I know that you could.
I used to have a reef tank about 12 years and
back then people used to make their base rock
out of a certain type of cement.
Sorry but not sure what specific one.
Been out of the hobby until 3 months ago.
Just looked it up and this is what i got.
If you're using it to do a DIY 3D background, try Dryloc (or Drylok?) instead. Lots of folks on the cichlid sites do - no curing time, doesn't effect water parameters, can be tinted with concrete colorant. I'm going to give it a try myself...I saw it at Lowes for about $22 a gallon, quarts were available too.
The concrete I intend to use will be buried below the substrate. I anticipe using about two 1'' thick pieces, with the diameter of about a hockey puck. Sticking drift wood into each piece before they harden.
Ahh, you're going to use them as weights... You can glue rocks to the DW with gorilla glue too, to save you time. I use slate because it's flat... You can use ceramic tiles too if you have some laying around.
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.
If you're using concrete, and making things small enough to fit, one of the best ways to cure it is to just sit it in the back of your toilet. It'll soak and do water changes for you for as long as you want without you having to worry about it. But regardless of how you do it, everyone is right to say make sure to cure it long enough.