Abstract
CaMnt is much less important than NaMnt due to its limited commercial applications. The time-dependent property of NaMnt gel has been studied extensively as it is exploited in many applications such as drilling mud and viscosity-modifier applications. In contrast, the time-dependent property of CaMnt suspension and the factors affecting it are largely unknown. The speciation of Ca2+ ions is one such factor to be evaluated. In the current study, pH and solids concentration were examined and then used to validate a recent theory on clay gel time dependency. The results supported the theory that a strong electrostatic double layer (EDL) repulsion in the 3-D network is needed to reorganize the structure and drive it toward the state of minimum free energy. The 12 wt.% CaMnt (STx-1b) gel displayed time-dependent behavior at pH 5 but not at its natural pH of 8.4. At pH 5, the interlayer Ca2+ ions became fully hydrated and desorbed from the platelet surface. This enhanced the EDL repulsion between the platelets. At pH >6.5, the positively charged hydrolysis product Ca(OH)+ began to form and was adsorbed, partially neutralizing the permanent layer charge and weakening the repulsion. However, at 19.4 wt.% solids, the gel displayed pronounced time-dependent behavior despite the high natural pH of 8. The platelets were much closer together, allowing the EDL force to operate and effect structural reorganization. The microstructure of these CaMnt gels showed high platelet concentrations interacting to form a relatively open structure. The microstructure of a kaolin (KGa-2) suspension which showed no time-dependent behavior even after 1 day of ageing revealed the importance of particle morphology and layer charge on time dependency. Its 3-D structure was formed by relatively thick, layered platelets with a low layer charge.
Highlights
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
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 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
Summary
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. The ageing or gelation behavior of bentonite and sodium montmorillonite (NaMnt) have been examined extensively (Broughton & Squires, 1936; Callaghan & Ottewill, 1974; Hauser & Reed, 1937; M’Ewen & Pratt, 1957; van Olphen, 1955), 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. 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. 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
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