Cements developed by precipitation of a hydrated magnesium carbonate activated by subsequent thermal treatment are described.During the precipitation stage, a solution containing HCO3 and/or CO32-and a solution containing Mg2+ are mixed in a lab reactor to form hydrated magnesium carbonates like nesquehonite,MgCO3·3H2O, which can be obtained at 25 ℃ and 1 atm.Nesquehonite is not self-cementing but its thermal activation leads to the formation of an amorphous precursor which, upon rehydration, reforms nesquehonite and gives a material which exhibits cementitious properties;it is readily formed into panels or blocks.The morphology of nesquehonite, as prismatic needles, is mostly preserved during the thermal activation.The cementitious materials formed contain a variable amount of water and around 30 wt% CO2;they are non-toxic and non-flammable and could be recycled.The properties of the materials depend on the experimental conditions used during the different stages but overall the bulk density is in the range 600-850 kg.m-3 and the unconfined compressive strength in the range 2-9 MPa.The production process has been scaled-up to kilogram quantity and a prototype of a continuous production facility is under development.In an industrial process, the source of CO2 could be any fixed-source combustion exhaust (e.g.from a cement plant);it is not necessary to separate CO2 from the other gases present as this occurs spontaneously in the course of dissolution into alkaline solution at the first step.The source of Mg could be any Mg-rich solution (e.g.seawater desalination brines);the presence of NaCl does not interfere with the precipitation and the resulting nesquehonite product can readily be washed to be chloride-free.The use of industrial wastes makes this type of material a carbon neutral or even carbon negative cement.