Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements.

The phase changes in alkali-activated slag samples when exposed to supercritical carbonation were evaluated. Ground granulated blast furnace slag was activated with five different activators. The NaOH, Na2SiO3, CaO, Na2SO4, and MgO were used as activators. C-S-H is identified as the main reaction product in all samples along with other minor reaction products. The X-ray diffractograms showed the complete decalcification of C-S-H and the formation of CaCO3 polymorphs such as calcite, aragonite, and vaterite. The thermal decomposition of carbonated samples indicates a broader range of CO2 decomposition. Formation of highly cross-linked aluminosilicate gel and a reduction in unreacted slag content upon carbonation is observed through 29Si and 27Al NMR spectroscopy. The observations indicate complete decalcification of C-S-H with formation of highly cross-linked aluminosilicates upon sCO2 carbonation. A 20-30% CO2 consumption per reacted slag under supercritical conditions is observed.

Thermodynamic Modeling Study of Carbonation of Portland Cement.

The assessment of the extent of carbonation and related phase changes is important for the evaluation of the durability aspects of concrete. The phase assemblage of Portland cements with different clinker compositions is evaluated using thermodynamic calculations. Four different compositions of cements, as specified by ASTM cements types I to IV, are considered in this study. Calcite, zeolites, and gypsum were identified as carbonation products. CO2 content required for full carbonation had a direct relationship with the initial volume of phases. The CO2 required for portlandite determined the initiation of carbonation of C-S-H. A continual decrease in the pH of pore solution and a decrease in Ca/Si is observed with the carbonation of C-S-H. Type II cement exhibited rapid carbonation at relatively less CO2for full carbonation, while type III required more CO2 to carbonate to the same level as other types of cement. The modeling of carbonation of different Portland cements provided insights into the quantity of CO2 required to destabilize different hydrated products into respective carbonated phases.