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.

Solid-state nuclear magnetic resonance spectroscopy of cements

The processes of oxygen tracer diffusion and oxygen surface exchange in acceptor-doped oxides displaying high rates of oxygen transport are examined. Literature data are analysed with the standard atomistic expression, in order to obtain chemically reasonable limiting values for the oxygen diffusion coefficient, D*, in the solid state. The analysis suggests a limiting value of D* that is characterised by a pre-exponential of 8.8 × 10−2 cm2 s−1 and an activation enthalpy of 0.5 eV. An (essentially temperature-independent) ultimate limit to the oxygen surface exchange coefficient, k*, of ca. 100 cm s−1 is obtained by assuming surface exchange to be limited by the incident flux of oxygen molecules from the gas phase. In addition, atomistic expressions are derived that quantitatively describe experimental k* data as a function of temperature and oxygen activity for surface exchange involving molecular oxygen at high temperatures. One expression is found for oxides with a bandgap, such as SrTiO3, Ce0.9Gd0.1O1.95, and La0.6Sr0.4Fe0.8Co0.2O3−δ; and an analogous expression is found for oxides with a partially filled conduction band, such as La1−aSraCoO3−δ and (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ. Conduction-band electrons are considered for both types of oxides to play a central role in determining the rate of oxygen surface exchange.

Limits to the rate of oxygen transport in mixed-conducting oxides

Defect dipoles are significant point defects in perovskite oxides as a result of their impact on oxygen vacancy dynamics. Electron paramagnetic resonance (EPR) was used to investigate the local defect structure of single crystal BaTiO3 doped with manganese. These results, along with a re-analysis of literature data, do not support the conclusion that transition metal-oxygen vacancy nearest neighbor defect dipoles (MnTi″−VO••)× in ferroelectric BaTiO3 are majority defect centers as previously reported. Local symmetry analysis of the zero-field splitting term of the spin Hamiltonian supports the assignment of fully coordinated defect centers as opposed to defect dipoles for resonance signals at geff ∼ 2. A newly discovered defect center with g⊥ ∼ 6 is observed in the manganese doped system, and it is argued that this defect center belongs to an associated defect complex or defect dipole. This newly reported strong axial defect center, however, is present in small, minor concentrations compared to the well-k...

Acceptor-oxygen vacancy defect dipoles and fully coordinated defect centers in a ferroelectric perovskite lattice: Electron paramagnetic resonance analysis of Mn2+ in single crystal BaTiO3

The catalytic oxidation of gaseous trichloroethylene (TCE), mediated by a cost effective mayenite support, was studied in a fixed bed reactor. Mayenite was synthetized by using hydrothermal method and characterized before and after catalytic oxidation experiments by using XRD, N 2 -sorption (BET), SEM-EDX, TEM and FTIR; the reaction products were analyzed by means of GC–MS, IC and a IR-based CO 2 probe. The results showed that mayenite promoted the total oxidation of TCE in the temperature range 300–500 °C, where TCE was quantitatively converted in CO 2 and chlorine. The mayenite catalyst showed high recyclability and could be used for several reaction cycles without any loss of activity and selectivity. Owing to its characteristics, mayenite was found to be a promising catalyst for the TCE total conversion and remediation.

Total oxidation of trichloroethylene over mayenite (Ca12Al14O33) catalyst

Tailoring the stability of Ni-Fe/mayenite in methane – Carbon dioxide reforming

Oxygen transport in mayenite single crystals was studied by means of 18O/16O isotope exchange experiments and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k* were determined as a function of temperature, 773 ≤ T/K ≤ 1273, at an oxygen activity of aO2 = 0.1, and as a function of oxygen activity, 0.01 ≤ aO2 ≤ 0.9, at a temperature of T = 1123 K. Two diffusion processes were observed: a fast diffusion process that is attributed to the interstitialcy diffusion of free oxygen ions (O2–) and a slow diffusion process that is attributed to the interstitialcy diffusion of superoxide ions (O2–).

Oxygen Diffusion in Mayenite

Oxygen transport in undoped and doped mayenite

18O/16O exchange annealing and subsequent Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis is used to investigate oxygen transport in dense, nanocrystalline (average grain size d E 300 nm) ceramics of nominally un-doped BaTiO3. Isotope penetration profiles are obtained as a function of temperature, 973 o T/K o 1173, at an oxygen activity aO2 = 0.20 and as a function of oxygen activity, 0.002 o aO2 o 0.20, at T = 1073 K. All isotope profiles show the same unusual shape: a flattened profile over the first B102 nm, followed by a short, conventional diffusion profile. We demonstrate that the entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation based on an increase in the local oxygen diffusion coefficient close to the surface. This position-dependent increase is attributed to additional oxygen vacancies that are generated by diffusion of chlorine impurities out of the ceramics. The presence of chlorine derives from the chemical route necessary to produce nanometric powders: it thus indicates a new manner in which nanocrystalline ceramics may differ from their microcrystalline counterparts.

https://hal.archives-ouvertes.fr/hal-01168945/document

Markus Teusner

Papers

Oxygen Diffusion in Mayenite

Oxygen transport in undoped and doped mayenite

Complex diffusion behavior of oxygen in nanocrystalline BaTiO3 ceramics

Diffusion of Nd and Mo in lanthanum tungsten oxide