AbstractPaper mill sludge ash (PMSA), sewage sludge ash (SSA), and desulfurization gypsum (DG) were mixed and calcined by microwave radiation to acquire a product with higher hydration activity. The resulting mixture was used as a supplementary cementitious material to prepare backfill materials for mining. The influence of the mixing ratio of raw materials, microwave parameters, and sample size on the properties and microstructure of the microwave-sintered sludge ash (MSSA) and MSSA-cement pastes was determined. Both the MSSA and the prepared pastes were characterized using X-ray diffraction (XRD), Fourier infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDAX). Results indicated that the minerals in the MSSA were rankinite, mayenite, and belite, which all exhibited relatively high hydration activity and significantly improved the degree of hydration of the MSSA–cement system. As the microwave power and irradiation time increased, the water demand of the MSSA–cement pastes decreased, the setting time decreased, and the compressive strength increased. The MSSA prepared with a small length-to-diameter ratio produced more crystals and gels compared with that prepared with a larger length-to-diameter ratio. The gels mutually fused to form a dense structure, and this further enhanced the strength of the MSSA–cement pastes.