Analysis of water droplet penetration in earth plasters using X-ray microtomography

Earth plasters are a very interesting solution for the coating of earthen construction, in terms of embodied energy, technical efficiency, while keeping the hygroscopic properties of the wall, unlike very little permeable coatings, as the cement-based ones. An experimental campaign has been conducted on several earth plasters which differs by their sand granulometry, earth origin and earth to sand ratio, leading to controlled surface topography and bulk properties. A novel X-ray microtomography technique adapted to such material has been relevant to observe directly the penetration of a water damping droplet inside the plaster with the 2D radiograph method, and enhance the understanding of the role of the surface and the bulk structure on it. A 3D scanning method was useful to understand the microstructure of the different samples and analyse their differences in terms of porosity. Two custom-made MATLAB codes have been developed in order to follow the droplet parameters inside the material, such as the spread, the penetration depth and the relative quantity of water along the time. Results show the consistent role of the surface topography on the water inertial penetration, while the penetration rate due to diffusion is directly linked to the bulk properties, and can be assimilated to a classic diffusion solution. The determination of the drying rate in the earth plasters highlights that the plasters which prevent the water to penetrate too deeply are also the ones which dry the slower, leading to a questioning on the role researched in an efficient coating material. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study found that the surface structure of earth plasters plays an important role in water penetration, while the penetration rate due to diffusion is directly linked to bulk properties. Additionally, plasters that prevent water from penetrating too deeply also dry the slowest, raising questions about the sought-after role of an efficient coating material.