# Evaluating the effect of slag variability on the properties of slag-based cement grouts

Faten Souayfan, Emmanuel Roziere, Manon Michaut, Christophe Justino

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Faten Souayfan, Emmanuel Roziere, Manon Michaut, Christophe Justino. Evaluating the effect of slag variability on the properties of slag-based cement grouts. Case Studies in Construction Materials, 2024, 21, pp.e04070. 10.1016/j.cscm.2024.e04070. hal-05092910

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# Evaluating the effect of slag variability on the properties of slag-based cement grouts

Faten Souayfan<sup>a,b</sup>, Emmanuel Roziere<sup>a,*</sup>, Manon Michaut<sup>a</sup>, Christophe Justino<sup>b</sup>

## ARTICLE INFO

**Keywords:**

Grout

Slag

Reactivity

Chemical composition

Water-to-binder ratio

Hydraulic conductivity

## ABSTRACT

Slag cements are used to design grouts with high water-to-cement ratio and low permeability, to improve the physical and mechanical properties of permeable soils. Because of the variable physico-chemical properties of slag, it can be challenging to predict their behavior. Most research on slag reactivity involves low water-to-binder ratios in concrete and strength. This study investigates grouts with higher water-to-binder ratio of 6, for water resistant barriers executed using soilmixing or self-hardening slurry cut-off wall techniques, where the main engineering property is the hydraulic conductivity. The effect of slag composition on grout properties was studied at two water-to-binder ratios (0.5 and 6) and varying slag proportions, focusing on compressive strength and hydraulic conductivity. The results showed that higher water-to-binder ratios influence hydration, phase formation, and pore distribution, preventing the formation of portlandite. Isothermal calorimetry revealed that the silicate peak in sulfate-rich slag-based grout, linked to C-S-H and Ca(OH)<sub>2</sub> formation, overlapped with the aluminate peak and appeared earlier, which was not observed at a water-to-binder ratio of 0.5. Higher water-to-binder ratio promoted the formation of ettringite due to high sulfate content, affecting compressive strength and pore distribution. Coarser pores in sulfate-rich slag-based grout led to low compressive strength and high permeability. In addition, results showed that the reactivity of slag could not be determined solely by its basicity index. The performance of slag-based grouts depends on sulfate content and short-term slag reactivity. However, the evaluation of long-term slag hydration is necessary to understand the potential influence of latent slag hydration on engineering properties and performance.

## 1. Introduction

Grouting is commonly used in geotechnical engineering to reduce the permeability or enhance the mechanical properties of soils and embankments [1,2]. Cement grouts are designed with high water-to-cement ratios. The production of Portland cement cause significant emissions of CO₂ and intense energy demand [3,4]. Alternative binders such as slag cement have gained attention in recent years to lower environmental impacts [5]. Substituting Portland cement with Ground Granulated Blast Furnace slag also enhances several properties involved in the durability of cement-based materials. The resistance of concrete and mortar against chloride and sodium sulfate attack is improved [6–8]. It was reported that slag cement grouts can meet the requirements of grouting practices for

<footnote><sup>a</sup> Nantes Université, Ecole Centrale de Nantes, CNRS, GeM, UMR 6183, F-44000, France</footnote>

<footnote><sup>b</sup> Soletanche-Bachy, Chemin des Processions, Montereau Fault Yonne 77130, France</footnote>

<footnote>* Corresponding author.
E-mail address: emmanuel.roziere@ec-nantes.fr (E. Roziere).</footnote>
geotechnical engineering [9–11]. The use of slag in cement-based grouts enhanced their flowability, limited bleeding and delayed their initial setting [12–15]. At hardened state, the use of slag at optimum proportion enhanced the compressive strength of grouts [9,13, 16]. Due to their high water content, bentonite is commonly used in grout systems to ensure their stability [17]. Low bentonite contents allow to reduce sedimentation and bleeding without impairing pumpability and penetrability [18,19].

The proportion of slag can represent up to 85 % of binder content. To ensure optimal performances, it is essential to understand the influence of slag processing and physicochemical properties on their reactivity in cement-based materials. Several factors, including the glass content, particle size distribution, and chemical composition of the slag, have been correlated to the reactivity of GGBS. The reactivity of slag is also influenced by its glassy phase, which is more reactive when the structure is more disordered and depolymerized. The degree of depolymerization of the structure can be determined by the number of non-bridging oxygen per tetrahedral network-forming element [20]. The CaO/SiO₂ ratio and the Al₂O₃ contents are major parameters that can affect glass reactivity [21, 22]. Higher Al₂O₃ and MgO proportions had a positive impact on slag reactivity [23]. Previous studies indicated that MgO in slag has relatively small ion radii, potentially conferring high capacity to break the links within the network structure [24]. The dissolution of slag is influenced by the TiO₂ content as reported in [25]. Because the titanyl bond requires calcium ions Ca²⁺ for charge-compensation, the availability of Ca²⁺ decreases during glass alteration in a modified random model of glass structure where Ca²⁺ ions are clustered in percolating cationic domains. As a result, the presence of five coordinated Ti slows down the dissolution of the slag. These unique structural properties of titanium could potentially clarify why Ti is harmful for many physical and chemical slag properties when its concentration exceeds 1 %. Despite the various parameters available to predict slag reactivity, none of the existing indices provide a reliable assessment of the reactivity of different slags [26,27]. They provide only rough guidelines that do not accurately reflect the reactivity of slag or its effects of the performances of cement-based materials.

The chemical composition of slag did not fully explain its reactivity or the physico-chemical properties of slag-based mixtures. The diverse origins of different types of slag significantly affect their chemical compositions [28]. Zhang et al. [29] reported that the origin of slag has minimal influence on the hydration process of the cement-slag system by assuming that the chemical composition and amorphous contain remain constant. They highlighted the role of sulfur in mixtures, which is involved in the formation of AFm-phase, such as calcium monosulfoaluminate. The RILEM Technical Committee demonstrated that assessing the reactivity of SCMs such as slag, metakaolin, and fly ash can rely on the R3 model system. The R3 model, through calorimetry and bound water tests, showed acceptable correlations with the 28-day relative compressive strength of standard mortar specimens [30]. A recent study proposed a graphical method to estimate the reactivity of slag by assessing its chemical composition that uses a cartesian coordinate system based on the (CaO/SiO₂) and (MgO + Al₂O₃) variables [23]. This method classifies the reactivity of slag based on different chemical tests (cumulative heat, chemically bound water, Si⁴⁺ concentration and dissolution rate). The method allows defining regions with slags of similar performance. Region I involved slag with low Al₂O₃ and/or MgO levels and exhibits low reactivity. Region III included slag with high Al₂O₃ and/or MgO concentrations and displays high reactivity. All other samples fall within Region II, which includes slags with moderate Al₂O₃ and/or MgO levels and moderate reactivity. However, these studies did not consider the reactivity of slags within diluted systems, such as grouts.

Based on previous works, the most influencing factors are not well defined, and they depend on the studied properties. Slag cements sometimes exhibit unexpected behavior in terms of mechanical strength [31] and hydraulic conductivity. Moreover, most existing studies were conducted at relatively low water-to-binder ratios for designing structural concrete mixtures from strength-based specifications, while there is limited research on the behavior of diluted mixtures used for soil improvement and designed from permeability criterion. This underscores the need for further research.

This study aims to design slag-based grouts for water resistant barriers executed using soilmixing or self-hardening slurry cut-off

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

Chemical composition (wt%) of the materials used.</caption><thead><tr><th rowspan="2">Material<br>Name</th><th rowspan="2">Portland cement<br>OPC</th><th colspan="4">GGBS</th></tr><tr><th>S1</th><th>S2</th><th>S3</th><th>S4</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>20.8</td><td>36.8</td><td>32.4</td><td>25.2</td><td>40.1</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>3.2</td><td>10.2</td><td>13.8</td><td>16.5</td><td>12.4</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>4.3</td><td>0.37</td><td>0.34</td><td>0.50</td><td>1.07</td></tr><tr><td>CaO</td><td>65.1</td><td>42.9</td><td>41.7</td><td>41.5</td><td>33.8</td></tr><tr><td>MgO</td><td>1.4</td><td>6.4</td><td>7.0</td><td>11.3</td><td>7.1</td></tr><tr><td>TiO<sub>2</sub></td><td>0.3</td><td>0.7</td><td>0.58</td><td>1.94</td><td>0.91</td></tr><tr><td>SO<sub>3</sub></td><td>2.7</td><td>0.1</td><td>1.59</td><td>3.03</td><td>0.17</td></tr><tr><td>Cl<sup>-</sup></td><td>0.04</td><td>0.01</td><td>0.01</td><td>0.014</td><td>0.01</td></tr><tr><td>S<sup>2-</sup></td><td>0.06</td><td>0.6</td><td>0.69</td><td>0.72</td><td>0.58</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.13</td><td>0.23</td><td>0.24</td><td>0.39</td><td>0.33</td></tr><tr><td>K<sub>2</sub>O</td><td>0.32</td><td>0.38</td><td>0.33</td><td>0.22</td><td>0.93</td></tr><tr><td>Density (kg/m<sup>3</sup>)</td><td>3200</td><td>2900</td><td>2880</td><td>2910</td><td>2900</td></tr><tr><td>Specific surface area (m<sup>2</sup>/g)</td><td></td><td>1.54</td><td>1.55</td><td>1.45</td><td>1.59</td></tr><tr><td>Median particle size d<sub>50</sub> (µm)</td><td></td><td>9.4</td><td>9.8</td><td>10.9</td><td>9.5</td></tr><tr><td colspan="6" style="text-align: left; font-weight: bold;">Hydraulic Index</td></tr><tr><td>CaO/SiO<sub>2</sub></td><td></td><td>1.17</td><td>1.29</td><td>1.64</td><td>0.84</td></tr><tr><td>(CaO+MgO)/SiO<sub>2</sub></td><td></td><td>1.34</td><td>1.50</td><td>2.09</td><td>1.02</td></tr><tr><td>(CaO+MgO+Al<sub>2</sub>O<sub>3</sub>)/SiO<sub>2</sub></td><td></td><td>1.62</td><td>1.93</td><td>2.75</td><td>1.05</td></tr></tbody></table>
wall techniques. One of the objectives of the study is to highlight the differences between slags from various sources that comply with existing standards on slag reactivity, at a water-to-cement ratio suitable for structural concrete (0.5), and their performance in a geotechnical application where hydraulic conductivity is the primary criterion. The choice of a water-to-cement ratio of 6 reflects industrial practices in cut-off wall construction [32]. Their specifications mainly consist in permeability and compressive strength. The impact of water content and slag composition on macroscopic and microstructural properties was examined. Isothermal calorimetry was used to investigate the reactivity at early age. The evolution of microstructure was analyzed using thermogravimetry, X-ray diffraction, and mercury intrusion porosimetry. The macroscopic properties and microstructure were correlated to investigate the effects of slag composition on the performance of the studied materials.

## 2. Materials and methodology

### 2.1. Materials

An ordinary Portland cement (CEM I 52.5 N) and four ground granulated blast furnace slag (GGBS) of different origins were used to design concentrated and diluted mixtures. The chemical and physical composition of cement and slags are given in Table 1. Slags had approximately similar median particle size values $d_{50}$.

The data indicate that the four slags were significantly different in terms of chemical composition. S3 had lower silica content but much higher alumina and magnesia oxide contents compared to the other slags. S4 had lower calcium oxide content, while slags S1, S2, and S3 had similar calcium oxide contents. Slags S2 and S3 had non-negligible sulfur content, and slag S3 also had a relatively high titanium oxide content. The presence of titanium oxide in GGBS is suspected to modify the properties of slag cement-based materials [25].

The hydraulic index derived from chemical composition analysis are also shown in Table 1. The results of the hydraulic index suggest that slag S3 should exhibit higher reactivity than the other slags. Slag S4 has the lowest hydraulic index, indicating lower reactivity.

As presented in Fig. 1, the four slags were primarily amorphous, indicating a lack of long-range structural order. Some crystalline peaks associated with the presence of basanite and gypsum were observed, corresponding to the higher sulfur contents in slags S2 and S3, respectively. For S4, the position of the amorphous hump shifted toward lower diffraction angles compared to the other slags, which may be related to its lower hydraulic index.

### 2.2. Mixture proportions and specimens preparation

Bentonite slag-based cement grouts are used as self-hardening slurry according to EN 1538 execution standard to build watertight cut-off walls. A water to cement (W/C) ratio of 6 is commonly applied for this type of application in order to achieve the required low strength and permeability [33]. Significant bentonite content was added to ensure grout stability. The rheological parameters that are relevant for bentonite cement slurries comply with the EN 1538 specifications, primarily the Marsh viscosity with a target flow time

<figure><img src="image_4.png" /><figcaption>Fig. 1. XRD scans of four slags. G: Gypsum /A: Akermanite /CO: Coesite /B: Bassanite.</figcaption></figure>
ranging from 32 to 50 s [34]. In this study, the bentonite dosages were adjusted to achieve a Marsh viscosity between 35 and 45 s.

Low hydraulic conductivity (k, less than $10^{-8}$ m/s in situ) and low unconfined compressive strength (UCS) are the primary required properties for these materials. Typical performance expected during laboratory trials and mix development is a minimum unconfined compressive strength at 28 days of 0.5 MPa and a hydraulic conductivity lower or equal to $10^{-9}$ m/s.

Four slags of different origins and physico-chemical properties were studied at a fixed water to binder ratio of 6. In accordance with preliminary results (Table 2), S1 and S2 slags met the typical laboratory requirements of the bentonite cement grout for this fixed water to binder ratio. Slag S3 is suitable for use in concrete according to ASTM C989, it did not produce similar grout performance and did not meet the hydraulic conductivity requirement at 28 days. In accordance with NF EN15167 standard, S3 slag was not suitable for concrete due to high sulfur content. Slag S4 was suitable for use in concrete but had a lower activity index (strength requirement) and did not meet the permeability criterion compared to slags S1 and S2. Therefore, the experimental program aims to understand and explain the different behaviors of these slags in grout systems.

The studied mixtures included concentrated mixes to establish a link with the specifications and methods used for concrete (water-to-binder ratio of 0.5). Other diluted mixtures, with a water-to-binder ratio of 6, were used to highlight the sensitivity of grout properties to binder content. The experimental program focused on three parameters (Table 3): the water-to-binder ratio (with two selected ratios: 0.5 and 6), the nature of the slag (four slags with different chemical and physical properties), and the proportion of slag in the mixture, which was a secondary concern in this work.

To prepare the grouts, the first step was to add bentonite to water and allow it to hydrate for an hour. Next, slag and cement were added to the mixture, which was stirred in a high-shear mixer for 5 minutes to ensure complete homogenization. The fresh mixture was then placed in sealed specimens and kept in water tanks at a temperature of 20°C before characterization in the hardened state. The amount of bentonite added was proportional to the quantity of water used; specifically, 30 g of bentonite was added to 1 liter of water. For mixtures with a water-to-binder ratio of 6, 47 g of bentonite was used, whereas only 6 g of bentonite was added to mixtures with a ratio of 0.5. The water-to-binder ratio refers to the proportion of water used relative to the sum of slag and cement contents.

## 2.3. Test procedures

### 2.3.1. Compressive strength

The grouts were cast in cylindrical molds with a diameter of 7 cm and a height of 14 cm, i.e. a shape and slenderness complying with NF EN 12390-1 standards [37]. Samples were demolded after 7 days and cured in a water tank at 20°C until compressive testing. The compressive strength tests were conducted using a 100 kN press at 37, 90, and 180 days for mixtures with a water/binder ratio of 6, and at 28 and 90 days for mixtures with a water/binder ratio of 0.5. The compressive strength values are presented in Table 3. The compressive strengths at 28 days could not be measured for mixtures with water-to-binder ratio of 6. They were extrapolated from the long-term values and strength vs. log(t) plots.

### 2.3.2. Water permeability test

Water permeability is the rate at which water flows through porous materials. It is tested according to ASTM D5084 [38]. The cells used to measure the permeability of 40 mm diameter and 100 mm height specimens were composed of two aluminum plates and a steel cylinder. A lateral confining pressure was applied to the specimens. Water flowed from the lower base to the upper side of the sample to avoid trapped air bubbles. The fluid circulation was ensured by a constant water pressure of 80 kPa at 20°C. The permeability *k* of the specimens was determined based on Darcy's law from the quantity of water collected for a given time of percolation (Eq. 1):

$$v = k * i \quad (1)$$

where in laminar flow conditions, the velocity *v* is proportional to the hydraulic gradient *i*.

### 2.3.3. Isothermal calorimetry

100 g grout samples were weighted and inserted into the calorimeter cell. An isothermal calorimeter of the TAM Air type was used to characterize the reactivity of the studied mixtures for 7 days at a constant temperature of 20°C. The heat flow was monitored, and the cumulative heat was deduced by the integration of the measured flux.

<table><caption>**Table 2**
Conformity of raw slags and slag cement grout to different specifications.</caption><thead><tr><th>Slag</th><th>S1</th><th>S2</th><th>S3</th><th>S4</th></tr></thead><tbody><tr><td colspan="5">Conformity with standard specification for slag cement for use as supplementary cementitious material in concrete and mortars</td></tr><tr><td>NF EN15167 [35]</td><td>yes</td><td>yes</td><td>no</td><td>yes</td></tr><tr><td>ASTM C989 [36]</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr><tr><td colspan="5">Compliance of grout with 28 days specifications (UCS ≥ 0.5 MPa, k ≤ 10<sup>-9</sup> m/s)</td></tr><tr><td>Compressive strength</td><td>yes</td><td>yes</td><td>yes</td><td>no</td></tr><tr><td>Permeability</td><td>yes</td><td>yes</td><td>no</td><td>no</td></tr></tbody></table>
<table><caption>**Table 3**
Components and compressive strength of different mixtures.</caption><thead><tr><th>Nomination</th><th>S1-6-85</th><th>S2-6-85</th><th>S3-6-85</th><th>S4-6-85</th><th>S1-05-85</th><th>S3-05-85</th><th>S1-05-75</th><th>S3-05-75</th></tr></thead><tbody><tr><td>Water/binder</td><td>6</td><td>6</td><td>6</td><td>6</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>Slag/binder (%)</td><td>85</td><td>85</td><td>85</td><td>85</td><td>85</td><td>85</td><td>75</td><td>75</td></tr><tr><td>Slag</td><td>S1</td><td>S2</td><td>S3</td><td>S4</td><td>S1</td><td>S3</td><td>S1</td><td>S3</td></tr><tr><td>Rc,28 days</td><td>1.19*</td><td>1.12*</td><td>0.77*</td><td>0.16*</td><td>17</td><td>15</td><td>15</td><td>17</td></tr><tr><td>Rc,37 days</td><td>1.22</td><td>1.17</td><td>0.80</td><td>0.30</td><td></td><td></td><td></td><td></td></tr><tr><td>Rc,90 days</td><td>1.38</td><td>1.30</td><td>0.86</td><td>0.76</td><td>20</td><td>18</td><td>20</td><td>18</td></tr><tr><td>Rc,180 days</td><td>1.44</td><td>1.40</td><td>0.93</td><td>1.35</td><td></td><td></td><td></td><td></td></tr></tbody></table>

### 2.3.4. Microstructure evaluation

The microstructural characterization was performed on samples taken from crushed cubes for thermogravimetric analysis (TGA), X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) at 90 days. The samples were ground to pass an 80 µm sieve. XRD was used to identify crystalline phases qualitatively. It was performed using an Aeris XRD at 30 Kv. A Cu Kα source (λ1 = 1.5405980 Å and λ2 = 1.5444260 Å) with a 0.38 mm slit was used. Analysis was performed for a 2θ range from 5° to 70°.

TGA was also used to identify solid phases from their transformation temperatures, using a Netzsch STA 409 PC Luxx device. The crushed sample was placed in the device, and its mass was monitored while heating. The temperature increased from 20°C to 1000°C at constant heating rate of 10 K/min, under a nitrogen atmosphere.

For the XRD and TGA tests, samples were placed in an oven at 40°C for 24 hours before testing, and the analyses were performed on the specified day.

The pore size distribution was characterized using a standard porosimeter allowing the application of pressures up to 400 MPa, which corresponds to a minimum pore radius of 1.5 mm. MIP was performed on small cubes sawn from the core of hardened grout specimen and dried in air-conditioned room at 20 °C and 50 % RH for 24 h before the test. The radius *r* can be calculated by the Washburn equation (Eq. 2):

$$r = \frac{2 * \sigma * \cos(\theta)}{P} \qquad (2)$$

With *P* the mercury penetration pressure, *σ* the surface tension of the mercury (0.475 N/m) and *θ* the contact angle between the solid surface and the mercury (141°) and by assuming that pores are cylindrical.

## 3. Results and discussion

### 3.1. Microstructural characterization

#### 3.1.1. Thermogravimetric analysis

Differential thermogravimetry (DTG) results of different slag mixtures are shown in Fig. 2. At the water-to-binder ratio of 0.5, the mass losses at 400–500°C and 700–800°C can be attributed respectively to the dehydroxylation of portlandite and the decarbonation of calcium carbonate. The S3-05-8 mixture showed no calcite peak. Mixtures with higher cement content (S1-05-75 and S3-05-75) exhibited a more pronounced portlandite peak. The hump at 350°C suggests the presence of hydrotalcite (H) formed with the provision of MgO and Al₂O₃ from slag. A shoulder appeared at 111°C in S3 based mixtures. Higher ettringite content can be expected to be formed in these mixtures (S3-05-75 and S3-05-85). The peak for ettringite (E) and C-(A)-S-H overlapped between 100 and 150 °C thus they cannot be quantified separately.

For mixtures with a water-to-binder ratio of 6, the DTG curves of grouts at water-to-binder ratio of 6 were similar. One large peak appeared around 125 °C. This peak is associated with the high-water content in capillary pores and the water bound within C-A-S-H. There is no peak indicating Portlandite formation. However, a calcite peak is observed in mixtures based on S3 and S4.

The percentage of water relative to the dry mass of the sample at 900°C was calculated for various temperature ranges and presented in Table 4. The water content in the grouts, calculated for various temperature ranges, was significantly higher for a water-to-binder ratio of 6, including the bound water between 105°C and 550°C [39]. This indicates that increasing the temperature with a heating rate of 10 °C/min may not be an appropriate methodology for measuring capillary water and bound water in cement grouts, unlike cement pastes, as a significant amount of water in these binders remained in the pores at temperatures above 105°C. More time is needed to reach thermal equilibrium at a specific temperature. A stepped temperature increase with extended time could be a more suitable methodology for grouts [39].

#### 3.1.2. X-ray diffraction

Fig. 3 displays XRD patterns of eight mixtures studied over a 90-day curing period. The primary hydration product was C-(A)-S-H, which appeared as a hump around 30° due to its amorphous nature. This hump corresponds to an aluminum-substituted tobermorite-like phase (Ca₅Si₅Al(OH)O₁₇.5H₂O) [40]. Calcite was detected in all mixtures, likely resulting from the carbonation of slag hydration products. Portlandite was observed at a water-to-binder ratio of 0.5, consistent with the results of DTG (Fig. 2). Mixtures with higher cement content (S1-05-75 and S3-05-75) exhibited a more pronounced portlandite peak. However, portlandite could not be

<footnote>* extrapolation from experimental data at 37, 90, and 180 days.</footnote>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Differential thermogravimetry (DTG) results after curing of 3 months, a) water-to-binder ratio = 0.5, b) water-to-binder ratio = 6, E: Ettringite, H: Hydrotalcite, P: Portlandite, C: Calcium carbonate.</figcaption></figure>
<table><caption>**Table 4**
Water losses (mass %) with respective to the dry mass of the sample at 900°C for different temperature ranges.</caption><thead><tr><th>% water</th><th>S1-6-85</th><th>S2-6-85</th><th>S3-6-85</th><th>S4-6-85</th><th>S1-05-85</th><th>S3-05-85</th><th>S1-05-75</th><th>S3-05-75</th></tr></thead><tbody><tr><td>20°C - 40°C</td><td>3.8</td><td>3.7</td><td>3.7</td><td>5.5</td><td>0.2</td><td>0.2</td><td>0.7</td><td>0.5</td></tr><tr><td>40°C - 80°C</td><td>15.9</td><td>16.8</td><td>15.9</td><td>20.1</td><td>2.5</td><td>2.5</td><td>3.8</td><td>4.5</td></tr><tr><td>80°C - 105°C</td><td>93.7</td><td>67.2</td><td>49.0</td><td>89.6</td><td>2.6</td><td>2.8</td><td>8.2</td><td>7.3</td></tr><tr><td>105°C - 250°C</td><td>344.8</td><td>342.3</td><td>342.8</td><td>321.0</td><td>6.7</td><td>9.1</td><td>11.0</td><td>8.9</td></tr><tr><td>250°C - 550°C</td><td>6.7</td><td>8.0</td><td>7.7</td><td>3.8</td><td>4.9</td><td>5.1</td><td>4.4</td><td>4.3</td></tr><tr><td>550°C - 900°C</td><td>2.5</td><td>1.9</td><td>12.8</td><td>12.4</td><td>3.3</td><td>2.5</td><td>2.4</td><td>1.0</td></tr></tbody></table>

identified in mixtures with a water-to-binder ratio of 6 (Fig. 3.c.), which is consistent with TGA results. Bentonite, a montmorillonite-type clay, was included in the mixtures to ensure grout stability, and traces of quartz from the bentonite were detected in all mixtures. (Figure A. 1). Ettringite was observed in slag 3 cement-based mixtures, with a higher peak in the S3-6-85 mixture, consistent with the relatively high sulfate content of the slag. (Table 1). Traces of monosulfate (MS) were also detected in slag 3-based mixtures. Ettringite was observed in the slag 2-based mixture (S2-6-85) as well. Peaks at 2θ ~11.5° and 22.9° can be attributed to a hydrotalcite-like phase, due to the significant MgO content of the slag (Table 1). The mixtures also showed traces originating from bentonite, with higher peaks observed at a water-to-binder ratio of 6.

### 3.1.3. Pore size distribution

Mercury intrusion porosimetry (MIP) was used to investigate the pore structure of the samples. The MIP results after 90 days of curing are presented in Fig. 4. The mixtures with a higher slag content (S1-05-85 and S3-05-85) had coarser pore distribution than materials with lower slag content (S3-05-75 and S3-05-75). The mixtures with higher slag content (S1-05-85 and S3-05-85) exhibited coarser pore distributions compared to those with lower slag content (S1-05-75 and S3-05-75). The slag composition affected the pore distribution, with more pronounced differences observed at 85% slag content compared to 75%. These results suggest that the type of slag significantly influences the microstructural properties. Furthermore, both the pore volume and the median pore radius increased with slag content, as shown in Fig. 5.

Similar results have been reported in the literature, where the pore distribution of slag mortars was coarser than that of OPC mortars at 28 days. Over time, the pore distribution gradually evolved towards smaller and finer pores, with porosity becoming comparable after 90 days and 360 days. [41,42]. This phenomenon is attributed to the latent hydration of slag, which requires activation by Ca(OH)₂ generated from clinker hydration. However, with higher slag content, an increase in porosity volume is expected, along with porosity refinement [41,43]. The C-S-H formed as a result of slag hydration has a higher porosity due to a greater fraction of small radius pores (ranging from 4 to 20 nm) [44].

The pore size distribution of materials with a water-to-binder ratio of 6 was coarser compared to mixtures with a water-to-binder ratio of 0.5. The pore radius was less than 10,000 nm for all mixtures, but it was less than 100 nm for mixtures with the lowest water-to-binder ratio. The water to cement ratio is known to impact the size, distribution, and connectivity of pores in cement-based materials [45]. An increase in the water to cement ratio is expected to generate an increase in capillary pores, whereas the intra and inter C-S-H remain unchanged [46]. The mixtures S2-6-85 and S1-6-85 show a monomodal pore distribution, whereas the mixtures S3-6-85 and S4-6-85 display a bimodal pore distribution and a higher median pore radius, as shown in Fig. 5.

The compressive strength is plotted as a function of median pore size after 90 days in Fig. 5. A correlation between the median pore size and compressive strength is observed: the larger the median pores, the lower the compressive strength. Mixtures S3-6-85 and S4-6-80 exhibited lower compressive strength with larger median pore radii. The mixtures with a water-to-solid ratio of 0.5 showed comparable compressive strength values and median pore radius after 90 days. S1-6-85 and S2-6-85 demonstrated relatively higher compressive strength after 90 days compared to mixtures S3 and S4 at the same water-to-binder ratio, with smaller median pore radii. This result is consistent with previous research showing that compressive strength is related to pore characteristics.

The difference in pore distribution could affect the transport properties of the different mixtures. Therefore, hydraulic conductivity was evaluated for mixtures with a water-to-binder ratio of 6.

### 3.2. Short-term reactivity

Fig. 6 shows the heat flow of different mixtures. According to previous studies on slag cement mixtures, hydration can be divided into five stages: initial period, induction period, acceleration period, deceleration period and steady period [47]. The recorded heat flow of slag 1 based grouts as a function of the water-to-binder ratio and slag content is presented in Fig. 6.a.

The initial peak occurring immediately after mixing with water is generally associated with the wetting and dissolution of the components. During the dormant period, a lower water-to-binder ratio induced higher heat flow intensities, which were similar across different slag contents. An acceleration stage, associated with the hydration of alite in cement (the “silicate peak”), was followed by a second exothermic peak (often referred to as the “aluminate peak”) or a shoulder on the first peak. Slag S1-0.5-75 exhibited a higher exothermic peak intensity and a noticeable shoulder in the deceleration stage after several hours, indicating a secondary aluminate reaction following sulfate depletion. The exothermic rate of the mixture with higher water content was significantly lower than that of the mixture with a high water-to-binder ratio. Increasing the water-to-binder ratio extended the dormant period and reduced heat flow intensities due to the dilution effect of water.
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** XRD scans of different mixtures after 90-day water curing. a) S1 slag and S1-based mixtures, b) S3 slag and S3-based mixtures, c) Mixtures with a water-to-binder ratio of 6. E: Ettringite/ H: Hydrotalcite/ N: Natrolite/ Q: Quartz/ C: Calcite/ V: Vaterite/ Ve: Vesuvianite/ D: Dolomite/ P: Portlandite/ M: Montmorillonite/ I: Indialite.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 4. Impact of different parameters on pore size distribution at 90 days.</figcaption></figure>

Slag 3 based grouts show different profiles of heat flow compared to slag 1 (Fig. 6.b.). It is noteworthy that slag 1 and slag 3 presented different chemical composition especially aluminate and sulfate contents. After the main hydration peak associated to the silicate reaction, the second aluminate peak occurred with higher intensity in S3-0.5-85 than S3-0.5-75. This peak is associated with the depletion of sulfate [48] which shows the expected significant dependence on the sulfate content in slag 3 compared to slag 1. The
<figure><img src="image_10.png" /><figcaption>Fig. 5. Compressive strength as a function of median pore radius of different mixtures at 90 days.</figcaption></figure>

intensity of the silicate reaction decreased with increasing sulfate content (higher slag content in S3-0.5-85) due to the reduced overlap with the aluminate peak. A hump could be distinguished after the second aluminate reaction peak in S3-0.5-85 and S3-0.5-75, which corresponds to the pozzolanic reaction between slag and Ca(OH)₂ when a sufficient of Ca(OH)₂ and alkalinity are reached as reported in [23]. This peak also includes the continuous transformation of ettringite (AFt) to calcium monosulfoaluminate (MS).

Comparing the different slag 3-based grouts, the mixture S3-6-85 exhibits a dominant sharp peak related to the aluminate reaction. The silicate peak, associated with the formation of C-S-H and Ca(OH)₂ in cement-based materials, likely overlapped with the aluminate peak and appeared first, which was not observed at a water-to-binder ratio of 0.5. The water-to-binder ratio seems to influence not only the advancement and rate of the reaction but also the stages of the reaction related to the formation of different products in the case of slag 3 cement grout.

The influence of slag origin on the rate of heat evolution is shown in Fig. 6.c at a constant water-to-binder ratio. The general shape of the heat evolution is similar for slag 2 and slag 3, as well as for slag 1 and slag 4. These findings support the idea that the main hydration reactions were influenced by sulfate content. The onset of the acceleration stage for slag mixtures S3-6-85 and S2-6-85 was independent of sulfate content and chemical composition. After the dormant period, the curves were similar, with only a slight difference in the main peak intensity. However, a small hump can be distinguished in the S2-6-85 grout before the aluminate peak, which is presumably related to the silicate reaction. The sharp aluminate peak preceding the C₃S peak corresponds to precipitation of ettringite (Aft) in S3-6-85 [49]. Slag S1-6-85 and S4-6-85 grouts show similar heat profile evolution, though it is less intense for the latter. This finding is consistent with the basicity index results, which indicate lower activity for slag 4 compared to the other slags.

## 3.3. Mechanical and hydraulic performance of grouts

The compressive strength of the samples was assessed in accordance with NF EN 12390 standard [37,50]. The permeability coefficient of mixtures at a water-to-binder ratio of 6 is plotted as a function of compressive strength at different ages (Fig. 7). The type of slag affected strength development. The S3-6-85 mixture presented lower compressive strength than S1 slag-based mixture. This can be related to the different reaction products in S1 and S3 based mixtures as discussed in Section 3.2. The sulfate content had a twofold impact on the microstructure with the formation of ettringite, having a positive impact on compressive strength, but also negatively affecting the pore size distribution. MIP results revealed a coarser pore distribution in S3-6-85 compared to S1-6-85, leading to lower compressive strength despite the increased volume of hydration products such as ettringite. For a given permeability value, S3-6-85 mixture showed higher strength than S4-6-85.

The mixture S4-6-85 exhibited lower strength values after 37 days. This can be related to the lower initial reactivity of S4 slag, consistently with its lower basicity index (Table 1) and isothermal calorimetry data (Fig. 6). Nevertheless, S4 based grout achieved a higher strength after 90 and 180 days than required by the technical specifications. This difference can be attributed to the slower hydration process of slag 4 compared to other types of slag. It should be noted that the compressive strength values of mixtures based on S1, S2, and S3 slags after 37 days are within the recommended strength ranges (0.5-1.5 MPa) for soil mixing applications and water-resistant barriers.

The permeability coefficient of mixtures is plotted as a function of the median pore radius determined from MIP after 90 days in Fig. 8. The results indicate two distinct sets of permeability and median pore radius values. The S1-6-85 and S2-6-85 mixtures exhibited lower permeability and smaller median pore radius compared to the other grouts. The permeability of the S3-6-85 and S4-6-85 mixtures exhibited a decrease over time (Fig. 7), with the permeability of the mixture S4-6-85 falling within the desired range after 180 days. However, the permeability of the S3-6-85 mixture consistently exceeded the limit value, indicating the
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 6. Heat flow evolution as a function of time for different mixtures. a) S1-based mixtures b) S3-based mixtures c) Mixtures with a water-to-binder ratio of 6.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 7. Time evolution of the compressive strength and permeability at water-to-binder ratio of 6 after 37, 90 and 180 days.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 8. Permeability versus median pore radius at 90 days.</figcaption></figure>

significant impact of ettringite formation on the microstructure and pore size distribution.

## 4. Discussion

The reactivity of slags can be initially estimated by using the hydraulic index and the degree of depolymerisation. In this study, the reactivity analysis of slags was first based on their chemical composition, as shown in (Table 1). Slags S1, S2, and S3 had satisfactory values for good reactivity based on the hydraulic index. Despite having lower basicity values, slag S4 fell within the ranges defined in the literature. Assessing slag reactivity based on its chemical composition is complex due to various influencing factors, such as the fineness of the slag, the structure of the glassy phase, and the water-to-binder ratio [26,27].

Therefore, the reactivity of slags was estimated by considering not only their chemical composition but also the physical and mechanical properties of slag-based materials. The primary goal was to evaluate the impact of different slag properties on material performance, particularly at high water-to-binder ratios, and to investigate the sensitivity of these properties to the chemical characteristics of the slag.

Two slags, S1 and S3, were selected to investigate how the water-to-binder ratio and slag content affect slag reactivity, microstructure, and mechanical properties. A higher water-to-binder ratio led to an increase in median pore size and a coarser pore distribution, while the released heat decreased significantly due to the dilution effect of water. This resulted in higher porosity, a lower content of reaction products, and consequently, reduced compressive strength. XRD and TGA results demonstrated that a high water-to-binder ratio inhibited the formation of portlandite. The slag content played an essential role in the microstructure by influencing
pore distribution and phase assemblage, although it had only a slight impact on compressive strength. It is important to study the influence of slag content over hydration time due to its latent hydration properties [43].

This study primarily focuses on the mixtures or grouts with a high water-to-binder ratio. Results showed that the evolution of heat flow is dependent on the initial composition, specifically the sulfate and aluminate content. A strong sulfate depletion peak was observed in S2 and S3 slags as shown by the heat flow evolution (Fig. 6). XRD results consistently revealed the presence of ettringite in these two mixtures, with S3-6-85 showing a higher intensity of ettringite peak compared to S3-0.5-85. In addition, portlandite could not be detected in S3-6-85 as shown by XRD and TGA results. These results seem consistent as the formation of ettringite from aluminate and sulfate-rich phases requires calcium and high pH, which could be provided by alite dissolution. This suggests that the formation of ettringite is favored over the production of other reaction products, as indicated by the early rise of aluminate peak in isothermal calorimetry results at high water-to-binder ratio of 6. For the mixture S2-6-85, the heat flow profile showed the two main peaks related to silicate and aluminate even if the peak related to silicate appeared as a shoulder (Fig. 6). XRD also showed the formation of ettringite in this mixture. The aluminate peak occurred earlier in S3-6-85 than in S2-6-85 (Fig. 6). The presence of one dominant sharp peak related to the aluminate reaction for S2 and S3 heat flow, correlated with well identified ettringite in both XRD patterns, confirms this suggestion. Thus, the water-to-binder ratio had a significant impact on the stages of reaction and reaction products. Comparing S2-6-85 and S3-6-85, it can be observed that the mixture S3-6-85 had a coarser pore distribution compared to S2-6-85, with higher median pore. Both mixtures presented satisfactory strength but S3 based mixture showed higher permeability than the limit permeability value defined in the technical specifications. The sulfate and aluminate content could explain this difference in behavior between these two slags. Ettringite results in higher strength due to its space filling effect [51]. On the other hand, it has been reported that higher sulfate content resulted in an increase of ettringite, coarser porosity, lower water content of the C-S-H and hence reduced strength of cement-based material [48]. Sulfate content can affect negatively the properties of the C-S-H matrix, including the fine porosity. The presence of sulfate dilutes calcium and alumina-silicate phases, leading to a decrease in C-S-H phase content and an increase in the coarse porosity volume [52]. The differing behavior of slags S2 and S3 is attributed to their higher sulfate content, which promotes the formation of ettringite, among other reaction products, at a high water-to-binder ratio. This phase assemblage results in satisfactory strength, as specified by grout standards, but it impacts the distribution of pores and therefore the permeability value. Thereby, it is crucial to consider the dual effect of the sulfate content of slag to optimize the mechanical and transport properties of slag-based cement materials.

The compressive strength and permeability of mixture S4-6-85 did not meet the requirements due to its coarse pore distribution and high median pores (Fig. 4, Fig. 5). Additionally, the mixture exhibited low short-term reactivity, as evidenced by its relatively low heat release (Fig. 6) and lower basicity index. Compared to other mixtures with a cumulative heat of 30-37 J/g, this mixture only reached 16 J/g. This lower initial reactivity leads to a reduced formation of reaction products and a decrease in the number of percolation paths. To reach the percolation threshold, a significant amount of product is needed and related to the development of elastic modulus, according to [53]. Regarding S1-6-85, the fine pore distribution and low median pores lead to a good compressive strength and permeability in the range of specifications.

Based on the long-term evaluation, slags can be divided into two categories depending on their changes in compressive strength and permeability. S1, S2, and S3 slags demonstrated higher initial reactivity and exhibited gradual evolution in their properties over time. Conversely, S4 slag was characterized by a lower basicity index and short-term reactivity. It experienced a significant improvement in

<figure><img src="image_16.png" /><figcaption>**Fig. 9.** Range of sulfate content and cumulative heat values at 7 days for the fulfillment of technical specifications of slag cement grouts, from experimental data at 37 days.</figcaption></figure>
both compressive strength and permeability with time. As a result, it is necessary to assess the long-term implications of latent slag
hydration on the performance assessment of slag-based grouts.

Finally, a diagram can be suggested to achieve satisfactory compressive strength and permeability required for grouts to be used in water-resistant barriers (Fig. 9). The hydraulicity index had some correlation with the cumulative heat at high water-to-binder ratio. Slags S2 and S3, which had a higher hydraulicity index, demonstrated a higher cumulative heat, while slag S4 had a lower hydraulicity index and lower short-term cumulative heat. A significant correlation has been reported between the hydraulic index and the cumulative heat generated by slags [54]. It has also been reported that the cumulative heat can be correlated to the initial strength development of slags [54]. The cumulated heat at 20°C after 7 days or the basicity index can provide insight into the short-term compressive strength and permeability; however, it is noteworthy that they are insufficient in assessing the long-term behavior. This was evidenced from the changes observed in the mechanical and hydraulic properties of slag S4 over time.

As previously discussed, sulfate content plays a significant role in the development of phase assemblage and pore distribution. However, since the study was conducted on only four slags, it is challenging to define definitive trends based on this small sample size. Further research with additional slag mixtures is essential to gain more comprehensive insights.

**5. Conclusion**

The experimental program aimed to investigate the influence of slag composition on the properties of diluted systems designed for soil improvement, particularly regarding permeability. Based on the experimental results of the study, the following conclusions can be drawn:

* According to the results, the reactivity of slag in diluted systems cannot be solely determined by the basicity index, as the hydration process is significantly influenced by the water-to-binder ratio. Even if the basicity index falls within the proposed range, the reactivity of slag may still be insufficient at high water-to-binder ratios.

* The findings also indicated a poor correlation between compressive strength and permeability at high water-to-binder ratios. Even if the compressive strength met acceptable criteria, the permeability did not align accordingly. Therefore, compliance with strength-based standards cannot be used as a reliable indicator for the suitability of slag in diluted systems or for meeting permeability requirements.

* The performance of slag cement-based grouts was significantly influenced by the water-to-binder ratio and the sulfate content of the slag. Higher water-to-binder ratios affected the hydration process, phase assemblage, and pore distribution, preventing the formation of portlandite. Additionally, high water-to-binder ratios combined with high sulfate content promoted the formation of ettringite, which in turn impacted compressive strength and pore size distribution. While the resulting phase assemblage met grout specifications for mechanical strength, it adversely affected permeability due to changes in pore distribution.

* Experimental results showed that the influence of slag proportion in the binder was of secondary importance. However, it did affect pore distribution and could alter the fraction of the reaction products.

* The time evolution of compressive strength and permeability at high water-to-binder ratio indicate the need for evaluating the long-term behavior of slag-based grouts. This assessment is required to understand the potential influence of latent slag hydration on different engineering properties and performance indicators.

Careful consideration of both the water-to-binder ratio and sulfate content is essential for assessing the performance of slag cement-
based materials. Achieving the appropriate balance between these factors requires proper mix design and testing to ensure that the
final product meets the required strength and permeability for its intended use. The experimental results also highlight the need for
testing procedures to evaluate the long-term performance of slags.

CRediT authorship contribution statement

**Manon Michaut:** Investigation, Data curation. **Christophe Justino:** Resources, Project administration, Methodology, Conceptualization. **Faten Souayfan:** Writing – original draft, Methodology, Investigation, Data curation. **Emmanuel Roziere:** Writing – review & editing, Supervision, Project administration, Funding acquisition, 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

Soletanche Bachy is gratefully acknowledged for their technical and financial support, Grant ID: Faten Souayfan's post doctoral
position.

Appendix
<figure><img src="image_17.png" /><figcaption>Fig. A1. XRD analysis of bentonite. M: Montmorillonite/ Q: Quartz.</figcaption></figure>

## Data availability

The data that has been used is confidential.

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