The mechanical properties of cement matrix composites can be severely affected at low temperatures, limiting the environment in which they can be used. To ameliorate this problem, the addition of silica fume and fly ash to the matrix is an effective method. This study used silica fume and flew ash as composite admixtures mixed with Portland cement to characterize the heat of hydration, mechanical properties, and microstructure of the composite cementitious materials. The test results show that the particle size distribution of fly ash and silica fume can influence the hydration reaction rate and the whole hydration process. By adjusting the particle size of the admixtures, the structural loosening and strength attenuation of the concrete caused by low temperatures can be improved. At low temperatures, when cement hydration is low, the coarser the particle size, the better the gradation between silica fume and fly ash and the better the filling of the pores. In addition, the specific surface area of silica fume was fixed at 19.1m²/g, and the correlation between fly ash particle size intervals in mineral admixtures was strongest in (30~40μm), (40~50μm) on the total porosity. Silica fume in the (>0.4μm) and (0.25~0.4μm) particle size intervals had the strongest correlation with porosity. Finally, a prediction model between the particle size interval of the mineral admixture and the total porosity of the concrete under standard and low temperature curing conditions was established.