INVESTIGATION ON THE SWELLING PROPERTIES AND MICROSTRUC-TURE MECHANISM OF COMPACTED GAOMIAOZI BENTONITE

Bentonite has the property of swelling when meeting water. It is a desirable buffer/backfill material in the deep geological disposal for high-level radioactive waste. The expansibility is one of the most important properties for bentonite as the buffer/backfill material, and is influenced by a number of factors. For study the expansibility of bentonite, Gaomiaozi(GMZ)bentonite was taken as the research object. Gaomiaozi(GMZ)bentonite had been proposed as the first choice of buffer/backfill material for the high-level radioactive waste disposal in China. Its expansibility was studied by the constant volume swelling test method which is one of the commonest methods for measuring the swelling pressure of bentonite. In these tests, water content and dry density were chosen as the control variable. Two types of dry density and three types of water content were adopted in the tests. Results of the swelling tests show that the shape of swelling curves and the maximum swelling pressure depend on the water content and dry density of the bentonite samples. There are conspicuous double-peak shapes of the swelling curve for the samples with low dry density. When the dry density is high, the swelling curves have different shapes with different water contents. The curve's shapes change from double-peak to smooth curve with the water content increasing. For analysis of the results of swelling tests, the mercury intrusion porosimetry(MIP)test has been carried out. MIP test results indicate that the pore size distribution curves of samples also depend on the water content and dry density, with the volume of inter-aggregate pores increasing as the water content or dry density decreases. In accordance with the relevant researches, the swelling curve of GMZ bentonite is deeply influenced by the volume of the inter-aggregate pores. When meeting water, the bentonite aggregates absorb water and swell quickly. When the inter-aggregate pores are large enough, there will be sufficient space for swelled aggregates to form a provisional structure. The provisional structure will collapse with the swelling pressure reach the limit load. Then the measured pressure fall and inner structure of bentonite recombination. The hydration is continuous so that the curve will get the second peak. Therefore, a double-peak structure can be observed when the space of inter-aggregate is great. With the volume of inter-aggregate pore decrease, the swell curve of bentonite changes from a double-peak structure to a smooth curve.