The functional and mechanical properties of steel slag (SS)–red mud (RM)–electrolytic manganese residue (EMR)-based composite mortar under different matching ratio conditions were investigated in this paper to examine the synergistic cementing effect among multiple solid wastes. The hydration characteristics of the composite mortar and its microstructure were characterized by the heat of hydration assessment, X-ray diffraction, thermogravimetric analysis, and other tests. The results of the study showed that compared with the pure cement group, 30% SS alone will inhibit the hydration reaction of the slurry, thus reducing the mechanical properties of the mortar, while compounding the appropriate amount of RM, and EMR can effectively reduce the negative impact of SS on the mechanical properties of the mortar. The flexural and compressive strengths of the composite mortar at 28 d were the highest when 15% of SS, 12% of RM, and 3% of EMR were mixed, which were 7.2 MPa and 41.4 MPa at 28 d, respectively. Compared with the test group with 30% SS alone, the flexural and compressive strengths increased by 18.0% and 25.5%. This is mainly because the incorporation of RM and EMR not only plays the role of physical filling, but the free alkali in RM and sulfate material in EMR can also compoundly stimulate the hydration activity of SS to produce more calcium alumina (AFt) and hydrated calcium silicate (C–S–H gel), thus improving the microstructure of mortar, which makes the overall decrease of 26.35% of multiharmful and harmful pores and the overall increase of harmless and less harmful pores of composite mortar specimens of 43.57%.

To prepare lightweight ultra high performance concretes used in the large-span and super-tall structure engineering fields, the effects of water-binder ratio on the mechanical performances, hydration products, and microstructure of the arched shell interface transition zone between the prewetting spherical lightweight aggregates and cement matrix were studied. The experimental results showed that adding prewetting spherical lightweight aggregates promoted the formation of an arched shell interface transition zone. And the hydration degree, microhardness, and elastic modulus values of the arched shell interface transition zone were still higher than the cement matrix. With the reduction of the water-binder ratio, the microhardness, elastic modulus, thickness, and compactness of the interface transition zone had an increase, and the internal curing action of the prewetting spherical lightweight aggregates was more obvious. Especially when the water-binder ratio was 0.18, the hydration degree of the arch shell interface transition zone increased by 18.27% compared with the cement matrix after 28 days curing time. It was concluded that the prewetting spherical lightweight aggregates could have better internal curing and arched shell effects in cement-based materials with a low water-binder ratio. Therefore, adding prewetting spherical lightweight aggregates was regarded as a potential measure to fabricate the lightweight ultra high performance concretes.