The surface I work on and you can see is 'Hebel' aerated concrete
block. It makes a great working surface for brazing and soldering. It won't
explode when wet. You can drill or cut it very easily to make jigs to hold
your work together when you solder. It is cheap. It comes in miriad of
sizes. Lasts for ages. It's very light and looks like 'Aero' chocolate.
I beleive it's a German invention where they aerate concrete slurry with
high pressure steam.
One slight drawback is that it's a little dusty.