MgO-Based Binder for Treating Contaminated Sediments: Characteristics of Metal Stabilization and Mineral Carbonation

We developed a novel mixed binder of MgO (magnesia) and supplementary cementitious materials that can solidify sediments contaminated with heavy metals as well as store CO2 through mineral carbonation reactions. The optimal MgO-based binder consisted of MgO, lime (L), fly ash (FA), and blast furnace slag (BFS) with a formula of MgO0.5–(L0.1–(FA0.4BFS0.6)0.9)0.5. The binder exhibited a compressive strength of 11.9 MPa, which was similar to that of Portland cement. Sequential extraction of treated sediments showed that the stabilization capacity of the MgO-based binder for heavy metals (Cu, Cd, Ni, Pb, and Zn) was two times higher than that of PC. Results also show that more than 50% of the stabilized heavy metals existed within very persistent solid phases that were not disintegrated during the final step of the sequential extraction procedure using a HNO3/HClO4/HF solution. The hydration products of MgO that contributed to strength development and metal stabilization included brucite (Mg(OH)2), magnesium–silicate–hydrates (M–S–H), and lansfordite (MgCO3 · 5 H2O). Lansfordite was a major carbonation product in the treated sediments. By use of thermogravimetric analyses, we found that 58 kg of CO2 could be sequestrated within the solidified sediment when a ton of dredged sediment was treated.