Revealing the intrinsic sintering mechanism of high-strength ceramsite from CFB fly ash: focus on the role of CaO

The preparation of high-strength ceramsite from circulating fluidized bed (CFB) fly ash is an effective approach for commercial utilization. The role of CaO plays a key factor in the performance of ceramsite. The impact of CaO on the sintering mechanism was studied at the sintering temperature from 1200 °C to 1300 °C and the CaO addition ratio from 2 wt.% to 8 wt.%. The in-situ visual furnace equipped with a high-resolution microscope, incorporated with TG-DSC-FTIR, SEM, XRD, and the semi-quantitative analysis method of XRD, was conducted to investigate the sintering process of ceramsite. The results verified that during the high-temperature sintering, CaO reacted with SiO2 and Al2O3 in CFB fly ash to constitute low-melting minerals like anorthite, and the elevation of sintering temperature expedited the conversion of anorthite and eutectics into the molten liquid phase. The expansion gas was wrapped by the molten liquid phase and reached equilibrium, which contributed to the closure of the pores and the elevation of the shrinkage rate of ceramsite. In addition, the rise in sintering temperature dropped the amorphous phase content and increased the mullite content, further enhancing the compressive strength. When the sintering temperature was in the range of 1250 °C-1300 °C and the CaO content was in the range of 2 wt.%-4 wt.%, the high-strength ceramsite with excellent physicochemical properties could be obtained. It suggested that the synergistic impacts of sintering temperature and CaO improved the performance of ceramsite.