Preparation of geopolymer type binder from Mongolian fly ash and its characterisation

Abstract

At presently there are five operating power plants in the central energy system of Mongolia which generate both heat and electricity. Together these burn about 5-6 million tonnes of coal resulting in more than 600,000 tonnes of coal combustion by-products per year. Three thermal power plants are located in Ulaanbaatar city and produce more than 80% of the total electricity in Mongolia. Within these plants, the 4th thermal power plant generates around 300,000 tonnes of fly and bottom ash and the 3rd thermal power plant around 100,000 tonnes of bottom ash. The thermal power plants in Ulaanbaatar city use coal from the Baganuur and Shivee-ovoo deposits. The chemical composition of these fly ashes indicate they belong to class C fly ashes. The physical and chemical characteristics of fly ash and boiler slags determine the how to make of use them. The physical properties of these residues will be determined by the technology, design and operational conditions of the power stations. The chemical composition of the fly ash and slags depends completely on the inorganic matter within the source coal deposits and the composition of the slags and fly ashes cannot be controlled. Comprehensive characterization of coal combustion by products were performed using XRD, XRF, SEM and gamma-spectroscopy methods. A novel geopolymer type binder was prepared using Baganuur and Shivee ovoo fly ashes from the 4th Thermal power plant. Sodium hydroxide solutions with a different molarity were used as the activating solutions. It was found that an alkali concentration increase resulted in an increase in the compressive strength of the geopolymer paste. Compressive strengths of greater than 20 MPa was achieved with Shivee ovoo fly ash based geopolymer while with Baganuur fly ash it was more than 30 MPa.