Formation of Zeolite Phases in Alkali-Activated Binders and Their Role in the Immobilization of Toxic Elements

Alkali-activated binders (AABs) have emerged as promising alternatives to Portland cement-based binders owing to their excellent properties and lower environmental impact. They are produced by the alkali activation of aluminosilicate precursors derived from natural minerals, calcined kaolinite clays, industrial waste, or mixtures thereof. The main reaction products are hydrated phases with predominantly amorphous to semicrystalline structures, while zeolites may also form as secondary reaction products, depending on the precursor composition, activator chemistry, and curing conditions. A subset of AABs (known as geopolymers) exhibits a highly cross-linked, three-dimensional network in which Si and Al are tetrahedrally coordinated. Alkali–metal cations are accommodated within the cavities of this framework, where they compensate for the negative charge arising from the substitution of Si4+ by Al3+. The geopolymer gel is often considered a “zeolite precursor”. These characteristics make AABs suitable for the immobilization of toxic elements. Unlike earlier reviews, this paper focuses on the formation of zeolite phases within AABs and critically examines their contribution to the immobilization of toxic elements. Particular attention is paid to distinguishing the immobilization attributable to zeolite phases from that associated with the binder matrix as a whole, since the detection of zeolites alone does not establish their specific contribution.