This study investigated the strength and microstructural development of MgO and MgO-microsilica (MS) systems under sealed and carbonated conditions. The influence of hydromagnesite seeds on the performance of each system was also evaluated. The hydration mechanisms were studied via isothermal calorimetry. A correlation between the strength development and formation of different phases was established. XRD, TG/DTG, FTIR and SEM were used for the identification and quantification of different hydrate and carbonate phases. MgO systems relied on the conversion of brucite into carbonate phases for their strength development, whereas M-S-H was the main source of strength in MgO-MS systems. The effect of seeding was evident in MgO-MS systems, where the extra space provided by the seeds increased the rate and degree of hydration. The formation of M-S-H was responsible for strength development and denser microstructures, which could be further improved via the increased utilization of unreacted MgO and MS.