Effects of sodium carbonate addition, heat and agitation on swelling and rheological behavior of Ca-bentonite colloidal dispersions

Abstract

The effects of the addition of sodium carbonate (Na2CO3) and the subsequent simultaneous heating and stirring on the rheological behavior of Ca-bentonite colloidal dispersions have been investigated in this study. Ca-bentonite dispersions were treated with Na2CO3 at various ratios (2, 4, 12 g/100 g bentonite), and then subjected to heating and stirring for variable periods. It was found that the swelling and the viscosity of the treated bentonite samples increased with increasing Na2CO3 content and the optimal Na2CO3 level (relative to the bentonite content) was found to be 4 g/100 g of Ca-bentonite (4%). The results also showed that simultaneous heating and stirring enhanced the bentonite swelling and ion exchange processes for sodium activation. The rheological properties of the Ca-bentonite suspensions depend on the strength of the bentonite gel structure and the inter-particle interactions. Although the treated and untreated bentonite samples showed gel and solid-like behavior in the linear viscoelastic region (LVR), the region where the storage (G′) and loss (G″) moduli are independent of applied strain and/or stress, the G′ values of the Na2CO3 treated bentonite samples were higher than those of untreated samples. All bentonite samples (whether Na2CO3 treated or untreated) showed more elastic than viscous response (i.e., G′ > G″) within the LVR. The results showed that bentonite dispersions treated with 4% Na2CO3 displayed the highest elastic behavior. The relationship between zeta potential and pH for the untreated and Na2CO3 treated Ca-bentonite dispersions were investigated. Bentonite colloids were stable in the examined range of pH (3 − 12); such wide pH range stability is essential for many industrial applications. Finally, the effect of heat treatment on the swelling behavior of 4% Na2CO3 treated bentonite dispersions was extracted from particle size measurements using a laser scattering particle size analyser. The results showed that the particle size increased with the combined heating and stirring treatment for the first 3 h then remained constant. This increase in particle size is likely due to improved swelling and the existence of some of weak particle-particle interactions bentonite (aggregates).