfates, qui provoquent le jaunissement des surfaces.

En effet, ce jaunissement, associé à la présence de gypse, a également été systématiquement détecté sur les pierres saines après exposition en laboratoire aux différentes atmosphères acides. Par ailleurs, **les analyses en tomographie à rayons X ont confirmé que lorsque les nitrates précipitent, ils cristallisent en profondeur et non en surface** (Sikiotis et Kirkitsos, 1995 ; Camaiti *et al.*, 2007 ; Grossi *et al.*, 2007).

Une prolongation des suivis sur sites apparaît nécessaire afin de confirmer les résultats préliminaires obtenus au cours de cette étude. Or, dans la perspective d'un suivi à long terme sur plusieurs dizaines d'années, il est alors indispensable de considérer les évolutions dues au **changement climatique** global.

Récemment, des études se sont intéressées à ces problématiques liées aux conséquences du changement climatique sur les types d'altération prépondérants en Europe d'après le modèle climatique Hadley HadCM3, de 1860 à 2100 (Johns *et al.*, 2003).
Bonazza et al. (2009) s'est basé sur la fonction de Lipfert (1989) pour prédire le taux de récession de surface annuel des pierres carbonatées au 21ème siècle en Europe, en prenant en compte l'effet karst, les pluies acides et les dépôts secs de polluants. Actuellement, l'augmentation de la concentration atmosphérique de CO2 est le principal facteur d'augmentation de dissolution chimique par l'effet karst et, d'après les prédictions, la récession devrait augmenter avec les futures concentrations en CO2 (Figure V-3) au point de largement dominer les effets du dépôt sec et de pluie acide dans les zones urbaines et rurales.

Avec des pluies composées d'acide sulfurique et nitrique en proportions équivalentes, comme c'est le cas actuellement en Europe (Irwin et al., 2002), les pertes de matières dues à la dissolution des carbonates sont équivalentes pour les pierres naturelles altérées ou non. A l'avenir, si les émissions de SO2 venaient à diminuer davantage et que la contribution à l'acidité des pluies viendrait majoritairement des NOx dissous en HNO3, les dissolutions seraient moins rapides mais les altérations esthétiques plus intenses (changements de couleur). Ces résultats sont confirmés par Brimblecombe & Grossi (2008) qui précise également que l'éclatement dû aux actions du gel devrait être moins observé à l'avenir du fait de la hausse globale de température.

D'autre part, concernant le dépôt sec de polluants en zones abritées des précipitations, Grossi et al. (2011) a introduit le concept de climatologie des sels, c'est-à-dire de quelle façon le climat influence les processus de dissolution/cristallisation en estimant la fréquence des transitions en fonction de la climatologie locale (Figure V-3).

<figure><img src="image_102.png" /><figcaption>Figure V-3. Phases transitions annuelles effectuées à Rouen de 2001 à 2099 ont été calculées à l'aide des modèles HadCM3a2 (Johns et al., 2003) et ARPEGE (Gibelin and Déqué, 2003) pour la halite (a) et la</figcaption></figure>
thenardite-mirabilite (b) (Grossi et al., 2011). (c) Différence du taux de récession annuel entre les périodes 1961-1990 et 2070-2099 estimée d'après le scenario HadCM3 (Bonazza et al., 2009).

En zones abritées de la pluie, la pierre artificielle semble alors la moins adaptée pour le remplacement de pierres naturelles sur monument car elle apparaît sujette aux efflorescences et subefflorescences, y compris sans émission de SO₂, car elle contient des sulfates de sodium (Thomachot-Schneider, et al. 2016) qui précipitent dans le réseau poreux et en surface lors des cycles humidification/séchage. Au contraire, la pierre de Savonnières s'est avérée très stable face au dépôt de polluant même une fois altérée, et a subi peu de changements visibles en raison de sa composition calcitique pure et de sa macroporosité, la substitution de la calcite par les sulfates étant alors peu expansive et les tensions de cristallisation minimales.

D'autre part, les résultats de Grossi *et al.* (2011) montrent que les régions européennes, dont l'est de la France, qui possèdent actuellement un climat humide vont devenir plus méditerranéennes. Par conséquent, il est envisageable, qu'a l'avenir, les dépôts de polluants se feront sur des surfaces sèches, ce qui diminuera considérablement le taux de réaction entre les polluants et la pierre (Haneef *et al.*, 1992 ; Johnson *et al.*, 1996). Les surfaces des monuments seront donc moins affectées par les problématiques d'altération par les sels.

Par ailleurs, Brimblecombe et Grossi (2009) ont développé une fonction de **prédiction du noircissement** qui relie la concentration en carbone élémentaire et en particules déposées sur les surfaces au taux de noircissement, et cela appliquée à la ville de Londres (Royaume-Uni). Les premiers résultats indiquent que dans le cas d'une diminution des concentrations en polluants primaires dans les décennies à venir, les monuments seront moins affectés par les problèmes d'encrassement. Dans ce contexte, les futures restaurations de monuments à Reims devraient être de plus en plus pérennes.
# CONCLUSION GENERALE
Conclusion générale

L'objectif de ce projet de thèse était de caractériser l'effet des ratios actuels de pollution au SO₂ et NOₓ sur des pierres calcaires de construction utilisées dans les monuments historiques ou en restauration, à Reims, par une approche multi-échelle : macro-, méso- et micro-échelles.

Ce projet de thèse a été financé par Grand Reims (anciennement Reims Métropole) dans le cadre de la conservation du patrimoine culturel.

A l'échelle du monument, deux réseaux de capteurs environnementaux ont été élaborés et mis en place à la chapelle Saint Joseph : Waspmote (Libelium) avec un nouveau système de transfert de données à distance par 3G et Smartmote, une plateforme qui fonctionne à partir d'un double transfert de données : à distance par Wi-Fi vers une base réceptrice, puis filaire vers le serveur (Martínez-Garrido et Fort 2014, 2016). Ces plateformes ont l'avantage de mesurer les variations climatiques (T, RH par Smartmote) et la pollution (SO₂, NOₓ, O₃ par Waspmote), et de transmettre ces données en temps réel sur un serveur accessible à distance.

L'utilisation de l'une ou l'autre des plates-formes sans fil doit être définie en fonction du besoin spécifique de l'étude en suivant les avantages et les inconvénients de chacun. Les contraintes pratiques rencontrées avec le système **Smartmote** (source d'énergie et connexion internet nécessaires au fonctionnement) le rendent peu adaptables. Néanmoins, une fois ces obstacles surmontés, sa consommation en énergie basse et son taux de transfert excellent en font un réseau de capteurs très fiable. Des problèmes d'instabilité du logiciel du système **Waspmote** (module 3G trop récent) ont été rencontrés. Les problèmes techniques sont en grande partie résolus et cette plateforme multiparamétrique, autonome (en énergie et en connectivité) et discrète la rendent parfaitement adaptée au suivi environnemental sur site culturel.

Les résultats obtenus avec ces deux systèmes ont révélés des variations climatiques et des concentrations en polluants différentes suivant la localisation des capteurs, et donc en fonction de l'orientation des façades.

Les analyses non destructives des surfaces des façades de la Chapelle Saint Joseph ont confirmé des différences en fonction de l'orientation des façades. La façade soumise à des concentrations en SO₂ et NOₓ plus élevées et à des variations d'humidité relative et de
température plus faible, a tendance à jaunir davantage, résultat de la réaction des polluants avec la pierre par formation de gypse.

Par ailleurs, les surfaces de la chapelle Saint Joseph et de l'église Saint André ont également évoluée en fonction de leur état initial (altérée ou restaurée), les surfaces altérées ayant tendance à noircir plus rapidement que les surfaces restaurées.

Ces résultats sont à prendre avec précaution car le délai de 2 ans entre la première et la dernière campagne de mesure est insuffisant pour confirmer ces tendances.

Par ailleurs, la station **ATMO** a enregistré des données de fond sur plusieurs années et a permis de déduire les niveaux de pollution et les ratios SO₂/NOₓ au niveau du centre de Reims. Cependant, ces données s'avèrent trop imprécises pour être adaptée au suivi d'un monument.

Le **vieillissement artificiel** et accéléré des matériaux en laboratoire permet de mieux comprendre les processus impliqués dans l'altération et la part de chaque facteur dans ces processus. Des tests de vieillissement accéléré en laboratoire simulant la pluie acide (tests en immersion) et le dépôt de polluants (tests en atmosphère) dans des milieux purs (H₂SO₃ en atmosphère, H₂SO₄ en immersion et HNO₃) et des mélanges (H₂SO₃+HNO₃ en atmosphère, H₂SO₄ + HNO₃ en immersion) ont été effectués dans ce but.

En **immersion** dans la solution H₂SO₄ pure, les porosités de chaque pierre sont fortement affectées par la dissolution, ce qui se manifeste par une ouverture du réseau poreux et une augmentation de la taille des pores. Cette dissolution est suivie de la cristallisation de gypse après séchage. La dissolution induite par l'immersion dans HNO₃ pure est moins intense que dans H₂SO₄ et seuls des changements de couleur sont observés après séchage. Le **mélange** de solution acides mène à une dissolution moins forte mais plus rapide que dans les solutions pures. De plus, aucune cristallisation n'a été observée.

L'exposition à des **atmosphères** composées à partir de H₂SO₃ pur a engendré la cristallisation de gypse et des changements de couleur (jaunissement et noircissement) pour tous les échantillons. La production de sulfates s'est révélée proportionnelle à la concentration initiale en [H₂SO₃]. L'exposition à HNO₃ pur a entraîné des changements de couleur, et la
Conclusion générale

cristallisation de nitrates a été observée. Les **mélanges** d'acides ont provoqués des altérations similaires mais plus intenses qu'avec H₂SO₃ pur.

Quatre types de pierre ont été testés permettant la comparaison de l'influence de leurs **propriétés intrinsèques** sur l'altération : une pierre artificielle (pierre reconstituée) et trois pierres calcaires naturelles dont une pierre de construction locale (Courville), une pierre de construction utilisée à l'est et en Belgique (Bajocien) et une pierre de remplacement (Savonnières). La pierre reconstituée est d'origine riche en sulfate de sodium et la pierre de Savonnières, déposée lors de la restauration de l'église Saint André, a été contaminée par du gypse sur site.

Une **porosité** totale élevée (Savonnières et Bajocien) favorise la dissolution des carbonates et la cristallisation en profondeur des sels. La distribution des tailles de pores et la connectivité jouent également un rôle clé dans l'altération. La bimodalité de la **distribution de taille des pores** (Bajocien, Savonnières et pierre reconstituée) tend à se compenser au cours des tests. Les macropores rétrécissent et passe dans la gamme de taille inférieure lors de la cristallisation de sels alors que les pores plus petits s'agrandissent lors de la dissolution. Les variations dans la distribution entière du réseau de pores dépendent des différentes tailles d'accès aux pores de chaque type de pierre. D'autre part, une bonne **connectivité** permet à la solution de pénétrer dans le réseau poreux et au taux de dissolution d'augmenter. Dans le cas d'une faible connectivité, comme c'est le cas pour la pierre reconstituée, la dissolution a lieu principalement en surface.

Concernant la comparaison entre le comportement des **pierres naturelles et artificielle**, la pierre reconstituée semble plus résistante à la dissolution que les pierres naturelles et, au contraire, la cristallisation de sel pourrait engendrer des dommages structurels plus préjudiciables sur la pierre reconstituée à cause de l'hétérogénéité de sa composition couplée à sa microporosité et à son enrichissement en sulfate.

En atmosphère, la formation de gypse est moins importante dans les **pierres altérées** (Savonnières et pierre reconstituée) que dans les **pierres fraiches** (Courville et Bajocien). En immersion, aucune tendance générale ne distingue le comportement des pierres du point de vue de leur concentration initiale en sulfate.
Les analyses microstructurales, effectuées sur la pierre de Savonnières et la pierre reconstituée soumises à H₂SO₃+HNO₃ en atmosphère et H₂SO₄+HNO₃ en immersion, confirment que la porosité et la composition chimique propres à chaque pierre s'avèrent des paramètres déterminants dans l'évolution de l'altération. Ainsi, la pierre de Savonnières, macroporeuse et calcitique pure, se dissout de manière homogène en immersion et le gypse cristallise par substitution de la calcite et la remplace progressivement de la surface vers le centre de l'échantillon. A l'inverse, la pierre reconstituée, microporeuse et de composition hétérogène, voit ses éléments et liants carbonatés se dissoudre préférentiellement en immersion menant à une forme de désagrégation granulaire. En atmosphère, le gypse cristallise en surface dans un premier temps et sa pénétration dans le réseau microporeux pourrait entrainer sa fissuration à long terme par la pression de cristallisation.

Suite à ces analyses, de nouvelles problématiques émergent :

* Au cours de cette étude, les interactions chimiques entre polluants et pierres ont été mises en évidence par dépôt simple. Quelle est alors l'influence des processus physiques, tels que la force du vent, sur les processus de dépôt, en particulier sur l'ordre des réactions mises en jeu (catalyse, dissolution et formation de sels) ?

* Dans quelle mesure les taux d'altération actuels sont-ils influencés par les niveaux de pollution du passé (effet de mémoire) ? Par exemple, même si les sources de pollution à l'origine des altérations passées étaient définitivement stoppées, les sulfates et les nitrates déjà présents dans la pierre continueront à causer des dommages en fonction des conditions climatiques et de la nouvelle composition atmosphérique à venir.

* Par ailleurs, d'après les résultats obtenus, la cristallisation de gypse devrait être de moins en moins observée *in situ* si les émissions de SO₂ continuent de diminuer. Cependant, il est reconnu dans la littérature que d'autres espèces chimiques prennent part aux altérations de la pierre (e.g. particules fines, COVs). Quelles seront alors les altérations résultantes de l'action prédominante de ce type de polluant ?

* La compatibilité des matériaux utilisés en construction joue également un rôle dans leur altération. Dans quelles mesures les nouvelles proportions de polluants pourraient affecter ces interactions entre matériaux de construction et de restauration ? Autrement
dit, les problèmes d'incompatibilité entre les matériaux seront-ils aggravés ou au contraire atténués suivant les nouvelles conditions environnementales ?

* Grâce à un suivi couplant analyses de surface et mesures environnementales à l'échelle du monument sur le long terme, les données obtenues, mises en parallèle aux résultats des tests, permettraient l'élaboration d'un modèle complexe de prédiction de l'altération. Plus un modèle comprend de variables, plus il s'adapte à des situations différentes.
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Yadav, S.K., Chakrapani, G.J., Gupta, M.K., **2008**. An experimental study of dissolution kinetics of Calcite, Dolomite, Leucogranite and Gneiss in buffered solutions at temperature 25 and 5°C. Environmental Geology 53, 1683-1694. doi:10.1007/s00254-007-0775-x

Yi, W., Lo, K., Mak, T., Leung, K., Leung, Y., Meng, M., **2015**. A Survey of Wireless Sensor Network Based Air Pollution Monitoring Systems. Sensors 15, 31392-31427. doi:10.3390/s151229859
Z

Zendri, E., Biscontin, G., Bakolas, A., Finotto, G., **1996**. Simulated study on the chemical and physical decay of the acid rain on carbonate stone, in: 8th International Congress on Deterioration and Conservation of Stone, Berlin, 30 Sept.-4 Oct. 1996.
# ANNEXES

<table><tr><td>Annexe 1 Caractéristiques des capteurs et appareils de mesure des polluants</td><td>295</td></tr><tr><td>Annexe 2 Code de programmation des modules Waspmote</td><td>305</td></tr><tr><td>Annexe 3 Plans des campagnes de mesures à l'église Saint André et à la chapelle Saint Joseph</td><td>313</td></tr></table>
Annexe 1
Caractéristiques des capteurs et appareils de mesure des polluants

<table><tr><td>1. Analyseur NOx APNA 370</td><td>297</td></tr><tr><td>2. Analyseur SO2 AF22M</td><td>298</td></tr><tr><td>3. Capteurs Waspmote</td><td>299</td></tr></table>
# 1. Analyseur NOₓ APNA 370

<figure><img src="image_103.png" /></figure>

## Ambient NOₓ Monitor
APNA-370

<figure><img src="image_104.png" /></figure>

According to EN14211 and VDI 4202/4203

TUEV Bericht 936/21204643C 07. Jul. 2006 U. S. EPA REFERENCE Equivalent Number RFNA-0506-157

## Features

The APNA-370 uses a combination of the dual cross flow modulation type chemiluminescence principle and the referential calculation method.

This gives it the advantages of the single-detector method plus the ability to do continuous measurements of NOₓ, NO, and NO₂. The design gives great stability and extremely high sensitivity (0.1 ppm F.S.)

Standard equipment includes a drier unit with an automatic recycle function to provide dry ambient air as the ozone source. This makes long-term continuous measurements possible.

The detector uses a silicon photodiode sensor to reduce size and prolong working life.

All the necessary features are built right into a single rack-sized unit, including a reference-gas generator, an ozone-source drier unit, an ozone decomposer, and a sampling pump. No supplemental gas is required.

## Principle

Cross flow modulation type, reduced pressure chemiluminescence (CLD)

The chemiluminescence method uses the reaction of NO with O₂

$$NO+O_2 \rightarrow NO_2+O_2$$

$$NO_2+NO \rightarrow NO_2+H_2$$

A portion of the NOₓ generated as the result of this reaction becomes NO₂. As these excited molecules return to the ground state, chemiluminescence is generated in the range of 600 nm to 3,000 nm. The light intensity is in proportion to the concentration of NO molecules and by measuring it we obtain the NO concentration of the sample. A deoxidation converter changes the NO to NO, which is measured. In other words, the NOₓ concentration can be obtained by the difference between (1) the NOₓ concentration measured when the sample gas is directed through a converter and (2) the NO concentration measured when the gas is not run through the converter.

## Specifications

Principle: Cross flow modulation type, reduced pressure chemiluminescence (CLD)

Application: NO, NO and NOₓ in ambient air

Range: Standard ranges: 0-0.1/0.2/0.5/1.0 ppm; auto range ~ manual range selectable; can be operated by remote switching.

Optional (measurable) ranges: 4 ranges selectable from 0-10 ppm, within 10 times range ratio; auto range ~ manual range selectable; can be operated by remote switching.

Lower detectable limit: 0.5 ppb(3 sigma)

Repeatability: ±1.0% of F.S.

Linearity: ±1.0% of F.S.

Zero drift: <LDL/day, at lowest range ±1.0 ppb/week at lowest range

Span drift: <LDL/day at lowest range ±1.5 % of F.S./week

Response time (Tref): Within 90 sec at lowest range

Sample gas flow rate: Approx. 0.8L/min

Indication: Measured value, range, alarm, maintenance screen

Alarms: During AIC, zero calibration error, span calibration error, temperature error in converter, etc.

On-screen messages are available in four languages: English, German, French, and Japanese.

Input/output: • 0-1 V/0-10 V/4-20 mA, to be specified (2 systems: either (1) momentary value and integrated or (2) moving average value) • Contact input/output • RS-232C (option)

Ambient temperature: 5-40 °C

Power: 100/110/115/120/220/230/240 VAC, 50/60 Hz (to be specified)

Dimensions: 430(W)×550(D)×221(H) mm

Mass: Approx. 21 kg.

<figure><img src="image_105.png" /></figure>
## 2. Analyseur SO₂ AF22M

$$SO_2 - H_2S$$

### UV Fluorescent Sulfur Dioxide Analyzer - model AF22M

**Specifications :**

* Ranges: 0-0.1/0.2/0.5/1/2/5/10 ppm or user selectable ranges

* Autoranging between two-user specified ranges

* Noise (σ): 0.5 ppb

* Lower detectable limit (2σ): 1 ppb

* Response time: automatic and program mable (minimum 10 sec)

* Zero drift: less than 0.5 ppb / 24 h less than 1 ppb / 7 days

* Span drift: less than 0.5 % / 24 h less than 1 % / 7 days

* Linearity: ± 1 % of F.S.

* Pressure and temperature compensation

* Internal sample pump

* Sample flow rate: 0.3 lpm

* Averaging time: programmable from 1min to 24h

* Data storage: 2 months quarterly average values

* Chassis: 19" rack mountable, 3U

* Dimensions: 545 mm x 483 mm x 133 mm (L x W x H)

* Weight: 10 kg (22 lbs)

* Power: 115 V, 60 Hz - 230 V, 50 Hz

* Power consumption: 60 VA

* Operating temperature: -5 to 40 °C (typical as per US EPA 10-35° C)

* Digital output: 2 RS 232 or RS 422 ports

* PVDF sample filter holder

* Valves block for selection of customer - supplied zero and span gas

**Options :**

* ESTEL electronic board (1 or 2) with :

- 4 independent analog inputs

- 4 independent analog outputs

- 4 remote control inputs

- 6 dry contacts

* Built-in permeation bench with SO₂ tube and Filter - Valves block

* Memory extension (1 year of 1/4 h average values)

* 24 V DC power supply for on-board applications

* Built-in H₂S → SO₂ converter for H₂S low level monitoring

- Ranges: 0-0.05/0.1/0.2/0.5/1 ppm or user selectable ranges

- Cycle time: SO₂/H₂S: 7 min

- SO₂ scrubber capacity: 500 ppm/h

- 3 selectable operating modes:

- continuous H₂S

- continuous SO₂

- cyclic SO₂/H₂S

**Main Features:**

The new model AF22M, sulfur dioxide analyzer, combines years of experience of a wide range of analyzers with an enhanced electronics package and a modular component parts design (measurement module, permeation module, 24V power supply module, analog input-output module, etc.).

The outcome is an ultra compact and light-rack 3U, easy-to-use analyzer capable of measuring sulfur dioxide at ppb levels. Applied to SO₂ measurement, the universally known UV fluorescent principle consists in detecting the characteristic fluorescence radiation emitted by SO₂ molecules. In the presence of a specific wavelength of UV light (214 nm) the SO₂ molecules reach a temporary excited electronic state. The subsequent relaxation produces a fluorescence radiation which is measured by a non-cooled photomultiplier tube (PM).

<figure><img src="image_106.png" /></figure>

The analyzer was developed to meet customers' requirement for reduced and easier maintenance. Equipped with an enhanced aromatic hydrocarbon scrubbing system that guarantees complete removal of these interferents, the AF22M also achieves very high sensitivity and stability through the use of an optical shutter to compensate for PM drift.

<figure><img src="image_107.png" /></figure>

The AF22M combines a powerful easy-to-use interface with quality components and design technology. Real-time calibration graphs can be displayed during span check operation. Multi-tasking software, combined with the LCD graphic display, gives a user-friendly access to the instrument set-up, as well as the status and maintenance parameters. Real-time synoptic, auto-diagnostic and maintenance data screens can be displayed while the instrument is operating. The new

electronics allow enhanced data storage of more than two months of 15 minutes averages, and total remote troubleshooting diagnostic capabilities via modem, using the analyzers complete display and functions emulation.

Equipped with the optional ESTEL I/O analog & digital board, the AF22M can be easily interfaced with other equipment and can be operated as a stand alone unit able to store several months of data.

<figure><img src="image_108.png" /></figure>
3. Capteurs Waspmote

Waspnote
Datasheet

<figure><img src="image_109.png" /></figure>

<figure><img src="image_110.png" /></figure>

<figure><img src="image_111.png" /></figure>

<figure><img src="image_112.png" /><figcaption>Figure: Humidity sensor operation diagram</figcaption></figure>

Pressure sensor

Measurement range: 30 ~ 150 hPa

Operation temperature range: -40 ~ +85°C

Full a course of temperature range: 0 ~ +65°C

Absolute a course: ±0.1 KPa (0 ~ -65°C)

Typical consumer: 2.8µA measuring

Maximum consumption: 42 µA measuring

<figure><img src="image_113.png" /></figure>

<figure><img src="image_114.png" /></figure>

5-81301S

5.2. Temperature, Humidity and Pressure sensor

The BME20 is a digital temperature, humidity and pressure sensor developed by Bosch
Sensor & Motion.

5.2.1. Specifications

Electrical characteristics:

Supply voltage: 3.3V

Shop current typical: 0.1 µA

Shop current maximum: 0.3 µA

Temperature/relative humidity:

Operating range: -40 ~ +85°C

Full accuracy range: 0 ~ +65°C

Accuracy: ±1 °C (range 0 ~ +65°C)

Response time: 1.65 seconds (68% response from +30 to +125 °C)

Typical consumption: 1 µA measuring

Humidity sensor

Measurement range: 0 ~ 100% of Relative Humidity (for temperature <0°C and >60°C, see figure below)

Accuracy: <0.3% RH (at 25°C, range 20 ~ 60%RH)

Measurement: ±1%RH

Operating temperature: -40 ~ +85°C

Response time (68% of max 90% to 0% or 0% to 90%): 1 second

Typical consumer: 1.8µA measuring

Maximum consumption: 2.8 µA measuring

<figure><img src="image_115.png" /></figure>

<figure><img src="image_116.png" /><figcaption>Figure: Image of the temperature, humidity and pressure sensor</figcaption></figure>
## 5.6. Ozone ($O_3$) Gas Sensor [Calibrated]

### 5.6.1. Specifications

Gas: $O_2$
Sensor: O3-A4

Performance Characteristics

Nominal Range: 0 to 5 ppm
Maximum Overload: 10 ppm
Long Term sensitivity Drift: -20 to -35 % change/year
Response Time (T90): ≤ 15 seconds
Sensitivity: -200 to -400 nA/pmm
Accuracy: as good as ±0.005 ppm* (ideal conditions)

<figure><img src="image_117.png" /></figure>

<figure><img src="image_118.png" /><figcaption>Figure: Image of the Ozone Sensor mounted on its APE module</figcaption></figure>

Operation Conditions

Temperature Range: -20°C to 50°C
Operating Humidity: 15 to 90%RH non-condensing
Pressure Range: 80 to 120 kPa

Storage Temperature: 0°C to 20°C
Expected Operating Life: > 18 months in air

Sockets for Wasp/moke OEM:

* SOCKET_3
* SOCKET_4

Sockets for Plug & Sense:

* SOCKET_A
* SOCKET_B
* SOCKET_C
* SOCKET_F

Average consumption: less than 1 mA

### 5.6.2. Cross-sensitivity data

<table><thead><tr><th>Gas</th><th>Formula</th><th>Concentration (ppm)</th><th>% measured gas</th></tr></thead><tbody><tr><td>Hydrogen Sulfide</td><td>H<sub>2</sub>S</td><td>5</td><td>&lt; -70</td></tr><tr><td>Nitric Dioxide</td><td>NO<sub>2</sub></td><td>5</td><td>70 to 120</td></tr><tr><td>Chlorine</td><td>Cl<sub>2</sub></td><td>5</td><td>&lt; 30</td></tr><tr><td>Nitric Oxide</td><td>NO</td><td>5</td><td>&lt; -5</td></tr><tr><td>Sulfur Dioxide</td><td>SO<sub>2</sub></td><td>5</td><td>&lt; -4</td></tr><tr><td>Carbon Monoxide</td><td>CO</td><td>5</td><td>&lt; 0.1</td></tr><tr><td>Hydrogen</td><td>H<sub>2</sub></td><td>100</td><td>&lt; -2</td></tr><tr><td>Ethylene</td><td>C<sub>2</sub>H<sub>4</sub></td><td>100</td><td>&lt; 0.1</td></tr><tr><td>An ammonia</td><td>NH<sub>3</sub></td><td>20</td><td>&lt; 1</td></tr><tr><td>Carbon Dioxide</td><td>CO<sub>2</sub></td><td>50000</td><td>&lt; 0.1</td></tr></tbody></table>

Table: Cross-sensitivity data for the $O_2$ Sensor

You can find a complete example code for reading the $O_2$ Sensor in the following link:
https://www.libelium.com/development/wasp/moke/examples/gp-04-03

<footnote>*Accuracy values are only given for the **optimum case**. See the "Calibration" chapter for more details.</footnote>
## 5.7. Nitric Oxide (NO) Gas Sensor [Calibrated]

### 5.7.1. Specifications

**Gas:** NO
**Sensor:** 4-NO-250

**Performance Characteristics**

**Nominal Range:** 0 to 250 ppm

**Maximum Overload:** 1000 ppm

**Long Term Output Drift:** < 2% signal/month

**Response Time (T90):** ≤ 30 seconds

**Sensitivity:** 400 ± 80 µA/PPM

**Accuracy:** as good as ±0.5 ppm# (ideal conditions)

**Operation Conditions**

**Temperature Range:** -20°C to 50°C

**Operating Humidity:** 15 to 90%RH non-condensing

**Pressure Range:** 90 to 110 kPa

**Storage Temperature:** 0°C to 20°C

**Expected Operating Life:** 2 years in air

**Sockets for Wasp mote OEM:**

* SOCKET_1

* SOCKET_2

* SOCKET_3

* SOCKET_4

* SOCKET_5

* SOCKET_6

**Sockets for Plug & Sense:**

* SOCKET_A

* SOCKET_B

* SOCKET_C

* SOCKET_F

**Average consumption:** less than 1 mA

<figure><img src="image_119.png" /></figure>

<figure><img src="image_120.png" /><figcaption>Figure: Image of the Nitric Oxide Sensor module on its AFE module</figcaption></figure>

### 5.7.2. Cross-sensitivity data

<table><thead><tr><td>Gas</td><td>Formula</td><td>Concentration (ppm)</td><td>Output Signal (ppm NO equivalent)</td></tr></thead><tbody><tr><td>Carbon Monoxide</td><td>CO</td><td>300</td><td>0</td></tr><tr><td>Sulfur Dioxide</td><td>SO<sub>2</sub></td><td>5</td><td>0</td></tr><tr><td>Nitric Dioxide</td><td>NO<sub>2</sub></td><td>5</td><td>1.5</td></tr><tr><td>Hydrogen Sulfide</td><td>H<sub>2</sub>S</td><td>15</td><td>-1.5</td></tr></tbody></table>

Table: Cross-sensitivity data for the NO Sensor

You can find a complete example code for reading the NO Sensor in the following link:
http://www.libellum.com/development/wasp mote/ examples/gp-05-no

<footnote>* Accuracy values are only given for the optimum case. See the "Calibration" chapter for more detail.</footnote>
## 5.8. Nitric Dioxide (NO₂) Gas Sensor [Calibrated]

### 5.8.1. Specifications

Gas: NO₂
Sensor: 4-NO2-20

Performance Characteristics

Nominal Range: 0 to 20 ppm

Maximum Overload: 250 ppm

Long Term Output Drift: < 2% signal/month

Response Time (T90): ≤ 30 seconds

Sensitivity: 600 ± 150 nA/ppm

Accuracy as good as ±0.1 ppm* (ideal conditions)

Operation Conditions

Temperature Range: -20°C to 50°C

Operating Humidity: 15 to 90RH non-condensing

Pressure Range: 98 to 110kPa

Storage Temperature: 0°C to 20°C

Expected Operating Life: 2 years in air

Sockets for Waspmote OEM:

* SOCKET_1

* SOCKET_2

* SOCKET_3

* SOCKET_4

* SOCKET_5

* SOCKET_6

Sockets for Plug & Sense:

* SOCKET_A

* SOCKET_B

* SOCKET_C

* SOCKET_F

Average consumption: less than 1 mA

<figure><img src="image_121.png" /></figure>

<figure><img src="image_122.png" /></figure>

Figure 1: Image of the Nitric Dioxide Sensor mounted on its APE module

### 5.8.2. Cross-sensitivity data

<table><thead><tr><td>Gas</td><td>Formula</td><td>Concentration (ppm)</td><td>Output Signal (ppm NO₂ equivalent)</td></tr></thead><tbody><tr><td>Carbon Monoxide</td><td>CO</td><td>300</td><td>0</td></tr><tr><td>Hydrogen Sulfide</td><td>H₂S</td><td>15</td><td>-1.2</td></tr><tr><td>Sulfur Dioxide</td><td>SO₂</td><td>5</td><td>-5</td></tr><tr><td>Nitric Oxide</td><td>NO</td><td>35</td><td>0</td></tr><tr><td>Chlorine</td><td>Cl₂</td><td>1</td><td>-1</td></tr></tbody></table>

Table: Cross-sensitivity data for the NO₂ Sensor

You can find a complete example code for reading the NO₂ Sensor in the following link: http://www.libellum.com/development/waspmote/samples/gp-06-n02

<footnote>*Accuracy values are only given for the **optimum case**. See the "Calibration" chapter for more details.</footnote>
## 5.9. Sulfur Dioxide ($SO_2$) Gas Sensor [Calibrated]

### 5.9.1. Specifications

Gas: $SO_2$
Sensor: 4-SO2-20

**Performance Characteristics**

Nominal Range: 0 to 20 ppm

Maximum Overload: 150 ppm

Long Term Output Drift: < 2% signal/month

Response Time (T90): ≤ 45 seconds

Sensitivity: 500 ± 150 nA/ppm
Accuracy: good as ±0.1 ppm² (ideal conditions)

**Operation Conditions**

Temperature Range: -20°C to 50°C

Operating Humidity: 15 to 90RH non-condensing

Pressure Range: 98 to 110kPa

Storage Temperature: 0°C to 20°C
Expected Operating Life: 2 years in air

**Sockets for Wasp/mote OEM:**

* SOCKET_1

* SOCKET_2

* SOCKET_3

* SOCKET_4

* SOCKET_5

* SOCKET_6

**Sockets for Plug & Sense:**

* SOCKET_A

* SOCKET_B

* SOCKET_C

* SOCKET_F

**Average consumption:** less than 1 mA

<figure><img src="image_123.png" /></figure>

<figure><img src="image_124.png" /></figure>

Figure: image of the Sulfur Dioxide Sensor mounted on its AFE module

### 5.9.2. Cross-sensitivity data

<table><thead><tr><th>Gas</th><th>Formula</th><th>Concentration (ppm)</th><th>Output Signal (ppm SO<sub>2</sub> equivalent)</th></tr></thead><tbody><tr><td>Carbon Monoxide</td><td>CO</td><td>300</td><td>3</td></tr><tr><td>Hydrogen Sulfide</td><td>H<sub>2</sub>S</td><td>15</td><td>0</td></tr><tr><td>Nitric Oxide</td><td>NO</td><td>35</td><td>0</td></tr><tr><td>Nitric Dioxide</td><td>NO<sub>2</sub></td><td>5</td><td>-5</td></tr></tbody></table>

Table: Cross-sensitivity data for the $SO_2$ Sensor

You can find a complete example code for reading the $SO_2$ Sensor in the following link:
http://www.libellum.com/development/wasp/mote/examples/gp-07-so2

<footnote>*Accuracy values are only given for the **optimum case**. See the "Calibration" chapter for more details.</footnote>
Annexe 2
Code de programmation des modules Waspmote

//--------------------------------------
// Variables definition
//--------------------------------------

#include <WaspSensorGas_Pro.h>
#include <WaspFrame.h>
#include <Wasp3G.h>

// Define parameters for 3G
char apn[] = "orange-mib";
char login[] = "orange";
char password[] = "orange";
int count=0;
uint8_t answer;

// Define object for sensors :
Gas O3(SOCKET_4);
Gas NO2(SOCKET_3);
Gas NO(SOCKET_2);
Gas SO2(SOCKET_6);

// Stores the concentration level of O3 in ppm
float concentrationO3;
int O3d1;
float O3f2;
long O3ld2;
char O3final[30];

// Stores the concentration level of NO2 in ppm
float concentrationNO2;
int NO2d1;
float NO2f2;
long NO2ld2;
char NO2final[30];

// Stores the concentration level of NO in ppm
float concentrationNO;
int NOd1;
float NOf2;
long NOld2;
char NOfinal[30];

// Stores the concentration level of SO2 in ppm
float concentrationSO2;
int SO2d1;
float SO2f2;
long SO2ld2;
char SO2final[30];

// Stores the temperature in Â°C
float temperature;
int tempd1;
float tempf2;
long templd2;
char tempfinal[30];

// Stores the realitve humidity in %RH
float humidity;
int HRd1;
float HRf2;
long HRld2;
char HRfinal[30];

// Stores the pressure in Pa
float pressure;
long Prd1;
float Prf2;
long Prld2;
char Prfinal[30];

char batlev[2];

// define file name: MUST be 8.3 SHORT FILE NAME
char filename[13];
char filename2[50];
char fileSD[20];
int sd_answer;

char* sdcat_answer;
char nothing[] = "";

// define hexadecimal data
char aux1[250];
char aux2[250];
SdFile file;
long sizeFile;
char node_ID[] = "RUE";

// define SMS variables
int8_t TotSMS;
int8_t last;
char SMS[2];
char Reset[2];
char order[12];
# Annexes

//--------------------------------------
// Setup
//--------------------------------------

void setup()
{
    // Set SD ON
    SD.ON();
    RTC.ON();
    RTC.getTime();
    // create the name of the file in function of time (e.g. files created at 16:23:52 will be named RU162352.CSV)
    sprintf(filename, "RU%02u%02u%02u.CSV", RTC hour, RTC minute, RTC second);
    SD.del(filename);
    SD.create(filename);

    sprintf(fileSD, "/%s", filename);
    sprintf(filename2, "/Wasp mote/%s", filename);

    // Sets operator parameters
    _3G.set_APN(apn, login, password);

    // activates the 3G module:
    _3G.ON();

    _3G.setPIN("0000");
}
//--------------------------------------
// Loop initiation
//--------------------------------------

void loop()
{
    SD.ON();
    RTC.ON();

iticat answer = SD.cat(filename,0,1);

// 1. Write the first line in the file (parameters name)

sdcat_answer = SD.cat(filename,aux1);

if (strcmp (sdcat_answer, nothing) == 0)
{
    sprintf(aux1, "Time [Day of week, YY/MM/DD, hh:mm:ss]; Temperature (°C); Relative
    Humidity (%c); Pressure (Pa); O3 concentration (ppm); NO2 concentration (ppm); NO
    concentration (ppm); SO2 concentration (ppm); Battery level (%c); DeepSleep%c", %', %', \n");
    SD.append(filename, aux1);
}

else
{

//%/2. Turn on the sensors

// Power on the gases sensors.
// If the gases PRO board is off, turn it on automatically.

O3.ON();
NO2.ON();
NO.ON();
SO2.ON();

// The sensor needs time to warm up and get a response from gas
// To reduce the battery consumption, use deepSleep instead delay
// After 2 minutes, Wasp mote wakes up thanks to the RTC Alarm
delay(120000); // 120000 msec = 120 sec = 2 min

//%/3. Read sensors

// Read the O3 sensor and get the value with 9 significant digits
concentrationO3 = O3.getConc();
O3d1 = concentrationO3;
O3f2 = concentrationO3 - O3d1;
O3ld2 = round(O3f2*1000000000);
sprintf(O3final, "%d.%09ld", O3d1, O3ld2);

// Read the NO2 sensor

concentrationNO2 = NO2.getConc();
NO2d1 = concentrationNO2;
NO2f2 = concentrationNO2 - NO2d1;
NO2ld2 = round(NO2f2*1000000000);
sprintf(NO2final, "%d.%09ld", NO2d1, NO2ld2);

// Read the NO sensor

concentrationNO = NO.getConc();
NOd1 = concentrationNO;
NOf2 = concentrationNO - NOd1;
NOld2 = round(NOf2*1000000000);
sprintf(NOfinal, "%d.%09ld", NOd1, NOld2);

// Read the SO2 sensor

concentrationSO2 = SO2.getConc();
SO2d1 = concentrationSO2;
SO2f2 = concentrationSO2 - SO2d1;
SO2ld2 = round(SO2f2*1000000000);
sprintf(SO2final, "%d.%09ld", SO2d1, SO2ld2);

// Read temperature

temperature = NO.getTemp(1);
tempd1 = temperature;
tempf2 = temperature - tempd1;
templd2 = tempf2*1000000000;
sprintf(tempfinal, "%d.%09ld", tempd1, templd2);

// Read relative humidity

humidity = NO.getHumidity();
HRd1 = humidity;
HRf2 = humidity - HRd1;
HRld2 = round(HRf2*1000000000);
sprintf(HRfinal, "%d.%09ld", HRd1, HRld2);

// Read atmospheric pressure

pressure = NO.getPressure();
Prd1 = pressure;
Prf2 = pressure - Prd1;
Prld2 = round(Prf2*1000000000);
sprintf(Prfinal, "%ld.%09ld", Prd1, Prld2);
sprintf(batlev, "%i", PWR.getBatteryLevel());

sprintf(aux2, "%s;%s;%s;%ld.%09ld;%s;%s;%s;%s;%i%c", RTC.getTime(), tempfinal, HRfinal, Prd1, Prld2, O3final, NO2final, NOfinal, SO2final, PWR.getBatteryLevel(), '\n');

rait Level())

// 4. Write a new line in the CSV file with measured datas
//的原因
// SD.append(filename, aux2);

// 5. Turn off the sensors
// 因为没有 sensors 的原因，这里直接返回。

O3.OFF();
NO2.OFF();
NO.OFF();
SO2.OFF();

// Check connection to the network and upload the file on the server
"files.000webhost.com"
_3G.check(60);
_3G.configureFTP("files.000webhost.com", "21", "login", "password", 1, "I");
_3G.uploadData(fileSD, filename2);

فة
// SMS RESCUE for restarting the system if necessary
//

//Configures info from incoming SMS
_3G.setInfoIncomingSMS();

//Get the number of the last SMS in the SIM card and stores them in the variable
"TotSMS"
TotSMS= _3Gfacilities();
_3G.readSMS(TotSMS); // lecture du dernier sms reçu

//Text message converted in variable char "SMS"
sprintf(SMS, "%.1s", _3G.buffer_3G);
sprintf(order, "%s", _3G.buffer_3G);

//Create "Reset" variable
sprintf(Reset, "%1s", "0");

//Comparison of variables "SMS" (number of SMS in the SIM card) with the variable
"reset" (0 SMS in the SIM card)
answer=strcmp(SMS,Reset);

if(answer == 0)//If SMS and Reset variables are identical, then launch the restarting
process after the indicated time (variable "order")

{  
    while (TotSMS > 0) // while the SIM card is not empty, the system deletes the sms  
    one by one until the SIM card is empty  
}

{  
    TotSMS= _3G voice.getVoiceSMS();  
    _3G.deleteSMS(TotSMS);  
    TotSMS= _3G voiceitetotSMS();
}

}

_3G.OFF();
PWR.setSleep(order, RTC_OFFSET, RTC_ALM1_MODE2,ALL_OFF);
PWR.reboot();

}

else //if SMS and Reset variables are different, that means the SIM card is EMPTY, no
SMS has been received

{  
    while (TotSMS > 0) //then the system deletes the sms one by one until the SIM card is  
    empty  
}

{  
    TotSMS= _3G voiceitetotSMS();  
    _3G.deleteSMS(TotSMS);  
    TotSMS= _3G voiceitetotSMS();
}

}

}

Male, female and child are allowed to enter the country, but only one is allowed to leave the country.

// 6. Sleep during 57 minutes before the next measurements

Male, female and child are allowed to enter the country, but only one is allowed to leave the country.

_3G.OFF();
// Go to deepsleep
// After 57 minutes, Waspbote wakes up thanks to the RTC Alarm
PWR.setSleep("00:00:57:00", RTC_OFFSET, RTC_ALM1_MODE1, ALL_OFF);

// 2. activates the 3G module:

_3G.ON();
_3G.setPIN("0000");

}
Annexe 3

Plans des campagnes de mesures à l'église Saint André et à la chapelle Saint Joseph

1. Eglise Saint André.................................................................................. 315

2. Chapelle Saint Joseph – Façade restaurée rue de Venise.................. 323

3. Chapelle Saint Joseph – Façade non restaurée côté cour ................ 328
# 1. Eglise Saint Andre

<figure><img src="image_125.png" /></figure>
1

<figure><img src="image_126.png" /></figure>
<figure><img src="image_127.png" /></figure>

3

<figure><img src="image_128.png" /></figure>
<figure><img src="image_129.png" /></figure>

5

<figure><img src="image_130.png" /></figure>

4

<figure><img src="image_131.png" /></figure>

6
7

<figure><img src="image_132.png" /></figure>

8

<figure><img src="image_133.png" /></figure>
9

<figure><img src="image_134.png" /></figure>

10

<figure><img src="image_135.png" /></figure>
11

<figure><img src="image_136.png" /></figure>

12

<figure><img src="image_137.png" /></figure>
<figure><img src="image_138.png" /></figure>
2. Chapelle Saint Joseph – Façade restaurée côté rue de Venise

<figure><img src="image_139.png" /></figure>
1

<figure><img src="image_140.png" /></figure>
2

<figure><img src="image_141.png" /></figure>

3

<figure><img src="image_142.png" /></figure>
4

<figure><img src="image_143.png" /></figure>

5

<figure><img src="image_144.png" /></figure>
6

<figure><img src="image_145.png" /></figure>

<figure><img src="image_146.png" /></figure>

7

7

<figure><img src="image_147.png" /></figure>

<figure><img src="image_148.png" /></figure>
<figure><img src="image_149.png" /><figcaption>### 3. Chapelle Saint Joseph – Façade non restaurée côté cour</figcaption></figure>
3

<figure><img src="image_150.png" /></figure>

4

<figure><img src="image_151.png" /></figure>

5

<figure><img src="image_152.png" /></figure>

6

<figure><img src="image_153.png" /></figure>
<figure><img src="image_154.png" /></figure>
<figure><img src="image_155.png" /></figure>

# Soizic Gibeaux

**Personal address** : 15 Avenue de la Mare à la Chaise
77330 Ozoir-la-Ferrière
France

**Email** : soizic.gibeaux@orange.fr

**Mobile** : +33 676-544-412

**Landline** : +33 164-400-522

## EDUCATION

**2014 - present ... Doctorate - Impact of the environmental agents on restored cultural heritage**

University of Reims Champagne Ardenne, Reims 51100, France

**2013 ...** **Master 2nd year** - Heritage Materials Ageing in the Environment

University of Paris-Est Créteil, Créteil 94010, France

**2012 ...** **Master 1st year** - Environmental Sciences

University of Paris-Diderot Paris 7, Paris 75013, France

**2011 ...** **Bachelor** - Chemistry

University of Paris-Diderot Paris 7, Paris 75013, France

## EMPLOYMENT

**Mar. – Aug. 2014 ...** **Research associate**

Pictorial matters exposed to UV-C Rays in underground environmental conditions.
**LRMH Laboratoire de Recherche des Monuments Historiques**, USR 3224, France.

**Oct. – Dec. 2013 ...** **Middle-School Teacher**

Mathematics to middle-school classes 6th, 7th and 9th grades, 24h/wk.
**Assomption Bondy College, Bondy 93140, France.**

**Mar. – Aug. 2013 ...** **Master Degree Research Internship**

* Climatic conditions of rock art conservation in underground environment.

* Identification and chemical characterization of pre-Columbian pigments and their support in rock shelters.
  **LRMH Laboratoire de Recherche des Monuments Historiques, USR 3224, France.**

**May – Jun. 2011 ...** **Bachelor Degree Research Internship**

Oxidation modeling of the VOCs pollutants in a cloudy environment.
**LISA Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583 University of Paris-Diderot Paris 7, Paris 75013, France.**
# TECHNICAL SKILLS

## Environmental monitoring and Sampling

* Programming and implementation of environmental sensors network;

* Inclusions in resin for stratigraphic section analysis;

* Preparation of clay fraction for identification by diffractometry.

## Analytical technics

### Imaging

* Optical microscopy

* SEM-EDS

* TEM

* X-Ray Computed microtomography

### Chemical analysis

* Portable X-Ray Fluorescence

* Ionic Chromatography

* ICP-AES

* Fourier Transform InfraRed spectroscopy

* X-Ray Diffractometry

### Physical analysis

* Colorimetry

* Magnetic Susceptibility

* 3D scanning (ZScan, RexScan)

* Mercury Porosimetry

# TRANSVERSAL SKILLS

* Desktop tools (Microsoft Office, OpenOffice...)

* Programming (C++, Fortran)

* Cartography (Surfer 11)

* Statistical processing (R, Matlab)

* Modeling (Runsalt, PhreeqC)

# TEACHING

**2015 - present ...** Geology, practical courses to bachelor's students, 64h/yr.
*University of Reims Champagne Ardenne, Reims 51100, France*

**2016-2017 ...** Co-supervision of Bachelor's and Master's trainees in Geosciences
*University of Reims Champagne Ardenne, Reims 51100, France*

**2013 ...** Mathematics, 6th, 7th and 9th grades students, 24h/wk.
*Assumption Bondy College, Bondy 93140, France.*

# REFEREES

**Céline Schneider**
*Research associate*
GEGENAA EA 3795

2 Esp. Roland Garros
51100 Reims, France

+33 326 773 639

celine.shneider
@univ-reims.fr

**Patricia Vázquez**
*Research associate*
GEGENAA EA 3795

2 Esp. Roland Garros
51100 Reims, France

+33 326 773 688

patricia.vazquez
@univ-reims.fr

**Stephanie Touron**
*Research engineer*
LRMH - CNRS 3224

29 Rue de Paris
77420 Champs, France

+33 160 377 780

stephanie.touron
@culture.gouv.fr

**Anne Chabas**
*Research associate*
LISA - CNRS 7583

61 Av. Gral de Gaulle
94010 Créteil, France

+33 145 171 674

anne.chabas
@lisa.u-pec.fr