Carbonation of alkali-activated materials: changes in mineralogical composition and pH, and qualitative estimation of corrosion risk

The aim of this paper is to investigate the effect of natural and accelerated (1% CO₂) carbonation on three different alkali-activated materials (AAMs), including a metakaolin geopolymer (GMK), a sodium silicate ground-granulated blast-furnace slag (GGBS), and a sodium carbonate GGBS, and compare their performances with those of Ordinary Portland cement (OPC). Mortar and paste samples were prepared and evaluated by using X-ray diffraction (XRD) and thermogravimetric analyses (TGA) before and after exposure to carbonation. The depth of carbonation was determined by using four pH-indicators, and the impact of carbonation on steel corrosion was assessed by examining steel flakes embedded in the pastes. The results showed that the carbonation kinetics of AAMs was higher than that of OPC under both carbonation conditions. Furthermore, mineralogical analyses indicated that carbonation mechanisms of AAMs were different from those of OPC, particularly for GMK. Interestingly, no steel corrosion was observed in carbonated GMK-based pastes due to the high pH of the carbonated zones. Overall, this study highlights the potential of AAMs as alternatives to OPC in the construction industry.