Hydration characteristics and expansive mechanism of MgO expansive agents

The microstructures of hardened cement pastes that contain different proportions of an MgO expansive agent were investigated via scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The influence of activity and dosage of MgO on the expansion and mechanical properties of the mortar was also investigated. The results indicated that brucite crystals grow on the walls of inferior pores of the confined zone and slightly on the surface of the MgO particles. The growth of brucite crystals is controlled by the supersaturation of Mg2+ in the pore solution. The crystal growth pressure of brucite forces the microvoid to expand and increases the porosity of the binder paste, thus causing the expansion of the paste and slightly reducing the mortar strength. The incorporation of supplementary cementitious materials (SCMs) can have either a positive or negative effect on the expansion of the mortar, depending on the curing temperature and its dosage.