Properties of concrete with high volumes of unprocessed coarser fly ash and nanosilica

Abstract

The limitation of fly ash as a cement substitute in concrete is often low due to its slow rate of strength development. The use of unprocessed coarser siliceous fly ash is also negligible. An attempt was made to produce a structural-grade concrete using a high volume of unprocessed coarser fly ash and colloidal nanosilica that was curable at ambient conditions. The fly ash content in the trial mixes varied from 50 to 70%. The temperature of fresh mixes during early age indicated that adding nanosilica accelerated the rate of hydration. Also, substantial improvements in early- and later-age compressive, split tensile and flexural strengths; modulus of elasticity; and stress–strain characteristics were observed for mixes with nanosilica. The compressive strength test results were corroborated with the inferences made from scanning electron microscopy and differential thermogravimetric analysis of the hardened cementitious pastes. The material cost of the mixes with nanosilica was found to be in a range similar to that of an ordinary Portland cement concrete, and the carbon dioxide equivalent emission of such concretes was 45–65% lower. It was concluded that with the addition of 3% colloidal nanosilica, it is possible to prepare a structural-grade concrete using unprocessed coarser siliceous fly ash that is curable at ambient conditions and has significantly improved early-age strength and also high later-age strength, while at the same time achieving sustainability and favourable cost benefits.