Quantitative characterization of the transition zone in high strength concretes

Synopsis The microstructure of the transition zone in high strength concretes, with and without silica fume, was characterized. Backscattered electron (bse) imaging combined with quantitative image analysis was used to measure the quantity and distribution of unreacted cement, pores larger than 0·5 μm and calcium hydroxide. For concretes of the same water/binder ratio, the silica fume was found to exert only a small influence on the porosity measured in the bulk paste. In the concretes without silica fume the transition zone was more porous than the bulk cement matrix. However, in the concretes with silica fume, the transition zone was much denser, having a level of porosity similar to that of the matrix. These results confirm and support the hypothesis that a major effect of silica fume in concrete is the improvement of the aggregate/matrix bond.