Effect of coal-gangue on chemical stability of cement mortar exposed to seawater was investigated compared with granulated blast furnace slag and fly ash by soaking cement mortar in artificial seawater.Results show that coal-gangue can remarkedly increase chemical resistance of cement mortar to seawater,but the effect is not better than that of granulated blast furnace slag or fly ash.The reason why admixture can increase chemical resistance of cement-based material to seawater is that it can reduce pore size of cement-based material.