Abstract
The use of raw earth as construction material can save embodied and operational energy because of low processing costs and passive regulation of indoor ambient conditions. Raw earth must however be mechanically and/or chemically stabilised to enhance stiffness, strength and water durability. In this work, stiffness and strength are enhanced by compacting raw earth to very high pressures up to 100 MPa while water durability is improved by using alkaline solutions and silicon based admixtures. The effect of these stabilisation methods on hygro-mechanical behaviour is explored and interpreted in terms of the microstructural features of the material. Stiffness and strength are defined at different humidity levels by unconfined compression tests while the moisture buffering capacity is measured by humidification/desiccation cycles as prescribed by the norm ISO 24353 (Hygrothermal performance of building materials and products determination of moisture adsorption/desorption properties in response to humidity variation. International Organization for Standardization, Geneva, 2008). As for the microstructural characterisation, different tests (i.e. X-ray diffractometry, Infrared Spectroscopy, Mercury Intrusion Porosimetry, Nitrogen Adsorption) are performed to analyse the effect of stabilisation on material fabric and mineralogy. Results indicate that the use of alkaline activators and silicon based admixtures significantly improves water durability while preserving good mechanical and moisture buffering properties. Similarly, the compaction to very high pressures results in high levels of stiffness and strength, which are comparable to those of standard masonry bricks. This macroscopic behaviour is then linked to the microscopic observations to clarify the mechanisms through which stabilisation affects the properties of raw earth at different scales.
Highlights
The use of raw earth as a construction material for load-bearing, infilling or partition walls can reduce environmental impact during both the construction and service life of buildings
Results from moisture buffering value (MBV) tests are typically presented in terms of moisture adsorption curves, where moisture adsorption is the ratio between the sample mass change and the sample area exposed to the ambient humidity
This is because exchanges of water vapour take place within the smallest nanoporous fraction, with diameters between 3 and 7 nm, which is not affected by compaction (Fig. 4)
Summary
The use of raw earth as a construction material for load-bearing, infilling or partition walls can reduce environmental impact during both the construction and service life of buildings. During service life, raw earth walls can passively regulate both indoor humidity, thanks to their high moisture buffering capacity, and temperature, through exchanges of latent heat, increasing environmental comfort for occupants while reducing air-conditioning needs [4,5,6,7,8]. To improve mechanical and durability properties, raw earth is often ‘‘stabilised’’ by either mechanical processes, e.g. through densification, or chemical processes, e.g. through mineral cementation. Some methods are more effective in improving stiffness and strength but less effective in enhancing durability, while other methods exhibit opposite results. As pointed out by Liuzzi et al [10] and McGregor et al [11], some stabilisation methods can induce undesirable side effects like a reduction of the material hygro-thermal inertia, defined as the ability of the material to store/release heat and moisture depending on the temperature and relative humidity of the surrounding environment
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