Effect of mineralogical composition on the compressive strength and microstructure of metakaolin geopolymer

The objective of this work is to investigate the impact of mineralogical composition (SiO₂ and Al₂O₃ content) on the properties of metakaolin-based geopolymers, including compressive strength and microstructure. Two kaolin samples with different mineralogical compositions were used: one has higher Al₂O₃ and lower SiO₂ content (coded as kH), compared to the second one, which is coded as kL. Post calcination at 700 °C for 1 h, the pozzolanic activity of both metakaolin samples was measured by the Chapelle test. The metakaolin samples (MkH and MkL) were activated with NaOH and Na₂SiO₃ to produce geopolymers. Four mixes from each metakaolin sample were prepared based on varying SiO₂/Al₂O₃ ratios at 3.1, 3.3, 3.5 and 3.7. Initial and final setting time and compressive strength were evaluated to assess the quality of the resulting geopolymers. The phase composition of the geopolymers was analyzed using X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG), as well as microstructure using scanning electron microscopy (SEM) and the Brunauer-Emmett-Teller/Barrett-Joyner-Halenda models (BET/BJH). The presence of Al₂O₃ in kaolin has significantly improved the metakaolin reactivity. MkH and MkL have pozzolanicity of 1006 and 767 mg CaO/g Mk, respectively. The MkH with a low SiO₂ content requires high Na₂SiO₃ to produce a geopolymer with the same SiO₂/Al₂O₃ ratio as the MkL-based geopolymer. The highest compressive strength was 64.24 and 18.6 MPa at a SiO₂/Al₂O₃ molar ratio of 3.5 for MkH- and MkL-based geopolymers, respectively. This suggests that adding SiO₂ externally with Na₂SiO₃ in the geopolymer would be more effective than relying on its presence in the original mineral makeup of the raw material metakaolin. The soluble active SiO₂ included in the matrix by Na₂SiO₃ promotes the rapid formation of a more densely packed network structure.