The use of high reactivity metakaolin in high performance concrete

Results are presented from laboratory investigations on the durability of concrete containing high-reactivity metakaolin (HRM), a pozzolan manufactured by processing purified kaolinitic clay. Expansion tests on concrete prisms and mortar bars containing reactive aggregates indicated that up to 15% HRM may be required to prevent deleterious expansion due to alkali-silica reactivity. The efficacy of HRM in this role is likely linked to the substantial reduction in pore solution alkalinity determined for pastes with 20% HRM compared to control specimens. Bulk diffusion testing indicated that HRM substantially reduced chloride ion penetration in concrete with W/CM = 0.30 and 0.40. Reductions in diffusion coefficients compared to control specimens were of the order of 50% and 60% for concrete with 8% and 12% HRM, respectively. Such reductions can be expected to have a substantial impact on the service life of reinforced concrete in chloride environments. Chloride binding tests indicated that 8% HRM increases the binding capacity of hydrated pastes cast with a moderate-C(3)A cement.