Cement-based materials are more and more recognized as fractal materials. From a practical point of view, fractality means that the pore size distribution can be described using fractal scaling law ...(i.e. power functions). Here, the simplest scaling law making use of a single fractal dimension was used to generate simple and virtual fractal porous media (based on bundles of parallel cylindrical pores). The capillary curve and permeability (relative and intrinsic) of the virtual porous media were then estimated through theoretical and numerical approaches. The results show that: (1) although bereft of any physical basis, van Genuchten equation bears some fractal information; (2) although highly flexible, van Genuchten equation fails to perfectly fit the capillary curves and can thus generate error in inverse analysis (for intrinsic permeability evaluation) and (3) there seems to be a relation between the intrinsic permeability and van Genuchten pressure parameter.
In the framework of the radioactive waste management in France, interim storage concrete structures should be submitted to temperatures up to 80
°C and subsequent desiccation. The impact of ...temperature on the sorption properties of concretes has been poorly studied and results are scarce. An experimental campaign was thus carried out to characterize the first desorption isotherms of a modern concrete at 30
°C and 80
°C. The results show a significant influence of the temperature increase that will have to be accounted for the durability assessment of the long-term interim storage concrete structures. Investigating the causes of these modifications, it appeared that desorption induced by temperature might be the principal mechanism rather than microstructure alteration and water properties evolution.
A simple, analytical model was proposed to account for the effect of moderate temperature on water retention in cement-based materials. The model takes advantage of the simplicity and flexibility of ...van Genuchten equation. It was first calibrated using experimental results taken from literature and then successfully validated on other results. The presented model provides a valuable tool for the description of water transport in unsaturated cementitious materials submitted to moderate temperatures (up to 85°C).
Thermal desorption is a critical process in cement-based materials subjected to temperature increase. C-S-H surface is the most likely surface available for thermal desorption in these materials. ...Here, we investigate surface thermal desorption in C-S-H. Molecular simulations are used to get systems equilibrated under two drained poromechanical conditions: liquid water-saturated and constant partial fluid pressure conditions. Suited fluctuation formulas are deployed to acquire properties at the adsorbed layer level. We show that thermal desorption is driven by thermal expansion of water and liquid-to-vapor phase transition (leading to cavitation). The potential energy, isochoric specific heat capacity, molar incremental enthalpy, bulk modulus, coefficient of thermal expansion, surface tension, and pressure tensor components exhibit a marked dependence on the distance from the C-S-H adsorbing surface. Parameters usually adopted in sorption models (e.g., BET family) such as monolayer thickness and adsorption energy need to be revisited using molecular scale evidence.
The Clausius-Clapeyron equation is an effective tool for describing the effect of temperature on water vapour adsorption isotherms in cementitious materials. The key information is the isosteric ...energy. This can currently be characterised experimentally using two approaches: (1) the isotherms method, which involves experimentally acquiring the desorption isotherm for two (or more) different temperatures, and (2) the hygrometric method, which involves monitoring the increase in vapour pressure at equilibrium with a small sample subjected to increasing temperature steps. It turns out that although each method has a fairly high uncertainty, the results obtained are similar. Finally, the results seem to suggest that the isosteric energy of Portland-based cementitious materials could be considered unique.
The consequences of accelerated carbonation at 3% CO2 were compared with those of natural carbonation (0.04%). Cement pastes (CEM I and CEM V/A) as well as the three major constitutive phases (C-S-H ...of different C/S ratios, portlandite and ettringite) were used and changes in the mineralogy, microstructure, water retention and cracking were investigated. The main conclusion was that accelerated carbonation at 3% CO2 was representative of natural carbonation although it promoted the precipitation of metastable calcium carbonate (aragonite and vaterite) in place of calcite. The results also showed that the presence of aragonite and vaterite were characteristic of the carbonation of ettringite and C-S-H respectively.
The impact of temperature on carbonation was investigated in laboratory conditions using a device developed for this purpose. Two hardened cement pastes (CEM I and CEM V/A) were tested between 20 °C ...and 80 °C at different levels of relative humidity (RH). The carbonation rate of the CEM I increased with temperature, whereas that of CEM V/A reached a maximum at around 50 °C.
In unsaturated conditions, the durability of concrete structures is strongly dependent on the evolution of the amount of free water within concrete porosity. Reliable durability assessment of ...concrete structures in relation to their environment thus requires accurate unsaturated water transport description as well as reliable input data. The effect of carbonation on water transport remains poorly studied and data are lacking. It was then the purpose of this article to acquire all the data needed to describe unsaturated water transport in carbonated cementitious materials (porosity, water retention and unsaturated permeability). Four hardened pastes made with four different binders were carbonated at 3% CO2 to ensure representativeness with natural carbonation. Beyond the modification of the water retention curve and porosity clogging, significant microcracking due to carbonation shrinkage was observed. The consequence on permeability highlighted a competition between porosity clogging and microcracking that was dependent on the initial mineralogical composition.
Water transport is a key issue for concrete structures durability; the most important parameter may be the intrinsic permeability to water. It is usually estimated through inverse analysis on the ...basis of the experimental weight loss of concrete samples in a controlled environment. By doing so, Mualem’s model together with van Genuchten’s equation are used to describe the water retention curve as well as the relative permeability evolution. To study the influence of such a restriction, the intrinsic permeability of a modern concrete was estimated using different mathematical expressions to describe the water retention curve. A simple numerical procedure was set up to approximate the relative permeability evolution according to Mualem’s model. The results show that the intrinsic permeability depends on the model used whereas all the effective permeability evolutions tend to describe the same evolution. It is believed that in this evaluation process the obtained intrinsic permeability to water is not a fully relevant parameter for the effective permeability evolution.
In the field of radioactive waste management the concrete structures are expected to undergo significant heating due to the waste thermal power and significant drying (in the French design, the ...temperature is not expected to exceed 80
°C). The durability assessment of such structures thus requires the knowledge of the evolution of the water vapour sorption properties versus temperature. The latter can be easily estimated using the Clausius–Clapeyron (CC) equation: the approach presented hereafter requires the knowledge of the isosteric heat of adsorption and one desorption isotherm (at ambient or any other temperature). Moreover, it is shown that using the CC equation and the desorption isotherms at two different temperatures, it is possible to estimate accurately the desorption isotherm at any other temperature.
PDF
