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This study compares the phase assemblage and pore structure variation of Portland cement and blended cements, containing limestone and burnt oil shale, during carbonation. Hardened cement paste was exposed to natural carbonation (400 ppm of CO2) at a relative humidity of 70 % for one year. Samples were prepared and cured at 95 % RH for 28 days. Reference mixes were exposed to CO2-free atmosphere to decouple the effect of ongoing hydration and drying from that of carbonation. The phase assemblage was investigated using XRD-Rietveld refinement and thermogravimetric analyses. The porosity and pore structure was assessed based on the results from mercury intrusion porosimetry. Additionally preliminary results of gas diffusion experiment of carbonated and non-carbonated samples are presented.