Despite being one of the oldest but still most used building materials on the planet, (micro-)structure-(macro-)property relations of fired clay bricks are still mostly limited to empirical laws formulated based on experimental campaigns. To meet the demands of modern bricks with a low CO2 footprint but improved performance, brick compositions have become more and more complex. Raw clays from different quarries are mixed with various pore-forming additives (e.g., sawdust or paper sludge), tempers (e.g., quartz sand or ?y ash) before the mixture is extruded, dried, and ?red at different temperatures. The complexity of mix design optimization, however, requires predictive tools based on microstructural features rather than time-consuming and cost-intensive laboratory testing. Such a tool is presented herein.
A micromechanics multiscale material model is developed [1,2,3] which resolves the material heterogeneities of fired clay bricks across three scales of observation. Mineral grains (such as quartz and feldspar) and pores are considered, at the three scales, as spheroidal phases with speci?c volume fractions, aspect ratios, and orientation distributions. In between the spheroids, a binding matrix phase is envisioned. Morphometry data are obtained from Archimedes’ principle, helium pycnometry measurements, mercury intrusion porosimetry, micro-computed tomography, and energy-dispersive X-ray spectroscopy in the scanning electron microscopy [4,5]. Based on continuum micromechanics homogenization [6], stiffness, elastic limits stresses, and thermal conductivity can thus be upscaled, whereby microscopic phase properties of the binding matrix and the mineral grains are obtained from small-scale testing (including nanoindentation [6]) or from the literature. Homogenized properties are very successfully validated by independent mechanical and thermal multiaxial tests, obtained from novel testing campaign as well as from literature results. After validation, the model is exploited to perform sensitivity analysis e.g. regarding the change of macroscopic properties with respect to changing pore shapes and pore orientations [8].
 
[1] Buchner, T., Kiefer, T., Königsberger, M., Jäger, A., & Füssl, J. (2021). Materials and Design, 212, 110212.
[2] Buchner, T., Königsberger, M., Jäger, A., & Füssl, J. (2022). Mechanics of Materials, 170.
[3] Kiefer, T., Füssl, J., Kariem, H., Konnerth, J., Gaggl, W., & Hellmich, C. (2020). Journal of the European Ceramic Society, 40(15), 6200–6217.
[4] Buchner, T., Kiefer, T., Zelaya-Lainez, L., Gaggl, W., Konegger, T., & Füssl, J. (2021). Applied Clay Science, 200
[5] Kariem, H., Hellmich, C., Kiefer, T., Jäger, A., & Füssl, J. (2018). Journal of Materials Science, 53(13), 9411–9428.
[6] Zaoui, A. (2002). Journal of Engineering Mechanics, 128(8), 808–816.
[7] Kariem, H., Füssl, J., Kiefer, T., Jäger, A., & Hellmich, C. (2020). Construction and Building Materials, 231, 117066.
[8] Buchner, T., Königsberger, W., Gaggl, A., & Füssl, J. (2023). Construction and Building Materials, Under Review