You could model the cement bag as a a bunch of solid elements using a material model appropriate for soils, then you can coat the lump with membrane elements. If you need to model the air, then use hydrostatic fluid elements above the cement powder.
I think I would start by just using a single 20 node brick to model the bag with the right mass, with simple elasticity, damping, and yield. If your not interested in the bag, but just it's effect, I would probably go this route, but that's just me. I tend to be very stingy with elements.