Strength, durability and microstructure of granulated blast furnace slag-modified magnesium oxychloride cement solidified waste sludge

Due to huge carbon emissions associated with the production of Portland cement (PC), the pursuit of sustainable and efficient cementitious materials is a major challenge to manage solid wastes including urban river sludge. As an eco-friendly alternative to PC, granulated blast furnace slag (GBFS)-modified magnesium oxychloride cement (MOC) is innovatively introduced in sludge solidification. The slag modification to MOC and their synergistic efficiency on sludge treatment is systematically evaluated by unconfined compressive strength (UCS), durability and microstructural tests, considering slag dosage, molar ratio of MgO/MgCl2 and curing age. The obtained results suggest that MOC-slag mixture is efficient in sludge solidification and the inclusion of slag overcomes the strength retraction of MOC solidified sludge. The mechanical behaviour is closely concerned with the formation of phase 5, brucite and C–S–H gels, which imposes a dominant impact on the morphology and microstructure of solidified sludge. Especially, the physic-chemical effect of slag is able to improve the durability of MOC solidified sludge, and induces an enhancement of its long-term stability. The C–S–H gels formed by alkaline activation of slag are the crucial cause of improvement in long-term strength and durability of MOC-slag solidified sludge. In brief, the combined agent of slag-modified MOC shall be deemed as an effective and eco-friendly approach for urban sludge disposal. •Magnesium oxychloride cement is innovatively introduced to dispose of waste sludge.•Slag overcomes strength retraction and improves durability of MOC solidified sludge.•Strength is highly dependent on MgO/MgCl2 molar ratio, slag dosage and curing time.•MOC-slag system governs strength, durability and microstructure of solidified sludge.•Phase 5, C–S–H and brucite are detected as mechanisms of MOC-slag solidified sludge.