Effect of Si/Al molar ratio on the strength behavior of geopolymer derived from various industrial waste: A current state of the art review

Geopolymer, also termed as alkali-activated material (AAM), is a promising concrete technology having the advantages of low carbon emission and resource reclamation. Intensive studies have verified the feasibility of using waste as an aluminosilicate precursor (AP) to synthesize AAM. However, the inconsistent characteristics of raw materials are an obstacle against its industry scale application. To address this issue, Si/Al molar ratio can serve as a non-dimensional index to standardize raw material and offer a potential solution to quality control of AAM feedstock supply chain. Even though the crucial role that molar ratios will play in the quality control of AAM, studies on the correlation between Si/Al molar ratio and AAM properties are limited. To fill this gap, this paper provided an in-depth review from the perspective of the Si/Al molar ratio and summarized their correlation with properties of various types AAMs, validating the feasibility of using the molar ratio to formulate AAM and to maintain stable performances. Based on these, a three-step strategy that can more effectively convert an ever-broader range of waste material with high variability into normalized AP was proposed, which may serve as a guide regarding waste valorization and AAM quality control.