You should have cut and screwed appropriate-sized slabs of "WOnderboard" to the existing treads first. This gives you a solid underlayment comparable to cement since the 'board' is a cement product. Use the appropriate sized Phillips screws. Cuphead ringshanks would also work probably, but I'd screw it because of the traffic. Make sure the treads themself are solid and screw-blocked underneath before you proceed. What you are trying to get is immobility of the underlayment. This would also work with slate or whatever. Another consideration is weight though. Make sure the stairs framework is up to it.