Mineralogical composition

The XRD pattern of bentonite mixed with small proportions of CaO (BC1 and BC3) showed characteristic peaks of Ca-montmorillonite, Na-montmorillonite, anorthite, K-feldspar, Na-feldspar, portlandite, calcite, and calcium silicate hydrate (C-S-H) (Fig. 2). The peak representing portlandite increased in intensity as the amount of lime added was increased. The peak at ~32.72 2θ (2.71 Å) appeared broader and less intense, probably due to the presence of C-S-H (Lothenbach et al., Reference Lothenbach, Nied, L’Hôpital, Achiedo and Dauzères2015).

Figure 2.

XRD pattern of bentonite mixed with variable amounts of CaO. CaMt = calcium montmorillonite; NaMt = sodium montmorillonite; An = anorthite; Cal = calcite; KFs = K-feldspar; NaFs = Na-feldspar; Por = portlandite; CSH = calcium silicate hydrate.

A strong base, Ca(OH)₂, was obtained by hydration of CaO. The solubility product of Ca(OH)₂ is equal to Ksp 5×10⁻⁶ (Dodds et al., Reference Dodds, Williams and Radcliffe2017). OH^– and Ca²⁺ ions were released into the intragranular solution of the bentonite aggregates. At pH >11 and in wet conditions, OH^– ions adsorbed to the surface of montmorillonite forming silanol (Si-OH) and aluminol (Al-OH) bonds. Ca²⁺ reacted with the tetrahedral silanol and octahedral aluminol to form the calcium silicate hydrate (C-S-H) and calcium alumino-silicate hydrate (C-S-A-H) gels. The tetrahedral silica evolves from a Q₃ to a Q₁ environment upon C-S-H formation (Vitale et al., Reference Vitale, Deneele, Paris and Russo2017). Calcite is formed by the carbonation reaction of lime in the presence of CO₂ according to the following reaction (Eqn 1):

(1) $$ \hskip-0.4em {\mathrm{CO}}_2+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\hskip0.5em \iff \hskip0.5em {\mathrm{Ca}}^{2+}+{{\mathrm{CO}}_3}^{2\hbox{--} }+{\mathrm{H}}_2\mathrm{O}\hskip0.5em \iff \hskip0.5em {\mathrm{Ca}\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}. $$

The precipitation of calcite on the surface of bentonite grains hinders the formation of C-S-H (Baldermann et al., Reference Baldermann, Baldermann, Furat, Krüger, Nachtnebel, Schroettner, Juhart and Tritthart2019).

The XRD pattern of bentonite-magnesium hydroxide mixtures (Fig. 3) showed the presence of Ca-montmorillonite, Na-montmorillonite, kaolinite, anorthite, K-feldspar, Na-feldspar, brucite, and periclase. In the mixtures BM1, BM2, and BM3, periclase (MgO) was entirely consumed to give brucite (Mg(OH)₂). MgO persisted in the BM4 mixture. The brucite-forming reaction is slow (Eqn 2):

(2) $$ \mathrm{Mg}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Mg}}^{2+}+2{\mathrm{OH}}^{\hbox{--}}\to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2 $$

Figure 3.

XRD patterns of BM1, BM2, BM3, and BM4 mixtures. CaMt = calcium montmorillonite; NaMt = sodium montmorillonite; An = anorthite; KFs = K-feldspar; NaFs = Na-feldspar; Brc = brucite; Per = periclase; Kao = kaolinite.

Likewise, the solubility product of brucite (Mg²⁺ and OH^–) is low with Ksp 2×10⁻¹³ (Dodds et al., Reference Dodds, Williams and Radcliffe2017). The peak at 7.22 Å (12.21°2θ) could correspond to kaolinite of low crystallinity in raw bentonite.

The XRD pattern of bentonite-sodium hydroxide mixtures (Fig. 4) showed the presence of Ca-montmorillonite, Na-montmorillonite, anorthite, K-feldspar, Na-feldspar, sodalite, and cancrinite.

Figure 4.

XRD pattern of BN1, BN2, BN3, and BN4 mixtures. CaMt = calcium montmorillonite; NaMt = sodium montmorillonite; An = anorthite; KFs = K-feldspar; NaFs = Na-feldspar; Sdl = sodalite; Ccn = cancrinite

The addition of 16% of NaOH to the bentonite (BN3) allowed the transformation of the bentonite to Na-montmorillonite by substitution of one calcium by two sodium ions (Fig. 4). This is due to neighboring cationic radii of Na⁺ and Ca²⁺. The XRD pattern of the bentonite mixed with 32 wt.% of NaOH (BN4) showed the formation of sodalite (Na₈(Al₆Si₆O₂₄)(OH)₂(H₂O)₂) and cancrinite (Na₆Ca₂Al₆Si₆O₂₄(CO₃)₂.2H₂O) (Fig. 4). The peak of montmorillonite disappeared, probably because of its dissolution in a highly sodium alkaline medium (pH >14). Pure hydroxysodalite can be formed from bentonite in the following conditions: (1) Na/Si molar ratio of 12; (2) Si/Al molar ratio of 1.0; (3) temperature of reaction of 90°C; (4) reaction time of 12 h (Liu et al., Reference Liu, Sun, Peng and He2018). It can also occur after a long stay of montmorillonite in a solution of NaOH (Elert et al., Reference Elert, Pardo and Rodriguez-Navarro2015). Zeolite, as cancrinite formed in sample BN4, can be obtained after sodium attack of montmorillonite (Seo et al., Reference Seo, Kim, Kim, Kim, Lee, Roh and Kang2018); its formation is favored by the CO₂ (Hackbarth et al., Reference Hackbarth, Gesing, Fechtelkord, Stief and Buhl1999).

In an aqueous and highly alkaline medium (pH >14), OH^– ions adsorb on montmorillonite to form Si-OH and Al-OH bonds. The excess sodium substitutes the proton and promotes the dissolution of the cancrinite to form metastable phases: Na₂SiO₃ and NaAlO₂. With time, sodalite is formed according to the following reaction (Eqn 3) (Zong et al., Reference Zong, Zhao, Chen, Liu and Cang2020):

(3) $$ {\displaystyle \begin{array}{l}6{\mathrm{Na}}_2{\mathrm{Si}\mathrm{O}}_3+6{\mathrm{Na}\mathrm{AlO}}_2+8{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Na}}_8\left({\mathrm{Al}}_6{\mathrm{Si}}_6{\mathrm{O}}_{24}\right){\left(\mathrm{OH}\right)}_2{\left({\mathrm{H}}_2\mathrm{O}\right)}_2\\ {}\hskip22em +\hskip2px 10\mathrm{NaOH}\end{array}} $$

The XRD pattern of bentonite-potassium hydroxide mixtures (Fig. 5) indicated the presence Ca-montmorillonite, Na-montmorillonite, kaolinite, anorthite, K-feldspar, Na-feldspar, hydrated potassium aluminum oxide, and potassium carbonate sesquihydrate.

Figure 5.

XRD pattern of BK1, BK2, BK3, and BK4. CaMt = calcium montmorillonite; NaMt = sodium montmorillonite; An = anorthite; Kao = kaolinite; KFs = K-feldspar; NaFs = Na-feldspar; KO = hydrated potassium aluminium oxide; KC = potassium carbonate sesquihydrate.

Ca-montmorillonite and Na-montmorillonite peaks decreased in intensity and shifted toward small angles when 32 wt.% of KOH was added (Fig. 5). KOH solution induced a mineralogical change in the 2:1 layer of montmorillonite that increased the layer charge by increasing the Al content (Bauer et al., Reference Bauer, Lanson, Ferrage, Emmerich, Taubald, Schild and Veld2006).

A small peak, characteristic of kaolinite, appeared at 7.32 Å for BK3 and BK4 (Fig. 5). The Si/Al molar ratio was, on average, equal to 0.5 for montmorillonite, and 1 for kaolinite. The progressive loss of the tetrahedral sheets in montmorillonite layers and the increase in hydroxyl content at the surface of the exposed octahedral sheet transformed smectite into kaolinite in highly alkaline conditions (pH >14) according to the following reaction (Eqn 4; Dudek et al., Reference Dudek, Cuadros and Fiore2006):

(4) $$ \mathrm{Smectite}+{\mathrm{Si}}^{4+}\to \mathrm{kaolinite} $$

Montmorillonite transformed into kaolinite and silica according to the following reaction (Eqn 5; Altschuler et al., Reference Altschuler, Dwornik and Kramer1963):

(5) $$ \mathrm{Montmorillonite}\to \mathrm{kaolinite}+\mathrm{silica} $$

With the addition of KOH, potassium carbonate sesquihydrate (K₂CO₃⋅1.5H₂O) and potassium aluminum oxide hydrate (K₂Al₂O₄ 3H₂O) appeared in BK3 and BK4 (Fig. 5). Potassium carbonate sesquihydrate formed during the synthesis of C-S-H gel in a 1.0 M KOH solution (Yan et al., Reference Yan, Yang, Miron, Collings, L’Hôpital, Skibsted, Scrivener and Lothenbach2022). Potassium aluminum oxide hydrate was obtained following the attack on alumina by KOH (Zhang et al., Reference Zhang, Zhang, Xu, Jing and Zhang2008).

The presence of minor phases in bentonite, such as feldspars and anorthite, did not affect the reactivity of natural montmorillonite with the various bases. These aluminosilicates seem to be unaltered under hyperalkaline conditions, as evidenced by their clear detection by XRD (Figs 2–5).

Evolution of the d ₀₀₁ reflection of Ca/Na-montmorillonite

The system studied was quite complex with several clay mineral phases undergoing dissolution. Dissolution-precipitation reactions could be dominant, leading to a broadening of the diffraction peaks due to the decrease in particle size. Several processes together lead to these changes, therefore the discussion related to the differences in cation hydration might account for part of the material, but not all of the processes.

The intensity of the montmorillonite peak in the XRD patterns (Fig. 6) decreased as the quantity of CaO increased. This could be attributed to the dilution of montmorillonite by the lime present in the sample or more significantly to its dissolution. The montmorillonite peak became flattened, shifting toward the peak of Ca-montmorillonite, while the Na-montmorillonite peak disappeared.

Figure 6.

Evolution of the position of the (001) peak of montmorillonite.

The addition of MgO did not show any change in the d ₀₀₁ position, but its intensity decreased with increasing the amount of MgO added (Fig. 6). The basal spacing (d ₀₀₁) is governed by the size and the degree of hydration. The basal spacings of non-hydrated montmorillonite exchanged with small cations Mg²⁺ and Ca²⁺ are similar (Berghout et al., Reference Berghout, Tunega and Zaoui2010). The decrease in peak intensity could be attributed to the dissolution of the counter cations in montmorillonite under alkaline conditions (pH=11.4).

With the addition of NaOH, the characteristic peak of montmorillonite flattened and decreased in intensity with increasing amount of NaOH added. Extreme sodification of montmorillonite was observed. With the addition of 2–8 wt.% of KOH (BK1 and BK2), the intensity of the montmorillonite peak decreased and moved toward high angles (2θ ~7). This decrease in the d ₀₀₁ value could be due to the ionic radius of K⁺ (133 pm), which makes it difficult to penetrate the cavities of the tetrahedral sheets (Bray and Redfern, Reference Bray and Redfern2000), causing shrinkage (Watanabe and Yokoyama, Reference Watanabe and Yokoyama2022). When large amounts of KOH (16–32 wt.%, BK3 and BK4 specimens) were added, the (001) peak of montmorillonite became broad and an expansion of the interlayer space was observed. Sodium-montmorillonite saturated with Na⁺ and K⁺ have larger (decrease in 2θ) d-spacings and broader reflections than Na-montmorillonite saturated with Ca²⁺ or Mg²⁺ (Bray and Redfern, Reference Bray and Redfern2000; Titiloye and Skipper, Reference Titiloye and Skipper2005). This could indicate that Na⁺ and K⁺ have lower hydration energies than Ca²⁺ or Mg²⁺ (Rao et al., Reference Rao, Dinadayalane, Leszczynski and Sastry2008) and lower hydrated radius (Nightingale, Reference Nightingale1959; Volkov et al., Reference Volkov, Paula and Deamer1997):

$$ \begin{array}{c}\Delta {\mathrm{H}}_{\left(\mathrm{K}\left(\mathrm{H}2\mathrm{O}\right)+\right)}\left(69.7\;\mathrm{kJ}\;{\mathrm{mol}}^{\hbox{--} 1}\right)<\Delta {\mathrm{H}}_{\left(\mathrm{Na}\left(\mathrm{H}2\mathrm{O}\right)+\right)}\left(102\;\mathrm{kJ}\;{\mathrm{mol}}^{\hbox{--} 1}\right)\\ {}<\Delta {\mathrm{H}}_{\left(\mathrm{Ca}\left(\mathrm{H}2\mathrm{O}\right)2+\right)}\left(220.1\;\mathrm{kJ}\;{\mathrm{mol}}^{\hbox{--} 1}\right)<\Delta {\mathrm{H}}_{\left(\mathrm{Mg}\left(\mathrm{H}2\mathrm{O}\right)2+\right)}\left(313.8\;\mathrm{kJ}\;{\mathrm{mol}}^{\hbox{--} 1}\right)\end{array} $$

The position of the Na⁺ and K⁺ interlayer cations was such that rehydration was possible. In contrast, Mg²⁺ and Ca²⁺ appeared to have penetrated the cavities of the outer tetrahedral sheets and could not readily form hydration spheres.

Infrared spectroscopy

FTIR was used to follow the modification of chemical composition of mortar specimens after 28 days of hydration (Table 3; Fig. 7).

Table 3.

Absorption bands from FTIR spectra of bentonite, BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4

L = low; I = intense; LL = low and large.

Figure 7.

FTIR spectra of BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4.

FTIR spectra of raw bentonite showed a small band at 3698 cm^–1 attributed to the -O–H stretch for Al-OH and Si-OH (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007). This absorption band persisted for BC2, BM2, BM4, BN2, and BK2, but disappeared for BC4, BN4, and BK4 when the structure of montmorillonite was strongly altered in a highly alkaline medium. In BM2 and BM4, the intensity of this absorption band increased due to the elongation vibration of the Mg–OH bond in brucite (Kloprogge et al., Reference Kloprogge and Frost1999). An absorption band was observed at 3620 cm^–1 due to the elongation vibration of the O–H bond of the Al-O-H group of montmorillonites (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007) appearing in raw bentonite and specimens containing 8% oxides (CaO, MgO) or hydroxides (NaOH, KOH). When 32% oxides or hydroxides was added, this absorption band weakened for BC4, BM4, and BK4, and disappeared for BN4. This decrease could be attributed to the destabilization of the structure of montmorillonite. A broad absorption band was observed at ~3423 cm^–1 corresponding to the vibration of OH bond of the C-S-H gel (Chukanov, Reference Chukanov2013) or to the O–H stretching vibrations of water physisorbed on the surface of matter (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007). An absorption band at 1639 cm⁻¹ is attributed to the H-O-H-bending vibrations of H₂O (Zhang et al., Reference Zhang, Cheng, Peng, Li, Gao and Wang2020). This absorption band disappeared or increased in size when the quantity of oxides or hydroxides increased.

A sharp absorption band appeared at 1436 cm^–1 for BC, BN, and BK, associated with the bending vibration of C-O in CO₃^2– (Muraleedharan et al., Reference Muraleedharan, Asgar, Hahn, Dasgupta, Gadikota and Van Duin2021). The intensity of this band increased with CaO and KOH added to bentonite. This may be due to the formation of calcite in BC and potassium carbonate in BK specimens as confirmed by XRD. Monocarboaluminate was only observed in BC4 highlighted by the split ν3-CO₃²⁻ at 1403 cm⁻¹ and ν2-CO₃²⁻ at 870 cm⁻¹ (Shiferaw et al., Reference Shiferaw, Habte, Thenepalli and Ahn2019). An absorption band was observed at 1115 cm⁻¹ in all samples expect in BN4, attributed to the Si-O stretching of montmorillonite (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007). A broad absorption band situated at 996 cm⁻¹ attributed to Si-O-Si stretching was observed in all samples with a slight displacement at 956 cm⁻¹ for BN4 (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007). This displacement could be attributed to the formation of sodalite and cancrinite. A broad absorption band was observed at 979 cm^–1 attributed to vibrational frequencies of (Si, Al)-O bond in tetrahedron [(Si,Al)O₄]^4– of hydrosodalite (Vaičiukyniene et al., Reference Vaičiukyniene, Baltakys and Kantautas2009). Absorption bands at 907, 698, and 520 cm^–1 were observed in all samples, attributed to the Si-O stretching, OH bonded to 2Al³⁺ and Al-Si-O bending respectively (Tabak et al., Reference Tabak, Afsin, Caglar and Koksal2007).

A narrow absorption band was observed at 870 cm^–1 for BC4 specimen fits to ν2 asymmetric CO₃ stretching due to the formation of calcite (Shiferaw et al., Reference Shiferaw, Habte, Thenepalli and Ahn2019). The absorption band observed at 862 cm^–1 for BN4 can be attributed to the bending bands of carbonate groups of cancrinite (Rozenberg et al., Reference Rozenberg, Rastsvetaeva and Chukanov2009). An absorption band was observed at 825 cm⁻¹ for BK4, indicating the increase in the tetrahedrally coordinated aluminum atom in K₂Al₂O₄.3H₂O (Padmaja et al., Reference Padmaja, Anilkumar, Mukundan, Aruldhas and Warrier2001). It may also be because of Al-O out-of-plane vibration and reflects Al-for-Si substitution in the tetrahedral sheet (Xu et al., Reference Xu, Ding and Cheng2014).

Thermal characteristics

DTA/TGA results of raw bentonite and specimens made by oxides and hydroxides after 28 days of hydration indicated the occurrence of endothermic peaks associated with a loss of mass (Table 4; Fig. 8a,b). The weight loss between 25 and 250°C increased with increasing amount of CaO, NaOH, and KOH added to bentonite, but not MgO. This loss in weight was associated with two endothermic peaks at 107 and 172°C. The first peak was attributed to dehydration of water physisorbed on the grain surface of bentonite (Bounab et al., Reference Bounab, Draoui, Ahrouch, Hadri, Bouchta and Barhoun2017) and the second is due to dehydration of interlayer water of montmorillonite (Ikari et al., Reference Ikari, Saffer and Marone2007). A small endothermic peak was observed at 172°C for BC2 and BC4 due to dehydration of the C-S-H gel (Li et al., Reference Li, Yu, Ma, Hu, Ge and Shen2018). This peak was intense for BK2 and BK4, attributed to dehydration to K₂CO₃·1.5H₂O, which is stable up to 153°C at ambient atmosphere conditions, through the following phase transition reaction (Eqn 6):

(6) $$ {\mathrm{K}}_2{\mathrm{CO}}_3\cdotp 1.5{\mathrm{H}}_2\mathrm{O}\to {\mathrm{K}}_2{\mathrm{CO}}_3+1.5{\mathrm{H}}_2\mathrm{O} $$

Table 4.

DTA/TGA results of raw bentonite, BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4

L = low; M = medium; I = intense.

^a Bounab et al., (Reference Bounab, Draoui, Ahrouch, Hadri, Bouchta and Barhoun2017);

^b Ikari et al., (Reference Ikari, Saffer and Marone2007);

^c Duan et al., (Reference Duan, Luebke, Pennline, Li, Janik and Halley2012);

^d Nied et al., (Reference Nied, Enemark-Rasmussen, L’Hopital, Skibsted and Lothenbach2016);

^e Chhaiba et al., (Reference Chhaiba, Blanco-Varela, Diouri and Bougarrani2018);

^f Caglar et al., (Reference Caglar, Afsin, Tabak and Eren2009);

^g Wang et al., (Reference Wang, Ren and Yang2018);

^h Dumańska-Słowik et al., (Reference Dumańska-Słowik, Pieczka, Heflik and Sikorska2016).

Figure 8.

(a) TGA curves of raw bentonite (Bent) and mixtures BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4; (b) DTA curves of raw bentonite (Bent) and mixtures BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4.

In the temperature range between 25 and 250°C, when CaO was added, loss of weight doubled from BC2 to BC4 due to dehydration of the C-S-H gel. For NaOH, the loss of mass was very important for BN2 and further increased for BN4, probably due to the departure of water molecules located in the channels and cages of the cancrinite and sodalite (Reyes et al., Reference Reyes, Williams and Alarcón2013). For KOH, the loss of weight increased from 7.61% to 11.3% for BK2 and BK4, respectively, due to dehydration of potassium carbonate sesquihydrate.

Slight and similar loss of mass (2–5%) was observed between 250 and 550°C, corresponding to the dehydroxylation of the phases formed, except for BM4 which was high (13%) due to the dehydroxylation of brucite. This loss of mass was associated with three endothermic peaks at 410, 475, and 509°C. The endothermic peak at 410°C was only observed in BM2 and BM4 with a highest intensity in BM4, attributed to the dehydroxylation of brucite (Nied et al., Reference Nied, Enemark-Rasmussen, L’Hopital, Skibsted and Lothenbach2016). The second endothermic peak at 475°C was observed for BC2 and BC4, due to the dehydroxylation of portlandite (Chhaiba et al., Reference Chhaiba, Blanco-Varela, Diouri and Bougarrani2018). A small endothermic peak at 509°C was observed for MC4 and BM4, corresponding to dehydroxylation of Ca-montmorillonite.

The third loss in weight occurred between 550 and 950°C, associated with three endothermic peaks at 509, 654, and 778°C. The peak at 509°C for raw bentonite, BC4, BM2, and BM4 is attributed to dehydroxylation of Ca-montmorillonite. The peak at 654°C was shown for all samples except for BN4 and BK4, due to the departure of structural water of Ca-montmorillonite (Xie et al., Reference Xie, Gao, Liu, Pan, Vaia, Hunter and Singh2001). This peak can be attributed to the decarbonation of amorphous carbonate from the C-S-H gel formed for samples BC2 and BC4 (Bandipally et al., Reference Bandipally, Cherian and Arnepalli2018). The peak at 778°C, only observed for BC4, is due to the loss of CO₂ of neoformed calcite (Wang et al., Reference Wang, Ren and Yang2018).

Between 550 and 950°C, the loss in weight for BM2, BM4, BN2, and BK2 was small (1.3–2.4%) due to the dehydroxylation of Ca-montmorillonite. This small loss in weight could be attributed to the low amount of adsorbed water in the interfoliar space of montmorillonite as shown by XRD pattern for specimens mixed with NaOH and KOH (Fig. 6). For specimens mixed with MgO, brucite formation probably minimized the number of intercalated water molecules in the interfoliar space, while the loss in mass was around 5% for BC2, BN4, and BK4, and reached 17.6% for BC4. This slight increase in mass loss observed was probably due to decarbonation of cancrinite in BN4 (Kriaa et al., Reference Kriaa, Ben Saad and Hamzaoui2012), and to the dehydration of the formed K₂CO₃⋅1.5H₂O and K₂Al₂O₄ 3H₂O in BK4.

The loss of total mass between 25 and 950°C increased with the increase in the amount added of CaO, MgO, NaOH, and KOH, indicating the formation of hydrates and carbonates. Bentonite represented 63% of total mass of specimens for BC2, BM2, BN2, and BK2, and 46.6% for BC4, BM4, BN4, and BK4 (10 g of the solid and 4.6 g of water, 8/14.6×100=63% or 32/14.6×100=46.6%). The calculation of the total loss in weight of bentonite in relation to the quantity of raw bentonite used to manufacture the specimens, allowed us to observe that losses came from other compounds (i.e. hydrates and calcareous phases). The contribution of bentonite to the loss (total loss of bentonite/16.8; 16.8 is the total mass loss of bentonite) decreased gradually as follows: BM2 ≈ BK2 <BN2 << BC2 and BM4 < BK4 <BN4 < BC4. Destabilization of bentonite structure occurs as follows: MgO < KOH << NaOH << CaO.

Microstructure

The SEM image of BC2 (Fig. 9a) showed a platelet morphology characteristic of smectites (González-Santamaría et al., Reference González-Santamaría, Justel, Fernández, Ruiz, Stavropoulou, Rodríguez-Blanco and Cuevas2021). Needle-shaped fibers formed in BC2 (Fig. 9a) with an average length of 4.34 μm. The EDX results of these needles showed 5.67% Ca, which could be attributed to the formation of aragonite, an unstable phase that subsequently transforms into the more stable calcite (Sharp et al., Reference Sharp, Al-Shehri, Horozov, Stoyanov and Paunov2014; Karunarathne et al., Reference Karunarathne, Paul and Šavija2019). The pozzolanic reaction developed from the surface of montmorillonite grains toward its inner part. The absence of portlandite and the presence of C-S-H gel as flakes confirmed the consumption of portlandite by pozzolanic reaction for BC2. The chemical compositions of these fibers indicated the presence of aluminum and calcium and the absence of sulfur (Table 5). Such morphology may affect the mechanical properties of the clayey paste by increasing cementation of the grains and decreasing their porosity and density.

Table 5.

Chemical EDX-derived data of BC2, BC4, BM2, BM4, BN2, BN4, BK2, and BK4

Figure 9.

(a) SEM/EDX images of BC2, BC4, BM2, and BM4; (b) SEM/EDX images of BN2, BN4, BK2, and BK4.

The SEM image of the BC4 mixture showed a deposit of portlandite on montmorillonite grains; note that the alteration of the morphology of montmorillonite was not obvious. An excess of lime, visible on the outer surface of montmorillonite grains, may affect the mechanical strength of the BC4 mixture (Fig. 9, BC4). The SEM image of BM2 showed that montmorillonite grains were eroded at the surface (Fig. 9a, BM2). This could be explained by the formation of brucite on the surface of the grains. Brucite is subparallel to aggregates of plates in irregular form on the outer surface and the pores (Fernández et al., Reference Fernández, Mäder, Rodríguez, Vigil De La Villa and Cuevas2009). By adding 32% of MgO, a precipitation of nanocrystal in the outer surface of bentonite was observed. This precipitation may be due to an excess of periclase and brucite.

The external surface of the bentonite grains of sample BN4 was modified by the addition of NaOH. The morphology of montmorillonite was transformed into the typical texture of sodalite, as shown by XRD results (Fig. 9b, BN4). Sodalite nanocrystals agglomerate to form micro-granules with diameters of 8–500 nm (Khajavi et al., Reference Khajavi, Kapteijn and Jansen2007). Montmorillonite treated with 5 M HCl and calcined for 4 h by hydrothermal pre-fusion gives sodalite (Belviso et al., Reference Belviso, Cavalcante, Niceforo and Lettino2017). The SEM images did not allow the observation of cancrinite grains.

SEM images of BK2 and BK4 showed that characteristic morphology of montmorillonite is well preserved despite the addition of KOH. An alteration of the outer surface of the montmorillonite grains was visible.

With the addition of CaO, the Ca/Si ratio increased from 0.16 to 1.23 for BC2 and BC4, respectively. The EDX results from SEM observations (Table 5) did not show any K and Na cations in the mixture BC4. This can be explained by the formation of C-S-H and C-A-H gels at the surface of bentonite and by the excess portlandite and calcite that precipitate on the outer area of bentonite. Likewise, with the addition of MgO, the Mg/Si ratio increased from 0.27 to 1.01 for BM2 and BM4, respectively, indicating the formation of periclase and brucite. The significant increase in Al/Si ratio of BN4 (0.67) compared to raw bentonite (Al/Si=0.54) is consistent with the formation of sodalite and cancrinite.

Mechanical behavior of bentonite in different alkaline environments

Testing the compressive strength of clay-based cementitious materials is crucial for assessing material strength, quality control, design decisions, and predicting structural longevity and durability. Bentonite-NaOH and bentonite-KOH specimens remained powdery without setting after 28 days due to the zeolitization of bentonite by NaOH and its transformation into kaolinite by KOH. Consequently, the compressive strength could not be measured for BN and BK. This lack of setting could be due to the absence of cementitious phase formation, influenced by the nature of the cation. There are several setting accelerators, such as chlorite and nitrate salts, that can be added to NaOH or KOH to allow the samples to set. The cation-accelerating strength of chloride and nitrate salts in the hydration of ordinary Portland cement, as observed in calorimetric experiments, follows this order (Wilding et al., Reference Wilding, Walter and Double1984):

$$ \begin{array}{c}{\mathrm{Ca}}^{2+}>{\mathrm{Ni}}^{2+}>{\mathrm{Ba}}^{2+}>{\mathrm{Mg}}^{2+}>{\mathrm{Fe}}^{3+}>{\mathrm{Cr}}^{2+}>{\mathrm{Co}}^{2+}\\ {}>{\mathrm{La}}^{3+}>>{{\mathrm{NH}}_4}^{+},{\mathrm{K}}^{+}>{\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}\end{array} $$

However, in a hyperalkaline environment, a comprehensive study should be conducted, controlling other factors such as temperature, pH, and humidity, and choosing suitable accelerators.

Alkaline chlorides, such as sodium and potassium chloride, are not particularly effective at accelerating the setting and hardening of cement (Vehmas et al., Reference Vehmas, Kronlöf and Cwirzen2018). Alkaline activation by sodium silicate, when applied to an aluminosilicate material, can dissolve silica and alumina in the soil (Muhammad and Siddiqua, Reference Muhammad and Siddiqua2022). This could potentially enhance the setting of BN and BK mixtures.

The uniaxial compression of bentonite-Ca(OH)₂ and bentonite-Mg(OH)₂ mixtures (Table 2) after 28 days of hydration (Fig. 10) increased with increasing lime content; it ranged from 0.44 MPa for BC1 (2% of CaO), to 2.27 MPa for BC2 (8% of CaO), to 2.47 MPa for BC3 (16% of CaO). This could be explained by the formation of C-S-H and C-A-H gel during the pozzolanic reaction. The longer the chains of C-S-H, the harder the clay paste became, which increased their compressive strength (Das et al., Reference Das, Ray and Sarkar2020). Ca²⁺ is the optimal accelerator for setting and hardening, which is why calcium salts (i.e. CaCl2 and Ca(NO₃)₂) are commonly used as accelerating admixtures in concrete (Dorn et al., Reference Dorn, Blask and Stephan2022). The addition of 32% of CaO on raw bentonite decreased the compressive strength of the BC4 mixture to a value of 0.87 MPa due to the neoformation of calcite. The precipitation of calcite on bentonite grains prevented the development of silicate chains on the surface of bentonite grains; therefore, the C-S-H chains were short. Similarly, the compressive strength of the samples decreased with the addition of Mg(OH)₂. The dissolution of Mg(OH)₂ is slow, which does not favor the formation of M-S-H gel. The occurrence of Mg(OH)₂ after 30 days of hydration stabilized the microstructure of the cemented paste (Gonçalves et al., Reference Gonçalves, Silva, De Brito, Fernández and Esquinas2020), which could explain the setting of the clay.

Figure 10.

Uniaxial strength of bentonite-CaO (BC1–BC4) and bentonite-MgO (BM1–BM4) mixtures.