Cleaner production of geopolymer materials: A critical review of waste-derived activators

Geopolymer materials as a sustainable building material have received ever-growing interest from researchers worldwide. Geopolymer binders have high early strength, improved economic benefits, the ability to withstand aggressive environments for long periods, and facilitate utilization of agricultural, industrial, and municipal wastes. However, the commercially synthesized NaOH and Na2SiO3 predominantly contribute to greenhouse gas (GHG) emissions and the cost of geopolymer materials. In order to promote the industrial acceptance of geopolymers, clean and sustainable alkaline activators are essential. Additionally, these alkaline activators are toxic and caustic in nature which can cause severe health issues. Researchers have been investigating the viability of various wastes as precursors for developing geopolymers since over a decade. Therefore, there is need for identifying and assessing waste derived activators for production of sustainable geopolymers. The purpose of this paper is to provide a brief discussion about the common precursors, geopolymer reaction mechanism, and environmental impacts of conventional activators and a critical review of waste-derived alkaline activators for the development of a robust and user-friendly geopolymer system. This literature review provides a comprehensive insight into the potential applications of waste-derived alkaline activators by comparing their effects on compressive strength and setting time, enabling an appropriate selection of activators based on availability and desired performance. This study highlights waste-derived activators that exhibit similar mechanical properties, if not better than geopolymers produced with conventional activators, which encourages these wastes for structural applications.It was found that the utilization of waste-derived activators, for example, biomass ashes and waste glass can significantly reduce the environmental impacts of geopolymer materials. However, further studies on feasibility and life cycle analysis are required to validate their sustainability potential on industrial scale, succeeded by assessment of long term durability.