Because of groundwater dissolution, the lining concrete of HLW geological repository release a “cement solution”, and its chemical composition undergoes a four-stage evolution process. A young cement solution standing for I-stage cement solution was mixed with Gaomiaozi (GMZ) bentonite powder, as suspensions to prompt static solid-liquid reaction in a batch style for evaluation of long-term chemical stability of the bentonite buffer material as used in China. During 256 d test, suspensions were harvested in different time intervals and separated into solid and liquid phases. The liquid phase were analyzed for pH, electrical conductivity and cation concentration. The solid phase was characterized by means of X-ray diffraction, Fourier transform infrared spectra and scanning electron microscope-energy spectrum. The analysis results of solid and liquid indicated both the dissolution of montmorillonite mineral and the formation of mixed layer illite-smectite occur in GMZ bentonite at the same time under the action of young cement solution. This tends to reduce the buffer performance of the GMZ bentonite.