Preparation of belite cement from stockpiled high-carbon fly ash using granule-hydrothermal synthesis method

Belite cement was prepared from lime and long-term stockpiled high-carbon fly ash by granule-hydrothermal synthesis at 97 °C followed by calcination at 800-1000 °C. The effects of the lime/fly ash ratio, dosage of Na2O and calcination temperature on the phase compositions of the synthesized precursors and clinkers, and the properties of the cements were investigated. Belite cements with the flexural and compressive strengths at 28-day higher then 6.1 MPa and 32.8 MPa, respectively, were prepared. The results show that the quartz and mullite in the fly ash basically do not take part in the reactions. Adding 0.5-1.5% Na2O promotes the hydrothermal reaction and formation of C-S-H and finally increases the formation of belite. α′L-C2S and C12A7 are the main cementitious minerals in the clinker calcined at 800 °C, while β-C2S and gehlenite present when the calcination temperature reached 900 °C or higher. Replacing ordinary portland cement partly for the belite cement might reduce the water requirement and increase the strengths.