Geopolímeros de tipo binario basados en una puzolana natural y escoria siderúrgica de alto horno

This study describes the synthesis at ambient temperature (25±3 °C) of binary geopolymer systems based on natural volcanic pozzolan and granulated blast furnace slag. Na2SiO3 and NaOH were used as alkaline activators. The effects of the SiO2/Al2O3, Na2O/Al2O3 ratio and the amount of slag added (from 0 to 30%) on the reaction kinetics, compressive strength and microstructure of the final product were studied. To characterise the geopolymer pastes, techniques such as X-ray diffraction (XRD), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used. The results indicate the possibility of obtaining a geopolymer cement with a compressive strength of up to 48.11 MPa after 28 days of curing at ambient temperature whose characteristics are comparable to those of commercial portland cement.Este trabajo describe la síntesis a temperatura ambiente (25±3 °C) de sistemas geopoliméricos de tipo binario basados en una puzolana natural de origen volcánico y escoria siderúrgica de alto horno usando activadores alcalinos basados en la combinación de Na2SiO3 y NaOH. Se estudió el efecto de la relación SiO2/Al2O3, Na2O/Al2O3 y la cantidad de escoria adicionada en niveles entre el 0 y 30% sobre la cinética de reacción, la resistencia a la compresión y la microestructura del producto final. Para la caracterización de las pastas geopoliméricas se utilizaron técnicas como difracción de rayos X (DRX), espectroscopia infrarroja (FTIR) y microscopia electrónica de barrido (MEB). Los resultados conseguidos revelan la posibilidad de obtener un cementante geopolimérico con una resistencia a la compresión de hasta 48,11 MPa a los 28 días de curado a temperatura ambiente cuyas características son comparables a las de un cemento portland comercial

Nuevo material compuesto de matriz geopolimérica activado con ceniza de cascarilla de arroz y KOH: Desempeño a alta temperatura

Geopolymers were produced using an environmentally friendly alkali activator (based on Rice Husk Ash and potassium hydroxide). Aluminosilicates particles, carbon and ceramic fibres were used as reinforcement materials. The effects of reinforcement materials on the flexural strength, linear-shrinkage, thermophysical properties and microstructure of the geopolymers at room and high temperature (1200 ÅãC) were studied. The results indicated that the toughness of the composites is increased 110.4% for geopolymer reinforced by ceramic fibres (G-AF) at room temperature. The presence of particles improved the flexural behaviour 265% for geopolymer reinforced by carbon fibres and particles after exposure to 1200 .C. Linear-shrinkage for geopolymer reinforced by ceramic fibres and particles and the geopolymer G-AF compared with reference sample (without fibres and particles) is improved by 27.88% and 7.88% respectively at 900 ÅãC. The geopolymer materials developed in this work are porous materials with low thermal conductivity and good mechanical properties with potential thermal insulation applications for building applications.Compuestos geopoliméricos fueron producidos usando un activador alcalino alternativo (basado en ceniza de cascarilla de arroz e hidróxido de potasio), partículas aluminosilicatos, fibras de carbono y cerámicas. Se estudió el efecto de fibras y partículas en la resistencia a la flexión, contracción lineal, propiedades termofísicas y microestructura de los geopolímeros a temperatura ambiente y 1200 °C. Los resultados indican que la tenacidad se incrementó 110.4% para el geopolímero reforzado con fibras cerámicas (G-AF) a temperatura ambiente. La presencia de partículas mejora el comportamiento a la flexión 265% para el geopolímero reforzado con fibras de carbono y partículas después de la exposición a 1200 °C. La contracción lineal para el geopolímero reforzado con fibras cerámicas y partículas y el geopolímero G-AF es mejorada 27.88% y 7.88% respectivamente a 900 ºC con respecto al material sin refuerzo. Los materiales geopoliméricos desarrollados en este estudio son materiales porosos de baja conductividad térmica y buenas propiedades mecánicas con potencial aplicación en la industria de la construcción como aislantes térmicos

Durabilidad del hormigón armado expuesto a condiciones agresivas

Reinforced concrete is the most widely used construction material in the world. The combination of high compressive strength, afforded by concrete and, and the excellent mechanical properties that characterise steel make it an ideal composite for all manner of structures. One of the main weaknesses of this material, however, is that when the concrete is exposed to aggressive environments, in particular high concentrations of chloride ions or carbon dioxide, reinforcing steel corrodes, shortening service life. This article reviews the state of the art on concrete durability, along with the conditions that affect its useful life by inducing reinforcing steel corrosion. It also discusses the prevention and control methods, in particular electrochemical prevention and rehabilitation techniques, that have been developed to mitigate the problem.El hormigón armado se ha convertido en el material más utilizado a nivel mundial en el sector de la construcción. La combinación de las altas prestaciones a resistencia a la compresión que ofrece el hormigón y las excelentes propiedades mecánicas del acero lo hacen el material compuesto ideal para aplicaciones estructurales. Sin embargo, uno de los mayores problemas que afecta la vida útil del hormigón es la corrosión del acero de refuerzo, la cual se presenta cuando el hormigón se encuentra expuesto a ambientes agresivos, en especial frente a la exposición a iones cloruros y/o dióxido de carbono. Este trabajo revisa el estado del conocimiento con respecto a la durabilidad del hormigón enfatizando en los fenómenos que afectan su durabilidad y generan la corrosión del acero de refuerzo, adicionalmente se incluyen los métodos de prevención y control que se han desarrollado para mitigar el problema, en particular los relacionados con las técnicas electroquímicas de prevención y rehabilitación

Teacher Development for Bilingual Education in Colombia: Towards Teacher Empowerment

This article examines aspects of teacher education in Colombia with regard to demands posed by bilingual education and foreign language programmes. There is an initial overview of developments in the Americas, followed by a discussion of research conducted in Colombia in relation to the teaching of language and content area knowledge in a bilingual education context. The results of a recent study carried out on teacher empowerment are presented and there is a fi nal discussion of how far teacher education programmes enable practitioners to cope with tensions in their classroom practice, both in bilingual education and in foreign language settings.Este artículo examina aspectos del desarrollo profesional en Colombia, con referencia a las exigencias de los programas de educación bilingüe y lenguas extranjeras. Inicialmente se da una visión general de desarrollos en las Américas, seguida de una discusión de investigaciones llevadas a cabo en Colombia sobre la enseñanza de lengua y contenidos en un contexto bilingüe. Se presentan los resultados de un estudio reciente sobre empoderamiento docente y una discusión fi nal sobre el alcance de los programas de desarrollo profesional que ayuda a los profesores a lidiar con las tensiones diarias en su práctica docente, tanto en contextos de educación bilingüe como en contextos de enseñanza de lenguas extranjeras

Galaxy properties from J-PAS narrow-band photometry

We study the consistency of the physical properties of galaxies retrieved from SED-fitting as a function of spectral resolution and signal-to-noise ratio (SNR). Using a selection of physically motivated star formation histories, we set up a control sample of mock galaxy spectra representing observations of the local universe in high-resolution spectroscopy, and in 56 narrow-band and 5 broad-band photometry. We fit the SEDs at these spectral resolutions and compute their corresponding the stellar mass, the mass- and luminosity-weighted age and metallicity, and the dust extinction. We study the biases, correlations, and degeneracies affecting the retrieved parameters and explore the r\^ole of the spectral resolution and the SNR in regulating these degeneracies. We find that narrow-band photometry and spectroscopy yield similar trends in the physical properties derived, the former being considerably more precise. Using a galaxy sample from the SDSS, we compare more realistically the results obtained from high-resolution and narrow-band SEDs (synthesized from the same SDSS spectra) following the same spectral fitting procedures. We use results from the literature as a benchmark to our spectroscopic estimates and show that the prior PDFs, commonly adopted in parametric methods, may introduce biases not accounted for in a Bayesian framework. We conclude that narrow-band photometry yields the same trend in the age-metallicity relation in the literature, provided it is affected by the same biases as spectroscopy; albeit the precision achieved with the latter is generally twice as large as with the narrow-band, at SNR values typical of the different kinds of data.Comment: 26 pages, 15 figures. Accepted for publication in MNRA

Fluctuations in Nonequilibrium Statistical Mechanics: Models, Mathematical Theory, Physical Mechanisms

The fluctuations in nonequilibrium systems are under intense theoretical and experimental investigation. Topical ``fluctuation relations'' describe symmetries of the statistical properties of certain observables, in a variety of models and phenomena. They have been derived in deterministic and, later, in stochastic frameworks. Other results first obtained for stochastic processes, and later considered in deterministic dynamics, describe the temporal evolution of fluctuations. The field has grown beyond expectation: research works and different perspectives are proposed at an ever faster pace. Indeed, understanding fluctuations is important for the emerging theory of nonequilibrium phenomena, as well as for applications, such as those of nanotechnological and biophysical interest. However, the links among the different approaches and the limitations of these approaches are not fully understood. We focus on these issues, providing: a) analysis of the theoretical models; b) discussion of the rigorous mathematical results; c) identification of the physical mechanisms underlying the validity of the theoretical predictions, for a wide range of phenomena.Comment: 44 pages, 2 figures. To appear in Nonlinearity (2007

Confinement Effects in Antiferromagnets

Phase equilibrium in confined Ising antiferromagnets was studied as a function of the coupling (v) and a magnetic field (h) at the surfaces, in the presence of an external field H. The ground state properties were calculated exactly for symmetric boundary conditions and nearest-neighbor interactions, and a full zero-temperature phase diagram in the plane v-h was obtained for films with symmetry-preserving surface orientations. The ground-state analysis was extended to the H-T plane using a cluster-variation free energy. The study of the finite-T properties (as a function of v and h) reveals the close interdependence between the surface and finite-size effects and, together with the ground-state phase diagram, provides an integral picture of the confinement in anisotropic antiferromagnets with surfaces that preserve the symmetry of the order parameter.Comment: 10 pages, 8 figures, Accepted in Phys. Rev.