In order to understand the influence of calcined trass on the cement hydrates， the mineral composition change of trass calcined at 700 ℃ for different time and its influence on hydrates were studied by means of X-ray diffraction（XRD）， thermogravimetric analysis（TG）， Fourier infrared spectroscopy（FTIR）， et al. The results show that the zeolite water， structural water and adsorbed water in the trass are removed after calcination at 700 ℃， and the framework structure of zeolite minerals such as clinoptilolite is destroyed， forming amorphous SiO₂ and Al₂O₃. Calcined trass contributes to the formation of calcium silicate hydrates（C-S-H）， ettringite（AFt） and single sulfur hydrated calcium sulfoaluminate （AFm） in cement， the reduction of the calcium-silicate ratio（n（Ca）/n（Si））of C-S-H， the consumption of Ca（OH）₂， and cement hardened paste is more readily to carbonation. After 28 days of hydration， the porosity of cement hardened paste added with calcined trass reduces， and the pore size distribution is finer， thus the cement strength is improved.