COMPACTION A N D SWELLING OF Ca-SMECTITE IN WATER A N D IN CaC12 SOLUTIONS: WATER ACTIVITY MEASUREMENTS A N D MATRIX RESISTANCE TO COMPACTION

Direct water-activity measurements were made for calcium-smectite-CaCl2 mixtures. The samples used were prepared by high-pressure filtration and had water contents <0.60% by weight of clay. For deionized water, the water activity (aw) ranged between 1 and 0.4 and was independent of the pressure history of the clay cake. Good agreement was found with the results of an adsorption isotherm. Samples were also prepared in l, 3, and 5 M CaCl~ solutions. The final concentration of chloride present in the cakes was measured: substantial anion exclusion was observed. The water activity of the clay-salt mixtures depended on both the water content and the concentration of CaCl2. The data can be well represented by the expression aw = min(awo, aw* x aw*), where awo is the water activity of the brine used to prepare the cake, and aw* and aw~, are water activities calculated for clay (in absence of salt) and salt (in absence of clay), respectively. The compaction and swelling behavior of a core of the same Ca-smectite was also investigated for pressures (P) between 0.5 and 800 bar. In two compaction-swelling cycles total recovery of the void ratio (e) was observed with, however, large hysteresis in the relationship between e and P. The osmotic pressure developed in the core at equilibrium, evaluated using the water-activity data, ,was intermediate between the compaction and swelling pressures. Apparently, another force, Pro, linked to irreversible changes in the structure of the matrix was contributing to the resistance to compaction. Pm values agreed with literature results for Ca-smectite suspensions. Key Words--Calcium chloride, Compaction, Matrix resistance, Rheology, Smectite, Swelling, Water activity. I N T R O D U C T I O N Smectite clays have been widely studied for their ability to disperse and form stable colloidal suspensions (see reviews by van Olphen, 1977; Hunter, 1987). Whereas this property can be advantageous in some industrial applications (e.g., drilling mud technology), the uptake of water by clay rocks and their subsequent swelling is a major problem in soil engineering, tunnelling operations, and oil well drilling. The long-range interactions between clay particles (typically for separations >50 A) has been the subject of numerous studies. The DLVO theory gives a quantitative interpretation of the swelling of Na-smectite, but the hysteresis observed between the first compaction and the rebound indicates that other forces are involved in the behavior of clay gels (Warkentin et aL, 1957). For alkali ions, agreement between swelling results and theory decreases with the polarizability and the hydration energy of the counterion (Lubetkin et aL, 1984): this can be explained by a surface adsorption mechanism (Pashley, 198 t); however, poor agreement has been found between experiment and theory for Ca-smectite (Warkentin et aL, 1957; Mesri and Olson, 1971), and modification of the DLVO to account for the experimental observations is an active research field (see, e.g. Kjellander and Mar~elja, 1988; Kjellander et aL, 1988; Attard et aL, 1988). The short-range interaction between clay particles has also been widely studied, van Copyright 9 1991, The Clay Minerals Society 35 Olphen (1954) discussed interlayer forces in bentonite for small interlayer separations and showed that hydration effects become important, leading to deviation from the D L V O theory. X-ray powder diffraction measurements indicate that the interlayer separations of smectite change in a stepwise manner at low relative humidity (Glaeser and Mrring, 1968), a phenomenon known as crystalline swelling. Direct forces measurements using the method developed by Israelachvili and Adams (1978) have also been used to investigate short range interactions between single sheets of mica. Comparison of experimental results for mica surfaces to the DLVO theory shows that the expected mi n i mum in the interaction potential is not observed for electrolyte concentration higher than a threshold (Israelachvi/i and Adams, 1978; Pashley, i 981). In this regime, hydration and solvent ordering are predominant, and the force between two interacting mica sheets varies in an oscillatory manner with separation (Pashley and Israelachvili, 1984). Thus, DLVO theory cannot describe crystalline swelling, and although significant progress has been made in understanding the phenomenon at a microscopic scale, the macroscopic properties of clay material have received less attention, Short-range interactions are of a paramont importance to the study of the properties of most shales and clay rocks, inasmuch as their water content is such that crystalline swelling is initially predominant. To give a better insight into 36 Denis Clays and Clay Minerals