<p>To elucidate the microstructure evolution of the compacted Gaomiaozi (GMZ) bentonite under different alkaline environments induced by cement degradation in Chinese high-level radioactive waste (HLW) repository conditions, the mercury intrusion porosimetry (MIP), scanning electron microscope coupled with energy-dispersive spectrometry (SEM–EDS) and X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms (NAI) tests were performed to determine its microstructure and mineralogy characterizations, etc. The permeability, saturated water content and particle density of GMZ samples were also investigated to analyse the macro-performance of bentonite. Results show that during the infiltration of alkaline cement waters, the GMZ bentonite displays markedly different pore evolution behaviours under the two alkaline conditions, indicating distinct underlying mechanisms. Illite is found in the bentonite after infiltrated by young cement water (YCW: K–Na–OH type); when increasing temperature, the illitization will be accolated. The formation of secondary minerals, including C–S–H, calcite and portlandite, was attributed to porosity changes in bentonite following its reaction after saturated by evolved cement water (ECW: Ca–OH type). The water content of GMZ bentonite decreases and the particle density increases with time identifying mineralogy alteration. The interlayer swelling volume of bentonite was calculated based on a novel method combining multiple microscopic techniques, which confirms that the layer swelling of bentonite was inhibited by increasing temperature. The hydraulic conductivity determined by MIP results is larger than the experimental ones. Additionally, the accuracy of MIP in characterizing bentonite porosity was evaluated. The results indicate that MIP maintained a measurement accuracy exceeding 80% in compacted GMZ bentonite after two months of reaction with alkaline solutions. However, prolonged reaction time led to a gradual decline in the reliability of the porosity analysis.</p>