Effect of carbonation curing on the physical, mechanical, and microstructural properties of metakaolin-based geopolymer concrete

•Geopolymer requires thermal cure preconditioning to allow CO₂ to enter.•The geopolymer exhibited carbonation depth similar to Portland cement, but with less CO₂ absorbed.•Carbonation curing improved the permeability properties of the geopolymer.•An increase in compressive strength occurred immediately after the carbonation curing step.•Carbonation curing did not affect the strength of the geopolymer after additional curing. Carbonation curing can absorb CO2 and fix it as a carbonate mineral in cementitious materials. Carbonation curing in Portland cement-based materials has been widely discussed in the literature, unlike carbonation curing in geopolymers. Therefore, the objective of this article was to evaluate the effect of carbonation curing on the physical, mechanical, and microstructural properties of metakaolin-based geopolymer concrete. Changes in pH value, carbonation depth, CO2 absorption, compressive strength, and microstructure formation were determined before and after carbonation curing. The results showed that the carbonation depth of Portland cement concrete is similar to the carbonation depth of geopolymer concrete. Furthermore, compressive strength after post-conditioning was not affected by carbonation curing, remaining statistically equal to reference concrete. Also, the formation of carbonates (Na2CO3 and CaCO3) refined the pore structure and increased closed pores, reducing the void ratio and the water absorption (permeability properties). A hybrid system containing N-A-S-(H) gel and carbonate crystals was identified.