Sustainable Improvement of Magnesium Oxychloride Cement Solidified Waste Sludge with Fly-Ash Inclusion

Fly ash is introduced to strengthen the durability of magnesium oxychloride cement (MOC) solidified sludge, developing a more sustainable method for waste sludge disposal. The effect of curing age, fly ash dosage, and MgO/MgCl2 molar ratio on strength, microstructure, durability, and cementation products of MOC-solidified sludge was identified. Fly ash weakens the compressive strength of solidified sludge and prevents its long-term strength deterioration. The amount of brucite and magnesium silicate hydrates (M-S-H) gels increases with curing age, and the crystal shape of phase 5 becomes more robust when fly ash is added. Fly ash grains fill the pore space, enhancing the long-term strength and durability, but an excessive dosage of fly ash weakens the porous structure. The higher the fly ash dosage, the better the mass stabilization and strength retention. In particular, the strength of solidified sludge exposed to freeze–thaw cycles was enhanced for the dewatering and ice hardening effect. The results indicate that fly ash promotes the durability of MOC-solidified sludge, and the fly ash-MOC blend is proved a profitable and sustainable material for sludge solidification.