Properties of cementitious systems containing silica fume or nonreactive microfillers

Abstract

The object of this work was to explore the effects of silica fume on the microstructure of hardened paste and of the transition zone between paste and aggregates in concrete. The significance of aggregates as reinforcing fillers and their impact on some properties of the transition zone and bulk paste were resolved. The experimental procedure was based on simultaneous studies of model concretes and paste matrices extracted from fresh model concrete mixes. In addition, continuously graded aggregate concretes were prepared. Three sizes of a nonreactive microfiller (carbon black) and one reactive microfiller (silica fume) were applied. On the basis of microstructural studies and compressive strength tests, it was concluded that the primary effect of silica fume was generated by its physical (microfiller) properties, since the strengthening provided by reactive silica fume was similar to that obtained with nonreactive carbon black of similar size and shape. This effect was more significant from the point of view of the concrete strength enhancement than the chemical (pozzolanic) activity of the silica fume. In concretes containing either silica fume or carbon black, aggregates of high quality could serve as reinforcing filler. This could take place when sufficient densification of the transition zone occurred in the presence of silica fume or carbon black. Significant refinement of pore structure was observed in both types of paste matrices (containing silica fume or carbon black). However, this led to a relatively small influence on the paste strength. Concretes containing reactive silica fume or an inert carbon black microfiller behaved as a composite material, unlike conventional concrete.