# Sulfate adjustment and early reactivity in cements containing kaolinitic calcined clays: investigation and assessment via a sulfate-limited model system

Tafadzwa Ronald Muzenda, Yannick Demeusy, Fabien Georget, Christophe Labbez, Thomas Matschei

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Tafadzwa Ronald Muzenda, Yannick Demeusy, Fabien Georget, Christophe Labbez, Thomas Matschei. Sulfate adjustment and early reactivity in cements containing kaolinitic calcined clays: investigation and assessment via a sulfate-limited model system. Cement and Concrete Research, 2026, 199, pp.108060. 10.1016/j.cemconres.2025.108060. hal-05400345

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# Sulfate adjustment and early reactivity in cements containing kaolinitic calcined clays: investigation and assessment via a sulfate-limited model system

Tafadzwa Ronald Muzenda<sup>a,*</sup>, Yannick Demeusy<sup>b</sup>, Fabien Georget<sup>a</sup>, Christophe Labbez<sup>b</sup>, Thomas Matschei<sup>a</sup>

## ARTICLE INFO

### Keywords:

- Calcined clay

- Early reactivity

- Hydration

- Enthalpy of formation

- Physical properties

## ABSTRACT

Understanding the early reaction kinetics of calcined clays (CCs) in calcined clay-limestone cements (CCLC) is required to optimize the formulations for optimal early performance. In this study, a clinker-free sulfate-limited model system (SLiM) is utilized to compare the early reactivity of 9 diverse natural CCs. The SLiM consists of an excess of CC and portlandite, and limited gypsum. It is demonstrated that the sulfate-carrier depletion time provides a rate of reaction for each clay which is characteristic of its reactivity in a blended cement. As such, it is shown that the SLiM test can be used to probe physico-chemical properties, including the standard enthalpy of formation of metakaolinite, and to understand the mechanical performance and hydration of blended cement with these clays up to 3 days. In conclusion, a foundation for a standard test allowing to adjust gypsum content in CCLC from a single calorimetry measurement is developed.

## 1. Introduction

The substitution of clinker by calcined clays (CCs) and limestone in calcined-clay limestone cement (CCLC also referred to as LC³ from the project of the same name [1]) significantly reduces the clinker factor and, in turn, the carbon emissions associated to cement production [1,2]. One of the preconditions of the successful use of CCLC is a sufficiently high reactivity of CC [1,3,4]. Previous studies have shown that CCLC binders can achieve mechanical performance comparable to OPC at 28 days [1,5,6]. Furthermore, it was also shown that associated minerals, such as quartz, hematite and other clay phases like illite should be considered during reactivity assessment, since they influence the properties of the calcined clay, and the early age properties such as rheology and compressive strength of the blended cement [7].

The assessment of early reactivity of CCs is a subject of industrial interest in the utilization of CCLC [8] due to the typical lower early-age performance of blended systems with lower clinker factors, and issue with slump loss. Regarding reactivity testing, a lot of progress has been achieved within the work of the RILEM TC 267, which led to the development of the so called R³ test, which is now also manifested in

various standards e.g. ASTM C1897 and EN 197. The ASTM C1897 standard is used to measure the reactivity of CCs within 7 days at an elevated temperature of 40 °C [9] by measuring the heat of reaction, or the chemical bound water. The results of this test after 7 d have been shown to be correlated relatively well with compressive strength development at 28 days [5,10]. In addition, a newly developed test based on dissolution of calcined clays in NaOH solution has shown good correlation to the R³ test [11]. However, considering the R³ test results at 7 d is not suitable for the investigation of the early reactivity of CCs [7,12]. The main underlying reason is that the R³ test results at 7 d mainly follow the (meta)kaolin content, and it does not significantly distinguish the impact of other factors which may impact early age reactivity such as the physico-chemical constitution of the clay and the role of secondary minerals [7,13]. Furthermore, recent research papers [14–16] indicate that CC does actively contribute to early hydration of CCLC, which may significantly impact early age performance in terms of strength and workability. As a consequence, there is a need to develop complementary tests that can be used to specifically address the early age reactivity of SCMs e.g. CCs to enable further optimization of blended cement formulations.

<footnote><sup>a</sup> Institute of Building Materials Research at RWTH Aachen University, 52062, Aachen, Germany</footnote>

<footnote><sup>b</sup> Laboratoire Interdisciplinaire Carnot de Bourgogne UMR 6303 CNRS, Université Bourgogne Europe, FR-21000, Dijon, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: muzenda@ibac.rwth-aachen.de, t.r.muzenda@gmail.com (T.R. Muzenda).</footnote>
Studies of clinker-free systems help to understand the intrinsic re-
action behaviour of activated clays [14,17,18]. The lime reactivity test,
based on a mix of SCM, portlandite (CH) and water is extensively used to
assess the reactivity of CC as part of the Indian standard [10,18,19].
Murat [17] observed that in a system containing metakaolinite, por-
tlandite and water (MK-CH-H₂O) there is a strong dependency of hy-
drate phase precipitation on the available calcium content. Calcium
aluminate silicate hydrates (C-A-S-H), calcium aluminate hydrates
(C₄AH₁₃ or C₃AH₆) or strätlingite are formed in the absence of sulfate
carriers and calcite as a function of portlandite content. In a study
comparing the reaction behaviour of three different meta-phyllosilicate-
CH mixtures in alkaline conditions, Scherb et al. [14] observed that in
presence of sulfate, C-A-S-H and ettringite formed in the MK-CH-CaSO₄-
K/NaOH-H₂O system. They demonstrate an intrinsic reactivity of the
clays in timescales comparable to early-hydration of cement pastes. This
indicates that the early reactivity of the CC also impacts the sulfate
required for an optimum hydration performance of the CCLC, and hence
needs to be considered within sulfate optimisation measures. This has
also been highlighted before by Zunino and Scrivener [20]. Unlike
Scherb et al., Zunino attributed the additional need for sulfate to
enhanced early C-A-S-H precipitation and increased sulfate sorption due
to the increase of SSA. However, some of these studies are focusing on
high-grade clays, or clays synthetized from high-grade clays, which as
demonstrated in this study, may have lower (early) reactivity than
natural/industrially-sourced clays. As such, there is a need for a more
systematic analysis of the reactivity of activated natural clays.

Sulfate balance is a critical part of the binder formulation. CCLC
require careful sulfate adjustment due to accelerated sulfate depletion
caused by the filler effect of calcined clay and limestone and the early
reactivity of calcined clays [20–23]. This ensures that the reaction of C₃S
precedes that of C₃A which improves early strength. Widely used tech-
niques for sulfate balance include calorimetry, strength and shrinkage at
different sulfate contents [23,24]. The application of some of these
techniques in cement plants is limited by equipment availability.
Recently, Canbeck et al. [22] proposed a methodology based on Kernel
smoothing of calorimetry measurements to determine optimal sulfate of
CCLC. Between carrying out many experiments to requiring advanced
modelling or computational methods that require experimental verifi-
cation, sulfate optimization of CCLC binders remains a time-consuming
task.

A practical test method that can be used to better assess the early
reactivity of CC and, in turn, aid in binder formulation, including opti-
mum sulfate prediction, is lacking. This is of utmost importance since it
significantly impacts the early hydration and hence the early perfor-
mance of CCLC in terms of mechanical strength and rheology. In this
study, we extend the use of a clinker free sulfate-limited model system
(SLiM) based on the components CC-CH-CaSO₄(-Cc)-H₂O to enable a
study of the intrinsic calcined clay reactivity, and the formulation of
calcined clay limestone cement. To complement previous studies, a new
thermodynamic approach that enables a quantification of the contri-
bution of reacted metakaolinite to the early age hydration of CCLC is
introduced, thereby giving direct guidance to parameters such as addi-
tional silica and alumina, resulting from clay dissolution and the need
for sulfate adjustment. The approach is validated with the help of 9
natural/industrially sourced CCs with varying mineral compositions (8
kaolinitic clays and 1 smectitic clay). This study focuses primarily on
kaolinitic clays. With the use of techniques such as calorimetry,
rheology, in-situ XRD, TGA and SEM of CCs, the effect of CCs on rheo-
logical changes and hydrate phase precipitation in the SLiM is investi-
gated. Finally, how the results of the SLiM test can be used to infer
additional gypsum requirements is shown. In conclusion, a pathway
towards a standardized method to assist with a more systematic
formulation of limestone calcined clay cements is proposed.

## 2. Materials and methods

2.1. Materials

9 raw clays with a wide range of mineral composition were supplied
by industrials partners. The sulfate-carrier supplied for this study is
gypsum. Before characterization and calcination, the clays were dried at
105 °C for at least 48 h until constant mass and crushed to less than 2
mm using a jaw-crusher. Representative samples of the raw clays were
then ground to pass 63 µm using a disk mill for characterization.
Chemical composition determined using X-ray fluorescence (XRF)
analysis is shown in Table 1. A detailed characterization of the raw clays
is provided in [13], as well as a presentation and discussion of the
experimental methods. In addition, the mineral composition of the raw
clays form [7] is provided in Table A1. The names of the calcined clays
(CCs) are based on metakaolinite content: >80 % (A), >60 % (B), >40 %
(C) and the other two about 23 % MK (D1) and the smectitic clay (D3).
The kaolinite amount is taken as the approximate metakaolinite amount
(though in practice the wt% would be slightly lower due to water loss,
for example 72 wt% of kaolinite, becomes about 69 wt% of meta-
kaolinite after full conversion at optimal calcination temperature).
Calcination of dried and jaw-crushed raw clays was carried out using a
laboratory muffle furnace at 800 °C for the kaolinitic clays and 900 °C
for D3 (smectitic clay). These temperatures were optimized in a related
study using the same raw materials [25]. In both cases, a sample depth of
less than 4 cm in the crucible and a residence time of 2 h was used.
Thereafter, the CCs were ground in batches of 3–4 kg using a laboratory
steel ball mill operating at 60 rpm, and steel balls having a total mass of
about 50 kg. The reference grinding time was 90 min.

In addition, a high-grade kaolinite powder (with a purity of 96 %)
and reagent-grade calcium hydroxide were purchased from Sigma-
Aldrich (03584 and 102,047, respectively). The high-grade kaolinite
was calcined at 700 °C for 2 h. Quartz powder was obtained from
grinding standard sand for mortar using a disk mill. The quartz powder
was used to substitute 30 wt% of the high grade metakaolinite (purity
95 %) to form a sample named K700 which contains about 70 %
metakaolinite.

Ground clinker, limestone, and gypsum were also supplied by in-
dustrial partners. The phase composition determined by X-ray diffrac-
tion (XRD) with Rietveld analysis for clinker, limestone and gypsum, as
well as the analysis of the CCs are shown in Table 2. The particle size
distribution of the powders (Fig. 1) and the d₁₀, d₅₀ and d₉₀ (Table 3)
were determined by laser diffraction (Malvern Mastersizer). Specific
surface area (SSA) was measured using the BET method and is also
shown in Table 2. A PCE-based superplasticizer (SP) formulated for
binders containing calcined clays was provided by industrial partners to
adjust flowability of CCLC-50 (CCLC with approximately 50 % clinker
composition, 30 % CC and 15 % limestone).

## 2.2. Methods

2.2.1. Clinker-free sulfate-limited system and cement paste

Two systems were investigated in this study:

(1) SLiM: A clinker-free model system, hereafter referred to as the sulfate-limited model system (SLiM) composed of calcined clay - portlandite - sulfate-carrier, with a ratio of 1:0.25:0.1 [28]. These conditions have been developed to obtain a sulfate-carrier and maximum ettringite time within 30 h for the natural/industrially-sourced kaolinitic clays, to be easily measurable by isothermal calorimetry, as well as to ensure an excess of portlandite. The use of limestone as part of the SLiM is introduced in section 3.4. A water to binder ratio (w/b) of 0.7 was used for rheology studies and 1.0 was used for the rest of the other experiments.

(2) CCLC: A calcined clay limestone cement (CCLC) made of 49 % clinker, 30 % CC, 15 % limestone and 6 % gypsum (hereafter
<table><caption>**Table 1**

Chemical composition of clinker, gypsum, limestone and raw clays (X-R, -R signifies sample before activation) materials determined by XRF.</caption><thead><tr><th>Oxide (wt%)</th><th>Clinker</th><th>Gypsum</th><th>Limestone</th><th>A1-R</th><th>B1-R</th><th>B2-R</th><th>C1-R</th><th>C2-R</th><th>C3-R</th><th>C4-R</th><th>D1-R</th><th>D3-R</th></tr></thead><tbody><tr><td>CaO</td><td>63.7</td><td>35.0</td><td>55.3</td><td>0.15</td><td>0.27</td><td>0.27</td><td>0.32</td><td>0.74</td><td>0.16</td><td>0.25</td><td>0.97</td><td>0.71</td></tr><tr><td>Al2O3</td><td>5.50</td><td>0.15</td><td>0.22</td><td>36.5</td><td>30.9</td><td>30.4</td><td>27.2</td><td>25.7</td><td>25.5</td><td>28.4</td><td>14.5</td><td>15.4</td></tr><tr><td>SiO2</td><td>21.0</td><td>0.69</td><td>1.16</td><td>49.4</td><td>52.5</td><td>37.0</td><td>27.5</td><td>49.9</td><td>55.0</td><td>54.8</td><td>70.7</td><td>54.0</td></tr><tr><td>P2O5</td><td>0.17</td><td>0.01</td><td>0.54</td><td>0.10</td><td>0.12</td><td>0.21</td><td>0.19</td><td>0.22</td><td>0.13</td><td>0</td><td>0.04</td><td>0.03</td></tr><tr><td>SO3</td><td>2.91</td><td>41.7</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.12</td><td>0.06</td><td>1.16</td></tr><tr><td>K2O</td><td>0.80</td><td>0.10</td><td>0.17</td><td>0.51</td><td>0.99</td><td>0.05</td><td>0.03</td><td>1.29</td><td>1.61</td><td>1.3</td><td>1.10</td><td>1.84</td></tr><tr><td>Na2O</td><td>0.04</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.21</td><td>0.01</td><td>0.88</td><td>0.01</td><td>1.34</td></tr><tr><td>MgO</td><td>1.51</td><td>0.57</td><td>0.98</td><td>0.11</td><td>0.27</td><td>0.20</td><td>0.62</td><td>1.13</td><td>0.44</td><td>1.34</td><td>0.38</td><td>3.26</td></tr><tr><td>TiO2</td><td>0.28</td><td>0.01</td><td>0.01</td><td>0.25</td><td>1.80</td><td>2.13</td><td>3.17</td><td>1.08</td><td>0.57</td><td>1.75</td><td>1.13</td><td>0.93</td></tr><tr><td>V2O5</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.13</td><td>0.04</td><td>0.04</td><td>0.12</td><td>0.04</td><td>0.02</td><td>0</td><td>0.02</td><td>0.01</td></tr><tr><td>Cr2O3</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.04</td><td>0.03</td><td>0.09</td><td>0.02</td><td>0.01</td><td>0</td><td>0.02</td><td>0</td></tr><tr><td>MnO</td><td>0.06</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.03</td><td>0.14</td><td>0.03</td><td>0.03</td><td>0.01</td><td>0.01</td><td>0.07</td></tr><tr><td>Fe2O3</td><td>2.63</td><td>0.09</td><td>0.18</td><td>0.79</td><td>1.55</td><td>14.84</td><td>26.24</td><td>7.49</td><td>5.18</td><td>2.31</td><td>4.55</td><td>9.72</td></tr><tr><td>LOI</td><td>1.43</td><td>21.7</td><td>41.4</td><td>13.6</td><td>11.6</td><td>14.2</td><td>14.5</td><td>11.4</td><td>11.3</td><td>8.50</td><td>6.60</td><td>11.0</td></tr></tbody></table>

<table><caption>**Table 2**

Phase composition of CCs, clinker, gypsum and limestone. ICSD numbers of the structures used for the Rietveld analysis are provided in [7,26]. The full characterization of the raw clays is provided in [13].</caption><thead><tr><th>Phase (wt%)</th><th>A1</th><th>B1</th><th>B2</th><th>C1</th><th>C2</th><th>C3</th><th>C4</th><th>D1</th><th>D3</th><th>Clinker</th><th>Limestone</th><th>Gypsum</th></tr></thead><tbody><tr><td>Quartz</td><td>7.3</td><td>19.4</td><td>2.0</td><td>0.8</td><td>23.3</td><td>23.8</td><td>35.7</td><td>50.6</td><td>13.5</td><td>-</td><td>0.6</td><td>1.5</td></tr><tr><td>Muscovite (/Illite) (high temperature [27])</td><td>1.7</td><td>2.6</td><td>-</td><td>-</td><td>3.3</td><td>3.4</td><td>3.8</td><td>1.5</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Hematite</td><td>-</td><td>-</td><td>15.7</td><td>22.5</td><td>3.1</td><td>4.9</td><td>-</td><td>0.7</td><td>4.2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Anatase</td><td>-</td><td>0.7</td><td>3.9</td><td>1.5</td><td>0.5</td><td>0.6</td><td>0.6</td><td>-</td><td>0.8</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Feldspar</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>7.2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Rutile</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.8</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>XRD Amorphous</td><td>90.9</td><td>77.4</td><td>78.5</td><td>75.3</td><td>67.3</td><td>65</td><td>58.7</td><td>47.2</td><td>74.2</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Clay minerals content before calcination</td><td>A1-R</td><td>B1-R</td><td>B2-R</td><td>C1-R</td><td>C2-R</td><td>C3-R</td><td>C4-R</td><td>D1-R</td><td>D3-R</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Kaolinite</td><td>80.6</td><td>70.5</td><td>71.3</td><td>51.2</td><td>50.1</td><td>51.0</td><td>42.1</td><td>21.2</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Kaolinite by TGA</td><td>86.5</td><td>72.2</td><td>74.2</td><td>52.4</td><td>50.9</td><td>46.6</td><td>44.2</td><td>23.3</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Illite</td><td>4.0</td><td>3.4</td><td>-</td><td>-</td><td>4.6</td><td>4.6</td><td>15.6</td><td>14.4</td><td>9.6</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Smectite</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>51.0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>C3S M3</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>66.8</td><td>-</td><td>-</td></tr><tr><td>C2S beta</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>14.5</td><td>-</td><td>-</td></tr><tr><td>C3A cubic</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.8</td><td>-</td><td>-</td></tr><tr><td>C3A orthorhombic</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.0</td><td>-</td><td>-</td></tr><tr><td>C4AF</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>11.6</td><td>-</td><td>-</td></tr><tr><td>Syngenite</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.1</td><td>-</td><td>-</td></tr><tr><td>Calcite</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.2</td><td>99.4</td><td>0.6</td></tr><tr><td>Gypsum</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>95.3</td></tr><tr><td>Dolomite</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.7</td></tr></tbody></table>

referred to as CCLC-50). A water to binder ratio of 0.4 with an SP dosage of 0.4 % by weight of binder (bwb) was used for CCLC-50 paste hydration studies while 0.7 was used (without SP) for the paste rheology study. The 6 % gypsum was adjusted in a previous study [7] to allow for a good hydration even for the most reactive clays.

Paste mixing was done using a high-shear overhead mixer (IKA MINISTAR) for 2 min at 1200 rpm for the SLiM paste due to high w/b ratio and 1600 rpm for CCLC-50 paste due to the low w/b ratio of 0.4.

### 2.2.2. Physical properties characterization of the binder components

The particle size distribution (PSD) of all binder components was assessed by laser diffraction using a Bettersizer S3 from 3P instruments. A dispersion time of 3 min in isopropanol at a speed of 2000 rpm was used. Specific surface area (SSA) was measured using the BET method using equipment from 3P instruments. Approximately 5 g of sample were first degassed for 16 h at 40 °C under a N2 flux and afterwards an average of three BET measurements was taken.

### 2.2.3. Calorimetry protocols

Isothermal calorimetry measurements of the SLiM test were done using I Cal 8000 calorimeter (Calmetrix) at 25 °C, similar to the temperature chosen for the rheology measurements. Metal plates matching

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Cumulative particle size distribution of the natural calcined clays, gypsum, clinker and limestone.</figcaption></figure>
<table><caption>**Table 3**
The D₁₀, D₅₀, and D₉₀, and the specific surface area (SSA) of CCs, clinker limestone and gypsum.</caption><thead><tr><th>Sample</th><th>A1</th><th>B1</th><th>B2</th><th>C1</th><th>C2</th><th>C3</th><th>C4</th><th>D1</th><th>D3</th><th>Clinker</th><th>Limestone</th><th>Gypsum</th></tr></thead><tbody><tr><td>D<sub>10</sub> (μm)</td><td>1.50</td><td>1.30</td><td>1.20</td><td>0.90</td><td>1.60</td><td>1.50</td><td>1.80</td><td>1.50</td><td>1.20</td><td>1.10</td><td>0.50</td><td>1.10</td></tr><tr><td>D<sub>50</sub></td><td>8.70</td><td>12.1</td><td>13.5</td><td>13.9</td><td>20.9</td><td>13.1</td><td>11.8</td><td>14.1</td><td>11.3</td><td>10.3</td><td>5.70</td><td>5.90</td></tr><tr><td>D<sub>90</sub></td><td>70.9</td><td>66.6</td><td>62.5</td><td>70.9</td><td>98.4</td><td>66.0</td><td>72.2</td><td>77.6</td><td>58.8</td><td>40.1</td><td>36.1</td><td>48.0</td></tr><tr><td>SSA (m²/g)</td><td>14.5</td><td>15.6</td><td>40.2</td><td>45.4</td><td>13.7</td><td>34.3</td><td>16.7</td><td>15.1</td><td>3.4</td><td>1.31</td><td>3.64</td><td>3.18</td></tr></tbody></table>

the heat capacity of the paste samples were used as references. Paste was mixed as described above and 18 g of paste was placed in plastic ampoules immediately after mixing and preparation which took a total of about 6 min. Measurements were done for 48 or 72 h. It was recognized during analysis of the results that for fast-reacting clays shorter times between mixing and measuring or internal-mixing, as well as more controlled temperature equilibration of reactants would be more accurate. To imitate internal-mixing due to lack of suitable accessories, hand-mixing for 50 s was carried out inside the ampoules for B1 and B2, and the ampoules were immediately placed in the calorimeter channels. Reproducible mixing can be achieved in the short time span due to the high w/b ratio. Other sets of calorimetry measurements were carried out at 10 and 40 °C for the purpose of testing the robustness of the test.

Isothermal calorimetry measurements of CCLC-50 pastes were carried out at 20 °C to follow the hydration of the cement pastes. A w/b of 0.4 was used and 22 g of paste were measured. External mixing at 1600 rpm for 2 min was used and all the samples were placed in the calorimeter at once.

### 2.2.4. Rheology

The stiffening of the paste due to the reactions was monitored for the cement paste and the SLiM using an Ares G-2 rheometer (TA Instrument). Samples were placed in an environment at 25 °C, with 100 % relative humidity to avoid water evaporation during measurement. The rheometer tool used had a plate-plate geometry with corrugated surfaces and a diameter of 25 mm and the gap between the plates was adjusted to 2 ± 0.05 mm during measurement. The storage or elastic modulus measurement was performed with an oscillation strain of 0.0008 %, so as to ensure to be in the elastic domain of the paste [29] (to avoid irreversible deformation) with an angular frequency of 10 rad.s⁻¹. The change of the storage modulus (which represents paste stiffening) was measured during the first 6 h of hydration. The same protocol was used for the SLiM and CCLC-50, and a similar water to solid ratio of 0.7 was used for both systems. The storage modulus (G') is measured and reported. G' is the elastic component of G* (complex shear modulus) which is used to describe visco-elastic materials [30]. G'' (loss modulus) represents the viscous component. Eq. 1 shows the relationship between these parameters.

$$G' = G' + iG'' \qquad 1$$

### 2.2.5. X-ray diffraction (XRD)

Quantification of crystalline phases of materials was accomplished by X-ray diffraction (XRD) with Rietveld analysis. A Bragg-Brentano configuration in a Panalytical X'pert pro diffractometer working at 40 kV and 40 mA with a copper source was used. An X'celerator detector was used to record the diffracted X-ray intensity signal.

Clay samples were first ground to pass 63 µm sieve and then further gently ground using an agate mortar with small amounts of isopropanol to increase fineness. The measurement was carried out at an angular range of 4 and 60°2θ with a step size of 0.0167°, and an equivalent scan speed of 0.0356°/s, corresponding to a total measurement time of about 28 min. An incidence slit of 1/4°, a divergence slit of 1/8° slit and a beam knife were used in the optical path. For cement, limestone and gypsum a similar step size and scan speed were used between 5 and 70°2θ and the total measurement time was 31 min. To obtain the absolute mass fractions of the crystalline phases, two clay samples were

measured separately using the internal and external standard method with rutile as the standard material [31]. For the internal standard method, 20 wt% of rutile was mixed with 80 wt% of the clay, further mixing was then gently carried out until it the mixture was homogeneous. For the external standard method, a sample of rutile was prepared separately and measured under the same conditions. Rietveld refinement was conducted using the HighScore Plus v4.8 software, according to the process highlighted in [31]. Chemical composition (XRF, Table 1) was used to calculate the X-ray mass absorption coefficient (MAC) used in the external standard method.

For the in-situ XRD, freshly prepared SLiM paste (1.0 w/b) or CCLC-50 paste (0.4 w/b with 0.4 wt% bwb of SP) was immediately loaded onto the in-situ sample holder and covered with a 12.7 µm thick Kapton film. The measurements were carried out up to 24–48 h with 15 min scans at specific intervals using the same setup as described above with a 1° soller slit and between 5 and 70°2θ. A step size of 0.0167° and an equivalent scan speed of 0.0727°/s for a total measurement time of about 15 min. A rutile sample covered with Kapton film was scanned under the same conditions soon after the in-situ measurements for use during the Rietveld refinement. The measurements were done without climate control and the temperature in the XRD instrument was about 27 °C (against 25 °C for calorimetry). The lattice and peak shape parameters for the clinker phases were taken from the refinement of anhydrous clinker and were kept constant during the Rietveld refinement of the hydrating sample. The external standard method was used to calculate the XRD amorphous content (as explained in the previous paragraph). The contribution of C-S-H, free water and Kapton film signals were covered using a combination of an analytical background function and a set of pre-defined broad peaks [31]. To avoid overestimating the portlandite content (compared to TGA results) due to the Kapton foil signals, the scale factor, cell parameters and peak width were refined first. After which the profile variable W (peak width) was refined only to fit the main portlandite peak sufficiently. The obtained results were normalized and reported per 100 g of solid/binder. Structures applied for the anhydrous cement and hydrated samples are detailed in [7,26].

### 2.2.6. Pore solution analysis

Pore solution, obtained from SLiM paste samples using centrifugation, was analysed for elemental concentrations of S, Ca, Si and Al using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Agilent 5100). The pore solution samples were first acidified to contain 2–5 % HNO₃ (using HNO₃ 69 %, Merck). The blank solution (2 % HNO₃) and multi-standard solutions were prepared in such a way to contain all the elements, S and Ca from 0 to 800 mg/L and Al and Si from 0 to 20 mg/L. The goal of this procedure was to minimize the matrix effect. The elemental concentration data was analysed by PHREEQC [32] using Cemdata18 [33].

### 2.2.7. Thermogravimetric analysis

Thermogravimetric analysis (TGA) was used to measure the remaining portlandite in the SLiM at the maximum ettringite time (MET) for B1 and B2. For this measure, SLiM pastes were prepared as detailed above, and at given times hydration was stopped by solvent exchange according the protocol described in [34]. 3 g of paste were dissolved in 250 ml of isopropanol for 15 min. The suspension was then vacuum filtered in 2 min and rinsed with diethyl ether. The filtered solid was
dried in an oven at 40 °C for 8 min. About 40 mg of the hydration-stopped sample was placed in an 85 µL alumina crucible immediately. A Netzsch STA 449F5 equipment with a heating rate of 10 °C/min from 30 °C to 1000 °C was used. The tests were carried out in open crucibles and the measurement cell was flushed with 50 mL/min nitrogen gas. Ca (OH)₂ content was determined using the step-wise method between 400 and 550 °C according to Eq. 2. Results were normalized to the sample mass at 550 °C, which was considered to be the dry binder mass that remains constant during cement hydration.

$$CH = \frac{M_{CH}}{M_{550}} \times \frac{74}{18} \times 100g \qquad (2)$$

Where CH is the portlandite content of SLiM paste samples. M₅₅₀ is the remaining percentage mass at 550 °C and M₄H is the percentage mass loss corresponding to portlandite.

### 2.2.8. Thermodynamic framework

At constant temperature and pressure, the heat of reaction is related to the standard enthalpy of formation by Eq. 3.

$$\Delta H_{r}^{\theta} = \prod_{i} \nu_{i} \Delta H_{f,i}^{\theta} = Q_{r} \qquad (3)$$

Where ΔHₜ is the standard enthalpy of reaction, νᵢ is the stoichiometry coefficient for species i, ΔHᵣᵢ is the standard enthalpy of formation of species i, and Qₜ is the heat of reaction as measured by isothermal calorimetry.

The standard enthalpies of formation used in the calculation are taken from PSI/NAgra [35,36] and Cemdata18 [33], and are listed in Table 4.

### 2.2.9. Simple early-age hydration model for CCLC binders

To relate the time of sulfate-carrier (gypsum) depletion measured in the clinker free system, to the time of gypsum depletion in the CCLC, a simple kinetic model of early-age hydration of CCLC-50 was designed. The main assumption of this model is that the sulfate consumption comes from two sources: the precipitation of ettringite, and the sorption of sulfate on C-S-H [42–45]. Based on S/Ca reported in [42,43] for cement paste, S/Si ratios of 0.05 to 0.11 are obtained, and a range of about 0.005 to 0.01 is reported in [45] for a saturated lime and gypsum system (Ca/Si = 1.5). S/Si of 0.05 and 0.01 were chosen for this study.

The other assumption is that the clinker phases mainly react before gypsum depletion in CCLC are C₃A and C₃S. Ettringite precipitates from the initial reaction of C₃A during mixing and the reaction of metakaolinite with Ca from the clinker phases and gypsum. C-S-H precipitates from the reaction of C₃Sand the reaction of metakaolinite. The C₃S is assumed to react at a constant rate between the end of the dormant period to the maximum ettringite time (MET). This constant rate is probably one of the major simplifications of this model. The reaction of metakaolinite with portlandite and sulfate to form ettringite and C-S-H is assumed to take place once C₃S starts to react, i.e. when Ca is available. Table 5 shows a summary and description of the parameters used in the kinetic model.

The gypsum content at time t is thus described as:

<table><caption>**Table 4**
The thermodynamic data of reactants and products.</caption><thead><tr><th>Reactants and products</th><th>ΔH<sup>θ</sup><sub>f,i</sub> (kJ/mol)</th><th>Reference</th></tr></thead><tbody><tr><td>Gypsum</td><td>-2023</td><td>[35,36]</td></tr><tr><td>Portlandite</td><td>-985</td><td>[35,36]</td></tr><tr><td>Water</td><td>-285.88</td><td>[35,36]</td></tr><tr><td>C<sub>1.67</sub>SH<sub>2.1</sub></td><td>-2723</td><td>[37]</td></tr><tr><td>C<sub>1.33</sub>SH<sub>2.17</sub></td><td>-2506.3</td><td>[38]</td></tr><tr><td>Ettringite</td><td>-17,535</td><td>[37,39]</td></tr><tr><td>Strätlingite</td><td>-6360</td><td>[37,39]</td></tr><tr><td>Monosulfate</td><td>-8758.6</td><td>[40,41]</td></tr></tbody></table>

<table><caption>**Table 5**
Parameters for the simple kinetic model of early-age hydration of CCLC-50.</caption><thead><tr><th>Parameter</th><th>Value</th><th>Source</th><th>Description</th></tr></thead><tbody><tr><td>R<sub>MK</sub></td><td>0.05–0.21 g/h</td><td>As determined using the SLiM and normalized as explained below</td><td>Rate of reaction of MK</td></tr><tr><td>R<sub>C3S</sub></td><td>2.5 g/h</td><td>From in-situ measurement, Fig. A5</td><td>Rate of reaction of C₃S</td></tr><tr><td>R<sub>C3A</sub></td><td>0.25 g/h</td><td>Average from in-situ measurements, Fig. A5</td><td>Rate of reaction of C₃A</td></tr><tr><td>m<sub>AFl,0</sub></td><td>3.5 g</td><td>From in-situ measurement, Fig. A5</td><td>Initial amount of ettringite due to C3A reaction during mixing</td></tr><tr><td>CaSi<sub>CSH</sub></td><td>1.65</td><td>[6,46]</td><td>Calcium to silica ratio of the C-S-H</td></tr><tr><td>SSi<sub>CSH</sub></td><td>0.05<br>0.01</td><td>[42,43]<br>[45]</td><td>Sulfate to silica ratio of the C-S-H during C3S reaction</td></tr><tr><td>AlSi<sub>CSH</sub></td><td>0.085–0.095</td><td>Fitted to the sulfate-carrier depletion time</td><td>Initial amount of gypsum in cement</td></tr><tr><td>m<sub>gypsum</sub></td><td>6 g</td><td></td><td></td></tr></tbody></table>

$$m_{gp,t} = m_{gp,0} - 3M_{gp} \left[ \frac{m_{AFl,0}}{M_{AFl}} + t^* \left( \frac{R_{MK}}{M_{MK}} + \frac{R_{C3A}}{M_{C3A}} \right) \right] - t^* M_{gp} \left( \frac{S}{Si} - 3 \frac{Al}{Si} \right) \left( \frac{R_{C3S}}{M_{C3S}} + 2 \frac{R_{MK}}{M_{MK}} \right) \qquad (4)$$

Where Mᵢ is the molar mass of solid phase i, and t* is time t minus the dominant period (taken to be 0.5 h in this study). Rₖₘ is the rate of reaction of metakaolinite as measured by the SLiM test. However, if the rate of consumption of portlandite from the metakaolinite reaction is higher than its rate of precipitation (from C₃S and C₃A reaction), then, the equivalent rate of portlandite precipitation is set to 0, i.e. Rₖₘ = Rₖ₃S. The rate of reaction of metakaolinite is obtained by normalizing the rate of dissolution of metakaolinite during the SLiM test (Eq. 9) by the mass of calcined clay in CCLC, i.e. the factor 0.3/0.741 representing the ratio of fraction of calcined clay in the blended cement and clinker free system is applied.

### 2.2.10. Apparent activation energy at sulfate-carrier depletion

An apparent activation energy was estimated from calorimetry measurements of the model system at 10, 25 and 40 °C. Initial estimation using the rate methods showed that the results at elevated temperatures such as 40 °C were not optimal [47,48]. As such a simplified approach using only the heat flow at sulfate-carrier depletion is used. For a similar reaction at different temperatures, the Arrhenius law can be rewritten as Eq. 5.

$$E_a = R \left( \frac{T_1 T_2}{T_1 - T_2} \right) \ln(x) \qquad (5)$$

Where x is the factor by which the rate changes when the temperature is changed to T₂ (283 K or 313 K), and T₁ and T₂ are the initial and final temperature (in K), respectively, and R, the molar gas constant.

### 2.2.11. Compressive strength

Mortar samples of CCLC-50 were also prepared and compressive strength was measured as a supplement to the other tests. The mortar was prepared according to EN196–1, with cement-sand-water proportions of 2/3/1 (450 g/1350 g/225 g), corresponding to a water to binder ratio of 0.5. The amount of superplasticizer required to achieve a spread of 200 +/- 10 mm was then determined using the standard mortar spread table. For compressive strength of selected samples mortar prisms (4 × 4 × 16 cm) were cast and demoulded after one day. The samples were then stored in a curing chamber at 20 °C and each sample set was separately immersed in water in small boxes. Measurements were carried out at 3 d and an average of 3 tests was taken.
# 3. Results

## 3.1. Early reactivity and hydration behaviour

### 3.1.1. Heat signature of the sulfate limited system

The SLiM consists of a calcined clay and portlandite in excess, and gypsum as the limiting reactant. The reactivity is evaluated by measuring the time needed to consume gypsum. This consumption can be measured by isothermal calorimetry, XRD and TGA, as well as conductimetry, as shown in our related study [28].

To better understand the results in this article, an overview of the expected reactions is presented and common vocabulary is defined. Fig. 2 shows a breakdown of a typical isothermal calorimetry curve obtained on the SLiM with a kaolinitic calcined clay. In agreement with previous studies [14,16,17,49], the heat flow curve can be split into 4 main stages which can be interpreted as follows:

I. **Initial dissolution:** this is the first phase that takes place soon after mixing. The initial dissolution peak might not be observed depending on the experimental protocol (e.g. temperature, internal or external mixing, and equilibration of reactants).

II. **Constant reaction rate:** This stage is dominated by the reaction of metakaolinite, gypsum and portlandite to form ettringite and C-(A-) S-H [14,17]. The main assumption behind the SLiM is that the rate of reaction is constant until depletion of gypsum, and the heat flow measured is related to the reaction of metakaolinite. This assumption has been shown to hold to be true in our recent study [28] and is further verified in Section 3.3. The reaction occurring during this step is described by the following equation:

$$AS_2 + (3 + 2x)CH + 3CsH_2 + (23 - 2x + 2y)H \rightarrow C_3A \ 3Cs \ 32H + 2C_xSH_y \quad (6)$$

III. **Sulfate-carrier depletion and maximum ettringite:** As the limiting reactant is the sulfate-carrier which is gypsum in this study, and the constant reaction rate stops with the consumption of gypsum. Two signatures are visible by calorimetry, a second peak on the heat flow curve (sulfate-carrier depletion, i.e. full dissolution of gypsum), and an inflection point on the cumulative

curve (maximum ettringite time, i.e. time at which maximum ettringite is reached due to the drop of sulfate concentration in pore solution see [28]).

IV. **Slower pozzolanic reaction:** After sulfate depletion, meta-kaolinite continues to react with portlandite to form C-A-S-H and AFm phases [14,17,49]. This reaction was found to be no longer only limited by the dissolution of metakaolinite but also to calcium availability (see Section 3.3 and [14,50]). Therefore, it is not investigated in details in this article.

Two times are defined from the reaction profile: the sulfate-carrier depletion time (ScDT) which is the maximum point on the heat flow curve and the maximum ettringite time (MET) which is the inflection point on the cumulative heat curve. The MET is also supported by in-situ XRD results (Fig. 5). The MET is slightly delayed (~2.5 h for B1) compared to ScDT. This delay is consistent with the cement-related phase diagrams shown by [51,52] which shows that the sulfate carrier (gypsum) depletes before the excess dissolved sulfate is fully consumed from the pore solution.

This test is primarily focused on the reaction up to MET. In reference to kaolinitic clays, the assumptions behind the test are: (1) the constant reaction rate is limited by metakaolinite reactivity, (2) the metakaolinite reactivity is sufficiently different for each calcined clay (CC), (3) meta-kaolinite is the only reactant from natural clays during the constant reaction rate, and (4) the only hydration products up to sulfate depletion are ettringite and C-(A-)S-H (Eq. 6). Thus, early reactivity measurement of kaolinitic clays through the SLiM test is primarily metakaolinite reactivity in a portlandite-gypsum system (similar to the cement early-age environment). These assumptions have been validated in our recent publication [28] and will further be verified in this article.

### 3.1.2. Early reactivity of natural calcined clays according to the SLiM test

The heat output of the SLiM test measured by isothermal calorimetry for the 9 natural CCs and K700 are shown in Fig. 3. The two characteristic peaks can be observed for the kaolinitic clay samples while it is not observed during the time frame of the experiment for D3, the smectitic clay. According to Fig. 3, the fastest depletion of gypsum is observed for C1 and B2, and C3 follows closely, then C4 and A1, followed by B1, C2, D1, K700, and, lastly, the smectitic clay D3 which does

<figure><img src="image_5.png" /><figcaption>Fig. 2. Measurement of the SLiM for the calcined clay B1 by isothermal calorimetry. The main phases, and features of the reaction profiles are identified on the heat flow curve.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Heat flow curves of the SLiM test at 25 °C for the tested calcined clays. Early reactivity is inversely proportional to sulfate-carrier depletion time (2nd peak).</figcaption></figure>

not reach sulfate-carrier depletion within 72 h. This result confirms our hypothesis that a calorimetry test based on the SLiM test can be used to classify the reaction rates of the different clays.

The second hypothesis to check is whether this result is related to the reaction kinetics of the CCs in CCLC-50 pastes (6 wt% gypsum). The relationship between the sulfate-carrier depletion time (ScDT) in CCLC-50 and the ScDT during the SLiM test is shown in Fig. 4. The ScDT of CCLC-50 is taken as the time at which the aluminate peak reaches a maximum [53,54]. A linear relationship can be observed. The slope of the correlation is related to the available gypsum in SLiM test and the real cement (considering the clinker reaction). In both systems, the ScDT is dependent on the available alumina to form ettringite, and the amount of formed C-S-H [14,20]. It indicates that alumina and silica availability in CCLC is strongly dependent on the reactivity of the clay, i.e. how fast it dissolves, and not only on physical effects such as the filler effect. This confirms our hypothesis that the reactivity of the CC in the proposed testing method is related to the reactivity of CC in a cement paste.

A contrast of B1 vs B2, C1 vs C2 or A1 vs C1/C3/C4 in Fig. 3 and Fig. 4 highlights that sulfate depletion or early reactivity is not directly

<figure><img src="image_7.png" /><figcaption>Fig. 4. Correlating sulfate-carrier depletion time (ScDT) for the SLiM test (time at the second peak, see Fig. 2) and ScDT of CCLC-50 (time at the peak of the aluminate peak).</figcaption></figure>

dependent on total metakaolinite content. These results agree with our previous study [7], in which rheology, compressive strength at 3 d and R³ test results at 12 h were shown to be not dependent on metakaolinite content. Rather, the initial dissolution rate of metakaolinite, which determines early reactivity or performance, depends more on SSA and the structural disorder of kaolinite. This is similar to what was observed by Bernal et al. [55], who concluded that the rate of the pozzolanic reaction at early ages is governed by the particle size, surface area, and local disorder of metakaolinite. Similar observations were recently reported by Panzer et al. [56]. In our companion paper [28], we further discuss these results in relation to changes in rheology.

### 3.1.3. Phase evolution of SLiM paste

To better interpret the classification of the calcined clays' reactivity, it is important to verify our assumptions on the reactions in the system. In-situ XRD was used to follow hydration up to 48 h for the SLiM. The quantitative analysis is presented for two calcined clays, B1 (medium reactivity) and B2 (high reactivity) in Fig. 5. In addition, the annotated diffractograms are shown in appendix (Fig. A2) for B1, B2, C1, and K700. C1 and B2 were chosen because they have the fastest early reactivity, and B1 and K700 provide a natural and lab-made calcined clay for comparison to B2 with comparable metakaolinite content (~72 %). K700 is composed of a lab-grade kaolinite (purity 95 %) mixed with 30 wt% of quartz and calcined at 700 °C. As the temperature during in-situ XRD experiments was not regulated, the time cannot be strictly compared to the calorimetry experiments. In the XRD chamber, the temperature is around 27 °C against 25 °C in the calorimeter. However, the high heat released by the reaction (as discussed in Section 3.3) indicates that the temperature is probably higher locally, especially for fast reacting clays.

In the XRD results, two main periods can be observed. In the first period, the gypsum and portlandite is being consumed while ettringite is formed. This is similar to the constant reaction rate shown in the cumulative heat curve in Fig. 2. The phase assemblage evolution is compatible with Eq. 6. The end of the first period is marked by the full consumption of gypsum (ScDT) and a maximum amount of ettringite (MET). The quantified amount of ettringite formed is around 18 g which is the expected amount from the initial 7.4 g of gypsum (Fig. 5). This indicates that in this particular system, the sorption of sulfate on C-S-H become negligible after sulfate-carrier depletion, i.e. the sulfate adsorbed on the C-S-H is released in the pore solution between the ScDT and MET.

After the ScDT, the consumption of portlandite by the pozzolanic reaction continues, albeit at a seemingly lower rate, especially for the most reactive clay B2. In addition to C-S-H, a product of this reaction is strätlingite as observed in Fig. 5. Despite thermodynamic modelling predicting strätlingite only after full portlandite consumption [28], it is a common observation in portlandite and metakaolinite system in the absence of calcite and sulfate [17]. This period is stage IV in Fig. 2, and the case in which strätlingite is formed is represented by Eq. 7. As a consideration of the case in which monosulfate is formed instead of strätlingite [28], Eq. 8 has also been provided. As stated earlier, the focus of this study is the reaction that occurs up to the MET or complete sulfate depletion. Further research is required to understand the reaction (s) that take place after sulfate depletion.

$$AS_2 + (2+x)CH + (6-x+y)H \rightarrow C_xSH_y + C_2ASH_8 \quad (7)$$

$$2AS_2 + (6+4x)CH + C_3A \cdot 3Cs \cdot 32H \rightarrow 4C_xSH_y + 3C_4AsH_{12} + (2+4x-4y)H \quad (8)$$

To further support the in-situ XRD results and the key assumptions concerning an initially gypsum and portlandite buffered chemical system up (see also Fig. 2), the composition of the pore solution of B1 and B2 samples was followed up to sulfate depletion. The subsequent thermodynamic calculation of the saturation indexes of gypsum, portlandite and ettringite, shown in Table A2, confirm that the systems are close to
<figure><img src="image_8.png" /><figcaption>Fig. 5. Quantification of phase changes of SLiM paste with hydration time using Rietveld analysis. The measurements were done without climate control (at a temperature of about 27 °C).</figcaption></figure>

equilibrium with respect to gypsum and portlandite during the initial stage of the test.

The stoichiometries of Eq. 6, and in-situ XRD experiments were further validated by stopping the reaction of the SLiM at specific times by solvent exchange. Fig. 6 shows the TGA results of B1 and B2 at the sulfate-carrier depletion time (ScDT) (Fig. 3). The results show a mass loss of 2.7 % for B2–6.8 h and 2.6 % for B1–14.2 h which corresponds to 6.0 and 6.5 g of reacted CH, respectively. The representative error on mass loss is about 0.1 % which equates to 0.5 g of CH. The portlandite consumption at the ScDT is slightly lower than at the MET (16.4 h for B1 and 8.7 h for B2) and almost linearly proportional, as shown by the linear precipitation of ettringite shown in Fig. 5 and shown by the cumulative heat curves (Fig. 8). Thus, about 7.5 g and 7.7 g are expected at the MET for B1 and B2, respectively. The mass loss observed between 600 and 800 °C (Fig. 6) corresponds to 0.84 g and 1.35 g carbonated portlandite for B2 and B1 respectively. After factoring in the carbonation, a portlandite consumption of ~6.2 and ~6.8 g was determined for B1 and B2 at the MET, respectively. The hypothesis was that a comparable mass loss of CH is expected at the ScDT and MET, and this is validated by the TGA results. According to Eq. 6, this portlandite consumption would correspond to a Ca/Si of around 1.5 ± 0.2 and as will be shown in Section 3.3, this mass difference has a negligible impact on the

assumptions and conclusions of this study.

## 3.2. Application of the sulfate-limited system

### 3.2.1. Rheology

Contact of the binder with water leads to colloidal and attractive forces within the first few seconds to minutes of cement hydration which governs the initial rheological behaviour [57]. However, when ettringite and CSH precipitates, the precipitation of hydrates is the governing factor [57,58]. In this sub-section it is verified whether the reactivity classification results shown in Section 3.1 are related to the rheology behaviour in CCLC-50 systems.

A correlation of storage modulus between the SLiM and CCLC-50 paste-systems after 1 h and 6 h is presented in Fig. 7. Results show a linear relationship for the natural clays, but K700 is an outlier showing a lower reactivity than expected in the cement paste. Previously, it has been reported that rheology or workability is influenced by meta-kaolinite content [59], however the results presented here and in [60] show that this trend does not hold true for natural CCs. These results agree with what was observed for mortar spread flow and change in static yield stress with hydration time in a related study [7]. Thus, illustrating that it is important to measure natural clays in method development and mechanistic studies and not rely only on chemical-grade clays. In addition, Fig. 7 also shows that the calcined clays, in the absence of limestone, are sufficient to reproduce the rheology of CCLC-50. This strongly suggest that the rheology of CCLC-50 is dominated by calcined clays as proposed in previous works [60,61] and demonstrated in our related study [28].

Rheology is a challenging measurement to compare between labs due to the influence of the measurement device [62]. Studying the SLiM provides a simple method to better understand the stiffening of the paste and the influence of the clays. Additionally, due to the absence of clinker in the SLiM, it is easier to reliably compare the impact of the early reactivity of different CCs on rheology. This subject is explored in further detail in our related study [28].

### 3.2.2. Enthalpy of formation of metakaolinite

In this section, focus is placed on verifying that the model system, up to ScDT, follows a well-defined reaction, and that it can be used to infer thermodynamic properties of metakaolinite. Fig. 8 presents the cumulative heat results obtained by isothermal calorimetry using SLiM test. It is observed that the heat at MET is similar for all kaolinitic clays, including K700: 69.4 +/− 1.6 J/g of CC (Fig. 9b). The cumulative heat

<figure><img src="image_9.png" /><figcaption>Fig. 6. TGA analysis at approximate sulfate-carrier depletion time for B1 and B2.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Relationship between the storage modulus obtained with the SLiM and CCLC-50 at 1 and 6 h of hydration.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Increase in cumulative heat of the SLiM test at 25 °C (a), and cumulative heat at maximum ettringite time for kaolinitic clays (b). Only the internal mixing of B2 is considered in (b) instead of the external mixing of B2 and C1 shown in (a), as explained in section 4.2.2 based on Fig. A1.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Degree of reaction of the calcined clays (a) normalized to the mass of the calcined clay, and (b) normalized to the wt% of metakaolinite.</figcaption></figure>
at the MET represents the point at which ettringite content reaches its maximum. It is determined as the intersection between the tangent lines corresponding to the two phases of the cumulative heat curve (phase II and IV in Fig. 2). After sulfate depletion, heat output continues due to the formation of C-(A-)S-H as shown in section 3.1.

The common heat of reaction at MET implies that although the kinetics are different, the reaction path is common for all calcined kaolinitic clays, i.e. the chemical reaction up to MET in the system is well-defined. As such, the enthalpy of formation of metakaolinite can be estimated from the heat of hydration as both a validation and extension of the SLiM test (see section 2.2.8 for details). A main parameter of this equation is the Ca/Si ratio. Attempt to measure it by SEM-EDS were unsuccessful due to the lack of dense C-S-H agglomerates. However, portlandite consumption was determined to be about 6.4 ± 0.4 g from TGA results (Section 3.1) and this equates to a Ca/Si ratio of approximately 1.5 ± 0.17 based on Eq. 6. This is close to the thermodynamic Ca/Si prediction of ~1.65 for a system at equilibrium with Ca(OH)₂, gypsum and ettringite. Additionally, it is assumed that only minor quantities of alumina and sulfates are included in C-S-H [28,63], as discussed in Section 3.1. Therefore, based on a Ca/Si ratio of 1.67, the system is described by Eq. 9 up to the MET.

$$AS_2 + 6.34CH + 3CsH_2 + 23.86H \rightarrow C_3A_3 3Cs 32H + 2C_{1.67}SH_{2.1} \quad (9)$$

Based on Eqs. 3 and 9, and the heat of reaction at the MET ($Q_r = 69.4$

± 1.6 J/g of CC), the standard heat of formation of metakaolinite, $\Delta H_{f,MK}^\circ$, is estimated at -3487 ± 8 kJ/mol. A Ca/Si of 1.33 gives a similar $\Delta H_{f,MK}^\circ$ of -3489 ± 8 kJ/mol, which highlights the robustness of the approach. The $\Delta H^\circ$ determined in this study from the sulfate-limited mode system is almost similar to the value calculated by Weise et al. ($\Delta H_{f,MK}^\circ$ of -3478 kJ/mol) from the heat of dehydroxylation of kaolinite [49].

This agreement of the calculation with a value from a different study is another validation of the assumptions made about the chemical reaction that occurs in the SLiM up to the MET. The heat of formation of metakaolinite is valuable in the modelling of isothermal calorimetry curve of clay-based cement materials like calcined clay limestone cement (CCLC). This is a key parameter in the ongoing effort to model the hydration of CCLC over time.

### 3.3. Optimization of the sulfate-limited model system

#### 3.3.1. Degree of reaction of metakaolinite

The enthalpy of formation of metakaolinite can be used to simply calculate the degree of reaction (DoR) of metakaolinite for different calcined clays. According to Eq. 9, the heat released when one gram of MK reacts is 1617 J/g of MK before MET. Using the same Ca/Si of 1.67, the heat released after the MET is 643 J/g of MK based on Eq. 7 (if strätlingite is formed), or 561 J/g of MK based on Eq. 8 (if monosulfate is formed). The heat of reaction as measured by isothermal calorimetry can be normalized by these values to calculate the DoR of metakaolinite. Eq. 7 was used for the reaction after the MET to illustrate the calculation of the DoR of metakaolinite from the $\Delta H_{f,MK}^\circ$ calculated in this study.

Fig. 9 presents the DoR of calcined clay (a), and DoR of metakaolinite in the calcined clay (b) as a function of reaction time during the SLiM test. It is observed that the degree of reaction at MET is less than 10 %. This degree of reaction corresponds to the amount of metakaolinite reacted. It means that gypsum is the limiting factor for all clays with a kaolinite content greater than 10 %. In particular, only a fifth of the metakaolinite in D1, the lowest grade clay in this study, has reacted at MET (the point at which the gradient of the curve changes). This analysis can help better optimize the composition of the sulfate-limited system.

#### 3.3.2. The sulfate-limited system at different temperatures

To test the robustness of our proposed early-reactivity test method,

the reaction of the SLiM test was followed up by isothermal calorimetry at 10 and 40 °C. The heat flow curves are shown in Fig. 10. As expected, it is observed that reactions are faster when the temperature increases. In particular, at 40 °C, zone IV (further pozzolanic reaction) consists of a broad shoulder, before heat output drops to near zero, indicating an apparent completion of the reaction, compatible with the observation of the R³ test. The relationship between compressive strength at 3 d and sulfate-carrier depletion times at the three investigated temperatures is shown in Fig. 11. Results show an overall correlation at the three temperatures, indicating that the temperature has little influence on the interpretation of the test. It is worth observing the systematic bias for each clay (a lower-than-average depletion time at 10 °C is correlated to a lower-than-average depletion time at 25 and 40 °C). This reinforces the validity of the proposed test method, but it also highlights the importance of other factors than the clay reactivity to determine the 3-day compressive strength. As such, by the reactivity before 3 days is defined as “early-age reactivity”. In addition, the average of rate change at 40 °C shows that the overall measurement time of the experiment can be reduced by about 2.8 times compared to 25 °C. However, the calorimetry experiment must be carried out more carefully to ensure that the heat release at the start of the reaction is captured.

### 3.4. The SLiM test with limestone: Phase assemblage compared to CCLC

A main difference of the SLiM paste compared to CCLC paste is the absence of calcium carbonate which promotes the precipitation of carbonate-AFm after sulfate depletion. A first attempt to mimic more closely CCLC was carried out by substituting 25 % of the calcined clay by limestone. The flow curves of B2 without and with 25 % limestone are in Fig. 12. A similar heat signature is observed with a significant delay in the ScDT due to the dilution effect of the replacement of calcined clay by limestone as the total surface of metakaolinite available for dissolution is decreased. On the other hand, the cumulative heat at MET is similar, but higher cumulative heat was observed for the system with limestone at 48 h. Table 6 shows the phase assemblage after addition of limestone (25 wt% of B2, ~18.5 g / 100 g of solid) and some of the in-situ XRD diffractograms are shown in Fig. A3. As expected, hemicarbonate is observed and it continues to increase with time. However, strätlingite is not detected and its peak at 31.1°2θ is not clearly visible, which suggests that the addition of limestone significantly reduces or eliminates the precipitation of strätlingite.

These results show that limestone can be included in the SLiM test to increase the similarity with the composition of modern blended cements. In addition, this helps reduce the uncertainty about which AFm, whether strätlingite (Eq. 7) or monosulfate (Eq. 8), forms at equilibrium after sulfate depletion. However, the substitution of the SCM with limestone leads to a delay in sulfate depletion (Fig. 12) due to a dilution effect. Given that only 11.4 % of limestone is consumed after 48 h (Table 6), a lower substitution of the SCM by limestone in the SLiM test is recommended.

## 4. Discussion

### 4.1. Linking reactivity to clay properties

Section 3 demonstrated that the ScDT and MET in the sulfate-limited system (SLiM) are meaningful measurements to investigate early reactivity of natural calcined clays (CCs). The ScDT and MET can be measured with different methods, including isothermal calorimetry and XRD. Based on the heat release of the SLiM test, the early reactivity of the investigated clays can be classified as follows: C1, B2, C3 > C4, A1 > B1, D1, C2 > K700 > D3. This classification is similar to that found in the CCL50 system (Fig. 4) and highlights the potential of the SLiM test in understanding the contribution of the clays to early hydration, in particular the intrinsic sulfate demand of each CC.

An important question for mechanistic study is to know whether this
<figure><img src="image_13.png" /><figcaption>Fig. 10. Calorimetry curves at (a) 10 °C, and (b) 40 °C for 8 natural CCs (excluding C2). Heat flow measurements at 10 °C were not very stable.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. Relationship between mortar compressive strength at 3 d for CCLC-50 and sulfate-carrier depletion time during the SLiM test at different temperatures (10, 25 and 40 °C) measured by isothermal calorimetry. The trend lines are meant to be visual guides only.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Heat flow curves and cumulative heat of the SLiM test using B2, and with substitution of the calcined clay by 25 wt% of limestone.</figcaption></figure>

reactivity can be related to intrinsic properties of the clays.

Two main properties of the clays are often proposed to explain reactivity: metakaolinite content and specific surface area. All the natural CCs in this study were processed with the same pre-calcination treatment, calcination, and post-calcination grinding protocol. Hence, the difference in physico-chemical properties is intrinsic to the clay and its mineral composition. The early-age reactivity classification does not simply correlate with the metakaolinite content as shown in Fig. 13a, nor the specific surface area as shown in Fig. 13b. A unique relationship exists between specific surface area (SSA) of the clay after calcination and the sulfate-carrier depletion time (ScDT) during the SLiM test (Fig. 13b). Two clusters are observed: (1) CCs with low SSA (< 20 m²/g) with high variation in ScDT (~x3) and (2) CCs with high SSA (> 20 m²/g) but comparable ScDT (<8 h). The variation of ScDT in the first cluster (low-SSA), and simultaneously the lack of ScDT variation with large SSA variations (20–40 m²/g) indicates that the measured SSA on calcined clays cannot fully explain their early-age reactivity, unlike the reactivity measured using the SLiM test. It should be noted that the lack of correlation between the reactivity and the SSA is not an indication that the reactivity is not related to the dissolution, and the surface area of metakaolinite. Rather, it is an indication that the method to measure the SSA is not appropriate, nor representative of the metakaolinite reactivity. Nevertheless, the SLiM test opens a new way to probe the reactivity in a more controlled manner, and allows for a more systematic approach to the study of clay reactivity.

Two outliers were observed in this study: the smectitic clay, D3, and the lab-made K700 which is a combination of 70 % almost pure metakaolinite and 30 % quartz. D3 reacted the slowest and did not reach the ScDT within 72 h. It has been shown [64] that smectite/montmorillonite clays release alumina at a much slower rate than kaolinitic clays, and this leads to slow sulfate consumption through the formation of ettringite. Additionally, the SSA of D3 is about 4 times less than that of D1. On the other hand, K700 with MK content comparable to B1 was expected to react faster than shown in the calorimetry and rheology results (Figs. 3 and 7). This is due to the low SSA (~10 m²/g) coupled with the high crystallinity of kaolinite in K700 [64,65] which seems to reduce the rate of early dissolution of metakaolinite. The degree of disorder of kaolinite in natural clays is the one of the reasons why lab-made clays sometimes done have similar reactivity as natural clays of similar individual mineral components [7].

Compressive strength at 3 d was also measured for D3 (~15.2 ± 0.5 MPa) and it was lower than all kaolinitic clays, but not as different as shown in Figs. 3 and 5. The reason for this is that the SLiM test measures sulfate consumption, which depends on the reaction of alumina and
<table><caption>**Table 6**
Phase composition in the SLiM for B2 with limestone (quantified by XRD-Rietveld analysis).</caption><thead><tr><th rowspan="2">Hydration time (h)</th><th colspan="5">Phase composition (g/100 g of solid)</th></tr><tr><th>Portlandite</th><th>Ettringite</th><th>Strätlingite</th><th>Hemicarbonate</th><th>Calcite</th></tr></thead><tbody><tr><td>24</td><td>7.6</td><td>18.0</td><td>0</td><td>1.6</td><td>17.4</td></tr><tr><td>48</td><td>2.6</td><td>18.1</td><td>0</td><td>3.4</td><td>16.4</td></tr></tbody></table>

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Relationship between (a) kaolinite content, and (b) BET SSA of calcined clays, and sulfate-carrier depletion time (2nd peak).</figcaption></figure>

sulfate ions to form ettringite (Eq. 6). Smectitic clays have a 2:1 ratio of silica to alumina compared to kaolinitic clays which have a 1:1 ratio. This results in lower alumina content and in turn slower sulfate consumption. Similar to other reactivity tests such as the R³ test, comparison between SCMs is best done within a similar group. Additionally, it must also be noted that the relatively high SO₃ content of D3 (Table 1) may prolong the ScDT of D3.

## 4.2. Activation energy and dissolution

In section 3.1, it was assumed that the reactivity classification is related to the dissolution of the clays, specifically the dissolution of metakaolinite and this is equal to the rate of precipitation of hydrates. Overall, the rate of dissolution of a mineral phase can be described by the combination of three terms, as shown in Eq. 9.

$$r = Ak(T)S(SI) = \frac{m_{CCreactedatScDT}}{ScDT} \qquad (9)$$

Where A represents the surface available for dissolution, i.e. the metakaolinite SSA, k(T) is a temperature-dependant kinetic term representing the energy barrier for dissolution, and S is the term representing the thermodynamic driving force, i.e. the deviation to the equilibrium state, represented by the saturation index (SI) of the dissolving mineral. $M_{CCreactedScDT}$ represents the mass of reacted calcined clay (specifically metakaolinite) at ScDT, which can be calculated similar to the degree of reaction (Fig. 9).

Section 3.2.2 demonstrated that the thermodynamic driving force is similar for all kaolinitic clays, since a unique standard enthalpy of formation of metakaolinite was calculated. The specific surface area was discussed in the previous subsection. Therefore, the temperature-dependent kinetic term will be explored next. As a proxy for this term, an apparent activation energy (Ea) can be calculated from the calorimetry results at different temperatures. The details of the calculation of Ea at ScDT are provided in Section 2.2.10 and Table A3. The relationship between the obtained Ea and the ScDT is shown in Fig. 14. The significant error in some of the results appears to be due to the measurements done at 40 °C (see section 4.3.2). Nonetheless, a trend can be observed,

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** The relationship between sulfate-carrier depletion time (at 25 °C) and activation energy calculated from the sulfate-carrier depletion times at 10, 25 and 40 °C.</figcaption></figure>

and it provides a better relationship than Fig. 13b, the relationship between SSA and ScDT. A possible explanation for the variation of the activation energy with the clay was investigated in a previous study [7]. In agreement with other studies [13,66,67], it was shown in [7] that clays with a high iron content had a low crystallinity. This low crystallinity was related to higher surface area and to higher reactivity, but it could also be related to lower activation energies. This effect has to be considered in future mechanistic investigations.

Fig. 14 highlights the practical value of the SLiM test as a measurement that accurately represents dissolution kinetics of metakaolinite and thus provides a sum of all the parameters such as SSA, kaolinite crystallinity, experimental temperature and departure from equilibrium, i.e. saturation index.
### 4.3. Towards sulfate optimization using the sulfate-limited system

Fig. 4 highlighted that the ScDTs observed for the SLiM test are related to what is observed in the CCLC-50 system for similar gypsum contents for all systems (7.4 % for SLiM paste and 6 % for CCLC-50). A comparison of in-situ XRD results of the SLiM test (Fig. 5) to in-situ XRD results of CCLC-50 (shown in appendix, Fig. A5) shows a similar linear ettringite formation (and gypsum consumption) with and without clinker. Hence, it can be concluded that calcined clays contribute significantly to sulfate-carrier depletion in CCLC-50. This is in line with what was recently shown for several clays and varying sulfate amounts by Panzer et al. [21].

A simple hydration model to highlight the contribution of the clays during early hydration, and to show that the reaction rate measured in our proposed test is an intrinsic property of the clay is proposed. This rate can be translated to a blended cement, and even used to predict the sulfate demand of the clay. As described in Section 2.2.9, the model assumes that C₃S and C₃A, and the clay reaction are concurrent, and that gypsum is consumed by two products: sorption on C-S-H (from C₃S and CC), and ettringite (initial C₃A reaction and continuous CC reaction). The rate of reaction of metakaolinite (for kaolinitic clays) is calculated from the ScDT of the SLiM test (Eq. 9), and normalized per gram of calcined clay in each system (i.e. the amount of metakaolinite surface available for dissolution).

The gypsum consumption in CCLC as predicted by the hydration model is shown in Fig. 15a. The relation between the estimated ScDT and the experimentally determined ScDT is shown in Fig. 15b. As stated in Section 2.2.9, two S/Si ratios were chosen for the hydration model, and corresponding Al/Si ratios were fitted to the ScDT. Given 7 data points, Fig. 15b shows that the assumptions made in the hydration model are largely valid. To improve the model, the S/Si and Al/Si ratios need to be reliably determined for very early ages, and the hydration model should be adapted to factor in the changes in C₃S reaction rate, S/Si and Al/Si up to sulfate-carrier depletion.

The linear trends at different S/Si indicate that the calcined clay reaction rate measured in SLiM is related to the rate of metakaolinite reaction in the blended cement. The other main implication of these results is that as long as portlandite and sulfate are present, the metakaolinite and the C₃S and C₃A reactions do not interfere with each other. Fig. 15c highlights that metakaolinite contributes to gypsum depletion for all CCs shown in Fig. 15a. This implies that if an activated clay is

sufficiently reactive, it can contribute to early hydrate formation and thus, compensate for the low clinker portion and provide early strength, as evidenced by Fig. 11.

Of course, this requires the sulfate content of a blended system to be correctly adjusted. As seen in Fig. 15a when a low reactive clay is used with a fixed gypsum content (here 6 %) it can dramatically delay the aluminate peak. However, our hydration model can be inversed, to find the gypsum content for a given ScDT, and calcined clay and cement reactivity. Fig. 16 shows the relation between the gypsum needed in the blended cement and the ScDT in the clinker-free system as measured by isothermal calorimetry. The relation has to be further validated for more clays and cement, and the accurate determination of the S/Si and Al/Si at early ages will also have an impact on this prediction. Nonetheless,

<figure><img src="image_18.png" /><figcaption>Fig. 16. Illustration of the potential of the SLiM test as a sulfate optimization tool. Relation between gypsum needed for an ScDT at 8 h in CCLC-50 for S/Si ratio of 0.05 and 0.066, and the ScDT measured during the SLiM test. The points are sampled from the reaction rates of the clays investigated in this study, as depicted in Fig. 15. The solid horizontal line corresponds to the optimum gypsum for the clinker in CCLC-50.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 15. Gypsum consumption in CCLC-50 as estimated through the hydration model (a), the relationship between the calculated ScDT and the experimentally determined (b), and a ratio of metakaolinite and clinker contribution (C₃S and C₃A) (c).</figcaption></figure>
this opens a new way to formulate calcined clay limestone cements using
only one calorimetry measurement per calcined clay. The solid line
representing the optimal gypsum content for the clinker portion of
CCLC-50 shows that there is a rate of dissolution of metakaolinite, r(MK)
based on the ScDT of the SLiM test, for which no additional gypsum will
need to be added to CCLC. Although the model is far from perfect, its
simplicity might provide a sufficient practical and engineering
description of the early-age hydration of calcined clay cements.

## 4.4. Implications for a standardized early-reactivity test

### 4.4.1. A standardized early-reactivity test

Our observations correlating the results from an isothermal calorimetry measurement on the clinker-free system to properties of standard mortars at early-age, and even gypsum demand hints at the possibility to develop a standardized reactivity test in addition to other reactivity test aimed at the long-term, such as ASTM C1897 (R³ test). This test could be used for clay selection, calcination optimization, quality control of the calcination, and help in cement formulation. However, before a standard protocol can be proposed, the strength and limitations of the approach needs to be discussed, and validated, if possible in an interlaboratory set of experiments.

### 4.4.2. Limitations of calorimetry measurements and error detection

Using the same measurement method as the R³ test has important practical implications for an optimized workflow. To both shorten the duration of the test, and optimize the utilization of the equipment, it would be best to use the same temperature (40 °C). However, as early-age behaviour is of interest, it is crucial to ensure that the most reactive clays can also be measured. Fig. 17 shows the cumulative heat at 25 and 40 °C with the measurement time normalized using maximum ettringite time (MET) for each CC. The comparison shows that at 25 °C C1 and B2 (externally mixed) are the main outliers, but at higher temperature the captured cumulative heat for most systems is lower than expected. As illustrated in Fig. A1, external mixing, or prolonged time between external mixing and measurement, leads to an underestimation of the cumulative heat of fast-reacting CCs like B2. Similarly, at 40 °C external mixing and differences in measurement and room temperature lead to an underestimation of cumulative heat. External mixing may also

<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Cumulative heat based on normalized time (measurement time, t, divided by the MET (maximum ettringite time) for each sample) for 25 and 40 °C. The shaded area, which is bordered by the internally mixed B1 and B2 systems, represents the measurements which are considered accurate (for enthalpy determination) in this study.</figcaption></figure>

affect accuracy of the MET for fast-reacting CCs (Fig. A1). Thus, the more reactive the CC and the higher the experiment temperature, the greater the necessity for equilibration of reactants and internal mixing. In addition, care has to be taken that the reactants are equilibrated at the measurement temperature to avoid the difference in temperature affecting the height and shape of the initial dissolution peak and the total cumulative heat. This will in turn affect other applications of the SLiM test such as the calculation of the enthalpy of formation of metakaolin.

The SO₃ content of the raw/calcined clays may be another source of error. High SO₃ content, such as 1.21 % for D3 (Table 1), is likely to prolong the ScDT and affect the total cumulative heat, if its water-soluble. Hence, it may be necessary to adjust the sulfate content in the model system for clays that have high soluble SO₃ contents.

Since it was demonstrated that metakaolinite has a unique standard enthalpy of formation in all calcined kaolinitic clays, Fig. 17 can be used to detect anomalous measurements. It is possible to define a master curve by normalizing by the MET. The shaded area in Fig. 17, which is bordered by the curves for internally mixed B1 and B2, is used to represent the correct measurement range. The outliers can be detected and repeated if necessary. However, the temperature, and the composition of the system, should be optimized to reduce the chance of incorrect measurements.

To increase reproducibility of the measurements a similar mixing technique should always be applied, and in the case of activation energy calculations where high precision is required, internal mixing should be chosen. The internal mixing does not need to be automatic but, as in part of this study, it can be done by weighting all the solid contents in one ampoule and the water in another, mixing by hand for 1 min and placing in the calorimeter immediately. In all the experiments equilibration of both the solids and water should be done at the measurement temperature for at least 12 h.

Apart from calorimetry, the proposed test could also be carried out by measuring the consumed portlandite by TGA at fixed intervals, or doing in-situ-XRD or interval XRD measurements until ScDT. TGA would be the most practical for industrial and routine lab applications. However, the precision is relatively low, as seen in Fig. 6.

### 4.4.3. Composition of the system

In the proposed clinker free system, the excess portlandite content (18.5 %) is sufficient for a measurement up to 72 h at 25 °C with the most reactive clay measured in this study. The duration of the test can be adjusted by the temperature or the gypsum content. A temperature of 20 °C may be more practical than the 25 °C used in this study, and increasing the temperature to 40 °C can reduce the measurement time by a factor of almost 3, and match the temperature of the R³ test. Increasing the temperature, or decreasing sulfate content reduces measurement time, but also increases the chance of errors during measurement or uncertainties. A water to solid ratio between 0.7 and 1.0 seems practical to avoid too much bleeding and the need of superplasticizers for calcined clays that have high water demand. Finally, limestone can be added to the system, but the content and impact on the results was not systematically investigated and requires further investigation. However, it should be noted that adding limestone does not affect the reaction up to sulfate depletion but significantly increases heat output after sulfate depletion, as seen in Fig. 12. Hence, for the focus of this study, which is the reaction up to sulfate depletion or maximum ettringite, the absence of limestone does not impact the conclusions.

In this article, only calcined clays were discussed, but it is, in principle, possible to test high-reactivity aluminum-rich SCM, e.g. slags. The extension of the model for this use-case should also be tested, to ensure that the assumptions behind the test still hold true. Similarly, it should also be tested for other activation methods, such as mechanically activated clays.
## 5. Conclusions

In this study, a clinker-free sulfate-limited model system (SLiM), consisting of an excess of calcined clay (CC) (74.1 %) and portlandite (18.5 %), and limited gypsum (7.4 %) was presented. This model system is applied to the investigation of the early reactivity of CCs in CCLC-50, including the required sulfate adjustment. The main observable is the sulfate-carrier depletion time (ScDT) and maximum ettringite time (MET), which can be followed by several techniques, in particular isothermal calorimetry. Through the study of this system, and its characteristic reaction time for several calcined clays, the following conclusions could be made:

* The reactivity in the clinker free system can be used to classify the calcined clays, and can be related to mechanical and hydration properties of the corresponding calcined clay limestone cement. The early reactivity of 9 different natural CCs is differentiated using the ScDT and MET. In particular, the ScDT can be easily determined with accuracy by isothermal calorimetry. This depletion time was related to the ScDT for CCLC-50 paste and the mechanical strength at 3 days.

* Results demonstrated that the reactivity of metakaolinite is not linked to its content, nor wholly to the specific surface area as measured by BET. As such, this free-clinker model system provides a unique piece of information not provided by other reactivity tests or physical properties.

* The reactions in the SLiM are well-defined and can be used to characterize the reactivity of metakaolinite. Up to the ScDT, gypsum and portlandite react with metakaolinite to form ettringite and C-(A-)S-H. It was observed that the cumulative heat at the MET is similar for all calcined kaolinitic clays. This fact was used to calculate the standard enthalpy of formation of metakaolinite: -3487 ± 8 kJ/mol, which agrees with previously published data, highlighting the robustness of the SLiM test.

* The reaction rate was also observed to be much higher up to the ScDT and implies a sulfate activation of the metakaolinite reaction.

* The reaction rate in the SLiM up to ScDT can be taken as an intrinsic reaction rate of the calcined clay, i.e. it is not sensitive to the presence or absence of cement clinkers. Using a simplified hydration

model, this rate was applied to predict the ScDT in a calcined clay limestone system. After further optimization, the clinker free system could be used to help with the formulation of blended system, in particular for sulfate adjustment.

Therefore, it is proposed that the clinker-free sulfate-limited system can be extended as a reactivity test for early-age performance for other activation methods or other high-aluminum SCMs, to complement other reactivity tests aimed for long-term properties. To move towards this standardized test, further investigations should focus on optimizing the content of the clinker free system and the protocols to be used to measure accurately the characteristic reaction time.

### CRediT authorship contribution statement

**Tafadzwa Ronald Muzenda:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. **Yannick Demeusy:** Writing – review & editing, Methodology, Investigation. **Fabien Georget:** Writing – review & editing, Conceptualization, Software. **Christophe Labbez:** Writing – review & editing, Conceptualization. **Thomas Matschei:** Writing – review & editing, Supervision, 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

This work was funded by the Global Cement and Concrete Research Network: Innovandi, under core projects 5-7 (2021). This support is most gracefully acknowledged. TRM would also like to thank Karen Alloncle and Prof. Sandrine Gauffinet for assistance in some of the experiments carried out at Université Bourgogne Europe. The authors would also want to thank the anonymous reviewers for their valuable comments and suggestions.

## Appendix

<caption>Table A1</caption>

Phase composition and BET SSA of raw kaolinitic clays, taken from [7]. Designation for raw clays is represented by K. * Other silicates and aluminosilicates quantified in minor quantities are cristobalite, anorthoclase, feldspar and bementite. The full characterization of the raw clays is provided in [13].

<table><thead><tr><th>Phase composition (wt%)</th><th></th><th>K23</th><th>K44</th><th>K47</th><th>K51</th><th>K52</th><th>K72</th><th>K74</th><th>K87</th></tr></thead><tbody><tr><td rowspan="2">Kaolinitic clay minerals</td><td>By XRD</td><td>21.2</td><td>42.1</td><td>51.0</td><td>50.1</td><td>51.2</td><td>70.5</td><td>71.3</td><td>80.6</td></tr><tr><td>By TGA</td><td>23.3</td><td>44.2</td><td>46.6</td><td>50.9</td><td>52.4</td><td>72.2</td><td>74.2</td><td>86.5</td></tr><tr><td>Illite/muscovite</td><td></td><td>14.4</td><td>15.6</td><td>4.6</td><td>4.6</td><td>–</td><td>3.4</td><td>–</td><td>4.0</td></tr><tr><td rowspan="2">Silicates and aluminosilicates</td><td>Quartz</td><td>56.8</td><td>25.6</td><td>21.6</td><td>22.4</td><td>–</td><td>13.9</td><td>4.4</td><td>5.4</td></tr><tr><td>Others*</td><td>0.8</td><td>–</td><td>–</td><td>2.9</td><td>0.2</td><td>1.2</td><td>–</td><td>0.9</td></tr><tr><td rowspan="2">Fe (hydr)oxides</td><td>Hematite</td><td>0.6</td><td>2.1</td><td>–</td><td>2.7</td><td>2.3</td><td>–</td><td>10.1</td><td>–</td></tr><tr><td>Goethite</td><td>–</td><td>–</td><td>4.3</td><td>–</td><td>18.5</td><td>–</td><td>5.1</td><td>–</td></tr><tr><td>Calcite</td><td></td><td>1.0</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td rowspan="2">Other (hydr)oxides</td><td>Anatase</td><td>0.4</td><td>4.9</td><td>0.3</td><td>0.6</td><td>0.2</td><td>2.0</td><td>4.5</td><td>0.2</td></tr><tr><td>Gibbsite</td><td>–</td><td>–</td><td>–</td><td>–</td><td>6.0</td><td>–</td><td>–</td><td>–</td></tr><tr><td>PXRD Amorphous</td><td></td><td>4.9</td><td>10.2</td><td>18.1</td><td>16.9</td><td>20.5</td><td>8.9</td><td>4.3</td><td>8.9</td></tr><tr><td>BET SSA (m²/g)</td><td></td><td>18.4</td><td>16.3</td><td>33.6</td><td>25.1</td><td>47.2</td><td>22.9</td><td>45.4</td><td>15.1</td></tr><tr><td>Calcined clay names</td><td></td><td>D1</td><td>C4</td><td>C3</td><td>C2</td><td>C1</td><td>B1</td><td>B2</td><td>A1</td></tr></tbody></table>
<figure><img src="image_21.png" /><figcaption>Fig. A1. Heat flow and cumulative heat curves for B1 and B2 with external and internal mixing.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Fig. A2. Qualitative analysis of in-situ XRD results (at 27 °C) at different times until ScDT. E-ettringite, G-gypsum, P-portlandite, Q-quartz.</figcaption></figure>

Table A2

Measured concentrations of Al, Ca, S, and Si for SLiM pore solutions obtained at different time intervals and the corresponding saturation indices (SI) for gypsum, portlandite, and ettringite as calculated using PHREEQC.

<table><thead><tr><th>Sample</th><th>Al (mmol/L)</th><th>Ca (mmol/L)</th><th>S (mmol/L)</th><th>Si (mmol/L)</th><th>Gypsum SI</th><th>Portlandite SI</th><th>Ettringite SI</th></tr></thead><tbody><tr><td>B1-1h</td><td>0.0014</td><td>34.6</td><td>14.6</td><td>0.0198</td><td>0.07</td><td>0.10</td><td>7.16</td></tr><tr><td>B1-8h</td><td>0.0022</td><td>34.0</td><td>14.4</td><td>0.0299</td><td>0.07</td><td>0.08</td><td>7.50</td></tr><tr><td>B1-14.2 h</td><td>0.0011</td><td>27.6</td><td>10.5</td><td>0.0180</td><td>-0.09</td><td>-0.07</td><td>6.10</td></tr><tr><td>B2-1h</td><td>0.0015</td><td>32.5</td><td>13.7</td><td>0.0260</td><td>0.04</td><td>0.04</td><td>6.98</td></tr><tr><td>B2-3h</td><td>0.0012</td><td>33.5</td><td>13.6</td><td>0.0269</td><td>0.04</td><td>0.09</td><td>6.92</td></tr><tr><td>B2-6.8 h</td><td>0.0013</td><td>28.2</td><td>10.3</td><td>0.0248</td><td>-0.09</td><td>-0.02</td><td>6.32</td></tr></tbody></table>
<figure><img src="image_23.png" /><figcaption>Fig. A3. In-situ XRD results of B2, after replacing 25 wt% of B2 with limestone. E-ettringite, G-gypsum, P-portlandite, Hc-hemicarbonate, C-calcite.</figcaption></figure>

Table A3

Steps and details for the calculation of activation energy of different CCs. K* represents reaction rate for a specific CC based on maximum ettringite time at 25 °C, for example for A1, 11.52 h represents the reaction rate K<sub>A1</sub>, and the reaction rate at 40 °C is 11.92/3.92 = 2.94K<sub>A1</sub>. Given maximum ettringite times at 3 different temperatures, a semi-empirical approach based on equations given in Section 2.2.9 was used to estimate the activation energy of the different CCs. Fig. A4 shows the modified Arrhenius plot and the variation of e<sup>-Ea/RT</sup> with temperature.

<table><thead><tr><th>Step no.</th><th>Details</th><th></th><th>A1</th><th>B1</th><th>B2</th><th>C1</th><th>C3</th><th>C4</th><th>D1</th></tr></thead><tbody><tr><td rowspan="3">Step 1</td><td rowspan="3">Maximum ettringite time</td><td>10 °C</td><td>44.7</td><td>49.9</td><td>21.45</td><td>19.47</td><td>25.8</td><td>37.25</td><td>85.8</td></tr><tr><td>25 °C</td><td>11.52</td><td>14.15</td><td>6.78</td><td>6.08</td><td>8.15</td><td>10.93</td><td>22.77</td></tr><tr><td>40 °C</td><td>3.92</td><td>5.47</td><td>2.62</td><td>2.23</td><td>3.02</td><td>3.80</td><td>7.63</td></tr><tr><td rowspan="3">Step 2</td><td rowspan="3">Take 25 °C as arbitrary rate (K*)</td><td>10 °C</td><td>0.258 K<sub>A1</sub></td><td>0.284 K<sub>B1</sub></td><td>0.316K<sub>B2</sub></td><td>0.312K<sub>C1</sub></td><td>0.316K<sub>C3</sub></td><td>0.293K<sub>C4</sub></td><td>0.265K<sub>D1</sub></td></tr><tr><td>25 °C</td><td>K<sub>A1</sub></td><td>K<sub>B1</sub></td><td>K<sub>B2</sub></td><td>K<sub>C1</sub></td><td>K<sub>C3</sub></td><td>K<sub>C4</sub></td><td>K<sub>D1</sub></td></tr><tr><td>40 °C</td><td>2.94 K<sub>A1</sub></td><td>2.59 K<sub>B1</sub></td><td>2.59 K<sub>B2</sub></td><td>2.73 K<sub>C1</sub></td><td>2.70 K<sub>C3</sub></td><td>2.88K<sub>C4</sub></td><td>2.98 K<sub>D1</sub></td></tr><tr><td rowspan="3">Step 3</td><td rowspan="3">Activation energy for different temp. Changes</td><td>10 to 25 °C</td><td>63.4</td><td>58.9</td><td>53.8</td><td>54.4</td><td>53.9</td><td>57.3</td><td>62.0</td></tr><tr><td>25 to 40 °C</td><td>55.7</td><td>49.1</td><td>49.1</td><td>51.8</td><td>51.3</td><td>54.6</td><td>56.5</td></tr><tr><td>10 to 40 °C</td><td>59.7</td><td>54.3</td><td>51.6</td><td>53.2</td><td>52.7</td><td>56.0</td><td>59.4</td></tr><tr><td rowspan="2">Step 4</td><td>Average</td><td></td><td>59.6</td><td>54.1</td><td>51.5</td><td>53.1</td><td>52.6</td><td>56.0</td><td>59.3</td></tr><tr><td>Error</td><td></td><td>3.8</td><td>4.9</td><td>2.3</td><td>1.3</td><td>1.3</td><td>1.3</td><td>2.7</td></tr></tbody></table>

<figure><img src="image_24.png" /><figcaption>Fig. A4. (a) Modified Arrhenius plot (based on step 2 in Table A2), and (b) variation of e<sup>-Ea/RT</sup> with temperature.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Fig. A5. Quantification of phase changes in CCLC-50 with hydration time using Rietveld analysis.</figcaption></figure>

## Data availability

Data will be made available on request.

## References

[1] K. Scrivener, F. Martirena, S. Bishnoi, S. Maity, Calcined clay limestone cements (LC3), Cem. Concr. Res. 114 (2018) 49–56, https://doi.org/10.1016/j.cemconres.2017.08.017.

[2] M. Sharma, S. Bishnoi, F. Martirena, K. Scrivener, Limestone calcined clay cement and concrete: a state-of-the-art review, Cem. Concr. Res. 149 (2021) 106564, https://doi.org/10.1016/j.cemconres.2021.106564.

[3] M.C.G. Juenger, F. Winnefeld, J.L. Provis, J.H. Ideker, Advances in alternative cementitious binders, Cem. Concr. Res. 41 (2011) 1232–1243, https://doi.org/10.1016/j.cemconres.2010.11.012.

[4] M. Antoni, J. Rossen, F. Martirena, K. Scrivener, Cement substitution by a combination of metakaolin and limestone, Cem. Concr. Res. 42 (2012) 1579–1589, https://doi.org/10.1016/j.cemconres.2012.09.006.

[5] F.H. Avet, Investigation of the grade of calcined clays used as clinker substitute in Limestone Calcined Clay Cement LC3 (2017) 169.

[6] F. Avet, K. Scrivener, Investigation of the calcined kaolinite content on the hydration of limestone calcined clay cement (LC3), Cem. Concr. Res. 107 (2018) 124–135, https://doi.org/10.1016/j.cemconres.2018.02.016.

[7] T.R. Muzenda, F. Georget, T. Matschei, The effect of the physico-chemical properties of (calcined) clays on the rheological properties and early hydration of calcined clay-limestone cement, Constr. Build. Mater. 492 (2025) 142864, https://doi.org/10.1016/j.conbuildmat.2025.142864.

[8] K. Scrivener, M.B. Haha, P. Juilland, C. Levy, Research needs for cementitious building materials with focus on Europe, RILEM Technical Letters 7 (2022) 220–252, https://doi.org/10.21809/rilemtechlett.2022.165.

[9] C1897 Standard Test Methods for Measuring the Reactivity of Supplementary Cementitious Materials by Isothermal Calorimetry and Bound Water Measurements, (n.d.). https://www.astm.org/c1897-20.html (accessed December 9, 2024).

[10] F. Avet, X. Li, M. Ben Haha, S.A. Bernal, S. Bishnoi, Ö. Cizer, M. Cyr, S. Dolenec, P. Durdzinski, J. Haufe, D. Hooton, M.C.G. Juenger, S. Kamali-Bernard, D. Londono-Zuluaga, A.T.M. Marsh, M. Marroccoli, M. Mrak, A. Parashar, C. Patapy, M. Pedersen, J.L. Provis, S. Sabio, S. Schulze, R. Snellings, A. Telesca, M. Thomas, F. Vargas, A. Vollpracht, B. Walkley, F. Winnefeld, G. Ye, S. Zhang, K. Scrivener, Report of RILEM TC 267-TRM phase 2: optimization and testing of the robustness of the R3 reactivity tests for supplementary cementitious materials, Mater. Struct. 55 (92) (2022), https://doi.org/10.1617/s11527-022-01928-6.

[11] Y. Min, H. Kabir, C. Kothari, M.F. Iqbal, N. Garg, UR2: ultra-rapid reactivity test for real-time, low-cost quality control of calcined clays, Cem. Concr. Res. 191 (2025) 107806, https://doi.org/10.1016/j.cemconres.2025.107806.

[12] T.R. Muzenda, F. Georget, T. Matschei, Early reactivity assessment of calcined clays and the impact of secondary phases, SP 362 (2024) 720–732, https://doi.org/10.14359/51742004.

[13] M. Dhar, T.R. Muzenda, Y. Demeusy, A. Koutsouradi, P.A. Jensen, C. Labbez, S. Bishnoi, F. Georget, T. Matschei, Evaluation of characterization techniques for selecting kaolinitic clays as supplementary cementitious materials: An interlaboratory study, Journal of the American Ceramic Society n/a (n.d.) e20512. doi: https://doi.org/10.1111/jace.20512.

[14] S. Scherb, M. Maier, N. Beuntner, K.-C. Thienel, J. Neubauer, Reaction kinetics during early hydration of calcined phyllosilicates in clinker-free model systems, Cem. Concr. Res. 143 (2021) 106382, https://doi.org/10.1016/j.cemconres.2021.106382.

[15] F. Zunino, K. Scrivener, Insights on the role of alumina content and the filler effect on the sulfate requirement of PC and blended cements, Cem. Concr. Res. 160 (2022) 106929, https://doi.org/10.1016/j.cemconres.2022.106929.

[16] S. Scherb, M. Maier, K. Mathias, N. Beuntner, K.-C. Thienel, Reaction kinetics of calcined smectite in a clinker-free model and a synthetic cement system in comparison with selected calcined phyllosilicates, Cem. Concr. Res. 189 (2025) 107766, https://doi.org/10.1016/j.cemconres.2024.107766.

[17] M. Murat, Hydration reaction and hardening of calcined clays and related minerals. I. Preliminary investigation on metakaolinite, Cem. Concr. Res. 13 (1983) 259–266, https://doi.org/10.1016/0008-8846(83)90109-6.

[18] P.F. Ash-Specification, Bureau of Indian Standards, New Delhi, India, 2003. https://www.sfindia.org/forum/files/CED2_7697_WC_193.pdf. accessed December 11, 2024.

[19] X. Li, R. Snellings, M. Antoni, N.M. Alderete, M. Ben Haha, S. Bishnoi, Ö. Cizer, M. Cyr, K. De Weerdt, Y. Dhandapani, J. Duchesne, J. Haufe, D. Hooton, M. Juenger, S. Kamali-Bernard, S. Kramar, M. Marroccoli, A.M. Joseph, A. Parashar, C. Patapy, J.L. Provis, S. Sabio, M. Santhanam, L. Steger, T. Sui, A. Telesca, A. Vollpracht, F. Vargas, B. Walkley, F. Winnefeld, G. Ye, M. Zajac, S. Zhang, K.L. Scrivener, Reactivity Tests for Supplementary Cementitious Materials, RILEM TC 267-TRM phase 1, vol. 51, Mater Struct, 2018, p. 151, https://doi.org/10.1617/s11527-018-1269-x.

[20] F. Zunino, K. Scrivener, The influence of the filler effect on the sulfate requirement of blended cements, Cem. Concr. Res. 126 (2019) 105918, https://doi.org/10.1016/j.cemconres.2019.105918.

[21] M. Panzer, S. Scherb, N. Beuntner, K.-C. Thienel, Effect of sulfate carrier addition on the strength of calcined clay blended cements, Constr. Build. Mater. 477 (2025) 140915, https://doi.org/10.1016/j.conbuildmat.2025.140915.

[22] O. Canbek, C. Szeto, N.R. Washburn, K.E. Kurtis, A quantitative approach to determining sulfate balance for LC3, CEMENT 12 (2023) 100063, https://doi.org/10.1016/j.cement.2023.100063.

[23] M.R.C.D. Silva, J.D.S. Andrade Neto, B. Walkley, A.P. Kirchheim, Exploring sulfate optimization techniques in limestone calcined clay cements (LC3): limitations and insights, Cem. Concr. Res. 175 (2024) 107375, https://doi.org/10.1016/j.cemconres.2023.107375.

[24] S. Mohammed, O. Safiullah, Optimization of the SO3 content of an Algerian Portland cement: study on the effect of various amounts of gypsum on cement properties, Constr. Build. Mater. 164 (2018) 362–370, https://doi.org/10.1016/j.conbuildmat.2017.12.218.

[25] M. Dhar, S. Bishnoi, Influence of calcination temperature on the physical and chemical characteristics of kaolinitic clays for use as supplementary cementitious materials, Cem. Concr. Res. 178 (2024) 107464, https://doi.org/10.1016/j.cemconres.2024.107464.

[26] T.R. Muzenda, F. Georget, T. Matschei, Mechano-chemical activation of kaolins – effect of mineral composition and triethanolamine on activation effectiveness and hydration, Constr. Build. Mater. 482 (2025) 141703, https://doi.org/10.1016/j.conbuildmat.2025.141703.

[27] M. Catti, G. Ferraris, G. Ivaldi, Thermal strain analysis in the crystal structure of muscovite at 700 °C, Eur, J. Mineral. 1 (1989) 625–632 (n.d.).

[28] Y. Demeusy, T.R. Muzenda, A. Guillet, S. Gauffinet, F. Georget, T. Matschei, C. Labbez, Reactivity of calcined clays and its impact on fresh cement paste rheology, Under Review (n.d.).
[29] Y. Demeusy, S. Gauffinet, C. Labbez, Paste Rheology and Surface Charge of Calcined Kaolinite (2023), https://doi.org/10.48550/arXiv.2310.07273.

[30] P.F.G. Banfill, Rheology of Fresh Cement and Concrete, 2006.

[31] R. Snellings, X-ray diffraction applied to cement, in: K.L. Scrivener, R. Snellings, B. Lothenbach (Eds.), A Practical Guide to Microstructural Analysis of Cementitious Materials, CRC Press, Boca Raton, 2016, pp. 179-213, https://doi.org/10.1201/B19074, n.d.

[32] D.L. Parkhurst, C.a.J. Appelo, Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, U.S. Geological Survey (2013), https://doi.org/10.3133/tm6A43.

[33] B. Lothenbach, D.A. Kulik, T. Matschei, M. Balonis, L. Baquerizo, B. Dilnesa, G. D. Miron, R.J. Myers, Cemdata18: a chemical thermodynamic database for hydrated Portland cements and alkali-activated materials, Cem. Concr. Res. 115 (2019) 472-506, https://doi.org/10.1016/j.cemconres.2018.04.018.

[34] R. Snellings, J. Chwast, Ö. Cizer, N. De Belie, Y. Dhandapani, P. Durdzinski, J. Elsen, J. Haufe, D. Hooton, C. Patapy, M. Santhanam, K. Scrivener, D. Snoeck, L. Steger, S. Tongbo, A. Vollpracht, F. Winnefeld, B. Lothenbach, RILEM TC-238 SCM recommendation on hydration stoppage by solvent exchange for the study of hydrate assemblages, Mater. Struct. 51 (2018) 172, https://doi.org/10.1617/s11527-018-1298-5.

[35] T. Thoenen, D.A. Kulik, Nagra/PSI Chemical Thermodynamic Data Base 01/01 for the GEM-Selektor (V.2- PSI) Geochemical Modeling Code: Release 28-02-03, in: Internal Report TM-44-03-04, 2003 http://gems.web.psi.ch/TDOC/pdf/TM-44-03-04-web.pdf. accessed January 28, 2025.

[36] T. Thoenen, W. Hummel, U. Berner, E. Curti, The PSI/Nagra Chemical Thermodynamic Database 12/07 - Google Scholar, (n.d.). https://scholar.google.com/scholar_lookup?title=%20PSI%2FNagra%20Chemical%20Thermodynamic%20Data%20Base%2012%2F07%2C%20PSI%20Report%2014-04&author=T.%20Thoenen&publication_year=2014 (accessed January 28, 2025).

[37] B. Lothenbach, T. Matschei, G. Möschner, F.P. Glasser, Thermodynamic modelling of the effect of temperature on the hydration and porosity of Portland cement, Cem. Concr. Res. 38 (2008) 1-18, https://doi.org/10.1016/j.cemconres.2007.08.017.

[38] D.A. Kulik, Improving the structural consistency of C-S-H solid solution thermodynamic models, Cem. Concr. Res. 41 (2011) 477-495, https://doi.org/10.1016/j.cemconres.2011.01.012.

[39] T. Matschei, B. Lothenbach, F.P. Glasser, Thermodynamic properties of Portland cement hydrates in the system CaO-Al2O3-SiO2-CaSO4-CaCO3-H2O, Cem. Concr. Res. 37 (2007) 1379-1410, https://doi.org/10.1016/j.cemconres.2007.06.002.

[40] L.G. Baquerizo, T. Matschei, K.L. Scrivener, M. Saeidpour, A. Thorell, L. Wadsö, Methods to determine hydration states of minerals and cement hydrates, Cem. Concr. Res. 65 (2014) 85-95, https://doi.org/10.1016/j.cemconres.2014.07.009.

[41] L.G. Baquerizo, T. Matschei, K.L. Scrivener, Impact of water activity on the stability of ettringite, Cem. Concr. Res. 79 (2016) 31-44, https://doi.org/10.1016/j.cemconres.2015.07.008.

[42] E.M.J. Berodier, Impact of the Supplementary Cementitious Materials on the Kinetics and Microstructural Development of Cement Hydration, (n.d.).

[43] F. Zunino, K. Scrivener, Factors influencing the sulfate balance in pure phase C3S/ C3A systems, Cem. Concr. Res. 133 (2020) 106085, https://doi.org/10.1016/j.cemconres.2020.106085.

[44] P. Hemstad, B. Lothenbach, K. De Weerdt, Distribution of sulphate and aluminium in hydrated cement pastes, Cem. Concr. Res. 180 (2024) 107467, https://doi.org/10.1016/j.cemconres.2024.107467.

[45] K. Ioannidou, C. Labbez, E. Masoero, A review of coarse grained and mesoscale simulations of C-S-H, Cem. Concr. Res. 159 (2022) 106857, https://doi.org/10.1016/j.cemconres.2022.106857.

[46] F. Zunino, K. Scrivener, Microstructural developments of limestone calcined clay cement (LC3) pastes after long-term (3 years) hydration, Cem. Concr. Res. 153 (2022) 106693, https://doi.org/10.1016/j.cemconres.2021.106693.

[47] J.L. Poole, K.A. Riding, K.J. Folliard, M.C.G. Juenger, A.K. Schindler, Methods for calculating activation energy for Portland cement, ACI Mater. J. 104 (2007), https://doi.org/10.14359/18499.

[48] W. Hanpongpun, Investigation of the use of Limestone Calcined Clay Cement (LC3) applied to Thailand, (n.d.) 140.

[49] K. Weise, N. Ukrainczyk, E. Koenders, Pozzolanic Metakaolin reactions: stoichiometric and kinetic modeling, Mater. Des. 239 (2024) 112747, https://doi.org/10.1016/j.matdes.2024.112747.

[50] M. Maier, S. Scherb, K.-C. Thielen, Sulfate consumption during the hydration of Alite and its influence by SCMs, Ce/Papers 6 (2023) 2-7, https://doi.org/10.1002/cepa.2885.

[51] D. Damidot, F.P. Glasser, Thermodynamic investigation of the CaOAl2O3CaSO4K2OH2O system at 25°C, Cem. Concr. Res. 23 (1993) 1195-1204, https://doi.org/10.1016/0008-8846(93)90180-H.

[52] A.L. Kelzenberg, S.L. Tracy, B.J. Christiansen, J.J. Thomas, M.E. Clarage, S. Hodson, H.M. Jennings, Chemistry of the aqueous phase of ordinary Portland cement pastes at early reaction times, J. Am. Ceram. Soc. 81 (1998) 2349-2359, https://doi.org/10.1111/j.1151-2916.1998.tb02631.x.

[53] J. da S. Andrade Neto, A.G. De la Torre, A.P. Kirchheim, Effects of sulfates on the hydration of Portland cement - a review, Constr. Build. Mater. 279 (2021) 122428, https://doi.org/10.1016/j.conbuildmat.2021.122428.

[54] S. Sim, S. Her, H. Suh, S. Cho, S. Im, P. Li, S. Bae, Influences of alumina type and sulfate content on hydration and physicochemical changes in Portland-limestone cement, Constr. Build. Mater. 426 (2024) 135989, https://doi.org/10.1016/j.conbuildmat.2024.135989.

[55] I.M.R. Bernal, M.A.G. Aranda, I. Santacruz, A.G. De la Torre, A. Cuesta, Early-age reactivity of calcined kaolinitic clays in LC3 cements: a multitechnique investigation including pair distribution function analysis, J. Sustain. Cem.-Based Mater. 12 (2023) 721-735, https://doi.org/10.1080/21650373.2022.2117248.

[56] M. Panzer, S. Scherb, N. Beuntner, K.-C. Thielen, An approach to estimate the strength contribution of calcined clays in blended cements, Constr. Build. Mater. 447 (2024) 137800, https://doi.org/10.1016/j.conbuildmat.2024.137800.

[57] N. Roussel, G. Ovarlez, S. Garrault, C. Brumaud, The origins of thixotropy of fresh cement pastes, Cem. Concr. Res. 42 (2012) 148-157, https://doi.org/10.1016/j.cemconres.2011.09.004.

[58] J. Link, T. Sowoidnich, C. Pfitzner, T. Gil-Diaz, F. Heberling, J. Lützenkirchen, T. Schäfer, H.-M. Ludwig, M. Haist, The influences of cement hydration and temperature on the thixotropy of cement paste, Materials 13 (2020) 1853, https://doi.org/10.3390/ma13081853.

[59] Y. Chen, Y. Zhang, S. He, M. Liang, Y. Zhang, E. Schlangen, O. Çopuroğlu, Rheology control of limestone calcined clay cement pastes by modifying the content of fine-grained metakaolin, J. Sustain. Cement-Based Mater. (2023) 1-15, https://doi.org/10.1080/21650373.2023.2169965.

[60] T. Matschei, T.R. Muzenda, F. Georget, Influence of clay impurities on the performance of calcined clay-limestone cements, Ce/Papers 6 (2023) 373-381, https://doi.org/10.1002/cepa.2776.

[61] T.R. Muzenda, P. Hou, S. Kawashima, T. Sui, X. Cheng, The role of limestone and calcined clay on the rheological properties of LC3, Cem. Concr. Compos. 107 (2020) 103516, https://doi.org/10.1016/j.cemconcomp.2020.103516.

[62] O.H. Wallevik, D. Feys, J.E. Wallevik, K.H. Khayat, Avoiding inaccurate interpretations of rheological measurements for cement-based materials, Cem. Concr. Res. 78 (2015) 100-109, https://doi.org/10.1016/j.cemconres.2015.05.003.

[63] E. Kapeluszna, Ł. Kotwica, A. Różyccka, J. Golek, Incorporation of Al in C-A-S-H gels with various ca/Si and Al/Si ratio: microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis, Constr. Build. Mater. 155 (2017) 643-653, https://doi.org/10.1016/j.conbuildmat.2017.08.091.

[64] A. Bauer, G. Berger, Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C, Appl. Geochem. 13 (1998) 905-916, https://doi.org/10.1016/S0883-2927(98)00018-3.

[65] N. Garg, J. Skibsted, Dissolution kinetics of calcined kaolinite and montmorillonite in alkaline conditions: evidence for reactive Al(V) sites, J. Am. Ceram. Soc. 102 (2019) 7720-7734, https://doi.org/10.1111/jace.16663.

[66] W.E.E. Stone, Structural iron in kaolinite, A study by proton magnetic resonance, Sciences Géologiques, Bulletins et Mémoires 86 (1990) 109-114. https://www.persee.fr/doc/sgeol_0302-2684_1990_act_86_1_2134. accessed May 1, 2025.

[67] M.M. Mestdagh, L. Vielvoye, A.J. Herbillon, Iron in kaolinite: II, The relationship between kaolinite crystallinity and iron content, Clay Minerals 15 (1980) 1-13, https://doi.org/10.1180/claymin.1980.015.1.01.