Carbonate de calcium | EPFL Graph Search

Le carbonate de calcium (CaCO) est composé d'ions carbonate (CO) et d'ions calcium (Ca). Cette substance blanche a une masse molaire de 100,1 g/mol. Le carbonate de calcium est le composant majeur des calcaires comme la craie, mais également du marbre. C'est aussi le constituant principal des coquilles d'animaux marins, du corail et des escargots, ainsi que des coquilles d'œufs des amniotes (à l'exception bien sûr des mammifères thériens dont les œufs, internes, sont sans coquille). :Ca + ⇌ . Le carbonate de calcium cristallise naturellement avec deux formes cristallines principales : l'aragonite et la calcite. Les deux autres formes cristallines sont plus rares : la vatérite et un hexahydrate, forme intermédiaire peu stable qui évolue vers la calcite. Il peut se présenter, dans les processus de cristallisation, sous forme de colloïde contenant de fins cristaux des formes plus stables. L'ion bicarbonate , également appelé hydrogénocarbonate, est à la base de la formation du carbon

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

Hydrological behaviour through experimental and modelling approaches

Daniela Balin

The present work is related to the recent research topics in hydrology devoted to the integration of field knowledge into the hydrological modelling. The study catchment is the Haute-Mentue experimental basin (12.5 km2) located in western Switzerland, in the Plateau region. In order to complete the existing knowledge about the hydrological behaviour of the study catchment, a field experimental approach has been conducted at two scales: catchment (environmental tracing) and local scale (TDR soil moisture measurements). The environmental tracing application has led to the same conclusion as previous researches: hydrological behaviour is strongly influenced by the catchment antecedent conditions and by the rainfall duration and intensity. The geology characteristics (moraine or molasse) explain the main differences in the hydrological behaviour that have been observed so far. As the environmental tracing does not allow easy identification of the mechanisms responsible for the runoff generation, TDR equipments have been installed on two hillslopes with different geological characteristics, which allowed monitoring of the soil moisture at different depths along the hillslope during two intensive campaigns in 2002 and 2003. Association of the environmental tracing and TDR technique has finally allowed precising the conceptual model of two head sub-catchments of the Haute-Mentue catchment. The second part of the research is devoted to the hydrological modelling. A simple conceptual model (TOPMODEL) has been considered as an appropriate representation of the hydrological processes on the Haute-Mentue catchment. In order to estimate TOPMODEL parameters and to take into account uncertainty associated with estimated parameters and model output, a Bayesian approach has been proposed and two Bayesian techniques have been compared: GLUE (Generalized Likelihood Uncertainty Estimation) and MCMC (Monte Carlo Markov Chains). The role of the statistical corrections on the resulting parameters and model output uncertainty has been assessed. In the last part of the present research, the Bayesian methodology has been extended to the case of multi-response calibration. Previous field acquired knowledge (i.e. soil storage saturation deficit, stream water silica and calcium concentrations) has been used to constrain parametrization of the classical and of a modified version of TOPMODEL. In both cases, multi-calibration led to trade-off behaviour of the efficiencies of the simulated responses. The total modelling uncertainty of the new introduced responses was considerably reduced at the expense of an increase in the total modelling uncertainty of the simulated discharges.

EPFL2004