Thermal stability of a waste-based alkali-activated material for thermal energy storage

Abstract

Moving into a carbon constrained world it is imperative that new forms of energy generation and storage are implemented to replace the fossil-fuel driven economy. Additionally, this dependence on fossil-fuels has generated large amounts of waste material (such as fly ash and black slag) which currently occupies large amounts of landfill area. Therefore, this current study investigated the thermal performance and stability of a waste-based alkali-activated material (AAM) for use as a high temperature storage material. Samples contained fly ash from a coal fire power plant, slag from an iron refinery, and commercially available sodium silicate solution. Thermo- and physical properties such as density and heat capacity were measured showing excellent properties as a storage material. Microstructure analysis by scanning electron microscopy, energy dispersive x-ray spectroscopy (SEM/EDS), and x-ray diffraction (XRD) was also conducted. Starting waste material indicated high concentrations of iron oxides, aluminosilicates, calcium hydrates, and minimal alkali activators such as sodium or potassium. A sodium silicate solution provided the necessary alkali activator. The thermal stability of this material was measured using a simultaneous thermal analyser (STA) indicating that the material is stable until 800 °C while the formation of aegirine, a mineral with thermal stability up to 990 °C, was found in the thermally cycled AAM suggesting this value could be higher. Lastly, the thermo- and physical properties, as well as the microstructure and thermal stability, of the material were reassessed after 50, 100, and 200 thermal cycles (635–800 °C) showing minimal weight loss and chemical change after the initial heating cycle. The desirable thermo- and physical properties coupled with excellent thermal stability and sample repeatability, indicate that this material could provide an option for high temperature thermal storage in the future.