Insights into the role of additives on the hydration and carbonation of magnesium oxide and stability of the carbonates

Magnesium carbonate-based minerals can capture and store CO2 as stable mineral carbonates and be utilized to produce construction materials with a low or negative carbon footprint compared to conventional Portland cement. However, several challenges exist in such binders that need to be addressed to enable their use in practical applications. This thesis aims to address two main issues. Firstly, the reactivity of MgO, the main raw material for such binders, is slow. Different ways to improve the reactivity of MgO have been proposed and tested, of which, the addition of organic additives such as acetate has been promising. Yet, several questions such as, “does it alter the rection product?” or “where does the acetate go?”, remain unanswered. In this thesis, the above-mentioned questions are investigated, for three different organic additives, acetate, formate and citrate. Secondly, on reaction with CO2, various hydrated magnesium carbonates minerals (HMCs) are formed based on reaction conditions. At ambient temperature and pressure, nesquehonite [MgCO3۰3H2O] is the most favored HMCs. However, nesquehonite is metastable and can transform to other stable HMCs, leading to reduction in solid volume and some loss of CO2. When using these materials as construction materials, the phase change from one HMC to another would cause structural instability and pose risk. To overcome this bottleneck, two different approaches to stabilize nesquehonite are explored as a part of this thesis. One, by dope with Zn2+, with the hypothesis that, cations having similar ionic radii with Mg2+ can partially replace Mg2+ in hydrated magnesium carbonates to form a solid solution which in turn, may enhance the stability of the Mg-carbonates. And the second, by tuning the chemical environment and pH of nesquehonite in an aqueous system.
The main findings of the thesis; The co-precipitation of organics with brucite, “organic-modified brucite”, in the presence of the various organic additives in magnesium carbonate-based binders has been reported for the first time. Further, the reaction of acetate-modified brucite with CO2, lead to the formation of a lesser known HMC, giorgiosite, as compared to nesquehonite in the reference sample without acetate. The findings reported here give insights into possibility to tune the formation pathways of different magnesium carbonates. In the second part of the thesis, doping Zn2+, did not stabilize nesquehonite, instead formed a solid solution with dypingite with a maximum Zn/(Mg + Zn) molar ratio of 0.01. The solid solution was found to be thermodynamically more stable than pure dypingite. By varying the chemical aqueous environment, nesquehonite was found to react with phosphate to form magnesium potassium phosphate (MKP) phases. This phase precipitates on the surface of nesquehonite, hindering further dissolution of Mg and thereby successfully stabilizing nesquehonite. The conclusions from this part of the thesis help build insights into stabilizing HMCs, to enable safe utilization as construction materials.