Carbon Sequestration and Metals Stabilization Characteristics of a Magnesium Oxide-Based Binder

Carbon sequestration and metals stabilization characteristics of an MgO-based binder that consisted of MgO and supplementary cementitious materials were investigated. Kinetics and equilibrium aspects of carbonation were initially investigated at various CO₂ concentrations using a thin plate specimen of the MgO-based binder. It was shown that an optimum water to binder ratio existed around 0.7 and that humidity did not affect the kinetics/extent of carbonation substantially. Kinetics of carbonation were reasonably described by a modified unreacted core model. Magnesite(MgCO₃), nesquehonite(MgCO₃×3H₂O), and lansfordite (MgCO₃×5H₂O) were identified as major carbonation products 

Sequential extraction showed that stabilization capacity of the developed binder for heavy metals (Cu, Cd, Pb, Zn and Ni ) was 2 times higher than that of Portland cement and that more than 50-60% of the stabilized heavy metals existed within the solid forms that was not disintegrated at the final step of the sequential extraction. The solid phases that contributed to the strength development and metal stabilization were identified to include portlandite, brucite, M-S-H (magnesium-silicate-hydrates), and landfordite.

When the MgO-based binder was applied to treat a contaminated sediment at ambient CO₂ condition, a compressive strength of 4.78 MPa was obtained that was comparable to that obtained by a Portland cement. However, when CO₂ concentration was higher than 50%, the solidifying capability of the binder was substantially compromised due to the formation of cracks that were likely to be attributable to carbonation reactions and a decrease in pH of the solidified matrix. Under the 100% CO2 condition, the CO₂ stabilization capacity of the binder during 28 days was 99.4 kg-CO₂/ton-treated sediment.