Shock compression of granular materials features prominently in manufacturing, defense, and planetary sciences. A significant amount of the total energy used to shock-compress powders is consumed in plasticity-induced pore collapse (Meyers 1999) followed by deformation of the grains. In this work, X-ray phase contrast imaging (XPCI) of shock compression experiments was performed at the Dynamic Compression Sector (DCS) of the Advanced Photon Source (APS) for ductile and brittle granular materials such as aluminum (Al), soda lime glass (SLG), and Ottawa sand (OS). Since the time-resolved in-situ experimental images can only provide 2D information, we combine them with mesoscale Finite Element (FE) modeling to capture the in-situ micro-mechanisms that dictate the compaction process in 3D. Different models incorporating plasticity, thermal effects, strain-rate effects, and EOS are integrated to capture different mechanisms that are activated during the rapid compaction of ductile and brittle granular materials. From these simulations, artificially generated phase contrast images are then compared with the experimental images to calibrate the model. The simulations coupled with experiments provide a holistic picture of the different mechanisms that are activated during such high strain-rate conditions. Overall, this study and the framework employed in it will advance our understanding of the dynamic response of heterogeneous materials subjected to shock loading.