Microscopy and microanalysis of alkali activated fly ash binder

Abstract

Large quantities of fly ash (FA) are produced worldwide by burning of pulverized coal in the thermal power plants. It is a dustlike material which usually contains significant amount of amorphous phase due to rapid cooling of the molten material. It is highly heterogeneous material, as FA particles show different chemical and phase composition. Alkali activation of FA with a concentrated alkali metal silicate or hydroxide solution transforms it to a high performance binder. Amorphous aluminosilicate phase present in FA dissolves in alkaline solution creating aluminosilicate gel as a final product of the reaction. Due to this three-dimensional highly interconnected aluminosilicate gel new binder shows excellent mechanical properties and high durability. In this work, FA samples from Serbian power plants were treated with sodium silicate solution. Alkali activation process was performed for 24h at 95°C and followed by curing of binder samples in humid chamber for 365 days. Microstructure of the samples was studied by electron microscopic analysis (scanning electron microscope, SEM, coupled with energy dispersive X-ray spectrometer, EDS). SEM/EDS analysis provided useful insight into the process of dissolution of specific FA particles by alkaline solution. Alkali solution dissolves amorphous aluminosilicate phase in FA, while crystalline phases remain unaltered. Iron and calcium rich FA particles remained unreacted in the alkali activation process. Some aluminosilicate particles were also unaffected by alkaline solution due to the presence of crystalline phases or due to the lower solubility of certain amorphous phase.