Use of ternary cementitious systems containing silica fume and fly ash in concrete

This paper reports the results from laboratory studies on the durability of concrete that contains ternary blends of portland cement, silica fume, and a wide range of fly ashes. Previous work has shown that high CaO fly ashes are generally less effective in controlling alkali silica reactivity (ASR) and sulfate attack compared with Class F or low lime fly ashes. Indeed, in this study it was shown that replacement levels of up to 60% were required to control expansion due to ASR with some fly ashes. However, combinations of relatively small levels of silica fume (e.g., 3 to 6%) and moderate lev els of high CaO fly ash (20 to 30%) were very effective in reducing expansi on due to ASR and also produced a high level of sulphate resistance. Concre tes made with these proportions generally show excellent fresh and hardened properties since the combination of silica fume and fly ash is somewhat sy nergistic. For instance, fly ash appears to compensate for some of the work ability problems often associated with the use of higher levels of silica f ume, whereas the silica fume appears to compensate for the relatively low e arly strength of fly ash concrete. Diffusion testing indicates that concret e produced with ternary cementitious blends has a very high resistance to t he penetration of chloride ions. Furthermore, these data indicate that the diffusivity of the concrete that contains ternary blends continues to decre ase with age. The reductions are very significant and have a considerable e ffect on the predicted service life of reinforced concrete elements exposed to chloride environments. (C) 1999 Elsevier Science Ltd. All rights reserv ed.