Effect of nano silica on properties of concretes containing recycled coarse aggregates

The effect of nano silica on mechanical and durability properties of concretes containing recycled coarse aggregates (RCA) is investigated using seven series of concretes: the first is the control series, containing all natural aggregates and no nano silica. In the second and third series, 25% and 50% (by weight) natural coarse aggregates are replaced by RCA, respectively. Effects of nano silica on concretes containing RCA are evaluated in the remaining series: in the fourth and fifth, 1% and 2% (by weight) nano silica is added to concrete containing 25% RCA, respectively. Similar nano silica additions are used in the sixth and seventh series, but the RCA content is 50%. Compressive and tensile strengths of all concretes are evaluated after 7, 28 and 56 d of water curing. Concrete durability properties, such as sorptivity, volume of permeable voids and chloride penetration, are also evaluated after 7 and 28 d of water curing. Addition of nano silica is found to significantly improve compressive strength of concrete containing 25% RCA at all ages; however, no such improvement is observed in concrete containing 50% RCA. An opposite scenario is observed for tensile strength, where nano silica addition showed improvement in concrete containing 50% RCA. The addition of nano silica also exhibited improvement in the durability properties of concretes containing 25% and 50% RCA.