Review of the materials composition and performance evolution of green alkali-activated cementitious materials

Abstract

Alkali-activated cementitious materials (AAMs) are a kind of green building materials compared to Portland cement components. This review considers the related research on the AAMs to systematically summarize the results on its materials composition and performance influencing factors. The precursor material is mainly industrial solid waste containing Al2O3 and SiO2. The alkali activator is compound containing caustic alkali and basic elements, which can provide an alkaline environment. Meanwhile, some new green precursors and alkali activators are summarized in this review. The main mechanism of alkali-activated reaction is that the hydroxide ion from the alkali activator nucleophilically attacks the covalent bonds of Al-O and Si-O in the precursor material, which generates Si(OH)(4) and Al(OH)(4)(-). They generate three-dimensional network gel of tetrahedral structure of [SiO4] and [AlO4](-) through polycondensation reaction, which forms cement stone-like block material after setting and hardening. Too high or too low concentration of alkali activator is detrimental to the workability and mechanical properties of AAMs. When the ratio of Na/K to Al is small, the mechanical properties of AAMs are reduced. When the Ca content in the AAM is high, calcium ions enter the polycondensation chain and reduce the degree of polymerization and mechanical properties of the aluminosilicate gel phase and increase the shrinkage deformation. The hydration products in the AAM are free of calcium hydroxide, calcium aluminate hydrate, ettringite, etc., and can resist the erosion of acid and sulfate media. The main aim is to provide an informed outlook on the advantages and drawbacks of AAMs and present a comprehensive review of the studies performed in this area.