Using Concrete as Aquarium Decor??? Safe or not?

Hey guys- need some help. My CHEAPEST LFS sells rock at 6$/lb. It looks just like the leftover concrete blobs we have laying around our house from it's construction 6 years ago, or our random fencing/etc. jobs over the years. It really does look the same So I chiseled a few holes in it- rinsed it off- poured a bit of vinegar on a piece "to see if it bubbles" and it didn't.

I put a huge piece of it into my son's 125 gallon Malawi/turtle tank a few days ago. PH is still a wonderful 8.0 (our tap PH). Water is clear. Fishies LOVING their new holey rock.

I cannot find a straightforward answer anywhere so far- Is well-cured concrete going to be safe in my aquarium? Is the risk only of elevated PH or do the chemicals that activate the cement continue to leech?

I'd love to hear from someone who has done this. I know there are websites where they are making live/base rock from concrete. Also- many large ponds/aquariums are made from concrete. Some say to seal/paint it, then it's "ok". My concrete is large and porous- I don't intend to try to accomplish sealing it all.

If I wash my old concrete well- can make my cichlids an awesome network of rock/caves with it without it making the water "toxic" in some way? Thank you very much for your thoughts!

Thanks for your response....yeah, the landscaping stores don't have much of anything besides gravel around here....

Home Depot has broken bits of slate I considered- but I don't like the look as well. We are in a small area- and I cannot even find "pool filter sand" in town despite having a Home Depot and Bomgaars here ): Ace Hardware closed last fall....

It is older concrete.... NOT as easy to put a few holes in as I'd hoped!!! But, for the pieces that haven't fallen apart with the drill/chisel- it looks really neat (but IS super heavy). My plan for now is to soak them all in a bucket of tap water- dump and re-fill a few times to soak it good. Then- let it set about 3-5 days and check the PH in the bucket. If all is well, I think I will give it a go! Thanks! Anyone else ever try or consider it?

Thanks, Heretik! It all seems to be going fine in the tank I already put the large chunk into. Really does look pretty good- looking forward to some algae taking residence on it. The cichlids will be in heaven then! They have all kinds of holes, nooks, and caves now...Thanks for your answers. It seemed crazy that I couldn't find a decent response anywhere on the web....just people suspecting NOT to do it because of leaching toxins.