Utilization of Magnesium Hydroxy-Carbonate Hydrate (MHCH) Materials Formed By Precipitation of CO2 in Alkaline Aqueous Solutions

Carbon dioxide is a relatively stable molecule and so its recovery from mixed gaseous emissions and subsequent utilisation has proven to be quite difficult, leading to proposals that recovered CO₂ should be stored rather than utilised. This paper describes a relatively simple route for the recovery of CO₂ and its conversion to useful solids that can be used to make low carbon and carbon neutral construction products. The carbon dioxide from a combustion gas waste stream (e.g. from a cement plant) containing nitrogen and water vapour is selectively sorbed into an alkaline aqueous solution at ~ 25 °C and 1 atm. The carbonate and bicarbonate ions thus formed react with Mg ions (e.g. from a seawater desalination brine) under conditions such that Magnesium Hydroxy-Carbonate Hydrate (MHCH) phases precipitate; NaCl (which is quite abundant in brines from seawater) is not found to interfere significantly with the precipitation process. Nesquehonite, MgCO₃·3H₂O, is the preferred product obtained at ~ 25 °C and 1 atm; it typically occurs as micrometric well crystallized prismatic needles. When the powder obtained from the thermal activation of nesquehonite is mixed with water, the resulting self-cementing product can be used to form panels or blocks. The hardened materials contain ~ 30 wt% CO₂ and their properties depend on the applied parameters; they are non-toxic, non-combustible and can be recycled at end of life.