Just had another thought about this and wondered if they still made Lytag blocks. They look like concrete blocks and they act like concrete blocks but they were significantly lighter and cheaper. The aggregate used in them was I believe slagg, may even have been ally slagg. Worth a look if they’re still about. This sounds like the perfect use for them. A few years back, that would have been there as a foremost thought, now it took a couple of days to clear the grey cell barriers. I think it’s called being over twenty one Ozziedog,,,,,,,,,,just wanted to say slagggggggg a couple of times