Impact of Chemical Impurities on the Crystalline Cement Clinker Phases Determined by Atomistic Simulations

Cement is the most utilized material in the world. It is a highly versatile material, with remarkable mechanical properties and chemical durability. However, concomitant to these positive attributes is the massive quantity of CO2 emitted during its manufacturing, accounting for roughly 5% of global anthropogenic carbon dioxide emissions. Researchers have long pursued modifications to the manufacturing process that would allow for a decrease in cement’s environmental footprint. Examination of the phase diagram of calcium and silicon oxides reveals a straightforward way of achieving this objective, namely, the use of belite (dicalcium silicate) instead of alite (tricalcium silicate) as the main clinker phase. The temperature necessary to produce belite is ∼1200C, while alite requires temperatures of 1500. Clearly, the reduction of the required energy to form belite would entail both economic and environmental benefits. Unfortunately, the strength development is much slower in belitic cements, rendering them essentially useless in construction. The search for a form of belite with higher reactivity has been carried out by a trial and for many decades. Several strategies have been attempted, for example, modifying the chemical structure of belite by thermal processing3!5 or including chemical impurities.3!5 Regarding the latter, and despite partial success, there is little control or understanding of where the atomic substitutions take place, their effect on the structure, and their role on the chemical reactions.Although little used todate in thisfield, atomistic simulation techniques could be a critical tool to overcome the aforementioned difficulties, as they allow one to determine the position of chemical impurities and examine the resulting properties on the clinker. To this end, in the present work we use a combination of force field and density functional theory methods to study the impact of the most common chemical substitutions in alite and belite, and suggest appropriate routes to modify their reactivity. Alite is the main component of Portland cement, accounting for 70 wt %. It is a chemically modified form of pure tricalcium silicate (Ca3SiO5 or C3S), which exhibits a set of reversible phase transitions upon heating.7!12 The crystal structures of the different alite polymorphs are similar, consisting of independent SiO4 4! tetrahedra and three ionic sites: one O and two Ca2þ positions (see figure 1). The polymorphs differ in the orientation of the SiO4 4tetrahedra, which affects the symmetry and the coordination of the Ca and O atoms. In this study we choose the alite M3 polymorph refined from a single crystal by Mumme et al., since it is the most abundant polymorph in cement clinkers. The most common substitutions are those of Mg2þ for Ca2þ, but 2#Al3þ or 2# Fe3þ for Ca2þþ Si4þ also take place. The role of the substitutions as stabilizers of the monoclinic phases and their solubility limits in alite have been extensively