1,721,016 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
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)
Get PDFThe 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 textCo- and Ni-exchanged ferrierite: The contribution of synchrotron X-ray diffraction data to siting of TMIs
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Application of the Rietveld method for the investigation of mortars: a case study on the archaeological site of Thamusida (Morocco)
No full textThis study presents a multi-technique approach to the investigations of mortars, involving the application of the Rietveld method. The quantitative mineralogical analysis of samples was aimed at (1) verifying its significance in relation to results obtained through other techniques and (2) evaluating its potential in mortars characterisation.
Six type of mortars used in the Roman settlement of Thamusida (50 km north of Rabat, Morocco) were investigated through X-ray powder-diffraction and Rietveld refinement, optical microscopy, image analysis, scanning electron microscopy, electron microprobe analysis, X-ray fluorescence (XRF) and combined differential thermal, and thermo-gravimetric analysis (DT-TGA).
All the adopted techniques yielded unique results and complementary data; however, the Rietveld method can actually Substitute thermal analysis accurately. CaO oxide contents obtained by DT/TGA and Rietveld are in good agreement, but both were only partly comparable to XRF results. In our case, XRF and Rietveld results diverge when binders contain more than 67 wt.% CaO. The discrepancies can mainly be ascribed to the presence of amorphous material and to the obtainable accuracy by QPA-Rietveld method; conversely, the nonstoichiometry of phases does not affect our results, because phases with variable compositions are present in small amounts.
Results further showed that the adopted methods were suitable for mortar characterisation. All studied mortars (1) have weak hydraulic properties; (2) employed binders characterised by highly variable CaO contents and by significant amounts of SiO(2); (3) used coarse to fine quartz-carbonate-rich sands as the aggregate; and (4) selectively mixed 2 or 3 parts of aggregate with I part of binder, depending on the destination of the mortar
Role of Polycarboxylate-ether superplasticizers on cement hydration kinetics and microstructural development
Get PDFPolycarboxylate-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
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