Porous media such as concrete physically represent a spherical packing of different components — in this case cement, rock and water. The mechanical properties of such mixtures are still difficult to calculate due to their discretized nature. A team has now been able to investigate an unexpected property of mixtures of granular media consisting of soft and stiff spherical particles. For this purpose, a combination of ultrasound investigations and X-ray computed tomographic imaging was employed, allowing a three-dimensional (3D) characterization and evaluation. The discovery could contribute to safer future building in earthquake zones.