Research On Carbonation Hardening Low Calcium Silicate Cementitious Materials

In order to reduce the energy consumption of cement and reduce the emission of CO₂,a low-calcium cementitious material with C₃S₂ andα-CS as main minerals was prepared.The preparation of the cementitious material,the formulation index,the mineral composition and the relationship between calcination temperature and mechanical properties were studied.The microstructure changes and the type of carbonation production of the cementitious materials before and after carbonation were studied.The sintering of C₃S₂ andα-CS were carried out respectively,which carbonation progress and carbonation kinetics was researched.The results are as follows:1.The Gibbs free energies of C₃S,C₂S,C₃S₂ and CS at different temperatures were calculated according to thermodynamic laws when the molar ratios of CaCO3 and SiO2were 1:1 and 3:2,respectively.Determine the minerals stability of the calcination process.During the calcination ofα-CS minerals,the formation ofα-CS is accompanied by the formation of C₂S and C₃S₂,and the final product isα-CS.During the calcination of C₃S₂ minerals,the formation and decomposition of C₃S₂ is accompanied,the optimum temperature for C₃S₂ is 1430℃.The trend of mineral formation at different temperature stages is consistent with the thermodynamic calculation results.2.The molding pressure has a great influence on the compressive strength and CO₂uptake ofα-CS,4 MPa is the optimum molding pressure;The CO₂ uptake reached13.16%and giving a compressive strength of 93.5 MPa after 24 h carbonation in 99.9%CO₂ concentration,the carbonation products ofα-CS are calcite,vaterite and SiO₂ gel.The CO₂ uptake of C₃S₂ after carbonation for 24 h reached 15.57%,and the compressive strength reached 74.8 MPa,the carbonation product is calcite and SiO₂.The carbonation kinetics equation for C₃S₂ andα-CS minerals is derived from the Jander diffusion model:（?）the values of K and C are related to the mineral.3.The phase range of the low-calcium cementitious material was determined by considering the calcination temperature,mechanical properties.The raw material ratio index of low calcium cementitious material,the alkalinity coefficient（Cm）,was designed,and its value range was 0.681≤Cm≤0.798.It is proved that the cementitious material was calcined within the designed alkalinity coefficient,and the sintering temperature was 1300℃.The size of paste block 20 mm×20 mm×20 mm was prepared,and after 24 h of carbonation,the compressive strength exceeds 80.0 MPa,showing excellent mechanical properties.