Effects of organic additives on calcium hydroxide crystallisation during lime slaking

Organic compounds, often used in cement systems as admixtures, may affect the crystallisation and carbonation kinetics of Ca(OH)2, an important phase of hydrated cement. Here, we investigated changes in Ca(OH)2 morphology in the presence of 3 organic compounds, commonly encountered in cement and lime-based materials: sucrose, pectin and calcium lignosulfonate. The additives were introduced either before or after lime slaking to determine the influence of temperature. Ca(OH)2 crystals and supernatant solutions were characterised at time of slaking and after 6 months of ageing using scanning electron microscopy, X-ray diffraction and optical emission spectroscopy. Our results indicate that the morphology of Ca(OH)2 crystals is modified by the characteristics of the organic molecules which promote formation of Ca(OH)2 with habits that can result in faster carbonation, an effect that is detrimental to cement used in reinforced concrete. These effects are enhanced when the additives are introduced before slaking, likely as a result of thermal degradation

Bones activitats per a la transició entre primària i secundària

En aquest article proposem tres activitats que creiem que poden servir demodel de treball a l'aula per ajudar els alumnes en el seu aprenentatge de les matemàtiques durant la transició entre aquestes dues etapes de l'ensenyament obligatori. Aquestes activitats promouen la resolució de problemes, la cerca de patrons i regularitats i el treball sistemàtic com a valors fonamentals per a aprendre matemàtiques.In this article we suggest three activities that we believe could serve asamodel for mathematics classes for pupils making the transition between the two key stages of compulsory education. The activities promote problem solving, the recognition of patterns and order and systematicwork as the basis for learningmathematics

Bones activitats per a la transició entre primària i secundària

En aquest article proposem tres activitats que creiem que poden servir demodel de treball a l'aula per ajudar els alumnes en el seu aprenentatge de les matemàtiques durant la transició entre aquestes dues etapes de l'ensenyament obligatori. Aquestes activitats promouen la resolució de problemes, la cerca de patrons i regularitats i el treball sistemàtic com a valors fonamentals per a aprendre matemàtiques.In this article we suggest three activities that we believe could serve asamodel for mathematics classes for pupils making the transition between the two key stages of compulsory education. The activities promote problem solving, the recognition of patterns and order and systematicwork as the basis for learningmathematics

Towards a Better Understanding of Hot-Mixed Mortars for the Conservation of Historic Buildings: The Role of Water Temperature and Steam During Lime Slaking

According to various historic accounts and material evidence, the practice of producing lime mortars by mixing the quicklime with the sand (i.e. hot-mixing) before first slaking it with water was much more common in the past centuries than appreciated by most contemporary academics, conservation professionals and craftsmen. However, in the last 10 years, there has been resurgence in interest in hot-mixing. In such systems, the steam developed during the mixing is supposed to be crucial in determining the superior characteristics of the mortars, but in-depth investigations on the role of steam in hot-mixing are very few. This study reports the results of some experimental work investigating the effects of water temperature and steam used for lime slaking on the characteristics of lime and related mortars. In these tests, calcic quicklime was slaked in water at 20 and 75 °C, and with steam at 90 °C. Microstructure and mineralogical characteristics of the hydrates were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Mortars produced with these limes were tested for fresh (water retention and flowability) and hardened (compressive and flexural strength) properties. Carbonation was assessed using SEM, XRD and phenolphthalein tests. Results show that steam-slaked lime is characterised by portlandite crystals with smaller crystallite size and significantly different microstructure compared to that of water-slaked lime. Results also show that mortars made with steam-slaked lime have higher water retention and flowability than the mortars produced with water-slaked lime. Under conditions of comparatively low relative humidity (c 40–50%), carbonation is slower in the steam-slaked lime mortar due to the lower water content compared to water-slaked lime mortars. Overall, these results confirm anecdotal reports of better workability and water retention and suggest that this production technology, which is only rarely used nowadays, can produce mortars with improved characteristics, and provide a means by which to match the performance of some historic mortars, and create compatible materials for conservation and restoration work

Effect of Lignosulfonates on the Morphology of Calcium Hydroxide Crystals

Lignosulfonates is commonly used in concrete as superplasticisers. Its presence can affect crystallisation and carbonation kinetics of calcium hydroxide (CH), which is an important phase in hydrated cement since it plays a crucial role in the durability and performance of concrete. In this work, lime pastes were produced by slaking quicklime in distilled water. Ca lignosulfonate (CaLS) was added to them, either before and after the slaking process to investigate the effects of temperature on the CaLS-CH interaction. All samples were characterised at 0 and 6 months from slaking with the aim of investigating the effects of CaLS on CH crystallisation. Scanning electron microscopy and X-ray diffraction were used to investigate CH crystal size and morphology, whereas micro-chemical analyses contributed to the investigation of the chemistry of pore solutions. Results suggest that the addition of CaLS (both, during and after slaking) affects the CH crystals habit and size, which, in turn, can have an effect on the reactivity of CH and, consequently, on the properties of fresh and hardened concrete

Tailoring lime properties to the modern construction industry. The effects of organic additives and alternative slaking methods on portlandite mineralogy

In a world where sustainability is a cardinal concept in the global public discourse, lime is becoming an increasingly valuable commodity for its importance in the development of more sustainable binders for construction. Lime has been manufactured and used since antiquity for a variety of applications including as binder for mortars and plasters. In the construction industry, its use gradually phased out during the 20th Century in favour of Portland cement. Nonetheless, a consensus on the beneficial properties of lime in construction has been growing over the past decades among practitioners, industry, and academics. The research described in this thesis focuses on the modifications induced to portlandite crystals by alternative slaking methods i.e., steam slaking and those entailing the use of organic additives. The goal of the research is to develop lime-based products with improved performances for the modern construction industry, able to overcome lime’s intrinsic limitations, such as the slow setting process and the need for a lengthy maturation to achieve optimal performance. In this study the effects of steam and of a range of organic additives (mono-, oligo- and polysaccharides as well as lignosulfonates) on portlandite crystals and air lime-based mixtures were assessed. Steam-slaked and additive-modified hydrated limes were characterised using a series of analytical techniques to obtain information on microstructural development, evolution of crystal growth and chemistry of the pore solution. Steam-slaked and additive-modified lime mixtures were prepared to study the influence of the slaking process on fresh- and hardened-state properties, and on the carbonation reaction. The results of this work demonstrate that both steam-slaking and the use of selected organic additives can affect the nucleation and growth of portlandite crystals and modify their size and shape, with an overall reduction in size and crystallinity. Such modifications are reflected in the behaviour of lime mixtures in terms of water demand, water retention, mechanical properties, and carbonation progression. These findings contribute to the understanding of the interaction mechanisms of lime with water vapour and with organic additives and their effects on the nucleation and growth of portlandite crystals. Furthermore, these results can be exploited to tailor the properties of lime to specific applications in the modern construction industry

Una visión del uso inteligente de las herramientas tecnológicas en el aula de matemática

Este trabajo presenta una serie de reflexiones acerca del uso inteligente de los recursos tecnológicos en el aula de matemática. Se analizan ejemplos de actividades planteadas a estudiantes de la carrera del Profesorado en Matemática en asignaturas de Análisis Matemático. Éstas muestran que es necesario reformular el tipo de actividades que se proponen en el discurso matemático escolar para que la tecnología sea incorporada como instrumento en el aula.Este trabajo presenta una serie de reflexiones acerca del uso inteligente de los recursos tecnológicos en el aula de matemática. Se analizan ejemplos de actividades planteadas a estudiantes de la carrera del Profesorado en Matemática en asignaturas de Análisis Matemático. Éstas muestran que es necesario reformular el tipo de actividades que se proponen en el discurso matemático escolar para que la tecnología sea incorporada como instrumento en el aula

Los cambios de variables y sus dificultades en el cálculo integral

La finalidad del presente trabajo es el análisis de las dificultades que presentan los alumnos para la utilización de los cambios de variables adecuados en la resolución de uno de los problemas clave del cálculo infinitesimal: la integración. Se estudiaron las características de la presencia de los cambios de variable en el discurso matemático escolar y situaciones concretas en las que se recurre al procedimiento de cambiar la o las variables con el fin de hacer más sencillo el camino de resolución de problemas clásicos

Formación Docente en Robótica

Este trabajo presenta una propuesta didáctica basada en proyectos diseñada para que docentes de primaria y secundaria sin experiencia previa en robótica educativa, puedan aprender sobre componentes básicos de hardware y su relación con el software en un curso de dos meses de duración con 4 clases virtuales y 3 presenciales. El curso es parte de una Especialización en Programación de dos años de duración. Analizamos los principales aspectos de la propuesta y los resultados en términos de las producciones que lograron los docentes y sus valoraciones del curso'

European natural cements - their key technical properties under standardised conditions

The first comparative study on 7 commercially available Roman cements has been undertaken. In the absence of a European Standard, the testing procedures have followed a protocol proposed by the EU FP7 funded ROCARE project, which itself is based upon various ENs for cement and building limes. Evaluation has been made of mineralogy, particle size distribution, setting time, strength, water absorption, pore size distribution and mortar microstructure. Five of the cements required retardation and citric acid was used. The use of a pre-hydration technique was also investigated to extend the workable life of mortars to some 2 hours. The results confirm the view that the term Roman cement refers to a broad family of cements with a diverse range of properties which will need to be accounted for in future work to produce an all-encompassing Standard