Fly ash based geopolymeric binders have potential for a wide range of high temperature applications and it is receiving more attention due to their economic and environmental advantages. This kind of Geopolymeric materials possess intrinsic fire resistance due to their rigid 3-D aluminosilicate structure. Geopolymer specimens remains dimensionally stable at high temperature and provides good compressive strength also. The percentage of Na2O and SiO2/Na2O ratio affect development of internal pore structure of Geopolymers and subsequently its performance at elevated temperature. The characterisation of internal pore structure can be made by using simple experimentation like measuring water absorption, apparent porosity and water sorptivity as well as some sophisticated mineralogical and microstructure studies like SEM, XRD, MIP etc. also. This book records a huge experimental observations and explained intricate behavioural aspects of fly ash based geopolymer exposed elevated temperature. It will help the researchers and technologists to produce high temperature resistant geopolymer for different engineering applications.