Slag Blended Cement Paste Carbonation under Different CO(2)Concentrations: Controls on Mineralogy and Morphology of Products

To investigate the effect of different CO(2)concentrations on the carbonation results of slag blended cement pastes, carbonation experiments under natural (0.03% CO2) and accelerated conditions (3, 20, and 100% CO2) were investigated with various microscopic testing methods, including X-ray diffraction (XRD),Si-29 magic angle spinning nuclear magnetic resonance (Si-29 MAS NMR) and scanning electron microscopy (SEM). The XRD results indicated that the major polymorphs of CaCO(3)after carbonation were calcite and vaterite. The values of the calcite/(aragonite + vaterite) (c/(a + v)) ratios were almost the same in all carbonation conditions. Additionally, NMR results showed that the decalcification degree of C-S-H gel exposed to 0.03% CO(2)was less than that exposed to accelerated carbonation; under accelerated conditions, it increased from 83.1 to 84.2% when the CO(2)concentration improved from 3% to 100%. In SEM observations, the microstructures after accelerated carbonation were denser than those under natural carbonation but showed minor differences between different CO(2)concentrations. In conclusion, for cement pastes blended with 20% slag, a higher CO(2)concentration (above 3%) led to products different from those produced under natural carbonation. A further increase in CO(2)concentration showed limited variation in generated carbonation products