Full-scale Microstructure and Volume Composition Changes in Hardened Cement Paste during Drying and Accelerated Carbonation by Water-2-propanol ¹H-NMR Relaxometry

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In this study, 1 mm-thick disk samples of hardened cement paste were carbonated under 60% relative humidity (RH) and a 1.0% CO₂ concentration. Uncarbonated and carbonated samples were impregnated with 2-propanol (IPA) and analyzed by ¹H Nuclear Magnetic Resonance (NMR) relaxometry to quantify the full-scale microstructure changes in hardened cement paste during carbonation process. Pore structure changes during drying and carbonation were discussed based on the mineral composition changes, as determined by X-ray diffraction/Rietveld analysis, and volume changes as measured by length and volume change measurements. Furthermore, this water and IPA ¹H NMR technique enabled the evaluation of volume fraction changes in hardened cement pastes during drying and carbonation, combined with changes in pore structure. In the drying process under 60% RH, gel pore water evaporated, increasing the coarse pore volume, and drying shrinkage was induced. During the carbonation process, calcium carbonates precipitated in coarse pores, thereby increasing the solid volume of cement minerals and decreasing the total pore volume. As C-(A)-S-H gel was decomposed due to carbonation, the volume fraction of C-(A)-S-H and silica gel agglomeration decreased, resulting in the macroscopic carbonation shrinkage in hardened cement paste.