Experimental proof of moisture clog through neutron tomography in a porous medium under truly one‐directional drying

Abstract

Structural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive spalling on partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in-operando drying behavior of a high-alumina refractory castable, proposing a novel experimental layout aimed at a truly one-dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.

Heating rate effect on the moisture clog while drying refractory castables: A neutron tomography perspective

“…The first experimental evidence of the moisture clog phenomenon using the neutron tomography was reported by Dauti et al., 20 and later conclusively proven by Moreira et al 17 . In the current work, the effect of the heating rate on the moisture transport while drying the calcium aluminate cement refractory castable is studied by neutron tomography and the results are compared with those in the literature.…”

“…Besides these localized effects, the overall drying speed can be described through the fraction of the sample whose attenuation has significantly changed, as described in Section 2 and detailed by Moreira et al 17 Figure 10 shows this evolution for both tests which highlights a marked difference in speed as well as the delay between the start of the test and the inception of drying.…”

“…The unidirectionality of the heat and mass transport is important to keep the boundary conditions reproducible and close to the industrial applications conditions and also to simplify the quantification of moisture transport from the resulting images. 17,18,20 This was attained by using mullite and alumina ceramic casings. These materials were selected because they are transparent to the neutron beam, impermeable to water vapor, and, most importantly, present a lower thermal expansion than the refractory castable.…”

“…Studies ranged from laboratory-scale thermogravimetric tests (TGA) [9][10][11] to those based on pressure and temperature sensors, [12][13][14] or even more advanced techniques, such as nuclear magnetic resonance (NMR) 15,16 and neutron tomography. 17 Full-field techniques, such as neutron imaging, describe the entirety of a sample and the evolving water distribution within it, rather than providing macroscopic or point-wise information as the other techniques. Additionally, nondestructive techniques do not affect the sample, as they do not require sensors placed within them.…”

“…It is well known that faster temperature increases lead to a greater likelihood of explosive spalling as shown by TGA analysis. 3,10,23 However, the effect of the heating rate on the water distribution within the sample itself is still unknown as the available studies tackle the macroscopic response of the sample instead of its full-field response provided by imaging tests, such as the neutron tomography setup used by Moreira et al 17 Additionally, the most common technique applied for studying these effects, the TGA, generates a thermal gradient that is not representative of unidirectional heating attained when drying refractory linings in industrial applications, as schematically shown in Figure 1A. The unidirectional heating of the sample via the neutron tomography setup is shown in Figure 1B.…”