A comparative study of alkali-activated fly ashes under heat-exposure

Abstract

Coal fly Ash (FA) is a by-product of combustion processes in coal fired power plants. Annually over 800 million tons of fly Ash is being produced all around the world. Only a small portion of this Ash is reutilized; a major portion goes to landfills or dumped at the sea. The disposal of FA brings significant economic and environmental concerns. Alkali-activation (AA) of fly ashes is a possible way to put some of the FAs in a beneficial use. Alkali-activated fly ashes (AAFA) possess good mechanical properties, acid and fire resistance and thermal stability. This study aims to evaluate the alkali-activation and heat-exposure performances of FAs with varying Ca-contents from three different thermal power plants in Turkey through compressive strength tests and XRD measurements. The strength development of the AAFA samples were followed by 3-, 7- And 28-day compressive strength tests. The heat-exposure performance of the AAFA samples were determined by measuring the residual compressive strengths of the 28-day old samples after being exposed to elevated temperatures (400,700 and 1000°C). It is concluded that not only the amorphous content, but also the fineness of the FA has a critical importance on the strength development of the AAFAs. The AA samples of the FA with highest Ca-content lost considerable strength after exposure to elevated temperatures, possibly due to decomposition of the Ca-bearing phases. The AA samples with medium Ca-content did not show significant changes during heat exposure. The AA samples with low Ca-content showed enhanced performance after being exposed to 400°C. The role of Ca in performance of AAFAs under heat-exposure needs to be further studied. The X-ray Diffraction (XRD) patterns of the FAs showed that zeolite type phases has formed during activation. Zeolite phases remained stable during heat treatment until 700°C. After 700°C zeolite type phases started to dehydrate and form stable phases such as Nepheline and Haüyne.