Development of high performance concrete using combinations of mineral admixtures.

Cement replacement materials are by-products used to produce high performance concrete. Published data on the effects of combinations of mineral admixtures in concrete on the microstructural and performance-related properties under different curing regimes are comparatively little. Further the correlation of strength of concrete to its permeability and pore structure is also not clear. The main objective of this research is to study the performance of various combinations of fly ash/silica fume and slag/silica fume concretes under three different curing regimes, viz. continuous moist curing, no moist curing after demolding and air drying after 7-days of initial moist curing. Six different concrete mixes were prepared with ordinary portland cement and a blend of portland cement and combinations of fly ash+silica fume and slag+silica fume The water-to-cementitious materials ratio of all the concrete mixtures was kept constant at 0.45. The properties investigated included workability of the fresh concrete, engineering properties such as cube and modified cube compressive strength, flexural strength, dynamic modulus of elasticity, pulse velocity, shrinkage and swelling, permeability and microstructural properties such as porosity and pore size distribution. The results show that prolonged dry curing results in lower strengths, higher porosity, coarser pore structure and more permeable concretes. It was found that the loss in early age compressive strength due to incorporation of fly ash or slag can be compensated for by the addition of small amounts of silica fume. The engineering and microstructural properties and permeability of concretes containing fly ash or slag appear to be more sensitive to poor curing than the control concrete, with the sensitivity increasing with increasing amounts of fly ash or slag in the mixtures. The incorporation of high volumes of slag in the concrete mixtures refined the pore structure and produced concretes with very low porosity and threshold diameters. The results emphasize that a minimum 7-day wet curing is needed for concrete with mineral admixtures to develop the full potential, and that continued exposure to a drying environment can have adverse effects on the long-term durability of inadequately cured slag or fly ash concretes. The results also confirm that compressive strength alone is not an adequate index to judge the performance of concrete, and the knowledge of the strength, pore structure and permeability are required for this purpose. Slag/silica fume concrete mixtures showed better performance than fly ash/silica fume concrete mixtures as regards the development of engineering and microstructural properties.