Exploring the formation of phosphoric acid-based metakaolin geopolymer and underlying mechanisms under various Liquid-to-Solid ratios

This study investigates the influence of a wide range of liquid-to-solid (L/S) mass ratios (L1–L2000) on the reaction mechanism of phosphoric acid-activated metakaolin geopolymers, presenting new insights into acid-activated geopolymer behavior that have not been fully explored in previous studies. Through chemical analyses and characterization techniques (titration, ICP-OES, XRF, XRD), we found that L/S < 2 promotes complete geopolymerization, resulting in dense AlPO₄-rich matrices (confirmed by XRD), an important observation in the context of phosphoric acid activation. In contrast, L/S > 2 leads to incomplete reactions or the disintegration of the geopolymer due to excess H₃PO₄, an aspect previously underappreciated. The study also reveals critical findings about chemical stability, including significant Al/P leaching (5900/6567 %) when exposed to 3 mol/L HNO₃ concentration, with silicon remaining largely intact. This demonstrates a new understanding of the acid vulnerability and silicate stability of phosphoric acid-activated geopolymers. By identifying the optimal L1 ratio, which shows minimal leaching and pH stability, our work provides novel practical guidelines for controlling L/S ratios to improve acid resistance in geopolymer applications. These findings offer crucial insights into acid-activated geopolymer degradation mechanisms and present a step forward in designing sustainable, acid-resistant materials and cement alternatives, surpassing prior studies in their focus on the precise L/S ratio control (<2) and its practical implications for industrial applications. Strong titration-ICP correlations validate the analytical reliability, further underscoring the robustness of our findings.