Carbonation of alkali-activated materials: A review of mechanisms, testing methods, and research gaps

Alkali-activated materials (AAMs) have attracted considerable attention as low-carbon alternatives to ordinary Portland cement (OPC) due to their resource utilisation potential, excellent mechanical properties, and durability. However, carbonation remains a critical factor affecting their long-term performance. Compared with OPC, carbonation in AAMs involves more complex physicochemical processes owing to differences in pore solution chemistry, gel composition, and calcium content. In addition, significant uncertainties remain regarding the applicability of accelerated carbonation testing and the relationship between accelerated and natural carbonation behaviour. This review provides a comprehensive assessment of the carbonation behaviour of AAMs. A bibliometric analysis is first conducted to identify major research trends over the past decade. Subsequently, carbonation mechanisms, testing methodologies, and key influencing factors are critically reviewed. Attention is given to the effects of environmental conditions, including CO 2 concentration, relative humidity, temperature, and pre-curing regimes, as well as mix design parameters such as precursors, activators, aggregates, and additives. The differences between accelerated and natural carbonation are also systematically discussed. Based on the available evidence, an integrated mechanistic framework linking CO 2 transport, pore solution reactions, gel evolution, pore structure development, and performance changes is proposed. The review highlights that carbonation resistance is governed by the coupled effects of chemical buffering capacity and transport properties, while the discrepancy between accelerated and natural carbonation arises from the interaction of transport, reaction, and microstructural evolution processes. Finally, current research gaps and future directions for carbonation testing standardisation, predictive modelling, and durability assessment are discussed.