1,720,962 research outputs found
Physical and nanomechanical properties of the synthetic anhydrous crystalline CaCO3 polymorphs: vaterite, aragonite and calcite
No full textThe synthetic anhydrous crystalline CaCO3 polymorphs—vaterite, aragonite and calcite—were tested using dilatometry and nanoindentation. Microstructural changes in the samples before and after measurements were observed under scanning electron microscope and their phase composition quantified with X-ray powder diffraction with the Rietveld method. The thermal expansion coefficients of vaterite and the hardness and elastic modulus of synthetic aragonite are reported for the first time. The physical and nanomechanical properties were measured under similar conditions for each CaCO3 polymorph. Aragonite, calcite and vaterite showed volumetric thermal expansion coefficient at 303 K of 49.2(8), 48.6(2) and 44.1(3) 10−6 K−1, respectively. The elastic modulus increased from 5(4), 16(7) to 31(8) GPa for aragonite, calcite and vaterite, respectively. Average hardness was found lower than values from the literature, ranging from 0.3 to 1.3 GPa. The results are considered of interest for the design of CaCO3-based materials for applications
Assessment of firing conditions in old fired-clay bricks: The contribution of X-ray powder diffraction with the Rietveld method and small angle neutron scattering
No full textFull characterization of fired-clay bricks is crucial for the improvement of process variables in manufacturing and, in case of old bricks, for restoration/replacement purposes. To this aim, five bricks produced in a plant in Czech Republic in the past have been investigated with a combination of analytical techniques in order to derive information on the firing process. An additional old brick from another brickyard was also used to study the influence of different raw materials on sample microstructure. The potential of X-ray diffraction with the Rietveld method and small angle neutron scattering technique has been exploited to describe the phase transformations taking place during firing and characterize the brick microstructure. Unit-cell parameter of spinel and amount of hematite are proposed as indicators of the maximum firing temperature, although for the latter, limited to bricks produced from the same raw material. The fractal quality of the surface area of pores obtained from small angle neutron scattering is also suggested as a method to distinguish between bricks produced from different raw clays
The use of mineralogical indicators for the assessment of firing temperature in fired-clay bodies
No full textFired-clay bricks are frequently object of conservative actions aimed at the preservation of cultural heritage. Information on firing conditions is relevant for the production of custom made replacement bricks, since, as a widely accepted principle, they should be close match to the pre-existing ones. In this work, the mineralogical and microstructural evolution of fired-clay bodies is described using a combination of analytical techniques, and an approach for the assessment of firing temperature using calibration curves built from the results of X-ray powder diffraction quantitative phase analysis with the Rietveld method, is presented. The weight fractions of hematite, mullite and the amorphous fraction, from two raw clays fired in the laboratory at different temperatures, have been used to assess the firing temperature of two industrially produced bricks. The values derived applying these three methods were in good agreement with the nominal temperatures of the industrial cycles. This approach might be of interest for the assessment of the firing conditions of a broader range of historical/archaeological fired-clay materials
Characterisation of historical fired clay bricks with small angle neutron scattering
No full textMechanical properties and performance of fired-clay bricks are strictly related to their microstructure and mineralogical composition. The need for meeting the requirements of a nondestructive or low impact approach aimed at the conservation of cultural heritage objects, is prompting for the search of new analytical methods for their characterisation. Small angle neutron scattering (SANS) is a powerful technique for evaluating and quantifying a statistically representative microstructure of heterogeneous materials. Five historical fired-clay bricks, produced in a manufacturing plant of the Czech Republic, have been investigated. SANS-derived parameters allowed to retrieve the shape of the pore size distribution, characterise the fractal surface of pores, and suggest an empirical relationship between surface area per unit volume of pores and firing temperature
Mg-phosphate ceramics produced from the product of thermal transformation of cement-asbestos
No full textAccording to recent European directives, the need for environmentally friendly alternative solutions to landfill disposal of hazardous wastes, such as asbestos-containing materials, prompts their recycling as secondary raw materials. In this work, magnesium phosphate ceramics were prepared using the product of inertization of cement-asbestos. Magnesium phosphate ceramics show interesting properties like good water resistance and high strength that make them attractive materials for several applications. Asbestos containing materials were mixed with magnesium carbonate and annealed at two different temperatures (1100 and 1300 °C). During thermal treatment complete destruction of asbestos minerals with their transformation into new phases, and crystallization of MgO from magnesium carbonate decomposition, occurs. Upon addition of potassium di-hydrogen phosphate and water, the magnesium oxide in the product of thermal treatment, contributes to the onset of a setting reaction whose product is magnesium potassium phosphate hydrate. The reactivity of periclase was found to be dependent on the calcination temperature. Lower reaction rates were observed for the MgO obtained at lower temperature. The setting reaction of the magnesium phosphate ceramic has been followed in time up to 6 months by means of the X-rays powder diffraction trechnique. Quantitative phase analysis was performed using the Rietveld method and both crystalline and amorphous phases were quantified. The amount of magnesium potassium phosphate was found to increase with time, and was accompained by a decrease in the amount of the amorphous fraction. This fact supports the hypothesis of the formation of an amorphous precursor of the crystalline MKP during the hydration reaction. SEM images showing elongated magnesium potassium phosphate hydrate crystals emerging from what appears as an amorphous matrix, further confirms this view. Since the mechanical properties of magnesium phosphate ceramics are known to increase with time, we can conclude that the main contribution to the development of strenght comes from the crystalline magnesium potassium phosphate hydrate. In this work, we describe a procedure for simultaneous destruction of asbestos minerals and formation of cementitious compounds, which represents a recycling opportunity for this class of hazardous wastes, bringing benefits in terms of energy requirements and preservation of natural resources in cement manufacturing
Preliminary study of the pozzolanic activity of dumped mine wastes obtained from the North Bohemian basin in the Czech Republic
Full text linkThree dumped raw materials, a tuff and two bentonites, obtained from two mining sites at the North Bohemian basin in the Czech Republic, have been studied in order to evaluate them as pozzolanic admixtures in lime mortars for employment in restoration of cultural heritage objects. After thermal activation (800 °C; 5 h), their pozzolanic properties were compared with those of commercial metakaolin. Quantitative phase analysis with the Rietveld method from X-ray powder diffraction patterns, morphological observations, as well as the Frattini and the modified Chapelle tests were performed. In addition, lime mortars, incorporating the fired materials, were prepared and subjected to simultaneous thermal analysis after a 28-day initial curing (20 ± 1 °C; 60 ± 5 % RH). The results showed that all three materials possess pozzolanic activity. However, when employed in lime mortars they did not result in formation of pozzolanic reaction products. Two methods were proposed to improve their reactivity; grinding to obtain finer particle size and removal of quartz content where necessary
Micro-Raman spectroscopy investigation of the carbonation reaction in a lime paste produced with a traditional technology
No full textMicro-Raman spectroscopy has been used to follow the carbonation reaction up to 16 weeks in lime paste produced with a traditional technology and cured under controlled conditions. The transformation of calcium hydroxide into calcium carbonate was visualised using Raman Imaging. Calcite was the only crystalline CaCO3 polymorph detected under the conditions of the experiment. Three sharp zones of pure portlandite (Ca(OH)2), partially carbonated portlandite and calcite (CaCO3), could be distinguished. Amorphous calcium carbonate was determined within the carbonation front. The carbonation rate was found to be higher between first and second week of ageing. After this time, the rate was continuously decreasing until full carbonation was achieved. Results have been compared with those obtained from quantitative phase analysis with X-rays powder diffraction. The proposed approach allows for a highly accurate description of the carbonation process in traditional lime-based systems. In general, it can be applied to assess the effectiveness of treatments with inorganic agents promoting carbonation, and aimed at the conservation of cultural heritage objects. Copyright © 2016 John Wiley & Sons, Ltd
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
