Utilisation of electrolytic manganese residue as a sulphate activator in producing concrete blocks with high-volume fly ash

Electrolytic manganese residue has been largely generated and accumulated, which poses a threat to the sus-tainable development of the electrolytic manganese metal industry. This paper explores the feasibility of utilising pretreated electrolytic manganese residue and fly ash in the production of concrete blocks. The pretreated electrolytic manganese residue is used as an activator to react with the alumina and portlandite in the mixture. The volume of permeable voids, compressive strength, and drying-wetting performance of the blocks were estimated, followed by the characterisation of their microstructure and the leaching behaviour of hazardous elements. The results show that replacing sand with 0-30 wt% pretreated electrolytic manganese residue can improve the blocks' compressive strength and reduce the volume of permeable voids due to the filler effect of pretreated electrolytic manganese residue and the promoted formation of ettringite. However, increasing the pretreated electrolytic manganese residue content compromises the blocks' water resistance due to the presence of gypsum. Substituting 0-30 wt% of ordinary Portland cement with fly ash in the blocks contributes reactive SiO2 and Al2O3 to the system with the pretreated electrolytic manganese residue as a sulphate activator. This further facilitates the formation of ettringite which can bridge micro-particles and fill pores, enabling the con-crete blocks with ordinary Portland cement and fly ash to attain comparable properties to those with ordinary Portland cement only. Moreover, leaching tests demonstrate that hazardous elements in electrolytic manganese residue can be efficiently immobilised in concrete blocks. The incorporation of 0-30 wt% fly ash enhances the solidification of Mn in the blocks due to the intensified ion substitution and physical encapsulation emanating from the facilitated formation of ettringite. Overall, this paper showcases a sustainable technical pathway for the utilisation of electrolytic manganese residue in concrete blocks with high-volume fly ash.