The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite

Textural properties of homoionic Na- and Ca-bentonite (MX-80 from Wyoming) were studied by nitrogen adsorption. Bulk samples, which contained 20% of nonclay minerals and purified samples, were taken for the study. Powder samples and compressed pellets, which were prepared by applying a uniaxial stress of 57 MPa, were examined. Presence of nonclay minerals in the bulk samples affects significantly textural properties of the system. In particular, nonclay minerals prevented full compression of the bulk samples. In comparison to Na-saturated purified samples, higher values of surface area and total pore volume were detected in Ca-saturated samples. In addition, amount of the trapped pores was considerably higher in Ca-saturated samples than that in Na-saturated ones. In Ca-saturated samples, values of micropore surface area and micropore volume were very low, or even equal to zero. In Na-saturated samples, on the other hand, considerable percentage of surface area was associated with micropores. Surface area and pore volume of the sample decreased following heating at 200 °C during 24 h, with no difference to the exchanged cation. Following compression, surface areas of Na-saturated samples decreased and those of Ca-saturated samples increased. With no differences to the exchanged cation, considerable reduction in pore volumes was observed for the purified samples following compression. In the case of the compressed nonheated samples, hydration at relative humidity of 100% during 2 days resulted in significant increase of total and micropore surface areas. The increase in surface area was contributed mainly by micropores, i.e., open surface areas of the samples increased only slightly. Behavior of heated Na-saturated sample was similar to that of nonheated one. Hydration capacity of Ca-saturated sample, however, diminished considerably after heating.