Performance of nano-Silica modified high strength concrete at elevated temperatures Morteza Bastami a,⇑ , Mazyar Baghbadrani b , Farhad Aslani c a Department of Structure, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran b Department of Civil Engineering, University of Kurdistan, Sanandaj, Iran c Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, University of New South Wales, Australia highlights  We studied effect of elevated temperature on HSC modified with nano-Silica (nS).  We studied compressive(f 0 c > 80 MPa) and tensile strengths, spalling and mass loss.  nS used in HSC can improve its mechanical properties at elevated temperature.  The presence of nS improved the tensile strength to prevent crack extension.  The addition of nS was more effective than SF for increasing residual strengths. article info Article history: Received 29 March 2014 Received in revised form 15 June 2014 Accepted 17 June 2014 Keywords: Nano-Silica Silica fume High strength concrete Elevated temperature abstract This research studied effect of elevated temperature on of high strength concrete (HSC) modified with nano-Silica (nS) and on its compressive and tensile strengths, spalling, and mass loss (f 0 c > 80 MPa). This research studied the effect of elevated temperature on the compressive and tensile strength, spalling, and mass loss of HSC modified with nS. Six sample mixtures contained varying amounts of nS and two samples did not contain nS are considered in the experimental program. The mechanical properties of the modified HSC were measured by heating 150  300 mm sample cylinders of concrete to 400, 600 and 800 °C at a rate of 20 °C/min. The obtained results demonstrate that nS efficiently used in HSC can improve its mechanical properties at elevated temperature. The results show that the presence of nS increased residual compressive and tensile strengths, and spalling and mass loss are decreased as penetrability increased. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. General effects of nano-Silica Different effects are produced by the addition of nano-Silica (nS) or Silica Fume (SF) powder to concrete. Adding micro-silica decreases the amount of cement required an effect that is more pronounced for nS [1–5]. The main mechanism of this working principle is the high surface area of nS, which acts as a nucleation site for the precipitation of CSH gel [5]. Bjornstrom et al. [6] assert that it is unclear whether the more rapid hydration of cement in the presence of nS is caused by its chemical reactivity upon disso- lution (pozzolanic activity) or increased surface activity. Viscosity test results have shown that cement paste and mortar with nS require more water to maintain the workability of the mixtures; moreover, nS exhibits a stronger tendency for adsorption of ionic species in the aqueous medium, thus, the formation of agglomer- ates is expected. In the latter case, a dispersing additive or plasti- cizer is required to minimize this effect [5]. At high nS concentrations, autogenous shrinkage occurs as self- desiccation increases, resulting in higher cracking potential. To avoid this effect, researchers have added high concentrations of superplasticizer and water and applied appropriate curing methods [5–7]. The superplasticizer was applied at 2.6–4.2% of binder mass depending on the ratio of nS. Microstructural analysis of concrete using electronic micro- scope techniques (SEM, ESEM, TEM) has revealed that the micro- structure of nS concrete is more uniform and compact than normal concrete. Particles of nS fill the voids of the CSH gel struc- ture and function as nuclei to tightly bond with CSH gel particles. This means that the application of nS decreases the calcium leach- ing rate of cement paste and increases its durability [3–11]. SEM has shown that nano-Fe 2 O 3 and nS particles fill the pores and http://dx.doi.org/10.1016/j.conbuildmat.2014.06.026 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +98 21 22831116 9; fax: +98 21 22803933. E-mail address: m.bastami@iiees.ac.ir (M. Bastami). Construction and Building Materials 68 (2014) 402–408 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat