# An experimental review of the reaction paths followed by alkali-activated slag pastes

C. Paillard, N. Sanson, J.-B. D'espinose de Lacaillerie, M. Palacios, P.

Boustingorry, M. Jachiet, C. Giraudeau, V. Kocaba

► To cite this version:

C. Paillard, N. Sanson, J.-B. D'espinose de Lacaillerie, M. Palacios, P. Boustingorry, et al. An experimental review of the reaction paths followed by alkali-activated slag pastes. Cement and Concrete Research, 2025, 189 (7), pp.107765. 10.1016/j.cemconres.2024.107765. hal-05076089

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Distributed under a Creative Commons Attribution 4.0 International License
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AN EXPERIMENTAL REVIEW OF THE REACTION PATHS FOLLOWED BY
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ALKALI-ACTIVATED SLAG PASTES

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C. Paillard<sup>a,c,*</sup> N. Sanson<sup>a</sup>, J.-B. d'Espinose de Lacaillerie<sup>a</sup>,* M. Palacios<sup>b</sup>,

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P. Boustingorry<sup>c</sup>, M. Jachiet<sup>c</sup>, C. Giraudeau<sup>c</sup> and V. Kocaba<sup>c</sup>

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a Soft Matter Science and Engineering Laboratory (SIMM), UMR CNRS 7615, ESPCI

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Paris, Université PSL, Sorbonne Université, Paris, France

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b Eduardo Torroja Institute for Construction Science (IETcc-CSIC), Madrid, Spain

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 CHRYSO Saint-Gobain Construction Chemicals, Sermaises-du-Loiret, France

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**Abstract**

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NMR, XRD, mechanical strength measurements, bound water quantification and
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isothermal calorimetry were combined to contrast the degree of reaction and the paths followed
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by slag under the influence of three activators: NaOH, Na₂CO₃, and Na₂Si₁.₇O₄.₄. NaOH-
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activation led to the concomitant formation of a very ordered C-A-S-H gel and of hydrotalcite,
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giving rise to early mechanical strength. For Na₂Si₁.₇O₄.₄, a N-A-S-H gel formed first due to the
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high quantities of silicon in solution. This led to quick setting but no mechanical strength. Later,
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an amorphous C-A-S-H gel provided mechanical strength, while an aluminate phase
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precipitated. Finally, Na₂CO₃-activation also led to the formation of N-A-S-H, formation
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favored by the initial consumption of calcium to form calcite. This did not bring any real
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structuration and mechanical strength. Only after a few days of hydration did the mechanical
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strength improve with the precipitation of amorphous C-A-S-H, an ill-defined hydrated
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aluminate phase, and of gaylussite.

22 **Keywords:** isothermal calorimetry, NMR, XRD, mechanical strength, bound water

23 **Corresponding authors:** clara.paillard@saint-gobain.com, jean-baptiste.despinose@espci.fr

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† Present address: Clara Paillard, Saint Gobain Research, 39 Quai Lucien Lefranc,
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93300 Aubervilliers, France

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<figure><img src="image_2.png" /></figure>
## Introduction

Responsible for around 8 % of global CO2 emissions, the cement industry needs to drastically reduce its carbon footprint.[1] One solution is to substitute the clinker of Ordinary Portland Cement (OPC) with oxides of lower calcium content. These oxides inevitably have a lower reactivity than clinker but, if they contain enough calcium, their reactivity can be activated by an external source of alkalinity that can either be a part of clinker or an added alkali activator. This latter case corresponds to alkali-activated cements[2–4] made from clays or various industrial by-products such as ground-granulated blast furnace slag. Slag, in particular, is already regularly used to partially replace clinker in cements[5]. It is consequently inherently interesting; however, it also represents a good model for cements formulations based on Ca-poor glasses of low hydraulicity. Regarding the activators, the most promising ones appear to be sodium hydroxide – a strong base –, sodium carbonate – a weak base –, and sodium silicate, as they can easily be supplied and have good activating properties[5].

During the alkali activation of slag pastes, reaction products, namely hydrates and carbonates, form in a different sequence than in more traditional cements and have different chemistry and microstructure. This has been reviewed in numerous textbooks[5,6] and articles,[7,8] and is still the subject of ongoing research. As in OPC, the primary hydration product which is formed is an Aluminum-inserted- Calcium-Silicate-Hydrate, C-A-S-H, phase gel[9–12]. The structure of the C-A-S-H formed depends on the slag composition but most importantly on the activator used. For example, the NaOH-activation usually leads to a more crystalline gel with a higher Al incorporation than with sodium silicates[13–16]. Regardless of crystallinity, Fernández-Jiménez et al.[16] found longer linear chains in the C-A-S-H precipitated in presence of NaOH. Also, amorphous cross-linked gels are more likely to form when sodium silicates or sodium carbonate are used as activators than when NaOH is used[16]. In the case of low Ca-environments, another type of aluminosilicate gel can even form: a fully amorphous hydrated sodium aluminosilicate gel, the N-A-S-H gel[7]. This amorphous reaction product has a complex structure with a high degree of cross-linking forming a three-dimensional network, similar to an amorphous zeolite. Compared to C-A-S-H, it contains a low quantity of Ca and significantly more alkali metals (Na or K). The secondary hydration products vary a lot depending on the slag composition and the activator used. The most present secondary hydration products[12,16–18] are Mg6Al2CO3(OH)16.4H2O (hydrotalcite) and phases of aluminate ferrite monosulfate (AFm) such as C4AH13[17], C4Ac3H11, C2ASH8 (strätlingite)[19],
or C₄Ac₀.₅H₁₂. CaCO₃ (calcite) and Na₂Ca(CO₃)₂.5H₂O (gaylussite) are also often found in Na₂CO₃-activated slag cements[16,17,20–24].

Several groups studied the hydration kinetics of slags activated with Na₂CO₃, NaOH, or SiO₂.Na₂O showing calorimetric curves present at most three thermal events [16,25,26]. Based on prior knowledge of the hydration mechanisms of Portland cement, these events were attributed to the initial dissolution of the slag under the effect of OH⁻ ions and water, to a pre-induction period, and finally to the acceleration of dissolution driven by the precipitation of reaction products. In SiO₂.Na₂O-activated slag, rapid setting occurs due to the reaction of the silicates of the activating solution with the calcium coming from the slag, forming a primary amorphous C-S-H[16,22,25,27] phase gel. In particular, this has been demonstrated by Fernández-Jiménez et al.[25] through magic angle nuclear magnetic resonance (MAS-NMR) analysis at 2h, 20h and 115h of reaction. At first, only NMR resonances of small intensity corresponded to silicates from the activating solution and to C-S-H were visible, while resonances corresponding to more cross-linked silicates present in a C-A-S-H structure appeared later. Parallelly, the amount of tetrahedral Al resonances progressively increased while phases with octahedral Al (AFm, hydrotalcite etc.) contributed to the spectrum only after 115h of reaction. The use of NaOH leads to rapid dissolution, and calorimetry curves often contain only one heat peak[22,28]. Ben Haha et al.[22] confirmed this with scanning electron microscopy (SEM), observing higher hydration degrees in the early stages for NaOH-activated slag compared to silicate-activated slag. Finally, when slag is activated with Na₂CO₃, calcium and sodium carbonates precipitate long before C-A-S-H appears[16,20,23,24,29,30]. Hence, X-ray diffraction (XRD) analyses by Bernal et al.[24] identified calcite, vaterite, aragonite, gaylussite, and zeolite NaA at one day, with C-A-S-H only forming after seven days.

As apparent from this rapid literature review, the reaction paths followed during activation have been studied independently for each activator and under varying conditions. Reviews attempting to compare different activators often concluded on the difficulty of taking into account the high numbers of experimental parameters influencing chemical and microstructural evolution (water to solid ratio, mixing, temperature, composition and fineness of the slag to name a few). This is exemplified in a recent review concluding that more comparative experimental work was needed[31]. Alternatively, others faced that complexity using statistical approaches (neural networks) to analyze data from the literature[32]. The present article aims at providing a more comprehensive and unified picture – an experimental
review - by investigating the influence of the three types of activators (NaOH, Na₂CO₃, and Na₂Si₁.₇O₄.₄) on the reaction path followed by slag activated under similar conditions. This was done by combining the results of different analysis and characterizing methods, namely isothermal micro-calorimetry, ²⁹Si and ²⁷Al one-pulse MAS NMR, {¹H}²⁹Si cross-polarized MAS NMR, X-ray diffraction (XRD), quantification of bound water by thermogravimetric analysis (TGA), and measurements of the mechanical strength. In particular, the link between the evolution of the microstructure and the macroscale properties will be illustrated.

# Materials and methods

## Materials

A ground granulated blast-furnace slag supplied by CHRYSO Saint-Gobain Construction Chemicals was used in this study. The same slag was used in our previous study of superplasticizers efficiency in slag pastes[33] but its characteristics are nevertheless recalled here for clarity. Its chemical composition, determined using X-ray fluorescence (XRF) by an external contractor (Université Paris Cité, France), is summarized in Table I. A specific surface area of 0.85 ± 0.02 m²/g was measured using krypton adsorption at 77 K following the BET method, by an external contractor (LIEC, Nancy, France). The measurements were conducted on an absorbometer Belsorp-Max II from MicrotracBEL Corp. The samples were degassed beforehand under vacuum at 120°C for 228 h.

<table><caption>**Table I:** The chemical composition of the slag determined by XRF (wt %). LoI.: Loss on ignition measured at 1000 °C.</caption><thead><tr><th></th><th>SiO<sub>2</sub></th><th>CaO</th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>MgO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>TiO<sub>2</sub></th><th>MnO</th><th>LoI.</th></tr></thead><tbody><tr><td>Slag</td><td>37.33</td><td>40.88</td><td>9.76</td><td>0.31</td><td>6.35</td><td>0.30</td><td>0.40</td><td>0.56</td><td>0.18</td><td>3.95</td></tr></tbody></table>

Figure 1a presents the particle size distribution of the slag dispersed in isopropanol and measured by laser light diffraction (Mastersizer, Malvern Panalytical), courtesy of Dr. Y. Keskin (Navier laboratory, Marne la Vallée, France), with a Dv50 value of 9.15 µm. The particles had irregular shapes with sharp edges, as shown on the SEM image (Figure 1b). The image was taken on a JEOL 6300F field emission electron microscope, at 10 kV, in high-vacuum mode.
<figure><img src="image_3.png" /><figcaption>**Figure 1:** (a) Particle size distribution of blast furnace slag. (b) SEM image of the blast furnace slag particles.</figcaption></figure>

Sodium hydroxide (NaOH) pellets EMPLURA®, sodium carbonate (Na₂CO₃) powder >99.5% from Sigma-Aldrich, or a Betol 52T solution (SiO₂/Na₂O molar ratio of 2.12 and 55.1 wt% of H₂O) from Woellner were used for the preparation of the activating solutions. NaOH and Na₂CO₃ were dissolved in ultrapure (Type III) water (conductivity of less than 18.2 mΩ/cm) in the appropriate amounts to obtain an alkaline solution with the target activator's concentration (wt% of Na₂O per mass of slag). In the case of the sodium silicate activation, Betol 52T was diluted with ultrapure water and a specific quantity of NaOH was dissolved to reach the desired SiO₂/Na₂O molar ratio. Except otherwise stated, the activating solutions had a composition corresponding to a paste formulation of 4.0 wt% Na₂O by weight of slag, a water/slag (w/s) weight ratio of 0.4 (including the water from the activator), and for sodium silicate activation a SiO₂/Na₂O molar ratio of 1.7 (Na₂Si₁.₇O₄.₄).

## Sample preparation and tests performed

To prepare the alkali-activated pastes, 50 g of slag were combined with activating solutions in a container with a diameter of 52 mm. Immediately after, the slag paste was mixed with an overhead stirrer, using a radial flow stirring blade of 29 mm diameter. The mixing procedure was as follows. First, the paste was blended at 500 rpm for 30 s. Then, for an additional 30 s, a fast (1500 rpm) mixing was applied. A rest time of 30 s and a final 1 min of fast mixing concluded the sequence. Some experiments were also conducted on unactivated slag pastes, meaning a mix of water and slag. The same mixing protocol was used for these pastes.
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Prior to XRD, TGA, and NMR analysis, the samples were placed in a plastic container in a thermal bath set at 20 °C. At the desired ages, the hydration was stopped following the protocol given by Palacios et al. [34]: crushing with a hammer, washing twice with ultrapure water and then with isopropanol. Finally, the samples were dried in a vacuum desiccator at room temperature and finely ground in a mortar.

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To prepare mortars for mechanical testing, standard clean sand (CEN EN 196-1 sand from Société Nouvelle du Littoral, France), slag, and activator were mixed in an automatic mixer (3R, France) with a water to powder ratio of 0.4 and a ratio of powders to sand of 0.67. After mixing accordingly to the protocol recommended by the EN 196-1, the fresh mortar was cast into 16x4x4 cm molds and left to set for at least 24 h before demolding. Specimens of longer ages were left to cure in a walk-in chamber set at saturation (20 °C, 100 % relative humidity) until the targeted curing time elapsed.

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## Isothermal micro-calorimetry

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The hydration heat of the alkali-activated slag pastes was measured at 20 °C with a TAM Air isothermal micro-calorimeter from TA Instruments. A total mass of 5.0 g of alkali-activated slag paste was weighed in the mix-ampoules immediately after mixing. The mix-ampoules were placed in the calorimeter at the same time as a reference composed of a precise mass of ultrapure water to have a calorific capacity like that of the samples. The resulting heat flows were normalized by the slag mass.

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## Characterization of the reaction products by X-ray diffraction

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Crystalline reaction products were characterized by XRD using a Phillips PW 1700 diffractometer equipped with an X'Celerator detector, in θ-2θ configuration. The X-rays corresponding to the Cupper Kα₁ line (1.540562 Å) were produced with applied voltage and intensity of 40 kV and 40 mA respectively. Data were collected between 5 and 60°, with steps of 0.0170°.

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## Characterization of the reaction products by Nuclear Magnetic Resonance

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The NMR spectra of the slag samples were obtained on a Bruker 500WB Avance II spectrometer (11.74 T) equipped with magic angle spinning probes (MAS NMR) of the same manufacturer. For the ²⁹Si spectra, the samples were placed in a 7 mm zirconia rotor, at a rotation speed of 6 kHz and a Larmor frequency of 99.3 MHz, following a one-pulse excitation with a recycle delay of 20 s and 2 000-12 000 scans. The pulse length was 3 µs, corresponding

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to a $\frac{3\pi}{8}$ angle. {$^{1}$H}{$^{29}$Si Cross-polarized experiments were also conducted to study more precisely the Si environments in the hydrates. A contact time of 1 ms, a recycle delay of 2 s, and a $^{1}$H pulse length of 3 µs were used, with a number of scans varying between 36 000 and 70 000 depending on signal to noise. The $^{27}$Al spectra were taken in 2.5 mm rotors, at a rotation speed of 26.5 to 29.5 kHz and a Larmor frequency of 130.06 MHz, following a one-pulse excitation with a recycle delay of 2 s and 8 000 scans. The pulse length was 0.5 µs, corresponding to a $\frac{\pi}{10}$ angle. The chemical shift was calibrated externally with a TMS solution for the $^{29}$Si spectra, a 0.1 M Al(NO$_3$)$_3$ solution for the $^{27}$Al spectra, and kaolinite for the cross-polarized experiments. The spectral intensities were normalized according to the integrated signal between -50 and -120 ppm for $^{29}$Si and between 150 and -90 ppm for $^{27}$Al. They were eventually fitted using the DmFit program[35]. All the parameters used for the fits are reported in the Supplementary Materials.

## Characterization of bound water by thermogravimetric analysis

The thermogravimetric analysis of the samples was performed with a TGA 2 device (Mettler-Toledo), using a simple temperature ramp of 10 °C/min from 25 °C to 1000 °C. The measurement environment was continuously flushed with nitrogen flow during the analysis. Total bound water was computed by adding up all the mass loss of the sample, except the mass loss related to calcite when it was identified by XRD.

## Measurement of mechanical strength on mortars

At the desired time, mortar prisms were broken into halves with a purpose-designed hammer and each half was tested for compressive strength in a testing machine (3R, France). The reported results are the averages of two measurements.

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# Results

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## Characterization of the anhydrous slag

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The unreacted slag was characterized by <sup>29</sup>Si and <sup>27</sup>Al NMR, and by XRD. The <sup>29</sup>Si MAS NMR spectrum (Figure 2a) could be fitted with a pseudo-Voigt line shape centered at -75.5 ppm and with a full width at a half maximum of 12 ppm[14]. A mixing parameter of 0.7 for the pseudo-Voigt line shape corresponded to a line shape closer to a Gaussian line shape than to a Lorentzian one, revealing the disordered environment of silicon in vitreous slag.

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To describe the environment of the Si tetrahedra, the usual notation is used: Si tetrahedra, which are linked to *n* other Si tetrahedra, are marked *Q*<sub>*n*</sub>. If some of these *n* tetrahedra are not SiO<sub>4</sub> but AlO<sub>4</sub>, the environment is designated as *Q*<sub>*n*</sub>(*mAl*), where *m* is the number of Al tetrahedra. Although unresolved, the broad resonance seen in the <sup>29</sup>Si MAS NMR spectrum being centered at -75.5 ppm thus corresponded to a distribution of *Q*<sub>0</sub>, *Q*<sub>1</sub> and *Q*<sub>2</sub> environments[36].

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<figure><img src="image_4.png" /></figure>

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<caption>**Figure 2:** <sup>29</sup>Si (a) and <sup>27</sup>Al (b) MAS NMR spectra of the anhydrous slag. The contributions used to model the spectra are shown in solid grey and the total fit obtained is shown in dashed red.</caption>

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The <sup>27</sup>Al MAS NMR spectrum (Figure 2b) of the anhydrous slag was also made of broad contributions. The Czjzek model[37] was used to fit the spectrum since it gives a good line shape prediction for spectra of quadrupolar nuclei such as <sup>27</sup>Al in poorly ordered solids. This

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model gives an expression of the joint probability density function of the quadrupolar parameters and allows one to obtain the mean square of the quadrupolar product $\sqrt{\langle C_{Q\eta}^2 \rangle}$ through the adjustable parameter (Equation 1(1)):

$$ \langle C_{Q\eta}^2 \rangle = \left(\frac{2}{3}I(2I-1)\right)^2 5\sigma^2 \quad (1) $$

Fitting the spectrum of the unreacted slag required a minimum of three contributions: two resonances corresponding to tetrahedral environments with isotropic chemical shifts of 73.2 ppm ($\sigma$ = 0.5 MHz) and 63.0 ppm ($\sigma$ = 0.2 MHz), and one resonance at 11.5 ppm ($\sigma$ = 0.3 MHz) assigned to an octahedral environment. Alternatively, this last resonance could be associated to a superposition of the resonances of an octahedral environment with the one of a small amount of coordinatively unsaturated aluminum in pentahedral coordination. Another possibility is that it did not correspond to a distinct structural environment but simply reflected the limitation of the Czjzek model as applied to slag, i.e. that slag is not a perfect glass and that atomic positions in slag might not be distributed in a fully Gaussian isotropic manner. In any case, it is the $^{27}$Al resonance at 73.2 ppm that dominated the spectrum. It corresponds to the tetrahedral site generally observed in slag and in percalcic aluminosilicate glasses[37–39]. The second minor tetrahedral resonance at 63 ppm is probably the sign of the presence of a very small quantity of reacted slag forming of amorphous alumina-silicate gel.

Going back to the 11.5 ppm resonance, an octahedral contribution has already been previously observed in slags by other researchers[16,40] and has been tentatively assigned to Al in melilite ((Ca,Na)₂(Al,Mg,Fe)[(Al,Si)SiO₇]), which is a solid solution of gehlenite (Ca₂Al₂SiO₇) and akermanite (Ca₂MgSi₂O₇). However, none of these crystalline phases were visible on the diffractogram of the unreacted slag (Figure 3), thus questioning the fact that this range of resonance truly corresponded to an octahedral environment. Only a large bump is visible at around 30°, showing that this material was mostly amorphous as only small peaks corresponding to vaterite appeared. It was surprising that CaCO₃ crystallized as vaterite since it is a less stable polymorph of calcite[41], but its occurrence has previously been reported in other slag samples[40].
<figure><img src="image_5.png" /><figcaption>**Figure 3:** X-ray diffractogram of the anhydrous slag. The peaks are referenced with the corresponding Miller indexes between brackets according to the PDF file of vaterite (noted V, file 01-072-0506).</figcaption></figure>

## Compressive strength of mortars and bound water in paste

A first leg of the study was dedicated to the observation of the evolution of mortar strength and total bound water over 28 days. Compressive strengths are reported at the top of Figure 4 for slag blended with activators at a dosage of 4 wt% equivalent Na₂O per mass of slag.

The three activators featured very discriminated behaviors. Sodium hydroxide induced a very fast hardening with almost 15 MPa after 24 h of curing, but the increase in strength subsequently slowed down to reach only 32 MPa at 28 days (672 h). Sodium carbonate had a completely different influence, with almost no measurable strength during about 9 days (approximately 200 h). The strength suddenly rose to overtake the sodium hydroxide-activated sample at 14 days (approximately 350 h) and ended at around 42 MPa at 28 days. Sodium silicate appeared to produce a slower hardening than hydroxide in the first two days. However,
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the strength became higher than the other two systems afterwards, to reach eventually the highest performance with 65 MPa at 28 days.

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To understand the link between microstructure development and strength, it is interesting to correlate the advancement of hydration – appreciated by the amount of bound water in the paste – to strength development in mortars at the same age. The kinetics of water binding over 28 days was thus followed by TGA, after hydration stoppage of pastes (Figure 4 bottom). The amount of bound water seemed indeed to follow the trends observed on strength as highlighted on Figure 5 where strength is plotted as a function of bound water. Still, each activator induced a very different behavior as explained below.

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Sodium hydroxide led to a very fast water binding on the first day, which subsequently increased slowly towards a final bound mass fraction of 0.16-0.17. At all times measured, mechanical strength increased in proportion to water binding.

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Sodium carbonate produced a two-step water binding process, with a first plateau around 0.06 wt% H₂O after 7 days, followed by sudden increase to 0.14 wt% H₂O at 14 days, with no apparent evolution later. The similarity with the succession of events observed in the time evolution of the compressive strength is striking. However, compared to the case of sodium hydroxide activation, there was no clear relation between water binding and mechanical strength. The first stage of water binding (1st week of curing) did not produce any hardening, and the strength did not correlate linearly with the water binding in the second stage.

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Sodium silicate induced a mixed behavior compared to the ones of sodium hydroxide and carbonate described above. The time evolution of the water binding was similar to the one observed with sodium hydroxide but with a more gradual water binding rate than with hydroxide. Eventually the same plateau of bound water mass fraction of around 0.16-0.17 was reached. However, as for sodium carbonate, water binding did not result immediately in compressive strength as the latter developed only after 20 h. Later on, a linear correlation between bound water and strength was again observed as for sodium hydroxide activation.

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Figure 5 summarizes and clearly contrasts the differing behavior of the pastes depending on the activators. Once the hydroxide and silicate pastes reached experimentally significant mechanical strengths (above 5 MPa), this property increased linearly with hydration, as expected from models linking strength to the amounts of hydrates formed[42]. Nevertheless,
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the slopes differ which shows that the amount of bound water is not the only parameter controlling mechanical performances, and other microscopic parameters must be considered – such as porosity. Porosity depends highly on the water to solid ratio of the paste[43] making comparison of the slopes reported in the literature difficult. The case of the carbonate is more complex as its hydration follows a two-step process as will be confirmed in what follows.

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This illustrated how different systems produce very different strength gain kinetics, which hints at discriminated microscopic mechanisms. This study precisely aims at explaining these differences.

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<figure><img src="image_6.png" /></figure>

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**Figure 4:** Compressive strength and total bound water of mortar specimens over 28 days of hydration. An activator dosage of 4 wt% of Na₂O and a w/s ratio of 0.4 were used. The initial water fraction upon mortar mixing is 0.286. (♦) NaOH, (×) Na₂CO₃, (+) Na₂Si₁.₇O₄.₄. Arrows at the top of the figure point to the beginning of each hydration plateau.

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<figure><img src="image_7.png" /><figcaption>**Figure 5:** Compressive strength versus total bound water of mortar specimens over 28 days of hydration. The data are the same as in Figure 4. An activator dosage of 4 wt% of Na₂O and a w/s ratio of 0.4 were used. The initial water fraction upon mortar mixing is 0.286. (♦) NaOH, (×) Na₂CO₃, (⊕) Na₂Si₁.₇O₄.₄. The dotted lines are least-square linear regressions for the points above 5 MPa.</figcaption></figure>

### Hydration kinetics of the alkali-activated slag pastes

The hydration kinetics of the non-activated slag paste and the slag pastes activated with NaOH, Na₂CO₃, or Na₂Si₁.₇O₄.₄ were followed by isothermal micro-calorimetry for 13 days. Samples were made with a water/slag weight ratio of 0.4 and with 4 wt% of Na₂O per mass of slag. The results presented in Figure 6 show very different behaviors depending on the sample. First, almost no heat was released when the slag paste was not activated (hydrated in pure water). Again, this was expected since slag requires alkaline activators to hydrate in a reasonable period.
<figure><img src="image_8.png" /><figcaption>**Figure 6:** Isothermal micro-calorimetry curves of the heat flow (a and c) and the total heat (b and d) over 309 hours (a and b) or 24 hours (c and d) of a hydrating slag paste. Activations at 20 °C in pure water (solid blue line), with NaOH (purple dashed lines), with Na₂CO₃ (green dashed-dotted lines) or with Na₂Si₁.₇O₄.₄ (orange dotted lines). Samples were prepared with a w/s ratio of 0.4 and 4 wt% Na₂O. Arrows in (a) point to the beginning of each hydration plateau as determined in Figure 4.</figcaption></figure>

The Na₂Si₁.₇O₄.₄-activated slag paste reacted the fastest with a first heat peak with a flow maximum at only 2 h. This was followed by a period of low thermal activity for around 20 h before a second heat peak appeared around 50 h. When the slag was activated with Na₂CO₃, the activation reaction also followed a two-step pathway with a first peak at 22 h and a second peak at 210 h (8.75 days). On the contrary, NaOH-activation led to only one relatively large heat release peak with a maximum at 11 h. The width and asymmetry of the peak suggested the overlay of two thermal events, which could correspond to the formation of two different reaction products or to different classes of slag particle sizes with differing reactivity. Moreover, even though the sample activated with Na₂Si₁.₇O₄.₄ reacted first, it was quickly
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caught up by the NaOH-activated sample, which did not present an observable thermally dormant period (Figure 6b). Finally, hydration with Na₂CO₃ was the slowest and the total heat released after 13 days was only about half that released for the two other samples.

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Comparing these measurements to bound water and strength shows how chemistry and physics connected here: the exothermic events described above fit nicely with the time events reported in Figure 4. More specifically, the two-step process of water binding obtained with sodium carbonate fits with the long dormant period of heat flow, with a very good respect of chronology (the second increase of water binding at around 200 h corresponds to the second exothermal peak in isothermal calorimetry). The cases of sodium silicate and hydroxide are also consistent, as their main exothermal events match their respective inflection points on the water-binding curves. This is highlighted by the arrows in Figure 6a that mark the beginning of the hydration plateau measured by TGA in Figure 4. There is a clear agreement between the decrease of the calorimetric activity and the reaching of a hydration plateau.

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## Effect of Na₂O quantities and of the SiO₂/Na₂O ratio in the activation solutions on the hydration kinetics

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It is well known from the literature that the acceleration depends on the Na₂O amount [26,44,45]. However, in the case of sodium silicates, the respective roles of the alkali and of the silicate in activation are not clear. Consequently, as a side study, the influence of the activator quantity and of the SiO₂/Na₂O molar ratio of the silicate solution was investigated. The results regarding the Na₂O dosage for the hydroxide, carbonate and silicate activators are gathered in Figure 7, and the ones regarding the SiO₂/Na₂O ratio of the silicate activator in Figure 8. The samples were mixed with a w/s ratio of 0.4. The Na₂O dosage was varied by changing the total amount of activating solution added to the samples. The SiO₂/Na₂O ratio of the silicate solution was reduced by adding NaOH to the activating solutions. Hence, for a constant Na₂O dosage, a higher SiO₂/Na₂O ratio corresponds to a higher silicate dosage relative to the quantity of slag.

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<figure><img src="image_9.png" /><figcaption>(a)</figcaption></figure>

Evolution of heat flow of Na₂CO₃-activated slag over 250h

<figure><img src="image_10.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(b)</figcaption></figure>

Evolution of total heat of Na₂CO₃-activated slag over 250h

<figure><img src="image_12.png" /><figcaption>(d)</figcaption></figure>

**Figure 7:** Isothermal micro-calorimetry curves of the evolution of the heat flow (a and c) and the total heat (b and d) of a slag activated with NaOH (a and b), or Na₂CO₃ (c and d), at 20 °C. Samples were prepared with a w/s ratio of 0.4 and varying wt% of Na₂O.

Whichever activator used, increasing the quantity of Na₂O led to acceleration and intensification of hydration, as previously observed in the literature[26,44,45]. More specifically, in the case of NaOH activation (Figure 7a and b), two hydration peaks appeared at 8 and 10 hours for activation with 2 wt% Na₂O, while a single broad peak was visible with a maximum at 6 h for slag pastes activated with 4 or 6 wt% of Na₂O. Therefore, increasing the amount of Na₂O would lead to an acceleration of the two stages that were hypothesized above (see section *Hydration kinetics of the alkali-activated slag pastes*), which would then fully overlap. The total heat measured increased with the concentration of activator, which is the sign of a higher degree of hydration. Furthermore, these observations are in line with those made in
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the Vicat prisometry (Figure S1 in Supplementary Materials), where an acceleration of the onset
of setting was observed with increasing activator quantity.

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With Na₂CO₃ (Figure 7c and d), an increase in intensity and an acceleration of hydration
is also visible at the second peak. However, it seems that the first peak was slightly delayed by
the increased amount of activator used. Overall, the induction period between the two hydration
peaks shortened as the activator quantity increased.

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When SiO₂.Na₂O (Figure 8) was used as activator, similar observations could be made
again. The position of the first hydration peak did not vary but its intensity increased when more
activator was added. The appearance of the second peak was accelerated when the amount of
Na₂O was increased. Furthermore, concerning the modulus of the activating solution, the only
significant difference was the position of the second peak, in pastes made with 4 wt% Na₂O.
Therefore, it seems that the variation of the amount of sodium plays a preponderant role
compared to the amount of silicon in solution. These results are in agreement with the literature,
in which they have been interpreted in terms of the relative availability of silicate, aluminate
and calcium ions in solution[26,44]. This will be commented hereafter in the discussion section
after identification of the phases formed at each reaction steps.

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<figure><img src="image_13.png" /><figcaption>Evolution of heat flow of Na₂Si₁.₅O₄-activated slag over 190h</figcaption></figure>

(a)

<figure><img src="image_14.png" /><figcaption>Evolution of total heat of Na₂Si₁.₅O₄-activated slag over 190h</figcaption></figure>

(b)

<figure><img src="image_15.png" /><figcaption>Evolution of heat flow of Na₂Si₁.₇O₄.₄-activated slag over 190h</figcaption></figure>

(c)

<figure><img src="image_16.png" /><figcaption>Evolution of total heat of Na₂Si₁.₇O₄.₄-activated slag over 190h</figcaption></figure>

(d)

**Figure 8:** Isothermal micro-calorimetry curves of the evolution of the heat flow (a and c) and the total heat (b and d) of a slag paste activated Na₂Si₁.₅O₄ (a and b), or Na₂Si₁.₇O₄.₄ (c and d), at 20 °C. Samples were prepared with a *w/s* ratio of 0.4 and varying wt% of Na₂O. The Na₂Si₁.₅O₄ solution was obtained from the Na₂Si₁.₇O₄.₄ one by adding NaOH. At a constant wt% of Na₂O, the silicate dosage is thus higher in the Na₂Si₁.₇O₄.₄ solution than in the Na₂Si₁.₅O₄ one.

## Hydration stoppage

To further study the reaction paths followed by the different samples, their hydration was stopped at specific times corresponding to the same total heat released; and the reaction products were characterized by NMR and XRD. The samples were made with a *w/s* ratio of 0.4 and 4.0 wt% Na₂O. The working hypothesis behind this choice of stoppage at given heat released rather than at fixed times is that it would allow comparing samples at similar
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advancement of reaction regardless of kinetics (the activators have differing activating efficiency). The validity of this hypothesis will be examined in the discussion section.

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Consequently, the Na₂CO₃-activated sample was stopped at 12, 133 and 309 h, which correspond to total heats of 0.5, 23 and 108 J/g of slag, respectively. These same total heats were reached at 1, 16.75 and 86.4 h for the Na₂Si₁.₇O₄.₄-activated paste, and at 2, 9.75 and 38.5 h in the case of NaOH-activation. Furthermore, an additional stoppage was performed at 309 h for the pastes activated with Na₂Si₁.₇O₄.₄ and NaOH to study the three pastes at the longest possible hydration age. These times encompassed each hydration peak as summarized in Figure 9.

<figure><img src="image_17.png" /></figure>

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**Figure 9:** Isothermal micro-calorimetry curves of the total heat of a slag paste activated with NaOH (purple solid lines), Na₂CO₃ (green dashed-dotted lines) or Na₂Si₁.₇O₄.₄ (orange dotted lines), at 20 °C. The diamond markers show the ages at which the hydration was stopped to study the hydrates by NMR and XRD (note the markers at the end of the measuring time at 309 h for each sample). The samples were prepared at a w/s ratio of 0.4 and with 4.0 wt% of Na₂O.
Characterization of the hydrates by $^{29}$Si MAS NMR

After hydration stoppage, the samples were first characterized by $^{29}$Si MAS NMR to study the C-(N)-A-S-H gels that formed as primary hydration products. The $^{29}$Si MAS NMR spectra at different reaction time are presented in Figure 10 (NaOH activation), Figure 11 (Na$_{2}$Si$_{1.7}$O$_{4.4}$ activation), and Figure 12 (Na$_{2}$CO$_{3}$ activation). The spectra were normalized with respect to integrated intensities between -50 and -120 ppm. For the decomposition of the spectra measured for the hydrated pastes, the two following hypotheses were used: (i) the dissolution of the slag was considered congruent, meaning that the same line shape was used to model the anhydrous slag contribution for all the spectral decomposition; (ii) a pure Lorentzian line shape was used for all the hydrates contributions except for the $Q_3$ environment in the Na$_{2}$Si$_{1.7}$O$_{4.4}$-activated pastes where a pure Gaussian line shape was used [14,46]. Moreover, the samples with the longest reaction time were fitted first because they were more resolved. The obtained contributions were then used to fit the spectra of the less hydrated samples. However, it must be recognized that the lower resolution of these spectra increases the uncertainty of the decompositions compared to the most hydrated samples and that they are not univocal. All the parameters used to fit the spectra of the hydrated slag samples are reported in Table SI of the Supplementary Materials. A summary of the chemical shifts observed in the $^{29}$Si MAS NMR spectra is reported in Table II.

<table><caption>**Table II:** Attribution of the $^{29}$Si MAS NMR approximate isotropic chemical shifts ($\delta$ ± 0.5 ppm) observed in the one-pulse and $^{1}$H-$^{29}$Si spectra. Exact values for each spectrum are given in Table SI&SII of the Supplementary Materials.</caption><thead><tr><th>Phase</th><th>Connectivity</th><th>Pulse sequence</th><th><sup>29</sup>Si δ<sub>iso</sub> (ppm)</th></tr></thead><tbody><tr><td>Slag</td><td>Q<sub>0</sub> &amp; Q<sub>1</sub> &amp; Q<sub>2</sub></td><td>One pulse</td><td>-75.5</td></tr><tr><td rowspan="7">C-A-S-H</td><td>Q<sub>1</sub>(I)</td><td>Cross polarization</td><td>-76</td></tr><tr><td>Q<sub>1</sub>(II)</td><td>One pulse and cross polarization</td><td>-79</td></tr><tr><td>Q<sub>2</sub>(1Al)</td><td>One pulse and cross polarization</td><td>-81.5</td></tr><tr><td>Q<sub>2</sub></td><td>One pulse</td><td>-84</td></tr><tr><td>Q<sub>2</sub><sup>b</sup></td><td>Cross polarization</td><td>-84</td></tr><tr><td>Q<sub>2</sub><sup>p</sup></td><td>Cross polarization</td><td>-86</td></tr><tr><td rowspan="2">N-A-S-H</td><td>Q<sub>3</sub>(nAl)</td><td>One pulse</td><td>-87.5</td></tr><tr><td>Q<sub>3</sub> &amp; Q<sub>4</sub></td><td>Cross polarization</td><td>-89 to -93</td></tr></tbody></table>
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Two parameters are often used to describe C-A-S-H gels: the mean chain length, *MCL*, and the Al to Si molar ratio. They are defined using Equations (2) and (3) below as in [47]. The values of *MCL* and *Al/Si* ratios are reported in Table SI of the Supplementary Materials.

$$MCL = \frac{2}{\left( \frac{Q_1}{Q_1 + Q_2(0Al) + \frac{3}{2}Q_2(1Al)} \right)} \quad (2)$$

$$\frac{Al}{Si} = \frac{\frac{1}{2}Q_2(1Al)}{Q_1 + Q_2(0Al) + Q_2(1Al)} \quad (3)$$

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The ²⁹Si NMR spectra of the NaOH-activated samples (Figure 10) presented three resonances in addition to the one for the anhydrous slag: a $Q_1$ contribution centered at -78.6 ppm, a $Q_2$ contribution at -84.1 ppm, and a contribution at -81.5 ppm which can be assigned to a $Q_2(1Al)$ environment. These resonances were typical of C-A-S-H gels[12–14,40,48–51] which was thus the only Si bearing species detected here by ²⁹Si NMR. The resonance attributed to $Q_2(1Al)$ could also contain some signals from Si in $Q_2^B$ (bridging) environments, as they usually give a chemical shift in that range[14,52]. The small widths used for these three contributions show that the C-A-S-H formed was quite ordered. Moreover, this gel seems to be constituted of small silicate chains since the *MCL* calculated is of around 4.

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The same three contributions ($Q_1$, $Q_2(1Al)$ and $Q_2$) could be found in the NMR spectra of the $Na_2Si_{1.7}O_{4.4}$ (Figure 11) and $Na_2CO_3$ slag pastes activated with the longest reaction time (Figure 12). However, the resonances were less resolved, and their widths were slightly higher, showing a lower ordering for the C-A-S-H gel than with NaOH activation. Moreover, these resonances were only visible in the most reacted samples (for total heats above 108 J/g). In both $Na_2Si_{1.7}O_{4.4}$ and $Na_2CO_3$ activated samples, a large band centered at -87.5 ppm appeared first. This contribution, which was particularly intense for the least hydrated samples, can be assigned to a sum of $Q_3$ environments with some Al incorporation[48,53,54]. In the absence of clear $Q_1$ or $Q_2$ resonances, observation of $Q_3$ environments cannot correspond to the well-establish tobermoritic structure of C-A-S-H. From these results, it seems that a very amorphous N-A-S-H gel (possibly including some Ca) was formed first, giving rise to the resonance at -87.5 ppm. As the intensity of the $Q_3$ contribution progressively decreased relatively to new ones attributed to $Q_1$ or $Q_2$, one could conclude that C-A-S-H gel then formed rather than N-A-S-H gel as time
460 proceeded. The *MCL* of the C-A-S-H gels was also estimated despite the low resolution of the
461 spectra, not considering the *Q*₃ contribution. The values obtained were slightly higher than in
462 the NaOH-activated pastes. On the other hand, the estimated Al/Si ratios were quite similar for
463 the three different activations.

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<figure><img src="image_18.png" /></figure>

Figure 10: <sup>29</sup>Si MAS NMR spectra of NaOH-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a *w/s* ratio of 0.4 and with 4 wt% of Na<sub>2</sub>O, and hydrated at 20 °C.
<figure><img src="image_19.png" /><figcaption>**Figure 11:** $^{29}$Si MAS NMR spectra of Na$_{2}$Si$_{1.7}$O$_{4.4}$-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na$_{2}$O, and hydrated at 20 °C.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 12:** $^{29}$Si MAS NMR spectra of Na$_{2}$CO$_{3}$-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared with a w/s ratio of 0.4 and a 4 wt% of Na$_{2}$O, and hydrated at 20 °C.</figcaption></figure>
475 Characterization of the hydrates by {¹H}²⁹Si CPMAS NMR

476 To better study the C-A-S-H gels formed in the hydrated samples, cross polarized
477 {¹H}²⁹Si CPMAS NMR measurements were conducted. This technique allows to edit the
478 spectra by only showing the resonances originating from the hydrates and removing the large
479 contribution of the unreacted slag which dominates the one-pulse spectra. The cross polarized
480 spectra for the three types of activation are presented in Figures 13, 14 and 15. The parameters
481 used for the fit are summarized in Table SII in the Supplementary Materials. As for the previous
482 one-pulse ²⁹Si NMR spectra, the most reacted samples were studied first; and their fits were
483 used to start the modelling of the less hydrated pastes. The spectra were normalized according
484 to the quantity of reacted slag measured in the one-pulse ²⁹Si NMR spectra. This means that
485 from the one-pulse ²⁹Si NMR spectra which provides a quantitative response of all silicon
486 environments, the relative amount of reacted slag was estimated from the decrease of the
487 relative intensity of the unreacted slag resonance. This estimation was used to weigh the
488 integrated intensities of the cross-polarized spectra in Figures 13, 14 and 15, to give an
489 estimation of the growth of each hydrate's resonance with time.

490 Although the resonances were generally very broad and ill-resolved, the following
491 analysis and decompositions can be proposed based on the previous one-pulse ²⁹Si NMR
492 observations and literature data.

493 For NaOH-activated slag pastes (Figure 13) at total heats above 23 J/g, five resonances
494 typical of C-A-S-H in cement pastes were observed: two for Q₁ environments and three for
495 Q₂'s. The resonance at -79 ppm is attributed to a Q₁ environment, and the one around -81 to -
496 82 ppm to Q₂(1Al). The Q₂ resonance, which appeared at around -84 ppm in the one-pulse ²⁹Si
497 NMR spectra, was separated into two different contributions at -83.9 ppm and -85.8 ppm. They
498 were attributed to bridging Q₂ᵇ and pairing Q₂ᶜ tetrahedra, respectively [51,52,55]. Finally,
499 another resonance had to be considered at -76.1 ppm to fit the spectra. This contribution could
500 be assigned to Si in Q₁ environments (Q₁(I)) that were charge-balanced by different ions from
501 the Si corresponding to the resonance at -79.0 ppm (noted Q₁(II)) [56]. Some authors have
502 suggested that Q₁(I) corresponds to the tetrahedra charge-balanced by Ca²⁺, while those that
503 give the resonance Q₁(II) are charge-balanced by Na⁺ or H⁺, leading to more shielding due to
504 their electronegativity [40,53,57]. In conclusion for NaOH-activated pastes, like the one-pulse
505 ²⁹Si MAS NMR ones, the {¹H}²⁹Si CPMAS NMR spectra, although better defined, showed
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only the occurrence of a C-A-S-H. In addition, no hydration products could be detected at the first stoppage (0.5 J/g).

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The {¹H}²⁹Si CPMAS NMR spectra obtained for the Na₂Si₁.₇O₄.₄ and Na₂CO₃ activated samples were more complex. The same resonances (*Q*₁(*I*), *Q*₁(*II*), *Q*₂(*IAl*), *Q*₂ᵇ and *Q*₂ᶜ) were apparent in the spectra, but additional contributions corresponding to the *Q*₃ and *Q*₄ environments were also present.

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For the first stoppage (corresponding to an integrated heat of 0.5 J/g) of the Na₂Si₁.₇O₄.₄-activated slag paste (Figure 14) and the first two stoppages (0.5 J/g and 23 J/g) of the Na₂CO₃-activated one (Figure 15), only a weak and broad resonance was visible with a maximum from approximately -90 ppm to -93 ppm. This contribution, partially hidden by the unreacted slag in the one-pulse ²⁹Si NMR spectra, corresponds to a diversity of *Q*₃ and *Q*₄ environments with some of the Si tetrahedra linked to Al[40,48,53].

<figure><img src="image_21.png" /></figure>

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**Figure 13:** {¹H}²⁹Si CPMAS NMR spectra of NaOH-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a *w/s* ratio of 0.4 and with 4 wt% of Na₂O, at 20 °C.
<figure><img src="image_22.png" /><figcaption>**Figure 14:** {¹H}²⁹Si CPMAS NMR spectra of Na₂Si₁.₇O₄.₄-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na₂O, at 20 °C.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 15:** {¹H}²⁹Si CPMAS NMR spectra of Na₂CO₃-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na₂O, at 20 °C.</figcaption></figure>

When the pastes were hydrated for longer times, this broad resonance, initially between -90 ppm to -93 ppm, shifted slightly towards -89 ppm, and other resonances started to appear. As for the previous one-pulse ²⁹Si NMR spectra, the two resonances at -79.0 ppm for *Q*₁, and -81.6 ppm for *Q*₂(1Al) environment were visible in the three other spectra of the paste activated longer with Na₂Si₁.₇O₄.₄ (stoppage at 23, 108 and 165 J/g) and in the last spectrum of the Na₂CO₃-activated paste (stoppage at 108 J/g). As for NaOH activation, the *Q*₂ resonance could
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be separated into the two different contributions of bridging $Q_2^b$ and pairing $Q_2^p$ tetrahedra at -83.9 ppm and -85.8 ppm, respectively.

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Considering that $Q_3$ and $Q_4$ environments correspond to a tridimensional silicate gel (most likely N-A-S-H), and $Q_1$ and $Q_2$ (with eventually minor occurrence of $Q_3$) environments to the tobermoritic structure of C-A-S-H, these results were in agreement with the previous analysis of the one-pulse $^{29}$Si NMR spectra. They also showed that an amorphous cross-linked N-A-S-H gel first formed in $Na_2Si_{1.7}O_{4.4}$- and $Na_2CO_3$-activated pastes. Moreover, the changes in intensity and position of the N-A-S-H resonance suggest again that this gel was progressively replaced by a more ordered C-A-S-H gel. However, the lower resolution of these spectra and the number of contributions used led to more uncertainty on the parameters used for the fits and, thus, on the *MCL* and *Al/Si* calculated (see Table SI&SII of the Supplementary Materials).

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## Characterization of the reaction products by $^{27}$Al NMR

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$^{27}$Al MAS NMR experiments were also conducted in order to study the aluminum incorporated in the C-A-S-H gels and other aluminates which could be formed during the hydration reaction (Figures 16, 17, and 18). The spectra were normalized with respect to the total surface area under the curve between 150 and -90 ppm. The spectra of the most hydrated samples were also fitted first as they are the most resolved. All the parameters used to fit the spectra of the hydrated slag samples are reported in Table SIII of the Supplementary Materials. A summary of the chemical shifts observed in the $^{27}$Al MAS NMR spectra and of their attributions is reported in Table III.
<table><caption>**Table III:** Attribution of the <sup>27</sup>Al approximate isotropic chemical shifts range observed in the MAS NMR spectra. Values of the Czjek parameters are reported when applicable. Exact values used to decompose each spectrum are given in Table SIII of the Supplementary Materials.</caption><thead><tr><th>Phase</th><th>Coordination</th><th><sup>27</sup>Al δ<sub>iso</sub> (ppm)</th><th>σ (MHz)</th></tr></thead><tbody><tr><td rowspan="2">Slag</td><td>IV</td><td>73.2</td><td>0.5</td></tr><tr><td>Not attributed</td><td>11.5</td><td>0.3</td></tr><tr><td>N-A-S-H</td><td>IV</td><td>62.2 to 64.2</td><td>0.2</td></tr><tr><td rowspan="2">C-A-S-H</td><td>IV</td><td>75.7</td><td>0.2</td></tr><tr><td>IV</td><td>67.7 to 68.3</td><td>0.2</td></tr><tr><td></td><td>V</td><td>32 to 36</td><td>0.4</td></tr><tr><td>Hydrated calcium<br>aluminate</td><td>VI</td><td>9.5 to 9.9</td><td></td></tr><tr><td>TAH</td><td>VI</td><td>5.1 to 5.6</td><td>0.2</td></tr></tbody></table>

When NaOH was the activator (Figure 16), the first resonance, not corresponding to the anhydrous slag, appeared at the stoppage time corresponding to a heat release of 0.5 J/g. Its chemical shift was 63.7 ppm, corresponding to Al in tetrahedral environments. It has previously been assigned to Al substituting Si in Q<sub>3</sub> environments in N-A-S-H gel[9,58]. This chemical shift is similar to the small resonance at 63.0 ppm, which was present in the anhydrous slag (Figure 2b). This result nicely complements the <sup>29</sup>Si and {<sup>1</sup>H}<sup>29</sup>Si MAS NMR spectra which could not detect N-A-S-H gel at stoppage corresponding to 0.5 J/g (Figure 10 and 13). This suggests that this structure was very amorphous and that it was only present in very small quantities. Therefore, it is difficult to know exactly how much was already present in the anhydrous slag and how much precipitated during the first two hours of hydration.

Four resonances were visible on the other spectra of the NaOH-activated pastes: two tetrahedral resonances at 75.7 ppm and 68.3 ppm, one broad pentahedral resonance at 33.8 ppm, and an octahedral resonance at 9.5 ppm. The first three contributions correspond to Al incorporated in the C-A-S-H gel. The pentahedral one is usually assigned to the replacement of Ca<sup>2+</sup> in the interlayer space by Al<sup>3+</sup> [40,49,59]. The first tetrahedral site at 68.3 ppm can be assigned to Al in Q<sub>2</sub><sup>b</sup> bridging sites charge-balanced by Ca<sup>2+</sup> in the interlayer [60–62].
<figure><img src="image_24.png" /><figcaption>**Figure 16:** $^{27}$Al MAS NMR spectra of NaOH-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na$_2$O, at 20 °C.</figcaption></figure>

The other tetrahedral resonance, at 75.7 ppm, has been assigned to different types of environments in the literature; including $Q_2^b$ bridging sites charge-balanced by Al in a pentahedral or octahedral environment[9]. However, since no correlation could often be found between the quantities of Al in this tetrahedral environment and the pentahedral and octahedral Al, other hypotheses have been explored. Even though the substitution of bridging site is supposed to be favored compared to the incorporation of Al in pairing sites, it has been suggested that this contribution would indeed correspond to Al in pairing sites, which is said to be more plausible in C-A-S-H with small silicate chains[52,58–61,63,64]. However, the relative intensities of these two contributions (see the Supporting Materials) suggest that there were more Al in the pairing than in the bridging position, which is contrary to the thermodynamic data[11,65–69]. Some authors[14,15,22] observed two C-A-S-H in SEM images of NaOH-activated slag and, thus, suggested that the two tetrahedral environments correspond to the substitution of Si bridging sites in each C-A-S-H. The peak at 75.7 ppm, with the smaller quadrupolar constants, corresponds to the more ordered phase.

The fourth resonance at 9.5 ppm was very symmetrical and was thus fitted with a Lorentzian line instead of Czjzek model. It is assigned to Al in a hydrated aluminate phase, such as hydrotalcite, AFm, CAH$_{10}$, C$_2$AH$_8$ or C$_4$AH$_{13-19}$[60,65].
In the last two spectra of the NaOH-activated paste, at 108 J/g and 184 J/g, a second octahedral resonance was visible at 5.4 ppm. Some authors consider that this Al is part of another amorphous aluminate phase, called the third aluminate hydrate (TAH)[49,67], made of Al(OH)₆³⁻ or OₓAl(OH)₆₋(³+х)⁻ units, and which precipitates either as a separate phase or on the surface of C-S-H. However, in some more recent work[70], using DNP-enhanced ²⁷Al CP MAS NMR and 2D ²⁷Al-²⁹Si DNP-enhanced correlation spectra, the authors assigned this resonance to [AlO₂(OH)₄]⁵⁻ units linked to silicates and, therefore, proposed that this peak corresponds to the octahedral Al substituting the bridging Si in C-A-S-H. This hypothesis might explain why the relative intensities of the tetrahedral pairing Al were higher than those of the tetrahedral bridging Al and could suggest that the assignment of the 75.7- and 68.3 ppm resonances to Al in pairing and bridging positions is not as far-fetched as initially assumed.

The spectra of the first two stoppage of the Na₂Si₁.₇O₄.₄-activated slag pastes (Figure 17), at 0.5 and 23 J/g, only showed one resonance on top of the ones of the anhydrous slag. This resonance had a chemical shift of 63.7 ppm, which is identical to the value obtained for the first spectrum of the NaOH-activated paste. As the ²⁹Si NMR results showed the presence of N-A-S-H at these times for the Na₂Si₁.₇O₄.₄-activated pastes, this resonance can indeed be assigned to tetrahedral Al incorporated in an N-A-S-H gel. Three other resonances, at 33.5, 9.8 and 5.4 ppm, were also present at longer hydration times and were assigned to the same sites as with the NaOH-activated pastes. The quantity of pentahedral aluminium was yet smaller than in NaOH-activated pastes, as has been observed before[40].

615
<figure><img src="image_25.png" /><figcaption>**Figure 17:** $^{27}$Al MAS NMR spectra of Na$_2$Si$_{1.7}$O$_{4.4}$-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na$_2$O, at 20 °C.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>**Figure 18:** $^{27}$Al MAS NMR spectra of Na$_2$CO$_3$-activated slag pastes at different hydration times. On the right, the curves used to fit the spectra are shown in gray. The samples were prepared at a w/s ratio of 0.4 and with 4 wt% of Na$_2$O, at 20 °C.</figcaption></figure>

Finally, the case of Na$_2$CO$_3$-activated slags (Figure 18) was similar to the one of the Na$_2$Si$_{1.7}$O$_{4.4}$-activated pastes. Indeed, the first two spectra, at 0.5 and 23 J/g, only showed the formation of a N-A-S-H gel with incorporated tetrahedral Al, giving rise to a resonance at
626 62.2 ppm. The spectrum of the most hydrated sample, at 108 J/g, also presented the same four
627 resonances as with Na₂Si₁.₇O₄.₄, namely a tetrahedral contribution at 67.7 ppm, a pentahedral
628 contribution at 36.2 ppm, and octahedral ones at 9.9 ppm and 5.3 ppm.

629

630 **Characterization of the reaction products by XRD**

631 Finally, XRD experiments (Figure 19) were carried out to better study the crystalline
632 phases formed during hydration. Very different products precipitated depending on the activator
633 used. Indeed, even though a broad diffuse peak around 30° attributed to amorphous C-A-S-H
634 appeared in the diffractograms of all the samples, hydrotalcite was only visible in NaOH-
635 activated samples, while calcite and then gaylussite formed in the Na₂CO₃-activated slag pastes.

636 These results allow us to assign the 9.8 ppm resonance of the <sup>27</sup>Al NMR spectra of the
637 NaOH-activated slag (Figure 16) to Al in hydrotalcite. This resonance was also present in the
638 <sup>27</sup>Al NMR spectra of the most hydrated samples activated with Na<sub>2</sub>CO<sub>3</sub> (Figure 18) and
639 Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub> (Figure 17), but none of the peaks of the diffractograms could be attributed to an
640 aluminate phase. Nonetheless, hydrotalcite has previously been found in cements activated with
641 sodium silicate without being noticeable on diffractograms; so it is most probably this phase
642 that precipitated[15,17].
<figure><img src="image_27.png" /></figure>

(a)

<figure><img src="image_28.png" /></figure>

(b)

<figure><img src="image_29.png" /></figure>

(c)

<caption>**Figure 19:** X-ray diffractograms of slag pastes activated with NaOH (a), Na₂Si₁.₇O₄.₄ (b), or Na₂CO₃ (c). The peaks are referenced with the corresponding Miller indexes between brackets according to the indicated PDF files. The different visible phases are CaCO₃ crystallized as calcite (noted C, file 00-005-0586) and as vaterite (noted V, file 01-072-0506), gaylussite Na₂Ca(CO₃)₂.5H₂O (noted G, file 00-021-0343), hydrotalcite Mg₂Al(OH)₄(CO₃)0.5H₂O (noted H, file 01-089-0460) and a C-A-S-H taken accordingly to the tobermorite-11Å structure (file 00-045-1480). The samples were made with 4.0 wt% Na₂O and a w/s ratio of 0.4, at 20 °C.</caption>

643
644
645
646
647
648
649
650
651
Discussion

Now that the results of mechanical strength, TGA, isothermal calorimetry, NMR and XRD have been presented, we will discuss how their combination can bring to light the effect of the activator type on the reaction path of the hydration of alkali-activated slag.

Estimation of the degree of reaction

The first question was how to find a way of estimating the advancement of reaction in the different alkali-activated pastes to see if the phenomena happening at a certain degree of reaction were independent from the activator used. The calorimetric degree of hydration for a given activator (NaOH, Na₂Si₁.₇O₄.₄ or Na₂CO₃) under the present experimental conditions is

$$r(Q(t))^{activator} = \frac{Q(t)^{activator}}{Q_{tot}^{activator}} \qquad (4)$$

where $Q(t)^{activator}$ is the heat released at time t, and $Q_{tot}^{activator}$ the total heat obtained at full completion of the hydration. It is usually calculated from isothermal calorimetry results[71–73].

The total heat can be correctly estimated for OPC using the enthalpies of reactions and the anhydrous phase composition from Bogue's formulas. However, this approach does not apply to alkali-activated slag pastes because the slag is not at thermal equilibrium and the formed hydrates remain difficult to identify quantitatively. Some authors[74] proposed to replace $Q_{tot}$ by $Q_{max}$ which is the experimental maximum heat released at the end of the reaction:

$$r(Q(t))^{activator} = \frac{Q(t)^{activator}}{Q_{max}^{activator}} \qquad (5)$$

Nonetheless, the question still remains on how to measure this maximum heat $Q_{max}^{activator}$. It is usually taken as the total heat measured at the end of the calorimetry measurement, considering that the end of the reaction is near completion and that the total heat will not increase significantly afterwards. However, it should be noted that most researchers do not take into account the first thirty minutes of heat flow to calculate the total or maximum heat because it is prone to experimental artifacts. In fact, the initial heat comes not only from the hydration reaction but also from the temperature differences between the laboratory and the calorimeter enclosure[75].
Here, within this empirical approach, two maximum heats could be used to calculate this degree of reaction (Table IV): (i) The total heat released at 309 h (the maximum length of our calorimetry measurements), i.e. $Q_{max}^{activator} = {}^{1}Q_{max}^{activator} = Q(t = 309 h)^{activator}$, and which is different for each activator; (ii) $Q_{max}^{activator} = {}^{2}Q_{max}^{activator} = Q(t = 309 h)^{NaOH}$ which was the maximum total heat that could be measured amongst all samples, presently the one measured for the NaOH-activated slag paste at 309 h.

For case (ii), by construction, regardless of the activation methods,

$$ {}^{2}Q_{max}^{NaOH} = {}^{2}Q_{max}^{Na_{2}Si_{1.7}O_{4.4}} = {}^{2}Q_{max}^{Na_{2}CO_{3}} = Q(t = 309 h)^{NaOH} = 186 \text{ J/g} \quad (6) $$

Since the stoppage were performed at times corresponding to identical heat released (but different times) for each activation (see Fig. 9), the calorimetric degree of reaction at each stoppage will be equal.
**Table IV:** Calorimetric degrees of reaction $r(Q(t))^{activator}$ calculated from the maximum heat release $Q_{max}^{activator}$ in isothermal calorimetry measurements. The $Q_{max}^{activator}$ values are given with an error of ± 1 J/g and the resulting numerical error on $r(Q(t))^{activator}$ is ± 0.01. Calculated $r(Q(t))^{activator}$ values are reported for the two methods discussed in the text, i.e. taking $Q_{max}^{activator} = 1$ and $Q_{max}^{activator} = Q(t = 309 \text{ h})^{activator}$ (total heat released at 309 h for each activator, thus differing for each activator) or $Q_{max}^{activator} = 2$ and $Q_{max}^{activator} = Q(t = 309 \text{ h})^{NaOH}$ (total heat released at 309 h with NaOH activation, regardless of activator used). The stoppage times are those of Figure 9.

<table><thead><tr><th></th><th>NaOH</th><th colspan="2">Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub></th><th colspan="2">Na<sub>2</sub>CO<sub>3</sub></th></tr></thead><tbody><tr><td></td><td><sup>1</sup>Q<sub>max</sub><sup>NaOH</sup><br>= <sup>2</sup>Q<sub>max</sub><sup>NaOH</sup><br>= Q(309 h)<sup>NaOH</sup><br>= 186 J/g</td><td><sup>1</sup>Q<sub>max</sub><sup>Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub></sup><br>= Q(309 h)<sup>Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub></sup><br>= 166 J/g</td><td><sup>2</sup>Q<sub>max</sub><sup>Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub></sup><br>= Q(309 h)<sup>NaOH</sup><br>= 186 J/g</td><td><sup>1</sup>Q<sub>max</sub><sup>Na<sub>2</sub>CO<sub>3</sub></sup><br>= Q(309 h)<sup>Na<sub>2</sub>CO<sub>3</sub></sup><br>= 108 J/g</td><td><sup>2</sup>Q<sub>max</sub><sup>Na<sub>2</sub>CO<sub>3</sub></sup><br>= Q(309 h)<sup>NaOH</sup><br>= 186 J/g</td></tr><tr><td><strong>1<sup>st</sup> stoppage</strong><br>Q(t)<sup>activator</sup><br>= 0.5 J/g</td><td>0.003</td><td>0.003</td><td>0.003</td><td>0.005</td><td>0.003</td></tr><tr><td><strong>2<sup>nd</sup> stoppage</strong><br>Q(t)<sup>activator</sup><br>= 23 J/g</td><td>0.12</td><td>0.14</td><td>0.12</td><td>0.21</td><td>0.12</td></tr><tr><td><strong>3<sup>rd</sup> stoppage</strong><br>Q(t)<sup>activator</sup><br>= 108 J/g</td><td>0.58</td><td>0.65</td><td>0.58</td><td>1.00</td><td>0.58</td></tr><tr><td><strong>4<sup>th</sup> stoppage</strong><br>Q(309 h)<sup>NaOH</sup><br>= 186 J/g</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Q(309 h)<sup>Na<sub>2</sub>Si<sub>1.7</sub>O<sub>4.4</sub></sup><br>= 166 J/g</td><td>1.00</td><td>1.00</td><td>0.89</td><td></td><td></td></tr><tr><td>Q(309 h)<sup>Na<sub>2</sub>CO<sub>3</sub></sup><br>= 108 J/g</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

An NMR degree of reaction (DOR) was also defined using the results from the <sup>29</sup>Si and <sup>27</sup>Al NMR experiments (Table V). Equations (7) and (8), as proposed by Le Saoût et al.[14], were used to this end. Equation (7) can be used for all the <sup>27</sup>Al NMR spectra. It can also be used for the <sup>29</sup>Si NMR spectra when the activators does not include a source of Si (case of NaOH and Na<sub>2</sub>CO<sub>3</sub>). It is based on the measurement of the contribution of non-reacted slag, $I_{slag}^{rel}$, relative to the total spectra normalized to 1, according to:
$$DOR = 1 - \frac{I_{\text{slag}}^{\text{rel}}}{1} \qquad (7)$$

In the case of the Na₂Si₁.₇O₄.₄-activation, the silicon brought by the activator must be taken into account, using Equation (8).

$$DOR'_{Si} = 1 - \frac{I_{\text{slag}}^{\text{rel}}}{1} * \left( 1 + \frac{y}{x} * \frac{WG_{\text{SiO}_2}}{\text{Slag}_{\text{SiO}_2}} \right) \qquad (8)$$

The parameters used in these two equations are the relative intensity of the unreacted slag $I_{\text{slag}}^{\text{rel}}$ obtained from the NMR spectral fits (Tables SI and SII of the Supplementary Materials), the formulation mass ratio between the activator and the slag $\frac{y}{x}$ (0.23 wt%), the percentage of SiO₂ in the sodium silicate activator solution WG<sub>SiO₂</sub> (28.8 wt%), and the percentage of SiO₂ in slag Slag<sub>SiO₂</sub> (37.3 wt%).

Firstly, the calorimetric degrees of reaction (Table IV:) are noticeably different from the ones obtained from the NMR results (Table V). To estimate $r(Q(t))^{\text{activator}}$, it was assumed that the hydration was completed at 309 h for all the pastes when using $^{1}Q_{max}$, and only for the NaOH-activated pastes when using $^{2}Q_{max}$. The NMR spectra actually show that there was still a significant amount (more than 60%) of unreacted slag in the pastes at that time. This is coherent with what has been observed by other researchers with 50 % of reacted slag at 180 days[15].

<table><caption>**Table V:** NMR degree of reaction calculated from the ²⁹Si and ²⁷Al NMR integrated intensities. In the case of the Na₂Si₁.₇O₄.₄-activated samples, DOR<sub>Si</sub> and DOR<sub>Al</sub> were calculated using Equation (7) from the one-pulse ²⁹Si MAS NMR spectra and ²⁹Si MAS NMR spectra, respectively. DOH<sub>Si</sub>' was calculated using Equation (8). The DORs are given with an error of ± 0.05 due to the uncertainty in the NMR decomposition of ill-defined peaks. The stoppages are those defined in Figure 9 (same as in Table IV).</caption><thead><tr><th rowspan="2"></th><th colspan="2">NaOH</th><th colspan="3">Na₂Si₁.₇O₄.₄</th><th colspan="2">Na₂CO₃</th></tr><tr><th>DOR<sub>Si</sub></th><th>DOR<sub>Al</sub></th><th>DOR<sub>Si</sub></th><th>DOR<sub>Si</sub>'</th><th>DOR<sub>Al</sub></th><th>DOR<sub>Si</sub></th><th>DOR<sub>Al</sub></th></tr></thead><tbody><tr><td>1<sup>st</sup> stoppage</td><td>0</td><td>0.03</td><td>0</td><td>0</td><td>0.06</td><td>0</td><td>0.02</td></tr><tr><td>2<sup>nd</sup> stoppage</td><td>0.13</td><td>0.17</td><td>0.21</td><td>0.08</td><td>0.12</td><td>0.11</td><td>0.09</td></tr><tr><td>3<sup>rd</sup> stoppage</td><td>0.22</td><td>0.31</td><td>0.33</td><td>0.21</td><td>0.26</td><td>0.19</td><td>0.26</td></tr><tr><td>4<sup>th</sup> stoppage</td><td>0.34</td><td>0.40</td><td>0.36</td><td>0.25</td><td>0.35</td><td></td><td></td></tr></tbody></table>
723

If we now consider that the heat measured at the third stoppage (108 J/g) corresponds
to only about 25 % of reacted slag, as suggested by the NMR results for all activators
investigated, we get a maximum heat of hydration $ ^{3}Q_{max} $ of approximatively 432 J/g on average.
This is much more than the heat released even for NaOH at the end of the calorimetric
measurements (186 J/g), confirming that the reaction was still very far from completion in all
cases. New calorimetric degrees of reaction $r(t)$ can be calculated with this value: 0.001 at the
first stoppage, 0.05 at the second stoppage, 0.25 at the third stoppage, and 0.43 at the fourth
stoppage for the NaOH-activated paste and 0.38 at the fourth stoppage for the Na$_{2}$Si$_{1.7}$O$_{4.4}$-
activated paste. These values are quite similar to the NMR degrees of reaction *DOHs* reported
in Table V, within experimental errors. The calorimetric and NMR analysis were thus
consistent. The only issue being the determination of the maximum heat of hydration.

724

725

726

727

728

729

730

731

732

733

734

When comparing the different degrees of reaction calculated from the NMR
decompositions for the NaOH- and Na₂CO₃- activated samples (DOR<sub>Si</sub> and DOR<sub>Al</sub> in Table V),
it appears that they are similar within experimental errors (± 0.05), whether they were calculated
using the <sup>29</sup>Si or <sup>27</sup>Al NMR spectra. Thus, it can be assumed that the dissolution of Al and Si
followed the same kinetics. If this hypothesis is applied to Na₂Si₁.₇O₄.₄-activated slags, it seems
that DOR<sub>Si</sub>' gives better values for the first two stops but not for the last one, where DOR<sub>Si</sub> is
closer to DOR<sub>Al</sub>. This result can seem surprising. However, it should be reminded that the
hydration of the samples was stopped by first washing them twice with pure water. This led to
a loss of some of the silicates coming from the activator that had not yet reacted, leading to a
bias in the integration of the <sup>29</sup>Si NMR spectra.

735

736

737

738

739

740

741

742

743

744

Finally, the degrees of reaction for each stoppage can be compared, depending on the
activators used. As explained before, the ages at which the hydration stoppages were done were
chosen so that the degrees of hydration $r(Q(t))$ calculated with $ ^{2}Q_{max} $ would be identical for the
three types of activation. In the end, this hypothesis seems reasonable since the NMR results,
that is the *DORs* calculated at each stoppage, were indeed similar for the three activators (about
0.01 for the first stoppage, about 0.1 for the second one and about 0.25 for the third one).

745

746

747

748

749

750
751 Summary of the effect of the activation chemistry on the hydration reaction
752 path

753 Summaries of the hydrates identified over time for the NaOH-, Na₂CO₃- and
754 Na₂Si₁.₇O₄.₄-activated slag pastes, with 4.0 wt% of Na₂O and a w/s ratio of 0.4, are presented
755 on Figure 20. If the three pastes did follow the same reaction paths, then, at a given advancement
756 of reaction, the same hydrates should be presented on Figure 20 for the three activators. It is,
757 however, not the case, meaning that the activation chemistry influenced the reaction path
758 followed by the reacting slag.

759 When slag pastes were activated with NaOH, very small quantities of N-A-S-H gel
760 initially formed, as evidenced by the ²⁷Al NMR spectra (Figure 16). Relatively quickly, after
761 only a few hours of hydration, the heat (Figure 20b) produced by the sample increased as an
762 ordered C-A-S-H gel and hydrotalcite precipitated. These events occurred simultaneously when
763 the activator dosage was above 4.0 wt% of Na₂O but, when the activator dosage was reduced
764 from 4.0 wt% of Na₂O to 2.0 wt%, a division of the heat flow peak was observed. This is
765 probably a sign of a separation of the formation of hydrotalcite and C-A-S-H into two steps.
766 Thus, it could be interesting to further study these samples with NMR and XRD to verify this
767 hypothesis. In any case, the formation of C-A-S-H was accompanied by a rapid increase of the
768 mechanical strength and of the quantity of bound water (Figure 4 and 5). These reactions also
769

slowed down rapidly, as shown by the low increase of the total heat, of the mechanical strength, and of the quantity of bound water after two days.

770

771

<figure><img src="image_30.png" /><figcaption>**Figure 20:** (a) Summary of the hydrates precipitated according to the advancement of reaction, taken as the total heat measured in isothermal calorimetry, for the three types of activation. (b) Assignment of the heat flow peaks to the hydrates precipitated in the slag pastes activated with NaOH, Na₂CO₃ and Na₂Si₁.₇O₄.₄, at 4 wt% of Na₂O and w/s=0.4.</figcaption></figure>

772

773

774

775

776

777

778

779

780

781

782

783

784

785

In the Na₂Si₁.₇O₄.₄-activated pastes, a N-A-S-H amorphous gel was also the first phase to precipitate, but in larger proportions than with NaOH, as evidenced by the NMR results (Figures 11, 14, and 17). This product usually forms in alkali-activated binders with low calcium contents, such as alkali-activated fly ash[6]. The precipitation of the N-A-S-H gel, instead of a C-A-S-H, can be explained here by the very high concentration of silicon and sodium brought about by the activating solution, compared to the calcium concentration, which only resulted from the slag dissolution. The formation of this first phase led to the first heat flow peak (Figure 20b) and a quick setting (see Figure S1 in the Supplementary Materials) but to only small values of mechanical strength and bound water (Figures 4 and 5).

786

787

788

789

Progressively, the formation of a N-A-S-H gel in the Na₂Si₁.₇O₄.₄-activated pastes led place to the formation of an amorphous tobermoritic C-A-S-H gel, as evidenced from the progressive decrease of the *Q*₃ resonances in the NMR spectra. As slag dissolved, more calcium was available, explaining this progressive shift from N-A-S-H to C-A-S-H gel. The
contributions in the NMR spectra (Figure 11, 14, and 17) and the diffractograms (Figure 19b) remained broad. Simultaneously, a hydrated aluminate phase precipitated, but its low crystallinity or quantity could not allow us to characterize it properly. The formation of these two phases, C-A-S-H and the hydrated aluminate, was accompanied by a second heat flow peak (Figure 20b) and a strong increase of the mechanical strength and of the quantity of bound water (Figure 4). Moreover, it appears that even if the same amount of bound water was measured in Na₂Si₁.₇O₄.₄⁻ and NaOH-activated pastes, probably mostly incorporated in the C-A-S-H gel, a higher mechanical strength was gained with Na₂Si₁.₇O₄.₄. This could be due to the more amorphous structure and cross-linking of the C-A-S-H formed in this system, or, more likely, to denser structures and different pore size distribution as proposed in the literature[12,15,76]. This statement is in agreement with the higher packing found in C-A-S-H in sodium silicate-alkali activated slag pastes found in [77].

This succession of events observed in sodium silicate activated slag pastes (initial N-A-S-H formation followed by a second thermal event due to aluminate and C-A-S-H precipitation) allows to rationalize the observations reported in the results section (*Effect of Na₂O and SiO₂/Na₂O ratio in the activation solutions quantities on the hydration kinetics*). As stated in [44], increased concentration of silicates accelerates the kinetics by driving the precipitation of silicate gels and thus the dissolution of the slag and Ca release. This is consistent with the observation reported here that, at constant Na₂O wt%, the second hydration peak (precipitation of C-A-S-H and of hydrated aluminates) appears earlier for higher silicate modulus (Figure 8). It must be understood nonetheless that, independently from the silicate modulus, this is the amount of hydroxides brought by the activator, and not the pH, that determines the completion of the dissolution of the slag and the total heat released as shown by Aupoil et al. [78] in geopolymers. This explains that regardless of the activator (NaOH, Na₂Si₁.₇O₄.₄ or Na₂CO₃), higher heat was released for higher Na₂O wt% (Figure 7 and 8).

Finally, Na₂CO₃-activation first led to the precipitation of a N-A-S-H gel with Si and Al in environments similar to those of Na₂Si₁.₇O₄.₄-activated pastes, as shown by NMR spectra (Figure 12, 15, and 18). Nonetheless, the second phase that precipitated was not a C-A-S-H gel, as with the silicate activation, but calcite, as shown by the XRD results (Figure 19c). In fact, the CO₃²⁻ anions brought about by the activator have a great affinity for Ca²⁺ ions, as suggested by the low solubility of CaCO₃ of only 14 mg/L. The calcium was thus consumed by the calcite phase and was at first not available to form a C-A-S-H gel. The formation of these two phases,
calcite and N-A-S-H, led to the first peak of heat flow, which appeared after a day of hydration (Figure 2b), and to low amounts of bound water (Figure 4). However, no mechanical strength developed at that time (Figure 3), which explains the variability and length of the measured setting times (see Figure S1 in the Supplementary Materials).

Later, the NMR spectra eventually showed the formation of C-A-S-H. This was accompanied by the precipitation of gaylussite, as evidenced by the XRD results, and of an aluminate phase similar to the one formed in Na₂Si₁.₇O₄.₄-activated pastes. Ultimately, these reactions, which happened after more than a week of hydration, were the ones that allowed the structuration of the paste and gave rise to some mechanical strength. However, this complex chemical path led to a slower hydration and gain in mechanical strength than in the hydroxide or silicate activated systems.

The mechanism presented here is in agreement with the literature results, even though the products formed are not always identical [23–25]. This is particularly true of the minor hydration products that depend strongly on the chemical composition of the slag. Indeed, Bernal et al. first saw the formation of different polymorphs of calcium carbonate, zeolite NaA Na₁₂Al₁₂Si₁₂O₄₈.18H₂O and gaylussite. However, the NMR and XRD results show a silicate phase too amorphous to be a zeolite, which we attributed to N-A-S-H. Moreover, the formation of gaylussite only happens after a few days in our samples. Also of interest is the fact that the formation of N-A-S-H prior to C-A-S-H was identified for all three activating systems. This had already been evidenced in slag pastes activated with sodium silicate[34] but, to the knowledge of the authors, this is the first time it is established when NaOH is used as activator.

843
844 Conclusions

845 From a methodological perspective, the combination of a stoppage method suitable for
846 alkali activated pastes and of characterization techniques such as NMR allowing to quantify
847 materials regardless of crystallinity has enabled to accurately determine the degree of reaction
848 of the slag, even in the case of alkali silicate activation. Other papers have already concluded
849 that calorimetry alone does not provide reliable DoR and that recourse to NMR is provides
850 better accuracy.[79]

851 The different characterizing methods allowed us to determine the reaction paths
852 followed by alkali-activated slag pastes depending on the activator used under consistent
853 reaction conditions of temperature, concentrations, mixing protocol and advancement of
854 hydration:

855 • NaOH-activation led to rapid dissolution of slag with the initial formation of very small
856 quantities of N-A-S-H gel followed by the concomitant precipitation of C-A-S-H and
857 hydrotalcite. Al was incorporated in the C-A-S-H structure in bridging and pairing
858 positions, as well as in the interlayer space. This gel was also particularly ordered, with
859 small silicate chains.

860 • The Na₂Si₁.₇O₄.₄-activation gave rise to a fast setting which was due to the formation of
861 N-A-S-H gel. Later on, a C-A-S-H gel precipitated with Al in the bridging positions and
862 in the interlayer space, bringing high mechanical strength. Another aluminate phase
863 formed at the same time, which could be a disordered hydrotalcite, an AFm phase, or
864 some other calcium aluminate.

865 • When Na₂CO₃ was used to activate the slag paste, it was first a N-A-S-H gel and calcite
866 that precipitated, without giving any real strength to the paste. A few days later, a C-A-
867 S-H gel was finally formed, as well as gaylussite and some other disordered aluminate.

868 This set of conclusions was reached with a high level of confidence because they resulted from
869 the analysis of the impact of various activators on a single slag and within a given set of paste
870 preparation conditions. This strength is also a limitation as one could question their particular
871 character with respect to preparation conditions such as water to solid ratio. In this respect, it
872 could be nicely completed by an analysis of porosity and a more precise determination of the
873 amorphous phases composition and density. Those measurements are however notoriously
874 difficult.
875 **Credit authorship contribution statement**

876 **Clara Paillard**: Conceptualization, Formal analysis, Investigation, Methodology,
877 Validation, Visualization, Writing – Original Draft; **Nicolas Sanson**: Conceptualization,
878 Methodology, Supervision, Writing – Review & Editing; **Jean-Baptiste d'Espinose de**
879 **Lacaillerie**: Conceptualization, Funding acquisition, Methodology, Project administration,
880 Supervision, Writing – Review & Editing; **Marta Palacios**: Methodology, Writing – Review
881 & Editing; **Pascal Boustingorry**: Conceptualisation, Methodology, Writing – Review &
882 Editing; **Marie Jachiet**: Conceptualization, Methodology, Writing – Review & Editing; **Claire**
883 **Giraudeau**: Conceptualisation, Methodology; **Vanessa Kocaba**: Conceptualization, Funding
884 acquisition, Methodology, Writing – Review & Editing.

885 **Declaration of competing interests**

886 The authors declare that there is no conflict of interest.

887 **ACKNOWLEDGEMENT**

888 This research was solely funded by CHRYSO Saint-Gobain Construction Chemicals,
889 and by the SIMM laboratory (CNRS, ESPCI Paris PSL and Sorbonne Université) and the IETcc
890 (CSIC) supervisory authorities. It did not receive any specific grant from funding agencies in
891 the public, commercial, or not-for-profit sectors. NMR equipment at ESPCI Paris PSL is funded
892 in part by the Paris Region.
893 **References**

894 [1] J. Lehne, F. Preston, Making Concrete Change Innovation in Low-carbon Cement and Concrete, Chatham House, London, 2018.

896 [2] J.L. Provis, Alkali-activated materials, Cement and Concrete Research 114 (2018) 40–48.
897 https://doi.org/10.1016/j.cemconres.2017.02.009.

898 [3] C. Shi, A.F. Jiménez, A. Palomo, New cements for the 21st century: The pursuit of an alternative to Portland cement, Cement and Concrete Research 41 (2011) 750–763.
900 https://doi.org/10.1016/j.cemconres.2011.03.016.

901 [4] A. Palomo, O. Maltseva, I. Garcia-Lodeiro, A. Fernández-Jiménez, Portland Versus Alkaline Cement: Continuity or Clean Break: “A Key Decision for Global Sustainability,” Front. Chem. 9 (2021) 705475. https://doi.org/10.3389/fchem.2021.705475.

904 [5] M. Moranville-Regourd, S. Kamali-Bernard, Cements Made From Blastfurnace Slag, in: Lea's Chemistry of Cement and Concrete, Elsevier, 2019: pp. 469–507.
905
906 https://doi.org/10.1016/B978-0-08-100773-0.00010-1.

907 [6] J.L. Provis, J.S.J. van Deventer, Geopolymers and Other Alkali-Activated Materials, in: Lea's Chemistry of Cement and Concrete, Elsevier, 2019: pp. 779–805.
908
909 https://doi.org/10.1016/B978-0-08-100773-0.00016-2.

910 [7] J.L. Provis, S.A. Bernal, Geopolymers and Related Alkali-Activated Materials, Annual Review of Materials Research 44 (2014) 299–327. https://doi.org/10.1146/annurev-matsci-070813-113515.

913 [8] L. Li, J. Xie, B. Zhang, Y. Feng, J. Yang, A state-of-the-art review on the setting behaviours of ground granulated blast furnace slag- and metakaolin-based alkali-activated materials, Construction and Building Materials 368 (2023) 130389.
914
915
916 https://doi.org/10.1016/j.conbuildmat.2023.130389.

917 [9] G.K. Sun, J.F. Young, R.J. Kirkpatrick, The role of Al in C–S–H: NMR, XRD, and compositional results for precipitated samples, Cement and Concrete Research 36 (2006) 18–29.
918
919 https://doi.org/10.1016/j.cemconres.2005.03.002.

920 [10] S.-D. Wang, K.L. Scrivener, Hydration products of alkali activated slag cement, Cement and Concrete Research 25 (1995) 561–571. https://doi.org/10.1016/0008-8846(95)00045-E.

923 [11] I.G. Richardson, G.W. Groves, The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes, Cement and Concrete Research 23 (1993) 131–138. https://doi.org/10.1016/0008-8846(93)90143-W.

926 [12] F. Bonk, J. Schneider, M.A. Cincotto, H. Panepucci, Characterization by multinuclear high-resolution nmr of hydration products in activated blast-furnace slag pastes, Journal of the American Ceramic Society 86 (2003) 1712–1719. https://doi.org/10.1111/j.1151-2916.2003.tb03545.x.
[13] F. Puertas, A. Fernández-Jiménez, M.T. Blanco-Varela, Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate, Cement and Concrete Research 34 (2004) 139–148. https://doi.org/10.1016/S0008-8846(03)00254-0.

[14] G. Le Saoût, M. Ben Haha, F. Winnefeld, B. Lothenbach, Hydration Degree of Alkali-Activated Slags: A 29Si NMR Study, Journal of the American Ceramic Society 94 (2011) 4541–4547. https://doi.org/10.1111/j.1551-2916.2011.04828.x.

[15] M. Ben Haha, G. Le Saout, F. Winnefeld, B. Lothenbach, Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast-furnace slags, Cement and Concrete Research 41 (2011) 301–310. https://doi.org/10.1016/j.cemconres.2010.11.016.

[16] A. Fernández-Jiménez, F. Puertas, I. Sobrados, J. Sanz, Structure of calcium silicate hydrates formed in alkaline-activated slag: influence of the type of alkaline activator, Journal of the American Ceramic Society 86 (2003) 1389–1394. https://doi.org/10.1111/j.1151-2916.2003.tb03481.x.

[17] S.-D. Wang, K.L. Scrivener, 29Si and 27Al NMR study of alkali-activated slag, Cement and Concrete Research 33 (2003) 769–774. https://doi.org/10.1016/S0008-8846(02)01044-X.

[18] R.J. Myers, S.A. Bernal, J.L. Provis, Phase diagrams for alkali-activated slag binders, Cement and Concrete Research 95 (2017) 30–38. https://doi.org/10.1016/j.cemconres.2017.02.006.

[19] A. Roy, P.J. Schilling, H.C. Eaton, P.G. Malone, W.N. Brabston, L.D. Wakeley, Activation of ground blast-furnace slag by alkali-metal and alkaline-earth hydroxides, J American Ceramic Society 75 (1992) 3233–3240. https://doi.org/10.1111/j.1151-2916.1992.tb04416.x.

[20] N.T. Dung, T.J.N. Hooper, C. Unluer, Accelerating the reaction kinetics and improving the performance of Na2CO3-activated GGBS mixes, Cement and Concrete Research 126 (2019) 105927. https://doi.org/10.1016/j.cemconres.2019.105927.

[21] A.R. Sakulich, S. Miller, M.W. Barsoum, Chemical and microstructural characterization of 20-month-old alkali-activated slag cements, Journal of the American Ceramic Society 93 (2010) 1741–1748. https://doi.org/10.1111/j.1551-2916.2010.03611.x.

[22] M. Ben Haha, B. Lothenbach, G. Le Saout, F. Winnefeld, Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3, Cement and Concrete Research 42 (2012) 74–83. https://doi.org/10.1016/j.cemconres.2011.08.005.

[23] B. Yuan, Q.L. Yu, H.J.H. Brouwers, Time-dependent characterization of Na2CO3 activated slag, Cement and Concrete Composites 84 (2017) 188–197. https://doi.org/10.1016/j.cemconcomp.2017.09.005.
[24] S.A. Bernal, J.L. Provis, R.J. Myers, R. San Nicolas, J.S.J. van Deventer, Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders, Mater Struct 48 (2015) 517-529. https://doi.org/10.1617/s11527-014-0412-6.

[25] A. Fernández-Jiménez, F. Puertas, Setting of alkali-activated slag cement. Influence of activator nature, Advances in Cement Research (2001) 7.

[26] A.R. Brough, A. Atkinson, Sodium silicate-based, alkali-activated slag mortars: Part I. Strength, hydration and microstructure, Cement and Concrete Research 32 (2002) 865-879. https://doi.org/10.1016/S0008-8846(02)00717-2.

[27] C. Shi, R.L. Day, A calorimetric study of early hydration of alkali-slag cements, Cement and Concrete Research 25 (1995) 1333-1346. https://doi.org/10.1016/0008-8846(95)00126-W.

[28] M. Ben Haha, B. Lothenbach, G. Le Saout, F. Winnefeld, Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part I: Effect of MgO, Cement and Concrete Research 41 (2011) 955-963. https://doi.org/10.1016/j.cemconres.2011.05.002.

[29] Y. Bai, N.C. Collier, N.B. Milestone, C.H. Yang, The potential for using slags activated with near neutral salts as immobilisation matrices for nuclear wastes containing reactive metals, Journal of Nuclear Materials 413 (2011) 183-192. https://doi.org/10.1016/j.jnucmat.2011.04.011.

[30] S.A. Bernal, R.S. Nicolas, J.S.J. van Deventer, J.L. Provis, Alkali-activated slag cements produced with a blended sodium carbonate/sodium silicate activator, Advances in Cement Research 28 (2016) 262-273. https://doi.org/10.1680/jadcr.15.00013.

[31] S. Tome, A. Nana, H.K. Tchakouté, J. Temuujin, C.H. Rüscher, Mineralogical evolution of raw materials transformed to geopolymer materials: A review, Ceramics International 50 (2024) 35855-35868. https://doi.org/10.1016/j.ceramint.2024.07.024.

[32] R. Al-Sughayer, H. Alkhateb, H. Yasarer, Y. Najjar, A. Al-Ostaz, Evaluation and prediction of slag-based geopolymer's compressive strength using design of experiment (DOE) approach and artificial neural network (ANN) algorithms, Construction and Building Materials 440 (2024) 137322. https://doi.org/10.1016/j.conbuildmat.2024.137322.

[33] C. Paillard, M.A. Cordoba, N. Sanson, J.-B. d'Espinose de Lacaillerie, G. Ducouret, P. Boustingorry, M. Jachiet, C. Giraudeau, V. Kocaba, The role of solvent quality and of competitive adsorption on the efficiency of superplasticizers in alkali-activated slag pastes, Cement and Concrete Research 163 (2023) 107020. https://doi.org/10.1016/j.cemconres.2022.107020.

[34] M. Palacios, S. Gismera, M.M. Alonso, J.B. d'Espinose de Lacaillerie, B. Lothenbach, A. Favier, C. Brumaud, F. Puertas, Early reactivity of sodium silicate-activated slag pastes and its impact on rheological properties, Cement and Concrete Research 140 (2021) 106302. https://doi.org/10.1016/j.cemconres.2020.106302.
1006 [35] D. Massiot, F. Fayon, M. Capron, I. King, S.L. Calvé, B. Alonso, J.-O. Durand, B. Bujoli,
1007 Z. Gan, G. Hoatson, Modelling one- and two-dimensional solid-state NMR spectra,
1008 Magnetic Resonance in Chemistry 40 (2002) 70–76. https://doi.org/10.1002/mrc.984.

1009 [36] K. Shimoda, Y. Tobu, K. Kanehashi, T. Nemoto, K. Saito, Total understanding of the local
1010 structures of an amorphous slag: Perspective from multi-nuclear (29Si, 27Al, 17O, 25Mg,
1011 and 43Ca) solid-state NMR, Journal of Non-Crystalline Solids 354 (2008) 1036–1043.
1012 https://doi.org/10.1016/j.jnoncrysol.2007.08.010.

1013 [37] J.-B. d'Espinose de Lacaillerie, C. Fretigny, D. Massiot, MAS NMR spectra of
1014 quadrupolar nuclei in disordered solids: The Czjzek model, Journal of Magnetic
1015 Resonance 192 (2008) 244–251. https://doi.org/10.1016/j.jmr.2008.03.001.

1016 [38] D.R. Neuville, L. Cormier, D. Massiot, Al coordination and speciation in calcium
1017 aluminosilicate glasses: Effects of composition determined by 27Al MQ-MAS NMR and
1018 Raman spectroscopy, Chemical Geology 229 (2006) 173–185.
1019 https://doi.org/10.1016/j.chemgeo.2006.01.019.

1020 [39] D.R. Neuville, L. Cormier, D. Massiot, Al environment in tectosilicate and peraluminous
glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation, Geochimica et
1021 Cosmochimica Acta 68 (2004) 5071–5079. https://doi.org/10.1016/j.gca.2004.05.048.
1022

1023 [40] B. Walkley, X. Ke, J.L. Provis, S.A. Bernal, Activator Anion Influences the Nanostructure
1024 of Alkali-Activated Slag Cements, The Journal of Physical Chemistry C (2021)
1025 acs.jpcc.1c07328. https://doi.org/10.1021/acs.jpcc.1c07328.

1026 [41] R. Chang, D. Choi, M.H. Kim, Y. Park, Tuning Crystal Polymorphisms and Structural
Investigation of Precipitated Calcium Carbonates for CO2 Mineralization, ACS
1027 Sustainable Chem. Eng. 5 (2017) 1659–1667.
1028 https://doi.org/10.1021/acssuschemeng.6b02411.
1029

1030 [42] F. Winnefeld, M. Ben Haha, G. Le Saout, M. Costoya, S.-C. Ko, B. Lothenbach, Influence
of slag composition on the hydration of alkali-activated slags, Journal of Sustainable
1031 Cement-Based Materials 4 (2015) 85–100.
1032 https://doi.org/10.1080/21650373.2014.955550.
1033

1034 [43] C. Gu, J. Yao, Y. Yang, J. Huang, L. Ma, T. Ni, J. Liu, The Relationship of Compressive
Strength and Chemically Bound Water Content of High-Volume Fly Ash-Cement Mortar,
1035 Materials 14 (2021) 6273. https://doi.org/10.3390/ma14216273.
1036

1037 [44] M. Criado, B. Walkley, X. Ke, J.L. Provis, S.A. Bernal, Slag and activator chemistry
control the reaction kinetics of sodium metasilicate-activated slag cements, Sustainability
1038 10 (2018) 4709. https://doi.org/10.3390/su10124709.
1039

1040 [45] A. Fernández-Jiménez, F. Puertas, Influencia de la concentración del activador sobre la
cinética del proceso de activación alcalina de una escoria de alto horno, Mater. Construcc.
1041 47 (1997) 31–42. https://doi.org/10.3989/mc.1997.v47.i246.505.
1042

1043 [46] I. Klur, B. Pollet, J. Virlet, A. Nonat, C-S-H Structure Evolution with Calcium Content by
Multinuclear NMR, in: P. Colombet, H. Zanni, A.-R. Grimmer, P. Sozzani (Eds.), Nuclear
1044
1045 Magnetic Resonance Spectroscopy of Cement-Based Materials, Springer, Berlin,
1046 Heidelberg, 1998: pp. 119–141. https://doi.org/10.1007/978-3-642-80432-8_8.

1047 [47] I.G. Richardson, G.W. Groves, The structure of the calcium silicate hydrate phases present
1048 in hardened pastes of white Portland cement/blast-furnace slag blends, Journal of
1049 Materials Science (1997) 4793–4802.

1050 [48] K. Arbi, A. Palomo, A. Fernández-Jiménez, Alkali-activated blends of calcium aluminate
1051 cement and slag/diatomite, Ceramics International 39 (2013) 9237–9245.
1052 https://doi.org/10.1016/j.ceramint.2013.05.031.

1053 [49] M.D. Andersen, H.J. Jakobsen, J. Skibsted, Incorporation of aluminum in the calcium
1054 silicate hydrate (C-S-H) of hydrated portland cements: a high-field 27Al and 29Si MAS-
1055 NMR investigation, Inorganic Chemistry 42 (2003) 2280–2287.
1056 https://doi.org/10.1021/ic020607b.

1057 [50] J. Schneider, M.A. Cincotto, H. Panepucci, 29Si and 27Al high-resolution NMR
1058 characterization of calcium silicate hydrate phases in activated blast-furnace slag pastes,
1059 Cement and Concrete Research 31 (2001) 993–1001. https://doi.org/10.1016/S0008-
1060 8846(01)00530-0.

1061 [51] J. Skibsted, M.D. Andersen, The Effect of Alkali Ions on the Incorporation of Aluminum
1062 in the Calcium Silicate Hydrate (C-S-H) Phase Resulting from Portland Cement Hydration
1063 Studied by ²⁹Si MAS NMR, Journal of the American Ceramic Society 96 (2013) 651–
1064 656. https://doi.org/10.1111/jace.12024.

1065 [52] X. Pardal, F. Brunet, T. Charpentier, I. Pochard, A. Nonat, 27Al and 29Si Solid-State
1066 NMR Characterization of Calcium-Aluminosilicate-Hydrate, Inorganic Chemistry 51
1067 (2012) 1827–1836. https://doi.org/10.1021/ic202124x.

1068 [53] R.J. Myers, S.A. Bernal, J.D. Gehman, J.S.J. van Deventer, J.L. Provis, The Role of Al in
1069 Cross-Linking of Alkali-Activated Slag Cements, Journal of the American Ceramic
1070 Society 98 (2015) 996–1004. https://doi.org/10.1111/jace.13360.

1071 [54] T. Schade, F. Bellmann, B. Middendorf, Quantitative analysis of C-(K)-A-S-H-amount
1072 and hydrotalcite phase content in finely ground highly alkali-activated slag/silica fume
1073 blended cementitious material, Cement and Concrete Research 153 (2022) 106706.
1074 https://doi.org/10.1016/j.cemconres.2021.106706.

1075 [55] E. L'Hôpital, B. Lothenbach, G. Le Saout, D. Kulik, K. Scrivener, Incorporation of
1076 aluminium in calcium-silicate-hydrates, Cement and Concrete Research 75 (2015) 91–
1077 103. https://doi.org/10.1016/j.cemconres.2015.04.007.

1078 [56] P. Rejmak, J.S. Dolado, M.J. Stott, A. Ayuela, 29Si NMR in cement: a theoretical study
1079 on calcium silicate hydrates, The Journal of Physical Chemistry C 116 (2012) 9755–9761.
1080 https://doi.org/10.1021/jp302218j.

1081 [57] R.J. Myers, S.A. Bernal, R. San Nicolas, J.L. Provis, Generalized structural description of
1082 calcium–sodium aluminosilicate hydrate gels: the cross-linked substituted tobermorite
1083 model, Langmuir 29 (2013) 5294–5306. https://doi.org/10.1021/la4000473.
1084 [58] G. Renaudin, J. Russias, F. Leroux, C. Cau-dit-Coumes, F. Frizon, Structural characterization of C–S–H and C–A–S–H samples—Part II: Local environment 1085 investigated by spectroscopic analyses, Journal of Solid State Chemistry 182 (2009) 3320–1087 https://doi.org/10.1016/j.jssc.2009.09.024.

1088 [59] P. Faucon, A. Delagrave, C. Richet, J.M. Marchand, H. Zanni, Aluminum Incorporation 1089 in Calcium Silicate Hydrates (C–S–H) Depending on Their Ca/Si Ratio, The Journal of 1090 Physical Chemistry B 103 (1999) 7796–7802. https://doi.org/10.1021/jp990609q.

1091 [60] E. Kangni-Foli, Study of the kinetics rates of low alkalinity cementitious materials 1092 carbonation: impact on the microstructure, the gas transport and the mechanical properties, 1093 Thèse de doctorat, Université PSL, 2019.

1094 [61] E. Lam, A. Comas-Vives, C. Copéret, Role of Coordination Number, Geometry, and Local 1095 Disorder on ²⁷ Al NMR Chemical Shifts and Quadrupolar Coupling Constants: Case Study 1096 with Aluminosilicates, The Journal of Physical Chemistry C 121 (2017) 19946–19957. 1097 https://doi.org/10.1021/acs.jpcc.7b07872.

1098 [62] J.C.C. Chan, M. Bertmer, H. Eckert, Site Connectivities in Amorphous Materials Studied 1099 by Double-Resonance NMR of Quadrupolar Nuclei: High-Resolution ¹¹ B ↔ ²⁷ Al 1100 Spectroscopy of Aluminoborate Glasses, Journal of the American Chemical Society 121 1101 (1999) 5238–5248. https://doi.org/10.1021/ja983385i.

1102 [63] H. Manzano, J.S. Dolado, A. Ayuela, Aluminum incorporation to dreierketten silicate 1103 chains, The Journal of Physical Chemistry B 113 (2009) 2832–2839. 1104 https://doi.org/10.1021/jp804867u.

1105 [64] B. Lothenbach, A. Nonat, Calcium silicate hydrates: Solid and liquid phase composition, 1106 Cement and Concrete Research 78 (2015) 57–70. 1107 https://doi.org/10.1016/j.cemconres.2015.03.019.

1108 [65] P. Faucon, T. Charpentier, D. Bertrandie, A. Nonat, J. Virlet, J.C. Petit, Characterization 1109 of Calcium Aluminate Hydrates and Related Hydrates of Cement Pastes by 27Al MQ- 1110 MAS NMR, Inorganic Chemistry 37 (1998) 3726–3733. 1111 https://doi.org/10.1021/ic9800076.

1112 [66] M.D. Andersen, H.J. Jakobsen, J. Skibsted, Characterization of white Portland cement 1113 hydration and the C–S–H structure in the presence of sodium aluminate by 27Al and 29Si 1114 MAS NMR spectroscopy, Cement and Concrete Research 34 (2004) 857–868. 1115 https://doi.org/10.1016/j.cemconres.2003.10.009.

1116 [67] M.D. Andersen, H.J. Jakobsen, J. Skibsted, A new aluminium-hydrate species in hydrated 1117 Portland cements characterized by 27Al and 29Si MAS NMR spectroscopy, Cement and 1118 Concrete Research 36 (2006) 3–17. https://doi.org/10.1016/j.cemconres.2005.04.010.

1119 [68] L. Black, A. Stumm, K. Garbev, P. Stemmermann, K.R. Hallam, G.C. Allen, X-ray 1120 photoelectron spectroscopy of aluminium-substituted tobermorite, Cement and Concrete 1121 Research 35 (2005) 51–55. https://doi.org/10.1016/j.cemconres.2004.08.005.
1122 [69] H. Manzano, J.S. Dolado, M. Griebel, J. Hamaekers, A molecular dynamics study of the
1123 aluminosilicate chains structure in Al-rich calcium silicate hydrated (C-S-H) gels, Physica
1124 Status Solidi (a) 205 (2008) 1324–1329. https://doi.org/10.1002/pssa.200778175.

1125 [70] A. Kunhi Mohamed, P. Moutzouri, P. Berruyer, B.J. Walder, J. Siramanont, M. Harris,
1126 M. Negroni, S.C. Galmarini, S.C. Parker, K.L. Scrivener, L. Emsley, P. Bowen, The
1127 atomic-level structure of cementitious calcium aluminate silicate hydrate, Journal of the
1128 American Chemical Society (2020) jacs.0c02988. https://doi.org/10.1021/jacs.0c02988.

1129 [71] A. Fernández-Jiménez, J.G. Palomo, F. Puertas, Alkali-activated slag mortars: Mechanical
1130 strength behaviour, Cement and Concrete Research 29 (1999) 1313–1321.
1131 https://doi.org/10.1016/S0008-8846(99)00154-4.

1132 [72] G. De Schutter, L. Taerwe, General hydration model for portland cement and blast furnace
1133 slag cement, Cement and Concrete Research 25 (1995) 593–604.
1134 https://doi.org/10.1016/0008-8846(95)00048-H.

1135 [73] G. De Schutter, Hydration and temperature development of concrete made with blast-
1136 furnace slag cement, Cement and Concrete Research 29 (1999) 143–149.
1137 https://doi.org/10.1016/S0008-8846(98)00229-4.

1138 [74] A. Fernández-Jiménez, F. Puertas, Alkali-activated slag cements: Kinetic studies, Cement
1139 and Concrete Research 27 (1997) 359–368. https://doi.org/10.1016/S0008-
1140 8846(97)00040-9.

1141 [75] L. Wadsö, F. Winnefeld, K. Riding, P. Sandberg, Calorimetry, in: K. Scrivener, R.
1142 Snellings, B. Lothenbach (Eds.), A Practical Guide to Microstructural Analysis of
1143 Cementitious Materials, CRC Press, 2016: pp. 38–74.

1144 [76] S. Aydın, B. Baradan, Effect of activator type and content on properties of alkali-activated
1145 slag mortars, Composites Part B: Engineering 57 (2014) 166–172.
1146 https://doi.org/10.1016/j.compositesb.2013.10.001.

1147 [77] F. Puertas, M. Palacios, H. Manzano, J.S. Dolado, A. Rico, J. Rodríguez, A model for the
1148 C-A-S-H gel formed in alkali-activated slag cements, Journal of the European Ceramic
1149 Society 31 (2011) 2043–2056. https://doi.org/10.1016/j.jeurceramsoc.2011.04.036.

1150 [78] J. Aupoil, J.-B. Champenois, J.-B. d'Espinose de Lacaillerie, A. Poulesquen, Interplay
1151 between silicate and hydroxide ions during geopolymerization, Cement and Concrete
1152 Research 115 (2019) 426–432. https://doi.org/10.1016/j.cemconres.2018.09.012.

1153 [79] S. Park, S.Y. Abate, H.K. Lee, H.-K. Kim, On the quantification of degrees of reaction
1154 and hydration of sodium silicate-activated slag cements, Materials and Structures 53
1155 (2020) 65. https://doi.org/10.1617/s11527-020-01505-9.

1156