Interdependence of the key molar ratios (SiO₂/Al₂O₃ and Al₂O₃/Na₂O) in metakaolin-based geopolymers: phase...

Recently, metakaolin-geopolymers have gained increasing importance as alternative building materials to ordinary Portland cement. Intensive research published in this field has revealed several contradictions concerning the oxide's molar ratios (SiO₂/Al₂O₃ and Al₂O₃/Na₂O) that influence the properties of the metakaolin geopolymers, complicating the understanding of the geopolymerization process. Accordingly, the main objective of this study is to resolve this discrepancy by measuring the impact of a wide range of SiO₂/Al₂O₃ (2.74 to 4.1) and Al₂O₃/Na₂O (0.75 to 1.50) molar ratios on the mechanical performance of the metakaolin geopolymers. In all specimens, H₂O/Na₂O was fixed at 9.50, and they were cured for 24 h at 60 °C and then kept in a humidity of 20 °C ± 5 °C for 28 days. The results were confirmed by XRD, FTIR spectroscopy, TGA/DTG, SEM/EDX and texture characteristics analyses. The data revealed that the mix-design, using SiO₂/Al₂O₃, Al₂O₃/Na₂O and water/solid ratios of 3.5, 1.40 and 0.32, respectively, achieves a superior mechanical performance (50.80 MPa). This mix-design showed that it was composed of thermally stable three-dimensional geopolymeric phases and the lowest amount of unreacted metakaolin. Its microstructure and texture characteristics reveal dense, interconnected and well-dispersed strength-giving-phases. Finally, it can be concluded that the quality of the geopolymer depends on the SiO₂/Al₂O₃ ratio, followed by the Al₂O₃/Na₂O ratio, as SiO₂/Al₂O₃ controls the nature of the formed geopolymeric phase. The main roles of Na₂O in the geopolymeric matrix are the charge balancing of [Al(OH)₄]^- and contributing to the alkalinity required for metakaolin dissolution. Therefore, the claim made in some previous studies that Al₂O₃/Na₂O = 1 is universally optimal is inaccurate as the value of this ratio must depend on the SiO₂/Al₂O₃ ratio.