Long ago in a galaxy far away (ok, in 2005) we temporarily blocked off the gaps in the block wall with spray foam. At the time, there was still talk of drywalling the block walls, which would have covered it (and we could have left it), but since we are not drywalling those walls, the spray foam had to come out - it isn't rated as an exposed covering in a firecode rated building. So, it got all scraped out and we put in concrete instead. Here a volunteer (I think it is Adam, but can't really tell) scrapes out the foam from one of the gaps.