Thanks for the info!
I'm not quite sure how much the added mass would help, as the solid material would still transfer vibrations. What is needed is material that absorbes energy. Fabreeka has some examples on dampening the vibrations with large hammers: http://www.fabreeka.com/applications/hammers.htm.
My original plan was to make 4.5(wide)x6x3(deep) foot base. My hammer is only 110 lb, but I think I will upscale my plan. I'll probably add also some sort of rubber matt underneath the concrete. I'll use EPS frost plates to the sides.