Geopolymer

A geopolymer is a binder formed when an aluminosilicate precursor, such as metakaolin or fly ash, reacts with an alkaline activator to produce a hardened aluminosilicate network. The term is often used loosely. Strictly, geopolymers are the low-calcium subset of alkali-activated materials; high-calcium systems such as alkali-activated slag harden mainly through calcium silicate hydrate phases and are chemically closer to conventional cement. Keeping the distinction matters because the two behave very differently in setting, shrinkage and durability.

In practice

Alkali-activated systems are attractive for printing because their setting can be tuned over a wide range and some show pronounced structural rebuilding at rest. Against that sit real handling issues: the activators are corrosive and require trained handling, efflorescence and drying shrinkage can be severe, and precursor variability is high, since fly ash and slag differ by source and are becoming less available in Europe. Standardisation is also incomplete, as European structural design and conformity rules are built around Portland-based cements, so a project using these binders usually needs project-specific testing and approval rather than routine compliance.

Related terms

• Supplementary cementitious materials — the same industrial powders, used to replace clinker rather than activated alone.
• LC3 — a low-clinker route that stays inside conventional cement chemistry.
• Set-on-demand — controlled late stiffening, achievable in several alkali-activated systems.