Influence of (Al, Fe, Mg) Impurities on Triclinic Ca3SiO5: Interpretations from DFT Calculations

Ca3SiO5 and its polymorphic representations are the dominant phase(s) present in ordinary portland cement (OPC). As environmental pressures bracket the production of OPC, there is increasing emphasis on designing newer, more efficient OPC chemistries. While minor impurities in the form of (Al, Fe, Mg) are long understood to have substantial influences on the structure and reactivity of the siliceous cementing phases, specific details at the atomistic level remain unclear. In this paper, we report the results of first-principles calculations performed at the density functional level of theory (DFT), on triclinic Ca3SiO5, a template phase of relevance to OPC doped with (Al, Fe, Mg) species. Focus is devoted toward understanding modifications induced in (a) the lattice and crystallographic parameters, (b) the mechanical properties, and (c) the electronic descriptors of the silicate. Special efforts are devoted to identify preferred atomic substitution sites and to rank the stability of different phases using thermochemical descriptors. The results suggest that the presence of (Al, Fe, Mg) impurities in the silicate lattice modifies charge localization and exchange, contributing a new means toward interpreting and steering cementing phase reactivity, by careful manipulations of their impurity distributions.