Hydrothermal preparation of lightweight calcium silicate powder from alumina-leaching residue of low-calcium sintering red mud

In order to realize the complete and high-value utilization of red mud, a hydrothermal process was proposed to prepare the lightweight calcium silicate powder from a sodium calcium silicate residue after alumina extraction. The synthesized products were characterized by XRD, XRF, SEM, EDS, and PSD, and the corresponding transition mechanism and physical properties were also studied. The main silicon- and aluminum-bearing phases in the clinker after the low-calcium sintering are Na2CaSiO4 and NaAlO2, and the former silicate keeps stable during the alumina leaching process under atmospheric pressure. In the hydrothermal process, Na2CaSiO4 in the aluminaleaching residue is converted into NaOH and Ca5Si6O16(OH)(2)center dot 4H(2)O in the alkali solution, and the residual alumina is converted into Ca 3 Al 2 Si0 8 .4H 2 0 which promotes the generation of Ca5Si6O16(OH)(2)center dot 4H(2)O. Increasing the reaction temperature, duration and liquid-solid ratio of the hydrothermal process can facilitate the alkali removal efficiency in calcium silicate powder and decrease its bulk density, while the excessive hydrothermal temperature and duration reduce the alkali removal efficiency. The Na2O residue and bulk density of the calcium silicate powder are 0.42% and 0.30 g.cm(-3) obtained at the optimal hydrothermal conditions. The flaky particles of Ca5Si6O16(OH)(2)center dot 4H(2)O are aggregated into a porous cauliflower structure, which improves the absorption and insulation properties of calcium silicate powder.