Actually, while we'll likely never know for certain, [it's quite debatable for the first phase](https://en.wikipedia.org/wiki/Washington_Monument#Excavation_and_initial_construction), and we're reasonably certain it wasn't used for the second phase. There's no "yes" or "no" to this question; even the historians collectively don't know. (Although if "built by slaves" is implying exclusively or near-exclusively, that's definitively wrong.)