Design optimization and interfacial characterization of geopolymer mixtures upcycling construction and demolition waste-based binders and aggregates using response surface methodology

This study integrates statistical modeling with experimental design to optimize the performance of geopolymer mortars (GPMs) primarily composed of construction and demolition waste (CDW), used both as precursors and as 100 % replacement of natural aggregates. A design of experiments framework, employing response surface methodology (RSM) and analysis of variance (ANOVA), was implemented to investigate the relationships among input variables, including SiO 2 /Al 2 O 3 , water-to-binder, and particle size distribution modulus (q), and target responses of compressive strength, bulk density, water absorption and apparent porosity. The study considered various recycled aggregates, including recycled concrete (RCA), recycled brick (RBA) and recycled ceramic-tile (RTA) aggregates, with natural sand (NSA) serving as a control. At 28 days, NSA-based GPMs achieved compressive strengths up to 56.4 MPa. Predictive modeling demonstrated strong statistical correlation between mixture parameters and target responses. Multi-objective optimization subsequently identified optimized CDW-GPMs with strengths exceeding 44 MPa, 38 MPa and 33 MPa for RCA-, RTA- and RBA-based systems, respectively. ANOVA results confirmed the predictive accuracy of the second-order quadratic models, with statistically significant correlations and coefficients of determination (R 2 ) reaching up to 0.99. Furthermore, a novel porosity-validation method was employed, integrating Python-based image segmentation and pixel-intensity mapping to quantitatively assess pore morphology, distribution and intensity, while visualizing geopolymeric products distribution within the interfacial transition zone (ITZ). Microstructural analysis revealed enhanced interfacial bonding in RCA-GPMs compared to those incorporating RBA and RTA, while the addition of slag significantly improved the ITZ quality.