What is used to join two related clauses that are not joined by a coordinating conjunction?

A semi-colon is normally used. A colon can be used in certain situations.

Examples:

The bus ran into the motel wall because the driver could not make the sharp turn.

The bus ran into the motel wall; the driver could not make the sharp turn.

We all knew the truth, which was that the company was doomed.

We all knew the truth: the company was doomed.

This can be done with a fragment, as well.

The boss needed one more thing to complete the job: a paint sprayer.