# Methodology to assess the activity of binders containing calcined clays getting rid of porosity variation

N. Lely, L. Lemesre, S. Guihéneuf, Maxime Clément, D. Rangeard, Arnaud Perrot

► To cite this version:

N. Lely, L. Lemesre, S. Guihéneuf, Maxime Clément, D. Rangeard, et al.. Methodology to assess the activity of binders containing calcined clays getting rid of porosity variation. Materials and structures, 2025, 58 (10), pp.332. 10.1617/s11527-025-02861-0 . hal-05357196

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Submitted on 11 Nov 2025

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# Methodology to assess the activity of binders containing calcined clays
getting rid of porosity variation

N. Lely<fnref n="1" />,<fnref n="2" />, L. Lemesre<fnref n="1" />, S. Guihéneuf<fnref n="1" />, M. Clément<fnref n="2" />, D. Rangeard<fnref n="2" /> and A. Perrot<fnref n="1" />, *,

## Abstract

Due to their high specific surface, the incorporation of calcined clays (CC) as supplementary cementitious materials (SCMs) reduces the workability of standard mortars, as Water/Binder ratio (W/B) and compaction energy are kept constant, while assessing their activity according to NF EN 196-1. Thus, mortars incorporating calcined clays display a different initial porosity than reference mortars made with Ordinary Portland Cement (OPC). In this study, the impact of the variation of the initial porosity on compressive strength at 28 days and consequently on measured CC activity is first assessed. Two types of binder are here studied: 85wt% OPC + 15wt% CC and LC3-50 (limestone calcined clay cement with 50wt% clinker) with three different kaolinitic clays, calcined at different temperatures. A method is proposed to accurately measure the air fraction in the mortars to be able to compare compressive strength of these at similar porosity, allowing to get rid of experimental variation in porosity for binder activity assessment. It seems that evaluating the activity of a binder with Strength Activity Index (SAI), regardless initial porosity can lead to an underestimation of the activity of binders containing calcined clays.

**Keywords:** Porosity, mortars, calcined clays, Limestone calcined clay cement, superplasticizer, Supplementary Cementitious Materials, Strength activity index, R3 test

<footnote n="1">¹IRDL, UMR CNRS 6027, Université de Bretagne Sud, 56100 Lorient, France</footnote>

<footnote n="2">²Research & Development Department, Laboratoire CBTP, 35530 Noyal-sur-Vilaine, France</footnote>

<footnote>* Correspondence: arnaud.perrot@univ-ubs.fr</footnote>
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# 1. Introduction

The cement industry accounts for approximately 6-8% of global greenhouse gas (GHG) emissions, according to various sources [1,2]. Among the strategies developed to reduce these emissions, lowering the clinker factor—from 0.71 in 2022 to 0.57 by 2050 [1]—through the use of supplementary cementitious materials (SCMs) is considered one of the most impactful.

A particularly promising approach is the Limestone Calcined Clay Cement (LC3) technology, which combines calcined clays (CC) and limestone (LS) as SCMs [3]. This type of binder enables the production of cements with a clinker factor as low as 0.5 (LC3-50) while maintaining mechanical performance comparable to that of ordinary Portland cement (OPC).

Such properties can be achieved when using calcined clays containing at least 40% calcined kaolinite [4]. The broad availability of these SCMs [5], especially in regions where cement demand is expected to rise [6], makes the LC3 technology highly relevant from both an environmental and resource-efficiency perspective. This paper focuses on calcined clays, as these materials are more widely available and exhibit greater variability—in terms of clay mineralogy, calcination conditions, and particle size distribution—than other SCMs such as slags or silica fume.

According to NF EN 196-1 [7], compressive strength of cement is measured on standard mortars made with standard sand at a water to binder ratio (W/B) of 0.5 for a constant compaction energy following a specific protocol for casting and material preparation. To evaluate the activity of SCM as potential material in binder system, standard mortars are made with partial substitution of OPC with SCM. Ratio of compressive strength of mortars with substitution compared to a reference (100% OPC) is called Strength Activity Index (SAI). For example, in French standards, evaluation of calcined clays as SCMs is evaluated through NF P18-513 [8] with 15wt% substitution of OPC by calcined clays. Calcined clays are labelled as
“Metakaolin” if SAI with 15wt% calcined clays in the binder is higher than 100% OPC at 28
days which means that SCM is sufficiently reactive and has a significant pozzolanic effect.

The reactivity of calcined clays (CCs) depends on the type and proportion of clay minerals
present [9,10]. Among them, kaolinite exhibits the highest reactivity after calcination [11,12].

An increase in particle fineness further enhances the chemical activity of calcined clays,
resulting in higher mechanical strength and a refined pore structure [13].

The calcination temperature, typically ranging from 650°C to 1050°C, also has a significant
influence on CC reactivity. For instance, although complete dehydroxylation of kaolinite occurs
at around 650°C, its optimum reactivity is generally observed near 800°C [14]. Particle fineness
similarly affects CC reactivity; however, even though temperature plays a key role in reactivity,
Dhar et al. [21] reported no significant variations in specific surface area (SSA) or specific
gravity of CCs calcined between 600°C and 800°C.

The incorporation of calcined clay (CC) into cementitious binders is known to reduce mortar
workability by increasing viscosity and yield stress [15–17], primarily due to the higher specific
surface area (SSA) of CC compared with OPC. The rheology of cement pastes containing
calcined clay depends on the replacement level, mineralogical composition, and particle
fineness of the clay [18–20]. This loss of workability can result in higher porosity when the
compaction energy is kept constant. It is therefore important to distinguish this detrimental
physical effect from the beneficial chemical contributions of CC when assessing its potential
as a supplementary cementitious material (SCM).

To counteract this reduction in workability, superplasticizers (SPs) are commonly used. The
required SP dosage to achieve a given consistency depends on the chemical structure of the SP,
the SCM content in the binder, and the mineralogical nature of the calcined clay (e.g., kaolinite,
illite, or montmorillonite) [18,22,23]. For binders containing kaolinitic calcined clays, the SP
demand is directly related to the metakaolin content, owing to its highly charged specific
surface [18].
The addition of SPs can also affect hydration kinetics [24], leading to different phase assemblages and potentially influencing final compressive strength. Consequently, the Strength Activity Index (SAI) of a calcined clay may vary depending on the type and dosage of SP used.

For example, Li et al. [18] reported an increase in compressive strength and higher densities in mortars containing OPC and OPC+CC binders as the SP dosage increased. However, even though higher densities were observed, the corresponding improvement in mechanical properties could not be attributed solely to densification.

During binder characterization according to NF EN 196-1 [7], and in the absence of superplasticizer (SP), the loss of workability induced by the incorporation of calcined clay (CC) generally leads to an increase in air content, which in turn reduces mechanical strength and results in an underestimation of the chemical potential of CC using the Strength Activity Index (SAI).

Although air content can be measured using an aerometer, this method may overestimate the actual value [25], as the sample preparation procedure [26] differs from that specified in NF EN 196-1. Such overestimation can significantly affect the apparent reactivity of SCMs, since compressive strength is strongly dependent on entrained air [27].

Therefore, to accurately evaluate the reactivity potential of cements containing calcined clays, it appears essential not to strictly apply the constant-energy compaction procedure prescribed in NF EN 196-1 [7]. Instead, a methodology enabling comparison at equivalent porosity should be developed to better assess the intrinsic strength activity potential of calcined clay-containing binders.

A methodology is proposed in this study to accurately estimate the air volume entrained in mortars, thereby addressing the main objective of this work: to design a method for comparing compressive strengths of mortars at iso-porosity.

To validate the proposed approach, three kaolinitic clays were calcined at different temperatures and incorporated into two binder systems: (i) 85 wt% OPC + 15 wt% CC and (ii) LC3-50. This
allows the assessment of the influence of SCMs on initial porosity under varying compaction energies, reactivity levels, replacement ratios, and hydration mechanisms, without using chemical admixtures. The effect of a polycarboxylate-based superplasticizer (SP) on initial porosity, compressive strength, and Strength Activity Index (SAI) was also investigated.

The relationship between initial porosity and compressive strength was first examined on a reference OPC mortar, using different forming procedures and the addition of an air-entraining agent. Based on these results, a new activity index, termed the Binder Activity Index (BAI), is introduced. The BAI quantifies the chemical reactivity of the binder and is independent of mortar porosity. It is determined using de Larrard's model [28], which relates binder paste packing density and granular skeleton packing fraction through the Maximal Paste Thickness (MPT) concept.

These parameters are computed using precisely measured air contents, obtained from accurate volumetric measurements of raw materials and hardened mortar prisms. The BAI and SAI values are then compared at 28 days for different binders to evaluate how initial mortar porosity, particularly when SCMs are incorporated, influences the assessment of SCM activity. Finally, a 28-day iso-air Strength Activity Index (SAI<sub>iso-air</sub>) is calculated using de Larrard's model by matching the air content of OPC mortars with that of mortars containing SCMs. Conventional SAI values are determined in accordance with NF EN 196-1 [7].

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## 2. Materials and methods

### 2.1. Materials

The Ordinary Portland Cement (OPC) used in this study is a CEM I 52.5 N CE PM-CP2 NF, containing gypsum as the sulphate source. The limestone (LS) was obtained from a quarry located in western France, while the gypsum was supplied by Thermo Scientific Chemicals,
with a purity greater than 98%. The sand used corresponds to the standard sand specified in NF EN 196-1 [7].

Three different clays (C1, C2, and C3), exhibiting various kaolinite contents and originating from quarry waste (washing slurries), were employed in this study. Two chemical admixtures were also used: an air-entraining agent (AEA) and a commercial polycarboxylate-based superplasticizer (SP) typically used in ready-mix concrete.

## 2.2 Tests for materials characterisation

### 2.2.1. Preparation of calcined clays and determination of kaolinite content

The clays were first dried at 50°C and then crushed in a rotating cylindrical jar (20 cm in diameter and 40 cm in height) at 100 rpm using 5000 g of micro-Deval abrasive charges and 2000 g of dried raw clay for 120 minutes. Subsequently, the clays were calcined in a muffle furnace for 1 hour at 600°C, 700°C, or 800°C, with a heating rate of 10°C/min. For instance, the sample CC1_800 refers to clay C1 calcined at 800°C for 1 hour.

The kaolinite content, wt%$_{kaolinite}$, was determined using thermogravimetric analysis (TGA) following Eq. (1) [29] and applying a stepwise method. Thermal analyses were performed using an ATG/DSC 1 instrument (Mettler Toledo, USA), heating up to 950°C at a rate of 5°C/min, under a nitrogen atmosphere. The powders were manually ground to pass through a 160 µm sieve. Kaolinite dehydroxylation was considered to occur between 400°C and 600°C [10,11].

$M_{kaolinite}$ and $M_{water}$ refer respectively to molecular weight of kaolinite ($Si_2O_5Al_2(OH)_4$) and water. Weight loss between 400°C and 600°C is normalized by weight at 200°C to avoid bias caused by moisture from raw clays.

$$wt\%_{kaolinite} = \frac{m_{400} - m_{600}}{m_{200}} \times \frac{M_{kaolinite}}{2M_{water}} \quad (1)$$

The calcined kaolinite content, wt%$_{calcined kaolinite}$, is defined as the difference between the kaolinite content before and after calcination (wt%$_{kaolinite,raw clay}$ and .
$wt\%_{kaolinite,calcined clay}$ respectively). It is calculated using Eq. (2) [29] in order to verify the adequacy of the calcination process.

$$wt\%_{calcined kaolinite} = wt\%_{kaolinite,raw clay} - wt\%_{kaolinite,calcined clay} \quad (2)$$

Kaolinite and calcined kaolinite content are presented in **Table 1**.

## 2.2.2 Physical properties

The specific surface area (SSA), particle size distribution (PSD), and specific gravity were determined for all raw materials (OPC, calcined clays, limestone, and gypsum). The PSD curves of the raw materials are presented in **Figure 1**, and the corresponding characteristic diameters—$d_{v10}$, $d_{v50}$, and $d_{v90}$, representing the particle sizes below which 10%, 50%, and 90% of the sample volume is found—are reported in **Table 2**.

The PSD measurements were carried out using a Malvern laser diffraction analyzer on dried powders. Ethanol was used as the dispersing medium for OPC and limestone, whereas a 1 g/L sodium hexametaphosphate solution was used for calcined clays. The suspensions were homogenized by magnetic stirring before being introduced into the measurement unit of the diffractometer, ensuring an obscuration rate between 7.5% and 8.5%.

The BET specific surface areas (SSA) were determined from nitrogen adsorption isotherms using a NOVAtouch instrument (Anton Paar, USA), and the results are presented in **Table 1**. Degassing procedures were carried out under a pressure of 760 torr, for 16 hours at 40°C for OPC and gypsum, and for 3 hours at 300°C for calcined clays and limestone.

The specific gravity was measured using a helium pycnometer with a precision of 0.001 g/cm³, employing a 25 cm³ sample cell. The specific gravity of the standard sand, as well as the density and dry extract content of the admixtures, were provided by the respective suppliers (**Table 3**).

## 2.2.3 Reactivity test
The reactivity of the calcined clays was evaluated using the R3 test procedure, based on the bound water method, in accordance with the recommendations of RILEM TC 267-TRM [30]. The bound water (BW) contents of all calcined clays are reported in **Table 1**.

<table><caption>**Table 1** - Mineralogical properties, reactivity of calcined clays (BW : bound water in g/100 g paste) and specific surface area (SSA in m²/g)</caption><thead><tr><td>Material</td><td>Calcination temperature (°C)</td><td>Kaolinite content (%)</td><td>Calcined kaolinite content (%)</td><td>SSA (m²/g)</td><td>BW (g/100g paste)</td></tr></thead><tbody><tr><td>CC1 700</td><td>700</td><td>37%</td><td>35%</td><td>17.2</td><td>8.36 ± 0.05</td></tr><tr><td>CC1 800</td><td>800</td><td>37%</td><td>36%</td><td>15.1</td><td>8.39 ± 0.02</td></tr><tr><td>CC2 600</td><td>600</td><td>47%</td><td>46%</td><td>20.2</td><td>10.25 ± 0.09</td></tr><tr><td>CC2 700</td><td>700</td><td>47%</td><td>45%</td><td>xx</td><td>10.04 ± 0.08</td></tr><tr><td>CC2 800</td><td>800</td><td>47%</td><td>46%</td><td>15.1</td><td>9.92 ± 0.03</td></tr><tr><td>CC3 800</td><td>800</td><td>30%</td><td>30%</td><td>15.5</td><td>7.38 ± 0.06</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Figure 1** - Particle size distribution of raw materials</figcaption></figure>
<table><caption>**Table 2** - Particle diameter of raw materials. $d_{v,i}$ is particle diameter (µm) corresponding to a 1% volumetric passing (%)</caption><thead><tr><th>Material</th><th>d<sub>v90</sub> (µm)</th><th>d<sub>v50</sub> (µm)</th><th>d<sub>v10</sub> (µm)</th></tr></thead><tbody><tr><td>OPC</td><td>32.7</td><td>13.5</td><td>3.7</td></tr><tr><td>LS</td><td>42.9</td><td>12.8</td><td>2.4</td></tr><tr><td>CC1_700</td><td>416.0</td><td>10.0</td><td>1.6</td></tr><tr><td>CC1_800</td><td>395.6</td><td>8.8</td><td>1.6</td></tr><tr><td>CC2_600</td><td>19.0</td><td>5.4</td><td>1.2</td></tr><tr><td>CC2_700</td><td>20.9</td><td>5.6</td><td>1.3</td></tr><tr><td>CC2_800</td><td>25.6</td><td>6.4</td><td>1.4</td></tr><tr><td>CC3_800</td><td>26.8</td><td>6.0</td><td>0.9</td></tr></tbody></table>

<table><caption>**Table 3** - Specific gravity (g/cm³) of raw materials. Dry extract of admixtures are given in brackets</caption><thead><tr><th>Material</th><th>Specific gravity (g/cm³)</th></tr></thead><tbody><tr><td>Sand</td><td>2.640</td></tr><tr><td>OPC</td><td>3.140</td></tr><tr><td>LS</td><td>2.689 ± 0.001</td></tr><tr><td>CC1_700</td><td>2.801 ± 0.006</td></tr><tr><td>CC1_800</td><td>2.833 ± 0.009</td></tr><tr><td>CC2_600</td><td>2.794 ± 0.004</td></tr><tr><td>CC2_700</td><td>2.730 ± 0.008</td></tr><tr><td>CC2_800</td><td>2.830 ± 0.014</td></tr><tr><td>CC3_800</td><td>2.836 ± 0.010</td></tr><tr><td>Gypsum</td><td>2.300 ± 0.003</td></tr><tr><td>AEA (1.0%)</td><td>1.000</td></tr><tr><td>SP (50.0%)</td><td>1.120</td></tr></tbody></table>
## 2.3 Methods

### 2.3.1. Mix design and procedure

The mortars were prepared in accordance with NF EN 196-1, using a water-to-binder ratio (W/B) of 0.5. Different compaction procedures were applied: (i) two layers with 60 strokes per layer (reference procedure from NF EN 196-1), (ii) one layer with 10 strokes, (iii) one layer with 5 strokes, or (iv) manual compaction. The moulds were covered to minimize moisture loss and stored at 20°C. After 24 hours, the specimens were demoulded and immersed in water at $20 \pm 1^{\circ}$C until testing.

The methodology developed in this study consisted of two consecutive steps. The first step aimed to assess the influence of initial porosity on the compressive strength of OPC mortars by varying the porosity through different compaction procedures or by adding an air-entraining agent (AEA). In the second step, mortars were prepared using different binders—85/15 (OPC + 15 wt% CC) and LC3-50—and both initial porosity and compressive strength were measured under a constant compaction procedure (reference method).

#### Step 1:

The initial porosity of the different samples was adjusted either by modifying the compaction energy or by adding an air-entraining agent (AEA) as an admixture. The water content of the admixture was taken into account in the mix design to maintain a constant water-to-binder ratio (W/B) of 0.5 for all tested mortars.

First, three reference mortars were compacted using the standard procedure described in NF EN 196-1 (denoted as OPC 1-3). A hand-compacted mortar (OPC HC) was also prepared in an attempt to obtain a denser microstructure than the reference mortars. Two additional mortars were produced with AEA dosages of 0.5% (OPC AEA 0.5%) and 1.0% (OPC AEA 1.0%) by weight of binder; in both cases, the water content of the AEA was included to preserve a constant W/B = 0.5. Finally, two mortars were compacted in a single layer with 10 strokes (OPC 10H) and 5 strokes (OPC 5H), respectively, to obtain samples with lower densities.
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The characteristics of the OPC mortars prepared using these different forming procedures are presented in **Table 4**.

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<table><caption>**Table 4** - Characteristics of OPC mortars made with different compaction energy or admixtures</caption><thead><tr><th>Sample</th><th>Δm (%)</th><th>Air content (%)</th><th>(1-n)paste</th><th>g</th><th colspan="2">Rc (MPa)</th></tr></thead><tbody><tr><td>OPC 1</td><td>0.6%</td><td>1.4% / 6.7%*</td><td>0.379</td><td>0.575</td><td>59.1 ±</td><td>1.2</td></tr><tr><td>OPC 2</td><td>0.6%</td><td>1.3%</td><td>0.380</td><td>0.576</td><td>60.3 ±</td><td>3.5</td></tr><tr><td>OPC 3</td><td>0.4%</td><td>1.6%</td><td>0.378</td><td>0.574</td><td>61.0 ±</td><td>1.4</td></tr><tr><td>OPC HC</td><td>0.9%</td><td>0.4%</td><td>0.389</td><td>0.581</td><td>63.8 ±</td><td>2.3</td></tr><tr><td>OPC AEA 0.5%</td><td>0.4%</td><td>7.0%</td><td>0.332</td><td>0.542</td><td>43.0 ±</td><td>2.9</td></tr><tr><td>OPC AEA 1.0%</td><td>0.8%</td><td>8.5% / 16.5%*</td><td>0.320</td><td>0.533</td><td>37.6 ±</td><td>2.6</td></tr><tr><td>OPC 10H</td><td>0.3%</td><td>3.5%</td><td>0.361</td><td>0.563</td><td>53.6 ±</td><td>3.5</td></tr><tr><td>OPC 5H</td><td>0.3%</td><td>4.1%</td><td>0.356</td><td>0.559</td><td>47.7 ±</td><td>2.9</td></tr></tbody></table>

* Measurements made with an aerometer according to NF EN 413-2

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Step 2:

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Four mortars composed solely of OPC (100 wt%), referred to as OPC, were tested and used as the reference system in this study. Samples OPC 1–3 were prepared using the first batch of cement, while OPC 4 was produced from a second batch.

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Two types of binders incorporating calcined clays were also investigated. The first one, designated as 85/15, consisted of 85 wt% OPC and 15 wt% calcined clay, in accordance with NF P 18-513 [8]. The second binder, referred to as LC3-50, was composed of 50 wt% clinker, 30 wt% calcined clay, 15 wt% limestone, and 5 wt% gypsum. The gypsum content was selected to be consistent with values commonly reported in the literature [31]. The limestone and gypsum fractions were partially derived from the OPC itself.

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All binders containing SCMs were formulated using the first cement batch and were compared to the OPC 1–3 series.

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For each type of binder containing calcined clays (85/15 and LC3-50), an additional mortar incorporating CC2_800 was prepared with the addition of a superplasticizer (SP) to evaluate the effect of admixtures on the technical properties. The SP was added progressively until
achieving the same consistency as the OPC mortars. These mortars were compared to OPC 4, and the compaction energy was kept constant throughout this phase using the reference procedure. **Table 5** presents the mix designs, including the SP dosage (by mass) for each mortar. For each mixture, the theoretical density without porosity (i.e., corresponding to 0% air content, denoted as $D_{0\%}$) was calculated according to Eq. (3), where $m_i$ and $d_i$ represent the mass and density of component $i$, respectively. The mass and density values used for this calculation are reported in **Table 5** and **Table 1**, respectively.

$$D_{0\%} = \frac{\sum_i m_i}{\sum_i \frac{m_i}{d_i}} (g/cm^3) \qquad (3)$$

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243 **Table 5** - Mix design by mass (g) of mortars. OPC: Ordinary Portland Cement. CC:

244 Calcined clay. LS: Limestone. G: Gypsum. SP: superplasticizer. All mortars are compacted

245 with reference procedure from NF EN 196-1

<table><thead><tr><th>Type of binder</th><th>Samples</th><th>Sand (g)</th><th>Water (g)</th><th>OPC (g)</th><th>CC (g)</th><th>LS (g)</th><th>G (g)</th><th>SP (g)</th></tr></thead><tbody><tr><td rowspan="4">OPC</td><td>OPC 1</td><td rowspan="4">1350</td><td rowspan="4">225</td><td rowspan="4">450</td><td rowspan="4">0</td><td rowspan="4">0</td><td rowspan="4">0</td><td rowspan="4">0</td></tr><tr><td>OPC 2</td></tr><tr><td>OPC 3</td></tr><tr><td>OPC 4</td></tr><tr><td rowspan="7">85/15</td><td>CC1_700</td><td rowspan="7">1350</td><td rowspan="7">225</td><td rowspan="7">382.5</td><td rowspan="7">67.5</td><td rowspan="7">0</td><td rowspan="7">0</td><td>0</td></tr><tr><td>CC1_800</td><td>0</td></tr><tr><td>CC2_600</td><td>0</td></tr><tr><td>CC2_700</td><td>0</td></tr><tr><td>CC2_800</td><td>0</td></tr><tr><td>CC2_800 + SP</td><td>0.364</td></tr><tr><td>CC3_800</td><td>0</td></tr><tr><td rowspan="7">LC3-50</td><td>CC1_700</td><td rowspan="7">1350</td><td rowspan="7">225</td><td rowspan="7">244.7</td><td rowspan="7">135</td><td rowspan="7">60.5</td><td rowspan="7">9.8</td><td>0</td></tr><tr><td>CC1_800</td><td>0</td></tr><tr><td>CC2_600</td><td>0</td></tr><tr><td>CC2_700</td><td>0</td></tr><tr><td>CC2_800</td><td>0</td></tr><tr><td>CC2_800 + SP</td><td>1.164</td></tr><tr><td>CC3_800</td><td>0</td></tr></tbody></table>

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247 2.3.2. Measurement method of air content
First, the mass of the fresh mortar placed in the mould ($m_{fresh}$) was determined by weighing the mould before and after casting. After 24 hours of curing in a controlled environment at 20 ± 2°C, the prisms were demoulded and weighed ($m_{i,24h}$; with $i = \{1,2,3\}$). To ensure that no desiccation occurred and to maintain the accuracy of the apparent density calculation, the mass loss of each batch was computed according to Eq. (4).

$$ \Delta m = \frac{m_{fresh} - \sum_{i} m_{i}}{m_{fresh}} (\%) \qquad (4) $$

After 28 days of curing, the apparent volume ($V_{app}$) of each prism was measured using a Scantech 3D metrology scanner, and the volume analysis was performed with Zeiss Inspect software. This scanning system provides dimensional measurements with a precision of 1 µm. For all binders, it was assumed that the volume change between 1 day (demoulding) and 28 days was negligible compared to the total prism volume. Among the binders studied, LC3-50 exhibits the highest autogenous shrinkage, reported to be below 1500 µm/m after 28 days [32–34]. Assuming isotropic shrinkage, the corresponding volume change for a LC3-50 prism (40 × 40 × 160 mm³) is only 3.84 × 10⁻⁵ cm³, which can be considered insignificant.

The mass loss between the fresh state and demoulding represents another potential source of error in the air content determination, as it affects both mass and volume. This effect was accounted for by measuring the corresponding mass variation ($\Delta m$), as expressed in Eq. (4). Minor measurement inaccuracies may also arise due to balance precision when weighing the materials and mortar specimens.

The accuracy of the proposed method for determining air content in mortars is discussed in detail in **Appendix 1**. Finally, the compressive strength of the mortars was measured using a compression testing machine operating at a loading rate of 2.4 kN/s.

The apparent density ($D_{app}$) and air content of each prism were determined according to Eq. (5) and Eq. (6), respectively. The air content of each specimen was calculated from the ratio between the apparent density ($D_{app}$) and the theoretical density without porosity ($D_{0%}$), since
the pore-free volume of the prisms cannot be directly measured. In this approach, D₀% is considered constant for a given mortar formulation, while Dₐpp can be experimentally determined by combining the 3D-scanned volume with the mass measured at demoulding.

$$D_{app} = \frac{m_{24h}}{V_{app}} (g/cm^3) \qquad (5)$$

$$Air content = 1 - \frac{D_{app}}{D_{0\%}} (\%) \qquad (6)$$

The air content was also measured on two OPC mortars (OPC 1 and OPC AEA 1.0%) in accordance with NF EN 413-2 [26], using an aerometer, in order to compare the results obtained with both methods.

### 2.3.3. Matrix packing density and maximal thickness of paste (MTP)

The compressive strength of concrete depends on the porosity of the cement matrix, the maximal paste thickness (MPT), and the quality of the aggregates, as demonstrated by de Larrard [28]. In the present study, the sand was kept identical for SAI measurements on all mortar formulations; therefore, only the first two parameters could vary from one mortar to another.

The packing density of the cement paste, $(1 - n)_{paste}$, was defined according to Eq. (7) as the ratio of the cement (binder) volume to the total paste volume. In this equation, $V_{binder}$, $V_{water}$, and $V_{air}$ correspond to the volumes of binder (OPC, 85/15, or LC3-50), water, and air, respectively, with air volume determined using the method described in Section 2.2.2.

$$(1 - n)_{paste} = \frac{V_{binder}}{V_{binder} + V_{water} + V_{air}} \qquad (7)$$

Where n is the paste porosity.

The maximal paste thickness (MPT) represents the maximum distance between the two coarsest grains within the granular skeleton [28]. It depends on the particle size distribution, particle shape, and the mortar mix design. The MPT was calculated using Eq. (8), where $d_{max}$ is the
293 maximum particle diameter of the sand, g* is the theoretical packing density of the sand, and g is the volumetric fraction of sand in the mortar [35].

295 The sand used in this study was a standard natural river sand, characterized by a maximum particle diameter ($d_{max}$) of 4 mm and a minimum particle diameter ($d_{min}$) of 0.063 mm. The theoretical packing density of this sand was calculated according to Eq. (9) [35].

$$MTP = d_{max} \left( \sqrt[3]{\frac{g^*}{g}} - 1 \right) \qquad (8)$$

$$g^* = 1 - 0,39 \left( \frac{d}{d_{max}} \right)^{0,22} = 0,844 \qquad (9)$$

298 In this study, it is assumed that the compressive strength of mortar ($R_c$) can be predicted using de Larrard's model [28], as expressed in Eq. (10). In this equation, $K$ represents the binder activity (in MPa), MTP is the maximal paste thickness, and $(1-n)$ denotes the packing density of the cement paste.

$$R_c \propto K \times MTP^{-r} \times (1-n)^2_{\text{paste}} \qquad (10)$$

302 where $r$ is a fitting coefficient, fixed at 0.13 in accordance with the work of de Larrard [28]. To evaluate the validity of this equation, different forming procedures were applied to the same binder (OPC) in order to produce mortars with various porosities (see Step 1, §2.3.1) and to assess their influence on compressive strength. The characteristics of the OPC mortars prepared with the different forming methods are presented in **Table 4**.

307
308 2.3.4. Strength activity index

309 In this study, the CEM I 52.5 N cement was supplied from two different batches by the manufacturer. The second batch was used exclusively for mortars containing superplasticizers, while all other mortars were prepared using the first batch. The compressive strengths of the 310 100% OPC mortars (tested according to NF EN 196-1) for the first and second batches were 311 60.1 ± 2.1 MPa and 56.0 ± 0.5 MPa, respectively. These values were taken as references for 312
calculating the Strength Activity Index (SAI) of the corresponding mix designs containing SCMs.

The SAI and its standard deviation were calculated using Eq. (12) and Eq. (13) to evaluate the reactivity of binders incorporating calcined clays relative to their OPC reference. In these equations, $R_{c,SCM}$ refers to the average compressive strength of mortars containing SCMs, and $R_{c,OPC}$ refers to that of the reference OPC mortar. $SD_{SCM}$ and $SD_{OPC}$ denote the standard deviations for the SCM-based and reference mortars, respectively. Six compressive strength measurements were performed for each formulation.

$$ SAI (\%) = \frac{R_{c,SCM}}{R_{c,OPC}} \qquad (12) $$

$$ SD (\%) = SAI \times \sqrt{\left(\frac{SD_{SCM}}{R_{c,SCM}}\right)^2 + \left(\frac{SD_{OPC}}{R_{c,OPC}}\right)^2} \qquad (13) $$

According to de Larrard [28], the compressive strength can also be expressed as shown in Eq. (14). In this equation, $K(MPa)$ represents the chemical activity of the binder, $K_g(mm^{0.13})$ is a parameter dependent on the aggregate characteristics, $(1-n)$ denotes the packing density of the cement paste, and MTP (mm) is the maximal paste thickness. Since the same aggregate was used for all mortars, $K_g$ is considered to be constant throughout this study.

$$ R_c = K \times K_g \times (1-n)^2_{\text{paste}} \times MTP^{-0.13} = \alpha \times (1-n)^2_{\text{paste}} \times MTP^{-0.13} \qquad (14) $$

Thus, the Binder Activity Index (BAI) was calculated according to Eq. (15), where $\alpha_{OPC}(MPa \cdot mm^{0.13})$ represents the chemical activity of the OPC, and $\alpha_{SCM}$ corresponds to the chemical activity of the binder containing SCMs. The value of $\alpha_{OPC}$ was determined through linear regression (see Figure 2), while $\alpha_{SCM}$ was obtained from the experimental compressive strength, the packing density of the paste, and the maximal paste thickness (MTP).

For each formulation, the average value and standard deviation were computed using three data sets (six compressive strength measurements and three paste packing density values). The BAI
is expected to more accurately reflect the intrinsic reactivity potential of calcined clays than the conventional Strength Activity Index (SAI), as it eliminates the influence of variations in mortar porosity and aggregate network arrangement. The definition and analysis of this Binder Activity Index (BAI) constitute the main objectives of the present study.

$$BAI (\%) = \frac{\alpha_{SCM}}{\alpha_{OPC}} \qquad (15)$$

To evaluate the relevance of the Binder Activity Index (BAI), a Strength Activity Index at identical air content (SAI<sub>iso-air</sub>) was calculated for each SCM and binder tested. The SAI<sub>iso-air</sub> corresponds to the ratio between the compressive strength of a mortar containing SCMs and that of an OPC mortar with the same air content.

The SAI<sub>iso-air</sub> was computed according to Eq. (16), where R<sub>c,SCM</sub> denotes the average compressive strength of the mortar formulation containing SCMs, and R<sub>c,OPC,calculated</sub> represents the calculated compressive strength of the OPC mortar obtained using Eq. (14). The value of R<sub>c,OPC,calculated</sub> was determined using the experimentally derived α<sub>OPC</sub>, while MTP and (1 - n)<sup>2</sup><sub>paste</sub> were calculated based on the air content of the corresponding SCM-containing mortar.

The standard deviation of SAI<sub>iso-air</sub> was evaluated using Eq. (17).

$$SAI_{iso-air} (\%) = \frac{R_{c,SCM}}{R_{c,OPC,calculated}} = \frac{R_{c,SCM}}{\alpha_{OPC} \times (1-n)^2_{paste} \times MTP^{-0.13}} \qquad (16)$$

$$SD_{SAI_{iso-air}} (\%) = \frac{1}{R_{c,OPC,calculated}} \sqrt{A^2 + B^2 + C^2 + D^2} \qquad (17)$$

With:

349 - A = (MTP<sup>-0.13</sup> x (1-n)<sup>2</sup><sub>paste</sub> x SD<sub>α</sub>)<sup>2</sup>

350 - B = (0.13α x MTP<sup>-0.13</sup> x (1-n)<sup>2</sup><sub>paste</sub> x SD<sub>MTP</sub>)<sup>2</sup>

351 - C = (2α x MTP<sup>-0.13</sup> x (1-n)<sub>paste</sub> x SD<sub>(1-n)<sup>2</sup>paste</sub>)<sup>2</sup>

352 - D = (R<sub>c,SCM</sub> x SD<sub>Rc,OPC,calculated</sub>)<sup>2</sup>
353 **3. Results and discussions**

354 **3.1 Compressive strength on OPC mortars with various initial porosity**

355 **3.1.1 Air content and initial porosity**

356 **Table 4** presents the results obtained for OPC mortars prepared with different compaction energies. The mass variation (Δm) during the first 24 hours of hydration ranged between 0.3% and 0.9%, which can be attributed to water loss by desiccation and minor material losses during the demoulding stage.

360 For OPC mortars prepared using the standard method (NF EN 196-1), the air content ranged between 1.3% and 1.6% (samples OPC 1 to 3), which is slightly lower than values reported by other authors for normalized OPC mortars [25,36]. This difference may be explained by the higher precision of volume determination achieved through the 3D scanning method.

364 Although all mortars were cast in 40 × 40 × 160 mm³ moulds, their measured volumes varied between 252.5 cm³ and 261.1 cm³, demonstrating that inaccurate volume estimation can lead to either overestimation or underestimation of the air content if the specimen volume is not precisely measured.

368 As shown in **Table 4**, the air content measured with an aerometer tends to be overestimated compared to the method proposed in this study. For instance, the air contents obtained for OPC 1 were 1.4% with the proposed method and 6.7% with the aerometer. A similar trend was observed for OPC AEA 1.0%, with 8.5% and 16.5%, respectively, in agreement with results reported by other authors [25].

373 The differences in measured air content between the two methods can be attributed to the lower compaction energy applied during aerometer testing, which leads to greater air entrapment. Consequently, air content measurements obtained with an aerometer are not representative of the standard mortar preparation procedure described in NF EN 196-1.
Changes in compaction energy had a significant effect on air content. Using the one-layer compaction method, the air content increased to 3.5% (sample 10H) and 4.1% (sample 5H) for 10 and 5 strokes, respectively. This resulted in a decrease in cement paste packing density of –5% and –6%, and a reduction in sand proportion of –2% and –3%, respectively, compared to the reference mortars (OPC 1–3).

The incorporation of the air-entraining agent (AEA) led to the highest air contents, reaching 7.0% and 8.5% for 0.5% and 1.0% AEA (by binder mass), respectively. Consequently, the cement paste packing density decreased by –12% and –16%, and the sand proportion by –6% and –7%, relative to the reference mortars.

In contrast, the hand-compacted mortar (OPC HC) exhibited a lower air content (0.4%) than the reference, resulting in a higher cement paste packing density (+3%) and a slightly higher sand proportion (+1%).

Overall, the air content directly influences the cement paste packing density by increasing the total paste fraction in the mortar and increasing the maximal paste thickness (MTP) through the reduction of the sand volume fraction.

### 3.1.2 Compressive strength and activity of OPC binder

For OPC 1–3, prepared in accordance with NF EN 196-1, the average 28-day compressive strength ranged between 59.1 MPa and 61.0 MPa, which is consistent with the nominal strength class (52.5 N). The initial porosity and forming method had a significant influence on the compressive strength of the mortars (see Table 4). For samples 10H and 5H, the average strengths decreased to 53.6 MPa and 47.7 MPa, respectively.

When AEA admixtures were used, the average compressive strengths were even lower—43.0 MPa and 37.6 MPa for 0.5% and 1.0% AEA dosages, respectively. These reductions in strength are mainly attributed to the decrease in cement paste packing density and the increase in maximal paste thickness (MTP), which leads to a lower granular skeleton packing density, both
403

resulting from the higher air content in the mortars. In addition, the AEA may also affect the
hydration kinetics of cement [37].

404

405

As shown in **Figure 2**, the compressive strength of OPC mortars is proportional to both the maximal paste thickness (MTP) and the packing density of the cement paste, with a coefficient of determination *R*² = 0.92, in agreement with the relationship proposed by de Larrard [35].

406

407

408

Based on Eq. (14), the activity of the OPC binder (α<sub>OPC</sub>) was determined by linear regression, yielding a value of 476 ± 24 MPa·mm<sup>0.13</sup>. The standard deviation was calculated from experimental data.

409

410

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

411

**Figure 2** - Compressive strength on mortars (MPa) as a function of maximal thickness paste (mm) and binder paste packing density (1-n)ₚaste. Each dot corresponds to a single prism (40mm x 40mm x 160mm). The following coding system has been used: Marks - Filled dots: CC1; Filled triangles: CC2; Empty triangles: CC2 with SP addition; Crosses: CC3. Colors - Red: 85wt% OPC + 15wt% CC binder; Green: LC3-50 binder. Shade of colors are related to calcination temperature (Lighter: 600°C - Darker: 800°C)

412

413

414

415

416

417

**Figure 3** compares the measured and calculated compressive strengths (using de Larrard's model) of OPC mortars prepared with different compaction procedures. The compressive strengths were calculated using air content values obtained either from the 3D scanning method or from the aerometer. As shown in **Table 4**, the aerometer measurements (NF EN 413-2) —

418

419

420
for example, for OPC 1 — overestimate the air content compared with the proposed 3D
scanning method.

Since the OPC activity ($\alpha_{OPC}$) was determined using air content values from the 3D scan,
calculating compressive strength with the aerometer-derived air contents leads to a systematic
underestimation of strength (–32% for OPC 1 and –39% for OPC AEA 1.0%).

When air content is determined using the 3D scanning method, the measured and calculated
compressive strengths show a close agreement, with an average error of 1.7 MPa and a standard
deviation of 1.2 MPa. The largest discrepancies were observed for OPC 5H and OPC AEA
1.0%, with differences of 3.0 MPa and 3.8 MPa, respectively.

Based on de Larrard's model, it appears that air content plays a critical role in the compressive
strength of mortars. For instance, an OPC mortar prepared with the reference compaction
procedure (NF EN 196-1) exhibits an air content of 1.4%, corresponding to a calculated
compressive strength of 59.0 MPa. In contrast, assuming 0% air content results in a calculated
strength of 63.5 MPa, representing an 8% increase compared to the reference condition.

Conversely, with an air content of 4%—a value typical of LC3-50 mortars with superplasticizer
(see §3.2)—the calculated compressive strength decreases to 52.7 MPa, i.e., an 11% reduction
relative to the reference compaction procedure.
<figure><img src="image_4.png" /><figcaption>**Figure 3** – Calculated compressive strength with de Larrard's model and measured compressive strength of OPC mortars with air content measured with 3D scan (filled dots) and air content measured with an aerometer (empty dots). One dot: average of 3 prisms</figcaption></figure>

**3.2 Impact of SCMs on initial porosity with and without SP**

As shown in **Figure 4**, the addition of SCMs leads to a reduction in mortar flowability, primarily depending on the SCM substitution level. The loss of workability is most pronounced for LC3-50 mortars, as this binder contains the highest proportion of calcined clay among those studied. For a given binder, the metakaolin content of the calcined clay appears to influence flowability. For instance, the flow spread of LC3-50 mortars containing CC1_700 and CC2_800 was 158 mm and 135 mm, respectively (see **Table 6**), while their calcined kaolinite contents were 36% and 46%, respectively (see **Table 1**). In comparison, OPC mortars exhibited a significantly higher average spread of 225 ± 3 mm (mean of OPC 1–4).

A similar loss of flowability was observed for the 85/15 binder, though with a smaller variation, due to the lower calcined clay content (15 wt%) compared with LC3-50 (30 wt%). The
calcination temperature did not appear to significantly affect flowability; for example,
CC1_700 and CC1_800 exhibited identical spread values for both 85/15 and LC3-50 mortars.
The loss of flowability is primarily attributed to the presence of calcined clays, which exhibit a
higher metakaolin content and a larger specific surface area than OPC or limestone. These
observations are in good agreement with previous studies [15,17,18].

Under a constant compaction energy, the decrease in flowability—caused by the addition of
SCMs and the increase in substitution level—results in a higher air content in mortars (see
Table 6) compared with the reference mortars. While the air content of the reference mortars
(OPC 1–4) averaged 1.4 ± 0.2%, it ranged between 2.4% and 2.8% for 85/15 mortars and
between 2.5% and 3.1% for LC3-50 mortars (without SP).

In the presence of superplasticizer (SP), mortars containing calcined clays achieved a
consistency comparable to OPC, but also entrapped more air (see Figure 4). For instance, in
LC3-50 with CC2_800, the air content increased from 3.1% to 4.0% following the addition of
SP. This indicates that the selected superplasticizer in this study was not fully suitable for
achieving both similar workability and comparable air content to the reference mortars.

The increase in air content observed in binders containing calcined clays affects both the cement
paste packing density and the maximal paste thickness (MTP), as illustrated in Figure 5 using
de Larrard's model. However, the calcination temperature does not appear to significantly
influence either the paste packing density or the MTP values (see Table 6).

The binder volume in LC3-50 mortars is higher than in OPC mortars due to the lower specific
gravity of the SCMs compared with OPC. Conversely, the air content is also higher in LC3-50
mortars, which partly offsets the effect of the increased binder volume.

In the 85/15 samples, the change in binder volume is less pronounced than in LC3-50, owing
to the lower substitution level, but it still affects mortar flowability. In this case, the binder
volume increase does not fully compensate for the rise in air content, resulting in a lower
product of MTP and paste packing density compared to OPC mortars (see Figure 5).
Furthermore, as shown in **Figure 4**, the addition of superplasticizer (SP) does not reduce the air content in binders containing calcined clays and may even have a negative impact on both the maximal paste thickness (MTP) and the paste packing density (see **Figure 5**, empty triangles).

This effect is more pronounced in LC3-50 mortars, where the addition of SP shifts the MTP–packing density product outside the OPC reference range, whereas without SP, this product remains within the OPC range.

Overall, the SP used in this study appears to be unsuitable for simultaneously improving flowability and limiting air entrainment in mortars containing calcined clays.

However, despite the higher air content, the paste packing density and MTP of LC3-50 mortars remain comparable to those of OPC. Two approaches can therefore be considered when comparing mortars made with OPC and binders containing SCMs: either at a given air content, or at a given product of MTP and paste packing density.

As illustrated in **Figure 5**, binders containing SCMs—particularly LC3-50—with air contents within the range observed for OPC mortars can exhibit higher MTP × packing density products than OPC. According to previous findings (see **Figure 2**), this could potentially lead to an increase in compressive strength.
<figure><img src="image_5.png" /><figcaption>**Figure 4** - Impact on air content and spread of normalised mortars with different type of binders (OPC, 85/15 and LC3-50) and with superplasticizer. The following coding system has been used: Marks - Filled dots: CC1 ; Filled triangles: CC2 ; Empty triangles: CC2 with SP addition ; Crosses: CC3. Colors - Red: 85wt% OPC + 15wt% CC binder ; Green: LC3-50 binder. Shade of colors are related to calcination temperature (Lighter: 600°C – Darker: 800°C)</figcaption></figure>

<table><caption>**Table 6** - Physical characteristics of mortars</caption><thead><tr><th>Type of binder</th><th>Samples</th><th>Spread (mm)</th><th>D<sub>app</sub> (g/cm<sup>3</sup>)</th><th>D<sub>0%</sub> (g/cm<sup>3</sup>)</th><th>Air content (%)</th><th>(1-n)<sub>paste</sub></th><th>MTP (mm)</th></tr></thead><tbody><tr><td>REF</td><td>OPC 1 to 4</td><td>226 ± 3</td><td>2.278 ± 0.006</td><td>2.309</td><td>1.4 ± 0.2%</td><td>0.379 ± 0.002</td><td>3.10 ± 0.03</td></tr><tr><td rowspan="7">85/15</td><td>CC1_700</td><td>195</td><td>2.246</td><td>2.302</td><td>2.4%</td><td>0.374</td><td>3.25</td></tr><tr><td>CC1_800</td><td>193</td><td>2.245</td><td>2.303</td><td>2.5%</td><td>0.373</td><td>3.26</td></tr><tr><td>CC2_600</td><td>185</td><td>2.247</td><td>2.302</td><td>2.4%</td><td>0.375</td><td>3.25</td></tr><tr><td>CC2_700</td><td>182</td><td>2.239</td><td>2.301</td><td>2.7%</td><td>0.373</td><td>3.29</td></tr><tr><td>CC2_800</td><td>184</td><td>2.239</td><td>2.303</td><td>2.8%</td><td>0.370</td><td>3.29</td></tr><tr><td>CC2_800 + SP</td><td>215</td><td>2.247</td><td>2.302</td><td>2.4%</td><td>0.374</td><td>3.25</td></tr><tr><td>CC3_800</td><td>192</td><td>2.249</td><td>2.303</td><td>2.4%</td><td>0.374</td><td>3.24</td></tr><tr><td rowspan="7">LC3-50</td><td>CC1_700</td><td>158</td><td>2.217</td><td>2.284</td><td>3.0%</td><td>0.380</td><td>3.40</td></tr><tr><td>CC1_800</td><td>155</td><td>2.217</td><td>2.286</td><td>3.0%</td><td>0.379</td><td>3.40</td></tr><tr><td>CC2_600</td><td>129</td><td>2.214</td><td>2.284</td><td>3.1%</td><td>0.379</td><td>3.41</td></tr><tr><td>CC2_700</td><td>132</td><td>2.223</td><td>2.281</td><td>2.5%</td><td>0.386</td><td>3.37</td></tr><tr><td>CC2_800</td><td>135</td><td>2.214</td><td>2.285</td><td>3.1%</td><td>0.378</td><td>3.41</td></tr><tr><td>CC2_800 + SP</td><td>228</td><td>2.192</td><td>2.284</td><td>4.0%</td><td>0.369</td><td>3.53</td></tr><tr><td>CC3_800</td><td>166</td><td>2.224</td><td>2.286</td><td>2.7%</td><td>0.382</td><td>3.36</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 5** - Evolution of MTP and binder paste packing density with Larrard's model in function of air content in mortars with different binders and with SP (for CC2_800). Between the 2 continuous black lines, range of reference mortars (OPC) made according to NF EN 196-1. The following coding system has been used: Marks - Filled dots: CC1; Filled triangles: CC2; Empty triangles: CC2 with SP addition; Crosses: CC3. Colors - Red: 85wt% OPC + 15wt% CC binder; Green: LC3-50 binder. Shade of colors are related to calcination temperature (Lighter: 600°C - Darker: 800°C)</figcaption></figure>

## 3.3 Mechanical properties of mortars including SCMs

### 3.3.1 Compressive strength and binder activity results

**Table 7** presents the compressive strength results for all tested mortars. The mean compressive strength of the reference mortars from the first cement batch (OPC 1-3) was 60.1 ± 2.1 MPa (nine measurements). In this case, the corresponding binder activity was 476 ± 24 MPa·mm⁻⁰.¹³ (see **Figure 2**). These values were used as references for comparison with binders containing SCMs prepared without superplasticizer (SP).

Mortars containing SP were compared with OPC 4, prepared from the second cement batch used in this study. The average compressive strength of this second batch was 56.0 ± 0.5 MPa, which is slightly lower than that of the first batch, despite a similar air content (see **Table 7**). In this second case, the corresponding binder activity was 446 MPa·mm⁻⁰.¹³.
The calcined kaolinite content in the calcined clays (CCs) strongly influences the 28-day compressive strength of the mortars. CC2_800 and CC3_800 contain 46% and 30% calcined kaolinite, respectively, and CC2_800 yielded higher compressive strengths for both 85/15 and LC3-50 binders. This improvement can be attributed to the greater formation of hydration products resulting from the pozzolanic reaction between the calcined kaolinite in the CC and the portlandite released by the OPC [9,38]. These findings are consistent with the R3 test results, as CC2_800 exhibited higher reactivity than CC3_800 (see **Table 1**).

The compressive strengths of binders containing CC1_800 and CC3_800 are comparable (see **Table 7**). However, CC3_800 exhibits a lower reactivity than CC1_800, with 7.38 g of bound water per 100 g of paste compared to 8.39 g/100 g of paste for CC1_800. This difference is attributed to the lower calcined kaolinite content of CC3_800 (30%) relative to CC1_800 (36%).

Since the paste packing density and MTP are not significantly different between these two clays (see **Table 6**), the similar compressive strengths cannot be explained by a lower initial porosity of the mortar containing CC1_800. This is further supported by the binder activity values, which are 532 ± 8 MPa·mm⁻⁰.¹³ for CC1_800 and 525 ± 10 MPa·mm⁻⁰.¹³ for CC3_800 (see **Table 7**).

Although both CCs exhibit similar specific surface areas (SSA), Figure 1 shows that CC3_800 has a finer particle size distribution (PSD) than CC1_800. This higher fineness may provide additional nucleation sites during binder hydration [13,39], thereby allowing CC3_800 to achieve comparable 28-day compressive strengths despite its lower calcined kaolinite content.

Furthermore, the calcination temperature significantly influences the mechanical properties of the mortars. For both CC1 and CC2, the highest compressive strengths were obtained at 800°C for both binders (see **Table 7**), with no noticeable change in the particle size distribution (PSD) of the CCs (see **Figure 1**). This trend is also reflected in the binder activity values, indicating
that the increase in mechanical performance is not related to differences in initial mortar porosity.

Several authors [10,11,14] have attributed this enhanced reactivity to a more disordered aluminosilicate structure, associated with the presence of five-coordinated aluminium in the calcined clays. However, in the present study, no significant difference in CC reactivity was detected by the R3 test (see **Table 1**), which may be explained by the lower calcined kaolinite contents of the clays investigated and/or by minor variations within the experimental uncertainty [40].

Finally, the LC3-50 mortar containing CC2 did not achieve mechanical properties comparable to the OPC mortars, despite a calcined kaolinite content greater than 40% [41]. The corresponding SAI and BAI values were 84% and 87%, respectively. This difference cannot be attributed to the initial porosity of the LC3-50 mortar. It may instead be related to the clinker fineness [13] and/or a possible excess of sulphates in the binder, which could alter the hydration kinetics and phase assemblage, thereby negatively affecting the compressive strength of mortars containing LC3 binders [42–44].

### 3.3.2 Comparison of SAI and BAI results

**Figure 6** presents the SAI and BAI values for all calcined clays (CCs) studied, for both 85/15 and LC3-50 binders. The standard deviations were calculated according to Eq. (13). For all CCs and both binders, regardless of calcination temperature, the SAI values were lower than the BAI values. This indicates that using the SAI to assess the reactivity of SCMs tends to underestimate the binder activity, mainly due to the increase in air content resulting from the loss of flowability caused by CC addition at constant compaction energy.

The addition of superplasticizer (SP) does not reduce this discrepancy between SAI and BAI. For example, for the LC3-50 binder with CC2_800, the difference between SAI and BAI is approximately 3 percentage points without SP and 7 percentage points with SP (see **Table 6**).
As discussed in §3.2, while the SP allows mortars containing CCs to achieve the same consistency as OPC mortars, it also promotes air entrapment, thereby increasing the initial porosity.

**Figure 7** shows the deviation of BAI relative to SAI. For LC3-50, this deviation is about 2% (without SP). Despite the higher air content, the larger binder volume due to the higher SCM proportion leads to a binder paste packing density x MTP product comparable to that of OPC (see **Figure 5**). In this study, the deviation is greater when SP is used, due to air trapping. For the 85/15 binder, the deviation between SAI and BAI is around 6%, which can be attributed to the increase in initial porosity caused by SCM addition and to the limited binder volume change resulting from the lower OPC substitution rate.

These trends are further confirmed in **Figure 8**, where the measured SAI appears to be underestimated compared to $SAI_{iso-air}$, particularly for LC3 binders, which exhibit the highest air contents under the reference compaction procedure. For the 85/15 binder, however, firm conclusions are more difficult to draw, as the difference between SAI and $SAI_{iso-air}$ remains within the 5% error margin associated with estimating the OPC compressive strength at equivalent air content. In contrast, for LC3 binders, the deviation is more pronounced, especially when SP is added, due to the higher air content in these mortars.

Nevertheless, if a suitably selected SP were to achieve similar consistency and air content to those of OPC mortars, the binder paste packing density of mortars containing SCMs would become higher than that of OPC, potentially resulting in higher compressive strengths for the same binder system. For example, for the LC3-50 binder with CC2_800, the binder paste packing density equals 0.378 and the MTP equals 3.41 mm with an air content of 3.1% (see **Table 6**). If the air content were reduced to 1.4% (average value for OPC mortars), the corresponding values would be 0.393 for packing density and 3.22 mm for MTP. Using Eq. (14) and de Larrard's model, with a binder activity of 413 MPa·mm⁻⁰.¹³, the calculated compressive strength would increase from 50.3 MPa (with SP and 3.1% air) to 55.0 MPa.
The use of admixtures in standardized mortars can have a significant impact on the measured mechanical properties. Future work should therefore explore different types of superplasticizers (SPs) to assess their ability to achieve both equivalent consistency and air content, and to confirm or refute their influence on mechanical performance.

<table><caption>**Table 7** – Compressive strength (MPa), chemical activity of binder (MPa.mm⁻⁰.¹³) and activity index (SAI, BAI and SAI<sub>iso-air</sub>). Standards deviations of SAI and BAI are calculated according to Eq. 13</caption><thead><tr><th>Type of Binder</th><th>Samples</th><th>R<sub>c</sub> (MPa)</th><th>α (MPa.mm<sup>-0.13</sup>)</th><th>SAI (%)</th><th>BAI (%)</th><th>SAI<sub>iso-air</sub> (%)</th></tr></thead><tbody><tr><td rowspan="2">OPC</td><td>OPC 1 to 3</td><td>60.1 ± 2.1</td><td>476 ± 24</td><td>-</td><td>-</td><td>-</td></tr><tr><td>OPC 4</td><td>56.0 ± 0.5</td><td>446 ± 1</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="7">85/15</td><td>CC1_700</td><td>58.6 ± 2.1</td><td>488 ± 16</td><td>97 ± 5%</td><td>103 ± 6%</td><td>105 ± 6%</td></tr><tr><td>CC1_800</td><td>63.5 ± 1.4</td><td>532 ± 8</td><td>106 ± 4%</td><td>112 ± 6%</td><td>114 ± 6%</td></tr><tr><td>CC2_600</td><td>61.5 ± 1.9</td><td>511 ± 11</td><td>102 ± 5%</td><td>107 ± 6%</td><td>110 ± 6%</td></tr><tr><td>CC2_700</td><td>62.0 ± 0.9</td><td>521 ± 5</td><td>103 ± 4%</td><td>109 ± 6%</td><td>112 ± 6%</td></tr><tr><td>CC2_800</td><td>66.3 ± 2.1</td><td>564 ± 4</td><td>110 ± 5%</td><td>118 ± 6%</td><td>121 ± 7%</td></tr><tr><td>CC2_800 + SP</td><td>59.3 ± 1.4</td><td>494 ± 13</td><td>106 ± 3%</td><td>111 ± 3%</td><td>113 ± 6%</td></tr><tr><td>CC3_800</td><td>63.1 ± 1.4</td><td>525 ± 10</td><td>105 ± 4%</td><td>110 ± 6%</td><td>112 ± 6%</td></tr><tr><td rowspan="7">LC3 - 50</td><td>CC1_700</td><td>48.6 ± 0.6</td><td>394 ± 2</td><td>81 ± 3%</td><td>83 ± 4%</td><td>89 ± 5%</td></tr><tr><td>CC1_800</td><td>47.8 ± 0.6</td><td>391 ± 3</td><td>80 ± 3%</td><td>82 ± 4%</td><td>88 ± 5%</td></tr><tr><td>CC2_600</td><td>49.0 ± 0.9</td><td>400 ± 8</td><td>82 ± 3%</td><td>84 ± 5%</td><td>91 ± 5%</td></tr><tr><td>CC2_700</td><td>47.9 ± 2.3</td><td>377 ± 12</td><td>80 ± 5%</td><td>79 ± 5%</td><td>86 ± 5%</td></tr><tr><td>CC2_800</td><td>50.3 ± 1.1</td><td>413 ± 10</td><td>84 ± 3%</td><td>87 ± 5%</td><td>93 ± 5%</td></tr><tr><td>CC2_800 + SP</td><td>45.9 ± 1.7</td><td>397 ± 16</td><td>82 ± 3%</td><td>89 ± 3%</td><td>95 ± 6%</td></tr><tr><td>CC3_800</td><td>47.2 ± 1.1</td><td>379 ± 2</td><td>78 ± 3%</td><td>80 ± 4%</td><td>85 ± 5%</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Figure 6** – Comparison of SAI and BAI for 85/15 and LC3-50 binders and for calcined clays studied (CC) at different temperatures</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 7** – Deviation of BAI compared to SAI on LC3-50 and 85/15 binder. Reference is OPC binder</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8** – Deviation of SAI iso-air compared to measured SAI. z = 0.05 corresponds to the sum of the average error (1.7 MPa) and standard deviation (1.2 MPa) between calculated and measured compressive strength of OPC mortars with Eq.(14), expressed in % of reference compressive strength (OPC 1 to 3, 60.1 MPa).</figcaption></figure>

**4. Conclusions and perspectives**

This study highlights the influence of initial porosity on the measured compressive strength of standardized mortars prepared in accordance with NF EN 196-1. A new experimental method was proposed to accurately quantify the air fraction in mortars, enabling the comparison of compressive strength at equivalent porosity and eliminating experimental variability related to the incorporation of SCMs. A new parameter, the Binder Activity Index (BAI), was introduced to provide a more reliable assessment of calcined clay (CC) reactivity, as confirmed by the SAI<sub>iso-air</sub> results. The impact of initial porosity on compressive strength was analyzed using de Larrard's model, and the relationship between porosity and binder activity was evaluated for mortars incorporating two different SCM-based binders.

Based on the results obtained, the following conclusions can be drawn:
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* Initial porosity has a significant effect on the compressive strength of mortars by reducing both the cement paste packing density and the granular skeleton packing density.

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* De Larrard's model [28] is valid for calculating binder activity.

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* The use of calcined clays as SCMs increases air content in standardized mortars due to reduced flowability under constant compaction energy. The calcination temperature has no significant effect on measured air content.

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* The increase in air content in binders containing SCMs does not necessarily improve paste packing density, as the higher specific surface area of SCMs leads to a decrease in skeleton packing density.

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* The use of superplasticizers (SPs) can enhance flowability but may simultaneously increase initial porosity.

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Further studies should aim to validate de Larrard's model for binders containing different cements and various SCMs, particularly those that strongly affect workability (e.g., ultrafine slags, silica fume). Additional tests using different superplasticizer types are also needed to determine whether standardized mortars incorporating calcined clays of varying grades can be formulated with the same air content as OPC mortars, thereby achieving higher compressive strengths and clarifying the influence of admixtures on the evolution of mechanical performance over time.

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**Author Contributions:** Conceptualization, M.C., A.P., L.L., N.L., S.G., D.R. ; methodology, A.P., M.C., N.L., L.L., S.G.; validation, A.P., S.G., L.L. ; formal analysis, A.P., N.L. ; investigation, N.L., L.L.; resources, A.P.; D.R.; writing—original draft preparation, A.P., N.L.; writing—review and editing, A.P., S.G., L.L. ; supervision, A.P., S.G., L.L. ; project administration, A.P., D.R. . All authors have read and agreed to the published version of the manuscript.
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655 Data will be made available on reasonable request.
The air content was calculated according to Eq. (A.1) by measuring the apparent density of the mortars ($D_{app}$) and computing the theoretical density at 0% air content ($D_{0%}$), based on the mix design of each mortar. The relative error on the air content was determined using Eq. (A.2), which represents the sum of the errors associated with both the apparent density ($D_{app}$) and the theoretical density ($D_{0%}$).

The relative error of the apparent density ($D_{app}$) is obtained by summing the relative errors of the apparent mass ($m_{app}$) and the apparent volume ($V_{app}$), the latter being measured using the 3D scanning method. This error is expressed by Eq. (A.3).

Similarly, the relative error of the theoretical density ($D_{0%}$) is defined as the sum of the relative errors associated with the total mass of materials used in the mortar ($m_{0%}$) and the void-free volume ($V_{0%}$), calculated using the specific gravity (measured by helium pycnometry) and the mix design. This relationship is expressed by Eq. (A.4).

$$ \text{Air content} = 1 - \frac{D_{app}}{D_{0\%}} \% \qquad (A.1) $$

$$ \frac{\Delta(\text{Air content})}{\text{Air content}} = \frac{\Delta(D_{app})}{D_{app}} + \frac{\Delta(D_{0\%})}{D_{0\%}} \qquad (A.2) $$

$$ \frac{\Delta(D_{app})}{D_{app}} = \frac{\Delta(m_{app})}{m_{app}} + \frac{\Delta(V_{app})}{V_{app}} \qquad (A.3) $$

$$ \frac{\Delta(D_{0\%})}{D_{0\%}} = \frac{\Delta m_{0\%}}{m_{0\%}} + \frac{\Delta V_{0\%}}{V_{0\%}} \qquad (A.4) $$

Errors on $m_{app}$ arise primarily from mass loss between the fresh state and demoulding (24 h).

The maximum mass loss was observed for the OPC HC sample, reaching 0.9%, while the average mass loss across all samples was 0.4%. This estimation involves a strong assumption, as part of the observed loss may be due to binder paste adhering to the mould surfaces.
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An additional source of uncertainty stems from the balance precision, which is ±0.1 g. Given the minimum recorded sample mass of 543.6 g, the corresponding maximum relative weighing error is 0.02%. Consequently, the maximum relative error on $(\Delta m_{\text{app}})/m_{\text{app}}$ in Eq. (A.3) is estimated at 0.92%.

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Errors on $V_{\text{app}}$ are associated with both scanner precision and sample shrinkage. The 3D scanner used in this study has a precision of 1 µm, making scanning errors negligible. However, during the first 24 hours (when mortars were kept in moulds, covered to minimize moisture loss, and stored at 20°C), volume changes may occur due to bleeding or evaporation. The maximum mass loss of 0.9%, corresponding to 602.8 g, would translate—if attributed entirely to water evaporation or bleeding—to a maximum volume error of 5.4 cm³. For the smallest sample volume (252.49 cm³), this represents a maximum relative error of 2.1%.

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During immersion curing, volume variations are mainly due to autogenous shrinkage. For the LC3-50 binder, autogenous shrinkage does not exceed 1500 µm/m [32–34]. Assuming isotropic shrinkage, the corresponding volume error for a 40 × 40 × 160 mm³ prism is only $8.64 \times 10^{-7}$ cm³, which is negligible compared to the total specimen volume. Therefore, the maximum relative error on $(\Delta V_{\text{app}})/V_{\text{app}}$ is estimated at 2.1%.

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By summing the mass and volume contributions, the maximum relative error on $(\Delta D_{\text{app}})/D_{\text{app}}$ (see Eq. (A.3)) is estimated at 3.02% (0.92% + 2.1%).

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Errors on $m_{0\%}$ originate from weighing inaccuracies of the binder constituents, primarily due to the balance precision (±0.1 g). For LC3-50 mortars, five separate weighings were required (water, OPC, CC, LS, and G), resulting in a maximum cumulative weighing error of 0.5 g. For the standard sand, the supplier specifies a maximum weighing uncertainty of 5.0 g.

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In the case of standard mortars, the total batch mass is 2025 g (comprising 1350 g of sand, 450 g of binder, and 225 g of water). Consequently, the maximum relative error on $(\Delta m_{0\%})/m_{0\%}$ in Eq. (A.4) is estimated at 0.27%.

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V₀% was determined from the specific gravity measured using a helium pycnometer, combined with the weighed mass of each constituent. The maximum pycnometer error was observed for CC2_800, amounting to 0.014 g/cm³ for an average specific gravity of 2.830 g/cm³, corresponding to a relative error of 0.49%. Since mass values are also used in the calculation of V₀%, this uncertainty propagates directly into the determination of V₀%. Consequently, the maximum relative error on (ΔV₀%)/V₀%) in Eq. (A.4) is 0.49% + 0.27% = 0.76%. Therefore, the maximum relative error on (ΔD₀%)/D₀%) in Eq. (A.4) is 0.76% + 0.27% = 1.03%.

The maximum relative error on air content, (Δ(Air content))/(Air content)) from Eq. (A.1), is thus 3.02% + 1.03% = 4.05%.

The uncertainty analysis presented in Appendix 1 shows that the main sources of error in mass, volume, and density measurements remain limited. The maximum relative error on apparent density was estimated at 3.02%, while that on theoretical density was 1.03%, leading to a maximum propagated error of approximately 4.05% for air content. In absolute terms, this corresponds to 0.35% for the highest measured air content (8.6% in OPC mortars with AEA), and between 0.09% and 0.17% for binders containing SCMs, where air contents range from 2.3% to 4.1%.

These results confirm that the methodology used to determine air content is sufficiently accurate to support the main conclusions of the study. In particular, the uncertainty on air content is significantly smaller than the deviations observed between SAI, SAI iso-air, and BAI. Hence, neglecting density variations in the calculation of binder activity indices would introduce a much larger bias than the quantified measurement errors. The error analysis therefore reinforces the relevance of correcting for porosity effects when assessing the activity of binders containing calcined clays.

723
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