I know this was already discussed and might be coming with the survival update but i'll just list it in here in case the idea was abandoned. This would make stone and gravel actually useful and not just an unwanted side-product of mining.
- Using Gravel & Iron as components
- Too heavy for ships but great for stationary bases
- As robust as blast doors (maybe even slightly stronger)
- Non-conductive (blocks can't be powered)
- Not colorable (so it's easier for players to differentiate between concrete and armor blocks)
Both the facts that they're non-conductive and not colorable are meant to prevent players to build bases out of concrete blocks only and rather just use them as a foundation. This way it won't be used as an "tier 3" armor block for stations as it would be highly impractical and unpleasant to watch.