Microstructural evolution of fly ash geopolymer: effect of alkali concentration

Abstract

Geopolymer is formed when a source of alumino-silicate is activated with highly concentrated alkaline hydroxide or silicate solution. Fly ash is a by-product of coal fired power plant and considered as suitable material for geopolymer due to its chemistry, spherical shape, and abundance availability. Microstructure of any synthetic material depends of nature of starting material and processing conditions. Similarly microstructure of fly ash based geopolymeric material also depends on fly ash characteristics, nature of activator, curing conditions and age of curing. Any change in the above parameters reflected in microstructure and developed properties accordingly. In this paper the effect of alkali activator concentration on microstructural evolution of fly ash geopolymer was studied. The reactivity of fly ash alkali mix was measured using isothermal conduction calorimeter with different alkali concentration from 4M to 10M NaOH solution. Micro-structural characterization such as X-ray diffraction, Fourier transforms infrared spectroscopy, FEG-SEM with EDAX analysis and TEM of selected samples were carried out. It was observed reactivity increased with alkali concentration and developed more reaction phases as gel. EDAX analysis of gel phase confirmed the presence of ASH geopolymer gel.