> nutrient-cement

That reminds me of a less grey-goo-adjacent idea from a Larry Niven book, in which the base-structure of houses were cheaply made by growing a kind of coral inside a watertight scaffolding.

Given the last few centuries of humans under-estimating nature, I predict that many "nanobots" predictions will turn out to be a kind of optimistic hubris. We'll end up making comparatively minor tweaks to the massive base of existing nanobots called biological life. Especially the multicellular varieties, which have many tested and integrated strategies for building things, such as the towering bipedal mega-fortress my hive mind currently inhabits.