Structure, Atomistic Simulations, and Phase Transition of Stoichiometric Yeelimite

ABSTRACT: Yeelimite, Ca4[Al6O12]SO4, is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and Xray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously reported tetragonal system, which we establish to likely be the acentric orthorhombic space group Pcc2, with a √2a × √2a × a superstructure based on the cubic sodalite structure. Final unit cell values were a = 13.0356(7) Å, b = 13.0350(7) Å, and c = 9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being Pcc2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470 °C by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudocubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group I43m. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by RF values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively.Universidad de Málaga. Campus de Excelencia Internacional. Andalucía Tech

29Si NMR in cement: A theoretical study on calcium silicate hydrates

The NMR spectra of 29Si in cement-based materials are studied through calculations of the isotropic shielding of silicon atoms within the density functional theory. We focus on the main component of cement, the calcium-silicate-hydrate gel, using widely accepted models based on the observed structures of jennite and tobermorite minerals. The results show that the 29Si chemical shifts are dependent not only on the degree of condensation of the (SiO 4) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. We find that the NMR spectra for models of the calcium-silicate-hydrate gel based on tobermorite are in better agreement with experiment than those for jennite-based models. © 2012 American Chemical Society.We acknowledge the support of the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Innovación, Ciencia y Tecnología (Grant Nos. TEC2007-68065-C03-03 and FIS2010-19609-C02-02), and the ETORTEK research program (NANO-IKER Grant No. IE11-304) funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa.Peer Reviewe

Clustering in engineering materials: Effects on cements materials

Trabajo presentado al CECAM Workshop on "Aging of Engineering Materials: a Computational Approach to Durability and Sustainability" celebrado en Zurich (Suiza) del 8 al 10 de Febrero de 2012.Cement has a highly complex structure when going bottom-up from nanometer to macroscopic scale. It is thus crucial to understand the nanostructure of Calcium Silicate Hydrates (CSH), composed of silica tetrahedra chains on calcium oxide layers. Furthermore, it is important to see how the silica chains affect especially the cement mechanical properties. Here, we review recent advances in modelling for cementitious materials, focusing on clustering of the silicate tetrahedra. We first apply to cement the traditionally computational and statistical physical chemistry approaches for the aggregation of atoms in clusters. The aggregation of silica tetrahedra in CSH gel is described to have certain stable lengths, in good agreement with 29Si NMR experiments. Longer chains obtained by adding silica nanoparticles increase the chain length, addition which improves cement mechanical properties and avoids the deterioration due to calcium leaching. We then investigated a widely accepted models of cement hydrates with finite silicate chains, in order to look at the cohesion forces in cement-based materials. At the nanoscale, we calculate not only the elastic properties of the CSH models, but their 29Si NMR spectra. These nanoscale elastic properties are used to get the mechanical properties of CSH gel at the mesoscale taking into account its porous structure: Longer chains seem to be linked to a stronger mesostructure of CSH gel, but their dependence is not straightforward [3]. We acknowledge the help and cooperation of I. Campillo, Y. R. de Miguel, E. Erkizia, D. Sánchez-Portal, A. Rubio, A. Porro, P.M. Echenique.Peer Reviewe

29Si chemical shift anisotropies in hydrated calcium silicates: A computational study

The 29Si chemical shift anisotropies are investigated for calcium silicate hydrates. The focus is on the naturally occurring minerals, jennite and 14 Å tobermorite and models derived from them to simulate calcium-silicate-hydrate gel, the main component of Portland cement. Our theoretical results show that the analysis of anisotropy and asymmetry of the 29Si chemical shift discriminates between different Si types, even if their isotropic chemical shifts are similar. Terminal and pairing silica tetrahedra are clearly distinguished and the chemical shift anisotropies set apart the Si tetrahedra that are hydroxylated. The chemical shift anisotropy measurements, although more challenging than the usual isotropic chemical shift experiments, could greatly improve our knowledge of not only cement materials, but silicate hydrates, in general. © 2013 American Chemical Society.We acknowledge the support of the Basque Departamento de Educación and the UPV/EHU (Grant IT-366-07), the Spanish Ministerio de Innovación, Ciencia y Tecnologıá (Grant Nos. TEC2007-68065-C03-03 and FIS2010-19609-C02-02), and the ETORTEK research program (NANO-IKER Grant No. IE11-304) funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa.Peer Reviewe

First-principles calculations on polymorphs of dicalcium silicate-belite, a main component of Portland Cement

We investigate systematically the polymorphism of stoichiometric dicalcium orthosilicates C2S using ab initio studies and interatomic potential functions. Apart from the structural data, here, we present the analysis of novel data on phonon dynamics in belite. The latter is related to the number and type of phase transitions in C2S, including the peculiar thermal hysteresis in the route between three low-temperature phases and inverse thermal expansion of the lowest temperature phase. The study on the polymorphism of stoichiometric C2S is the necessary step to understand the behavior of doped belite-based materials.This work was partially supported by Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness MINECO, the Basque Government under the ELKARTEK project (SUPER), and the University of the Basque Country (grant no. IT-756-13). This work was supported in part by PLGrid infrastructure (ACK Cyfronet facility)

Water/methanol solutions characterized by liquid μ-jet XPS and DFT—The methanol hydration case

The advent of liquid μ-jet setups as proposed by Faubel and Winter – in conjunction with X-ray Photoemission Spectroscopy (XPS) – has opened up a large variety of experimental possibilities in the field of atomic and molecular physics. In this study, we present first results from a synchrotron-based XPS core level and valence band electron spectroscopy study on water (10-4 M aqueous NaCl solution) as well as a water/methanol mixture using the newly commissioned ALBA liquid μ-jet setup. The experimental results are compared with simulations from density functional theory (DFT) regarding the electronic structure of single molecules, pure molecular clusters, and mixed clusters configurations as well as previous experimental studies. We give a detailed interpretation of the core level and valence band spectra for the vapour and liquid phases of both sample systems. The resulting overall picture gives insight into the water/methanol concentrations of the vapour and liquid phases as well as into the local electronic structure of the pertinent molecular clusters under consideration, with a special emphasis on methanol as the simplest amphiphilic molecule capable of creating hydrogen bonds.The ICN2 is funded by the CERCA Programme/Generalitat de Catalunya (grant no. I/410002100/0000; Spain). The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706; Spain).Peer reviewe

²⁹Si Chemical Shift Anisotropies in Hydrated Calcium Silicates: A Computational Study

The ²⁹Si chemical shift anisotropies are investigated for calcium silicate hydrates. The focus is on the naturally occurring minerals, jennite and 14 Å tobermorite and models derived from them to simulate calcium–silicate–hydrate gel, the main component of Portland cement. Our theoretical results show that the analysis of anisotropy and asymmetry of the ²⁹Si chemical shift discriminates between different Si types, even if their isotropic chemical shifts are similar. Terminal and pairing silica tetrahedra are clearly distinguished and the chemical shift anisotropies set apart the Si tetrahedra that are hydroxylated. The chemical shift anisotropy measurements, although more challenging than the usual isotropic chemical shift experiments, could greatly improve our knowledge of not only cement materials, but silicate hydrates, in general

²⁹Si NMR in Cement: A Theoretical Study on Calcium Silicate Hydrates

The NMR spectra of ²⁹Si in cement-based materials are studied through calculations of the isotropic shielding of silicon atoms within the density functional theory. We focus on the main component of cement, the calcium-silicate-hydrate gel, using widely accepted models based on the observed structures of jennite and tobermorite minerals. The results show that the ²⁹Si chemical shifts are dependent not only on the degree of condensation of the (SiO₄) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. We find that the NMR spectra for models of the calcium-silicate-hydrate gel based on tobermorite are in better agreement with experiment than those for jennite-based models

²⁹Si NMR in Cement: A Theoretical Study on Calcium Silicate Hydrates

The NMR spectra of ²⁹Si in cement-based materials are studied through calculations of the isotropic shielding of silicon atoms within the density functional theory. We focus on the main component of cement, the calcium-silicate-hydrate gel, using widely accepted models based on the observed structures of jennite and tobermorite minerals. The results show that the ²⁹Si chemical shifts are dependent not only on the degree of condensation of the (SiO₄) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. We find that the NMR spectra for models of the calcium-silicate-hydrate gel based on tobermorite are in better agreement with experiment than those for jennite-based models

²⁹Si Chemical Shift Anisotropies in Hydrated Calcium Silicates: A Computational Study

The ²⁹Si chemical shift anisotropies are investigated for calcium silicate hydrates. The focus is on the naturally occurring minerals, jennite and 14 Å tobermorite and models derived from them to simulate calcium–silicate–hydrate gel, the main component of Portland cement. Our theoretical results show that the analysis of anisotropy and asymmetry of the ²⁹Si chemical shift discriminates between different Si types, even if their isotropic chemical shifts are similar. Terminal and pairing silica tetrahedra are clearly distinguished and the chemical shift anisotropies set apart the Si tetrahedra that are hydroxylated. The chemical shift anisotropy measurements, although more challenging than the usual isotropic chemical shift experiments, could greatly improve our knowledge of not only cement materials, but silicate hydrates, in general