Investigation on Mg content in calcite when magnesium calcite and nesquehonite co-precipitate in hardened cement paste

The Mg content incorporated in calcite is crucial to the formation of nesquehonite in a system where magnesium calcite and nesquehonite co-precipitate. Because of the importance of nesquehonite on the mechanical properties of paste and mortar containing reactive MgO, the Mg content that can be incorporated in calcite impacts the design of concrete containing reactive MgO. However, the maximum incorporation of Mg in calcite and co-precipitation behaviour of magnesium calcite and nesquehonite in a paste system are different from those in a solution system. Therefore, this study exposes blends of reactive MgO and calcium hydroxide to carbonation environments (50%-95% relative humidity, 75%-99% CO2 concentration and temperature of 20 ± 3 °C) to investigate the precipitation behaviour of magnesium calcite and nesquehonite and factors influencing the Mg content in calcite in a paste system. The results reveal that the initial Mg:Ca ratio in the original system greatly determines the average Mg content incorporated in calcite, while the influence of curing condition (relative humidity and CO2 concentration) is relatively negligible. The highest mole% of MgCO3 in calcite can be ∼30% in the study, and the high stability of this high magnesium calcite is attributed to the co-existence of nesquehonite in the system. Moreover, the thermal decomposition of nesquehonite in this research is further characterized in detail.