Environmental assessment of aqueous alkaline absorption of carbon dioxide and its use to produce a construction material

The use of carbon dioxide emissions as a raw material, e.g. for the chemical industry and for the production of synthetic fuels, is regarded as an alternative to its storage, as the potential benefits are dual: emissions are removed at the same time as greenhouse gases from the substitution of conventional goods are avoided. However, there is an outstanding lack of information on the overall carbon dioxide balance for these new approaches. This paper performs the greenhouse gases balance and resource efficiency assessment, in terms of water and energy use, of the production of a new construction material intended to have a negative or neutral carbon footprint, which composition is mainly magnesium carbonate trihydrate, known as nesquehonite. Desalination brines are considered the main source of Mg for the aqueous precipitation, which requires and alkaline agent. The overall carbon dioxide emissions balance of the process strongly depends on the amount of alkali consumed and the efficiency of its use. Additionally, the substitution of commercial materials with relatively high environmental footprint by the obtainable magnesium carbonates would yield a net reduction of emissions, embodied energy and water footprint. The capture process would have a net negative carbon footprint if the associated emissions linked to the alkalinity source were in the order of 22-27 kg of CO₂ per kmol of alkalinity. However, no commercial alkali source reaches such a low associated emissions, so alkaline wastes can be used.