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Carbonation is one of the durability concerns for fly ash blended concrete. Unlike concrete made with ordinary Portland cement (OPC), the microstructure of fly ash blended concrete evolves in a different pattern during carbonation. In this research, paste samples blended with variant amounts of fly ash were taken for accelerated carbonation, and based on an extend X-ray attenuation method (XRAM), spatial distribution of porosity for partly-carbonated fly ash-blended pastes was investigated for the first time. Based on XRAM, the porosity before carbonation is 0.42, 0.35, 0.36 and 0.46 respectively for OPC, FA30, FA50 and FA70, and the porosity changed to 0.36, 0.42, 0.46 and 0.49 respectively after carbonation. The evolved microstructure subjected to carbonation was verified by mercury intrusion porosimetry (MIP). For quantitative porosity results, very sharp porosity gradients were observed on the carbonation front, which attests that all carbonations in this study were diffusion controlling processes. The results of current research can be used for the verification of carbonation models and for the understanding of carbonation mechanism. (C) 2018 Elsevier Ltd. All rights reserved.