Predicting Electronic Structure in Tricalcium Silicate Phases with Impurities Using First-Principles

Tricalcium silicate (Ca3SiO5) is heavily used in industry as it is the most predominant constituent in Portland cement clinkers. In this work, using ab-initio calculations, we assess the ability of a large selection of substitutions to modify the electronic structure in the M3 polymorph of tricalcium silicate. We demonstrate the relation between electronic structure, hybridization of the impurity orbitals, and charge transfer from impurity atoms to the bulk material. Our work suggests that charge localization upon introducing impurities can passivate the reactive sites and several such substitutions are identified. ■ INTRODUCTION Tricalcium silicate (Ca3SiO5 or C3S) is considered to be the most predominant compound in ordinary Portland cement clinkers, at ∼50−70 wt %. C3S is usually chemically modified upon substituting with impurities, forming a solid solution which is called alite. The amount and content of the impurities in C3S can determine the physical and chemical properties of the resulting cement such as hydration reactivity, polymorphism, corrosion resistance, and elastic properties. From the energy consumption perspective, among other phases in cement, producing C3S requires the highest processing temperature, which accounts for roughly 5% of global CO2 emissions. As it is also the most reactive, a substantial amount of C3S is necessary to have satisfactory early setting properties. Much effort has been spent developing cement using industrial waste products that can provide the same reactivity properties as ordinary Portland cement clinkers, although these products introduce a wide range of impurities to different phases in the cement. Although several impurities have been found to decrease the reactivity of C3S, thus decreasing performance even when used in trace amounts, a comprehensive understanding of the chemical behavior of different impurities in C3S is still lacking. 9 Experimental studies in the field mostly focus on the effect of different impurities by examining their volatility during production as a measure of their incorporation into different phases. However, probing different phases in cement with such impurities can be challenging, and conflicting results can be open to interpretation due to the complexity of the material itself. In this paper, we use quantum mechanical computational methods to analyze the structural and electronic properties of C3S. We examine the changes in the electronic structure, upon introduction of a wide range of impurities. Such properties of different substitutions in the dicalcium silicate (Ca2SiO4 or C2S) structure including a preliminary comparison between alite and belite (second most compound in portland cement) structures for the Mg, Al, and Fe substitutions has already been examined by Durgun et al. In this work we build upon this methodology and extend the range of impurities investigated in alite substitution. We additionally support these hypotheses with partial density of states analyses and suggest a relation between the electronic structure of some impurities and their atomic radii. ■ COMPUTATIONAL DETAILS We performed calculations using density functional theory (DFT), using the projector augmented plane wave method (PAW) implemented in the VASP package. The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation is used to calculate the exchange correlation energy with a 500 eV kinetic energy cutoff for the plane wave basis set. A 4 × 4 × 4 k-point grid centered at the gamma point is chosen for sampling the Brillouin zone, which was sufficient to converge energies to within 10−5 eV per atom in the unit cell. For supercell calculations the density of k-points was scaled accordingly. Structural minimization using a conjugate gradient approach was carried out until the maximum force component on each atom was smaller than 10−3 eV/Å. For atomic basin charge analysis of the relevant structures, we use the Bader charge analysis. ■ RESULTS AND DISCUSSION In order to examine the effect of impurities on the C3S phase, we examine its monoclinic M3 (M3-C3S) polymorph, which is considered to be the most frequently observed in industrial applications, constituting 50−70%. In this unit cell there are 18 Ca, 6 Si, and 30 O atoms(see Figure S1 in the Supporting Received: October 21, 2014 Revised: January 29, 2015 Published: January 30, 2015 Article