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Calcium silicates are abundant, but sparingly soluble, feedstocks of interest for making low-carbon alternative cements. Under hydrothermal and alkaline conditions, they can form crystalline calcium silicate hydrate (CCSH) products, which are abundant in Roman concrete, or they can form carbonates when CO2 is present. To understand when co-precipitation of CCSH and carbonate phases is possible, we studied the hydrothermal carbonation of a model calcium silicate, pseudowollastonite (-CaSiO3), at 150ºC and high pH as a function of CO2 source (CO2(g) or Na2CO3) and different concentrations of sodium, alumina, and silica. Our experiments produced a range of CCSH phases including tobermorite – 13Å, rhodesite, and pectolite, as early as one day after the start of our experiments. About 10.7% hydrated product was observed after 7 days of curing in 2 M NaOH solution. We also observed the formation of CaCO3 as both aragonite and calcite when carbon was introduced to our experimental system. The carbon source impacted the ratio of CaCO3 to CCSH phases in the reaction products. Availability of Na2CO3 produced a balance between CaCO3 and CCSH phases whereas CO2(g) produced more CaCO3 at about 36.4% by mass at the highest. Higher concentrations of Na+ increased precipitation of both CaCO3 and/or CCSH phases. The presence of excess silica, in the form of dissolved borosilicate glass from our reaction vessels under alkaline reaction conditions, also enhanced the formation of CCSH phases formed in some experiments. Supplemental Al2O3, a common constituent in many silicate feedstocks, also enhanced CCSH formation, likely by forming aluminum substituted phases under the conditions tested here. These chemical insights can be enabling in designing formulation and curing guidelines for novel cementitious materials.
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This content will become publicly available on February 27, 2026
Thermal Ca2+/Mg2+ exchange reactions to synthesize CO2 removal materials
Most current strategies for carbon management require CO2 removal (CDR) from the atmosphere on the multi-hundred gigatonne (Gt) scale by 2100. Mg-rich silicate minerals can remove >105 Gt CO2 and sequester it as stable and innocuous carbonate minerals or dissolved bicarbonate ions. However, the reaction rates of these minerals under ambient conditions are far too slow for practical use. Here we show that CaCO3 and CaSO4 react quantitatively with diverse Mg-rich silicates (for example, olivine, serpentine and augite) under thermochemical conditions to form Ca2SiO4 and MgO. On exposure to ambient air under wet conditions, Ca2SiO4 is converted to CaCO3 and silicic acid, and MgO is partially converted into a Mg carbonate within weeks, whereas the input Mg silicate shows no reactivity over 6 months. Alternatively, Ca2SiO4 and MgO can be completely carbonated to CaCO3 and Mg(HCO3)2 under 1 atm CO2 at ambient temperature within hours. Using CaCO3 as the Ca source, this chemistry enables a CDR process in which the output Ca2SiO4/MgO material is used to remove CO2 from air or soil and the CO2 process emissions are sequestered. Analysis of the energy requirements indicates that this process could require less than 1 MWh per tonne CO2 removed, approximately half the energy of CO2 capture with leading direct air capture technologies. The chemistry described here could unlock Mg-rich silicates as a vast resource for safe and permanent CDR.
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- Award ID(s):
- 2011924
- PAR ID:
- 10589410
- Publisher / Repository:
- Springer Nature Publishing
- Date Published:
- Journal Name:
- Nature
- Volume:
- 638
- Issue:
- 8052
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 972 to 979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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