AbstractThis study investigated the efficiency of CO₂ curing in blast-furnace slag pastes activated with three concentrations of KOH solution (3, 4, and 5 M). The CO₂ curing was applied into the alkali-activated slag paste for 3 days. The CO₂-cured pastes were subjected to subsequent curing in a water bath or exposed to seawater. The mechanical properties and characteristics of the reaction products were compared. Full strength can be obtained within 3 days of the early CO₂ curing. The strength remained almost constant regardless of the activator concentration. The CO₂-cured samples produced more CaCO₃ after exposure to the seawater. The carbonates filled the micropores of the samples, and no strength reduction was observed. The hydrates Ca(OH)₂ and C–S–H(I) are carbonated, forming calcite and amorphous CaCO₃ during the subsequent curing and exposure to seawater as well as the CO₂ curing. A less dense matrix with a lower activator concentration facilitated the diffusion of CO₂ and promoted early carbonation in the paste. The polymerization of C–S–H(I) was relatively slow with low activity, and within the period of CO₂ curing, more carbonates were produced in the sample prepared at a low activator concentration.