Alison M. Coats

Influence of silica fume on diffusivity in cement-based materials: I. Experimental and computer modeling studies on cement pastes

Experimental and computer modeling studies are applied in determining the influence of silica fume on the microstructure and diffusivity of cement paste. It is suggested that silica fume modifies the inherent nanostructure of the calcium silicate hydrate (C-S-H) gel, reducing its porosity and thus increasing its resistance to diffusion of both tritiated water and chloride ions. Because the pores in the C-S-H are extremely fine, the relative reduction in diffusion depends on the specific diffusing species. Based on the NIST cement hydration and microstructural model, for tritiated water diffusion, the reduction in the diffusivity of the gel caused by silica fume is about a factor of five. For chloride ions, when a diffusivity value 25 times lower than that used for conventional high Ca/Si ratio C-S-H is assigned to the pozzolanic lower Ca/Si ratio C-S-H, excellent agreement is obtained between experimental chloride ion diffusivity data and results generated based on the NIST model, for silica fume additions ranging from 0% to 10%. For higher addition rates, the experimentally observed reduction in diffusivity is significantly greater than that predicted from the computer models, suggesting that at these very high dosages, the nanostructure of the pozzolanic C-S-H may be even further modified. Based on the hydration model, a percolation-based explanation of the influence of silica fume on diffusivity is proposed and a set of equations relating diffusivity to capillary porosity and silica fume addition rate is developed. A 10% addition of silica fume may result in a factor of 15 or more reduction in chloride ion diffusion and could potentially lead to a substantial increase in the service life of steel-reinforced concrete exposed to a severe environment.

Charge Compensation Mechanisms in La-Doped BaTiO3

The mechanism of doping BaTiO3 with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350–1400°C are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350–1400°C. We find no evidence for any changes in cationic composition of the BaTiO3 solid solutions arising from changes in oxygen content.

A theoretical study of the structure and vibrations of 2,4,6-trinitrotolune

Abstract Theoretical calculations of the structure, internal rotations and vibrations of 2,4,6-trinitrotolune, TNT, in the gas phase were performed at the B3LYP/6-31G* and B3LYP/6-311+G** levels of theory. Two genuine energy minimum structures were found. In both structures the 4-nitro group is planar to the phenyl ring, while the 2,6-nitro groups are slightly out of plane with the phenyl ring due to steric interaction with the methyl group. The two structures are related by internal rotations of the methyl and 2, or 6-nitro group. The lowest energy route for interconversion between them is a concerted motion of the methyl group and 2 or 6 nitro group in a ‘cog wheel’ type of mechanism. The geometry of the low energy structure A is closest to that observed in the crystal structures of TNT, where all three nitro groups are out of plane with the phenyl ring. FTIR and Raman spectra of solid TNT and 13C, 15N enriched TNT are presented and assigned with the help of the B3LYP/6-311+G** calculations on A. The lower level B3LYP/6-31G* calculation fails to predict the correct vibrational coupling between the nitro and phenyl groups. The B3LYP/6-311+G** calculation gives a good prediction of the nitro vibrations and the isotopic shifts observed for TNT isotopomers.

Chloride ingress in cement paste and mortar

In this paper chloride ingress in cement paste and mortar is followed by electron probe microanalysis. The influence of several paste and exposure parameters on chloride ingress are examined (e.g., water-cement ratio, silica fume addition, exposure time, and temperature). The measurements are modelled on Ficks law modified by a term for chloride binding. Inclusion of chloride binding significantly improves the profile shape of the modelled ingress profiles. The presence of fine aggregate and formation of interfacial transition zones at paste-aggregate boundaries does not significantly affect diffusion rates.

Infrared intensities of ν3 and ν4 in SiH4, GeH4 and SnH4

Abstract New infrared intensity data for the bands owing to ν 3 and ν 4 in SiH 4 , GeH 4 and SnH 4 are reported. Substantial changes from earlier estimates are found, particularly for SnH 4 . The intensities in SiH 4 are also calculated ab initio using 6–31G* and 6–31G** basis sets. With the signs of ∂ p /∂ Q fixed by these and prior calculations, ∂ p /∂ S values are calculated and interpreted in terms of zero order electro optical parameters, charge fluxes and atomic polar tensors. The atom polarisation P A found for SiH 4 agrees well with that from refractive index and dielectric constant measurements. The infrared intensities found correlate well with the frequency shifts from gas to high temperature crystal phase. The Haas-Ketelaar equation accounts for 45% and 70% respectively of these shifts in ν 3 and ν 4 , after appropriate estimates of density and refractive index have been made.

The crystal structures of Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8; palmierite derivatives

The title phases Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8, synthesised by solid state reaction of oxides at 1350°C, have structures derived from that of the palmierite-type of Ba3V2O8; the stoichiometry is often written as A3RV3O12. Ba2La2/3V2O8 is hexagonal, spacegroup R―3m, a = 5.75271(7), c = 21.0473(5) Å, Z = 3; cation ordering was determined by joint Rietveld refinement using X-ray and neutron powder diffraction data, Rwp = 4.45%, Rp = 6.33%, χ2 = 6.847. In the Ba3V2O8 structure, Ba occupies two sites: 3a and 6c. In Ba2La2/3V2O8, Ba wholly occupies the 3a site; the 6c site contains both Ba, La and vacancies. Bond valence analysis was inconclusive, but tends to support the presence of Ba on the 3a site. Sr2La2/3V2O8 and Ba2Nd2/3V2O8 are isostructural, as confirmed by Rietveld refinement using X-ray powder diffraction data. Ba2R2/3V2O8 phases could not be synthesised for lanthanides, R, smaller than Sm.

Synthesis, phase stability and electrical characterisation of BINAVOX solid solutions

The compositional range of ‘Bi4V2O11’ solid solutions containing Na has been determined by means of a phase diagram study using X-ray diffraction and electron probe microanalysis. The locus of the solid solution indicates that Na replaces V in the crystal structure and the BINAVOX family has the overall formula Bi4+yV2–y–xNaxO11–y–2x. The solid solution limits are temperature dependent and for samples prepared and air-cooled from 830 °C the limits are defined as 0.00 0.10 are thermodynamically unstable below ca. 720 °C but can be stabilised kinetically by rapid cooling from temperatures above ca. 750 °C. ac Impedance measurements demonstrate that BINAVOX materials are electrically inhomogeneous, exhibit temperature- and time-dependent conductivities and do not offer any significant advantages over the parent solid solution.

Stoichiometry and defect structure of ‘NdMnO3'’

The phase ‘NdMnO 3 ’ has been synthesised and characterised by a combination of electron probe microanalysis, X-ray and neutron diffraction and H 2 -reduction thermogravimetry. NdMnO 3 forms, at ≈1400 °C, over the range Nd 1.00(1) Mn 0.95(1) O 2.92(1) to Nd 0.88(1) Mn 1.00(1) O 2.72(1) . Oxygen contents vary in air over the range 700 to 1400 °C and can be varied further, either by high pressure O 2 treatment or by reduction in H 2 . The structure of ‘NdMnO 3 ’ is based on the GdFeO 3 structure with a Jahn-Teller distortion associated with the high proportion of Mn 3+ ions present. However, it also shows a very varied defect structure; depending on composition and heat treatment, vacancies can form on any one or any two of the three sublattices, Nd, Mn and O and the overall Mn oxidation state can include 2+, 3+ and 4+ contributions. Data on compositional range and defect crystal structure are presented in the form of a novel phase diagram-defect structure map.

Chloride ingress profiles measured by electron probe micro analysis

Abstract Traditional techniques for measuring chloride ingress profiles do not apply well to high performance cement paste systems; the geometric resolution of the traditional measuring techniques is too low. In this paper measurements by Electron Probe Micro Analysis (EPMA) are presented. EPMA is demonstated to determine chloride ingress in cement paste on a micrometer scale. Potential chloride ingress routes such as cracks or the paste-aggregate interface may also be Characterized by EPMA.

NaBi3V2O10: a new oxide ion conductor

The new phase NaBi3V2O10 is reported; it was synthesised by oxide reaction at 600 °C and is triclinic with a=7.2026(10), b=7.0600(9), c=5.5312(6) A, α=84.542(12), β=113.318(11) and γ=112.267(12)°; it is an oxide ion conductor with conductivity of ca. 1.5 mS cm–1 at 675 °C.