Supercritical drying of cementitious materials

Abstract

Techniques to characterize the microstructure of hydrated cement require dried materials. However, the microstructure of hydrated products is significantly altered by high capillary forces during drying when using the conventional drying methods. To avoid drying stresses when preparing samples, we have employed supercritical drying (SCD) which has been used for decades to prepare aerogels that undergo no shrinkage during drying, but has rarely been used for cementitious materials. The pore solution is first replaced with isopropanol, and then with trifluoromethane (R23). The temperature and pressure are raised above the critical point, where no menisci or capillary pressure can exist; therefore, the dried samples are free of artifacts created by stresses. Images from scanning electron microscope show less compact morphology for supercritically dried samples than that dried by conventional methods, while BET surface areas of SCD samples are very close to samples dried by isopropanol replacement method. This can be explained by the fact that isopropanol and supercritical fluid enter the micropores and block them. The nature of the chemical interactions of isopropanol and R23 with cement pastes are still not clear, but no reaction products were identified in the present study. of evaporation is controlled by diffusion through a boundary layer [4,5]. The flowing N 2 creates the zero-humidity environment to force liquid water to evaporate, and the higher the velocity of the nitrogen, the thinner the boundary layer, which accelerates the kinetics of drying. Oven drying, either at 60 • C or 105 • C, speeds up the evaporation by raising the vapor pressure and diffusivity of the water vapor, but usually does not permit strong convection. Ovendrying above 40 • C may lead to dehydration and rearrangement of hydration products [6], and consequently more coarsening of pore structure than other methods. Vacuum drying is also an acceleration process which decreases the surrounding vapor pressure, but it is slow because the absence of convection results in a thick boundary layer. Direct drying methods do not eliminate or reduce capillary pressure (except for the small effect of temperature on surface tension), so materials still suffer high capillary forces.Capillary pressure can be reduced by replacing water with low-surface-tension solvents. Solvent replacement drying is considered as the best technique with respect to preservation of the pore structure [7,3]. Commonly used solvents, such as acetone, ethanol, isopropanol, methanol, tetrahydrofuran and dimethyl sulfoxide, are miscible with water. When samples are immersed in the solvent, the interdiffusion of water and solvent reduces water content with time. Hydration can be arrested in a short time by reducing the water activity in the cement matrix [8] while it may take a long time to completely replace water, depending on solvent diffusion coefficient, sample size, etc. After solvent replacement, samples can be subjected to normal drying methods, either low RH drying or ove...

Determination of water permeability for a moisture transport model with minimized batch effect

“…TGA measurements were performed by PerkinElmer R Pyris 1, in which specimens were heated from room temperature to 1000 • C. The measured mass change between 105 • C and 1000 • C was used to calculate the degree of hydration (DoH). The detailed procedure and equation were reported in our previous studies [72,71,69]. The NAD measurements were conducted using an ASAP 2010 apparatus, from Micromeritics (see [71] for more details).…”

“…The detailed procedure and equation were reported in our previous studies [72,71,69]. The NAD measurements were conducted using an ASAP 2010 apparatus, from Micromeritics (see [71] for more details). The Micromeritics 9410 apparatus was used for MIP tests (see [73] for details).…”

An experimental study on adhesion between steel and cement pastes using particle probe scanning force microscopy

“…X‐ray diffraction analysis was performed to elucidate the compositions of new and old cement specimens. Based on our facilities and time schedule, we prepared the new cement specimens using the solvent exchange method . First, small cement pieces with weights of ~10 g were separated from the major cement bulk.…”

A critical comparison of 3D experiments and simulations of tricalcium silicate hydration

“…Sometimes they are also compared with 2D micrographs of actual hydrated cement paste microstructure either qualitatively [2, 4] or by statistically analyzing the spatial distribution of the solid phases or capillary porosity using radial distribution functions or autocorrelation functions [8–10]. However, 2D micrographs are typically acquired only after interrupting hydration by solvent exchange or drying, and under the high vacuum conditions of a scanning electron microscope (SEM) These conditions may alter the microstructure considerably compared to that in a fully saturated system, especially the C−S−H [11–13], although a recent report showed that solvent exchange with isopropyl alcohol followed by supercritical drying, which should exert no drying stresses on the structure, yield C−S−H morphologies observed by SEM that are at least qualitatively like those obtained by normal solvent exchange and drying [14]. In contrast, the advent of nanoscale X-ray computed tomography (nCT) has made it possible to examine the 3D microstructure of porous cementitious media with a spatial resolution on the order of tens of nanometers, and coupling tomography with nanoscale X-ray fluorescence spectrometry (nXRF) has enabled the mapping of the elemental concentrations with about the same spatial resolution [15].…”

“…These conditions may alter the microstructure considerably compared to that in a fully saturated system, especially the C-S-H, [11][12][13] although a recent report showed that solvent exchange with isopropyl alcohol followed by supercritical drying, which should exert no drying stresses on the structure, yield C-S-H morphologies observed by SEM that are at least qualitatively like those obtained by normal solvent exchange and drying. 14 In contrast, the advent of nanoscale X-ray computed tomography (nCT) has made it possible to examine the 3D microstructure of porous cementitious media with a spatial resolution on the order of tens of nanometers, and coupling tomography with nanoscale X-ray fluorescence spectrometry (nXRF) has enabled the mapping of the elemental concentrations with about the same spatial resolution. 15 Several papers published in the last few years [15][16][17][18] have shown the rich 3D data that can be acquired and analyzed with these methods, which make it possible for the first time to compare 3D microstructure against predictions of microstructure-based computer models of cement hydration.…”