Dec 20, 2012· Clay bricks, hollow block materials from cheap materials « Aug 18, 2010 · Concrete mixture for hollow blocks making … coconut coirdust or sawdust. … Hollow Blocks Made from Cheap Materials Formula for cementless blocks … »More detailed
You don't say what the block will be used for so I'll just answer generally. Yes you can use a router to hollow the back. You might consider making a tortion box out of the block.