Abstract: Magnesium-based cements (in particular, magnesium oxychloride (MOC) cement) are synthesized with a salt (magnesium chloride for MOC cement), water, and magnesium oxide. In this invention the salt is sourced from a waste brine. To the brine is added a base, for example a hydroxide or magnesium oxide. Carbon dioxide is then bubbled through the brine. Calcium carbonate precipitates out of solution. The brine is then left to cure and form cement. Optionally, calcium carbonate precipitate can be added back to (or left in) the brine prior to curing as a filler.