Thermodynamic Modeling and Experimental Study of Carbonation of Alkali-Activated Slag Cements

The present study investigates the carbonation of slags activated by NaOH, Na2SiO3, Na2CO3, and Na2SO4. The main hydrates in those activated with NaOH or Na2SiO3 were C (N-)A-S-H, stra''tlingite, and MgAl-layered double hydroxide phases as observed using different experimental characterization techniques and thermodynamic modeling. The use of a Na2CO3 activator led to the formation of calcium carbonate and monocarbonate instead of stra''tlingite, and Na2SO4 to ettringite formation. Carbonation of the alkali-activated slags in a powdered form at 1 vol % concentration of the CO2 environment proceeded rapidly, while little change was observed after 7 d. The main carbonation products commonly identified were calcium carbonate, carbonate containing-MgAl-layered double hydroxide, and N-A-S-H. In Na2SiO3- and Na2SO4-activated slags, M-S-H formation was predicted, which destabilized the MgAl-layered double hydroxide phases. Stra''tlingite and monocarbonate, which were initially present or formed as a transient phase, were transformed to N-A-S-H during carbonation, while C-(N-)A-S-H was only partially decalcified and destabilized.