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1. INTRODUCTION
The Portland cement (PC) clinker manufacture consumes about 2900 to 3300 MJ per ton of clinker (1), which is considered to heat the raw meal to a temperature exceeding 1450 °C that allows alite phase to form (2). Moreover, around 830-970 kg CO2 per ton (3, 4) are emitted of clinker in direct (5, 6, 7) and indirect ways (8, 9), which brings about enormous environmental footprint (6). In short, the cement industry is responsible for 5-7% of all anthropogenic emissions (10, 11) world wide. In order to attain sustainable development for manufacture of PC, there has been a revival of intensive interests in exploring high belite cement with low lime saturation factor (LSF), which leads to an increase in belite amount and a decrease in alite phase content in the clinker (12, 13, 14, 15, 16).
However, C2S, even with high activity form (17, 18), is less active than C3S, which has become the most significant limitation for the extensive application of belite cement (17, 19). Thus, many investigations were performed in an attempt to introduce another high early-strength mineral with low CaO such as C4A3$ (20, 21, 22, 23) or C2.75B1.25A3$ (2.75CaO•1.25BaO• 3Al2O3•SO3) (24, 25) into belite cement clinker system. Compared with C4A3$, C2.75B1.25A3$, it possesses much higher early-strength (26, 27). Consequently, belite-C2.75B1.25A3$ system, with the designed composition of 9.0% C2.75B1.25A3$, 75% silicate mineral and 16% intermediate phase (by weight, as following) (28, 29), has the potential to receive attention nowadays.
In belite-C2.75B1.25A3$ system, the formation of C2.75B1.25A3$ governed directly the early-strength of cement clinker. Consequently, it is necessary to reveal the influence of silicates and intermediate phases on the formation of C2.75B1.25A3$, which is not explored in previous work. On account of the belite as one of the main phases and the complication of phase composition for this new type of cement clinker system, the binary system of C