Effects of nano-SiO2 on early strength and microstructure of steam-cured high volume fly ash cement system

The utilization of fly ash in concrete production can reduce pollution as well as decrease greenhouse gas emission. However, a high replacement ratio of cement with fly ash largely reduces early strength, and this is the main obstacle for industrial application. In order to utilize high volume fly ash in concrete, nano-SiO2 (NS) was introduced in a steam-cured cementitious system to induce the early hydration, with intention to promote the early strength. The mortar (50 wt% fly ash and 50 wt% cement) with various dosages of NS were prepared and cured under steam with the temperature 60 +/- 2 degrees C for 9 h. The flexural and compressive strength results revealed that NS could largely increase the early strength of the high volume fly ash mortar. Paste samples were prepared in parallel for microstructure test to illustrate the mechanism of the addition of nano-silica. It can be seen from Si-29 NMR and SEM that both cement hydration and pozzolanic reaction of fly ash were accelerated with NS. Fillability of NS and more C-S-H gel resulting from pozzolanic reaction of NS also decrease the porosity and refine the pore structure, as seen in MIP test. Interfacial transition zone (ITZ) was improved due to the reaction of NS with CH, resulting in a more compact structure and less Ca/Si ratio. The results show that NS contributes to the early strength of high volume fly ash concrete by an increased hydration degree of cement and fly ash as well as pore refinement. (C) 2018 Elsevier Ltd. All rights reserved.