Abstract: The construction of a geological repository for high radioactive waste requires a large amount of concrete. A large amount of heat will be released because of the decay of radionuclide, which will accelerate the aging of cement, releasing a plenty of alkaline pore water which has a negative impact on the buffer performance of bentonite. The Gaomiaozi (GMZ) Ca-bentonite is used as the experimental material and NaOH solution is selected to simulate the alkaline pore water. Swelling pressure experiments of bentonite with different dry density saturated in NaOH solution with different concentration are carried out by self-developed corrosion resistant consolidometer. The results show that the final swelling pressure of bentonite which is saturated in the same solution increases with the initial dry density of bentonite. Under the same initial dry density of bentonite, ,the low concentration of NaOH solution can strengthen the swelling pressure of bentonite, while high concentration of NaOH solution can weaken the swelling pressure of bentonite. The bentonite samples saturated in three kinds of NaOH solution (i.e., 0.3, 0.5 and 1.0 mol/L s), the evolution curves of swelling pressure decrease after to a maximum value. The decrease degree of the swelling pressure relates with the solution concentration but not with the initial dry density of bentonite. The XRD analysis on the mineral composition and interlayer spacing show that the content of montmorillonite and cristobalite decrease with the increase of alkaline solution concentration, while the content of feldspar increases with the increase of alkaline solution concentration. The exchange of Na will occur in the Ca-bentonite soaked in NaOH solution and the degree of Na-modification is related to the alkaline solution concentration. Finally, the relationship between the montmorillonite void ratio and the final swelling pressure of the bentonite sample is linearly fitted in a double logarithmic coordinate system, and the expression for predicting the swelling pressure of Ca-bentonite saturated in alkaline solution is given.

Key words: Gaomiaozi(GMZ) bentonite, alkaline pore water, swelling characteristics, void ratio of montmorillonite, prediction