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Innovation in Cements for Sustainability
No full textThis eBook is a collection of articles from a Frontiers Research Topic. Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: frontiersin.org/about/contac
Binder chemistry – Low-calcium alkali-activated materials
Full text linkEarly developments in the developments of low-calcium (including calcium-free) alkali-activated binders were led by the work of Davidovits in France, as noted in Chap. 2. These materials were initially envisaged as a fire-resistant replacement for organic polymeric materials, with identification of potential applications as a possible binder for concrete production following relatively soon afterwards [1]. However, developments in the area of concrete production soon led back to more calcium-rich systems, including the hybrid Pyrament binders, leaving work based on the use of low-calcium systems predominantly aimed at high-temperature applications and other scenarios where the ceramic-like nature of clay-derived alkali-activated pastes was beneficial. Early work in this area was conducted with an almost solely commercial focus, meaning that little scientific information was made available with the exception of a conference proceedings volume [2], several scattered publications in other conferences, and an initial journal publication [3]. Academic research into the alkaline activation of metakaolin to form a binder material led to initial publications in the early 1990s [4, 5], and the first description of the formation of a strong and durable binder by alkaline activation of fly ash was published by Wastiels et al. [6-8]. With ongoing developments in fly ash activation, which offers more favourable rheology than is observed in clay-based binders, interest in low-calcium AAM concrete production was reignited, and work since that time in industry and academia has led to the development of a number of different approaches to this problem. A review of the binder chemistry of low-calcium AAM binder systems published in 2007 [9] has since received more than 350 citations in the scientific literature, indicating the high current level of interest in understanding and utilisation of these types of gels
Atomistic simulations of geopolymer models: the impact of disorder on structure and mechanics
Full text linkGeopolymers are hydrated aluminosilicates with excellent binding properties. Geopolymers appeal to the construction sector as a more sustainable alternative to traditional cements, but their exploitation is limited by a poor understanding of the linkage between chemical composition and macroscopic properties. Molecular simulations can help clarify this linkage, but existing models based on amorphous or crystalline aluminosilicate structures provide only a partial explanation of experimental data on the nanoscale. This paper presents a new model for the molecular structure of geopolymers, in particular for nanoscale interfacial zones between crystalline and amorphous nanodomains, which are crucial for the overall mechanical properties of the material. For a range of Si–Al molar ratios and water contents, the proposed structures are analyzed in terms of skeletal density, ring structure, pore structure, bond-angle distribution, bond length distribution, X-ray diffraction, X-ray pair distribution function, elastic moduli, and large-strain mechanics. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with an impact on mechanical properties. This offers a new starting point for the multiscale modeling of geopolymers
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
Cinética da hidratação do aluminato tricálcico cúbico e ortorrômbico na primeira hora : estudo com fases cristalinas sintéticas e extraídas do clínquer Portland
Full text linkDurante o processo de produção do clínquer Portland ocorrem fusões parciais da matéria prima e nódulos de clínquer de cimento Portland são produzidos. As características físico-químicas destes nódulos influenciam diretamente a cinética de hidratação do cimento, sendo a fase cristalina do aluminato tricálcico (C3A) uma das principais responsáveis pelas reações iniciais de hidratação. O C3A puro tem sua estrutura cristalina cúbica. A presença de elementos menores, principalmente o Na+, K+, Mg2+, Fe3+ e Si4+, derivado das matérias primas e/ou combustíveis, durante a clinquerização podem modificar a estrutura cristalina do C3A, em substituição aos íons Ca2+. Quando há incorporação destes íons em teores suficientes na estrutura cristalina do C3A, esta pode ser modificada de cúbica para ortorrômbica, sendo os compostos Na-C3A e K-C3A as principais e mais comumente fases polimórficas encontradas nas composições de clínqueres comerciais. A reação do C3A com a água é quase instantânea, com grande liberação de calor, caracterizando um processo exotérmico. O processo de liberação de calor é controlado com adição de uma fonte de sulfato de cálcio ao material, tendo como produto formado pela hidratação na reação com C3A, as fases trissulfoaluminato de cálcio hidratado (AFt ou etringita) e monossulfoaluminato de cálcio (AFm). Estudos apresentam resultados contraditórios quanto à influência dos tipos de estrutura cristalina do C3A durante o processo de hidratação, mediante o sistema C3A-Ca∙SO4-H2O. A influência de cada uma dessas estruturas é preferencialmente estudada de forma isolada, sendo estas produzidas sinteticamente, o que gera questionamentos quanto à validade destes resultados quando comparados à hidratação do cimento Portland, visto que as fases sintéticas produzidas em laboratório, diferentemente das encontrados no cimento, são puras. Apesar de todo conhecimento acumulado, ainda existem lacunas importantes a serem preenchidas quanto à hidratação das fases C3A na ausência ou na presença de sulfato de cálcio. Esta pesquisa visa utilizar fases de C3A sintéticas produzidas em laboratório e extraídas de clínqueres Portland (com diferentes teores de C3A cúbico e ortorrômbico) através da técnica de dissolução seletiva (DS), no estudo da hidratação na ausência e na presença de gipsita (CaSO4∙2H2O) e anidrita (CaSO4). Foram testados dois métodos de DS, sendo um deles adaptado após os resultados preliminares. A validação dos resultados exigiu uma gama de técnicas as quais, em associação, permitiram o avanço na pesquisa, tendo sido utilizada a Difração de Raios X (DRX), Fluorescência de Raios X (FRX) e espectroscopia RAMAN para tal objetivo. Os resultados indicaram a completa extração das fases aluminatos das amostras de clínquer Portland analisadas mediante modificação do processo com solução de ácido maleico/metanol (AMM). Os estudos de hidratação comprovaram uma maior reatividade do C3A cúbico na ausência de uma fonte de sulfato de cálcio, e na presença, a maior reatividade para C3A ortorrômbico até a primeira hora de hidratação, sendo que para as amostras sintéticas a presença da anidrita teve maior influência na reação de hidratação. Os resultados evidenciaram a eficiência das análises com uso de fases sintéticas, no entanto os dados experimentais comprovaram que as fases C3A extraídas apresentam maior reatividade, quando comparadas a fases sintéticas em condições similares. Entretanto, algumas fases do clínquer que não foram possíveis de ser extraídas, como periclásio e ferroaluminato, podem ter influenciado os resultados, o que indica o necessário aprimoramento da técnica de DS ou utilização de clínquer branco para esse método modificado.During clinker production partial fusions of the raw materials occurs and nodules of clinker are produced. The physicochemical characteristics of these nodules can directly influence the cement hydration kinetics, and the crystalline phase of tricalcium aluminate (C3A) is the main responsible for the initial hydration reactions. The pure C3A has cubic crystal structure. The presence of minor elements, mainly Na+, K+, Mg2+, Fe3+ and Si4+, derived from raw materials and/or fuels during clinkerization can modify the crystalline structure of C3A. When sufficient amount of these ions is incorporated into the C3A crystalline structure, it can be modified from cubic to orthorhombic. Na-C3A and K-C3A are the main polymorphs compounds and the most commonly found in the commercial clinker composition. The C3A reaction with water is almost instantaneous with large heat release featuring an exothermic process. The heat release is controlled adding a calcium sulphate source, the main hydrate products of the reaction are ettringite (AFt) and calcium monosulfate (AFm). Previous research shows conflicting results regarding the influence of the C3A crystalline structure types during the hydration in the C3A-Ca∙SO4-H2O system. The influence of each of these structures are rather studied using pure phases produced in laboratory, which leads to questions about the validity of these results against the real Portland cement phases hydration due to the high purity of these materials produced in laboratory. Despite all accumulated knowledge, there are still significant gaps to be filled about the hydration of C3A phases in the absence or presence of calcium sulfate sources. This research aims to study the hydration of C3A phases produced in laboratory and extracted from Portland clinkers using selective dissolution technique, in the absence and presence of gypsum (CaSO4 2H2O) and anhydrite (CaSO4). To elucidate the reactivity between the cubic and orthorhombic C3A phases, investigations will be carried out in clinkers with different amounts of cubic and orthorhombic C3A in their compositions. Two selective dissolution methods were tested and after previous results one of them was adapted. X ray diffraction (XRD), X ray fluorescence (XRF), isothermal calorimetry among other techniques were used. The results indicated the complete extraction of the aluminate phases of the Portland clinker samples. The hydration studies indicated higher reactivity for cubic C3A in the absence of calcium sulphate sources and for orthorhombic in presence of calcium sulphate sources in the first hour of hydration, in addition, for synthetic samples, the presence of anhydrite had a major influence on the hydration reaction. Results corroborate the efficiency in the use of synthetic phases for hydration tests, but the samples using the extracted aluminate phases showed higher reactivity. However, non-extracted phases such as periclase and C4AF may have influenced the results. Further studies are necessary to improve the DS method or the use of white Portland clinker in the dissolution
Immobilisation of simulant nuclear-grade spent ion-exchange resins in alkali-activated cement: Fresh state properties of the cement matrix
No full textThis study evaluates the fresh-state properties of alkali-activated cements (AACs) for the solidification of nuclear-grade spent ion-exchange resins (SIERs), a challenging waste stream in the nuclear industry. The formulations assessed use blast furnace slag and fly ash as precursors and Na2SiO3 and Na2CO3 as alkaline activators, for the immobilisation of SIERs. Setting time, workability, rheology, and reaction kinetics are assessed to optimise the cementitious matrix for effective waste encapsulation.
The incorporation of higher ash content slows reaction kinetics of the cement, delays setting, and reduces viscosity and yield stress, while slag accelerates polymerisation, increasing viscosity. Na2CO3 activation promotes early calcium carbonate precipitation, leading to faster setting and higher viscosity, whereas Na2SiO3 maintains fluidity over time. Resin incorporation is the dominant factor affecting fresh-state properties, significantly delaying reaction kinetics and reducing viscosity and yield stress, attributed to chemical interactions between Ca2+ from the pore solution and boron compounds released from the resin. Maintaining a setting time below 24 h, AACs with >85% slag allow more resin content than Portland cement. Slower kinetics may improve the stability of the resin encapsulation by allowing it to adapt to deformations.This work was carried out within the framework of the IRINEMA project supported by Ayudas de Atracción de Talento Investigador de la Comunidad de Madrid ( 2019-T1/AMB-13672 ).Peer reviewe
Pozzolanic potential of the calcined clay-lime system
No full textSeven reference clays, 4 kaolinitic and 3 smectitic clays, were purified and thermally treated in order to estimate their pozzolanic potential in cementitious materials. The clays were calcined in a fixed-bed electrical furnace at temperatures ranging between 500°C and 900°C. Both raw and calcined clays were characterized by XRD, XRF and BET techniques. Their pozzolanic activity was evaluated using thermogravimetry (TGA) on pastes after 3, 7, 14 and 28 and 56 days. The results indicate that all kaolinitic clays are highly active at a broad range of firing temperatures (500-900°C), with no clear differences in activity. The crystallinity of the raw clay only influences the short-term activity of the calcined clay-lime reaction. The smectitic clays possess a clear optimal firing temperature at 800°C. Ca-rich smectites are proven to be somewhat more reactive than Na-rich smectite and hectorite, however, even at 800°C, it’s activity is only mediocre compared to kaolinite.status: Publishe
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