I'm not doubt that. What I'm concerned about is mode of failure.

The reason why steel is preferred in reinforced concrete is because of its failure mechanism.

When a concrete beam (without steel) is over stressed, it can suddenly give way and collapse without any warning.

When it is paired with steel, the steel elongates over a period of time. Cracks develop but the structure still stands. The beam begins to heavily sag and eventually the steel snaps and then collapses.

The former is brittle failure, the latter ductile.if basalt is brittle, it has little use in construction (look at cast iron. It's as strong as steel, until you decide to stretch it).