524 research outputs found
Quantitative study of hydration of C3S and C2S by thermal analysis. Evolution and composition of C-S-H gels formed
This research is part of a European project (namely, CODICE project), main objective of which is modelling, at a multi-scale, the evolution of the mechanical performance of non-degraded and degraded cementitious matrices. For that, a series of experiments were planned with pure synthetic tri-calcium silicate (C3S) and bi-calcium silicate (C2S) (main components of the Portland cement clinker) to obtain different calcium–silicate–hydrate (C–S–H) gel structures during their hydration. The characterization of those C–S–H gels and matrices will provide experimental parameters for the validation of the multi-scale modelling scheme proposed. In this article, a quantitative method, based on thermal analyses, has been used for the determination of the chemical composition of the C–S–H gel together with the degree of hydration and quantitative evolution of all the components of the pastes. Besides, the microstructure and type of silicate tetrahedron and mean chain length (MCL) were studied by scanning electron microscopy (SEM) and 29Si magic-angle-spinning (MAS) NMR, respectively. The main results showed that the chemical compositions for the C–S–H gels have a CaO/SiO2 M ratio almost constant of 1.7 for both C3S and C2S compounds. Small differences were found in the gel water content: the H2O/SiO2 M ratio ranged from 2.9 ± 0.2 to 2.6 ± 0.2 for the C3S (decrease) and from 2.4 ± 0.2 to 3.2 ± 0.2 for the C2S (increase). The MCL values of the C–S–H gels, determined from 29Si MAS NMR, were 3.5 and 4 silicate tetrahedron, for the hydrated C3S and C2S, respectively, remaining almost constant at all hydration periods
Recent studies of cements and concretes by synchrotron radiation crystallographic and cognate methods
The portfolio of available synchrotron radiation techniques is increasing notably for cements
and pastes. Furthermore, sometimes the terminology is confusing and an overall picture highlighting
similarities and differences of related techniques was lacking. Therefore, the main
objective of this work is to review recent advances in synchrotron techniques providing a
comprehensive overview. This work is not intended to gather all publications in cement chemistry
but to give a unified picture through selected examples. Crystallographic techniques are
used for structure determination, quantitative phase analyses and microstructure characterization.
These studies are not only carried out in standard conditions but synchrotron techniques
are especially suited to non-ambient conditions: high temperatures and pressures, hydration,
etc., and combinations. Related crystallographic techniques, like Pair Distribution Function,
are being used for the analysis of ill-crystalline phase(s). Furthermore, crystallographic tools
are also employed in imaging techniques including scanning diffraction microscopy and
tomography and coherent diffraction imaging. Other synchrotron techniques are also reviewed
including X-rays absorption spectroscopy for local structure and speciation characterizations;
small angle X-ray scattering for microstructure analysis and several imaging techniques for
microstructure quantification: full-field soft and hard X-ray nano-tomographies; scanning
infrared spectro-microscopy; scanning transmission and fluorescence X-ray tomographies.
Finally, a personal outlook is provided.I am grateful to all my coauthors, collaborators, colleagues and PhD students, for all our work together
during more than two decades. I thank the University of Malaga and ALBA Synchrotron Light Source
for the support and the stirring environments. I acknowledge the Spanish science funding agencies (they
change the name quite often) for funding my studentship, to do the PhD and the three summer research
stays at Oxford University, to the last ongoing research project. To all synchrotrons I have been allowed to
enjoy carrying out experiments: SRS, ESRF, Max-Lab, DLS, APS, SLS and ALBA. Finally, this work has
been supported by the Spanish MINECO through the BIA2014-57658-C2-1-R research grant
Study of alkaline hydrothermal activation of belite cements by thermal analysis
The effect of alkaline hydrothermal activation of class-C fly ash belite cement was studied using thermal analysis (TG/DTG) by determining the increase in the combined water during a period of hydration of 180 days. The results were compared with those obtained for a belite cement hydrothermally activated in water. The two belite cements were fabricated via the hydrothermal-calcination route of class-C fly ash in 1 M NaOH solution (FABC-2-N) or demineralised water (FABC-2-W). From the results, the effect of the alkaline hydrothermal activation of belite cement (FABC-2-N) was clearly differentiated, mainly at early ages of hydration, for which the increase in the combined water was markedly higher than that of the belite cement that was hydrothermally activated in water. Important direct quantitative correlations were obtained among physicochemical parameters, such as the combined water, the BET surface area, the volume of nano-pores, and macro structural engineering properties such as the compressive mechanical strength
NLL resummation of jet mass
Starting from a factorization theorem in effective field theory, we present
resummed results for two non-global observables: the invariant-mass
distribution of jets and the energy distribution outside jets. Our results
include the full next-to-leading-order corrections to the hard, jet and soft
functions and are implemented in a parton-shower framework which generates the
renormalization-group running in the effective theory. The inclusion of these
matching corrections leads to an improved description of the data and reduced
theoretical uncertainties. They will have to be combined with two-loop running
in the future, but our results are an important first step towards the
higher-logarithmic resummation of non-global observables.Comment: 32 pages, 12 figures. v2: journal versio
Lime stabilisation for earthworks: a UK perspective
Lime stabilisation is a versatile technique applied during earthworks operations. Modern soil recycling units are much more efficient at pulverising fill material and intermixing the added binder/water than machinery available 20 years ago. While supplier innovation adds flexibility to the site working method, specifications have not been sufficiently updated to permit optimal application. This review paper details the physico-chemical changes instigated through the lime-clay soil reaction, updating previous reviews. It aims to assist scientific debate, current practitioners and future specification changes. For example, the application of the minimum 24 h mellowing periods (mandatory to UK specifications) with high reactivity, quicklime powders is concluded to cause increased air voids in the compacted fill. Increased air voids are associated with reduced long-term strength and potential volume change from water ingress, which is of particular concern for sulfate swelling. Shorter mellowing periods and/or use of hydrated lime may lesson this issue; however, a 'one size fits all' approach is discouraged in preference to site-specific methodologies refined to suit the fill material and project requirements. The discussion also summarises working methods which may lower the risk of sulfate swell and defines areas requiring further practical research
Effective Dark Matter Model: Relic density, CDMS II, Fermi LAT and LHC
The Cryogenic Dark Matter Search recently announced the observation of two
signal events with a 77% confidence level. Although statistically inconclusive,
it is nevertheless suggestive. In this work we present a model-independent
analysis on the implication of a positive signal in dark matter scattering off
nuclei. Assuming the interaction between (scalar, fermion or vector) dark
matter and the standard model induced by unknown new physics at the scale
, we examine various dimension-6 tree-level induced operators and
constrain them using the current experimental data, e.g. the WMAP data of the
relic abundance, CDMS II direct detection of the spin-independent scattering,
and indirect detection data (Fermi LAT cosmic gamma-ray), etc. Finally, the LHC
reach is also explored
Search for gamma-ray emission from magnetars with the Fermi Large Area Telescope
We report on the search for 0.1-10 GeV emission from magnetars in 17 months
of Fermi Large Area Telescope (LAT) observations. No significant evidence for
gamma-ray emission from any of the currently-known magnetars is found. The most
stringent upper limits to date on their persistent emission in the Fermi-LAT
energy range are estimated between ~10^{-12}-10^{-10} erg/s/cm2, depending on
the source. We also searched for gamma-ray pulsations and possible outbursts,
also with no significant detection. The upper limits derived support the
presence of a cut-off at an energy below a few MeV in the persistent emission
of magnetars. They also show the likely need for a revision of current models
of outer gap emission from strongly magnetized pulsars, which, in some
realizations, predict detectable GeV emission from magnetars at flux levels
exceeding the upper limits identified here using the Fermi-LAT observations.Comment: ApJ Letters in press; Corresponding authors: Caliandro G. A., Hadasch
D., Rea N., Burnett
Detection of Gamma-Ray Emission from the Starburst Galaxies M82 and NGC 253 with the Large Area Telescope on Fermi
We report the detection of high-energy gamma-ray emission from two starburst
galaxies using data obtained with the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Steady point-like emission above 200 MeV has been
detected at significance levels of 6.8 sigma and 4.8 sigma respectively, from
sources positionally coincident with locations of the starburst galaxies M82
and NGC 253. The total fluxes of the sources are consistent with gamma-ray
emission originating from the interaction of cosmic rays with local
interstellar gas and radiation fields and constitute evidence for a link
between massive star formation and gamma-ray emission in star-forming galaxies.Comment: Submitted to ApJ Letter
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