I know that from building concrete backgrounds using cement a mixed myself it has to be soaked first in water for at least a week with a few water changes before adding to tank or you will get a major PH change. if its small enough pieces to fit in a 5 gallon bucket then soak it in there.
you could fill up bucket add pieces, soak for a day, then tip to the side if outside and dump out water and then refill, soak for two days, tip and refill, soak for 3 days, repeat and then you should be good to go.
larger pieces a plastic bin would work.
but you need to soak to leach out anything that is in the cement that will effect your water chemistry.