Yes, apparently that's how they make the foam cement bricks and it doesn't seem to take much. I often wondered how they made them.

I guess the resonance you need to damp in a machine tool isn't the ringing of the tube itself but of the structure formed by it. So if you were to stick a large mass (gantry, machine head etc) on the end of said tube and give it a knock, you'd get a much lower resonant frequency as the mass swung back and forth. That's the motion you'd need to damp in a machine tool isn't it? I expect the damping effect of that different structure at the lower frequency is likely to be different.

Nice equipment to be able to play with, not least the analyser for logging and plotting the measurements! Can you do another experiment now, with a mass on the end of one of those beams and the other end anchored? Easier said than done, I appreciate.