In order to realize the high-value utilization of coal gangue, the thermal activation parameters of kaolinite in coal gangue were optimized by orthogonal test, and the compressive strength and microstructure of paste consisting of cement and metakaolin formed under the optimal activation conditions was studied. The experimental results showed that the optimum thermal activation conditions of metakaolin are calcination temperature 800℃, holding time 2h and grinding fineness 2.82μm. Calcination temperature has the greatest influence on the activity

of metakaolin, followed by grinding fineness and holding time. Using metakaolin instead of 20%wt cement can effectively improve the compressive strength of cement paste at all curing ages, especially in the early age. XRD and SEM test results showed that the pozzolanic reaction of metakaolin consumes a lot of calcium hydroxide, and forms calcium silicate hydrate with lower calcium-silicon ratio and more dense structure.