Introduction

The time-dependent behavior of calcium montmorillonite (CaMnt) gels characterized by an increasing yield stress with ageing time has not been researched thoroughly and so knowledge gaps exist. In contrast, the ageing or gelation behavior of bentonite and sodium montmorillonite (NaMnt) have been examined extensively (Broughton & Squires, Reference Broughton and Squires1936; Callaghan & Ottewill, Reference Callaghan and Ottewill1974; Hauser & Reed, Reference Hauser and Reed1937; M'Ewen & Pratt, Reference M'Ewen and Pratt1957; van Olphen, Reference Van Olphen1955), with this behavior being first reported almost 100 years ago. These gels displayed pronounced thixotropic behavior at a very low concentration of a few percent solids. Surface chemistry factors, such as pH and salt concentration, are important gelation factors for bentonite or NaMnt (Abend & Lagaly, Reference Abend and Lagaly2000; Brandenburg & Lagaly, Reference Brandenburg and Lagaly1988; Heller & Keren, Reference Heller and Keren2001; Tombacz & Szekeres, Reference Tombacz and Szekeres2004). CaMnt gels were reported to display time-independent behavior (Keren, Reference Keren1988; Sueto & Nakaishi, Reference Sueto and Nakaishi1992) and pH-independent rheological behavior at a low concentration of a few percent solids (Keren, Reference Keren1988). Sueto and Nakaishi (Reference Sueto and Nakaishi1992) also found that their CaMnt gel did not display time-dependent behavior over a wide salt concentration of between 0.001 M and 1.0 M NaCl. They used the flow hysteresis loop method to determine the time-dependent behavior. Abu-Jdayil (Reference Abu-Jdayil2011), however, reported a relatively weak time-dependent behavior for unpurified CaMnt gel at 6 wt.% solids as reflected by an increasing yield stress with ageing time. The yield stress was derived from constitutive models used to fit the flow data. This CaMnt, used in drilling mud formulation, has a relatively high Na₂O/CaO ratio of 0.8, suggesting the presence of NaMnt. The Na₂O and CaO contents were 3.215 wt.% and 3.986 wt.%, respectively. The experimental protocol employed is not ideal for a study of ageing. The structural state of the aged gel will have suffered damage during: (1) its transfer to the rheometer; and (2) the preshearing step adopted prior to each flow characterization. This could be the reason for the very short ageing time of 24 h reported to attain the fully recovered state. This is in contrast to an ageing time of months and years which was reported for three different thixotropic clay gels (Leong et al., Reference Leong, Liu, Clode and Liu2021). Leong et al. (Reference Leong, Liu, Clode and Liu2021) also discussed in detail the various methods for characterization of time-dependent behavior and why the results are usually not quantitatively comparable. One of the reasons is that the structural state of the gel at the start of the time-dependent experiment was not well defined. The sparse investigation of CaMnt gel rheology is due to its limited commercial applications (Choo et al., Reference Choo, Choi, Lee and Lee2020). One of the knowledge gaps in factors affecting the time-dependent rheology is the state or speciation of Ca²⁺ ions. At low pH, Ca²⁺ is fully hydrated as Ca(H₂O)₆ ²⁺ and this ionic species appeared not to adsorb. At alkaline pH it formed the hydrolysis product Ca(OH)⁺ which was adsorbed readily onto the particle surface, affecting the zeta potential and rheology of the suspension (Zhu et al., Reference Zhu, Avadiar and Leong2016). The strength of the platelet–platelet interactions should, therefore, be affected by the nature of the Ca²⁺ ions. This postulate was examined in the current study in which the time-dependent behavior of CaMnt (STx-1b) gels was evaluated as a function of pH and solids concentration. The pH chosen was such that the Ca²⁺ ions were in either the fully hydrated or the hydrolysis product form. The microstructures of these gels were also captured and their relationship with the time-dependent behavior discussed.

Leong et al. (Reference Leong, Du, Au, Clode and Liu2018) and Du et al (Reference Du, Liu, Clode and Leong2018) identified the conditions required for time-dependent behavior. These conditions are a 3-D network structure formed by heterogeneous charge attraction between negative faces and positive edges (plus contributions from van der Waals attractive force) and a strong face–face repulsion between platelets in the structure. The strong face–face electrostatic double layer (EDL) repulsion governs the development of the microstructure and is responsible for the time-dependent behavior. The EDL repulsive force causes the platelets to break apart, orientate, move, and reform stronger bonds to produce a stronger network structure with time. The structure moves progressively towards a lower free energy state, the driving force for the time-dependent behavior. The platelet charge properties, therefore, have an important effect. The pH was shown to have a pronounced effect on the thixotropic and ageing behavior of NaMnt and bentonite gel (Lee et al., Reference Lee, Chandra and Leong2012; Tombacz & Szekeres, Reference Tombacz and Szekeres2004). Low pH tended to produce a stronger effect, as it gives rise to a stronger edge–face attraction as a result of a higher positive charge density of the edge. Face–face repulsion remained strong as the negative layer charge was permanent. A stronger network structure thus forms with time. The effect of layer charge density on the gel time-dependent property is, however, unclear. The use of CaMnt will help provide a better understanding of this effect by exploiting the different adsorption capacity of Ca(H₂O)₆ ²⁺ and its hydrolysis product Ca(OH)⁺. The effects of these Ca²⁺ species on the time-dependent rheology of CaMnt gel should reveal definitive information on the effect of the layer charge.

Ice crystals were believed to have affected the microstructure of STx-1b CaMnt gels obtained by cryo-SEM imaging published by Au and Leong (Reference Au and Leong2016). These hexagonal crystals fractured the clay gel microstructure, producing large pores and channels (Au et al., Reference Au, Clode, Smart and Leong2015a; Au & Leong, Reference Au and Leong2016; Du et al., Reference Du, Liu, Clode and Leong2018; Leong et al., Reference Leong, Du, Au, Clode and Liu2018). The purpose of the present study, therefore, was to obtain artefact-free microstructures of STx-1b CaMnt gels by preventing ice-crystal formation by means of subjecting the gel samples to very high pressure and ultra-fast cryo-freezing (Du et al., Reference Du, Liu, Clode and Leong2018; Leong et al., Reference Leong, Du, Au, Clode and Liu2018; Wyss et al., Reference Wyss, Tervoort, Meier, Müller and Gauckler2004).

The microstructure and time-dependent properties of high-defect KGa-2 kaolin suspensions will be examined also for comparison purposes. The morphology and charge properties of kaolin particles are very different from CaMnt platelets. Yet, under certain conditions, both clay suspensions will be shown to display time-independent ageing behavior.

Results and Discussion

The ageing of 12.3 wt.% purified CaMnt gels at their natural pH of 8.4 and at  pH ~5 (4.5–5.3) displayed very dissimilar behaviors (Fig. 1). The gel at its natural pH did not exhibit time-dependent behavior; the yield stress remained essentially unchanged even after 4 days of ageing. The yield stress was close to 0 Pa. The lower pH gel, however, demonstrated pronounced ageing behavior. The yield-stress increase was rapid at the start of ageing. The rate of increase slowed considerably after the first day. The yield stress of 9 Pa after 1 min of ageing increased to 50 Pa after 18,787 min (13 days) of rest, a 5-fold increase. At pH 8.4, the Ca²⁺ ions existed as a positively charged hydrolysis product Ca(OH)⁺ (Baes & Mesmer, Reference Baes and Mesmer1986), according to the zeta potential-pH results obtained for alumina suspension at various CaCl₂ concentrations (Zhu et al., Reference Zhu, Avadiar and Leong2016). This hydrolysis product Ca(OH)⁺ adsorbed readily on the clay platelets, thus reducing the value of its negative face charge density, weakening the face–face repulsion. This condition was, thus, not conducive for the time-dependent behavior to occur. At pH < 6, all the Ca²⁺ ions were in the hydrated form with 6 to 12 water molecules per cation (Rudolph & Irmer, Reference Rudolph and Irmer2013; Teich-McGoldrick et al., Reference Teich-McGoldrick, Greathouse, Jové-Colón and Cygan2015; Zavitsas, Reference Zavitsas2005). These bulky hydrated ions were unable to adsorb strongly and would spend most of their time in the diffuse layer. The negative face–charge density increased, therefore, producing stronger face–face repulsion that could break bonds and move and orientate platelets to form a stronger structure.

Fig. 1 The ageing behavior of CaMnt gels at various pH and solids loading

The zeta potential provides information on the dominant charge determining the surface properties of a clay. It cannot be used as a measure of the strength of the particle–particle repulsive interaction because of the localized heterogeneous charge properties of the clay platelets. Both the layer or face and edge charges and their interactions with the diffuse layer ions contribute to the zeta potential. Adsorbed charged additives also affect this zeta potential.

The zeta potential data of 1 wt.% CaMnt suspension prepared from the 19.4 wt.% gel (Fig. 2) were all negative from pH 2.5 to 8 and showed a weak dependence on pH. The permanent negative charge in the octahedral sheet dominated the CaMnt net surface charge at all pH values. The magnitude of –25.9 mV at pH 7.6 and electrical conductivity of 0.4 mS/cm decreased to –25 mV at pH 2.6 and 1.24 mS/cm. The high proton mobility and concentration at pH 2.6 contributed to the high conductivity. The initial solution ionic strength was relatively low as reflected by a conductivity of 0.4–0.45 mS/cm, which is equivalent to a KCl concentration of only ~0.003 M . Addition of 0.1 g Ca²⁺/100 g CaMnt or 0.1 dwb% Ca²⁺ (dwb –dry weight basis) at pH 2.5 caused the pH to increase slightly to 2.6 and the negative zeta potential value to decrease to –22 mV, representing a 12% reduction. The corresponding conductivity was increased by 14% but this change was regarded as small for a 'normal' ionic strength-effect study where orders of magnitude change in salt concentration are normally employed. The smaller zeta potential was due to the effect of solution-based Ca(H₂O)₆ ²⁺ shielding the surface charge. Between pH 6 and 9 the zeta potential was essentially constant and so was the conductivity, 1.12–1.2 mS/cm. The positively charged hydrolysis product Ca(OH)⁺ began to form at pH ~6.5 and increased with pH. The constant zeta potential suggested its increased formation and adsorption being compensated exactly by a corresponding increase in the concentration of the edge pH-dependent charge sites acquiring more negative charges or becoming neutral. The relatively small effect of 0.1 dwb% Ca²⁺ on the zeta potential was due to an already high Ca²⁺ content existing as interlayer ions in the STx-1b. The Ca²⁺ content of STx-1 was 1.14 wt.%, or in terms of CaO, 1.59 wt.%, of the clay according to the source clay data. A 7 wt.% bentonite suspension in 1 M CaCl₂ (112 mS/cm) was reported to display a negative zeta potential of –18 mV at pH 7.2 (Au et al., Reference Au, Pillai and Leong2015b), which is another example showing the dominant influence of the negative layer charge on the platelet surface property. For suspensions with negligible Ca²⁺ present, the effect of Ca²⁺ on zeta potential at pH above 6.5 was quite pronounced such as in the case of alumina suspensions (Zhu et al., Reference Zhu, Avadiar and Leong2016). Ca²⁺ caused the trend of the zeta potential-pH behavior to deviate sharply at this pH.

Fig. 2 The zeta potential-pH behavior of STx-1b CaMnt suspensions with and without added CaCl₂ . The dosage of Ca²⁺ added was 1 dwb% (g Ca²⁺/100 g CaMnt solids)

The microstructure of the thixotropic 12.3 wt.% CaMnt gel at pH ~5 (Fig. 3) showed irregularly shaped, flexible, ~1 μm platelets with a curling feature, forming an open 3-D network structure. The platelets were well separated with most adopting an almost vertical orientation. The EDL repulsive force between these platelets in the face-face configuration was responsible for opening up the structure. Heterogeneous charge and van der Waals attraction between the face and edge formed the platelet–platelet bonds and network junctions. The structure showed pores being quite large in diameter and deep. The EDL repulsive force must be controlling the development of the 3-D structure and driving it towards a minimum free energy state (Du et al., Reference Du, Liu, Clode and Leong2018; Leong et al., Reference Leong, Du, Au, Clode and Liu2018). This state was achieved by the platelets undergoing processes of bond breaking, orientating, moving, and establishing stronger bonds. The strength of the structure reflects the free energy state of the gel. The stronger the structure, the lower its free energy.

Fig. 3 The microstructure of thixotropic 12.3 wt.% STx-1b CaMnt at pH 5.32 and conductivity of 0.97 mS cm^–1 aged for 3 months. This conductivity is equivalent to 0.007 M KCl

At the higher CaMnt concentration of 19.4 wt.%, the suspension also displayed time-dependent behavior (Fig. 1) despite its high pH of 8.0. The yield stress increased from 5.4 Pa at 1 min ageing to 240 Pa after 13 days. This was an enormous, 44-fold increase. Its corresponding microstructure (Fig. 4a) captured at the same magnification of 12.3 wt.% gel at pH ~5 showed an open structure with a much higher platelet density and smaller pores. Many of the platelets were interacting both attractively and repulsively in a near-vertical orientation forming the open structure (Fig. 4a, b). A schematic of the platelet–platelet interactions forming the open structure, viewed from the top is shown in Fig. 4c. The pore diameters were relatively small but appeared to be deep. The EDL repulsive force governed the development of the 3-D structure and drove it towards a minimum free energy state despite a lower negative charge density of the face, a result of partial neutralization by the positively charged Ca²⁺ hydrolysis products. The smaller face–face separation distance brought the platelets to within the operational range of the weakened EDL repulsive force.

Fig. 4 The microstructure of thixotropic 19.4% CaMnt gel at pH 8 with conductivity 0.77 mS/cm aged for 3 months imaged at different magnifications

The microstructure of an even more concentrated CaMnt suspension at 25 wt.% solids in Fig. 5a and at a higher magnification in Fig. 5b showed a relatively large concentration of interacting platelets adopting a flat-lying configuration. The solids volume fraction was only 0.12. Some of the pores or void spaces are highlighted by red crosses (Fig. 5b). Each platelet in the structure has more neighbors within the operational range of their repulsive and attractive forces. The configuration adopted is, thus, the result of the complex interactions of these forces with all their nearest neighbors. At high platelet concentration, adopting a vertical orientation was, thus, not possible for many platelets. This suspension displayed a yield stress of 56.3 Pa. Its pH and conductivity of 8 and 0.87 mS/cm were similar to that obtained for the 19.4 wt.% suspension. A few compact agglomerates can also be seen clearly in Fig. 5b. In contrast to the NaMnt platelets in Fig. 5c, the CaMnt platelets appeared to be thicker and more rigid. These CaMnt platelets were likely to be a few layers thick.

Fig. 5 The microstructure of 25 wt.% STx-1b CaMnt suspension imaged at a magnification of a 20 kX and b 50 kX, and c the microstructure of 4 wt.% NaMnt (SWy-2) at 80 kX

Despite the low solids volume fraction of 0.12, the high density of irregularly-shaped platelets, mostly ~1 μm in size (Fig. 5 and b), was due to platelets interacting at different depth levels being projected onto the 2-D image. The many flat-lying platelets interacted with others in all configurations, lying in flat and upright vertical positions at all angles, to form the 3-D structure. Curly and irregularly shaped edges enhanced the opportunity to form bonds and network junctions. Most pores were obscured by platelets lying flat but some of the observable deep ones are highlighted by red crosses in Fig. 5b.

In contrast, the microstructure of 4 wt.% NaMnt gel (Fig. 5c) showed thinner and more flexible platelets. These platelets were a single layer thick (Lagaly, Reference Lagaly, Bergaya, Theng and Lagaly2006). The strong face–face EDL repulsion, edge-face attraction, and high platelet concentration forming the network structure were responsible for the high yield stress and the pronounced time-dependent behavior displayed by the gel at a few weight percent solids (Leong et al., Reference Leong, Du, Au, Clode and Liu2018). It is thus not possible to produce a homogeneous, handleable NaMnt suspension at 25 wt.% solids as the yield stress will be infinite. In contrast, the layered CaMnt platelets with their reduced negative charge density caused by adsorbed Ca²⁺ hydrolysis products required a much higher concentration to  display appreciable yield stress and time-dependent behavior.

Kaolin particles in suspension also displayed weak face–face repulsion. The 38.5 wt.% KGa-2 kaolin suspension at pH 5.2 with a conductivity of 2.03 mS/cm (Fig. 6) did not display any time-dependent behavior after 1 day of ageing. The yield stress remained unchanged, 212 and 210 Pa at 3 min and 23 h of ageing. The microstructure and particle morphology of the KGa-2 suspension were very different from that found in the CaMnt gel. These differences in time-dependent, particle and microstructure properties were used to validate the theory on the mechanism or cause of immediate or undelayed thixotropy (Leong et al., Reference Leong, Du, Au, Clode and Liu2018, Reference Leong, Liu, Clode and Liu2021).

Fig. 6 Ageing behavior of high-defect KGa-2 kaolin suspension showing the absence of time-dependent behavior

The microstructure of a 26.5 wt.% or 12.2 vol.% kaolin suspension at pH 4.9 with a yield stress of 78 Pa (Fig. 7a and b) was formed by bulky, multilayered, irregularly shaped clay particles. The edges were jagged with platelets of various sizes bonded together in layers to form the particles. This increased the edge area relative to the face area compared to that of a single unattached platelet. The edge charge sites will also be high. For many of these particles, a few small bonded platelets formed the outermost face (red arrows in Fig. 7b). These particles with a small outer face area are not conducive to producing strong face–face repulsion. The kaolin has a small (positive) layer charge, 0.16 e per formula unit, located in the alumina octahedral sheet. The EDL repulsive interaction for all kaolin particles in the network is unlikely to be strong. An essential condition for time-dependent behavior is, therefore, not met.

Fig. 7 The microstructure of high-defect 26.5 wt.% KGa-2 kaolinite suspension imaged at a 30 kX and b 20 kX

Many studies have examined the time-dependent properties of clay gels between different  shear-dependent equilibrium states. One study employed a stepdown constant-stress approach in which the gel was sheared at a high constant stress until a constant shear rate was attained and then stepped down abruptly to a lower stress and the shear rate was monitored continuously (Coussot et al., Reference Coussot, Nguyen, Huynh and Bonn2002, Reference Coussot, Tabuteau, Chateau, Tocquer and Ovarlez2006). After stepdown, the shear rate decreased as the structure recovered to the equilibrium state of the applied constant stress. Sometimes the flow stopped, i.e. the shear rate decreased to zero. This occurred when the recovered structure had a yield stress greater than the applied shear stress. A similar approach can be used to determine whether a suspension is time dependent. This was the stepdown constant shear rate approach with the shear stress being monitored as a function of time (Leong et al., Reference Leong, Liu, Clode and Liu2021). The gel was subjected to a high constant shear rate until the equilibrium state was reached which was reflected by a constant shear stress response. After that the shear rate stepped down abruptly to a lower value. The high shear equilibrium structure will reorganize upon stepdown to produce a stronger equilibrium structure associated with the low shear rate. This process takes time and was reflected by an increasing shear stress commencing immediately upon stepdown. In the present study, the gel was sheared at a constant rate of 1000 s^–1 to an equilibrium state and then stepped down  to a lower rate of 10 s^–1. A 1° cone-plate viscometer was used so that everywhere in the gel in the gap experienced the same shear rate.

The stepdown shear-rate results for 19.4 wt.% CaMnt and 38.5 wt.% kaolin suspensions (Fig. 8) showed very different behaviors. The shear stress of the 19.4 wt% CaMnt gel displayed immediate increase upon stepdown to 10 s^-1. It was still increasing after 3 min of shearing. This is a reflection of the slow kinetics of the structural reorganization process. At 1000 s^–1, the equilibrium state comprised of many platelets aligned in the shear flow direction. The abrupt decrease in shear rate caused the structure to reorganize to the new equilibrium state. Platelet–platelet repulsion drives this reorganization. The immediate and sustained shear stress increase after stepdown showed that the CaMnt gel is very time-dependent. In contrast, the 38.5 wt.% kaolin suspension displayed no immediate increase in the shear stress after stepdown. Attractive force between the kaolin particles governed the structure formed.

Fig. 8 A stepdown shear-rate method for determining whether a suspension or gel is time-dependent

However, both CaMnt and kaolin suspensions displayed a decreasing shear stress or viscosity with time at 1000 s^–1 . The decrease was pronounced for the CaMnt gel and small for the kaolin suspension. By definition, both of these suspensions are thixotropic (Guven, Reference Guven, Guven and Pollastro1992), but the kaolin suspension was so weakly thixotropic that the structural recovery mode was not observed over the time span of the experiments. Upon stepdown to 10 s^–1, the CaMnt gel displayed immediate rheopectic behavior, an increasing shear stress or viscosity with time. All thixotropic suspensions displayed rheopectic behavior upon stepdown in shear. The gel needed to develop a stronger structure at low shear rate so that it was able to display thixotropic behavior upon shearing at a higher rate. The kaolin suspension just needed a longer time for the stress to grow to a significant value at the low shear rate. The weak EDL repulsion is the cause.