Cleaner production of waste-derived alkali activators from industrial and agricultural by-products for sustainable alkali activated binders

Alkali activated binders are gaining worldwide acceptance as a sustainable alternative to conventional cement. Although industrial by-products are utilised as precursors, the manufacture of alkali activated binders is not entirely a sustainable and low-cost process due to the expensive commercial activators. Therefore, it is imperative to identify potential eco-friendly and economical alkali activators. Several studies have reported that silica-rich waste materials such as waste glass, rice husk ash etc., can effectively resolve the cost and environmental concerns caused by commercial activators. However, research studies on the performance of waste-derived alkali activators are highly limited. The present study is a systematic review of the performance of waste-derived activators in alkali activated binders. The setting time, compressive strength, flexural strength and water absorption characteristics of binders with waste-derived alkali activators are presented. In addition, the influence of waste-derived activators is methodically compared with commercial activators. Alkali activated concrete using potassium-based activators has better compressive strength than sodium-based activators. Based on the detailed review, an optimum alkali concentration of the activator solution in the range of 8–10 molarity is recommended. It is observed that the reaction products of rice husk ash and waste glass based activators used specimens are similar to that of alkali activated specimens with commercial activators. The significance of activator modulus in the depolymerisation of precursors is also evident from the denser microstructure of alkali activated binder (AAB) activated using activator solutions of higher Ms modulus. Moreover, the environmental impact of using waste-derived activators is discussed to assess the performance of AAB from a sustainability point of view.