Bentonite has been adopted as a buffer/sealing material in most high-level radioactive waste disposal programs. After emplacement in the repository, bentonite will undergo alkaline chemical effects due to cement degradation, changing its hydro-mechanical behaviour. In this study, the swelling pressure of compacted GMZ bentonite at an initial dry density of 1.7 Mg/m3 was investigated with two synthetic cement solutions (Young Cement Water -YCW and Evolved Cement Water -ECW) at temperatures of 20 and 60 °C. After swelling tests environmental scanning electron microscope (ESEM), mercury intrusion porosimetry (MIP) and X-Ray diffractometry (XRD) tests were also performed to analyse the morphology, microstructure and mineralogy characteristics of the specimens. Results show that the swelling property of bentonite would degrade with time under the in-situ pore water and temperature conditions of repository. With cement solutions injection, a mass of macro-pores in compacted bentonite transforms into micro-pores, especially, in case of YCW with higher temperature. Cation exchanges, smectite dissolution and secondary minerals formation (some zeolites and C-S-H gels) are found to occur in compacted GMZ bentonite. Increasing temperature accelerates the smectite dissolution rate. The dissolution of montmorillonite and the formation of secondary minerals should be the fundamental reasons for the swelling pressure decline, and then microstructure change.