Strength and microstructural evaluation of sustainable geopolymer mortars using calcium carbonate sludge and fly ash as precursors

This study investigated the effects of alkaline activation on geopolymer mortars synthesized from fly ash and calcium carbonate sludge precursors. The research systematically examined key parameters including NaOH molarity (5 M, 10 M, 15 M), sodium silicate-to-sodium hydroxide ratio (0:100 to 90:10), fly ash-to-calcium carbonate sludge ratio (100:0 to 70:30), and liquid-to-binder ratio (0.3, 0.4, 0.5). Results demonstrated optimal performance at a liquid-to-binder ratio of 0.4, with several mix designs exceeding the reference cement mortar strength of 24.5 MPa. Higher NaOH concentrations (15 M) consistently produced shorter setting times and enhanced compressive strength compared to lower molarities. The incorporating calcium carbonate sludge (an underutilized industrial by-product) as a novel calcium-rich precursor with fly ash resulted in a sophisticated hybrid binding system containing both N-A-S-H and C-A-S-H gels, creating more homogeneous microstructures. Microstructural analysis revealed denser matrices at higher NaOH concentrations, particularly in specimens containing both fly ash and calcium carbonate sludge. Mineralogical analysis confirmed complex reaction product formation, with higher NaOH molarities producing more pronounced amorphous characteristics. Furthermore, this study uniquely integrates machine learning (XGBoost and SHAP analysis) to predict compressive strength and revealed that the sodium silicate-to-sodium hydroxide ratio was the most influential parameter, followed by NaOH molarity. These findings demonstrated the potential for optimizing geopolymer mortar performance through careful control of composition and processing parameters, highlighting the beneficial effects of incorporating calcium carbonate sludge and using higher NaOH concentrations in developing sustainable construction materials.