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>that must take a bunch of energy - although presumably not as much as smelting steel?

You'd be surprised. Steel is 20MJ/kg[1], epoxy resin is 80MJ/kg[2]. FWIW, aluminium is 150MJ/kg.

[1]https://en.wikipedia.org/wiki/Embodied_energy#In_common_mate... [2] https://compositesuk.co.uk/composite-materials/faqs/embodied...