Hey,
I've tried using slates before (even sharp ones) and I didn't encountered any problems; albiet I only used one large slate to build a cave-like structure. But Bill is right in saying that using the slates as substrate would cause a great of havoc when cleaning.
You could always use a few for decorative purposes. However, because the slates are from the builder's yard, I would be inclined to take extra precaution. Before I put the slates that I purchased into the tank, I tested them rigourously by scrubbing them thoroughly with sodium bicarbonate and warm water. After doing that a few times, I sat the slates in declorinated water for about a week and did before and after tests for compounds such as phosphates, carbonates, and the routine nitrogen and pH tests. The last thing you want is for these slates to leak salts that will affect the overall pH or the hardness of the tank water.
Hope this helps.
- Jay