As a DIYer plus working on less than optimal scaffolding I thought a lighter block would suit me better now I'm working a bit higher so I ordered some medium dense concrete blocks. A quick trawl around suggests to me these should be around 12-14kg per block rather than 18kg for a standard. Is this correct or is there more to it? I've just had completely standard blocks (18kg) delivered. No mention of "medium" on the paperwork.