Drying | EPFL Graph Search

Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered "dried", the final product must be solid, in the form of a continuous sheet (e.g., paper), long pieces (e.g., wood), particles (e.g., cereal grains or corn flakes) or powder (e.g., sand, salt, washing powder, milk powder). A source of heat and an agent to remove the vapor produced by the process are often involved. In bioproducts like food, grains, and pharmaceuticals like vaccines, the solvent to be removed is almost invariably water. Desiccation may be synonymous with drying or considered an extreme form of drying. In the most common case, a gas stream, e.g., air, applies the heat by convection and carries away the vapor as humidity. Other possibilities are vacuum drying, where heat is supplied by conduction or radiation (or microwaves),

Impact of water activity on the mineralogy of hydrated cement

Luis Guillermo Baquerizo Ibarra

Cement hydrates and their chemically bound water content are sensitive to changes in relative humidity (RH) and temperature. This may cause specific solid volume changes affecting dimensional properties of hydrated cement paste such as shrinkage, swelling and expansion, and therefore impact the performance and the transport properties of cementitious materials. The present thesis studies the impact of drying conditions on the structure and thermodynamic properties of crystalline cement hydrates in different hydration states (i.e. varying molar water contents). The cement hydrates studied include the most important AFm and AFt phases present in different types of cements. A novel multi method approach, including XRD, TGA, DSC, sorption balance measurements, sorption calorimetry and the hydrate pair ¿ humidity buffer method, was used to derive physico-chemical boundary conditions and the thermodynamic properties of the studied hydrated phases. The stability and hydration states of AFm phases depend on the anion content and the exposure conditions. Some phases such as monosulfoaluminate and hydroxy-AFm are very sensitive to different relative humidities. On the other hand monocarboaluminate and strätlingite present very good volume stability with varying RH. Ettringite shows a strong hysteresis during desorption/adsorption, and the thermodynamic properties associated with its decomposition and reformation were determined. The experimental results were included into a thermodynamic model capable of predicting the response of cementitious systems considering the ¿true¿ crystal water content on the final phase assemblage in two different scenarios: i) during hydration, even at conditions where the amount of water added was insufficient for complete hydration, and ii) during drying and re-wetting processes in already hydrated systems. Our work hence opens the possibility to model the response of various cement materials exposed to different climatic conditions and to engineer cementitious systems with respect to minimizing volume changes in the course of drying, which may positively impact properties like drying shrinkage and total porosity.

EPFL2015

Evolution of microstructural changes in cement paste during environmental drying

Fabien Jacques Michel Georget, Hamed Maraghechi, Karen Scrivener, Wioletta Soja

Exposure of concrete to the atmosphere changes the microstructure of the cement paste. However, knowledge about the alteration of the porosity due to drying is typically not considered in deterioration processes of the hardened cement paste. The purpose of this article is to emphasize the importance of the porosity changes due to drying. 1 mm thick discs specimens made with four different pastes including three blended cements were exposed to CO2-free atmosphere at 70% relative humidity. The consequences of drying during one year of exposure were studied using XRD, and SEM for the phase assemblage, while porosity alteration was examined by MIP and high-resolution microscope. This study showed that the significant coarsening of the pores occurs during exposure at 70% RH which should be taken account of in durability studies.

PERGAMON-ELSEVIER SCIENCE LTD2020

The role of cavitation in drying cementitious materials

Mohsen Ben Haha, Monisha Rastogi, Karen Scrivener

This study examines the role of cavitation in drying cementitious materials. Classical nucleation theory is used to understand and study the parameters that influence cavitation pressures. The water desorption isotherms indicate that both heterogeneous and homogeneous cavitation occur during the drying of saturated cement pastes and the commonly observed large moisture loss at 0.3 pv/psat can be explained by homogeneous cavitation. The homogeneous cavitation is suggested to occur in the gel porosity of the inner product that is accesible by the interlayer spaces. The findings of this study indicate that it is important to consider the effects of cavitation for characterization of pore sizes from moisture desorption isotherms.

PERGAMON-ELSEVIER SCIENCE LTD2022

Impact of water activity on the mineralogy of hydrated cement

Luis Guillermo Baquerizo Ibarra

EPFL2015