Development of a ternary blend geopolymer from coal fly ash, coal bottom ash and rice husk for low heat transmission and fire resistant material

Geopolymers have been considered a promising replacement for ordinary Portland cement (OPC) as a binding material. As geopolymers can be produced from alumina- and silica- rich industrial and agro-industrial waste materials and by-products such as blast furnace slags, mine tailings, coal ashes, etc., geopolymer technology has a great potential for environmental management of these types of wastes towards closing the loop in a circular economy. Further, the geopolymerization process has characteristic lower emissions and lower energy consumption vis- à-vis OPC which translates to positive environmental initiatives. In this study, the ternary mixture of the alumina- silica- rich materials coal fly ash, coal bottom ash and rice husk ash was evaluated for developing low heat transmission and fire resistant materials. Conventional geopolymer formulations use mainly coal fly ash with the coal bottom ash usually relegated to the ash ponds/ landfills. But in this study, coal bottom ash has been shown as a significant combining element in the geopolymer mixture. The rice husk ash being light and having very low thermal conductivity was able to pull lower both the heat transmissibility and volumetric weight of the resulting geopolymer. Via optimization using response surface methodology (RSM) with desirability functions, an optimum ternary mixture of 85% coal fly ash - 10% coal bottom ash - 5% rice husk ash met the performance requirements of OPC concrete according to ASTM standards for lightweight, low heat transmission, medium load masonry applications. Fire resistance tests based on ASTM E119 show better performance than similar samples of OPC specimens. Also, environmental impact analysis via embodied energy and CO2 estimation obtained 63% reduction in embodied energy and 84% reduction in embodied CO2 vis-à-vis OPC. The study has thus presented two main elements: (1) the development of a lightweight, low heat transmission, and fire-resistant material and (2) a waste management option for coal ashes and rice husk waste