Abstract: To overcome the poor mechanical strength of copper tailing powders- Portland cement, diatomite was added into this cementitious system.Diatomite replaced the copper tailing powders- Portland cement with 0.5%, 1.0%, 1.5% and 2.0% in weight.The fluidity, setting time, water absorption, porosity, bulk density, mechanical properties and microscopic properties of the composite cement materials were tested. The results showed that with the increase of the dosage of diatomite, the fluidities of the pastes decrease, the setting time shortens, water absorptions and porosities decrease, bulk densities increase, compressive strength and flexural strength of the mortars increase. After curing for 28 days, the addition of diatomite makes the SiO₂ and Ca(OH)₂ content in the hydration products decrease, the C-S-H (Calcium Silicate Hydrate) gels increase, and the whole is denser.