lnspite of the fact that portland cement concrete is incombustible and give good resistance to elevated temperatures, heating due to fire or different causes will bring about physical and chemical changes in the structure of concrete. To repair fire damaged concrete, firstly, it is necessary to understand the extend of the effects of high temperatures on the concrete. Considering this, the current study is designed to visualise the effects of elevated temperatures on concrete microstructure and compressive strength. For this purpose, mortars with normal portland cement and three different aggregates were prepared. These aggregates were quartzite, limestone and pumice. The mortar specimens for compressive strength tests and SEM investigations were subjected to 100, 250, 500, 700 and 85O$circ C$ for period of 4 hours including the temperature increase period. For SEM observations the same places in the samples before and after heating were investigated. This provides with the main advantage of the study which clearly shows the effect of heating on microstructure. As a result, changes in CSH, CH and aggregate, crack propagation and formation can be exactly identified on the same samples.