Opportunities to improve sustainability of alkali-activated materials: A review of side-stream based activators

Abstract

Alkali-activated materials (AAMs) are alternative binders that can be used instead of Portland cement in construction. One of the main drivers in their utilization is that AAMs can be designed to have lower CO2 emissions using industrial residues. The main component of AAMs is aluminosilicate precursor, which is frequently waste based. However, the sustainable profile and cost-efficiency of AAMs are greatly affected by the selection of the alkali activator. Commonly used activators include bulk chemicals, such as sodium hydroxide or silicates, which have a relatively high carbon footprint and cost that can inhibit AAM applicability for large-scale construction applications. Consequently, several locally available, but underutilized, agricultural and industrial by-products or wastes have been investigated as alternative alkali activators. Globally, millions of tons of these residues are produced annually, and currently, they are mostly landfilled. Utilization of these residues as alternative alkali activators not only solves the residues-management issue, but also has noteworthy economic and environmental benefits. Utilization and properties of these waste-based activators in AAMs are comprehensively reviewed in this manuscript. Various studies showed the formation of alkali silicates from amorphous silica residues, alkali hydroxides, carbonates, and aluminates sourced from biomass and industrial residues. The resulting binder exhibited similar reactivity to commercial activators and considerable binder strength. However, the durability, shrinkage, and workability properties were not investigated in most of the reviewed studies.