CO2–Binder Reaction Mechanisms in Geopolymer Wellbore Cements: Alternatives to API Class G Cement in CO2-Rich Environments (CCS)

API Classes of cement are susceptible to three major problems: carbonation, decalcification, and increased porosity of cement sheaths in CO₂-rich environments. These degradation pathways in American petroleum institute (API) Class/ordinary Portland cement (OPC) systems are well documented in laboratory and field observations for CO₂-rich wellbore service. In contrast, while geopolymer/alkali-activated binders have been increasingly studied as alternatives, the evidence remains distributed across different precursor chemistries, exposure conditions, and test protocols, and a consolidated, mechanism-based synthesis specific to CO₂ sequestration wells is still limited. Accordingly, this article presents a critical, narrative (non-systematic) review that synthesizes published laboratory and field studies on geopolymer/alkali-activated binders for CO₂ sequestration wells, with emphasis on permeability, strength retention, and microstructural stability under CO₂-rich exposure. The main outcome of this review is a mechanism-based synthesis that links CO₂-binder reaction pathways (gel chemistry/phase evolution) to pore-network and transport changes, and consolidates quantitative performance benchmarks (permeability and strength retention) relative to API Class G/OPC, while defining the key validation gaps for qualification (HPHT, cyclic/tensile integrity, mixed fluids, and long-term monitoring). Laboratory tests have already demonstrated that geopolymer samples have ultralow permeability and preserve 90% of their strength after being treated with supercritical CO₂ concentrations, while OPC loses its strength and produces macropores causing substantial growth of cement sheath porosity. Microstructural studies have shown that geopolymers do not contain portlandite but only N-A-S-H/C-A-S-H gels with low Ca content in concentrations high enough to create N-A-S-H/C-A-S-H gels, but do not suffer from multi-zone carbonation, as occurs for OPC concrete. Key challenges being tackled include slurry rheology, setting control and variability of precursors by designed admixture use and new performance specifications for higher-quality geopolymers. On the whole, geopolymers emerge as a sustainable and reliable alternative to traditional well cementing techniques for their sustainability well integrity.