The Effect of Polyaluminum Chloride (PAC) Residue on the Mechanical Properties and Freeze-Thaw Resistance of Cement Mortar

Abstract The accumulation of polyaluminium chloride (PAC) residue seriously restricts the sustainable development of water purification agent industry. This study proposes the utilization of PAC residue as a mineral admixture in cement-based materials, systematically investigates its effects on the physical, mechanical, and frost resistance properties of cement mortar, and employs scanning electron microscopy (SEM) microstructural analysis to elucidate the underlying mechanisms. The experimental results demonstrate that increasing PAC residue content leads to a gradual reduction in mortar workability, while the 7-day compressive strength exhibits a steady decline with the increment of PAC residue content. At a PAC residue content of 5%, the material exhibits optimal compressive strength at 28-day and 56-day curing ages, achieving 26.6 MPa and 29.1 MPa, respectively. Additionally, at 5% PAC residue content, the pore structure parameters of the mortar reach their optimal values, while the frost resistance achieves the highest level. SEM analysis demonstrates that a modest incorporation of PAC waste residue effectively fills the internal pores of mortar, thereby enhancing matrix compactness. Furthermore, the active components in PAC waste residue participate in secondary reactions with cement hydration products, optimizing both the composition and distribution of hydration phases to refine the mortar’s microstructure.