Paint Sludge Ash in Ecofriendly Mortar: Toward Optimization Using Response Surface Methodology

This study evaluates the mechanical and durability performance of mortar incorporating paint sludge ash (PSA) as a partial replacement for ordinary Portland cement (OPC), with emphasis on predictive modeling and multi-response optimization using response surface regression techniques. PSA was produced through controlled calcination of industrial paint sludge and characterized by a high content of pozzolanic oxides (SiO2 + Al2O3 + Fe2O3 > 70%), indicating pozzolanic potential. Its contribution to mortar performance is attributed to filler effects and pozzolanic reactions. Experimental dataset covering PSA contents (0–20%) and curing age (3–91 days) using Design-Expert software to evaluate their combined effects on compressive strength, ultrasonic pulse velocity (UPV), bulk density, water absorption, porosity, and sulfate resistance. The developed models demonstrated good predictive capability, with coefficients of determination (R2) ranging from 0.9073 to 0.9777, although significant lack-of-fit was observed for some responses, suggesting localized deviations that were not fully captured by the global polynomial models. Model adequacy was supported by ANOVA results, close agreement between adjusted and predicted R2 values, low coefficients of variation (<10%), high adequate precision values (>4), and satisfactory residual diagnostic analyses. Results indicate that curing age enhances the measured performance parameters due to continued hydration and pozzolanic reactions, while PSA content exhibits a nonlinear effect governed by competing mechanisms. At moderate replacement levels (≈10–11%), through densified matrix and pore refinement, higher replacement levels reduce performance due to dilution of cementitious phases and slower reaction kinetics. Desirability-based multi-response optimization predicted an optimum PSA replacement level of approximately 10.57%, corresponding to a predicted compressive strength of 39.78 MPa at 91 days with prediction errors below 5%. Durability indicators were also improved within this range. Overall, PSA is demonstrated to be a viable supplementary cementitious material for sustainable mortar production.