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% CO(2)) and accelerated conditions (3, 20, and 100% CO(2)) were investigated with various microscopic testing methods, including X-ray diffraction (XRD), (29)Si magic angle spinning nuclear magnetic resonance ((29)Si 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.