I believe you are talking about thermit welding. This is commonly used to join railroad rail sections, rebar, and copper grounding wires or lightning protection wires in buildings. The process uses a mold around the ends to be joined and igniting a magnesium charge to melt the filler and pour into the mold. Kind of like casting in place. I think CadWeld is actually a trade name used by Erico. Check out their website for more info:

http://www.cadweld.com/product.asp?name=IndCadweld&level=Product

Marty Sims