Crystal structure, photoluminescence and cathodoluminescence of Ba1-xCaxAl2O4 doped with Eu2+

© 2019 The Author(s). The synthesis, crystal structure, photoluminescence (PL) and cathodoluminescence (CL) spectra of Ba1-xCaxAl2O4 doped with 3 mol% Eu2+ between x = 0 and x = 1 are described. The molar fractions of the alkaline earth elements were varied in steps of 0.1. The materials have been synthesized by all solid state reactions at 1300°C in mixed gas (H2/N2). The identification of the crystal phases in the samples was based on analyses of the X-ray diffraction patterns. The Ba1-xCaxAl2O4 system contains one dominant monoclinic phase, one dominant hexagonal phase and two different cubic phases that were present in low concentrations. The main characteristic of the PL spectra was that the intensity of the Eu2+ photoluminescence decreased upon adding a second alkaline earth ion in the aluminate lattice. The hexagonal and monoclinic phases in the Ba1-xCaxAl2O4 samples showed an unexpected behaviour, namely increasing their unit cell volumes upon decreasing the mole fraction of Ba2+. For the hexagonal phase this behaviour has been explained qualitatively in terms of enhanced spontaneous polarization of the uncompensated anti-ferroelectric state.Engineering and Physical Sciences Research Council (CONVERTED (JeS no. TS/1003053/1), FAB3D, PRISM (EP/N508974/1), PURPOSE(TP11/MFE/6/1/AA129F; EP-SRCTS/G000271/1)); TECHNOLOGY STRATEGY BOARD (CONVERT)

Estimation of standard molar entropy of cement hydrates and clinker minerals

It is not straightforward to experimentally measure the standard molar entropy of cement hydrates or clinker minerals. This is further compounded by the controversies surrounding the entropy values reported in established thermodynamic datasets for cements. The purpose of this study is to assess the reliability of standard entropies compiled in those datasets. To this end, a simple but robust method is used in which the standard entropy of an inorganic solid is correlated to its formula unit volume via a linear equation. The results of this analysis show that the standard entropies and/or molar volumes (and in cases solubility products) of the following phases deserve closer scrutiny: meta-ettringite phases; magnesium/aluminium layered double hydroxide solid solutions; almost all iron-bearing monosulfate and hydrogarnet phases; and several calcium silicate hydrate solid solution end-members. In addition, this study reports the provisional estimates for the standard entropies of minerals ternesite and ye'elimite

Solid-state nuclear magnetic resonance spectroscopy of cements

Cement is the ubiquitous material upon which modern civilisation is built, providing long-term strength, impermeability and durability for housing and infrastructure. The fundamental chemical interactions which control the structure and performance of cements have been the subject of intense research for decades, but the complex, crystallographically disordered nature of the key phases which form in hardened cements has raised difficulty in obtaining detailed information about local structure, reaction mechanisms and kinetics. Solid-state nuclear magnetic resonance (SS NMR)spectroscopy can resolve key atomic structural details within these materials and has emerged as a crucial tool in characterising cement structure and properties. This review provides a comprehensive overview of the application of multinuclear SS NMR spectroscopy to understand composition–structure–property relationships in cements. This includes anhydrous and hydrated phases in Portland cement, calcium aluminate cements, calcium sulfoaluminate cements, magnesia-based cements, alkali-activated and geopolymer cements and synthetic model systems. Advanced and multidimensional experiments probe 1 H, 13 C, 17 O, 19 F, 23 Na, 25 Mg, 27 Al, 29 Si, 31 P, 33 S, 35 Cl, 39 K and 43 Ca nuclei, to study atomic structure, phase evolution, nanostructural development, reaction mechanisms and kinetics. Thus, the mechanisms controlling the physical properties of cements can now be resolved and understood at an unprecedented and essential level of detail

Role of a small addition of acetic acid on the setting behavior and on the microstructure of a calcium aluminate cement

International audienceIn order to be able to monitor dispersion and setting of cements, admixtures can be added, which usually can consist of large organic molecules. Here, the choice is different since small organic molecules have been used as in ceramic processing. This work concerns the preparation of calcium aluminate cement paste in the presence of acetic acid; the water to cement mass ratio is equal to 0.6 and the acid to cement mass ratio, mHOAc=mcement, ranges between 0 and 0.05. This admixture can have either a retarding or an accelerating effect on cement setting. The retarding effect is because of adsorption of acetate complexes, CaCH3CO2 1 positively charged, at the surface of CA particles, negatively charged, which leads to the delay (slowing down) of dissolution; it can reach 52 h (case of mHOAc=mcement ¼ 0:005). When mHOAc=mcement 0:01, there is a remarkable accelerating effect. The setting starts 2 h after mixing the cement with the liquid and is fairly constant for higher acetic acid contents. This rapid setting in acid conditions is because of the formation of hydrated calcium acetate in very low quantities and possibly gibbsite; the setting mechanism is quite different since there is no formation of conventional calcium and aluminum hydrates. As an example, with the highest amount of HOAc (mHOAc= mcement ¼ 0:05), no CxAyHz type of calcium aluminum hydrate is formed after aging for 4 days at 201C and 95% relative humidity. There is also a densifying effect of acetic acid; the open porosity of set samples left to age for 4 days at 201C and 95% relative humidity decreases from 35 to 25 vol% when mHOAc=mcement goes from 0 to 0.005 and remains relatively constant afterwards. Lastly, this decrease in the porosity continues with aging time; at 6 months, we obtain values as low as 9 vol% in samples where mHOAc=mcement 0:005