Interdependence of key molar ratios (SiO2/Al2O3 and Al2O3/Na2O) in metakaolin-based geopolymers: phase composition, microstructure and mechanical insights

Recently, metakaolin-geopolymer has gained increasing importance as an alternative building material 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 metakaolin geopolymers, which complicates 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 (1.0 to 1.81) molar ratios on the mechanical performance of metakaolin geopolymer. In all specimens, H2O/Na2O was fixed at 13 and they were cured for 24 hrs at 60°C, then kept in a humidity of 20±5°C for 28-days. These results have been confirmed by XRD, TGA/DTG, SEM and texture characteristics analyses. The data revealed that the mix-design achieves a superior mechanical performance (50.80 MPa) was prepared from SiO2/Al2O3, Al2O3/Na2O and water/solid ratios at 3.5, 1.7 and 0.37, respectively. This mix-design showed that it was composed of the least amount of unreacted metakaolin and thermal stable three-dimensional geopolymeric phases. Its microstructure and texture characteristics yield dense, interconnected and well-dispersed strength-giving-phases. Finally, it can be concluded that the quality of the geopolymer depends on SiO2/Al2O3, followed by Al2O3/Na2O, as SiO2/Al2O3 controls the nature of formed geopolymeric phase (poly(sialate) or poly(sialate-siloxo/disiloxo)). One of the main roles of Na2O in the geopolymeric matrix is charge balancing of [Al(OH)4]-. Therefore, the claim of some previous studies that reported that Al2O3/Na2O = 1 is universally optimal is inaccurate, as the value of this ratio must depend on the SiO2/Al2O3 ratio.