Preparation of Paste Backfill Material from Mix-Calcined Sludge Ash

Paper mill sludge ash (PMSA) and sewage sludge ash (SSA) were mixed and calcined at high temperatures to improve the hydration activity of the sludge ash. The resulting mixture was utilized as an auxiliary cementing material to prepare backfill material for mining. Factors, including sludge ash mixing ratio and calcination temperature, affecting the properties and microstructure of the mixed calcinated sludge ash (MCSA) and cement paste were analyzed. Both raw materials and prepared cement pastes were characterized by different techniques, including XRD, FTIR, TG–DSC, SEM, and EDS. Results show that the major reaction products of PMSA and SSA mixture calcined at high temperature are gehlenite and belite, which exhibit high hydration activity in the pozzolanic reaction and significantly improve the degree of hydration of the MCSA–cement system. Compared with trials involving separate calcination of the two sludge ashes, pastes prepared by mixing calcination had a lower water demand, shorter setting time, and higher compressive strength. The amounts of Ca(OH) 2 and C–A(F)–S–H gel produced by the MCSA–cement hydration reaction increased, and the degree of polymerization of the gel phase improved with the increase in calcination temperature as well as the decrease in PMSA mixing ratio, which resulted in the formation of a dense microstructure at the later stage of hydration. This research also suggested that the best modulus range values of MCSA should be controlled to within 1.42 ≤ HM ≤ 1.48 and SM = 0.72, the optimal calcination temperature was 1200 ℃, and the appropriate replacement rate of MCSA for cement should be equal to or smaller than 40% in laboratory conditions. Graphic Abstract