Microstructure of High Performance Concrete
An important characteristic of the microstructure is the homogeneous matrix due to the pozzolanic ractivity of the silica fume. In concretes with no silica fume, the map cracking spreads out gradually but it remains less dense than that of silica fume concrete. The low mechanical performances of concrete have been attributed to the capillary porosity and excess of water needed for the workability of fresh concrete. During the cement hydration, silica fume spheres are sites of nucleation for cement hydrates and then react as a pozzolanic material giving an homogeneous and like amorphous C–S–H. In the hardened material the polymer is highly bound to cement grains and the final porostiy is about 1 % by volume. With limestone aggregates which are considered as reactive, cracks progress along aggregates but at a distance corresponding to the thickness of the reacted zone. The largest difference between normal, high strength and very high strength concretes occur in the microporosity.