![]() ![]() The enhanced bond between paste and aggregate or steel rebar has been attributed to both the chemical reaction between CH and pozzolanic materials (pozzolanic reaction) and the filling effect (physical action) that are considered to significantly densify the interfacial transition zone and reduce the water accumulation beneath rebar and aggregate. The bond-enhancing effect of SF has also been previously discussed in several researches, ,. Furthermore, SF is constituted of smaller particles than cement and can thus fill the gap between cement grains, leading to micro-filling or particle packing which contributes towards an increase of compressive strength and a decreased permeability. The high content of amorphous SiO 2 reacts with calcium hydroxide (CH), which is a weak part of the hydration product of ordinary Portland cement. The high content of amorphous silicon dioxide (more than 85%) and very fine spherical particle are the main reasons for its high pozzolanic reaction. Numerous studies have proved that the use of SF which is in the nano-range can significantly improve the mechanical and durability properties of Portland cement composite and concrete, ,. The typical particle size of silica fume is around 0.1–0.5 μm and the nitrogen BET surface is 20,000 m 2/kg. SF is a very fine amorphous silica powder produced in electric arc furnaces as a by-product of the manufacture of alloys with silicon or elemental silicon. Silica fume (SF) has been recognized as one of the most effective SCMs which contribute to concrete durability improvement through pozzolanic reactions with free lime, pore size refinement and matrix densification, as well as cement paste–aggregate interfacial improvements. It is well known that supplementary cementitious materials (SCM) in the form of natural Pozzolan or industrial by-products are able to modify the microstructure and interfacial zones of aggregate-paste or concrete (paste)-reinforcement, and improve durability of concrete. The different dispersion and the improvement of the interfacial transition zone are the main factors causing the different role of silica fume in paste, mortar and concrete. The crystalline orientation degree, the crystalline size and the content of calcium hydroxide at the interface are obviously decreased by adding silica fume. The silica fume can improve the interface bond strength between hardened cement paste and aggregate. The agglomeration of silica fume has been found in blended paste which is hardly seen in concrete. The addition of silica fume trends to increasing the compressive strengths of hardened pastes, mortars and concretes, and the strength activity index of densified silica fume in concrete is the highest while that in paste is the lowest. The results show that silica fume can significantly increase the hydration degree of paste. ![]() ![]() The differences of effect of silica fume in paste, mortar and concrete were studied by determining the non-evaporable water content of pastes, measuring the compressive strengths of pastes, mortars and concretes containing 5% and 10% raw silica fume or densified silica fume with water-to-binder ratios (W/B) of 0.29 and 0.24 and investigating the properties of interfacial transition zone between hardened paste and aggregate. ![]()
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