Valorisation of thermally treated polyaluminum chloride residue in moisture-buffering mortar: mechanical–moisture performance and pore-throat characteristics

Valorising thermally treated polyaluminum chloride (PAC) residue as a partial cement replacement provides a potential route for reducing cement consumption and promoting residue reuse, while powdered activated carbon may further impart moisture-buffering functionality. However, the relationships among cement replacement, mechanical performance, liquid water absorption, and water-vapour buffering remain insufficiently understood. A two-factor full-factorial experimental program comprising 32 mortar mixtures was conducted, with PAC residue replacement levels of 0%–50% and activated carbon dosages of 0%–1.5%. Fresh properties, flexural and compressive strengths, 28 d water absorption, and practical moisture buffer value (MBV) were evaluated, while X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy were used to characterise phase composition, pore-throat structure, and interfacial morphology. The mortar strength was generally maintained at PAC residue replacement levels of 5%–10%, whereas marked deterioration occurred above 20%. At the highest investigated PAC residue replacement level and activated carbon dosage, the 28 d flexural and compressive strengths of P50C1.5 showed numerical decreases of 54.5% and 53.9%, respectively, relative to P0C0. In contrast, P10C1.0 achieved flexural and compressive strengths of 7.21 and 48.7 MPa, respectively, while maintaining relatively low water absorption. P15C1.5 exhibited the highest mean MBV of 0.678 g/(m2·%RH), corresponding to a numerical increase of 98.8% relative to P0C0, but this improvement was accompanied by higher liquid water absorption and reduced mechanical performance. Compared with P10C0, the principal mercury intrusion peak diameter of P10C1.0 decreased from 1417 to 343 nm, while its MIP-accessible porosity decreased from 12.7% to 9.8%. Higher activated carbon dosages, however, resulted in pore-throat coarsening and localised interfacial loosening. These results demonstrate that moisture-buffering enhancement is governed by the balance among potential adsorption capacity, transport pore-throat structure, and matrix integrity, providing a multiscale experimental basis for the design of PAC residue–based functional cement mortars.