The changes in the physical appearance of the geopolymer concrete cylinders of group GPN and GPH are shown in Figs. 7and 8 respectively. There was an obvious colour change in geopoly- mer concrete after exposure to the four different temperature ranges. At 400 oC there was not a huge change in colour for the geopolymer concrete samples, with both the GPN and GPH samples displaying a very similar surface colour with the GPH sample having only a slight light brown tinge to it. At 650 oC, the geopolymer concrete specimens displayed a very similar surface colour, but it had changed to a light brown colour. At 800 oC, there was a clear difference from the original colour with both the high and normal strength concretes displaying a dark red colour. Parts of the surface were quite black, but this was able to be rubbed off to reveal the earthy red colour below. At 1000 oC, the red colour became very prominent, with the high strength geopolymer con- crete being more distinctive than the normal strength geopolymer concrete. The colour changes of the geopolymer concrete samples were because of the high iron oxide content of the fly ash. A similar colour change to brown in fly ash geopolymers subjected to high temperature heat was also observed by Zhao and Sanjayan [30].