Originally Posted by
Goemon
I think that some of the confusion about what people are using epoxy / sand mixes for is that they aren't using it fill the cavities in aluminum extrusions. It's the opposite. They are using epoxy / sand mixes to create a machine base around an aluminum or steel skeleton. They use this term "epoxy granite".
The purpose of the epoxy granite machine base is vibration dampening and weight. They increase the thickness of the epoxy walls to make up for the reduced strength compared to aluminum or cast iron etc.
Anyway, the key point is that aluminum is the filling, not the thing being filled with this design. A lot of the structural strength still comes from the metal.
Sand is added because it's a lot cheaper than epoxy. It would cost a fortune if pure epoxy was used for such a large structure.