A review: Reaction mechanism and strength of slag and fly ash-based alkali-activated materials

Alkali-activated materials (AAM) are known to be environmentally friendly alternatives to cement-based materials because they can potentially reduce greenhouse gas emissions and reutilize industrial by-products/wastes. To study the factors influencing the strength of slag based alkali-activated materials (BFS-AAM), fly ash based alkali-activated materials (FA-AAM), slag, and fly ash-based alkali-activated materials (BFS/FA-AAM), and clarifying their reaction mechanisms, this paper reviews current knowledge about the mechanical properties and the reaction mechanisms of BFS-AAM, FA-AAM, and BFS/FA-AAM. The precursor requirements and the strength control factors are summarized. The control factors for the strength of BFS/FA-AAM are the BFS/binder ratio, the Na₂O/binder ratio, the SiO₂/Na₂O ratio, and the w/binder ratio. Ion concentrations, determined by these control factors, play a decisive role in the development of strength. Generally, the strength is proportional to the BFS/FA ratio. The optimal values of the Na₂O/binder ratio of BFS-AAM and FA-AAM are between 5.5% and 8% and between 7 and 10%, respectively. The optimal values of the SiO₂/Na₂O ratio of BFS-AAM and FA-AAM are between 0.85 and 1.4 and between 0.6 and 1, respectively. Increasing the w/binder ratio will only benefit workability but will affect the strength negatively. A w/binder ratio of around 0.4 may strike a balance between strength and workability.