Abstract
Internal insulation of the building envelope is a prime topic in building physics, due to the risk of moisture problems that this technique entails. As a remedy to these problems, the application of a water-repellent agent, which reduces the amount of absorbed wind-driven rain, has become popular in recent years. When such an agent is applied on a building material, it penetrates the pore network of the material, hereby attaching itself to the pore surfaces and rendering them hydrophobic. It is generally believed that some smaller pores can remain hydrophilic due to the inability of the agent to enter. An in-depth microscopic investigation towards these hydrophilic pores, however, has never been performed. Since direct visualisation of the polymer chains was proven impossible, this paper locates the hydrophilic (parts of) pores in a material, hydrophobised with 3 different water-repellent agents, by imaging the moisture storage at pore level using X-ray computed tomography images at different stages of the desaturation process. While completely hydrophilic pore bodies and throats are not found in the studied material, water storage remains possible in hydrophilic corners of hydrophobised pore bodies and throats. These corner islands are less present than in hydrophilic media and do not form a continuous liquid flow path. Therefore, they provide possible locations for little moisture storage but do not contribute notably to moisture flow.
Article Highlights
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When porous materials are hydrophobised, the water-repellent agent hydrophobises most of the pore surface, but it leaves some parts hydrophilic
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Little to no completely hydrophilic pore bodies or throats have been located, but several hydrophilic corners in hydrophobised elements allow moisture storage
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The hydrophilic sites in the hydrophobised pore networks are not connected and hence do not provide a continuous and fast liquid flow path
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Data availability
The XCT images and the other data used in this study are available from the corresponding author on reasonable request.
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Acknowledgements
The authors thank Jeroen Soete for the fruitful discussions regarding the acquisition of the X-ray computed tomography images.
Funding
Daan Deckers is a doctoral fellow of the Research Foundation (FWO) - Flanders, Belgium [FWO project 1117421N]. This work was also supported by KU Leuven [C2 project, Grant Number C24/18/039]. The financial support of both FWO and KU Leuven is gratefully acknowledged.
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DD and HJ developed the methodology of the study. DD acquired the XCT images and was responsible for the first analysis of the data after which the analysis was discussed with HJ. YZ and EK helped develop the laboratory made water-repellent agent. The first draft of the paper was written by DD and revised by HJ. Further remarks on the manuscript were provided by YZ and EK. All authors read and approved the final manuscript.
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Appendices
Appendix A: Preparation of the Laboratory Made Water-Repellent Agent
Hexadecyltrimethylammonium bromide (CTAB, 99.0%) was purchased from Fluka. Polyethylene Glycol Stearylamine (POE, n=approx... 15) was purchased from TCI Chemicals. N-octylamine (99.0%) was purchased from Merck. Silane was purchased from TCI Chemicals. All listed chemicals were used without further treatment.
In a typical preparation of 10 g of the lab-made hydrophobic agent, 0.45 g of CTAB is dissolved with 2.85 g of Milli-Q water in a 5 mL glass vial. 6.65 g of silane is weighed in a 25 mL capped PP plastic cup, followed by 0.05 g of POE and mixed using a Dual Asymmetric Centrifuge mixer (SpeedMixer DAC 150.1 FVZ-K) at 3500 rpm for 5 min. After this initial mixing, the dissolved CTAB aqueous solution and 100 µL of n-octylamine are added to the plastic cup. A second round of mixing is done at 3500 rpm for 15 min. Finally, the content in the plastic cup is further dispersed using an ultrasound probe (BRANSON Digital Sonifier model 250) for 15 s at 10% amplitude with cycles of 0.5 s on and 9.5 s off. The final product is a viscous white cream-like emulsion.
Appendix B: Algorithm to Identify the Filled Pore Bodies and Throats
To label the different water islands as capillary filled pore elements or corner islands, both the water island network and pore network are extracted. These networks exist of a list of elements with their geometrical (e.g. radius, volume) and topological (e.g. coordinates, connections) properties. When an element is capillary filled with water, it should be present in both networks with identical properties. Due to unavoidable errors in the segmentation, processing of the images and slight variation in the extraction of the networks, however, an element is often not characterised identical in both networks. In other words, the water island network and pore network possess several elements with approximately the same geometrical and topological properties (mainly coordinates, radius and volume), which can be classified as the capillary filled elements. This is exemplified by a capillary filled pore and one with only corner islands in Fig. 13 where a 2D representation of the extracted pore bodies in both networks is drawn. The algorithm iterates over the individual elements of the water island network and checks if they can be linked to an (almost) identical element in the pore network and hence be classified as capillary filled.
Example of the extraction of a capillary filled element (a) and a water island in a pore corner (b) in the pore network (red line) and the water island network (green dotted line)
The algorithm behaves slightly different for the classification of either pore bodies or pore throats. It is first explained for the pore bodies, after which the difference with the algorithm for pore throats is clarified. Starting off with the pore bodies, their coordinates, radii and volumes are used for their classification. Consider the pore body of the water island network represented by the red element in Fig. 14a. Firstly, the distances between pore body in the water island (WI) network and the elements in the pore elements (PE) network, indicated by the green lines, are calculated. When these distances are smaller than a certain limit, the two pore bodies are classified as a possible match. Establishing an absolute limit is difficult, however, since a distance between pore centers equal to 5 µm has a much larger influence on a small pore with a radius of 2 µm than on a large pore with a radius of 30 µm. Therefore, the distance is normalised with respect to the pore radius and expressed in percentages relative to its radius. For example, a center-to-center distance of 5 µm for a pore body with a radius of 40 µm results in 12.5%. This error is henceforth called the distance error. Secondly, the difference in radii between both elements is calculated, once again normalised with respect to the pore’s radius. This error is henceforth called the radius error. Finally, the difference in volume between the two elements is calculated and normalised with respect to the pore volume.
Representation of the overlapped water island network (WI) and pore network (PN) when comparing pore bodies (a) and pore throats (b)
To establish limits on the distance, radius and volume errors to which an element should adhere to be classified as capillary filled, all pore bodies of several XCT images are visually classified as capillary filled or corner islands and they are plotted in Fig. 15a in function of their distance and radius errors. The green and red dots represent the visually assessed capillary filled pore bodies and corner islands. Based on this visual assessment, the limits on the distance and radius errors are set to 100% and 30%, respectively, and the volume error should remain between 50 and 200%. It can be seen that some elements are wrongly classified in the algorithm (i.e. green dots in the red zone or red dots in the green zone), however, since manual labeling the hundreds to thousands of pores of every XCT scan is too labor intensive, this is inevitable. Nevertheless, these wrongly classified elements typically account for less than a few percent of the total moisture in the sample. Using these limits, the algorithm is used to classify water islands extracted from different XCT scans as capillary filled or corner islands.
Distance error in fuction of radius error of the pore bodies visually categorised as capillary filled pores (a) and throats (b) (green) and corner islands (red)
Identifying throats as capillary filled follows the same procedure, however, the boundary on the distance error is different. Since the radius of the throats is typically quite small, the distance error, which is normalised with respect to the element’s radius, is larger than for pore bodies as indicated in Fig. 15b. Therefore, the distance error is limited to 200% which seems much, but it is relative to the small throat radius. Once again, some throats are inevitably characterised incorrectly, however, the moisture in these elements is negligible compared to the total moisture content. It is important to mention that the smallest pore throats (with the smallest throat radii) are highly influenced by segmentation errors and slight differences in network extraction. Capillary filled throats with radii smaller than 10 µm are therefore sometimes mislabelled as corner islands when they are capillary filled. This is important to keep in mind when analysing results.
Appendix C: Water Island Size Distributions of Filled Pore Bodies and Throats
The water island size distributions, composed of only the capillary filled pore bodies and throats of the samples hydrophobised with agents B and LM can be found in Figs. 16 and 17, respectively.
The water island size distributions composed using the capillary filled pore bodies (a) and throats (b) of the samples hydrophobised with water-repellent agent B, preconditioned to different moisture contents
The water island size distributions composed using the capillary filled pore bodies (a) and throats (b) of the samples hydrophobised with water-repellent agent LM, preconditioned to different moisture contents
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Deckers, D., Zhu, Y., Koos, E. et al. Microscopic Localisation of Hydrophilically Oriented Pore Bodies and Throats in Hydrophobised Porous Materials. Transp Porous Med 151, 773–793 (2024). https://doi.org/10.1007/s11242-024-02069-w
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DOI: https://doi.org/10.1007/s11242-024-02069-w