Mechanical and microstructural properties of metakaolin geopolymer

Fly ash (FA) is a commonly used aluminosilicate raw material for the synthesis of geopolymers (GPs). Metakaolin (MK) is also an aluminosilicate-rich material that is relatively pure and has a huge potential as a GP precursor. In this research, FA is replaced by MK at quantities of 0, 25, 50 and 75% by mass (MK0, MK25, MK50 and MK75, respectively). The silicon (Si)/aluminium (Al) ratio of the MK precursor is significant in strength development. For a silicon/aluminium ratio equal to 0·86 (M1), the compressive strength of GP concrete (GPC) decreased by 57% for MK75 at 120 d. For a silicon/aluminium ratio of 1·11 (M2) of the MK75 mix, the compressive strength is almost same as that of MK0 at 120 d, and for a silicon/aluminium ratio of 1·21 (M3), the compressive strength increases by 16% for MK75 at 120 d. Nevertheless, the volume of permeable voids (VPV) percentage is reduced to 0·638 for MK75 (M3) compared to 18·58 for MK0. Scanning electron microscopy images of hardened MK-modified GPC support the results showing reduced VPV compared to that of FA-GPC. Energy-dispersive X-ray spectroscopy analysis of the hardened MK50 mix of M3 GPC confirms the presence of Si–O–Si bonds, resulting in higher strength.