Calcium sulfoaluminate based cements
Knowledge of energy requirements, solid solutions and phase compatibilities of high temperature cement phases is essential to the economics and performance of cement clinkers. Both chemical engineering and chemical approaches have been used to determine phase compatibilities and solid solutions of calcium sulfoaluminate, C4A3 S, C3S and C2S as well as to develop a new type of cement based on calcium monosulfoaluminate, C4A3S, tricalcium silicate, C3S, and dicalcium silicate, C2S. Investigation of solid solutions, substitution mechanisms and phase compatibilities of C4A3S, C3S and C2S form the basis of this thesis, and the subject is addressed as follows: (i) Investigation of the structures, solid solutions, phase compatibilities, polymorphism and stabilisation of C4A3S (literature review, theoretical and experimental research). This research discloses the existence of a high temperature polymorph (a) of C4A3S. It can be stabilised to ambient temperature by Fe or Na-Si coupled substitution. Limits of Na, Fe, Si and P solubilities in the C4A3S structure were investigated. When limits of Na2O or NaF in C4A3S solid solutions (~3%) is exceeded, C4A3S becomes unstable as a result of Na2SO4 or Na2Ca(SO4)2 formation. C4A3S is compatible with C2S, with CaO, with C3A, with C12A7, with CA, with CA2, with Al2O3, with C2AS and with Ca3(PO4)2 at 1300 C. (ii) Investigation of the microstructure and microchemistry of commercial calcium sulfoaluminate cement (literature review, theoretical and experimental research). Because of extensive A1 and S substitution, a large Si deficiency can occur in C2S solid solutions. The open structured texture of calcium sulfoaluminate cement explains why it is easily ground. (iii) Investigation of A1 and sulfur substitution into C2S solid solutions (literature review, theoretical and experimental research). A1 and S substitute into C2S by several mechanisms, the most important of which are 2A1+S for 3Si and Ca+S for 2Ca+Si. (iv) Investigation of the NaF-CaO-SiO2 phase equilibria and its resultant effect on C3S formation (literature review, theoretical and experimental research). This investigation revealed that NaF not only acts as flux but mainly it has a mineralising effect on C3S formation and results in extensive solid solutions, NaxCa3.xSiO5.xFx. C3Sss, can be obtained as low as ~1000 C in the presence of NaF. In the course of this study, I phase, structurally similar to that of 9 C3S but consisting of 15 and 24 layer variants, has been characterised. (v) Investigation of the NaF-CaO-Al2O3-SiO2-SO3 system and its resultant effect on both C3S and C4A3S formation. Cement based on C3Sss and C4A3S can be made at ~1300 C if F and SO3 concentrations in the raw materials are controlled. SO3 in excess relative to that required for the formation of C4A3S should be added to inhibit C11A7.CaF2 and C3A formation. In the course of this study, large amount of Si deficiency in the C2S structure, such as that of belite in commercial calcium sulfoaluminate cement (as a result of 2A1+S for 3Si substitution), has been found.