Recycling waste materials in geopolymer concrete

The widespread industry adoption of geopolymer concrete has the potential to positively contribute to environmental sustainability in both the industrial and construction sectors, through the recycling of waste materials, and the reduction in carbon emissions. Extensive research has been conducted into geopolymers during the past two decades, demonstrating the potential for the alkali-activated cement to replace ordinary Portland cement. However, there are a number of challenges facing the adoption of geopolymers. Much of the research into geopolymers uses sodium silicate solution as the alkali activator. Studies have noted that sodium silicate solutions are highly corrosive, and, as such, can be defined as user-hostile systems. Alternative alkali activators, such as potassium silicate solutions, have been proposed as more user-friendly and therefore more favourable for industry adoption. The highly variable nature of waste materials needs to be a focus of future research, with mix designs that focus on locally available waste materials with minimal processing. Much research has focused on heat-cured geopolymers; however, this increases the embodied energy while reducing the environmental benefit, which also acts as a limiting factor for in situ applications. Research into ambient temperature curing, addressing the issues of compressive strength, the rate of strength development, and curing time is required. Durability issues need to be addressed with studies finding the compressive strength of geopolymers being reduced after relatively short time periods of immersion in water, and potential problems relating to chloride induced corrosion of reinforcing steel. Further research is recommended for developing standardized leachate analysis for geopolymers containing recycled waste materials. The objective of this paper is to review the research into waste-incorporated geopolymers and highlight the barriers to industry adoption with a view to pointing the way forward for future research.