Determining the Reactivity of Low and High Carbon Fly Ashes for Application in Geopolymer Concrete

Determining the Reactivity of Low and High Carbon Fly Ashes for Application in Geopolymer Concrete

Abstract

            Portland cement, the basic ingredient in concrete, is the most commonly used construction material in the world. However, portland cement production results in the generation of greenhouse gases from both energy consumption and chemical changes associated with the production process. In general, production of one ton of portland cement results in approximately one ton of the greenhouse gas, CO₂, released into the atmosphere. In an effort to reduce our dependence on traditional portland cement-based concrete, ongoing research is concentrating on development of new low environmental impact alternative cements. Geopolymer cement concrete is one recently developed alternative. Geopolymer cement concrete uses different raw materials and different production processes than portland cement, which enable a five to six times reduction in greenhouse gas emissions.

The process for geopolymer production includes mixing an anhydrous alumniosilicate material with an alkaline solution such as sodium or potassium hydroxide. The industrial waste: fly ash can be used as potential sources of the alumniosilicate materials required in geopolymer cement production.

The mineralogy of the fly ashes, including the proportions of crystalline phases (quartz, mullite, hematite, magnetite etc.) and amorphous phases directly affects the rate of the geopolymerization reaction. Therefore, quantitative characterization of crystalline and amorphous phases in the fly ash is an essential requirement for the accurate study of the geopolymerization reaction.  Traditionally, fly ash composition is given as bulk elemental oxide content, generally determined by X-ray fluorescence spectroscopy. However, such analysis does not give exact idea about highly reactive amorphous phases of similar elemental composition.

In this research, the proportion of amorphous (glassy) material in one low and one high loss on ignition (LOI) fly ash has been evaluated by X-ray powder diffraction (XRD) using the Rietveld quantitative phase analysis method. The method calculates the amorphous content in fly ashes from the small overestimation of an internal crystalline standard in a Rietveld refinement of an appropriate mixture. Two different internal standards (corundum and rutile) were used for calculation of amorphous content. Of these two standards studied, rutile gave the best results.

A good estimate of the percentage of amorphous phases present in the low and high LOI fly ashes was made using the Rietveld XRD method. The results of quantitative XRD analysis of low LOI fly ashes were found to be consistent with published data for fly ash samples. The obtained results will be helpful to understand reactive (amorphous) components of the fly ashes. Using the amorphous composition improves the development of fly ash- geopolymer mixture formulation and will give insight on the production of the geopolymer cements.