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To investigate the effect of different CO₂ 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), ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and scanning electron microscopy (SEM). The XRD results indicated that the major polymorphs of CaCO₃ 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% CO₂ was less than that exposed to accelerated carbonation; under accelerated conditions, it increased from 83.1 to 84.2% when the CO₂ 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 CO₂ concentrations. In conclusion, for cement pastes blended with 20% slag, a higher CO₂ concentration (above 3%) led to products different from those produced under natural carbonation. A further increase in CO₂ concentration showed limited variation in generated carbonation products.