Recycling of water treatment sludge into magnesium potassium phosphate cement component by a combination of silica fume

Abstract

Supplementary cementitious materials (SCMs) are often used to improve the performance of magnesium potassium phosphate cement (MKPC). The synergistic influence of silica fume (SF) and calcined water treatment sludge (CWTS) instead of partial magnesium oxide on the setting time, fluidity and early-ages compressive strength of MKPC system have been investigated. The results show that although the incorporation of CWTS reduces the fluidity and setting time of MKPC paste, the addition of SF can significantly improve the adverse effect of CWTS on workability, thereby making it meet the performance requirements as repair material. As the content of SF or CWTS increases, the compressive strength of the specimens increases first and then decreases. The optimum contents of single-doped SF and CWTS are 10% and 40%, respectively. The strength gain of composite SCMs (≤20%) to MKPC sample is significantly higher than that of single SCM (SF or CWTS), which is attributed to the fact that composite SCMs refines the pore structure of the specimen and produces hydration products such as irregular crystalline MKP and amorphous aluminosilicate phosphate products. In the nano-scale characteristics, the pore phase of the MKPC matrix prepared by replacing MgO with an appropriate amount of CWTS (30%) is significantly reduced, while the hydration phase is relatively increased. Therefore, a new type of green low-carbon MKPC system is prepared, the sludge waste is effectively recycled, and the early-ages performance of MKPC is improved.