Investigation of Nano-SiO2 Effects on High-Volume Fly Ash Concrete

In this paper, laboratory studies have been done on the mechanical properties and microstructure of high-volume fly ash concrete incorporating nano-SiO2. The results were compared with those of ordinary Portland cement (OPC) and of high-volume fly ash concrete (HFC). In this study, in order to determine effects of nano-SiO2 additives on properties of y ash mortar, 1%, 2% and 4% of nano-SiO2 was added to the mixes as a replacement of cement to investigate their physical properties and micro-structures. In all mixtures, water-binding ratios of 0.5 and 0.6 have been used. Also in all mixtures except OPC samples, constant substitution amount of 40% of y ash was proposed. Assessments of these concrete mixes were based on short-term and long-term performance. Compressive strength tests on specimens cured at 3, 7, 28, 56, 90 and 120 days, tensile strength tests on specimens cured at 7 and 28 days and finally SEM test on selected specimens cured at 7 and 120 days were carried out. Results indicated that because of low pozzolanic reaction of fly ash, early strength of HFC decreased considerably. Besides, nano-SiO2 in addition to its effects as a ller, because of considerable increase of surface-to-volume, promoted pozzolanic reaction. So, it results in developing the strength of HFC especially in early ages. Also, by increase of curing age of concrete because of activation of fly ash and decrease of Ca(OH)2 due to hydration, strength was developed relative to OPC sample.