Empreinte carbone | EPFL Graph Search

L'empreinte carbone ou le contenu carbone d'une activité humaine est une mesure des émissions de gaz à effet de serre d’origine anthropique, c'est-à-dire qui peuvent lui être imputées. Elle dépend des facteurs d'émission des intrants liés à cette activité et en particulier des facteurs d’émission associés aux sources d'énergies utilisées. Les facteurs d’émission associés aux sources d’énergie correspondent pour l'essentiel à des émissions de . Ils s’expriment en général en grammes d’équivalent par kilowatt-heure () ; ils peuvent également s’exprimer en grammes d’équivalent carbone par kilowatt-heure. La mesure réelle ou l'évaluation modélisée des facteurs d'émissions des différentes sources d'énergies permettent d’établir des bilans d'émissions et par la suite d'investir dans des équipements ou dans des procédés consommant moins de ressources de combustibles fossiles de manière à moins affecter le climat. Les contenus en par activité peuvent être regroupés par ensembles d’activit

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

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

Hydration and durability of fast hardening binders incorporating supplementary cementitious materials

Sarra El Housseini

The use of supplementary cementitious materials (SCMs) is widely considered the most promis-ing approach to reduce CO2 emissions relative to cement production. SCMs are not commonly present in binders used for special applications. On the other hand, due to their low reactivity at early age, cements incorporating SCMs such as Limestone calcined clay cement (LC3) exhibit low early age strength when compared to PC. This could restrict their field of applications and therefore solutions have to be found.This thesis focused on two main axes: The first one is to lower the CO2 footprint of ternary blends composed of Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate (Cs) using calcined clay as clinker substitute. The hydration of simple systems composed of PC with an increas-ing amount of CAC and Cs with and without calcined clay from 3 hours up to 90 days of hydration helped to understand and identify the contribution of each component. The results showed that in-creasing the CAC and Cs content in a PC system with and without calcined clay led to an increase of the ettringite content and a decrease of the portlandite content due to the portlandite consumption to form ettringite. The C3S hydration was not delayed in presence of CAC and Cs. However, the pres-ence of CAC, Cs and calcined clay led to slightly lower degree of hydration especially at later ages. The hydration and strength of a more realistic system "a fixing mortar formulation" with and without calcined clay was investigated. Overall, results showed that calcined clay can replace Portland cement by up to 20% with acceptable strength results. The second aspect on which this thesis focused was the influence of incorporating CAC and Cs as limestone substitute in LC3. The effect of the C/A molar ratio of the used CAC and its content were the main parameters investigated. The incorporation of 10% (CAC, Cs) improved the strength com-pared to LC3 at all ages. The massive ettringite formation during the first hours of hydration explained the early age strength improvement. Results showed that the C3S hydration was hindered when the C/A ratio of the CAC is equal to 1.7. The calculation of saturation indices indicated that a super satu-ration with respect to portlandite is likely needed to trigger the C3S hydration. The high late age strength in LC3-CAC blends was explained by a high volume occupied by hydrates in comparison with LC3 blends. In terms of durability, the resistance to chloride ion penetration was investigated through the mini-migration method. LC3-CAC blends showed a better chloride resistance than LC3, which is mainly explained by the porosity refinement compared to PC, and the decrease of the total porosity in comparison with LC3. On the other hand, the natural carbonation study showed that LC3-CAC blends have a higher carbonation rate when compared to PC. Contrary to PC, accelerated car-bonation of LC3 and LC3-CAC blends resulted in a decrease of the strength. However, the strength of carbonated LC3-CAC specimens is still at least similar to the non-carbonated PC specimen.

EPFL2022

Supercritical water gasification of effluents from composted human excreta and recycling of the aqueous phases for microalgae cultivation

Dorian Tosi Robinson

Effluents from composted human excreta were sampled and characterized. Results show a constancy in the characteristics and nutrient content of these effluents. Then, these effluents were treated with supercritical water gasification, producing hydrogen rich gas (67-69%). Carbon efficiencies of 41-43% were achieved. Microalgae Chlorella vulgaris (CV) and Phaeodactylum tricornutum (Ph.t) cultivation were studied in the gasification aqueous phase (AP). No growth was observed in the raw AP even with adjusted pH. CV could grow in the activated carbon treated AP with similar results for cell concentration and pigment content, but with higher biomass production than in its control medium. Ph.t was unable to grow even in the treated AP. These results show promising development in recuperating energy and nutrients from effluents from composted human excreta, combining energy production, microalgae valorization and contributing to close the nutrient loop.

2017

Hydration and durability of fast hardening binders incorporating supplementary cementitious materials

Sarra El Housseini

EPFL2022

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

EPFL2022