Abstract: In the high-level radioactive waste repository,excavation process would affect the surrounding rock and result in cracks,which will inevitably develop into preferential infiltration channels for groundwater and affect the barrier efficiency of compacted bentonite.In light of this,the usage of alkali activated metakaolin-based polymer as grouting material to fill cracks was proposed in this study.Unconfined compressive strength tests were conducted to understand the parameters such as alkali activator composition,liquid to solid ratio,curing condition,and curing period on the mechanical properties and fracture evolution of geopolymer buffer materials,and the methods to avoid the formation of cracks was proposed accordingly.The results show that high temperature,low humidity and alkaline environment were beneficial to the strength formation of metakaolin-based geopolymer,however,these conditions also tend to induce cracks with the geopolymer samples.The formation of geopolymer cracks can be suppressed by reducing the alkali content (i.e.,n(SiO₂)/n(Al₂O₃)=2.4,n(Na₂O)/n(Al₂O₃)=0.4),changing the curing conditions (i.e.,temperature=85℃,relative humidity=70%),and improving the heating regime (i.e.,temperature increasing rate of 10 ℃/24h).The unique 3D network structure of geopolymers not only has a bonding/filling effect,but also acts as a micro-reinforcement to enhance the cementation between particles,thus exhibiting strong crack resistance.Based on the mercury intrusion porosimetry tests,the crack inhibition mechanism of alkali-activated metakaolin-based polymers was revealed from micropore structure the perspective of.The results can provide a guidance for the selection of grouting materials in surrounding rock of the repositories in China.