Cement filled block

Muriatic acid eats concrete, but not most metals.
If there is any way you can submerge the block in a 50/50 solution of muriatic acid and water and turn it every hour or so, that would do it.
Don't breath the fumes that it will start making immediatly when it makes contact with the concrete. (NASTY)
Otherwise you will have to spend hours pouring it through the block and be right in the fumes.