The combined effect of silica fume and fly ash on microstructure and properties of high-strength concrete

High strength concrete has many advantages and will be the development trend of the construction industry. Pozzolans, such as silica fume and fly ash, have been gradually recognized and accepted as the desirable and useful ingredients for producing high strength concrete. The difference in particle size and pozzolanic reactivity between silica fume and fly ash results in their unique effects on the microstructure- and properties of concrete. A combined use of silica fume and fly ash was proposed in order to utilize the maximum short-term and long-term benefits of these pozzolans in high strength concrete.; An experimental program was designed to study the combined effect of silica fume and fly ash on the microstructure and properties of high strength concrete. In a comparative study the following microstructural characteristics and material properties of the plain and pozzolan mixes were investigated: characteristics of the interfacial zone, pore structure, content and orientation of calcium hydroxide, compressive strength, flexural strength, shear bond strength, modulus of elasticity, stress-strain relationship, and resistance to freezing and thawing cycling, deicing salt scaling, sulphate attack and alkali-silica reaction.; The results of this study demonstrate that silica fume and fly ash significantly and beneficially affect many of the material properties and performance of the high strength concrete through modifying its microstructure. The major effect of silica fume occurs at earlier ages, while the effect of fly ash is more significant at later ages. The combined use of silica fume and fly ash leads to a denser interfacial zone, lower content of calcium hydroxide and finer pore structure of the high strength concrete. The overall mechanical properties and durability characteristics of the combined mix are superior to those of the plain mix or the mixes where silica fume or fly ash is used individually.