1,720,958 research outputs found
Assessing the dimensional stability of alkali-activated calcined clays in the fresh state: a time-lapse X-ray imaging approach
Full text linkAlkali-activated calcined clays are promising candidates for playing a prominent role in the future construction industry. These binders may achieve excellent mechanical performance, but one issue deserving attention is the proneness to plastic shrinkage and surface cracking. Tackling this issue requires the deployment of laboratory techniques that allow shrinkage-inducing mechanisms to be quantitatively assessed. Here, we demonstrate that time-lapse X-ray imaging can be used to quantify shrinkage immediately after mixing, when the binder is still in its fresh state, with excellent time and space resolution. The numeric quantification of strain is complemented by the real time visual inspection of the displacing sample interface and of the bleed aqueous solution layer that may form. Implementation of this method to a set of alkali-activated cement pastes, prepared by combining calcined clays having different mineralogical composition with sodium silicate activating solutions having different SiO 2/Na 2O ratios, suggests that two main mechanisms control the early dimensional stability of alkali-activated calcined clays. These mechanisms are: (a) volumetric contraction occurring in response to capillary stress arising from water evaporation and (b) segregation by particle settling, favoured in the water-saturated regime
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Mitigating the ecological footprint of alkali-activated calcined clays by waste marble addition
Full text linkLow-CO2, Portland-free cement binders can be produced by reaction of calcined clays in alkaline solutions, although the environmental footprint associated with alkaline activators may represent a point of concern. The amount of alkaline activator, and its embodied CO2, can be reduced by partial replacement of calcined clay with waste marble. Fresh and hardened state properties, as well as compositional and microstructural evolution of alkali-activated blends of calcined clay and waste marble, were investigated by combining experimental methods with thermodynamic modelling. It was shown that substitution of calcined clay with 30% waste marble was beneficial to the workability and mechanical properties of alkali-activated binders and can mitigate drying shrinkage. The physical and chemical role of calcium carbonate and the composition of the reaction products were discussed, along with the role of carbonation reactions occurring by uptake of atmospheric CO2. This approach can contribute to both reducing the CO2 footprint of cement and upcycling potentially hazardous waste from the dimension stone industry
Use of Waste Calcium Carbonate in Sustainable Cement
Full text linkThe quest for a new-generation concrete, designed to be compatible with the need of mitigating the effect of greenhouse gas emissions on the climate, has prompted applied research to define a broad range of low-CO2 cement-based materials. While minimizing CO2 emissions is a goal of the utmost importance, research into sustainable building materials must also tackle the issue of raw material depletion (including limestone, clay and aggregate deposits, as well as water resources) in favor of secondary raw materials. One possible solution is that of minimizing the impact of quarrying by a circular economy approach that envisages the reuse of waste from stone extraction and processing. It is estimated that 200 Mt waste are produced by the stone industry worldwide each year. This includes slurries obtained from the quarrying, cutting and polishing of marble, which can be used as a source of calcium carbonate, alternative to primary limestone. This contribution illustrates the use of waste calcium carbonate, obtained from marble slurry (waste marble, WM), in sustainable cement materials alternative to Portland cement. The possibility of exploiting locally available resources is explored, and the effect of WM additions up to 50% by total mass on the macroscopic properties is investigated experimentally. It is shown that binders with adequate fresh and hardened state properties can be obtained by moderate additions of WM, which greatly enhances the environmental performance by reducing the amount of primary resources used in the mix. By reducing the amount of thermally treated clay in alkali-activated blends, the use of WM also results in a net decrease of the embodied energy
Performance and Properties of Alkali-Activated Blend of Calcined Laterite and Waste Marble Powder
Full text linkThis contribution reports on some preliminary studies on the use of lateritic soils from Cameroon as raw materials for the production of alkali-activated binders. These soils contain about 40–60% kaolinite and variable amounts of quartz, hematite, and other minor phases. After calcination at 800 °C, this material is blended with up to 30% waste marble powder, which is produced in large amounts during quarrying, cutting, and processing of marble. The results of our tests show that a careful mix design allows a good mechanical performance to be achieved, with the values of the cubic compressive strength larger than 30 MPa after 28 days. The role of Fe on the performance of this material is investigated by comparison with Fe-free blends of commercial metakaolin, waste marble powder, and quartz. Calorimetric data suggest that the use of alkanolamines as Fe chelating agents may accelerate the early-age reactivity, depending on dosage, although the effect on the development of mechanical properties is minor. It is argued that alkali-activated calcined laterite represents a viable option for the development of sustainable binders, especially for the African market, where it could be used, for example, to produce compressed stabilized earth blocks, in substitution of masonry units based on Portland cement or fired clay bricks. The use of waste marble powder adds further environmental value to this material
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