A Microstructural Insight into the Hygrothermal Behaviour of Unstabilised and Cement Stabilised Compressed Earthen Bricks Exposed to High Temperatures

Earthen materials has received a lot of attention as a sustainable building material over the past few decades because of its eco-friendliness. As consequence of this renewed interest, numerous works have been recently developed on earthen materials, mostly with a focus on their mechanical behaviour. Conversely, there is still a significant gap in the literature on the understanding of the fire behaviour of earthen materials. Only a handful of studies has dealt with this subject and a systematic analysis on the causes of thermal instabilities in earth materials at high temperatures is still missing from the literature. The role played by the mineralogy of the earth materials on these thermal instabilities is still poorly understood and it requires further investigation. To fill these gaps, the present study presents the results from both mineralogical and hygrothermal tests performed on earth samples, which were either left unstabilised and compacted at 50 MPa or stabilised with cement and compacted at the optimum standard Proctor. Interestingly, unstabilised earthen samples exhibited thermal instabilities when exposed to high temperatures. To investigate deeper into the mineralogical causes of such thermal instabilities, samples were either equalised at the ambient temperature of about 25 °C or exposed at the high temperature of 350 °C before being subjected to thermogravimetric and differential scanning calorimetry (TG-DSC) tests, Fourier Transform Infrared (FTIR) spectroscopy tests, isothermal adsorption-desorption tests, and nitrogen gas permeability tests. Results show that cement stabilisation induced the formation of fresh the calcium-silicate-hydrate (C-S-H) and calcium-aluminate-hydrate (C-A-H) gels while reducing the water adsorption capacity. Both effects have rendered the material less prone to thermal instabilities compared to the unstabilised material. The main mineralogical and moisture-related changes influencing thermal and fire behavior might not be well captured by traditional studies. Though lacking in information to perform a thorough study, they do provide initial indications of possible internal alterations within the materials.