One of the concerns regarding the elastic behavior of self-compacting concretes (SCC) is their lower stiffness tendency compared to conventional concretes (CC). Because of mixture design methods applied, SCC mixtures, in general, contains higher amounts of fine materials. The use of high volume of fine materials causes a decrease in the amount of total aggregates which have an important role on the strength and elastic properties of concretes. This paper discusses the results of an experimental study which investigated the strength, elastic and microstructural properties of SCC mixtures modified with different particle sizes of colloidal nano-SiO2 (CNS). CNS samples, having 35, 17, and 5 nm average particle size, were used with the aim of reducing the total fine material (fly ash) content in SCC designs. With the help of CNS, the amount of fly ash in reference SCC mixture design was gradually reduced and replaced with aggregates. The mixtures which fulfill desired fresh SCC properties were subjected to compressive tests at 28th and 120th days. Moduli of elasticity of concrete specimens were also measured. Results have shown that, although the use of CNS in reduced-fly ash content mixtures could not compensate the strength decrement, the stiffness of SCC mixtures containing CNS together with fly ash has been strikingly improved even the increase in total aggregate content is not noteworthy. Microscopic investigations has indicated the development of a dense C-S-H gel and interfacial transition zone. (C) 2017 Elsevier Ltd. All rights reserved.