Effect of elevated temperatures on microstructure of high strength concrete based-metakaolin

The present experimental study evaluates the residual mechanical properties and the microstructure evolution of high-strength concrete (HSC) after the exposure to elevated temperatures. Metakaolin (MK) is used as supplementary material that is expectable to produce fire-resistant HSC and environmentally friendly. Three major groups of testing have been carried out herein. First, testing six mixes of hardened cement paste (HCP) using different ratios of MK replacements. Second, evaluating the optimum MK dosages at both ambient and after elevated temperatures in three concrete mixes. Finally, an attempt to define the correlation between the residual properties of HCP and HSC after elevated temperatures carried out as well. Results indicated that the influence of aggregate on concrete in the presence of MK is relatively less significant after elevated temperatures exposure. Inclusion of MK has shown beneficial changes on the interfacial transition zone (ITZ) of aggregate with HCP, in which ITZ between aggregate and mortar are clearly merged in the bond region, and the joints observed in the normal HSC are eliminated in the presence of MK. In addition, thermogravimetric and microscopy investigations demonstrated the high pozzolanic activity and microstructure development of HCP and of concrete. Finally, general equations are proposed for the residual compressive strength and flexural strength correlation of HSC after elevated temperature.