Not really "concrete".
http://www.hardblok.com/
It's usually used on race-only applications because it reduces the amount of water inside the block - and the cooling ability. But one can fill the block to different levels. Heat isn't as big of a factor at the lower ends of the cylinders.
Do some research & testing before trying to "dissolve" whatever is in the block. The chemical make-up may be too different from real concrete for it to work properly.