Microstructure of reactive magnesia cement pastes subjected to high carbon dioxide concentration

Reactive MgO cement (RMC) is a low-carbon cementing material, which is usually prepared by mixing a relatively large amount of industrial wastes, appropriate addition of reactive MgO and a relatively small amount of Portland cement. In contrast to conventional cement, the RMC obtains its mechanical properties as a result of carbonation reactions. This paper investigated the RMC mixtures that consist of 20%-40% reactive MgO, 0-40% slag and Portland cement, subjected to 99.9% concentration of CO₂ curing. The microstructure was characterized by dynamic vapor sorption, scanning electronic microscopy and back-scattered electronic microscopy, respectively. The carbonation mechanism was discussed based on the results. The results show that the carbonation of calcium silicate hydrated (C-S-H) leads to the formation of amorphous CaCO₃ as well as SiO₂ gel, which slightly increases the volume of meso-pores ranging from 0-11 nm as well as the corresponding specific surface areas. A large amount of inter-connected round shape Mg-Ca-bearing carbonates were formed due to the carbonation of RMC due to the densification of cement pastes. The carbonation of RMC probably proceeds mainly through the crystallization in the supersaturated solution containing both Ca²⁺ and Mg²⁺ in the RMC pastes when subjected to CO₂ curing.