Could reactive MgO cement be a green solution? The effect of CO2 mineralization and manufacturing route on the potential global warming impact

Abstract

Reactive MgO (rMgO) is considered a promising alternative to Portland cement (PC), because its capacity to mineralize carbon dioxide (CO2) can be used to reduce the life cycle global warming intensity (GWI) of concrete. However, the greenhouse gases (GHG) emitted during the manufacture of rMgO may outshine its environmental benefits of CO2 mineralization. The objective of this study is to compare the life cycle GWI of traditional PC binder to rMgO binder and rMgO-PC blended binder systems. The life cycle assessment (LCA) considers the actual amount of CO2 mineralized by the binders and their 28-day compressive strength, as well as different manufacturing routes (dry and wet) and energy sources (fossil fuels and electricity) to produce rMgO. Based on these variables, best- and worst-case scenarios were identified and analyzed. The results reveal that the quantity of CO2 stored in hydrated Mg carbonates increases as the rMgO replacement level increases, while the quantity of CO2 stored in Mg-calcite decreases. Mortar with rMgO-PC as binder exhibits a lower net GWI per unit of strength than mortars with either 100% PC or 100% rMgO as binder in the best-case scenarios. Using low carbon energy sources (such as renewables and nuclear) for rMgO production and on-site waste CO2 would significantly reduce the overall GHG emissions (by 82%). In the optimal situation, a net GWI reduction of almost 50% per unit of strength can be achieved by the rMgO-PC mortar. The wet route to produce rMgO looks particularly promising for a future decarbonized construction sector since no CO2 is chemically released during its manufacture. The properties and GWI of concrete containing rMgO carbonated in flowing CO2-rich gas warrant future study.