The impact of industrial waste on hydration behavior, mechanical performance, and water resistance of magnesium oxychloride cement

Magnesium oxychloride cement (MOC) is a low-carbon alternative to traditional Portland cement, however its poor water resistance limits practical application. This study investigates the use of massively produced industrial waste materials: red mud, biomass fly ash, and copper slag, as supplementary cementitious materials (SCM) to improve the water resistance of MOC. A commercial precursor, metakaolin, was included for comparison. This work reports, for the first time, the systematic exploration of these underutilized SCM, particularly biomass fly ash, to synergistically enhance both mechanical strength and water resistance of MOC composites. The hydration behavior, reaction kinetics, and microstructural evolution were comprehensively analyzed via isothermal calorimetry, XRD, SEM, and TG. Results reveal that the incorporation of 20 wt% biomass fly ash optimizes hydration reactions, leading to a compressive strength of 44.29 MPa after 7 days of curing. The softening coefficient of MOC with 30 % biomass fly ash is 45 %. Metakaolin exhibited comparable performance, with even greater water resistance at higher dosages. The formation of secondary M-S-H gel phases and densified microstructures were found to be key mechanisms behind the compressive strength and water resistance. These findings provide new insights into sustainable MOC formulation using industrial by-products, offering a practical strategy for improving material performance while supporting circular economy and carbon reduction in the construction sector. • Innovative use of red mud, biomass fly ash and copper slag wastes into MOC cements. • Development of high-strength MOCs with significantly improved microstructure. • Synthesis of waste-containing MOCs with enhanced water resistance.