Microstructure Study of Early In Situ Reaction of Fly Ash Geopolymer Observed by Environmental Scanning Electron Microscopy (ESEM)

Abstract

The alkali activation of waste materials can produce a binder with similar properties as Portland cement but without the drawback linked to greenhouse gas emissions. Geopolymer pastes were made using coal fly ash as precursor and sodium hydroxide and solid sodium silicate powder as alkaline activators. One sample was activated with sodium silicate only while the second sample used a 50:50 mass% mixture of sodium silicate and 8 M NaOH solution. Fresh geopolymer paste was immediately placed in the Environmental SEM chamber for microstructure observation using the ESEM mode. The sodium silicate activator dissolves rapidly and begins to bond fly ash particles. Open porosity can be observed and is rapidly filled with gel as soon as the liquid phase is able to reach the ash particle. The liquid phase is important as a fluid transport medium permitting the activator to reach and react with the fly ash particles. During this ESEM experiment, the reaction was limited to the surface of the fly ash particles. The reaction products examined had a gel like morphology and no crystallized phase was observed. The effect of the activator on the gel composition was investigated by quantitative microanalysis. The gel analyzed in the sample activated with the mixture of sodium silicate and NaOH solution is enriched in Na and Al. In that sample, the fly ash reaction rate is more advanced considering that the gel is richer in Al and that this element results from the fly ash. In fly ash-based geopolymers, the aluminum content of the aluminosilicate gel is an indicator of the fly ash reactivity.