Replacing cement in backfill materials with cementitious solid waste can reduce mine backfilling costs and mitigate environmental pollution.However,the cementitious properties of solid waste are influenced by various factors,among which particle size is a crucial one.This study conducted experimental research on backfill materials using  solid waste with different particle size ranges,using uniaxial compressive strength as the evaluation index to explore the impact mechanism.Microscopic structure analysis was performed to understand the underlying mechanism.Results show that as particle size decreases,the uniaxial compressive strength of the filling body increases at different curing ages,enhancing the cementitious properties of the solid waste.Notably,the uniaxial compressive strength of filling body increases significantly at later stages.Microscopic analysis reveals the formation of a large number of hydration products in the later stages,which are similar to those of cement hydration products.