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

Abstract The objective of this work is to investigate the impact of mineralogical composition (SiO 2 and Al 2 O 3 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 Al 2 O 3 and lower SiO 2 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 Na 2 SiO 3 to produce geopolymers. Four mixes from each metakaolin sample were prepared based on varying SiO 2 /Al 2 O 3 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 Al 2 O 3 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 SiO 2 content requires high Na 2 SiO 3 to produce a geopolymer with the same SiO 2 /Al 2 O 3 ratio as the MkL-based geopolymer. The highest compressive strength was 64.24 and 18.6 MPa at a SiO 2 /Al 2 O 3 molar ratio of 3.5 for MkH- and MkL-based geopolymers, respectively. This suggests that adding SiO 2 externally with Na 2 SiO 3 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 SiO 2 included in the matrix by Na 2 SiO 3 promotes the rapid formation of a more densely packed network structure.