# Effect of pre-crack on the degradation of cement pastes exposed to a low concentration external sulfate attack

Julie Pouya, Mejdi Neji, Laurent de Windt, Frédéric Péralès, Adrien Socié

► To cite this version:

Julie Pouya, Mejdi Neji, Laurent de Windt, Frédéric Péralès, Adrien Socié. Effect of pre-crack on the degradation of cement pastes exposed to a low concentration external sulfate attack. Construction and Building Materials, 2025, 496, pp.143818. 10.1016/j.conbuildmat.2025.143818. hal-05316078

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# Effect of pre-crack on the degradation of cement pastes exposed to a low concentration external sulfate attack

Julie Pouya<sup>a,b</sup>, Mejdi Neji<sup>a,*</sup>, Laurent De Windt<sup>b</sup>, Frédéric Perales<sup>c,□</sup>, Adrien Socié<sup>d,□</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Sulfate attack

Crack

CEM I paste

Ettringite

Gypsum

Diffusion

Fracture

## ABSTRACT

Pre-cracked cement pastes were exposed to a 30 mmol/L Na₂SO₄ solution to assess the influence of pre-existing cracks on degradation caused by a low concentration external sulfate attack. A global chemo-mechanical investigation was performed through SEM-EDS and microindentation analyses in the pre-cracked zone. These results were compared to analyses conducted in a distant zone from the pre-existing cracks to evaluate the effects of the pre-crack on the microstructure evolution. The chemical attack propagated in a characteristic V-shaped pattern around the pre-crack, indicating accelerated ion diffusion and localized degradation. Microtomography acquisitions were performed to monitor crack evolution during degradation. New cracks develop at the boundary between the decalcified and sound zones, with a higher density of cracks occurring near the initial pre-crack. The results suggest that the microcracks are caused by differential strain between the degraded and sound zones of the cement matrix, coalescing with the pre-crack and leading to its widening, promoting further decalcification, mineral precipitation and expansion.

## 1. Introduction

In the context of nuclear facilities, cement-based materials are frequently employed for construction and waste materials. The ageing of the containment building and cementitious materials used to manage nuclear safety is a key issue that is being studied in the frame of several international projects such as ODOBA [1] and VeRCoRS [2]. Long-term environmental exposure of cementitious materials induces both delayed deformation and cracking of concrete. Cracks have a significant impact on the transport properties of ions, as they create preferential pathways for ionic diffusion and the ingress of aggressive agents [3,4]. The study and modeling of cracking phenomena have been extensive due to their critical influence on the durability and performance of materials [4–6]. For instance, the impact of cracking on the diffusion of chloride ions in cementitious materials has been thoroughly investigated [4,7–10] and it was found that the opening of the crack is a key parameter influencing the local diffusion of chloride ions [10–12], as highlighted by the work of Djerbi et al. [13]. The results demonstrated that the wider the crack, the greater the diffusion is. The diffusion coefficient through the crack

was found to be independent of material parameters and became constant when the crack width exceeded approximately 80 µm, with the obtained value representing the diffusion coefficient in free solution. Apart from crack opening, self-healing can also influence the diffusion coefficient. Ismail et al. [10], found that at crack openings greater than the threshold value, diffusion along the crack path depends on mortar age.

These studies demonstrated the significant impact of cracks on ionic diffusion, which can also be applied to other ingressive agents such as sulfates, which can penetrate the material and lead to external sulfate attack (ESA). ESA is a degradation process caused by the penetration of sulfate ions into the interconnected porosity and their subsequent interaction with the initial hydrated components [14,15]. As a result of this reaction, ettringite and gypsum crystals precipitate within the pores of the material, leading to an increase in internal swelling pressure, expansion, cracking, spalling, and ultimately a reduction in material strength [16–18]. The formed cracks, in turn, serve as sites for ionic diffusion, contributing to their propagation. The experiments conducted on the influence of cracking on chloride diffusion provide results that

<footnote><sup>a</sup> Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSE-ENV/SPDR/LETIS, Fontenay-aux-Roses F-92260, France</footnote>

<footnote><sup>b</sup> Mines Paris, PSL University, Centre for Geosciences and Geoengineering, 35 rue Saint-Honoré, Fontainebleau 77300, France</footnote>

<footnote><sup>c</sup> Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSN-RES/SEMIA/LSMA, Saint-Paul-Lez-Durance F-13115, France</footnote>

<footnote><sup>d</sup> CEA, DES, IRESNE, DEC, SESC, Saint-Paul-Lez-Durance F-13108, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: juliepouya@yahoo.fr (J. Pouya), mejdi.neji@asnr.fr (M. Neji), laurent.de_windt@minesparis.psl.eu (L. De Windt), frederic.perales@asnr.fr (F. Perales), adrien.socie@cea.fr (A. Socié).</footnote>
can be applied to the study of the ESA. In fact, based on the experimental findings of Djerbi et al. [13], Idiart et al. [19] proposed a relationships to model diffusion within the crack in the case of external sulfate attacks, assuming a critical crack width of 100 µm. The work of Socié [20] and Socié et al. [3] builds upon the numerical method developed by Idiart et al. [19] and Segura et Carol [21] to study the influence of cracks on reactive transport and their consequences on swelling reaction. Moreover, as emphasized in studies on chloride diffusion, self-healing can also impact physicochemical processes such as mineral formation or mechanical processes, including the blocking of cracks by corrosion products or external particles [22,23]. Therefore, the new sulfate products formed within the crack as a result of sulfate diffusion into the crack can influence the diffusion coefficient. This may lead to a decrease in its value due to clogging or, conversely, a widening of the crack caused by the formation of sulfate products, thus increasing the diffusion coefficient.

Therefore, improving the understanding of the coupled effects between ESA and material cracking requires knowledge of the factors that significantly influence the species diffusion. The work conducted by Escadeillas et al. [24] focused on the influence of pre-existing microcracks within mortars made from Portland CEM I cement on the formation of delayed ettringite. They observed that the expansion measured in mortars that had undergone pre-damage was similar to that in sound mortars. However, they noted that degradation progressed more rapidly in the presence of microcracks. Consequently, they concluded that the pre-existence of cracks within the material resulted in an acceleration of the reaction. These observations were corroborated by the research of Leklou et al. [25]. Nevertheless, these tests concern delayed ettringite formation which is an internal sulfate reaction. The mechanisms behind these phenomena remain mostly unexplored, and to the best of our knowledge, there appear to be no documented experimental tests in the literature regarding the progression of ESA in a fractured environment.

Furthermore, despite extensive research, numerous questions persist regarding the contribution of gypsum on the expansion [26]. Traditionally, ettringite has been attributed to macroscopic expansion in concrete, primarily due to its substantial molar volume, resulting in significant swelling and material cracking. Nevertheless, ongoing debates persist due to conflicting observations and differing theories regarding the mechanisms of expansion. Some studies propose that gypsum precipitation could be even more detrimental than ettringite [27,28] and should be regarded as a contributing factor to degradation [29]. Nevertheless, the precise mechanism through which gypsum influences expansion remains uncertain, and discussions continue regarding its adverse effects in sulfate attacks.

The effects of ESA also depend on the sulfate concentration; a high sulfate concentration has the effect of accelerating the reaction rate, as well as generating a greater formation of sulfate products [30] particularly gypsum [31], thus impacting self-sealing of cracks. Usually, the test solutions contain much higher sulfate concentrations than those encountered by concrete in realistic conditions. These elevated sulfate levels are deliberately used to speed up the deterioration processes [18, 32], which would otherwise take an impractical amount of time to observe. However, this approach raises concerns about whether obtaining test results within a relatively short timeframe compromises the accuracy of the deterioration process compared to the actual service life of a structure. Sulfate ions can come from the waste itself or from groundwater in contact with the waste package structure, and generally, groundwater concentrations do not exceed 30 mmol/L [33–35].

The present paper aims at contributing to a better phenomenological understanding of the effect of pre-cracking on the ESA by conducting degradation tests on pre-cracked CEM I cement. The experimental campaign involves immersing cement pastes in a low-concentration Na₂SO₄ solution (30 mmol/L). This concentration was selected to accurately reflect concentrations found in groundwater and to allow for a precise study of the reaction process while avoiding a quick

<table><caption>**Table 1**

Composition of the CEM I pastes.</caption><thead><tr><th>Normative phase composition (g/100 g)</th><th></th></tr></thead><tbody><tr><td>C<sub>3</sub>A</td><td>11</td></tr><tr><td>C<sub>3</sub>S</td><td>66</td></tr><tr><td>C<sub>2</sub>S</td><td>10</td></tr><tr><td>C<sub>4</sub>AF</td><td>8</td></tr><tr><td>CaCO<sub>3</sub></td><td>4</td></tr><tr><td>Water</td><td></td></tr><tr><td>W/C (mass)</td><td>0.5</td></tr></tbody></table>

degradation of the sample [36]. The chemical and mechanical degradation is assessed through SEM-EDS, microtomography and microindentation analysis. The chemical and mechanical degradation of these samples was the subject of a previous publication [36], in which the investigation focused on parts far from the influence of the pre-crack. The results from experimental tests combined with numerical simulations provided valuable insights into the impact of gypsum and ettringite on cracking and expansion mechanisms. Building on these results, the present study focuses on the phenomena in the pre-cracked zone to better understand the influence of the pre-crack on the observed chemical and mechanical degradation. The progression of degradation is examined at the microstructure level, followed by an investigation at the macrostructure level.

## 2. Materials and methods

### 2.1. Pre-cracked cement pastes preparation and exposure conditions

#### 2.1.1. Materials

The tests involve hardened cement pastes with a water-to-cement ratio (W/C) of 0.5. A CEM I formulation with a C₃A content of 11 % was used, specifically the Beffes CEM I 52.5 N CE CP2 NF cement. The cement composition is given in Table 1. The samples had a cylindrical shape, with a diameter of 3 cm and a thickness of 1 cm. The cylindrical shape is the least sensitive to edge effects and represents the simplest configuration to analyze, as the primary objective of this study is to provide insights into the experimentally observed phenomena while minimizing the number of influencing factors. The samples were cured in sealed bags at a constant temperature (20 °C) for 28 days. A resin-epoxy Resoltech 3030 was applied to the sample's perimeter to enable unidirectional diffusion, leaving only the two largest parallel faces exposed to the external environment (Fig. 1). The samples were intentionally pre-cracked using the protocol explained in the subsequent section. Each cement paste contains two through cracks at its center, each with a width of approximately 150–300 µm.

#### 2.1.2. Protocol used to pre-crack the samples

The method was splitting-induced cracking and was chosen for its ease of implementation and the resulting crack morphology, characterized by through-cracks with relatively uniform widths, typically ranging between 100 and 300 µm. The tests were conducted on cylindrical cement specimens measuring 10 cm in length and 3 cm in diameter. Several metal mesh reinforcements were strategically positioned at three different heights within the specimens to maintain contact between the two cracked faces.

During the crack splitting test, the cylindrical sample was placed horizontally between the press plates, and the load was incrementally increased until indirect tensile failure occurred, resulting in cracking along the vertical diameter of the cylinder.

Subsequently, the cracked cylindrical cement paste was coated with Resoltech 3030 resin. Finally, to prevent the separation of the two cracked faces during sectioning, the specimens were embedded in epoxy resin, allowing to produce 1 cm thick samples. Once the resin had hardened, the sections were cut (Fig. 1).
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Protocol used to pre-crack the samples which involved a crack splitting test, resulting in two through cracks. Cracked samples were then coated with Resoltech 3030 resin and embedded in epoxy resin to prevent separation of the cracked faces during sectioning, resulting in 1 cm thick samples.</figcaption></figure>

Five cylindrical samples were immersed in a 60 L tank at ambient temperature of 20°C. The tanks were filled with a $30.10^{-3}$ mol/L $Na_2SO_4$ solution. Sufficient solution volume was used to ensure a consistent chemical composition throughout the duration of the experiment. The solution was stirred throughout the entire experiment using a magnetic stirrer placed under the tank. Nitric acid was added manually to maintain the pH between 7 and 9. Prior to sealing the tanks, nitrogen gas was injected to prevent carbonation. Sample collection took place at 0.5, 2, 4, and 6 months for analysis.

## 2.2. Experimental characterization techniques

### 2.2.1. Scanning electron microscopy

The scanning electron microscopy (SEM) was used to determine the chemical element distribution in the sample during the sulfate attack. Both sound and degraded zones were analyzed with a Hitachi S3500N SEM with two Bruker SDD detectors for energy-dispersive spectrometry (EDS). Prior to analysis, the samples were embedded in epoxy resin and then prepared as polished thin sections, which were subsequently coated with a layer of carbon. The analyses were performed under a voltage of 15 keV at a working distance of 16.7 mm. The SEM-Beam interaction pear had an approximate depth of 3 µm, while the spot size for EDS measurement was about 1.5 µm. The software ESPRIT (Bruker) was used for spectrum and EDS map acquisition. Hyperspectral images were captured at a rate of 8 s per pixel. The identification of gypsum and ettringite from the EDS elemental maps was enabled by using their spectra [37].

### 2.2.2. X-ray microtomography

A Bruker SkyScan 1272 computer tomography scanner was used, operating at 100 keV and 100 µA. The acquisitions were conducted with a 360° rotation and a step size of 0.1°, frame averaging of 16 with a voxel size of 4 µm. After data reconstruction, a median filter with a radius of 2 was applied. A K-means deconvolution [38] was then performed to threshold the reconstructed 3D data and distinguish areas of different densities within the sample.

Additionally, a Bruker Skyscan 1173 microtomograph was employed for the analysis of larger-sized samples. The operating parameters were set at 130 keV and 61 µA, with a voxel size of 23.1 µm. The acquisitions were performed with a 360° rotation and a step size of 0.1°.

The analyses were conducted immediately after collecting the sample from the solution, without any drying or resin impregnation.

To estimate the crack opening, the Euclidean Distance Transform (EDT) was computed on the binarized crack phase using the *scipy.ndimage* module in Python. This method provides a voxel-wise measure of the minimum distance to the nearest boundary, which can be interpreted as a local crack aperture [39].

### 2.2.3. Microindentation

Microindentation analysis was conducted using the Anton Paar NHT3 Nanoindenter, which was equipped with a Berkovich diamond indenter and connected to a Step 700 stabilization platform. The objective was to assess the evolution of the Young's modulus within the degraded paste at various depths. Measurements were performed on the same polished sections prepared for SEM-EDS analysis. The indenter was vertically pressed onto the sample surface, and its displacement and corresponding load were recorded. The corresponding curve displaying displacement versus load was interpreted to calculate the elastic modulus of indented zones. The elastic modulus values of the material at the indented zone were estimated via Oliver and Phar approach [40]. Adapted indentation parameters were chosen in a way to consider the roughness differences between the sound and degraded zones as these variations could affect the reliability of the elastic modulus values.

The load was first raised from 0 to 100 mN with a constant loading rate of 100 mN/min, then maintained constant for 10 sec at the maximum value to limit creep phenomena and, finally, decreased to 0 with an unloading rate of 100 mN/min. The acquisition rate was set at 15 Hz, with an approach distance of 8500 nm and a speed of 24000 nm/min. To ensure a statistically significant analysis of the measured mechanical values, an 8 × 8 grid of indentation points was employed, with a spacing of 300 µm between each point. This grid configuration followed the recommendation by Constantinides et al. [41], which suggests
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Polished cross-section of the sample degraded for 15 days. Elemental maps of Ca and S, along with SEM-EDS images taken from the pre-cracked zone and from a zone distant from the pre-crack.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Polished cross-section of the sample degraded for 4 months; elemental maps of Ca, S, and images resulting from SEM-EDS analyses conducted in the pre-cracked zone.</figcaption></figure>
maintaining a distance between points at least 10 times greater than the maximum displacement before unloading to prevent interaction between neighboring points.

## 3. Results and discussion

### 3.1. Impact on the chemical and mechanical degradation of cement paste at the microstructure level

#### 3.1.1. After 15 days

Fig. 2 shows the polished cross-section of the C₃A-rich sample after 15 days of immersion in the Na₂SO₄ solution. In the polished cross-section, the leached zone appears clearly as a straight band parallel to the exposed surface, lighter than the other part of the sample. SEM-EDS analyses were conducted in two zones: the first around the pre-crack and the second in a zone further away, outside the influence of this pre-crack. These two zones are outlined in different colors in Fig. 2. In the non-pre-cracked zone, the elemental map of Ca highlights a decalcification depth of approximately 500 µm, evidenced by a noticeable reduction in Ca signal intensity. The elemental map of S indicates a highly enriched zone, which presents as a band parallel to the exposed surface. Previous publications on sulfates attacks of cement pastes [36, 37] demonstrated that this S-rich band was characteristic of gypsum formation, as confirmed by XRD analyses. The SEM-EDS image shows an increase in porosity in the decalcified part of the sample, visible as darker and more irregular along the decalcification front, consistent with the dissolution of calcium-bearing phases.

The analyses conducted in the pre-cracked zone reveal a crack exhibiting a relatively constant opening of approximately 150 µm. This crack is transverse and perpendicular to the attacked surface. The degradation observed in this zone differs from that observed in the non-pre-cracked section. Notably, the chemical fronts no longer advance linearly in the direction of diffusion. Instead, both the decalcification and the sulfate-enriched zones expand laterally around the crack, with a maximum decalcification depth of about 1800 µm, and an S-enriched zone extending up to 900 µm. These values are substantially greater than those observed in zones further away from the crack, where the depths of decalcification and gypsum precipitation are 500 µm and 300 µm, respectively, and are similar to the ones in the non-pre-cracked zone.

The crack's presence increases the chemical degradation depth in the zone around it. The morphology of the degradation front adopts a "V" shape, suggesting a preferential pathway for ion diffusion within the crack and, subsequently, diffusive exchanges with the sound cement matrix along the crack. This shape has also been measured and simulated by Kato et al. [42] for chloride ion penetration experiments in the cement matrix of concrete (W/C = 0.55). Their calculations considered a higher apparent diffusion coefficient within the crack compared to that in the matrix.

Additionally, the SEM image reveals an increase in porosity along the decalcification front, visible as darker regions with irregular texture and lower electron backscatter signal. These features correspond to voids or pore clusters formed as a result of calcium leaching, leading to a loss of matrix density and contrast in the affected zone. In this same part, numerous small cracks are observed, revealing the weakening of the cement paste in the degraded zone. However, this microcracking is not observed in the non-pre-cracked zone. Altogether, the SEM-EDS images provide evidence of the influence of the pre-crack on degradation mechanisms. They show how the presence of a crack facilitates deeper sulfate ingress, amplifies decalcification and gypsum formation and induces additional microcracking.

#### 3.1.2. After 4 months

After 4 months of exposure, the leached zone is significantly affected by the pre-crack, which becomes difficult to distinguish from the network of newly formed cracks surrounding it in the polished cross-

<caption>**Table 2**</caption>

Maximum depths of decalcification and S-enriched zones and gypsum band thickness measured by SEM-EDS at locations away from and at the pre-crack after 15 days and 4 months of exposure. See the arrows shown in Fig. 2, which indicate as an example the location of the measurement zones for the maximum depth of decalcification.

<table><thead><tr><th>Parameter</th><th>Location</th><th>15 days (µm)</th><th>4 months (µm)</th><th>Observation</th></tr></thead><tbody><tr><td rowspan="2">Maximum depth of decalcification</td><td>Away from pre-crack</td><td>500</td><td>1400</td><td>Linear front profile</td></tr><tr><td>At the pre-crack</td><td>1800</td><td>4500</td><td>V-shaped front; maximum depth reported</td></tr><tr><td rowspan="2">Maximum depth of S-enriched zone (gypsum)</td><td>Away from pre-crack</td><td>300</td><td>1300</td><td>Linear front profile</td></tr><tr><td>At the pre-crack</td><td>900</td><td>2200</td><td>V-shaped front; maximum depth reported</td></tr><tr><td rowspan="2">Thickness of gypsum band</td><td>Away from pre-crack</td><td>150</td><td>500</td><td>Measured as a continuous layer</td></tr><tr><td>At the pre-crack</td><td>Cannot be precisely measured</td><td>800</td><td>Localized thickening near crack</td></tr></tbody></table>

section (Fig. 3). The SEM-EDS analyses conducted in this zone display the pre-crack which has an opening of approximately 250 µm at the center of the sample and becomes narrower near the exposed surfaces (Fig. 3). Since the initial crack's opening in this zone is unknown, it is not possible to accurately characterize its evolution since the beginning of the attack. The elemental map of Ca shows the propagation of the decalcification front from the attack surface toward the center of the cement paste, which is faster near the pre-crack. This propagation occurs symmetrically on both sides of the sample, extending up to 4500 µm from the exposed surface. Several levels of decalcification can be identified, especially near the crack edges, where portions of the matrix appear delaminated or detached, suggesting local mechanical weakening or flaking.

The S elemental map reveals some parts with considerable S content, corresponding to gypsum precipitation. Beyond the influence zone of the pre-crack, the gypsum band exhibits a thickness of around 400 µm (Fig. 3). However, near the pre-crack, this band expands significantly, reaching up to 800 µm. This localized thickening, already observed after 15 days of exposure, is even more pronounced in this case, highlighting the accelerating effect of the crack on sulfate ingress and mineral precipitation.

The secondary electron image highlights the initiation of cracks in the matrix, following the gypsum precipitation front. The initiation of these cracks in the gypsum zone, at the interface between the decalcified and non-decalcified part, has also been observed in other studies [36,37, 43]. They propagate toward the center of the sample near the pre-crack, following the gypsum formation front, which suggests that significant stress associated with gypsum precipitation contributes to crack development in the matrix.

Microcracks originating from the main crack and following the decalcification front possess openings of a few tens of micrometers. These may have been created during sample preparation; however, a significant S enrichment is observed in one of them (around 2000 µm deep), corresponding to the precipitation of a sulfate mineral in the crack. This suggests the crack may have formed during degradation or pre-existed in the material. Similar sulfate deposits are also observed inside the pre-crack, confirming its role as a preferential pathway for aggressive ions. This point will be addressed in Section 3.2.

Overall, the SEM-EDS analyses reveal extensive degradation after 4 months of immersion, characterized by the deep progression of chemical
<figure><img src="image_7.png" /><figcaption>Fig. 4. Polished cross-section of the cement paste degraded for 6 months; elemental maps of Ca, S, and pictures resulting from SEM-EDS analyses, along with a distribution map of the Young's modulus measured by microindentation in the pre-cracked zone.</figcaption></figure>

fronts, the development of microcracking within the matrix, and the presence of sulfate reaction products in both the pre-existing and newly formed cracks. These results highlight the combined effects of chemical degradation and mechanical damage in sulfate-exposed cracked cement paste.

Table 2 summarizes the degradation depths measured after 15 days and 4 months of exposure, comparing the results obtained in the pre-cracked zone with those from the zone located away from the pre-crack. It includes the maximum depths of decalcification and S-enriched zones, as well as the thicknesses of the gypsum band. The S-enriched band detected by SEM-EDS corresponds to gypsum precipitation according to data from [36,37].

### 3.1.3. After 6 months

The sample is heavily damaged. The leached zone has detached from the cement paste, with only a small part remaining on the lower and upper edges of the sample, observable on the polished section in Fig. 4. An increase in the pre-crack opening is noted, measuring several millimeters after 6 months of degradation. Visible to the naked eye, the chemical degradation fronts form arc-shaped patterns surrounding the initial crack. The pre-crack appears to be filled; however, it could be the result of debris from sample polishing that became embedded during preparation. SEM-EDS acquisitions and microindentation were performed under the lower edge of the pre-crack (outlined in dark yellow, Fig. 4).

The elemental maps of Ca and S reveal that the decalcification and gypsum precipitation fronts progress inward from the exposed surfaces toward the core of the sample, converging near the center.

The whole cement paste is decalcified, except for a small zone near the sample center that remains relatively sound. Additionally, the sample exhibits numerous microcracks that are sites of delamination, indicating an overall loss of material cohesion.

Notably, cracks appear to initiate again on the gypsum precipitation front located upstream and downstream of it. This phenomenon has been previously studied in the sound zone of the sample, as documented in a prior publication [36]. These cracks tend to occur at locations where gypsum precipitation had occurred earlier. The mechanisms of cracking and expansion were investigated in a previous study [36], which focused on the same samples but in zones located away from the influence of the pre-crack. In that study, crack initiation was observed in the region where gypsum, ettringite, and portlandite coexist, specifically along the gypsum-rich band. Experimental results combined with numerical simulations led to the formulation of a hypothesis suggesting that cracking occurs in this zone due to the limited available porosity (as portlandite was not fully dissolved) and the accumulated volume of precipitated gypsum and ettringite. A similar cracking pattern, following the gypsum band, is observed in the pre-cracked zone. Therefore, the same hypothesis is proposed here to explain the formation of this crack.

Microindentation testing was carried out on regions surrounding the pre-crack to evaluate changes in local mechanical properties induced by chemical degradation and associated microcracking. The Young's modulus values obtained show a significant reduction compared to sound material, with measurements generally below 5 GPa in the most degraded zones. This pronounced decrease in stiffness is attributed to the loss of portlandite, which is entirely dissolved, and to the C-S-H phases, which are present in a highly decalcified form. These two mineral phases are primarily responsible for the mechanical strength of the cement matrix. Their substantial degradation leads to a severe weakening of the material, resulting in a marked loss of cohesion and disintegration of the surface layer (this deterioration with extensive desquamation of the degraded sample is further discussed in Section 3.3.2).

In zones with less pronounced decalcification, particularly toward the sample center, the modulus increases to values between 10 and
<figure><img src="image_8.png" /><figcaption>Fig. 5. SEM-EDS picture of a pre-cracked and degraded cement paste exposed to a Na₂SO₄ solution for 4 months. Detection of ettringite and gypsum using their spectra given in Pouya et al. [37].</figcaption></figure>

15 GPa but still remains well below the approximately 20 GPa measured in intact, unaffected regions of the same specimen [36,37]. This gradient in stiffness values highlights the spatial variability of damage intensity. The low modulus values suggest the presence of microcracks within the matrix, which, while often below the resolution of imaging techniques, nonetheless lead to substantial weakening of the material. These microcracks likely originate from the propagation of cracks emanating from the main pre-crack and evolve progressively with exposure time (as confirmed by observations in the sample degraded for 4 months, see Section 3.3.1).

Globally, microindentation provides critical quantitative evidence of the mechanical degradation occurring at the microscale, supplementing microstructural data obtained via SEM and X-ray microtomography. These results reinforce the hypothesis that chemical degradation not only modifies the material chemistry but also significantly compromises its mechanical integrity through the nucleation and growth of microcracks, ultimately leading to the observed macroscopic damage.

Thus, after 6 months of immersion in sulfate-rich water, the cement paste is significantly altered. The progression of the decalcification and gypsum precipitation fronts is much more pronounced near the main crack compared to samples degraded for 15 days and 4 months. However, in all cases, the exact location of secondary ettringite is difficult to identify. Indeed, its presence, confirmed by X-ray diffraction analyses conducted in the non-pre-cracked zone [36], cannot be solely determined from the elemental maps of Ca, S and Al obtained by SEM-EDS (no zone enriched in Al is evident). This issue will be addressed in the following subsection, and the mineral precipitation identified in the pre-crack of the sample degraded for 4 months will be studied.

## 3.2. Mineral precipitation in the crack

### 3.2.1. Chemical composition analysis

Fig. 5a displays the SE image from SEM-EDS acquisitions and Fig. 5b the post-processing results which were carried out from the SEM-EDS analysis to identify the sulfate phases. The ettringite and gypsum spectrum provided in Pouya et al. [37] are used to identify pixels containing these minerals from the SEM-EDS results in this zone since the conditions for analysis were the same.

The SE image highlights increased porosity associated with decalcification along the crack. Gypsum formation is detected within the crack,

representing an extension of gypsum precipitation observed in the surrounding matrix. In fact, gypsum formation within the crack begins at around 1600 aligning with the onset of the precipitation front in the surrounding matrix and extends up to around 3600 µm. Over this 2000 µm span, the mineral fully fills the crack, which has an average width of about 250 µm. Additionally, several microcracks branching from the main crack are observed, although their exact origin remains unclear. However, it is worth noting that they primarily occur in regions of mineral precipitation within the crack.

The extensive formation of ettringite along the crack edges and gypsum precipitation within the crack are well confirmed. These findings provide new insights into the secondary ettringite formation: the mineral is clearly identified in the matrix between 300 µm and 3400 µm in depth. According to the sulfur elemental map (see Fig. S1 in Supplementary information), no detectable S signal is observed from 0 to 300 µm according to the S elemental map confirming the absence of ettringite crystals larger than 3 µm in this zone. Furthermore, ettringite formation appears to be limited to the decalcified zone and is not detected beyond the boundary with the sound matrix.

The formation of sulfate minerals in the pre-crack results from the reaction of SO₄²⁻ ions diffusing into the crack with the solid phase on the crack surface. The large size of gypsum crystals indicates a high sulfate concentration in the pre-crack, which supports crystal growth through Ca²⁺ ions supplied by matrix decalcification. The formation of ettringite is constrained by the limited availability of Al³⁺, potentially explaining its occurrence in a smaller proportion within the crack compared to gypsum. Furthermore, at the thermodynamic equilibrium, the ion concentration of aluminum hydroxide is lower (10⁻⁶ mol/L) than that of calcium (around 10⁻² mol/L or 10⁻³ mol/L) which reduces the diffusion gradient. Finally, as depicted in Socié et al. [3], in the crack area, portlandite dissolves first, followed by the aluminate solid phases. This sequence lowers the aluminum concentration and promotes gypsum precipitation within the crack.

Moreover, the extensive precipitation of ettringite at the edges of the crack evokes findings observed in tests conducted within confined environments [37], in which substantial ettringite formation was detected at the exposed surface in a high-pH environment (pH 12). Since the crack chemistry is influenced by the cement matrix, the solution inside the crack likely has a high pH, effectively making the crack a confined chemical environment. This elevated pH thus promotes ettringite
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Ettringite formed in the degraded zone of the cement paste (a) and ettringite crystals formed on the surface of the newly formed fractures (b). Results of SEM acquisitions on recently developed fractures.</figcaption></figure>

precipitation on crack edges. Nonetheless, it remains difficult to quantify the formation of ettringite and confirming its relative abundance compared to the rest of the cement paste.

Finally, the results of this work are consistent with the observations of El-Hachem et al. [44] where gypsum was found preferentially in cracks and ITZ while ettringite was found deeper in the material and inside the capillary pore.

### 3.2.2. Morphology of ettringite

Another proposed hypothesis concerning this observation is linked to the morphology of ettringite, suggesting that the formation of large crystals at the surface is a result of the high porosity in this zone. Indeed, the morphology of ettringite depends on the available space for its precipitation [45,46]. To further support this hypothesis, SEM analyses were conducted on fresh fractures made in the matrix of the sample, compared to those conducted at the crack edges (Fig. 6).

The ettringite formed on the surface of the crack exhibits a highly crystalline structure, with clearly discernible needles measuring from 10 to 30 µm in length (Fig. 6b). In contrast, ettringite within the matrix is scarcely visible, given its smaller size, and seems to blend with the matrix (Fig. 6a). Its shape is less defined compared to the ettringite developing within the crack and measures only a few micrometers at most. These crystalline ettringites are likely too small to be detected by the SEM used, as recently formed within the hardened paste, encapsulated by C-S-H gel, may lack sufficient space to grow freely [45,47]. The detection of secondary ettringite in the decalcified zone (Fig. 5b) is consistent with the presence of larger crystals growing in zones of higher porosity, due to the strong decalcification of this area.

The precipitation of ettringite as spherical clusters along the edges of the crack is clearly evident (Fig. 6b). This specific morphology aligns with observations reported in the study conducted by Tosun and Baradan [46]: they investigated the impact of ettringite formation location and morphology on expansion, using SEM secondary electron images of fractured mortar surfaces. They identified two different morphologies of secondary ettringite: the first one, well crystallized and detectable,

forms in a "bal" shape primarily within larger pores corresponding to air voids. The diameter of the largest and most mature ball-shaped ettringite observed is about 5–10 µm. They referred to the second form as "massive" describing it as less crystalline and smaller in size than the first morphology, typically forming within the smaller pores of the cement paste. They explained that ettringite formation initially occurs as "ball-shaped" crystals, regardless of the location. These spherical ettringite clusters remain stable when formed in large pores and do not cause expansion until the pore becomes saturated. In contrast, they observed the conversion of "ball-shaped" ettringite into a massive form within confined spaces (particularly in microcracks induced by shrinkage) due to limited available space. This massive ettringite formation was considered as the main cause of expansion in the mortars. Therefore, they highlighted that ettringite morphology is an important factor influencing its expansive effect, supporting the conclusions of Yan et al. [45], which stated that the greatest expansion occurs only with the later development of abundant microcrystalline ettringite within the dense, hardened paste under water-saturated conditions.

Thus, based on these studies, well-crystallized ettringite forming at the crack edges may only have an expansive effect when they completely fill the crack, leaving no additional space for further precipitation and converting into massive and expansive ettringite.

### 3.3. Crack propagation and macroscopic degradation

#### 3.3.1. Crack propagation

The analysis of pre-cracked samples was conducted at the macroscopic scale, offering insights into the observed macroscopic expansion of the sample after 6 months of degradation. X-ray microtomography was used to characterize the cement paste before and after 4 months of immersion in the Na₂SO₄ solution. The scans were performed immediately after sampling to avoid artifacts introduced during sample preparation. Fig. 7 and Fig. 8 present the results of these analyses.

The sound sample exhibits two parallel through-going cracks spaced approximately 2 cm apart, each with an opening of around 125 µm.
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** $Ca_2SO_4$ rich cement paste (a) before and (b) after 4 months of degradation in $Na_2SO_4$ solution, analyzed by X-ray microtomography (voxel size: 23 µm). Panels (a1) and (b1) show representative cross-sectional images, while (a2) and (b2) display 3D renderings of macroscopic cracks in both samples. Panel (c) presents the histogram of crack opening distributions before and after degradation.</figcaption></figure>

These two pre-existing cracks remain visible in the sample after degradation. However, a new crack appears within one of the original cracks, extending from the sample edge and joining the pre-crack at its center. This new crack highlights progressive mechanical damage induced by chemical degradation. Additionally, the widening of the second pre-crack is noticeable after the degradation, although it is difficult to quantify at the given resolution. Crack initiation and propagation within the swelling zone have been observed experimentally and numerically by several authors [15,43,44,48]. Based on the numerical analysis of Bary et al. [15] and Socié et al. [48] crack initiates due to the differential strain between the degraded zone and the internal sound zone. Then, due to structural deformation, cracks propagate toward the central part of the sample. In this case, the minimum energy required for crack propagation is reached in the region of the initial crack, which promotes crack coalescence. This explains why the distribution of crack openings (Fig. 7c) shifts from an initial value around 125 µm to higher values between 200 and 300 µm. In parallel, mineral precipitation within the cracks is evidenced in the µCT analysis of Fig. 7b, where the initial

pre-crack appears progressively filled.

This precipitation would most likely correspond to the formation of gypsum, based on the volume it occupies within the crack as revealed by SEM-EDS analyses (Fig. 3 and Fig. 5b). The microtomography analyses were performed with a resolution of approximately 23 µm, allowing visualization of cracks and precipitates down to this scale, but limiting the detection of finer microcracks or nanoscale crystals.

To provide a more detailed understanding of the evolution of these pre-existing cracks, a section of the original sample was extracted at the crack site and analyzed using microtomography. By reducing the sample size and employing a more advanced X-ray microtomography. The resolution was improved from 23 to 3 µm. As a result, new cracks were detected near the pre-crack zone (Fig. 8c). These microcracks appear to develop almost parallel to the pre-crack forming an interconnected network. Notably, one microcrack extends toward the outer part of the sample. This crack network spans approximately 2 mm from the pre-crack, highlighting localized damage propagation.

Fig. 8a,b shows cross-sectional images of the degraded sample
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Results of X-ray microtomography analyses conducted on a portion of the C₃A-rich cement paste degraded for 4 months. (a) Representative cross-sectional images; (b) magnified view of the pre-crack; (c) further magnification of the pre-crack border with crack opening values represented using a plasma colormap and highlighting 3D renderings of (d) micro-crack in yellow and (e) macro-porosity in cyan.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Picture of the cement paste degraded for 6 months and 3D reconstruction of the sample after its characterization using X-ray microtomography.</figcaption></figure>

acquired at a voxel size of 23 µm, revealing the presence of new cracks parallel to the pre-crack on both sides. The higher-resolution analysis performed at a voxel size of 3 µm (Fig. 8c) allows estimation of the crack openings, ranging from 20 to 100 µm after 4 months. Fig. 8c,d further indicates in 3D rendering that cracks with openings of about 40 µm tend to connect these parallel cracks corresponding to a coalescence of cracks. A similar mechanism seems to occur between the main pre-crack and the newly formed cracks, as suggested by Fig. 8a,b. The high porosity observed in these regions (Fig. 8c,e) highlights the influence of decalcification processes, as also evidenced by SEM-EDS analyses (Fig. 3). The formation of these new cracks, combined with the increased porosity, facilitated ionic diffusion within the material, thereby promoting the propagation of decalcification and further porosity development.

### 3.3.2. Macroscopic expansion of the cement paste

After 6 months of exposure to sulfates, the cement paste shows significant expansion in the crack zones (Fig. 9). This expansion is

substantial enough to break the part composed solely of resin. At the original pre-crack locations, where the initial openings measured several hundred micrometers, a void band measuring 2–5 mm is evident in the 3D microtomography image. This observation indicates a significant expansion and widening of the pre-cracks. The most severely degraded zone lies between the two pre-cracks, where significant delamination of the specimen occurs. The two main cracks are interconnected by smaller secondary cracks located in the center of the sample. Additional cracks are also visible on the outer edge of the cement paste. These secondary cracks joining the pre-crack can already be discerned in Fig. 7b1 and in Fig. 8a.

Thus, as observed after 4 months of exposure to sulfates (Fig. 7), the most degraded zones of the cement paste are located at pre-crack sites. It can be inferred that the network of new cracks, initially observed near the pre-crack after 4 months, has evolved around it, ultimately contributing to the observed extension after 6 months. However, the precise origin of these newly formed cracks remains uncertain. One possible explanation lies in the cracking observed at the interface
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Diagram illustrating the hypothesized mechanisms of external sulfate attack in a pre-cracked environment and the relationship between the degradations observed at the microscopic and macroscopic scales. **15 days:** The degraded zone is characterized by decalcification, ettringite precipitation and gypsum accumulation along the decalcification front. While degradation progresses linearly away from the pre-crack, it adopts a V-shaped pattern near the pre-crack, following the crack edges and indicating enhanced ion diffusion. An increase in porosity is observed along the decalcification front. **4 months:** New cracks form at the interface between sound and degraded zones where portlandite, gypsum, and ettringite coexist, as degradation progresses. These cracks multiply over time, coalesce with the pre-crack and become visible at the macroscopic scale. Sulfate phases also precipitate within the pre-crack. **6 months:** The main crack widens as numerous secondary cracks progressively form along the degradation fronts and coalesce with it. Surface flaking is observed and millimeter-wide crack openings are visible to the naked eye.</figcaption></figure>
between sound and degraded zones, as revealed by SEM-EDS analyses at 4 months. (Fig. 3). Indeed, these interface cracks, located near the gypsum front, suggest localized swelling of the material in the degraded zone. They tend to follow the fronts of chemical degradation fronts which are accelerated at the pre-crack and eventually merge with the initial crack. Over time, this mechanism may have contributed to the continued widening of the pre-crack.

Additionally, the advancement of the decalcification front results in a highly porous region near the pre-crack (Fig. 8b), promoting enhanced ionic diffusion which in turn accelerates both decalcification and the precipitation of ettringite and gypsum during the degradation process. These combined phenomena likely contribute to the development of microcracks around the pre-crack. Furthermore, decalcification is expected to reduce the local elasticity and tenacity of the material, which have an impact on crack propagation and material swelling. Indeed, the macroscopic fracture energy and ultimate stress (tensile strength) of a pre-cracked beam decrease due to calcium leaching and above all due to the effect of the crack on the location of the degradation at the pre-cracked area [49,50]. Moreover, the work of Socié et al. [51] on carbonation highlights that crack propagation localizes and accelerates the chemical degradation which induces a heterogeneous degradation impacting the elasticity of the material and the strain. These combined effects must amplify the macroscopic strain.

Finally, the mineral precipitation within the crack may also play a role in crack propagation. The filling of cracks by gypsum and ettringite could impose internal stresses and mechanical constraints, potentially accelerating crack opening and propagation. Nevertheless, these hypotheses cannot be conclusively validated solely based on the presented observations.

A diagram is presented in Fig. 10 summarizing the hypothesized mechanisms of the reaction based on experimental data presented in this paper, illustrating the coupling between cement phase decalcification, mineral precipitation and cracks.

## 4. Conclusion

This study offers new insights into the impact of pre-existing cracks on degradation mechanisms in cement paste subjected to external sulfate attack. Through a combination of multiscale experimental analyses, the investigation highlights how the presence of a pre-crack accelerates ion diffusion, modifies degradation front morphology and promotes localized microcracking and expansion around the cracked zone.

In particular, the development of a microcrack network was observed surrounding the pre-crack, demonstrating progressive coalescence with the main fissure, leading to its widening. This coalescence also significantly influences local microporosity, which is notably increased in this region. Hypotheses have been proposed regarding the microscale mechanisms driving these phenomena and their relationship to macroscopic degradation patterns.

Nevertheless, several limitations of the experimental approach must be acknowledged. The use of simplified cylindrical specimens, while deliberately chosen to minimize edge effects and facilitate analysis, does not fully replicate the complex crack geometries and stress states found in real concrete structures. Furthermore, some characterization techniques employed (such as SEM-EDS and microtomography) are essentially qualitative, limiting the accuracy in assessing mineral concentration and porosity. Finally, degradation monitoring was conducted through destructive sampling at predetermined intervals on different specimens, which introduces uncertainty in the exact timing and progression of cracking due to the material's inherent heterogeneity.

Future work should focus on developing coupled chemoporemechanical models to quantitatively link microstructural damage with macroscopic behavior and to clarify the long-term effects of mineral precipitation within cracks.

## CRediT authorship contribution statement

**Frédéric Perales:** Writing – review & editing, Supervision. **Adrien Socié:** Writing – review & editing, Supervision. **NEJI Mejdi:** Writing – review & editing, Supervision, Data curation. **Laurent De Windt:** Writing – review & editing, Supervision, Data curation. **Julie Pouya:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

## 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.

## Acknowledgements

We gratefully acknowledge Patrick le Bescop, now retired from CEA (France), for fruitful discussions, as well as three anonymous reviewers for their relevant comments.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.conbuildmat.2025.143818.

## Data availability

Data will be made available on request.

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