Engineering Geopolymers for Sustainable Construction: A Critical Review from Synthesis and Performance to Life-Cycle Impact

Background: Low-calcium geopolymers, alkali-activated aluminosilicate binders cured below 100 °C, are widely studied as substitutes for Portland cement, a major source of anthropogenic CO₂. The evidence on their performance, durability, environmental benefit and cost is scattered and often quoted without the system it came from. Objective: To follow geopolymers from synthesis to life-cycle impact and to separate robust findings from system-specific ones. Methods: Narrative critical review based on keyword searches and citation tracing. The graphics replot published values, several from the authors’ own studies. No new experiments were performed. Findings: Geopolymers harden by dissolution and polycondensation into an amorphous alkali aluminosilicate network, not the calcium–silicate–hydrate gel of Portland cement. Sulfate, acid and fire resistance is generally good (in one replotted study, concrete kept 72% of its strength after 800 °C, against 33% for Portland concrete), but carbonation, efflorescence, shrinkage and workability depend strongly on precursor and activator dose, and RILEM TC 247-DTA round-robin tests show that Portland-cement test methods need validation for alkali-activated concretes. In two life-cycle assessments of 80 MPa concretes, a peer-reviewed slag-based geopolymer reached 23% of the Portland global warming potential but had higher ozone depletion, and a non-peer-reviewed sludge-based preprint reached 27%. A meta-analysis reports average greenhouse-gas savings of 12%, 30% and 50% for low-, normal- and high-strength concrete, at about twice the cost. Conclusions: Geopolymer concrete is a promising but conditional low-carbon binder. Its benefit depends on the activator, the system boundary and the strength grade, and cost, mixed results in other impact categories and missing performance-based standards limit routine use.