Influence of accelerated hydration and carbonation on the performance of reactive magnesium oxide concrete

The low dissolution of magnesium oxide (MgO) limits the performance of carbonated reactive magnesium oxide cement (RMC)-based formulations. The initial acceleration of hydration (IAH) of RMC samples involving the use of sodium bicarbonate (NaHCO₃) (SBC) was enabled by way of the adjustment of curing conditions. The influence of IAH and SBC on hydration kinetics was evaluated by isothermal calorimetry, hydration degree and pH measurements. Mechanical performance results were supported by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy, which identified the amount and morphology of the final phases. The use of IAH increased Mg²⁺ concentration and enhanced hydration and carbonation at early ages. The introduction of SBC increased CO₃²⁻ concentration by providing higher initial pH and additional HCO₃⁻ that stimulated carbonation at later ages. RMC samples involving IAH and SBC outperformed others, resulting in ∼130% higher 28 d compressive strengths than the control. This increased performance was associated with improvements in the morphology and amount of carbonates, which were assessed by thermal analysis.