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> The oldest buildings in the world[0] are as old as they are because they use materials with lifespans of much less than 100 years.

Rather apropos counter-example: Rome's Pantheon is several hundred years older and built in part of concrete.

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The ancient Roman concrete was very different from modern: it was more thick, could not ne poured etc. But more importantly, it had no rebar, which is the main thing that corrodes over time. Ancient buildings could use molten lead to join together pieces of stone, but steel could not be made in the volume needed for reinforced structures.

Thing is we can use rebar in concrete for indefinately long use if we plan ahead, use sacrificial annodes to stave off corrosion or use stainless steel or use carbonfiber rebar as they won't rust. Hell using a polymer coating on rebar can prevent moisture contacting the surface of steel and rusting but that would require added cost and care when preping to prevent the polymer from being damaged prior to being embedded in concrete.

Are the thermal expansion characteristics the same as for regular steel rebar?

Reinforcing rebar and fibers can also be made out of basalt, which takes care of both corrosion and thermal expansion issues.

Un reinforced.

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