making cement base for large rock
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Old 08-24-2012, 02:35 PM   #1
thesawguy
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making cement base for large rock


I have a large rock that I want to put almost vertical in a new tank and it would be too precarious to support with just more smaller rocks and substrate. So last night I mixed up some quikrete portland cement sand topping mix and set the rock in the position I want in order to create a stable base. Assuming it works and attaches to the rock securely, how long will the cement take to cure enough to put in with live fish.

On some forums discussing DIY backgrounds I've read that people will set it in water and keep making water changes until the ph is steady.

On one forum someone said to give it two months. I'd like to set this up in two to four weeks. If it would really take that long is there some other technique I could use like using spray foam and sealer?

I also saw that some thought adding salt to the water sped up the curing process.

Does anyone have any experience doing something like this?
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Old 08-24-2012, 02:46 PM   #2
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Hi thesawguy,

I would not put concrete in my tank, the limestone and calcium will continuously effect the water chemistry.

Quote:
Cement is made from limestone, calcium, silicon, iron and aluminum, plus lesser amounts of other ingredients.
However, I would consider using a big glob of pottery clay to hold the stone upright, especially if the clay is under the substrate so it isn't clouding the water.
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Old 08-24-2012, 03:06 PM   #3
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Hi Roy,
I kind of like the clay idea. I called a local craft store to see if they have pottery clay and they said they only have self hardenning clay. Would that work or do you think it would have harmful ingredients?
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Old 08-24-2012, 03:19 PM   #4
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The clay will settle over time. You'd have to fire the clay to make it permanent.
You can use concrete but you have to put it in a bucket of water to let the unwanted chemicals leach out with periodic water changes, maybe a month.
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Old 08-24-2012, 03:24 PM   #5
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Hi thesawguy,

Self hardening clay is not the same as pottery clay. Depending upon the manufacturer's formula it may contain toxic elements.

Here is a location near you that carries a variety of clay types; note the non-toxic label.
http://www.ceramicsupplypittsburgh.c...FireClays.html
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Old 08-24-2012, 03:40 PM   #6
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I think you are making this too hard for yourself. Get a flat piece of tile or slate and just silicon or epoxy the rock to it. Best to have some kind of flat bottom on the rock (I used an angle grinder, your metal working buddies will have one), but maybe you could arrange some larger gravel and glob it into an appropriate support with the epoxy. It might take two steps, but each one would be ready in a day or two for whatever is next.
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Old 08-24-2012, 03:41 PM   #7
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Thanks for the link but that clay is a minimum 50 lb order and an hour drive. Clay was a cool thought though but also this base needs to grab onto not just cradle the bottom of the rock.

So I'm back to my current course. I think I can let it leach for a month in water. And I'm not planning any super sensitive fish or plants. Also it's a relatively small amount of cement (about 6" X 10" X 2" in a 38 gallon tank) compared to what some people use for a whole DIY background.
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Old 08-24-2012, 03:45 PM   #8
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Quote:
Originally Posted by tomfromstlouis View Post
I think you are making this too hard for yourself. Get a flat piece of tile or slate and just silicon or epoxy the rock to it. Best to have some kind of flat bottom on the rock (I used an angle grinder, your metal working buddies will have one), but maybe you could arrange some larger gravel and glob it into an appropriate support with the epoxy. It might take two steps, but each one would be ready in a day or two for whatever is next.
Yes this could work, I did not think of grinding off the bottom of the rock (which would definitely be necessary for this one) but I do think that would be quite a chore (with this particular rock there would be a fair amount of material to remove) and I'd lose some height.
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Old 08-24-2012, 04:12 PM   #9
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Well then create a cupped base with epoxy or silicon and large bits of stone. I do not like the cement idea at all unless you coat over it to keep it from leaching bads into the tank.
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Old 08-24-2012, 05:00 PM   #10
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Well... now it's 2 to 1 against the cement. Is there a specific brand/type of epoxy you recommend?
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Old 08-24-2012, 05:51 PM   #11
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I would use great stuff expanding foam. They make a black kind that is used in pond s. It is water proof and safe for fish. I have used it to make a background for a tank and I didn't have any problems with it.
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Old 08-24-2012, 06:09 PM   #12
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Thanks Mr. Catfish!
I did not know about that type of great stuff. Seems like a darn good solution. Adheres to stone and rock, fish safe, and cures in 8 hours. It's a little more expensive than just using some cement I had laying around and I might waste most of the can but maybe I could think of some other fun things to do with it.

I found info on it here if anyone else is interrested:
http://greatstuff.dow.com/products/pond-and-stone/
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Old 08-24-2012, 06:26 PM   #13
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I like the expanding foam idea too. Careful, it R E A L L Y expands!

You might test it with some other rock to see if it traps too many air bubbles to stay sunk. Doubt it, but possible.
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Old 08-24-2012, 06:28 PM   #14
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use some of that pink styrofoam and hollow out the center to support the rock. May not even need to silicone it.

I'm assuming its not top heavy. If so, you'll need weight at the base.
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Old 08-24-2012, 07:00 PM   #15
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Quote:
Originally Posted by Seattle_Aquarist View Post
Hi thesawguy,

I would not put concrete in my tank, the limestone and calcium will continuously effect the water chemistry.



However, I would consider using a big glob of pottery clay to hold the stone upright, especially if the clay is under the substrate so it isn't clouding the water.
People have been using Concrete in DIY backgrounds for a long time. Eventually they will cure and stop effecting pH. Soaking in water with water changes helps to speed this up.

Pottery clay will require a kiln to harden and dry it. Note the link says "high-fire" in the title of the page.....

I've made DIY cement backgrounds and had no problem at all. Just cure it. there is PLENTY of information on this process on the internet. "diy cement background curing" should give you plenty of info in google.
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