RHEOLOGY OF SODIUM AND POTASSIUM ILLITE SUSPENSIONS IN RELATION TO COLLOIDAL STABILITY

To further understand the colloidal behavior of clay minerals in soils , rheological properties of illite suspensions were investigated and r elated to colloidal stability. Using a concentric cylinder viscometer to measure shear stress as a function of shear rate, Bingham yield str esses (tau(B)) and plastic viscosities (eta(pl)) were determined for h omoionic Na- and K-saturated illite suspensions varying in pH, NaClO4 or KClO4 concentration, and suspended solids concentration. For a give n Na or K concentration, tau(B) typically decreased with increasing pH . At a given pH >5.5, both tau(B) and eta(pl) increased with increasin g electrolyte concentration; but for Na-illite, constant values of tau (B) and eta(pl) were observed at higher electrolyte concentrations. Th e Na or K concentration yielding the greatest increase in tau(B) or et a(pl) per unit increase in concentration was usually less than, but co rrelated with, published critical coagulation concentrations (CCCs) of Na- or K-illite. With increasing solids concentration at pH 7, tau(B) increased curvilinearly for unstable suspensions and linearly for sta ble suspensions. In light of published models of non-Newtonian flow, d ata for unstable suspensions at pH 7 suggested that interparticle bond ing energy or the number times energy of bonds within flocs was greate r for K-illite in 25 mol K m-3 than for Na-illite in 60 mol Na m-3.