Early-age hydration dynamics in MgO/nesquehonite cement

With the growing interest in Mg-based cements, the early-age hydration mechanisms of Mg-carbonate binders, such as MgO/Nq, remain insufficiently studied. This study investigates the early hydration of MgO/Nq binder, focusing on the hydration kinetics, carbonate phase formation, and water partitioning within the paste. These processes were followed using isothermal calorimetry, internal relative humidity measurements, XRD, TGA, FTIR, total carbon analysis, 13 C NMR, and 1 H NMR. The results reveal two distinct hydration regimes. During the first 20–24 h, MgO and Nq react slowly, forming mainly an amorphous Mg-hydroxycarbonate, MHC. After total consumption of Nq, MgO hydration accelerates markedly, and MHC reacts with MgO and water to form hydrous carbonate-containing brucite, HCB. This transition is accompanied by pronounced heat release, increased water consumption, pore refinement, and a sharp decrease in internal relative humidity. The findings highlight the key role of transient amorphous Mg-hydroxycarbonate phase in controlling the early hydration and microstructural development of MgO/Nq binders.