Elspru, I’m not going to argue with you. I’m pretty sure you could make good cases against your own arguemnts perfectly well. I have better things to do than bandy words.
They say the plastic sargasoo can’t be recycled, but that just means it’s not commercially feasable. All that plastic can be recovered somehow, or at least used for filler.
I have started a forum on renewable materials and communes: http://www.hombreranch.com/commune/
Hemp will rot over time. Use smoked bamboo. Bamboo, even if it’s just harvested and dried, will lighten and greatly toughen concrete structures.
You’re right, metals should be avoided altogether. Magnesium is fairly corrosion resistent though, and its abundance in sea water is over a kg per m3.