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The romans fabricated concrete by mining volcanic material which hardens just like portland cement. In fact the roman volcanic version of concrete is a bit better in material quality than the portaland cement todays civil engineering is using.

Cement industry is fabricating cement rather locally in industrual rotating ovens to avoid high land transport costs. This is where seasteading and free mobility of big structures comes in.

If you think that trough there might be a “sweetspot on the planet” maybe a remote island where nobody bothers you – where volcanism offers you incredible amounts of almost ready to use cement for free. You go there build a enourmous structure at low cost.

Volcanism not only offers you cement it also offers a floating stone (pumice) used by the romans as light weight aggregate for magnificent dome structures.

Again location is the most important thing. As we are on the modular island thread – i assume we talk about a island size seastead – many hundreds thousands of tons of material – here is where the cost of material and building becomes really important.

At the moment we are building floating concrete shell structures from materials provided by the land based construction market. And the cost plantet we are talking about is 331 Euro/ton – but for a real floating island of city size it would probably be worth to develop a own material production industry independent from from the existing cement industry – possibly skiping the mayor cost factors like transport, packing, energy cost.

Wil