This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200°C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800°C. Nepheline (NaAlSiO 4 ) and carnegieite (NaAlSiO 4 ) form at 900°C in the geopolymer phase and after exposure to 1000°C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (∼275MPa) and high Vickers hardness (∼350 HV). Between 1000 and 1200°C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.