This difference in the temperature gradients has an effect onthe cracking of specimens. Surface cracking of the geopolymer con- crete specimens was not as predominant as in the OPC concrete samples. The number and width of cracks occurred after exposure to 1000 oC fire were much smaller in geopolymer concrete specimens as compared to the OPC concrete specimens. These cracks were very small and difficult to see with the naked eye, as shown in Figs. 7 and 8. However the cracks were still evident, but were generally observed to only temperatures above 800 oC for the geopolymer concrete specimens.