Effect of fly ash and silica fume on transition zone, pore structure and permeability of concrete

The effect of fly ash (10, 20 and 30%) and silica fume (5 and 10%) as a substitute for cement in binary and ternary mixtures on the properties of fresh and hardened concrete are investigated. To determine mechanical properties, the compressive strength, splitting tensile strength and modulus of elasticity tests are used. Also, water penetration depth, water sorptivity, specific electrical resistivity, rapid chloride permeability and rapid chloride migration tests are applied to evaluate concrete durability. To examine the pore structure and transition zone morphology of concrete specimens, mercury intrusion porosimetry and scanning electron microscopy are used, respectively. The fresh concrete results show that the presence of silica fume in binary and ternary mixtures reduces workability and air content, but fly ash increases them. Adding silica fume to mixtures containing fly ash increases the mechanical strength at the ages of 7, 28 and 90 d. In addition to increasing the quality of the aggregate-cement matrix bond and improving the pore structure, the presence of supplementary cementitious materials has a considerably positive effect on the transport properties of concrete at all ages.