All you need is
Portland cement(not pre mix concrete)
oyster shells
Aragonite sand
I dont remember the ratios but they are easy to find. Ive never tried it but some people put rock salt in it so that when you cure it it dissolves and leave big holes in it. Looked a little better i thought.
It would be wiser to cure it in a separate container. Just submerge it in water and change the water regularly. Give it a couple weeks then check the ph and see if it rises above normal. It is fully cured once it does not affect the ph.
Ive made my own rock and, like theyve said, DO NOT add it to your tank without curing.