Alcali

Résumé

Un alcali, écrit alkali à la fin du pour marquer l'origine arabe via le latin médiéval, est un terme de l'alchimie puis de la chimie décrivant différents composés chimiques, parfois en mélange, à propriétés dites alcalines ou basiques. Depuis le , le terme est employé de manière générique pour désigner des bases, des sels ou des solutions basiques concentrées. En chimie industrielle moderne, un alcali désigne une base forte dans l'eau. L'adjectif associé alcalin, qui indiquait une saveur amère ou agressive typique, la présence d'une base forte ou une fonction basique réduisant l'acidité d'un milieu, a fini par désigner les ions ou atomes de lithium, sodium ou potassium, plus rarement rubidium, césium ou francium de la première colonne du tableau périodique. Ces métaux alcalins ont la propriété de former avec l'oxygène ou l'eau des bases fortes, nommées « alcalis ». Ainsi une pile alcaline, faisant mouvoir des ions Li+ ou Na+. D'un point de vue pratique et commercial, le terme alca

Source officielle

https://fr.wikipedia.org/wiki/Alcali

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To reduce the CO2 footprint of construction materials, concrete producers blend their cement with Supplementary Cementitious Materials (SCMs). SCMs such as fly ash or blast furnace slag are mostly the byproducts of other industries. And while SCMs are chosen to match the properties of the common Ordinary Portland cement (OPC), their addition to the cement recipe may alter the chemistry of the system. These changes may lead to different mechanical and transport properties of the cement-based structure which may, in turn, affect its long-term resistance to harmful external agents. Therefore, understanding the relationship between cementâs microstructure and the degradation mechanisms is key for optimizing the design of new cementitious materialsIn this context, chloride attack is the most common reason for steel rebars to corrode especially when exposed to external chloride (seawater, deicer saltsâŠ). At low w/c ratios (typically 13) pore solution, the C-S-H surface develops a negative surface charge density. Electrostatic interactions between the surface and the ions in the pore solution result in a redistribution of the ionic species in two layers of charge at the interface of the solid i.e. the Electrical double layer (EDL). In nanoscopic pores, the EDL is dominated by atomic phenomena which are thought to interfere with the mobility of ions and chloride in particular. In order to understand and quantify the surface effects and their influence on ionic transport, we firstly propose an atomistic model of the EDL formation based on the use of the Metropolis Monte Carlo algorithm. This model is used to compute the ionic distributions and electrochemical potentials of electrolytes at equilibrium in nanoscopic pores. These quantities constitute the main driving forces of ionic transport at the pore scale. The microstructure parameters including the surface charge density of C-S-H, the ionic strength (and pH) of the pore solution, and the pore size are equally investigated and their effect on chlorideâs behavior quantified. Among the other parameters, the model also provides quantitative information on the effect of calcium ions which are thought to play a major role in the binding of chloride on C-S-H. The next step consists in using the calculated atomic-scale properties of the EDL in order to resolve the transport problem at the pore scale and compute microscopic diffusivities of chloride. This is achieved by using the molecular computations from the Monte Carlo (MC) engine in order to implement a modified version of the classical Poisson-Boltzmann system. The method is compared to the classical Finite element analysis of the Poisson-Nernst-Planck (PNP) equations and the data are discussed in the light of established experimental results in the literature.

EPFL2022

Chargement

Effect of alkali, aluminium and equilibration time on calcium-silicate-hydrates

Yu Yan

Calcium silicate hydrate (C-S-H) is the main hydration product in Portland and blended cements, and greatly affects durability and mechanical properties of the hydrated cement. In the presence of Al-rich supplementary cementitious materials (SCMs), C-(A-)S-H can contain more Al than C-S-H in plain Portland cements. The use of SCMs in cementitious system not only lowers CO2 emission, but may also enhance the mechanical properties in the long term, its durability and can help to suppress alkali silica reaction. However, important questions regarding the effect of Al, alkali and equilibration time on C-A-S-H structure and solubility are still not fully understood. This thesis focuses on the interplay between aluminium, alkali hydroxides in solution, the structure of C-A-S-H and aluminium, alkali uptake by C-A-S-H and kinetics of C-A-S-H formation. For aluminium free C-S-H, KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.For C-A-S-H with Ca/Si=1, the presence of Al led to higher amounts of secondary phases, larger interlayer distance, longer silicate chain, higher dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases is reduced at higher pH; strätlingite and Al(OH)3 dominate at lower pH, and katoite at higher pH values. High pH is also associated with shortened silicate chain length, increased Al and Si concentrations, maximum Al/SiC-S-H and lower Ca concentrations. The Al uptake in C-A-S-H increased with aluminum concentration in solution, indicating the predominance of a common Al sorption mechanism independent of dissolved Al concentration or pH values. XANES data showed the presence of both Al(IV) and Al(VI) with different chemical environments in C-S-H.The effect of equilibration time on C-A-S-H was investigated from 6 hours to 3 years. Rapid changes within the first day were observed in both the solid phase and the aqueous phase. Portlandite was formed in all samples with different Ca/Si and its amount decreased rapidly within the first day and remained stable after more than 7 days. Gismondine-P1 precipitated between 1 and 3 years and destabilized the C-A-S-H phases at target Ca/Si 0.6. In the solid phase, the Al migration from secondary phases to C-A-S-H and an increasing fraction of Al(V) and Al(VI) in C-S-H were observed with longer equilibration time. The Ca/Si ratio of C-S-H played an important role on the extent of changes in aqueous concentration: an increasing of Al concentrations with time was observed in C-A-S-H with Ca/Sitarget=0.6, while a decrease was observed for C-A-S-H with Ca/Sitarget=1.0. The uptake of Al and Na in C-A-S-H was observed to decrease after 4 days and 1 day separately, which indicates a rearrangement of C-A-S-H with time.

EPFL2022

Chargement

When using natural or waste wood in thermo-chemical conversion processes, the presence of a number of heteroatoms (e.g. sulfur, chlorine, potassium and sodium or heavy metals) may hinder the processes themselves as well as pose a threat to equipment and/or the environment. Due to the often strongly heterogeneous composition of the feedstocks and high dynamics of thermo-chemical reactions, the elemental concentrations in process gases show a high temporal variability. In order to accurately characterize such processes efficient and representative sampling methods and analytical instruments with a high sensitivity and time resolution are necessary. This thesis presents the development and application of devices to analyze trace element contents of product gases from thermo- chemical conversion processes and to characterize the thermal behavior of feedstocks such as wood pellets. In the first part the development of a mobile sampling and measurement system for the analysis of gaseous and liquid samples in the field is presented. An inductively coupled plasma optical emission spectrometer (ICP-OES) which was built into a van, was used as detector. The analytical system was calibrated with liquid and/or gaseous standards. It was shown that identical mass flows of either gaseous or liquid standards resulted in identical ICP-OES signal intensities. In a field measurement campaign trace and minor elements in the raw flue gas of a waste wood combustor were monitored. Sampling was performed with a highly transport efficient liquid quench system which made possible the observation of temporal variations in the elemental process gas composition. After a change in feedstock an immediate change of the element concentrations in the flue gas was detected. A comparison of the average element concentrations during the combustion of the two feedstocks showed a high reproducibility for matrix elements that are expected to be present in similar concentrations. Elements showing strong differences in their concentration in the feedstock were also represented by a higher concentration in the flue gas. Following the temporal variations of different elements revealed strong correlations between a number of elements, such as chlorine with sodium, potassium and zinc, as well as arsenic with lead, and calcium with strontium. Most notably, an online detector for alkalis (SID II) based on the principle of surface ionization has been designed and constructed within the framework this thesis. The detector includes a number of improvements compared to previous designs. Due to a fixed filament geometry, the sensitivity of the SID II is highly reproducible between different filaments. Improvements to the flow geometry of the device, and the installation of an auxiliary heateing element have minimized the overall tar depositions. In contrast to previous designs, the electrical feedthroughs are kept outside the sample gas stream and are therefore protected against tar contamination which can lead to signal artifacts. In numerous measurements at thermal processes up to industrial scale the SID II has demonstrated a high sensitivity and improved characteristics to conduct measurements in tar and particle laden process gases. By using an ultrasonic nebulizer as a source for alkali aerosols it was possible to perform a calibration of the alkali detector over 4 orders of magnitude. In combination with a dilution setup and a sampling lance, we could take online measurements of gasifier product gas with a high degree of reproducibility and a high time resolution of 1 s. In an experimental series with a miniature thermo-chemical reactor the release of alkalis and formation of product gases during pyrolysis and combustion of single commercial-sized wood pellets could both be recorded with a time resolution of approximately 1 s for with the SID II and the a mass spectrometer, respectively. The thermal processing was conducted by using compressed air at temperatures between 400 °C and 900 °C. During the experiments at temperatures of 700 °C and below the majority of the alkali are released after the initial pyrolysis stage. In contrast to that, in the experiments at 800 °C and 900 °C most of the alkali release happened during the pyrolysis stage (during the first 200 s after the insertion of the pellet). The alkali release rates collected at 800°C during the char combustion phase were reproduced using an extended version of the random pore model, resulting in similar numeric values of the fitted model parameters as reported by Struis et al. (2002) for thermo-chemical conversion experiments with pulverized wood char and carbon dioxide at 800°C well. During the late stage of combustion the alkali release rates exhibit the same phenomena as previously determined for char pore surface area. These phenomena, related to pore coalescence and char structure disintegration, gave rise to a number of varied attempts to modify the original random pore model derived by Bhatia and Perlmutter.

EPFL2012

Chargement

EPFL2022

Effect of alkali, aluminium and equilibration time on calcium-silicate-hydrates

Yu Yan

EPFL2022

EPFL2012