Effect of natural and accelerated carbonation on microstructure and pH of sodium carbonate alkali-activated slag

This study aims to understand the effect of the natural and accelerated carbonation on the alkali-activated slag (AAS) activated by sodium carbonate. Carbonation increased AAS paste total porosity but reduced slag containing materials (CEM III) porosity. The pH after carbonation was higher in AAS than in CEM III, which was explained by the buffering effect of the alkalis and the carbonation of the hydrotalcite. A higher CO2 percentage promoted the precipitation of nahcolite (NaHCO3) rather than natron (Na2CO3) in AAS, which caused a decrease in the pH after carbonation in accelerated conditions. The decrease in the pH in accelerated conditions could also be due to a more advanced carbonation than in natural conditions. •The pH after carbonation is higher in AAS than in slag-based cement.•The alkalis have an important buffering effect on the pH at early carbonation age.•The carbonation of the AAS caused an increase in the total porosity.•It is recommended to maintain CO2 percentage