62,973 research outputs found
In-situ XRPD of hydrating cement with lab instrument: reflection vs. transmission measurements
No full textThe complex system of reactions taking place during cement hydration can be investigated by means of in-situ XRPD. Time-resolved synchrotron XRPD is the optimal source although it is hardly accessible for routine measurements. The present work was aimed to test the feasibility of in situ measurements of hydrating cement pastes using a labo-ratory diffractometer (PANalytical X’Pert PRO) in reflecting and focusing transmission geometries. Ordinary CEM-I 52.5 R paste (w/c = 0.5) was measured using three different instrumental settings: reflection Bragg-Brentano geometry, focusing transmission capillary geometry and focusing transmission flat sample geometry. Semi-quantitative phase analysis were performed using a Rietveld full profile fitting approach. Advantages and disadvantages of the three different experimental configurations are critically discussed
Role of Polycarboxylate-ether superplasticizers on cement hydration kinetics and microstructural development
No full textPolycarboxylate-ether (PCE) superplasticizers are a fundamental constituent of modern cementbased materials due to their impact on the rheology of the fresh mix and mechanical performance of the hardened material. The effect of PCEs on cement hydration kinetics has been known since their introduction in the early 1980s. However, detailed knowledge of the role played by PCE macromolecules on the basic mechanisms of cement hydration (dissolution, diffusion, precipitation) is still lacking. A better understanding of how such mechanisms are influenced by the addition of PCE is no doubt beneficial to the design of novel superplasticizing admixtures. Here, I report on some recent findings about the role of PCE superplasticizers on cement hydration kinetics and microstructural development. The interaction between PCE and C3S pastes was investigated by an ad-hoc kinetic model based on a combination of generalized forms of the Avrami and BNG (Boundary Nucleation and Growth) models. The model is used to fit the rate of C-S-H precipitation measured by in-situ X-ray powder diffraction combined with mass balance calculations. The results show that a switch from heterogeneous to homogeneous C-S-H nucleation occurs in the presence of PCEs and that the C-S-H growth rate decreases proportionally to the amount of PCE used. The predicted switch to homogeneous nucleation is in agreement with experimental results obtained by XRD-enhanced micro-tomography imaging, showing that, in the presence of PCE, C-S-H preferentially forms in the pore space rather than at the surface of clinker particles.</jats:p
In situ time resolved synchrotron powder diffraction study of mordenite
No full textThe step by step thermal dehydration process of the zeolites mordenite from Pashan (Poona, India) (Na3.51K0.14Ca1.89Mg0.09Sr0.01)[Fe0.033+Al7.40Si40.53O96].27.26H(2)O, has been studied in situ by synchrotron powder diffraction. The time-resolved experiment was performed using a translating imaging plate system. The structure refinements by full profile Rietveld analysis were performed in the Cmcm space group in the temperature range from 25 to 830degreesC. The results of structure refinements indicate that the slight cell-volume contraction (similar to1.9%) is related to the release of water molecules from the channels: above 375degreesC, the water loss lead to an enlargement of the 8-membered ring parallel to [010], which occurred by a flattening of T3-O9-T3 and T4-O4-T3 bridges. The dehydration process is reflected not only in the content of water molecules in the zeolite channel, but also in the temperature behaviour of the unit cell parameters. The cell parameters b and c decreased regularly as the temperature rose, whereas a decreased up to 400degreesC then increased up to 630degreesC, and finally decreased until the end of experiment. The removal of water molecules was accompanied by a spreading of the initial Ca sites into many positions bonded to the framework oxygens. The increased interaction with the framework oxygens of Ca sites was intimately related to the distortion of the 12-ring which is in turn related to the lengthening of the a cell parameter
Over-loaded Cu-ZSM-5 upon heating treatment: A time resolved X-ray diffraction study
No full texthe over-loaded Cu-ZSM-5 under heating treatment (50-910 degrees C) in oxidative conditions exhibits the formation of copper aggregates, which show different oxidation states as a function of temperature (Cu-0, Cu2O and CuO). At room temperature the excess of copper is not detectable by X-ray diffraction (XRD). It is likely present as divalent copper species (Cu-x(OH)(y)) dispersed on the zeolite surface that appear amorphous at XRD. This is confirmed by the UV-Vis diffuse reflectance spectroscopy data. From Rietveld refinement of room temperature and high temperature diffraction data, there are no indications of the presence of copper clusters inside the zeolite cavities. As the heating treatment proceeds, the dispersed copper species transform to metallic copper particles with increasing diffracting domains as a function of temperature. Above 510 degrees C the metallic copper starts to progressively oxidise to Cu2O and finally CuO
Multifractal Analysis of Calcium Silicate Hydrate (C–S–H) Mapped by X-ray Diffraction Microtomography
No full textAn investigation of Calcium Silicate Hydrate (C–S–H) topological distribution and modes of precipitation is performed by combining X-ray powder diffraction tomography, performed on an ordinary Portland cement sample, with a numerical scheme that simulates C–S–H precipitation. C–S–H distribution maps obtained both experimentally and numerically are analyzed by means of a quantitative method based on the principles of multifractal systems. The combination of these methods allows C–S–H spatial distribution and modes of precipitation and aggregation to be assessed quantitatively. In particular, the multifractal spectra obtained from the digital images of the cement paste microstructure, act as a structural probe, which is able to quantify the tendency of C–S–H to form clusters. The results of this combined approach suggest that a multifractal network forms by aggregation of C–S–H clusters heterogeneously nucleated on clinker grains and preexisting clusters that are partly homogeneously nucleated in the porous space
L'influenza del gelo-disgelo e della cristallizzazione salina sul comportamento elettrico di campioni di marmi
No full textThe causes of deterìoration of the physical and mechaniical proprìeties of stane mateiials in the Italian climate are chiefly due to two factors: -changes in the status of the water inside the pores following varìations in air temperature;
- capillary absorption of water with high load of dissolved salts. These processes promote the formation of saline solutions inside the materials due to condensation.
The state of aggregation of stane materìah is represented by porosity, a parameter that affects their physical and mechaniral proprìeties. Porosity impìies thè presence of empty spaces (pores and cracks) through which water and saline solutions can penetrate. Important modifications dìie to wecithmng phenomoia mai oca.tr. Of hese salts are distinctively importuni thè diasulphates (abundant in earth), thè chlorìdes (sea water) and thè nitrates (waste waters). Their presence under certain conditions provoke thè formation of crystals, wich detennines an increase in thè pore water pressure (pressure of crystallisation) that if contini/ed far long perioda of (ime would resuìt in devasta-iing effects (sudi as their qualità deterìomtion). Addi-tionally, many of these salts may exist in different states of hydration (Le. different volumes), so that "hydration pressure" is alno exerted on thè wnlls of thè pore. The precipitation of thè salts ma.y take piace on thè exter-nal surface ofthe storie mate-rìal or within thè porous. If thè veloci ty ofdiffiision ofthe water rapour through thè surface is loiver tìian thè velociti of migmtion of thè solution thmugh thè inside pores, thè salts will cr\slallise on thè surface under thè forni of effloresccnces. Alter-natively. crystfillisntion ma} occur beneath thè external surface giving ?isc lo thè formation of suì>florescences. Saline efflorescences altack and destro} thè surface parta, ti'hile thè formation o
Arsenic in the groundwater aquifers of the Venetian Plain: geochemical modelling and occurrence of As-sulfides minerals, a review of data from the medio Brenta domain (Italy)
No full textThe Venetian Plain is known for areas with high concentrations of arsenic (As) in groundwater (up to more than 400 AgIL; exceptionally 647 mu gIL, in selected areas). A study area was chosen, north of Padua, which exhibits typical residential, industrial, and agricultural characteristics similar to most Western countries and lacks hydrothermal, volcanic, or anthropogenic sources of arsenic. The pilot area was the focus of several studies which are reviewed in this note. The objectives of the studies were to verify the distribution of As concentrations in groundwater and sediments (mineralogical and geochemical analysis of groundwater sediments and of filtered and unfiltered groundwater) and to model the mobility of arsenic arising from water-rock interaction. The grain size of aquifer reservoirs includes gravel, sand, silt, and clay. The amount of organic matter in the aquifer sediments of the study area seems peculiar (higher) compared to other plains in the world; it influences the redox potential and the relative concentration of As in groundwater. Arsenic contamination in groundwater and redox conditions varied greatly in the area. Groundwater under oxidizing and highly reducing conditions had much lower arsenic concentrations compared to groundwater under intermediate reducing conditions. Arsenic minerals (such as realgar-pararealgar) occur in aquifer sediments and they were documented in the studied materials by different analytical techniques for the first time in the context of the Italian plains. Since these minerals are rare throughout the world in plain sediments not affected by volcanic or hydrothermal activity, their occurrence is a distinctive feature of the Venetian Plain aquifer. These arsenic minerals were found in peat sediments of the study area, consistent with geochemical modeling results, which require highly reducing conditions for their precipitation from groundwater. Modeling suggests that under oxidizing and up to slightly reducing conditions (from 200 mV to -50 mV), arsenic is adsorbed on solid phases, but a further decrease in redox potential leads arsenic desorption from solids and consequent groundwater contamination (from -50 mV to -250 mV). If the redox potential becomes even more negative (below -250 mV), geochemical conditions are favorable to the formation of arsenic sulfides. The precipitation of the realgar-pararealgar phases, predicted by the geochemical model, proceeds by extracting arsenic from the groundwater and quantitatively accounts for the lower arsenic concentration measured in the highly reducing groundwater of the study area
Study of dehydration-rehydration processes in zeolite A: possible application of hydrophylic zeolites to improve the selectivity of gas sensors
No full textPt NPs Supported on CeO2/C as Electrocatalysts for Oxygen Reduction Reaction: Novel Physicochemical Insights on the Synthesis and on the Improved Activity and Stability
No full textThis study emphasizes the effect of CeO2 on the Pt nanoparticle (NP) dimension, stability, and activity versus the oxygen reduction reaction. It is demonstrated that the one-pot synthesis of Pt NPs along with CeO2 NPs over carbon support produces small Pt NPs (2 nm) with higher activity, than the sole Pt NPs, thanks to the cooperative interaction exerted by CeO2. This is nicely demonstrated by using synchrotron wide-angle X-ray total scattering and advanced data analysis, monitoring the in situ nucleation and growth of Pt NPs in the presence of preformed CeO2 NPs or of a Ce precursor. Raman, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy analyses are carried out to support the formation of oxygen vacancies responsible for the metal–support interaction. Moreover, the most effective catalyst, PtCeO2/C250 (mass activity: MA0.9 V = 423 Ag−1; specific activity: SA0.9 V = 446 μAcm−2), exhibits activity comparable to the commercial benchmark Pt/C, yet significantly greater stability as demonstrated by accelerated stress tests conducted on gas diffusion electrode. Specifically, PtCeO2/C250 retains 62% ± 7% of its MA0.65 V and 79% ± 9% of its SA0.65 V, compared to 43% ± 5% and 62% ± 7%, respectively, for the benchmark
- …
