Carbonation of blast furnace slag concrete at different CO2 concentrations: Carbonation rate, phase assemblage, microstructure and thermodynamic modelling

The carbonation behavior of concrete with different blast furnace slag replacement ratios (0 %, 50 % and 70 %) at different CO2 concentrations (natural carbonation at 0.04 % and accelerated carbonation at 1 % and 2 %) was investigated in terms of carbonation rate, carbonation products assemblage and pore structure. The results show that the porosity of concrete decreases with the increase in CO2 concentration regardless of mix proportion. The lower porosity at a higher CO2 concentration can lead to an underestimation of the natural carbonation coefficient from accelerated carbonation tests. The carbonation degree of the C-S-H is one of the key factors that determine the carbonation behavior at different CO2 concentrations. A higher CO2 concentration enhances the consumption of the high-density C-S-H and therefore the carbonation of unreacted clinker. However, the C-S-H formed from GGBFS hydration is carbonated to a more advanced state (considerable silica gel is formed) under natural carbonation.