making cement base for large rock - Page 2 - The Planted Tank Forum
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post #16 of 20 (permalink) Old 08-25-2012, 04:26 AM
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An even easier solution would be to buy thick foam core/board. Cut to size and stick your rock on Top. Done.

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post #17 of 20 (permalink) Old 08-25-2012, 04:40 AM
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Concrete: The more water changes the better, but allow a month.

Black expanding foam, sold for ponds, water falls and similar: Great material! Does not expand as much as the largest expanding construction material, but it does expand. It is much stickier than the construction material, too. Once it gets onto your skin you are marked for several weeks! I do not find it irritating to the skin, but if you are sensitive to things, wear gloves. Completely fish safe once is has cured. Sticky enough to act as a glue to hold the rock in some other firmer, shaped base. I would not use it by itself. Maybe it would make a skin over the concrete, and the color sure is better than concrete!

Epoxy (2 part) paint over concrete. Look for the potable water type. Very expensive, but it will seal in the minerals from the concrete.

Other types of concrete sealers: I would not trust them under water. They are for above ground use for sealing patios and driveways. In this use they need to be re-applied every few years. They wear off. I have no idea if they are fish safe, or if they would even seal the concrete enough to prevent leaching of the minerals.

Styrofoam: Is OK in the tank, if you can make it work for the rock. The expanding foam might make a nice cover/sealer for this. Styrofoam is used for aquarium backgrounds, usually with colored concrete for texture and color. And weight.
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post #18 of 20 (permalink) Old 08-25-2012, 12:36 PM
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I enjoy these engineering "think-tank' threads.

I think I might try a pc of foam insulation board, I believe it's 1.5 " thick Maybe rated R7?

Cut a rectangle base, the hollow-out what I need to set my large rock in place as I wanted it. I'd use silicone in the holed out area and cut blocks of foam as supports as need for structure or for aesthetics .

The foam board could be simply hidden in the gravel but, I think a quickcrete coating and shaping would be better looking.

I've not seen many fusions of natural rock/quickcrete foam faux rock, That could be a fantasic look if done well.
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post #19 of 20 (permalink) Old 08-25-2012, 04:22 PM Thread Starter
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So my concrete didn't stick to the rock, maybe I needed to wet the rock first or maybe it was just not the right kind of concrete. I will buy a can of Pond and Stone Great Stuff. I'm also going to hunt for a few more similiar rocks since you only get one shot to use up the can and it will be nice to have some choices when it comes time to actually put together my hardscape.

Someone recommended combining this approach with rigid insulation foam which I happen to have. It's 2" and has a reflective coating on one side. Could this stuff really be okay to use in a tank? It sure doesn't smell non toxic.

Give me a couple of weeks since I am supposed to be working on remodelling my bathroom and I will post the results of however I end up doing this.
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post #20 of 20 (permalink) Old 08-25-2012, 09:39 PM
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On the rigid foam you have, the reflective coating is probably attached with glue. I wouldn't use it unless you can slice off the coated part.

Do you have a pic of the rock in question? Are you putting it a very unbalanced position?
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