Thermodynamic, mechanical, and electronic properties of ettringite and AFm phases from first-principles calculations

The aluminate containing hydration products of Portland cement, mainly referring to ettringite and AFm phases (AFm, Mc, Hc and Friedel's salt), are important for chloride chemical binding and microstructure development of cement pastes since they are easily transformed with different additives and solutions. First principles calculations are employed to find their thermodynamic equilibrium crystal structures. Various equivalent hydrostatic pressures are applied onto these structures to study their volume change, thermodynamic and elastic moduli. Among them, Mc shows the smallest interlayer space, highest volume stability, density, elastic modulus, and stiffness property. The volume of hydrated phase can increase by 12.5 % due to the conversion from Mc to Friedel's salt. Electronic properties suggest that there are non-bonding electrons in the O-p orbitals of CO₃^2– in the Mc and charge transfer from CO₃^2– to the Ca²⁺ made the bonding states between O-p of CO₃^2– and Ca-d stronger. This could provide a theoretical basis for improving the mechanical and durability properties of concrete by adding supplementary cementitious materials, such as limestone powders appropriately.