Atomic and nano-scale characterization of a 50-year-old hydrated C3S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C-S-H fibers are composed of particles that are 1.5-2 nm thick and several tens of nanometers long. 29Si NMR shows 47.9% Q1 and 52.1% Q2, with a mean SiO4 tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50 years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5 year old hydrated C3S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L3,2-edge indicates that Ca2 + in C-S-H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO44 - tetrahedron chain

Absolute single photoionization cross-sections of Br3+: Experiment and theory

Absolute single photoionization cross section measurements for Br3+ ions are reported in the photon energy range 44.79-59.54 eV at a photon energy resolution of 21 ±3 meV. Measurements were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams technique. Numerous resonance features in the experimental spectrum are assigned and their energies and quantum defect values are tabulated. The cross-section measurements are also compared with Breit-Pauli R-matrix calculations with suitable agreement over the photon energy range investigated. Analysis of the measured spectrum including Rydberg resonance series identifications produced a new emperical determination of the ionizational potential of Br3+ of 46.977 ± 0.050 eV, which is 805 meV lower than the most recently published value of 47.782 eV. This disparity between our determination and the earlier published value is similar to an 843 meV shift in the accepted ionization potential published for iso-electronic Se2+ as part of this same research program

Fossil biomass preserved as graphitic carbon in a late paleoproterozoic banded iron formation metamorphosed at more than 550°C

Metamorphism is thought to destroy microfossils, partly through devolatilization and graphitization of biogenic organic matter. However, the extent to which there is a loss of molecular, elemental and isotope signatures from biomass during high-temperature metamorphism is not clearly established. We report on graphitic structures inside and coating apatite grains from the c. 1850 Ma Michigamme silicate banded iron formation from Michigan, metamorphosed above 550°C. Traces of N, S, O, H, Ca and Fe are preserved in this graphitic carbon and X-ray spectra show traces of aliphatic groups. Graphitic carbon has an expanded lattice around 3.6 Å, forms microscopic concentrically-layered and radiating polygonal flakes and has homogeneous δ13C values around −22‰, identical to bulk analyses. Graphitic carbon inside apatite is associated with nanometre-size ammoniated phyllosilicate. Precursors of these metamorphic minerals and graphitic carbon originated from ferruginous clayrich sediments with biomass. We conclude that graphite coatings and inclusions in apatite grains indicate fluid remobilization during amphibolite-facies metamorphism of precursor biomass. This new evidence fills in observational gaps of metamorphosed biomass into graphite and supports the existence of biosignatures in the highly metamorphosed iron formation from the Eoarchean Akilia Association, which dates from the beginning of the sedimentary rock record

A marine biogenic source of atmospheric ice nucleating particles

The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties1,2. The formation of ice in clouds is facilitated by the presence of airborne ice nucleating particles1,2. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice3-11. Sea spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer12-19. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice nucleating material is likely biogenic and less than ~0.2 μm in size. We find that exudates separated from cells of the marine diatom T. Pseudonana nucleate ice and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol in combination with our measurements suggest that marine organic material may be an important source of ice nucleating particles in remote marine environments such as the Southern Ocean, North Pacific and North Atlantic

Nanometer-Resolved Spectroscopic Study Reveals the Conversion Mechanism of CaO·Al2O3·10H2O to 2CaO·Al2O3·8H2O and 3CaO·Al2O3·6H2O at an Elevated Temperature

The main binding phases of calcium aluminate cement (CAC) concrete, CaO·Al2O3·10H2O (CAH10) and 2CaO·Al2O3·8H2O (C2AH8), slowly convert to 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3). This reaction significantly speeds up at a temperature higher than ∼30 °C, and over time leads to significant strength loss in CAC concrete. Because of the lack of direct evidence that simultaneously probes morphological and chemical/crystallographic information, intense debate remains whether the conversion is generated by a solid-state or through-solution reaction. The conversion of CAH10 at an elevated temperature is studied herein using synchrotron-radiation-based X-ray spectromicroscopy capable of acquiring near edge X-ray absorption fine structure data and ptychographic images with a resolution of ∼15 nm. We show that, when stored at 60 °C, CAH10 first converts to C2AH8 by solid-state decomposition, followed by the through-solution formation of C3AH6. The C3AH6 crystallizes from both the relics of dissolved C2AH8 and from the surface of existing C3AH6 crystals. The solid-state decomposition of CAH10 occurs in multiple sites inside the CAH10 crystals; the spatial range of each decomposition site spans a few tens of nanometers, which overcomes the kinetics barrier of ion transportation in the solid-state. Our work provides the first nanoscale crystal-chemical evidence to explain the microstructure evolution of converted CAC concrete

Atomic and nano-scale characterization of a 50-year-old hydrated C3S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C-S-H fibers are composed of particles that are 1.5-2 nm thick and several tens of nanometers long. 29Si NMR shows 47.9% Q1 and 52.1% Q2, with a mean SiO4 tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50 years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5 year old hydrated C3S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L3,2-edge indicates that Ca2 + in C-S-H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO44 - tetrahedron chain

Absolute single-photoionization cross sections of Se2+: Experiment and theory

Absolute single-photoionization cross-section measurements for Se2+ ions were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams photo-ion technique. Measurements were made at a photon energy resolution of 24±3 meV in the photon energy range 23.5-42.5 eV, spanning the ground state and low-lying metastable state ionization thresholds. To clearly resolve the resonant structure near the ground-state threshold, high-resolution measurements were made from 30.0 to 31.9 eV at a photon energy resolution of 6.7±0.7 meV. Numerous resonance features observed in the experimental spectra are assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are compared with large-scale, state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb R-matrix method. Suitable agreement is obtained over the entire photon energy range investigated. These results are an experimental determination of the absolute photoionization cross section of doubly ionized selenium and include a detailed analysis of the photoionization resonance spectrum of this ion

Absolute single photoionization cross-section measurements of Rb2+ ions: Experiment and theory

Absolute single photoionization cross-section measurements of Rb2+ ions were performed using synchrotron radiation and the photo-ion, merged-beams technique at the Advanced Light Source at Lawrence Berkeley National Laboratory. Measurements were made at a photon energy resolution of 13.5 2.5 meV from 37.31 to 44.08 eV spanning the 2P ground state and 2P metastable state ionization thresholds. Multiple autoionizing resonance series arising from each initial state are identified using quantum defect theory. The measurements are compared to Breit-Pauli R-matrix calculations with excellent agreement between theory and experiment