I am part of a wave of trademen who don’t agree with the use of stone dust in building stairs with interlock.

Why?

Simply because stone dust absorbs moisture and that moisture leads to a greater heave and thaw, a greater expansion and contraction due to temperature changes.   I’ve re-set a lot of interlock stairways.  You can say that’s one of the benefits of interlock, that they can be re-set.  But my question concerns how often you want to do that.

I’m more in favour of pea gravel.  It compacts really well, but it doesn’t hold moisture.   Water drains through it.  Voila, less expansion and contraction.