# Degradation of concrete panels in air scrubbers

Marie Giroudon, Marlène Fourré, Isabelle Frigout, Alexandra Bertron

► To cite this version:

Marie Giroudon, Marlène Fourré, Isabelle Frigout, Alexandra Bertron. Degradation of concrete panels in air scrubbers. Case Studies in Construction Materials, 2025, 23, pp.e05035. 10.1016/j.cscm.2025.e05035. hal-05170077

HAL Id: hal-05170077

https://hal.science/hal-05170077v1

Submitted on 18 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Degradation of concrete panels in air scrubbers

Marie Giroudon<sup>a, *</sup>, Marlène Fourré<sup>a</sup>, Isabelle Frigout<sup>b</sup>, Alexandra Bertron<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Air scrubbers

Biodeterioration

Concrete

Livestock buildings

## ABSTRACT

Air scrubbers installed in livestock buildings play a critical role in mitigating ammonia and particulate emissions, thereby improving air quality and helping to meet environmental regulations. However, concrete structures within these systems exhibit premature degradation, posing financial risks to farmers. The mechanisms behind this degradation, as well as the composition of the runoff water, remain poorly understood. This study aims to analyse runoff water compositions over 12 months and to evaluate the degradation mechanisms of two types of altered concrete: XA2 (exposed for 12 months) and XF1 (exposed for 10 years), through microstructural, chemical, and mineralogical analyses. Techniques used include inductively coupled plasma/optical emission spectrometry (ICP/OES), high-performance ion chromatography (HPIC), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Results showed that the runoff water contained ammonium in large amount (up to 995 mg/L), sulphates (up to 338 mg/L), and magnesium (up to 54.5 mg/L), all of which are aggressive to concrete. These components led to leaching and carbonation of the panels, with the XF1 concrete also suffering from sulphate attack (expansive secondary ettringite) and magnesium attack. The XF1 and XA2 type concretes exhibited degraded depths of 15 mm and 5 mm, respectively, after 10 years and 12 months of exposure. The findings indicate that air scrubbers create highly aggressive environments for concrete, highlighting the need for more suitable materials, such as calcium aluminate cement coatings, to improve durability and prevent structural failure.

## 1. Introduction

Agriculture, like many other sectors, significantly contributes to air pollution, particularly through the emission of ammonia, nitrogen and sulphur oxides and volatile organic compounds. These emissions negatively affect air quality, human and animal health, the economy, the environment and biodiversity. In France, agricultural alone accounted for 93.4 % of national ammonia emissions and 9 % of emissions of fine particulate matter with a diameter of less than 2.5 µm (PM<sub>2.5</sub>) in 2019 [1,2]. As one of the leading ammonia emitters in the European Union, France is required by the European National Emission Ceiling (NEC) Directive [3] to reduce its ammonia emissions by 13 % and PM<sub>2.5</sub> emissions by 57 % by 2030.

Among agricultural practices, livestock rearing is a major emitter of ammonia, with animal housing and manure storage systems accounting for 25–55 % of total emissions [1,4]. In this context, pig farming facilities must comply with air quality regulations derived from several frameworks: (i) the Gothenburg Protocol under the Convention on Long-Range Transboundary Air Pollution, which targets reductions in five key air pollutants (sulphur dioxide, nitrogen oxides, volatile organic compounds, ammonia, and fine

<footnote><sup>a</sup> Univ Toulouse, INSA, LMDC, Toulouse, France</footnote>

<footnote><sup>b</sup> CELTYS, ZI du Quillivaron, Landivisiau, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: marie.giroudon@insa-toulouse.fr (M. Giroudon).</footnote>
particles) [5,6], (ii) the European NEC Directive, which sets emission ceilings for each Member State [3], (iii) the Industrial Emission Directive, which outlines best available techniques for emission reduction; and (iv) the national Labor Code concerning occupational health [2]. Accordingly, the air quality in pig farms must meet strict thresholds: ammonia must not exceed 20 ppm over 15 min or 10 ppm over 8 h carbon dioxide must not exceed 0.3 % of air volume; hydrogen sulphur content must not exceed 0.5 ppm; and fine particulate matter must be below 0.9 mg/m³ over an 8-h period.

To meet these requirements, exhaust air from livestock housing is increasingly treated using end-of-pipe that target pollutants such as ammonia, odours and dust [7]. One widely adopted method is air scrubbing, which reduces emissions from pig housing by 40–90 % for ammonia and 60–80 % for particulate matter [1], while also mitigating odour nuisances [8,9]. Although this technology has been used in the Netherlands for many years due to environmental regulations, its implementation in France is relatively recent [8,10].

The air scrubbing is based on the solubility of certain pollutants in water. For ammonia, the following reversible reaction occurs between ammonia (NH₃) and ammonium (NH₄⁺):

$$NH_{3(l)} + H_2O(l) \rightleftharpoons NH_{3(l)} + H_2O(l) + NH_{4(l)}^+ + OH_{(l)}^- \quad (1)$$

The air from the housing facilities is directed into a central duct and introduced into the air scrubber, where water is continuously sprayed. The purpose of the nozzle ramp is to spray the evenly wet the entire mesh with recirculated water and to humidify the incoming air. In this way, soluble compounds and dust are captured by the water before the treated air is released to the outside (Fig. 1). These air scrubbers are sometimes referred to as “biological air scrubbers” due to the bacterial population that develops within the mesh, resulting from the accumulation of dust (primarily organic in the case of pig farms). These microorganisms degrade odorous chemical components present in the air, thereby enhancing the effectiveness of the scrubbing process, particularly in terms of odour removal [8,11].

A greater reduction in ammonia levels can be achieved by lowering the pH of the runoff water through the addition of acid (typically sulfuric acid), as practiced in the Netherlands. However, this approach also results in the destruction of microbial populations that are highly active in degrading odorous compounds. In addition, it introduces challenges related to acid storage and handling on farms. As a result, air scrubbers that use only water are predominantly employed in France and are cleaned infrequently or not at all to preserve microbial diversity [2,8,11]. Occasionally, cleaning with a water gun may still be performed.

While the mesh is made of an inert, structured plastic material such as polypropylene in a honeycomb configuration [11,12], the walls of the air scrubbers are constructed from concrete panels. The concrete is therefore exposed to (i) recirculating water, which may contain, according to the literature review of typical background concentrations in the outcoming air compiled by Van der Heyden et al. [13], ammonium, dissolved CO₂, S-bearing species, volatile fatty acids (VFA) and microorganisms (although no direct data on water composition are available) and (ii) polluted air, which contains gases such as CO₂ and H₂S. The presence of these compounds in the liquid phase is known to be detrimental to concrete, as they promote leaching (by VFAs and ammonium) and/or carbonation (by dissolved CO₂), with microbial activity further accelerating degradation [14,15]. In the gas phase, CO₂ may also cause carbonation, while H₂S can lead to biogenic acid attack, due to the biological formation of sulfuric acid in the presence of oxygen [16,17]. A number of studies have demonstrated the highly aggressive nature of agricultural effluents, which can cause severe deterioration of

<figure><img src="image_3.png" /><figcaption>Fig. 1. Air scrubbers (vertical or horizontal), adapted from [8].</figcaption></figure>
concrete structures at various stages of the process, including production, storage, disposal and treatment. Although the composition of agricultural effluents varies widely, most share common characteristics—namely, elevated concentrations of volatile fatty acids, microorganisms, and occasionally ammonia [18–21]. These conditions result in a form of degradation comparable that caused by strong acids, characterized by pronounced mineralogical and chemical zonation within the cement paste. The affected zone is typically highly porous, exhibits low mechanical strength, and becomes progressively and almost completely decalcified. The environmental conditions present in air scrubbers are also analogous to those found in anaerobic digestion silos. In these systems, the liquid phase contains organic acids, microorganisms and ammonium, while the walls in contact with the gas phase are exposed to a highly humid atmosphere, rich in CO₂ and H₂S. Under such conditions, materials undergo decalcification and carbonation in the liquid phase, and are also susceptible to carbonation and/or sulfuric attack in the gas phase [21–23].

However, there is a notable lack of data regarding both the composition of runoff water and the specific degradation mechanisms affecting the concrete components of air scrubbers. Structural deterioration has already been observed on-site, as our team has documented visible damage to air scrubbers at several farms (Fig. 2), which would ultimately result in structural failure before the end of the building's expected service life—posing a significant financial risk for farmers. Thereby, this study aims to: (i) investigate the degradation mechanisms of concrete panels that have been exposed to the aggressive environment of an air scrubber for 12 months or more, using physical, microscopic, and mineralogical analyses; and (ii) conduct chemical analyses of the runoff water at multiple time intervals.

## 2. Materials and methods

### 2.1. Materials

Runoff water was collected for analysis after 3, 6, 9, and 12 months of operation in an air scrubber. The frequency of cleaning and water replacement in the air scrubber during this period is not documented.

Three concrete samples were characterised in this study: two degraded samples and one unexposed control sample. All samples were supplied by Celtyss. The first degraded specimen is an XA2-type concrete sample that was exposed to the aggressive air scrubber environment for 12 months (hereafter referred to as XA2/12 m). The "XA2" designation indicates that the concrete is designed to withstand a moderately chemically aggressive environment, in accordance with the French documentation booklet FD P18-011 and the NF EN 206+A2/CN standard [24,25]. The second degraded sample is a XF1-type concrete sample that was exposed to runoff water from an air scrubber for 10 years (hereafter referred to as XF1/10 y). The "XF1" classification indicates that the concrete to withstand light to moderate frost conditions, without the use of de-icing agents, based on the non-harmonized European Standard NF EN 206+A2/CN [25]. Fig. 3 presents the XF1/10 y sample, which consists of a layer of concrete on the surface (approximately 6.5 cm), followed by a layer of polystyrene core (approximately 10 cm), and another concrete layer beneath. The control sample is an unexposed XA2-type concrete, hereafter referred to as XA2/c.

### 2.2. Methods

The aim of this study was to understand the mechanisms driving concrete deterioration in these aggressive environments, which have not yet been thoroughly investigated, by analyzing the chemical and mineralogical changes occurring in the materials. For this type of degradation, which is primarily governed by reactive transport processes, a bulk or global analysis of the material is not sufficient. Therefore, analytical techniques that provide access to the degradation gradient are required to gain insight into the underlying processes. This approach involved applying existing knowledge of the degradation of individual phases to interpret the overall degradation behaviour of the material.

As there are no established standards for assessing chemical and mineralogical degradation gradients, a solid-phase analytical approach was employed. This involved the combination of spectroscopy with microscopy for micrometric-scale characterization, as well as the observation of newly formed phases at the tens-of-micrometres scale.

To complement these methods—which provide insight into the material's microstructure, morphology and chemical composition—X-ray diffraction (XRD) was also used. Although XRD typically provides a bulk (global) analysis, in this study it was applied to extract mineralogical information at varying depths along a plane parallel to the surface exposed to the aggressive environment. The

<figure><img src="image_4.png" /><figcaption>Fig. 2. (A) Surface layer (“Skin”) of concrete panels from a degraded air scrubber after several years of operation; (B) Close-up view showing the gravelly texture of the concrete surface subjected to the spray from the air scrubber - Celtyss ©.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. (A) Sampling of the XF1-type concrete after 10 years of exposure, (B) Surface view of the same sample, (C) Cross-sectional profile of the sample.</figcaption></figure>

degradation gradient was thus captured by progressively advancing into the cementitious material through successive surface abrasion.

In addition, the phenolphthalein indicator test was used to provide a macroscopic visual representation of the degradation gradient, while the results of the liquid-phase analyses were correlated with the observed chemical and mineralogical changes in the solid material.

### 2.2.1. Liquid analyses

The pH of the solutions was measured using a pH-meter calibrated within the range of 6 and 9. After filtration through a 0.45 µm membrane and appropriate dilution, the liquid samples were analysed using inductively coupled plasma/optical emission spectroscopy (ICP/OES) (Optima 7000DV ICP/OES Perkin Elmer) to determine concentrations of major and minor elements (Si, Ca, Al, Fe, K, Na, Mg, Ti, Mn). Additionally, high-performance ion chromatography (HPIC) (Dionex ICS-3000) was used to quantify volatile fatty acids (acetate, lactate, butarate, propionate, valerate, oxalate, formate), anions ($Cl^-$, $SO_4^{2-}$, $NO_3^-$, $PO_4^{3-}$, $NO_2^-$, $SO_3^{2-}$, $S_2O_3^{2-}$) and the $NH_4^+$ cation concentrations.

### 2.2.2. Concrete samples analyses

2.2.2.1. *Loss of alkalinity.* The loss of alkalinity was determined in accordance with GranDuBé recommendations [26] for identifying carbonation fronts. This was performed by spraying a phenolphthalein solution onto freshly sawn or fractured section of the XF1/10 y and the XA2/12 m concrete samples. A colour change from purple (pH>9; indicating an alkaline zone) to colourless (pH<9) indicated the loss of alkalinity. In addition, alkalinity was evaluated using the "Rainbow Indicator", a commercial multi-range pH indicator (German Instruments), which offers visual differentiation across multiple pH thresholds (Fig. 4).

2.2.2.2. *Microscopic characterization.* A diamond saw was used to cut the original core samples into smaller segments to allow for detailed microstructural characterization. Samples prepared for Scanning Electron Microscopy (SEM) (JEOL JSM-LV, 15 kV) were sectioned along the longitudinal axis of the initial core, embedded in epoxy resin (Mecaprex Ma2 + from Presi), and dry-polished using silicon carbide polishing disks (Presi). The polished cross-sections were then carbon-coated and analyzed in backscattered electron (BSE) mode, as well as by energy dispersive X-ray spectrometry (EDS, Bruker Xflash 6/30 - SDD) for chemical composition. Additional specimens were cut from the same cores to perform mineralogical analyses at various depths using qualitative X-Ray Diffraction (XRD) (Bruker D8 Advance, Cu anti-cathode, 40 kV, 40 nA) on solid samples.

## 3. Results

### 3.1. pH and composition of runoff water

Table 1 shows the pH values, visual appearance, and odour characteristics of the runoff water collected at different time intervals.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Commercial “Rainbow Indicator” product and its pH colour chart.</figcaption></figure>

<table><caption>**Table 1**
pH, visual appearance, and odour of the runoff water samples over time.</caption><thead><tr><th>Runoff water</th><th>pH</th><th>Temperature (°C)</th><th>Visual appearance and odour</th></tr></thead><tbody><tr><td>After 3 months in the air scrubber</td><td>6.95</td><td>21.0</td><td>Dark brown colour</td></tr><tr><td>After 6 months in the air scrubber</td><td>7.33</td><td>23.6</td><td>Brown color with suspended matter</td></tr><tr><td>After 9 months in the air scrubber</td><td>8.46</td><td>22.8</td><td>Dark brown colour with suspended matter; very strong smell of ammonia</td></tr><tr><td>After 12 months in the air scrubber</td><td>9.49</td><td>26.8</td><td>Dark brown colour with suspended matter; smell of ammonia</td></tr></tbody></table>

The pH of the runoff water increased progressively with exposure time, rising from 6.95 after 3 months to 9.49 after 12 months. Additionally, suspended matter becomes apparent after 6 months of exposure, while a noticeable ammonia odour was first detected after 9 months in the air scrubber.

**Table 2** shows the concentrations of major and minor elements, organic acids, anions and cations measured in the runoff water samples collected at 3, 6, 9, and 12 months.

Between 3- and 9-months of exposure, the composition of the runoff water remains relatively stable, with only minor variations observed. During this period, the water primarily contains silicon (average concentration of approximately 18 mg/L), magnesium (≈ 44 mg/L), and alkali elements such as sodium (≈ 57 mg/L), calcium (≈ 132 mg/L), and potassium (≈ 28 mg/L). After 12 months of exposure, however, the concentrations of these elements—with the exception of magnesium—increase significantly. Silicon levels triple (reaching nearly 60 mg/L), and calcium concentrations increase sixfold, reaching 741 mg/L compared with earlier samples. The concentration of organic acids remains very low and is generally below the limit of quantification. Only oxalate, at 1.7 mg/L, could be quantified in the runoff water after 12 months of exposure. Levels of sulphates, phosphates and chlorides remain relatively constant over time, averaging approximately 300 mg/L, 88 mg/L, and 12 mg/L, respectively. However, nitrate and nitrite concentrations double after 12 months of exposure, and a low concentration of thiosulphate is also detected at this stage. Finally, ammonium concentrations remain stable throughout the study, ranging between 800 and 900 mg/L.

## 3.2. Loss of alkalinity of the XF1/10y and XA2/12m concretes

Fig. 5 shows a fresh fracture of the XF1/10 y sample sprayed with phenolphthalein. The measured loss of alkalinity (pH < 9) extended to a depth of approximately 6–7 mm.

Fig. 6 shows the fresh section of the XF1/10 y concrete sample sprayed with the Rainbow Indicator. Alkalinity loss is observed over a depth of approximately 15 mm, with 3 distinct pH zones identified from the surface inward:

- A 6 mm red zone, where the pH is between 5 and 7;

- A 2–3 mm green zone, where pH = 9;

- A 5–6 mm green/violet zone, where the pH is between 9 and 11.

Fig. 7 shows the fresh section of the XA2/12 m concrete sample sprayed with phenolphthalein. The colour change indicating loss of alkalinity (pH < 9) occurs a depth of approximately 4.5–5 mm.

Fig. 8 shows the fresh section of the XA2/12 m concrete sample after spraying with the Rainbow Indicator. A single uncoloured zone— sometimes with an orange tint—measuring approximately 4.5–6 mm in thickness is observed at the exterior surface, likely indicating a pH of 4 or lower. The remaining portion of the sample appears purple, corresponding to a pH greater than 11.
<caption>**Table 2**</caption>

Concentrations of major and minor elements (*: below the limit of quantification), and concentrations of organic acids, anions and cations in runoff water samples (<1: below the limit of quantification / -: not detected).

<table><thead><tr><th rowspan="2"></th><th rowspan="2"></th><th colspan="4">Runoff water</th></tr><tr><th>After 3 months in the air scrubber</th><th>After 6 months in the air scrubber</th><th>After 9 months in the air scrubber</th><th>After 12 months in the air scrubber</th></tr></thead><tbody><tr><td rowspan="8">Content of major and minor elements (mg/L)</td><td>Si</td><td>16.6</td><td>19.7</td><td>19.1</td><td>59.8</td></tr><tr><td>Ca</td><td>108.5</td><td>148.0</td><td>140.0</td><td>741.0</td></tr><tr><td>Al*</td><td>&lt; 0.1</td><td>&lt; 0.1</td><td>&lt; 0.05</td><td>&lt; 0.05</td></tr><tr><td>Fe*</td><td>&lt; 0.1</td><td>&lt; 0.1</td><td>0.1</td><td>0.02</td></tr><tr><td>K</td><td>28.3</td><td>28.3</td><td>27.3</td><td>49.1</td></tr><tr><td>Na</td><td>46.5</td><td>60.2</td><td>63.8</td><td>822.0</td></tr><tr><td>Mg</td><td>36.9</td><td>41.9</td><td>54.5</td><td>38.1</td></tr><tr><td>Ti*</td><td>&lt; 0.01</td><td>&lt; 0.05</td><td>&lt; 0.002</td><td>&lt; 0.002</td></tr><tr><td rowspan="10">Organic acids (mg/L)</td><td>Mn</td><td>0.3</td><td>0.5</td><td>0.19</td><td>0.01</td></tr><tr><td>Acetate (CH<sub>3</sub>COO<sup>-</sup>)</td><td>&lt; 1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Lactate (CH<sub>3</sub>COHCOO<sup>-</sup>)</td><td>&lt; 1</td><td>-</td><td>-</td><td>&lt; 1</td></tr><tr><td>Butyrate (CH<sub>3</sub> (CH<sub>2</sub>)<sub>2</sub>COO<sup>-</sup>)</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Propionate (CH<sub>3</sub>CH<sub>2</sub>COO<sup>-</sup>)</td><td>&lt; 1</td><td>-</td><td>-</td><td>&lt; 1</td></tr><tr><td>Valerate (CH<sub>3</sub> (CH<sub>2</sub>)<sub>3</sub>COO<sup>-</sup>)</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Oxalate (COOHCOO<sup>-</sup>)</td><td>&lt; 1</td><td>-</td><td>-</td><td>1.7</td></tr><tr><td>Formate (COOH<sup>-</sup>)</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Anions (mg/L)</td><td>Chlorides (Cl<sup>-</sup>)</td><td>63.2</td><td>92.6</td><td>93.2</td><td>103.2</td></tr><tr><td></td><td>Sulphates (SO<sub>4</sub><sup>2-</sup>)</td><td>329.9</td><td>217.1</td><td>320.0</td><td>338.2</td></tr><tr><td></td><td>Nitrates (NO<sub>3</sub><sup>-</sup>)</td><td>2255.0</td><td>1581.3</td><td>2301.2</td><td>5183.1</td></tr><tr><td></td><td>Phosphates (PO<sub>4</sub><sup>3-</sup>)</td><td>9.0</td><td>18.3</td><td>16.9</td><td>3.0</td></tr><tr><td></td><td>Nitrites (NO<sub>2</sub><sup>-</sup>)</td><td>427.3</td><td>662.2</td><td>372.6</td><td>1079.0</td></tr><tr><td></td><td>Sulphites (SO<sub>3</sub><sup>2-</sup>)</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td></td><td>Thiosulphates (S<sub>2</sub>O<sub>3</sub><sup>2-</sup>)</td><td>-</td><td>-</td><td>-</td><td>1.0</td></tr><tr><td rowspan="2">Cation (mg/L)</td><td>Ammonium (NH<sub>4</sub><sup>+</sup>)</td><td>812.5</td><td>789.6</td><td>994.8</td><td>884.0</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Fresh fracture of XF1/10 y after phenolphthalein spraying.</figcaption></figure>

### 3.3. Microstructural, chemical and mineralogical alterations of the XF1/10 y and XA2/12 m concretes

#### 3.3.1. XF1/10 y concrete

Fig. 9 shows the BSE-SEM image of the XF1/10 y concrete section with corresponding elemental maps for sulphur, calcium, silicon and magnesium. Variations in chemical composition and microstructure reveal three distinct degradation zones (Zones 4, 3, and 2), which were further characterized using higher magnification SEM imaging and EDS. A fourth zone—Zone 1, representing the un-
<figure><img src="image_8.png" /><figcaption>Fig. 6. (A) Fresh section of the XF1/10 y concrete after spraying with the Rainbow Indicator; (B) Close-up view highlighting colour variations that correspond to different pH zones within the degraded outer layer of the sample.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. (A) Fresh section of the XA2/12 m concrete sample after spraying with phenolphthalein; (B) Close-up view of the external portion of the sample.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. Fresh section of the XA2/12 m concrete sample after spraying with the Rainbow Indicator.</figcaption></figure>

altered core of the concrete, stable composition and microstructure—was identified at a depth of approximately 15 mm, consistent with the pH transition observed in Fig. 6. The four zones are defined as follows:

- Zone 4 (surface layer, ~4.83 mm): This outermost zone exhibits high porosity and pronounced decohesion between the cement paste and aggregates. The paste is completely decalcified and enriched in magnesium, while the aggregates remain largely unaltered. According to EDS analyses, the cementitious matrix primarily consists of calcium, and the aggregates are siliceous, silico-aluminous, and calcareous composition.

- Zone 3 (subsurface layer, ~6.79 mm): Located beneath Zone 4, this zone is characterized by the presence of is microcracks, magnesium depletion, and sulphur enrichment, along with partial decalcification. The matrix is composed mainly of silicon and calcium. Numerous sites of massive, expansive secondary ettringite formation are observed in this zone (Fig. 10).

- Zone 2: Located beneath Zone 3, this region displays a higher density compared to the overlying layer and shows localized calcium enrichment over a few micrometres. The cement paste primarily consists of calcium and silicon. Massive and expansive secondary ettringite formations are still present in significant quantities within this zone.

- Zone 1 (from ~15 mm depth): This innermost zone shows a micro-cracked cement matrix with a cement paste exhibiting its typical, unaltered chemical composition, rich in calcium and silicon. Despite the relatively intact chemistry, massive and expansive secondary ettringite remains abundant (Fig. 11).

Since the XRD analyses (not shown) were conducted after the SEM-EDS characterisation, the mineralogical analyses were carried out in the previously identified zones, following a three-step approach: a surface analysis (Zone 4), and two additional analyses at greater depths corresponding to Zone 3 and the combined Zones 2 and 1.

The diffractogram of Zone 4 primarily reveals the presence of quartz (originating from the aggregates), along with Berlinite, muscovite, magnesian calcite and brucite. In Zone 3, quartz remains the predominant mineral phase, while traces of ettringite, albite and calcite emerge, and Berlinite and brucite are no longer detected. At greater depths—corresponding to the combined Zones 2 and 1 as identified by SEM—quartz continues to dominate, and portlandite becomes detectable.
<figure><img src="image_11.png" /><figcaption>Fig. 9. BSE-SEM image and corresponding elemental maps (S, Ca, Si, Mg) in the XF1/10 y concrete sample, SEM-EDS, polished section, HV, BSE, x22.</figcaption></figure>

### 3.3.2. XA2/12 m concrete

Fig. 12 shows a BSE-SEM image of the XA2/12 m concrete sample, along with elemental mapping for sulphur. Other elemental maps (e.g., magnesium, silicon, potassium or calcium) are not shown, as no visible chemical fronts were detected for those elements. The BSE-SEM image reveals a distinct darker outer zone, approximately 4.5 mm thick, characterized by lower density and sulphur depletion.
<figure><img src="image_12.png" /><figcaption>Fig. 10. Sites of massive and expansive secondary ettringite in Zone 3 in XF1/10 y concrete, SEM, polished section, HV, BSE.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11. (A) XF1/10 y concrete sample observed in Zone 1, 25 mm deep; (B) Massive and expansive secondary ettringite observed in Zone 1 of XF1/10 y concrete, SEM, polished section, HV, BSE.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 12. BSE-SEM image and elemental mapping of Sulphur (S) in the XA2/12 m sample, SEM-EDS, polished section, HV, BSE, x25.</figcaption></figure>

Mineralogical analyses (not shown) were conducted at three different depths in the XA1/12 m concrete sample: the surface, 2.5 mm, and 10 mm deep. At the surface, quartz was the dominant mineralogical phase, consistent with the presence of aggregates. Additional detected phases included berlinite, muscovite, calcite, albite, phlogopite and clinochlore. At a depth of 2.5 mm, quartz remained predominant with increased intensity of albite, berlinite and calcite signals. Phlogopite was also identified. At 10 mm depth, quartz continued to be the principal mineralogical phase, with all previously observed minerals still present, along with the appearance of copper salts. This deeper mineralogical composition closely resembles that of the XA2/c concrete, which contains quartz, biotite, albite, calcite, rutile, clinochlore, portlandite, along with traces amounts of phlogopite and muscovite. It is important to note that clinochlore, biotite, phlogopite and muscovite are non-cementitious minerals likely originating from the aggregates.

# 4. Discussion

## 4.1. Composition of runoff water and aggressiveness of the medium

According to the European standard EN 206 + A2/CN [25] and the French documentation booklet FD P 18-011 [24], the chemical aggressiveness of liquid environments toward concrete is classified from XA1 to XA3, based on the presence and concentration of specific aggressive agents. These include aggressive $CO_2$, $SO_4^{2-}$, $Mg^{2+}$, $NH_4^+$, low total alkali strength (TAC), and pH. Based on the re-
sults of the chemical analyses, the runoff water sampled in this study can be classified as very aggressive (XA3), primarily due to its
ammonium concentrations (800–900 mg/L), which significantly exceeds the XA3 threshold of 100 mg/L. In addition, sulphate con-
centrations fall within the range associated with XA1-classificed environments (200–600 mg/L), while magnesium levels remain be-
low the XA1 threshold. Although the measured pH is not considered aggressive according to the standards [24,25], it is relatively
close to XA1 class (pH between 6.5 and 5.5) in the first sample (measured pH 6.95). The French documentation booklet FD P 18-011
[24] also notes that microbial growth on concrete surfaces (biofilm) can induce localized chemical conditions that are more severe
than those in the surrounding environment, and that temperature and relative humidity cycles (daily and seasonal variations, tidal
cycles) considerably increase the damaging effects of aggressive agents. The runoff water also contains high levels of nitrates and ni-
trites, which are not aggressive to concrete. Overall, the composition of the runoff water is relatively close to that of the liquid phase
of anaerobic digestion, which typically contains several hundred mg/L of ammonium, a few tens of mg/L of magnesium, neutral pH,
and little or no volatile acids under steady-state conditions (when production and consumption are balanced) [22,27].

Sampling of the runoff water revealed an increase over time in the concentrations of major chemical elements found in concrete, including silicon, calcium, sodium, and potassium. This trend may result from the progressive leaching of the surrounding concrete or from a continuous external input of one or more alkaline-based salts.

## 4.2. Deterioration mechanisms of the XF1/10 y and XA2/12 m concretes

After 10 years of exposure, the XF1/10 y concrete was the most degraded, with a combination of several degradation mechanisms.
In the external part (Zone 4), the concrete was very porous, decalcified, but also carbonated and enriched in magnesium. These dam-
ages resulted from the combination of:

(i) Leaching from the runoff water (whose pH is sometimes very low compared to the pH of concrete) and ammonium attack, which lead to high porosity, decalcification, and the dissolution of initial phases of the concrete such as portlandite, along with a drop in pH [14,28]. It cannot be excluded that organic acids (produced and then immediately consumed) were also involved in the degradation of the cementitious matrix.

(ii) Carbonation by CO₂ in air, possibly favoured by the high moisture content of the permanently humid environment (carbonation kinetics reach a maximum at relative humidity levels between 40 % and 80 % [29]), and the very likely presence of microorganisms on the surface of the panel – corroborated by the precipitation of Berlinite (AlPO₄) which may result from biological interaction with fungi [30] that metabolise CO₂ directly at the concrete surface. Moreover, the dissolution of calcium-rich phases due to leaching makes calcium ions available for carbonation.

(iii) Migration of magnesium ions from the runoff water into the cement paste, a phenomenon commonly observed when concrete is exposed to water rich in magnesium and sulphates [31]. The presence of magnesium leads to the precipitation of brucite and magnesian calcite, both of which are detected in the outer part of the sample. In addition, the combined effects of decalcification and magnesium enrichment may have caused the replacement of calcium by magnesium in C-S-H, resulting in the formation of magnesium silicate hydrates (M-S-H) —a non-binding, amorphous phase [31–34]. Elemental mapping (Fig. 9) suggest the possible presence of M-S-H at a depth of 1–3 mm, where magnesian concentration is highest, in association with calcium depletion and a normal silicon content.

In zone 3, the XF1/10 y concrete was decalcified and enriched in sulphur, with the precipitation of secondary expansive ettringite, which led to formation of microcracks. Sulphur enrichment at depth is a phenomenon commonly observed in leached samples subjected to various aggressive environments [14,18,23,35]. However, the strong presence of expansive secondary ettringite, along with the sulphate concentrations measured in the runoff water, suggests that sulphates from the external medium penetrated deep into the concrete to form ettringite. In addition, the likely presence of H₂S in the gas phase [13] could contribute to a coupled attack by H⁺ and sulphates ions, leading to decalcification and the precipitation of secondary sulphate-based phases, such as gypsum or ettringite, at depth—as commonly observed in sewer environment [16,36,37]. Finally, the phenomena observed in Zones 2 and 1 are likely the result of the deterioration mechanisms described above.

For the XA2/12 m concrete, the main phenomena were leaching, and carbonation to a lesser extent. For this sample, SEM observations revealed a microstructure typically found in cases of ammonium attack, with an external zone of lower density, resulting from progressive decalcification [14]. As for the XF1/10 y sample, the carbonation could be attributed to both CO₂ from the air and CO₂ from microbial activity, and may have been favoured by the dissolution of calcium-bearing phases and the release of calcium ions.

## 5. Future recommendation on the choice of concrete formula

Based on its composition, and in accordance with standard EN 206+A2/CN [25] and the FD P 18-011 [24], the runoff water is classified as a very aggressive environment to concrete (XA3). Moreover, the presence of microorganisms on the surface of concrete (biofilm), along with variations in relative humidity occurring during the washing cycles of air scrubbers, could considerably increase the deleterious action of aggressive agents. However, in the two samples studied, the designers selected XF1 and XA2 type concretes, which showed degraded depths of 15 mm and 5 mm, respectively, after 10 years and 12 months of service life—the XF1/10 y sample being particularly damaged.
Thus, it may be relevant to consider the use of XA3 type concrete, or to apply coatings that are stable under chemo-biological attack. Calcium aluminate cement and geopolymers have already been tested in other aggressive environments such as sewers and bio-gas plants, where they demonstrated low deterioration [23,38,39]. These materials could be considered for use as panels coatings.

## 6. Conclusion

This case study showed that air scrubbers in agricultural livestock buildings, particularly in pig farms, create an environment that is highly aggressive to concrete. The gas phase, especially containing H₂S and CO₂, the type of exposure (including rare drying during maintenance and runoff), the sometimes-abrasive cleaning methods, and the presence of microorganisms—favoured by the thin, constantly aerated water film on the concrete surface—all contribute to amplifying the degradation caused by the runoff water composition, which includes aggressive agents such as magnesium, sulphates, and ammonium. This environment leads to significant degradation of concrete panels, including decalcification, surface enrichment in magnesium, and in-depth carbonation, resulting in high degraded depth—up to 15 mm in 10 years. Consideration should be given to the recommendations for binder selection in such environments, particularly by exploring the use of thin panel coatings made from resistant binders.

### CRediT authorship contribution statement

**Alexandra Bertron:** Supervision, Project administration, Methodology, Funding acquisition. **Isabelle Frigout:** Resources, Funding acquisition. **Marlène Fourré:** Investigation. **Marie Giroudon:** Writing – original draft.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

No data was used for the research described in the article.

### References

[1] ADEME, Guide des bonnes pratiques agricoles pour l'amélioration de la qualité de l'air, 2020. (https://agriculture.gouv.fr/un-guide-des-bonnes-pratiques-pour-ameliorer-la-qualite-de-lair). (accessed September 4, 2024).

[2] D. Creusy, N. Guingand, Bâtiment innovant en faveur de la qualité de l'air en élevage porcin, ADEME, 2021. (https://librairie.ademe.fr/5454-batiment-innovant-en-faveur-de-la-qualite-de-l-air-en-elevage-porcin.html). (accessed September 4, 2024).

[3] The European Parliament, The European Union Council, Directive 2001/81/EC of the European Parliament and of the Council of 23 October 2001 on national emission ceilings for certain atmospheric pollutants, 2001. (http://data.europa.eu/eli/dir/2001/81/oj/fra).

[4] A.H.W. Beusen, A.F. Bouwman, P.S.C. Heuberger, G. Van Drecht, K.W. Van Der Hoek, Bottom-up uncertainty estimates of global ammonia emissions from global agricultural production systems, Atmos. Environ. 42 (2008) 6067-6077, https://doi.org/10.1016/j.atmosenv.2008.03.044.

[5] Gothenburg protocol, Protocol to the 1979 convention on long-range transboundary air pollution to abate acidification, eutrophisation and ground-level ozone - amendment, (2012)

[6] Gothenburg protocol, Protocol to the 1979 convention on long-range transboundary air pollution to abate acidification, eutrophisation and ground-level ozone, (1999)

[7] Joint Research Centre (European Commission), K. Georgitzikis, G. Giner Santonja, S. Roudier, B.M. Scalet, P. Montobbio, L. Delgado Sancho, Best Available Techniques (BAT) reference document for the intensive rearing of poultry or pigs: Industrial Emissions Directive 2010/75/EU (Integrated Pollution Prevention and Control), Publications Office of the European Union, 2017. (https://data.europa.eu/doi/10.2760/020485) (accessed September 6, 2024).

[8] N. Guingand, Le lavage d'air en élevages porcins, Tech. Porc Rev. Tech. L'IFIP 31 (2008) 23-27.

[9] M.A. Van Geelen, K.W. Van Der Hoek, Odour control with biological air washers, Agric. Environ. 3 (1977) 217-222, https://doi.org/10.1016/0304-1131(77)90014-5.

[10] R.W. Melse, P. Hofschreuder, N.W.M. Ogink, Removal of particulate matter (PM10) by air scrubbers at livestock facilities: results of an on-farm monitoring program, Trans. ASABE 55 (2012) 689-698, https://doi.org/10.13031/2013.41378.

[11] S. Lagadec, P. Landrain, F. Bellec, L. Masson, C. Dapello, N. Guingand, Enquête sur 31 laveurs d'air de porcherie en Bretagne, clés d'amélioration de l'efficacité sur l'abattement de l'ammoniac, 2015.

[12] N. Guingand, Influence de la fréquence de vidange des eaux de lavage sur l'efficacité d'unités de lavage d'air sur les émissions d'ammoniac, d'odeurs et de poussières dans le cas de porcheries d'engraissement, Journ. Rech. Porc. (2014) 193-198.

[13] C. Van der Heyden, P. Demeyer, E.I.P. Volcke, Mitigating emissions from pig and poultry housing facilities through air scrubbers and biofilters: state-of-the-art and perspectives, Biosys. Eng. 134 (2015) 74-93, https://doi.org/10.1016/j.biosystemseng.2015.04.002.

[14] M. Giroudon, M. Peyre Lavigne, C. Patapy, A. Bertron, Laboratory assessment of the contribution of aggressive to concrete chemical compounds to the degradation of Portland cement-based materials during anaerobic digestion, Mater. Struct. 54 (2021) 218, https://doi.org/10.1617/s11527-021-01810-x.

[15] C. Magniont, M. Coutand, A. Bertron, X. Cameleyre, C. Lafforgue, S. Beaufort, G. Escadeillas, A new test method to assess the bacterial deterioration of cementitious materials, Cem. Concr. Res. 41 (2011) 429-438, https://doi.org/10.1016/j.cemconres.2011.01.014.

[16] A. Aboulela, M.P. Lavigne, A. Buvignier, M. Fourré, M. Schiettekatte, T. Pons, C. Patapy, O. Robin, M. Bounouba, E. Paul, A. Bertron, Laboratory test to evaluate the resistance of cementitious materials to biodeterioration in sewer network conditions, Materials 14 (2021) 686, https://doi.org/10.3390/ma14030686.

[17] A. Buvignier, C. Patapy, M.Peyre Lavigne, E. Paul, A. Bertron, Resistance to biodeterioration of aluminium-rich binders in sewer network environment: study of the possible bacteriostatic effect and role of phase reactivity, Cem. Concr. Res. 123 (2019) 105785, https://doi.org/10.1016/j.cemconres.2019.105785.

[18] A. Bertron, J. Duchesne, Attack of Cementitious Materials by Organic Acids in Agricultural and Agrofood Effluents, in: M. Alexander, A. Bertron, N. De Belie (Eds.), Perform. Cem.-Based Mater. Aggress. Aqueous Environ. Springer, Dordrecht, 2013, pp. 131-173, https://doi.org/10.1007/978-94-007-5413-3_6.

[19] E. Sánchez, A. Moragues, J. Massana, A. Guerrero, J. Fernandez, Effect of pig slurry on two cement mortars: changes in strength, porosity and crystalline phases, Cem. Concr. Res. 39 (2009) 798-804, https://doi.org/10.1016/j.cemconres.2009.06.008.
[20] J. Massana, A. Guerrero, R. Antón, M.A. Garcimartín, E. Sánchez, The aggressiveness of pig slurry to cement mortars, Biosyst. Eng. 114 (2013) 124-134, https://doi.org/10.1016/j.biosystemseng.2012.11.003.

[21] A. Koenig, F. Dehn, Biogenic acid attack on concretes in biogas plants, Biosyst. Eng. 147 (2016) 226-237, https://doi.org/10.1016/j.biosystemseng.2016.03.007.

[22] M. Giroudon, C. Perez, M. Peyre Lavigne, B. Erable, C. Lors, C. Patapy, A. Bertron, Insights into the local interaction mechanisms between fermenting broken maize and various binder materials for anaerobic digester structures, J. Environ. Manag. 300 (2021) 113735, https://doi.org/10.1016/j.jenvman.2021.113735.

[23] C. Voegel, M. Giroudon, A. Bertron, C. Patapy, M. Peyre Lavigne, T. Verdier, B. Erable, Cementitious materials in biogas systems: biodeterioration mechanisms and kinetics in CEM I and CAC based materials, Cem. Concr. Res. 124 (2019) 105815, https://doi.org/10.1016/j.cemconres.2019.105815.

[24] AFNOR, FD P18-011. Concrete - Definition and classification of chemically aggressive environments - Recommendations for concrete mix design, Paris, France, 2022.

[25] AFNOR, NF EN 206 + A2/CN. Concrete - Specification, performance, production and conformity - National addition to the standard NF EN 206 + A2, Paris, France, 2021.

[26] G. Arliguie, H. Hornain, GranDuBé - Grandeurs associées à la durabilité des bétons, AFGC-AUGC, Presses des Ponts, 2007.

[27] M. Giroudon, M. Peyre Lavigne, C. Patapy, A. Bertron, Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: interactions and biodeterioration mechanisms, Sci. Total Environ. 750 (2021) 141518, https://doi.org/10.1016/j.scitotenv.2020.141518.

[28] C. Carde, R. François, J.-M. Torrenti, Leaching of both calcium hydroxide and C-S-H from cement paste: modeling the mechanical behavior, Cem. Concr. Res. 26 (1996) 1257-1268, https://doi.org/10.1016/0008-8846(96)00095-6.

[29] M. Vénuat, Relations entre la carbonatation du béton et les phénomènes de la corrosion des armatures du béton, Ann. L'ITBTP (1978) 42-47.

[30] L.G. Sequeda-Castañeda, A.E. Ortiz-Ardila, J.P. Correa-Cuadros, C. López-Pérez, Chemical and microbiological comparison of biodeterioration in Colombian heritage constructions, Univ. Sci. 18 (2013) 51-63.

[31] M. Rosenqvist, A. Bertron, K. Fridh, M. Hassanzadeh, Concrete alteration due to 55years of exposure to river water: chemical and mineralogical characterisation, Cem. Concr. Res. 92 (2017) 110-120, https://doi.org/10.1016/j.cemconres.2016.11.012.

[32] D.R.M. Brew, F.P. Glasser, Synthesis and characterisation of magnesium silicate hydrate gels, Cem. Concr. Res. 35 (2005) 85-98, https://doi.org/10.1016/j.cemconres.2004.06.022.

[33] M. Santhanam, Magnesium Attack of Cementitious Materials in Marine Environments. Perform. Cem.-Based Mater. Aggress. Aqueous Environ., Springer, Dordrecht, 2013, pp. 75-90, https://doi.org/10.1007/978-94-007-5413-3_3.

[34] A. Dauzères, G. Achiedo, D. Nied, E. Bernard, S. Alahrache, B. Lothenbach, Magnesium perturbation in low-pH concretes placed in clayey environment—solid characterizations and modeling, Cem. Concr. Res. 79 (2016) 137-150, https://doi.org/10.1016/j.cemconres.2015.09.002.

[35] A. Bertron, J. Duchesne, G. Escadeillas, Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect, Cem. Concr. Res. 35 (2005) 155-166, https://doi.org/10.1016/j.cemconres.2004.09.009.

[36] C. Grengg, F. Mittermayr, A. Baldermann, M.E. Böttcher, A. Leis, G. Koraimann, P. Grunert, M. Dietzel, Microbiologically induced concrete corrosion: a case study from a combined sewer network, Cem. Concr. Res. 77 (2015) 16-25, https://doi.org/10.1016/j.cemconres.2015.06.011.

[37] M. Peyre Lavigne, A. Bertron, C. Botanch, L. Auer, G. Hernandez-Raquet, A. Cockx, J.-N. Foussard, G. Escadeillas, E. Paul, Innovative approach to simulating the biodeterioration of industrial cementitious products in sewer environment. Part II: validation on CAC and BFSC linings, Cem. Concr. Res. 79 (2016) 409-418, https://doi.org/10.1016/j.cemconres.2015.10.002.

[38] C. Grengg, N. Ukrainczyk, G. Koraimann, B. Mueller, M. Dietzel, F. Mittermayr, Long-term in situ performance of geopolymer, calcium aluminate and Portland cement-based materials exposed to microbially induced acid corrosion, Cem. Concr. Res. 131 (2020) 106034, https://doi.org/10.1016/j.cemconres.2020.106034.

[39] M. Giroudon, C. Patapy, M. Peyre Lavigne, M. Andriamiandroso, R. Cartier, S. Dubos, C. Bacquié, L. André, S. Pommier, X. Lefebvre, M. Cyr, A. Bertron, Potential of low carbon materials facing biodeterioration in concrete biogas structures, Mater. Struct. 56 (2023) 80, https://doi.org/10.1617/s11527-023-02174-0.