| Summary: | The energy required to develop and process the high temperature phases essential to the performance of ordinary Portland cement (OPC) and to process the resulting clinker, represents a substantial fraction of the overall energy input. Both chemical engineering and chemical approaches have been used to reduce specific energy inputs. In this thesis the chemical engineering advances are reviewed, and the use of fluxes to reduce production costs is extensively investigated. Investigation of borate-assisted clinkering reactions forms the basis of this thesis, and the subject is addressed as follows: (i) Investigation of the strcutures, polymorphism and stabilisation of C3S and C2S (literature review, theoretical and experimental research). On the basis of this research, a structural model was proposed for alpha-C_2S and a novel transformation mechanism was proposed for the alpha'C2 Sphase transition. (ii) Review of the physical properties of the clinkering melt and its modification by fluxes or mineralisers (literature review). This review revealed that borates have potential as fluxing agents, because they reduce both the surface tensions and viscosity of the melt. (iii) Investigation of the CaO-SiO2-B2O3 phase equilibria and its resultant effect on clinker mineralogy (literature review and experimental research). This investigation revealed that borates not only act as fluxes but in addition they have a mineralising effect, which alters the C3S:C 2S ratio in favour of dicalcium silicate. The main conclusion of this thesis is that the use of borates as fluxing agents in ordinary Portland cement production is disadvantageous: borates rapidly reduce the amounts of the essential constituent tricalcium silicate, and can hence be classed as deleterious mineralisers.
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