Neutron Radiographic Study of the Effect of Heat-Driven Water Transport on the Tensile Strength of Bentonite-Bonded Moulding Sand

Wet tensile testing is a common method to assess the stability of bentonite bonded moulding sands. For wet tensile testing, a specimen is first heated from above in order to simulate heat-driven moisture transport induced by the casting process. Then, tensile stress is applied until rupture. In this study, neutron radiography imaging was applied to moulding sands in-situ during heating and wet tensile testing in order to investigate the effects of water kinematics on the tensile strength. Neutron radiography allowed the localization of the rupture plane and the quantitative determination of the local water content with sub-mm resolution. Quantification of the temperature at the rupture plane and of the heat kinematics within the specimen was accomplished by temperature measurements both in-situ and ex-situ. In this way, experimental data correlating the wet tensile strength with the specific conditions of moulding sands at the rupture plane were obtained for the first time. Series of experiments with different initial sand moisture contents were conducted. The results show that the weakest location within a sand profile can be pinpointed at the interface between evaporation and condensation zone (i.e., at the 100 °C isotherm), where water vaporisation starts and the water bridges connecting the sand grains collapse. This weakest location has maximum strength, if the local water content at the rupture plane is between 5 and 9 wt.%. Less water leads to a strong decrease of wet tensile strength. More water requires an initial water content above 5 wt.%, which leads to a decrease of the tensile strength of the unheated sand

A Study on Ikaite Growth in the Presence of Phosphate

Phosphate is a common component in natural growth solutions of ikaite. Although phosphate often occurs as a minor constituent, its presence may promote the formation of ikaite as it significantly inhibits the precipitation of calcite. The interactions of phosphate with ikaite and the role of a potential uptake of phosphate by ikaite, however, are poorly understood. In this study, the influence of phosphate on ikaite growth at 1 °C was investigated. Ikaite and calcite seeded growth experiments were conducted in cryo-mixed-flow reactors at saturation ratios 1.5 ≤ Ωikaite ≤ 2.9 (Ω = ionic activity product/solubility product). From these growth experiments, the rate constant k = 0.10 ± 0.03 µmol/m2/s and the reaction order n = 0.8 ± 0.3 were derived for ikaite. The reaction order implies a transport or adsorption controlled growth mechanism which supports a low energy pathway of ikaite growth via an attachment of hydrous CaCO30 complexes without any extensive dehydration of aqueous species as, for instance, required for calcite growth. A potential depletion of aqueous phosphate due to an uptake by ikaite growth was not detectable. Furthermore, growth retardation by phosphate, as known for calcite growth, was not evident. Thus, a significant incorporation of phosphate into growing ikaite could be precluded for the conditions applied in this study. The observed lack of incorporation of phosphate agrees with the previously suggested growth mechanism via the attachment of hydrous CaCO30 complexes which likely does not facilitate substantial substitution of carbonate by phosphate ions

Micromechanical properties and structural characterization of modern inarticulated brachiopod shells

[1] We investigated micromechanical properties and ultrastructure ofthe shells of the modern brachiopod species Lingula anatina, Disciniscalaevis, and Discradisca stella with scanning electron microscopy (SEM,EDX), transmission electron microscopy (TEM) and Vickers microhardnessindentation analyses. The shells are composed of two distinct layers, anouter primary layer and an inner secondary layer. Except for the primarylayer in Lingula anatina, which is composed entirely of organic matter,all other shell layers are laminated organic/inorganic composites. Theorganic matter is built of chitin fibers, which provide the matrix forthe incorporation of calcium phosphate. Amorphous calcium phosphate inthe outer, primary layer and crystalline apatite is deposited into theinner, secondary layer of the shell. Apatite crystallite sizes in theumbonal region of the shell are about 50 x 50 nm, while within thevalves crystallite sizes are significantly smaller, averanging 10 x 25nm. There is great variation in hardness values between shell layers andbetween the investigated brachiopod species. The microhardness of theinvestigated shells is significantly lower than that of inorganichydroxyapatite. This is caused by the predominantly organic materialcomponent that in these shells is either developed as purely organiclayers or as an organic fibrous matrix reinforced by crystallites. Ourresults show that this particular fiber composite material is veryefficient for the protection and the support of the soft animal tissue.It lowers the probability of crack formation and effectively impedescrack propagation perpendicular to the shell by crack-deviationmechanisms. The high degree of mechanical stability and toughness isachieved by two design features. First, there is the fiber compositematerial which overcomes some detrimental and enhances some advantageousproperties of the single constituents, that is the softness andflexibility of chitin and the hardness and brittleness of apatite.Second, there is a hierarchical structuring from the nanometer to amicrometer level. We could identify at least seven levels of hierarchywithin the shells

On the nucleation of ikaite (CaCO3x6H2O) – A comparative study in the presence and absence of mineral surfaces

The formation of ikaite (CaCO3x6H2O) was studied in the presence and absence of quartz and mica surfaces using desupersaturation curves from cryo-mixed-batch-reactor experiments. Upon nucleation and growth within the reactor, the solution approached solubility of the precipitating carbonate phase. For ikaite, a solubility constant of log Ksp ikaite = -7.3 ± 0.1 was found (T = 0 °C). At supersaturations Ωikaite < 15, the nucleation of ikaite was significantly promoted by the presence of quartz or mica. This promotion prevented a competing nucleation of anhydrous calcium carbonates. In the presence of quartz or mica, therefore, ikaite forms over a much broader supersaturation range than in the absence. Similarly strong promotors of ikaite nucleation rather than anhydrous carbonate nucleation were previously attributed to calcite-inhibiting substances only. At supersaturations Ωikaite ≥ 8, application of classical nucleation theory on induction periods of ikaite formation yielded an effective interfacial energy of 15 ± 3 mJ/m2. Compared to data of anhydrous CaCO3 phases, this interfacial energy is low and expresses the highly hydrated character of ikaite. At supersaturations Ωikaite ≥ 18, a transient amorphous phase appeared besides ikaite. Our results show that a comprehensive understanding of ikaite formation in natural settings requires consideration not only of supersaturation and presence of calcite-inhibitors but also of the presence or absence of mineral surfaces capable of promoting heterogeneous nucleation of ikaite

Hierarchical structure of marine shell biomaterials: biomechanical functionalization of calcite by brachiopods

Biologic structural materials for skeletons or teeth show a hierarchicalarchitecture, where organic macromolecules and mineral substance form ahybrid composite material with its components inter-weaved on manylength scales. On the nanostructure level brachiopods form hybridcomposite mesocrystals of calcite with occluded organic molecules. Onthe microstructure level several types of materials are produced, onwhich the electron back-scatter diffraction (EBSD) technique givesinsight in texture and architecture. We describe the calcitesingle-crystal fiber composite architecture of the secondary layerinvolving organic matrix membranes, the competitive-growth texture ofthe columnar layer and the nanostructuring of the primary layer. In theoverall skeleton the organic biopolymers provide flexibility and tensilestrength while the mineral provides a high elastic modulus, compressivestrength, hardness and resistance to abrasion. The hierarchicalcomposite architecture, from the nanostructure to the macroscopic levelprovides fracture toughness. The morphogenesis of the biomaterial as awhole and of the mineral crystals is guided by the organic matrix andmost probably involves amorphous calcium carbonate (ACC) precursors. Inthis paper we review the hierarchical architecture of rhynchonelliformbrachiopod shells, which is very distinct from mollusk nacre

Nasszugfestigkeitsprüfungen in der Neutronenradiografie

Bentonitgebundene Formsande gehören aufgrund ihrer Umweltfreundlichkeit und Wiederverwendbarkeit zu den wichtigsten Formstoffen in Gießereien. Es liegt daher nahe, das Verhalten und die Eigenschaften dieser Formsande genau zu untersuchen. Im durchgeführten Forschungsprojekt wurden die Nasszugfestigkeiten von Formstoffproben in Echtzeit während einer Durchstrahlung mit Neutronen gemessen. Dabei konnten auch der Riss abgebildet und die lokalen Wassergehalte ermittelt werden

Raman Study of Synthetic Witherite–Strontianite Solid Solutions

Characterization of zoned crystals of a synthetic witherite–strontianite solid solution (BaxSr1-xCO3) was carried out using electron microprobe analysis and Raman spectroscopy. The sample was obtained by coprecipitation using the silica gel method. As each carbonate crystal from this preparation showed the whole range of intermediate compositions BaxSr1-xCO3, 0.1≤x≤0.9, the solid solution could be studied for single crystals. Peak-shape analysis of the Raman bands showed that the peak shifts depend on the replacement of the Ba and Sr cations introducing different radii and masses. We observed a shift to higher wave numbers for an increase of the SrCO3 content

Growth of Calcium Carbonate in the Presence of Cr(VI)

The extended use of hexavalent chromium Cr(VI) compounds in industrial processes caused a significant increase of the concentration of this highly toxic heavy metal in natural environments. In order to investigate the influence of Cr(VI) in the formation of CaCO3, crystallization experiments were carried out in a double diffusion system, using silica hydrogel with different Cr(VI) contents as the growth medium. Crystalline products were examined by scanning electron microscopy, Raman spectroscopy, electron microprobe analysis, and single crystal X-ray diffraction. Increasing Cr(VI) concentration caused inhibition of the nucleation and growth of calcite and promoted the formation of the metastable polymorphs aragonite and vaterite. This effect correlated with a decrease of crystal size. Furthermore, the habit of calcite crystals changed with increasing Cr(VI) concentrations from {104} to forms increasingly elongated parallel to the c-axis. Raman spectroscopy, single crystal X-ray diffraction (XRD), and electron microprobe analysis (EMPA) gave strong indications of an isomorphic anion substitution of trigonal planar carbonate by the tetrahedral chromate within the calcite lattice. The apparent partitioning coefficients of Cr(VI) into calcite determined in this work suggest that the fate of this pollutant in natural environments can be significantly influenced by CaCO3 precipitation processes

Neutronenradiographie an bentonitgebundenem Formstoff

Die Wiederverwendbarkeit von bentonitgebundenem Formstoff ist begrenzt. Die Ursachen sind vielfältig. Eine Hauptursache liegt in der Abnahme der Bindefähigkeit der Tonminerale. Wird diese Abnahme ausschließlich durch Bedingungen in der industriellen Anwendung ausgelöst, wie zum Beispiel durch Totbrand oder Versalzung, oder könnte sie auch Folge einer inhärenten Eigenschaft der Tonminerale sein, nämlich einer Minderung der Reversibilität des Wasseraustausches schon bei Temperaturen unter 200 °C? Mit der Neutronenradiographie kann Wasser im Formstoff während einer Hitzeeinwirkung quantitativ und in Echtzeit detektiert werden. Untersuchungen mit reinen Quarz-Bentonit-Wasser-Mischungen zeigten nach vier Umläufen keine signifikante Abweichung der Wasserkinematik während der Hitzeeinwirkung. In Formstoffbereichen, die Temperaturen über 100 °C ausgesetzt waren, verlief die Wassergabe in den verschiedenen Proben vergleichbar. In den Bereichen, die unter 100 °C blieben, hing jedoch der momentane lokale Wassergehalt deutlich von der anfänglichen Feuchte des Formstoffs und von der Qualität der Durchmischung ab. Letztere beeinflusste wiederum deutlich die Ausprägung der Kondensationszone im Formstoff