Study On High Belite-sulphoaluminate Cement

During the process of clinker production, roughly65%of the CO2emission is from the decomposition of limestone, and only around35%is from the combustion of fuels. Some levers were used to reduce the CO2emission, including increasing energy efficiency by optimizing processes, and increasing cementitious additions into the cement, etc. In terms of the fact that most of the emission is from limestone, there is less potential for conventional levers to mitigate CO2. Research on new cementitious binder has become a global focus.According to low calcium and low carbon design on clinker minerals composition, a type of C2S-C4A3S-C4AF clinker system (BCSA) with low energy consumption and low emission is developed. High energy consumption and high emission mineral C3S is replaced by low energy consumption and low emission minerals, belite (C2S) and sulphoaluminate (C4A3S). Through optimizing and activating the clinker minerals, the cement made of BCSA clinker can attain better properties with much less emission than OPC. Compared with OPC, BCSA cement consumes28%less limestone and16%less coal, and generates20%less CO2.Many aspects of the BCSA clinker are studied through the tests of XRD, Infrared, SEM, etc. in this paper. The following main parts are researched in this paper:(1) Low calcium design of clinker and clinkering of BCSAThe new clinker system is based on low calcium and low carbon minerals principle. C4A3S and C2S are providing the early strength and later strength of the clinker respectively in the C2S-C4A3S-C4AF clinker system.The clinkering temperature ranges of the minerals are studied. Considering the formation of well crystallized β-C2S and C4A3S, as well as the nucleation quantity of crystals, the optimum temperature range is1280～1320℃to ensure better properties.(2) Optimization of clinker mineral composition and clinker-gypsum reaction mechanismIt is defined that the ideal range for mineral composition of BCSA clinker is C2S36-56%, C4A3S32-42%and C4AF5-9%. The clinker can reach34.3MPa and55.1MPa respectively at3days and28days.The reaction mechanism between BCSA clinker and gypsum is studied. It is showed that the best range for gypsum content is12.5-15%. Gypsum reacts with C4A3S to form ettringite, and then the ettringite forms a compacted network with other hydrates.(3) Activating technologies for BCSA clinker minerals through ion dopingLow hydration rate of β-C2S and low early strength has been bottleneck of the effect of C2S in the clinker. The measures to increase the reactivity of P-C2S and its mechanism are studied. It is shown that foreign ions of Ba2+, P5+and Zn2+have effects on stabilization of β-C2S and activation of both β-C2S and C4A3S. Distortion defects of the lattice are occurred by the foreign ions via entering into the lattice of the β-C2S crystal structure, thus to prevent the transformation from β-C2S to γ-C2S, and accelerate the nucleation of P-C2S. In addition, the foreign ions can easily concentrate on crystal boundary and prevent the growth of crystal grains, eventually enhance the reactivity of P-C2S. Further study shows that the foreign ions enter into the lattice of C4A3S and cause distortion, as a result, the reactivity of C4A3S is enhanced as well. The sequence of combined effects of different ions on the reactivity of β-C2S and C4A3S:Ba2+>P5+>Zn2+>B3+.The compressive strength of cement from Ba2+doped clinker can reach69.7MPa at28d, nearly15MPa higher than reference one.(4) Key technology for trial production of BCSA cement on a line with NSP processBCSA cement is produced using existing NSP process and equipment, conventional raw materials and fuel. It was the first time in the world to produce BCSA cement on a real line. Main parameters for production are researched during the trial production. The compressive strength of BCSA cement produced can reach43.7MPa at3d and66.9MPa at28d.(5) Mechanical properties and durability of BCSA concreteIt is showed that BCSA-30concrete has good mechanical properties. And BCSA-30concrete has higher durability than OPC concrete in terms of frost resistance, sulphate resistance, drying shrinkage, etc. That is mainly due to the lower water demand, lower porosity, micro-expansion of ettringite and much denser hydration products.