Influence of Magnesium Oxide on Carbonation of Cement Paste Containing Limestone and Metakaolin

Abstract

One of the major durability concerns for limestone calcined clay cement (LC3) concrete is its high susceptibility to atmospheric carbonation that could lead to an early onset of electrochemical corrosion of reinforcing steel in concrete structures. Aimed at designing innovative LC3 formulations with potentially enhanced carbonation resistance, this preliminary study investigates the influence of reactive magnesia (MgO) on the early-age strength development, hydrates assemblage, and atmospheric carbonation resistance of ternary ordinary Portland cement-metakaolin-limestone blends with a constant 45% ordinary Portland cement (OPC) replacement level. The results show that the MgO addition impedes the formation of AFm phases (hemicarbonate and monocarbonate), likely through interfering reactions between metakaolin and portlandite. The formed brucite due to MgO hydration can uptake atmospheric CO2 to some extent, but at a considerably slower rate, in comparison with other hydrates in LC3 including AFm, AFt, and portlandite. The enhancement of carbonation resistance of LC3 pastes is insignificant by MgO addition of less than 5%.