If I were going to try this I would get a bottle of hydrochloric acid and do a thorough job of acid washing it first. Once it doesn't fizz from the acid it could work ok. Concrete isn't waterproof, so any paint would have to be water tight to avoid flaking off as water soaks in behind it. I would enjoy reading about how well this works, if you try it.

 

Would there be any other material I could use?

I want to do a handmade volcano about 6"-8" tall with lots of texture for plants to cling to.

 

Many DIY backgrounds are made of Styro coated with concrete, which will work depending on how you want your centerpiece. It does contain limestone and will raise hardness -- for what its worth, people say you can speed up the curing time by using softened water and daily water changes, but I still waited 28 days with the pH well beyond the limit of my test kit.

I think the paint is risky. Maybe you could seal it with an epoxy though.

Here's the best resource I have found online: http://www.cichlid-forum.com/articles/diy_list.php (se aquascaping ideas)
and the... coolest... background... ever: http://www.aquariumadvice.com/viewtopic.php?t=31078&highlight=

 

Concrete is often used for municipal water storage reservoirs. Back in my engineering days we'd build million gallon tanks, wait a month, fill and empty them once, and then put water in for human use. It takes about a month to cure completely. Once this is done it's quite inert, assuming you didn't use limestone or another soluble gravel in the mix.

Making something out of concrete that actually looks nice...... now that's a different story.

 

Making something out of concrete that actually looks nice...... now that's a different story.

A lot of trial and error. :madgrin: The great thing is the moss I plan on having in the tank will eventually cover the centerpiece. You'll still be able to tell what it is.

 

what about plaster. that is what they use in swimming pools so i am assuming it has to be moderatlely water proof

 

Swimming pool plaster would be interesting to try. As I recall from the time I had one built, it sheds material for a few weeks into the water - when you brush it you brush it off. It is made to cure under water too.

 

Burks if you find something and get your centerpiece setup, I think we all would love to see it. Sounds very interesting. Like "Mount Hawk-a-loogy" (Finding Nemo).

I would be most concerned about the paint flaking off. In terms of material, concrete sounds like a better choice after it's condition per Guaiac Boy's sugguestion. The pool plaster might be hard to paint no?

-John N.