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What the world doesn't need is yet another page on making aquarium rocks from concrete, but here's mine anyway. I make mine from perlite concrete. My first batch was made from just perlite and Portland cement, but they floated!
Cavities can be made by placing a small balloon in the middle, keeping the air in with a small rubber band instead of a knot. After the concrete has set up you can take off the rubber band, deflate the balloon, and re-use it. I also tried using plastic drinking straw to make channels for baby fish to live in, as suggested by the fake reef rock folks. I guess I didn't use the right straws or something, because they had to be removed by winding them up on pliers which made it impossible to re-use them. One even broke off inside a rock while I was trying to remove it.
After some more research online I changed the composites recipe to 4 volumes perlite and 1 volume crushed limestone. This gives a light rock that does not float.
I age my rocks for a week in tap water with daily water changes. My tap water has a lot of bicarbonate in it, and it reacts with the lime in the concrete to form a precipitate of carbonate. By the time a week is up the perlite concrete is no longer raising the pH of the water. If I have no immediate need for the rocks I then place them in a vat of stagnant water for further ageing. Otherwise they can be placed, a few at a time, into tanks and will not cause a pH change. This would not work did I not have water with so much buffering in it (KH about 400 ppm, pH 8.2). Above is one of my 30-gallon tanks in which I was ageing some perlite concrete rocks with water circulation when I suddenly had too many new fish and needed the tank for quarantine. You can see the deposits of carbonate on the walls of the tank. Subsequently I have used plastic tubs for the ageing.