1,720,998 research outputs found
Shrinkage characterization of alkali-activated slag/fly-ash blends
No full textAlkali-activated materials offer the potential for more durable, sustainable and low-CO2 construction and building materials with reduced environmental footprints when compared to Portland cement concrete. However, this new concrete technology suffers substantially from early-age autogenous shrinkage and micro cracking. The aim of this work is to illuminate the intrinsic reasons that are responsible for larger autogenous shrinkage in alkali-activated slag/fly-ash (AASF) blends by understanding the essential link between solidification process (reaction mechanism, kinetics, phase formations and binder structures) and early-age autogenous shrinkage deformations.
In this study, six different compositions of AASF are studied by varying the type and the concentration of three different activators: sodium metasilicate, combinations of sodium metasilicate with sodium carbonate and sodium sulfate . The results show that the solidification process strongly depends on both the alkalinity and anion type of the alkaline solution. A higher alkalinity (high amount of Na2O) accelerates the reaction process, as the presence of OH- ions enhances the dissolution of slag and also increases the solubility of silica and alumina. Therefore, the intense autogenous shrinkage of alkali-activated slag at early-ages can be attributed to the high amount of chemical shrinkage. Autogenous shrinkage of alkali-activated slag is not only caused by well-known self-desiccation process in hardened state, but related to the condensation shrinkage. The larger autogenous shrinkage in AAS may also be attributed to refined pore structure and silica polymerization, which is controlled by the nature of anion presence in the solution.status: Publishe
Competition between Carbonation and Hydration on the Hardening of Calcium Hydroxide and Calcium Silicate Binders
No full textLime-based mortars composed of calcium silicate binders harden as a resu lt of a combined reaction of hydration and carbonation. Understanding th is combined reaction mechanism and the consequences on the hardened mort ar properties are essential while assessing how they affect the overall durability of historic masonry. In this research, this combined reaction is studied in mortars with lime-pozzolana, hydraulic lime and cement-li me binders, and the consequences on the physical properties, mechanical properties and microstructure of the mortars are discussed. In-depth res earch has been performed on the hydration and carbonation reactions to p rovide fundamental insight on the reaction mechanisms. Carbonation reaction of calcium hydroxide proceeds as initial carbonatio n at the early stage and increased carbonation at the later stage. Durin g initial carbonation, CO2 molecules are rapidly absorbed by the alkalin e water, resulting in the precipitation of calcite crystals instantly. T his phase involves a dormant period in which the reaction controlling fa ctor is the dissolution rate of calcium hydroxide and CO2 in the pore wa ter. Increased carbonation starts once open pore space is created with t he evaporation of the water, allowing diffusion paths for CO2 through th e pore structure. Therefore, at later stage diffusion of CO2 is the rate controlling factor in the carbonation process. CO2 concentration, relat ive humidity conditions and physical properties of the lime particles ar e found to have an important impact on the habit, size and morphology of the calcite by altering calcium and carbonate ions concentration in the pore water. The consequence of this on the mechanical properties of the mortar is negligible. Hydration reactions are the first reaction order in hydraulic lime and l ime-pozzolana mortars, which proceed more gradually when compared with f ast cement hydration. Carbonation is the complementary reaction in the s trength gain of the mortars and starts once pore structure is formed to allow CO2 diffusion. Competitive reaction order between hydration and ca rbonation can occur in lime-pozzolana mortars depending on the pozzolani c reactivity of the pozzolana. As a consequence of the consumption of th e calcium hydroxide by the carbonation reaction, the degree of the pozzo lanic reactions is lowered. The consequence of this on the mechanical pr operties is remarkable while the same impact is not observed on the poro sity. The degree and the order of hydration and carbonation reactions ar e strongly influenced by the moisture content. Hydration reactions are p romoted under moist conditions while carbonation reaction is retarded. H ydraulic lime and lime-pozzolana mortars require at least 28-day moist c uring to improve the hydration reactions and to ensure sufficient streng th development. In cement-lime mortars, an interaction has been found between the unhydr ated or hydrated calcium aluminates and calcium carbonate originating fr om the addition of lime. Complementary effect of the carbonation reactio n of free lime on the strength development of these mortars is more pron ounced than the lime mortars. Competitive reaction order between cement hydration and carbonation of lime does not occur under atmospheric condi tions because the fast cement hydration is always the first-reaction ord er building up a sound hydrated microstructure. It is the CO2-rich condi tions under which carbonation prevails without negatively affecting the mechanical properties of cement-lime mortars despite decalcification of the hydrated phases.status: Publishe
The kinetic effect of carbonic anhydrase on CaCO3 phase transformations during carbonation of Ca(OH)2 solutions
No full textstatus: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
In situ monitoring of mineral waste carbonation under high CO₂ pressure
No full textThe treatment of fine grained steel slags with CO2 at elevated pressure and temperature conditions is a process that allows to produce high quality building materials. In this process the mineral waste is stabilized and CO2 is permanently sequestered in the form of carbonates. However, in order to enhance the sequestration potential and further improve the quality of the building materials, it is essential to understand the carbonate formation and the impact of this process on the pore network in the building material. A better understanding of the process will translate in improved building materials and enhanced CO2 sequestration. In this study, the process of carbonate formation stainless steel slags is investigated. The carbonate formation in the pore network of the fine grained steel slags is monitored during CO2 exposure in a specially designed mini reactor using high resolution X-ray tomography (HRXCT). The use of this non-destructive technique allows to visualize and quantify the amount carbonate that is precipitated inside the building material. The technique also provides a 3D representation of the pore network from which the porosity and permeability reduction through time is deducted. This new approach using HRXCT provides new insights in the carbonation process and provides better input parameters for modelling of carbonation models
Variations on the Author
Get PDF“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
Assessing the Autogenous Shrinkage of Alkali-Activated Slag/Fly Ash Mortar Blends
No full textThe purpose of this study is to investigate the autogenous shrinkage of alkali-activated slag/fly ash (AASF) mortar blends. A series of tests was performed to determine the effect of type and dosage of activators on autogenous shrinkage deformation. Heat progression in AASF systems was characterized by means of isothermal calorimetry. The reaction products of alkali-activated slag/fly ash (Class F) blends was characterized using X-ray diffraction (XRD). From those results, two main phases (C-A-S-H and N-A-S-H gels) are detected in slag/fly ash blended systems and with increase in fly ash content, the amount of the C-A-S-H gel decreases and the amount of N-A-S-H gel increases. Test results show that the slag/fly ash mass ratio, type and dosage of activator are the significant factors influencing the autogenous shrinkage and rate of reaction in AASF system. With increasing fly ash content and decrease in activator-to-cementitious materials ratio, the autogenous shrinkage (up to 7 days) of the AASF system decreases.status: Publishe
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Effect of grinding on the quantification of the bound water in tricalcium silicate
No full textThe effect of grinding of hydrated tricalcium silicate (C3S) pastes on the bound water content measured by TGA is investigated. The samples were subjected to different curing regimes at 20°C and 40°C at a constant water to cement ratio of 0.5. Hydration reactions were stopped using a freeze drier. To study the effect of grinding, samples were either roughly crushed or finely ground. Although the amount of water measured from the dehydroxylation of portlandite is not affected with grinding, the amount of total bound water increases with grinding. This increase in measured bound water is not affected by the studied curing regimes and degrees of hydration of C3S. Fine grinding before hydration stoppage using freeze dryer measured similar bound water content compared to crushed samples. It is proposed that the increase in measured bound water content after grinding of the sample is due to the confinement of physically bound water in the interlayers of C-S-H.status: Published onlin
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