Properties of High Performance Concrete Containing High Reactivity Metakaolin Aboozar Bonakdar, Mehdi Bakhshi and Mehdi Ghalibafian Synopsis : High Reactivity Metakaolin (HRM) is an engineered pozzolanic mineral admixture, reacting aggressively with calcium hydroxide which results in significant performance of concrete. HRM has been introduced to be a beneficial alternative for silica fume, required in the formulation of high strength/performance concrete. In this study, different aspects of concrete mechanical behaviors have been studied including compressive, flexural and splitting tensile strengths. Also some characteristics of concrete durability were investigated including water absorption, water penetration and gas permeability. In mixture proportioning, 5%, 10% and 15% of cement content is replaced by HRM or silica fume for comparative study. It was observed that both concrete with HRM and silica fume would perform almost the same in improving the mechanical properties of the materials. However in the case of workability and durability, a better performance was obtained in concrete with HRM. It was concluded from the investigation that HRM could be an appropriate substitute for silica fume in producing high performance concrete. Keywords: durability; Gas permeability; high performance concrete; Metakaolin; silica fume