Surface chemistry, rheology and microstructure of as-received SHCa-1 hectorite gels

The zeta potential and yield-stress behaviour of gels prepared from as-received SHCa-1 hectorite (Ht) containing 50 wt.% impurities, mainly of calcite, dolomite and quartz, were characterized. The zeta potential is negative in the pH range 4-12. At pH <5, reaction of CaCO3 producing CO2 and Ca2+ increased the solution conductivity significantly and reduced the zeta potential. The buffering effect due to this chemical reaction was also observed in the yield stress-pH characterization below pH 8. Low yield stress was observed below pH 6 and above pH 12. The yield stress peaked at pH similar to 8, where Ht displayed a zeta potential of -20 mV. The relationship between maximum yield stress and Ht content (wt.%) was linear and was not affected by impurities. The ageing behaviour was characterized by an initial phase of rapid yield-stress increase and then by a phase of gradual increase. The Leong model performed better than the two-parameter glassy polymer ageing model in describing this ageing behaviour. However, the empirical power law model was more accurate. The timescale of the ageing process is long, lasting several days. An open cellular microstructure was observed to form in the gels. Impurities appeared not to interact with or bind to the Ht platelets.