# Background Concrete Questions



## Eb0la11 (Feb 29, 2008)

Hey guys, so I finally am planning to concrete up my background. Its been a couple months but I think Im finally ready.

So I got some questions.

- What experience when applying the concrete can people give me for those that have done it? I have some details in my styrofoam I'd like to keep so thin layers of concrete interest me.

- What kind of concrete should I use? I cannot find quickwall quick crete so what stuff would be best for under water that is aquarium safe?

- What kind of sealing do I need to apply to the concrete? When do I apply it?

- I'll have styrofoam on the backside, what do I need to use to seal this off?

- How long should I let the concrete cure inbetween coats?

- What should I do to ensure a nice hard cure? I've heard keeping it wet is good. I was thinking of laying a damp towel on it while at work and spritzing it when I get home daily.

- Is it possible, once I've got a thin layer of concrete (3 mm or so) to melt away some areas of styrofoam and fill in this now "shell" of concrete?

Thats all I got right now. Any info provided would be great! Thanks so much in advance.


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## Eb0la11 (Feb 29, 2008)

Anyone?


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## iceblue (Apr 27, 2004)

> What kind of concrete should I use? I cannot find quickwall quick crete so what stuff would be best for under water that is aquarium safe?


Just about any mortar mix will work. Some of them have fiberglass mixed in them to to prevent cracking and are a better option. Avoid anything with a high lime content such as stucco mixes. I prefer mixing my own using type II Portland cement and sand.



> What experience when applying the concrete can people give me for those that have done it? I have some details in my styrofoam I'd like to keep so thin layers of concrete interest me.


Unfortunately, the mortar will cover any small detail you have carved into your styro. Detail work should be done in the cement itself with the styrofoam just giving you a basic shape to work off. I would recommend a minimum of a 1/4" to 1/2" to cover and give a solid front for your project. Any thinner and you run the risk of it flaking off or cracking. Another option is to just paint the whole thing with Dryloc if you don't want to lose your fine detail.



> Is it possible, once I've got a thin layer of concrete (3 mm or so) to melt away some areas of styrofoam and fill in this now "shell" of concrete?


Again, imo this is just too thin a layer to be effective and the dryloc would be a better option although you would not be able to melt the styro out from behind it.



> What should I do to ensure a nice hard cure? I've heard keeping it wet is good. I was thinking of laying a damp towel on it while at work and spritzing it when I get home daily.





> How long should I let the concrete cure in-between coats?


Others have had success on styrofoam by applying a slurry coat first. Just apply it with a brush as you would paint. Allow it to dry and apply your first coat of mortar mix on top of that approximately a 1/4" thick and do your rough-in details. After this has hardened I would apply another 1/4" coat adding even greater detail. Any thinner areas you have should be minimized and surrounded by thicker areas. You don't want a thin area that would go from say, top to bottom or side to side. Especially if you plan to remove the styrofoam at a later date.

Keeping the mortar moist helps keep it from setting up too quickly and reduces the chances of cracking. Keeping it covered with a damp towel (not soaking wet) and spritzing the structure with water will help get a more even cure.

I used a cold spray humidifier to keep the moisture high when I built my background in tank. It worked really well to keep things from drying out while I was at work or sleeping.

Other then the slurry coat you can test whether it's ready for your next coat with your fingernail. If you can push it into the mortar it's not ready. You should be able to barely scratch it before applying your next coat.



> What kind of sealing do I need to apply to the concrete? When do I apply it?


It's really not necessary if you plan a long leaching process and/or are going to do a fishless cycle. Once the "concrete" is covered with bacteria, algae, etc... it will seal itself.

If you plan to color your project you can use a liquid concrete polymer such as SikaTopseal mixed with thin-set and coloring and paint that over the structure to seal it further.



> I'll have styrofoam on the backside, what do I need to use to seal this off?


Styrofoam is inert and doesn't need to be sealed. As a matter of fact I would recommend not sealing it if you plan to silicone it to your tank.


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## Eb0la11 (Feb 29, 2008)

Thanks again IceBlue. I'm trying to think of a way to make my situation work. Still isnt an option to silicone in place.

I'm thinking my most likely option is get a layer of concrete on the outside of it, melt away parts of the styro, clean it up, pour in more concrete. I'm still kinda mulling things over though.

What kind of product is drylok? I couldnt figure out how to describe it other than a styrofoam sealant to the guy at the store and he hadnt heard of it. Is it sold in Canada??

Also, they didnt have cement tint lol. I thought that was kind of funny for a home depot not to have that. Guess I'll have to go to a specialty cement place?


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## Eb0la11 (Feb 29, 2008)

The liquid concrete polymer might be my best option for painting it. I'll definitely be painting it.


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## iceblue (Apr 27, 2004)

If you melt the styro out you could use a lightweight concrete mix like I used in my project to fill in the void. The recipe's on the first page. http://www.cichlid-forum.com/phpBB/view ... highlight=
You wouldn't need anything to attach the background using this method.

The Drylock is a brand name for a masonry paint. http://www.shop.com/+-a-drylock+paint-p ... 6-st.shtml

Strange Home Depot wouldn't have concrete coloring. They normally carry the liquid type somewhere near their bagged concrete products. It would be on the same shelves as their other concrete additives.


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## Eb0la11 (Feb 29, 2008)

So I did get some quickcrete as HD now had it.

I might do some tests with the concrete and styrofoam. I'll probably make a block of cement and see how much styrofoam a 1 inch thick 6"x6" block of cement can hold submerged. Then I'll do a rough estimate of how much cement/styro I'll need to have as a "balance" point to my project. That way its as light as possible while still staying submerged. I'm not sure if this'll work yet but theres a good deal of my styro I can melt out.

I think Im going to opt to use a torch to melt out the styro instead of acetone. What do you guys think? I think I'll have more control. It'd be a small pocket torch, not some honkin welding torch. This way I could be very intentional with the sections I melt out. Then I can pour in the concrete to fill the void and just not let it protrude further than any remaining styro so that the outsides of my BG that will be against the glass will still be soft and not scratch it up.

I still havent really figured out how Im gonna be able to paint or dye the concrete. I'll have to make a return trip to HD and see if they have any masonry paint.

Hopefully Im ready to go for the August 30,31, sept 1 long weekend. This is when Im planning on doing most of this work.


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## Mcdaphnia (Dec 16, 2003)

Fumes from burning styrofoam are toxic, and you will have much less control with a torch than with acetone. Toasting or scorching the foam may cause chemical changes producing a lingering odor. I used to kill hornets by making styrofoam frisbees and lighting them just before winging them toward the nest. If these landed on the ground, that soil would not support any life, not even grass or lawn weeds into the second growing season. I finally dug up the affected areas and replaced all the soil there before anything would grow again. I would not burn or melt styrofoam with heat or flame after that experience.

The acetone is volatile and evaporates away leaving the styrene the foam is made from chemically unchanged. More control and no chance of accidentally making anything banned by the Geneva Convention.


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## Eb0la11 (Feb 29, 2008)

Hmmm that is interesting to say the least. I thought it'd have less of a chemical change than acetone would. I'll have to do some tests or something and figure this out but Im not leaning towards acetone. I bought a 1 litre container already.

Side note. Im going to look for some masonry paint today. Im thinking of getting a black, white, maybe green and red depending on pricing. I'd like some redish/brownish colorations similar to this pic:

http://www.renderosity.com/mod/rrfilelo ... 1&key=5139


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## Eb0la11 (Feb 29, 2008)

Anyone know if Behr masonry paint is aquarium safe? Thats the only stuff that Home Depot has... I still dont like the idea of painting my concrete as much as tinting it


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## iceblue (Apr 27, 2004)

I don't think it is but I could be wrong.

Have you checked with local concrete suppliers or contractors to find out where they get their concrete coloring? It would be in a liquid or powder form. I would also look around in other home improvement stores besides Home Depot. Somebody has to have it.

This is what I used for mine. http://www.homedepot.com/webapp/wcs/sto ... 036+502846

It comes in 4 different colors and can be mixed and thinned to achieve the colors your looking for.


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## Eb0la11 (Feb 29, 2008)

That link didnt work for whatever reason. What was the product called?


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## iceblue (Apr 27, 2004)

Quikrete liquid cement color. Try this link. http://www.quikrete.com/ProductLines/CementColor.asp


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## Eb0la11 (Feb 29, 2008)

Ok so I'm hoping to do my first layer this weekend. I'll be doing a bit of a slurry mixture and it'll be pretty thin as people have said to do. After that layer and I good to make it a bit thicker or whats the best method?

I'm going to try to get the tint for my last layer or two. I havent had any luck so far which is super annoying but I havent tried any specialty places yet.


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## Eb0la11 (Feb 29, 2008)

Ok so I put on my initial layer on 1 of 3 sections of my BG tonight. I think *** got the wrong quikcrete mix.

Am I suppose to have topping mix?

I had to sift out a lot of the pea sized pebbles so I just did one section and will see how it dries.

Should I start spritzing it tonight or let it sit for a bit before I spritz?


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## iceblue (Apr 27, 2004)

If it has gravel in it it's the wrong mix. This is what is normally used. http://www.quikrete.com/ProductLines/MortarMix.asp

You should start spritzing after it has set up enough that the cement won't wash out. It will still look wet but not be soft and will absorb the water you spray on it.


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