In the present study, split tensile strength together with pore structure, thermal behavior and microstructure of concrete containing ground granulated blast furnace slag and SiO2 nanoparticles have been investigated. Portland cement was replaced by different amounts of ground granulated blast furnace slag and the properties of concrete specimens were measured. Although it negatively impacts the properties of concrete at early ages, ground granulated blast furnace slag was found to improve the physical and mechanical properties of concrete up to 45 wt% at later ages. SiO2 nanoparticles with the average particle size of 15 nm were partially added to concrete with the optimum content of ground granulated blast furnace slag and physical and mechanical properties of the specimens were studied. SiO2 nanoparticle as a partial replacement of cement up to 3 wt% could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH)(2) amount at the early age of hydration and hence increase split tensile strength of concrete specimens. The increased the SiO2 nanoparticles' content more than 3 wt% causes the reduced the split tensile strength because of the decreased crystalline Ca(OH)(2) content required for C-S-H gel formation. SiO2 nanoparticles could improve the pore structure of concrete and shift the distributed pores to harmless and few-harm pores. (C) 2010 Elsevier B.V. All rights reserved.