Fresh, Setting, and Hardened Properties of Fly Ash Concrete with Nano-Silica

Nano-silica (NS) may counteract the drawbacks of fly ash (FA), such as delayed setting and low early strength, by accelerating hydration and providing higher early strength in concrete. In this study, concrete mixtures having 4 different FA replacement ratios (0%, 20%, 35%, and 50% by vol.) and 3 different NS dosages (0%, 1.7%, and 3.4% by vol.) were prepared. Effect of NS on the rheology, setting times and temperature evolution during the setting period, microstructure, compressive strength, and modulus of elasticity (MOE) of concrete at constant slump (20 ± 1 cm) were investigated. Plasticizers influenced the rheological and setting properties of concrete designed at constant consistency. In terms of these properties, although there have been cases where the use of NS has shown controversy results compared to the common knowledge in literature, this situation has been associated with the demand for the plasticizers consumed to obtain constant slump. In general, NS accelerated the setting times of concrete, however, it could not completely tolerate the delay caused by FA. According to 7-day mechanical test results, using 3.4% NS almost fully recovered the 20% strength loss caused by 20% FA replacement. Even if compressive strength close to NS-free REF mixture could not be achieved with high volume FA replacement (50%), at 7 days, MOE results, comparable to NS-free REF concrete, could be achieved using 3.4% NS. At later ages, although the compressive strength varied over a wide range, i.e., from 30 to 75 MPa, MOE of FA concrete have become almost independent of the strength.