The microstructures of the geopolymer concrete specimens subjected to high temperature fire were investigated by scanning electron microscopy (SEM). The SEM images of the GPN specimens after exposures to 650, 800 and 1000 oC are shown in Figs. 14(a) to (c). It can be seen from these figures that the geopolymer micro- structures became denser with the increase of fire temperature up to 1000 oC. This change has occurred in the microstructure because of sintering and further geopolymerisation of fly ash with the increase of temperature. Thus, the geopolymer microstructure remained stable after exposure to high temperature fires. This is consistent with the observation of Kong et al. [32] that reported a 6% increase in strength of fly ash based geopolymer paste after exposure to heat of 800 oC. The strength of geopolymer paste increased with the increasing compactness of the microstructure at this temperature. However, the strength loss of the concrete specimens after high temperature exposure is mainly because of the thermal shock and the incompatibility between thermal expansions of the geopolymer matrix and the aggregates. It was shown by Kong and Sanjayan [25] that the thermal expansions of geopolymer paste and aggregates were different at high tempera- ture exposure. This difference in thermal expansions initiates the damages in concrete which eventually results in the reduction of compressive strength.