Determining the extent of bentonite alteration at the bentonite/cement interface

Abstract Bentonite is investigated as a geotechnical barrier material in repositories for high-level radioactive waste (HLRW). One of the most important research topics concerns the stability of bentonite at interfaces such as bentonite/metal or bentonite/cement. Alkaline cements are particularly hazardous because they may dissolve at least part of the bentonite. This reaction is, therefore, investigated extensively. Bentonite properties and performances in different applications vary from one deposit to another and even within some deposits. The hypothesis, therefore, was, that the stability of different bentonites in contact with cement could be different. Hence, the aim of the study was to compare the reactivity of a significant set of well characterized bentonites with Portland cement and to understand the reason for the differences. For bentonite comparison standard reaction parameters were fixed: 3 months, 80 °C, 25% cement (or 33%). The resulting degree of degradation was compared with basic bentonite parameters. A fairly good correlation (R2 = 0.7) of the extent of alteration with the amount of soda soluble silica (reactive silica which to variable amounts is present in natural bentonites) was obtained which proved, that soda soluble silica was able to buffer the reaction. Such bentonites with a higher amount of soda soluble silica showed less smectite degradation and hence will perform better at the cement/bentonite interface than others with less soda soluble silica. Varying the clay/cement ratio showed that smectite degradation requires 10–20% cement. No reaction was observed below this value. Linear curves were observed which allow to conclude that 1 g cement is able to degrade about 0.6 g of bentonite (smectite) at the selected solution/solid ratios. The reaction was almost complete after 1 month and no difference was observed between 60 and 80 °C.