One of the points made in a number of recent questions regarding underwater races has prompted me to review one of my older pieces of world-building. Namely: the difficulty of producing common structural materials like steel or concrete in an aquatic environment.
I have a species of intelligent, fishlike aliens that live on an ocean world - a world much like ours, only the sea level is high enough to submerge all but the highest peaks.
This species is the product of a prolonged uplifting program by a much more advanced civilisation - their technology is by now distantly descended from a wide variety of prefabricated factories introduced to them during the initial uplift. Accordingly the bootstrap problem isn't a big deal, since it can be assumed they acquired any necessary knowledge and equipment from their erstwhile benefactors. Similarly necessary raw materials such as ores can be safely assumed to be present.
Technologically they're what we might think of as a contemporary - they're comfortably industrialised and have turned their attentions to spaceships. What components they can't fabricate underwater are produced in aerated industrial complexes built in shallow parts of the world, supported by vast logistical networks. Suitable locations are limited, though, and I'd prefer not to rely on shallow plateaus for all industrial production. In general for this species, air-breathing manufacturing is expensive and inefficient but nevertheless possible - their motivation for moving industry to aquatic manufacture is a matter of efficiency more than for lack of alternatives.
What I'm looking for is a structural material that can be produced underwater with little to no reliance on atmospheric gases or access to the surface. Optimally it would be usable in ways comparable to steel, but it should at least be suitable for construction. Typical applications would be shelters/homes (against wildlife and aquatic weather), storage, workplaces, and so on.