Effects of Metakaolin and Silica Fume on Properties of Concrete

Abstract

Metakaolin is a relatively new mineral admixture for concrete. It is comparable to silica fume in pozzolanic reactivity, but is lower in price. The effects of metakaolin and silica fume on various properties of concrete were investigated and compared in this study. Seven concretes were cast at a water/binder ratio of 0.35 with 0, 5, 10, and 15% cement replaced by either metakaolin or silica fume. The concretes were tested for slump, compressive strength, free shrinkage, restrained shrinkage cracking, and chloride diffusivity by ponding. Metakaolin-modified concrete showed a better workability than silica fume-modified concrete. As the replacement level was increased, the strength of the metakaolin-modified concrete increased at all ages similarly to that of the silica fume-modified concrete. Both mineral admixtures reduced free drying shrinkage and restrained the shrinkage cracking width. However, the cracking time was earlier for these two concretes. The two admixtures also greatly reduced the chloride diffusivity of the concrete.