Microstructural evolution of cement pastes under long-term carbonation: new insights in nanopores from water vapor sorption and 1H NMR relaxometry

The long-term microstructural evolution of hardened cement paste under nearly two years of enforced carbonation was examined using water vapor sorption and 1 H NMR relaxometry. The results show that nanopore evolution, including interlayer spaces and gel pores, in carbonation at different relative humidity (SDC) was reduced significantly by as much as approximately 50 % degree of carbonation (DoC). In contrast, wet carbonation (WC) showed distinct pore changes and rapid matrix degradation from structural breakdown, with pore evolution following a trend similar to pH. The specific surface area (SSA) in WC is higher than in SDC owing to the formation of highly polymerized Al–Si gel, which also increases the SSA measured by 1 H NMR relaxometry. Lower relative humidity (RH) leads to a much greater reduction in nanopores. The reduction in interlayer spaces and gel pores and metastable CaCO 3 polymorph transformation retards further carbonation in SDC. In SDC, vaterite tends to form at DoC <50 %, whereas stable forms, such as aragonite and calcite, dominate at later stages, depending on the RH. At 60 % RH, aragonite and vaterite were depleted at a DoC of approximately 50 %, owing to the disappearance of the interlayer spaces and gel pores. By contrast, at 90 % RH, the interlayer spaces decreased to approximately 50 % DoC, whereas the gel pores decreased significantly beyond 70 % DoC. Vaterite is depleted around 50 % DoC as it transforms into aragonite alongside nanopore reduction, and aragonite seeds promote further aragonite growth. This behavior is attributed to aragonite crystal growth through Ostwald ripening and polymorph transformation. • Long-term accelerated carbonation reveals significant interlayer spaces and gel pore evolution. • Reduced interlayer spaces in C-S-H retard further carbonation at an RH of 60 %. • Transformation of CaCO 3 polymorph during long-term carbonation alters the carbonation rate. • Collapsed interlayer spaces and gel pores limit the formation of metastable CaCO 3 crystals at 60% RH. • Evolution of SSA during long-term carbonation is determined by 1 H NMR relaxometry and WVS.