Chemical durability of sodium disilicate glasses in different corrosion media at neutral to basic pH values

The chemical corrosion of sodium disilicate glasses in neutral to basic media was investigated. H2O, 1 mol/l NaOH, 1 mol/l LiOH, and a dilution series of sodium silicate solutions with a molar SiO2:Na2O ratio of 2.18 were used as corrosion media in static corrosion tests at temperatures of 30 °C and corrosion times up to 180 d. The corrosion process was characterized gravimetrically and in some cases by chemical analysis. Additionally, the temperature dependence of the corrosion in H2O at temperatures up to 70 °C was investigated. The nature of the corrosion process depends on the character of the corrosion media: The corrosion in 1 mol/l NaOH and 1 mol/l LiOH follows predominantly a square root time law, whereas corrosion in H2O or in sodium silicate solutions can be described by a linear time law. An additional result is that the corrosion rates increase in all cases with pH: in the case descnbed here corrosion in H2O and in alkaline silicate solutions proceeds faster than corrosion in sodium or lithium hydroxide solutions with higher pH values. The dilution series of sodium silicate solutions was used to check the dependence of the corrosion process on the concentration of glass dissolution products in the leachate. The results indicate the importance of surface adsorption processes

MOOC y el aprendizaje de una lengua extranjera: ¿Back to the future?

Cada vez se ofertan más MOOC, y poco a poco también en el campo de las lenguas extranjeras. Debido a las características de los MOOC como una forma de enseñanza-aprendizaje que se dirige a un público potencialmente ilimitado, su metodología está sujeta a dichas particularidades, que necesariamente influyen en la consecución de las competencias en las destrezas comunicativas como la expresión oral y escrita y la comprensión oral y lectora. Nuestra contribución versa sobre estos límites de los MOOC y plantea la duda de hasta qué punto la metodología relacionada constituye un paso atrás hacia enfoques fundamentalmente formales ya superados en el campo de las lenguas extranjeras

Hydrous sodium silicate glasses obtained by drying sodium silicate solutions

Concentrated sodium silicate solutions with a SiO2:Na2O molar ratio of 3.3 and a SiO2 content of 27 wt% were dried at temperatures between 30 and 100 °C to residual water contents between 11 and 45 wt%. The materials are brittle, transparent and X-ray amorphous. Their structure is discussed, with the conclusion that it is vitreous. The materials were investigated by thermal analysis techniques (e.g. differential scanning calorimetry in open and closed crucibles, thermogravimetry, hot stage microscopy), optical and electron microscopy, and X-ray diffraction. Up to six thermal events detected by thermal analysis are discussed. Two of these thermal events were analysed with respect to their heat rate dependence and are discussed with respect to structural transitions (glass to gel and gel to sol). Other events are also discussed, such as the temperatures of foam formation during heating and the boiling temperatures of the investigated samples

Wetting angle: New parameter indicating the reactivity of alkaline solutions and geopolymer binders

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