Abstract: The composite cementitious materials were prepared using molybdenum tailings for enhancing the utilization of solid waste resources.In order to solve the environmental and safety problems caused by tailings accumulation，and provide the corresponding theoretical basis，the research focuses on effects of grinding time and contents of molybdenum tailings on properties of cementitious materials and hydration mechanism of composite cementitious materials were conducted by means of particle size analysis， mechanical properties testing， X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicate that: ① when the grinding time was 80 min and the specific surface area was 500 m2/kg， the 28 d activity index of the molybdenum tailings was close to 1.2. The mortar block of 28 d compressive strength is about 52 MPa when cementitious materials is consisting of 40% molybdenum tailings， and the mass ratio of cementitious materials to standard sand is 1∶3 with the water-cement ratio of 0.5. ② The hydration products of composite cementitious materials are calcium silicate hydrate and ettringite at early stage， which provided the early strength for the mortar block strength. The main products in the later hydration reaction are C—S—H gels， calcium aluminate hydrate and ettringite （AFt）， and mortar block strength is provided by the aggregation effect of residual tailings particles and hydration products.

Key words: molybdenum tailings, composite cementitious materials, grinding time, addition, ettringite