Filler effects of CaCO3 polymorphs derived from limestone and seashell on hydration and carbonation of reactive magnesium oxide (MgO) cement (RMC)

This study investigated the influence of CaCO3 polymorphs from limestone and seashell on the hydration and carbonation kinetics of reactive MgO cement (RMC). Replacement levels at 5 and 10 wt% were adopted, and the samples were exposed to ambient curing before carbonation under 20 % CO2. The superior filler effects imparted by the limestone powder were demonstrated with an enhancement to the normalized heat flow at an early stage of hydration, leading to an increase in the compressive strength under air curing. Its nucleation effect did not lead to more brucite but involved the formation of amorphous phases, which encapsulated and passivated the MgO substrate, consequently restricting its hydration. Under carbonation, nucleation induced by both the CaCO3 forms improved the composite strengths by producing carbonation products of larger morphologies, resulting in pore filling and matrix densification. Amorphous phases of a hydromagnesite-like and fused structure were observed.