DIY cement 3D background curing advice

[Tool13x]

I have just recently attempted a DIY 3D background made from polystyrene and coated in cement, basically the same idea as you see here from the King of DIY. The construction went well and I am in the curing process but I am on week 6 and the pH is still rising. I let the cement cure for a few days and then filled it with water and have been doing water changes about every 3 days for the past 6 weeks.
I have been doing lots of research and it seems like this is a pretty common project and also a pretty common problem. I have seen claims from 1 week to 2 months before the pH stabilizes and I am just looking for some advice to help this along. I read that adding salt can help stabilize the concrete so I tried adding salt for the last 2 water changes ( about 10 days total) and it didnt seem to have any effect, when I went back to freshwater the pH started rising again.
I have read adding vinegar or adding an air stone for the cement to react with CO2 can help stabilize the cement, any experience with these methods?
I am willing to give this a few more weeks but I am getting antsy to get some fish in this tank. I am beginning to think I may be forced to seal the cement, epoxy resin seems to be the most common product. Although I really dont want to do that, I would much rather the cement be cured and stable as is.
I would appreciate any suggestions.

 

[Colin_T]

I haven't put concrete on foam but concrete fish ponds are generally always alkaline due to the lime in the cement.
Adding salt would not make any different because sodium chloride raises pH. I would just seal the polystyrene and be done with it. Either that, or keep Rift Lake cichlids or livebearers

 

[IHaveADogToo]

What about using tannins to lower the pH?

 

[seangee]

Cement is very alkaline and will stay that way. If you want to use it you will have to seal it.

 

[Colin_T]

Tannins won't drop the pH, they simply make the water turn brown. If you have enough rotting leaf litter in the tank it might lower the pH a little bit, but you would need a lot of rotting plant matter and that would probably affect the fish. Sealant is probably the easiest solution

 

[Tool13x]

I would just seal the polystyrene and be done with it. Either that, or keep Rift Lake cichlids or livebearers

Funny you say that. I was considering a Tanganyikan setup because of this mess, too bad its only a 30gal otherwise I might go for it. Shell dwellers could still be an option i suppose...

Cement is very alkaline and will stay that way. If you want to use it you will have to seal it.

That was what I thought originally, I must have read more than a dozen times that it will eventually cure after a couple weeks and pH will stabilize. If this is the case then I really wish I would have sealed it before installation. Being siliconed to the back of the tank is going to make it a much more tedious process.

 

[Colin_T]

I was considering a Tanganyikan setup because of this mess, too bad its only a 30gal otherwise I might go for it. Shell dwellers could still be an option i suppose...

Julidochromis species or Cyprichromis leptosoma or Paracyprichromis nigripinnis would fit in a 30gallon tank. Just make sure the water is hard enough and you're good to go

 

[Tool13x]

Well this is a learning experience for sure, I was toying with the idea of trying to seal the cement with an epoxy or other waterproof sealer and decided that wasnt the best route. With the background siliconed to the back of the tank it would be pretty difficult to make sure all the crevices would be sealed. I also discovered that the reason the water was so alkaline was because the concrete was dissolving where it was very thick and had gathered in the cracks between the "rocks" . I think this is because I had mixed it very thin so I could paint it on easier and the thick areas never cured correctly. Where the concrete was only 1/8" thick everywhere else it had cured fine. Unfortunately I decided to cut my losses and tear it out of the tank, I bought a manufactured 3d background to replace the DIY. Maybe on the next tank I will take what Ive learned and try again. Thanks for everyones suggestions. I will upload some pictures as soon as I have a chance to dowload my albums.

Suggestions for anyone trying this:
1) Make sure the concrete is a fairly uniform thickness of about 1/8" throughout the background, do not fill in cracks and gaps like I tried doing.
2) I wouldnt use hydraulic stop cement even though many people suggest it, the working time stated by the manufacturer is only 2-3 minutes and that is not nearly enough time to apply a quality uniform coat.
3) Quickrete quick setting cement has a 15 minute working time and is much easier to work with.
4) Do not mix the concrete too thin when brushing on. A thick milkshake consistancy will still brush on ok and will cure much better.

 

[StevenF]

I would take it out of the tank and let it air cure. Cement is basically calcium oxide. When mixed with water it reacts with CO2 in the air and water and is converted to Calcium carbonate. If it is in the tank it has basically removed all the CO2 it can from the water and enough isn't getting in to convert it to carbonate at a reasonable rate. The exterior has cured but the interior has not. Util it cures the PH will stay high. However once it cures it will keep your water PH at 7.

 

[Tool13x]

I would take it out of the tank and let it air cure. Cement is basically calcium oxide. When mixed with water it reacts with CO2 in the air and water and is converted to Calcium carbonate. If it is in the tank it has basically removed all the CO2 it can from the water and enough isn't getting in to convert it to carbonate at a reasonable rate. The exterior has cured but the interior has not. Util it cures the PH will stay high. However once it cures it will keep your water PH at 7.

I agree, that would have been a good step to take, unfortunately that ship has sailed. I did let it cure for about 5 days before I filled it with water but in hindsight It probably could have used some more time and it might have prevented this mess. When I removed the background I found some pretty big gaps that had opened up from where the uncured concrete had dissolved.

 

[Tool13x]

Here are a few progress pics of the background before the curing disaster.