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Concrete underwater.

Kelvin12

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16 Nov 2020
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NSW Australia
Can't find anything about this in a search but what are the thoughts about using concrete underwater. I am thinking of sealing it some sort of water proof paint. Not a massive amount but about 200mm square or a bit more. I need a small plith for under a very old pumice statue.
Any thoughts please.

Dirk
 

Simon Cole

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25 Dec 2018
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Buckingham
You will get a bit of carbonation when the water reacts with the concrete over time: CO2 + Ca(OH)2 → CaCO3 + H2O. It was more of an issue years ago when people seldom changed their aquarium water and would use considerable amounts of concrete décor and might observe increased alkalinity. Many of us struggle to get enough calcium to our plants anyway if we have softer water. The two most common paints for waterproofing concrete in construction are resin-based or chlorinated rubber coatings. Cured cement takes around 10 days.
 
Last edited:

seedoubleyou

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29 Mar 2022
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Windsor
Can't find anything about this in a search but what are the thoughts about using concrete underwater. I am thinking of sealing it some sort of water proof paint. Not a massive amount but about 200mm square or a bit more. I need a small plith for under a very old pumice statue.
Any thoughts please.

Dirk
Should be completely fine, I don’t think there’s even a requirement to seal it.
 

alnitak

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9 Feb 2022
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France - Essonne
Hello,
I would seal the concrete. In the manufacturing process of cement, many additives are used, that may leach in water (grinding agents like triethanolamine acetate for eample). Not only additives, but also mineral fillers (residues from mineral industries). I know that concrete is commonly used for water supply piping, and a lot of tests are carried out on cement and concrete for acceptation. But, the main problem is : from where does my cement come from ? For water supply, manufacturers of pipes have very strict specifications on the composition of the cement. But is it the case for the cheap cement bag you buy for DIY applications ? I'm quite sure not...
 

seedoubleyou

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I’m fairly sure that the aquascaping cement branded and sold by many aquatic companies is just Portland cement no?
 

alnitak

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I agree, cement itself is not an issue, the problem is what could be inside the cement.... Portland is a kind of cement, a manufacturing process, it dos not grant the fact that there are no additives or fillers or other heavy metals inside that could leach into the water.
Maybe it could be safe... My opinion is maybe not completely objective, because I've been working for years in the construction materials and more precisely in the field of the cements and concrete additives... And I swear, I would never put one piece of concrete in a tank, unless I know precisely what is inside the black box :)
 

seedoubleyou

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29 Mar 2022
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Windsor
I agree, cement itself is not an issue, the problem is what could be inside the cement.... Portland is a kind of cement, a manufacturing process, it dos not grant the fact that there are no additives or fillers or other heavy metals inside that could leach into the water.
Maybe it could be safe... My opinion is maybe not completely objective, because I've been working for years in the construction materials and more precisely in the field of the cements and concrete additives... And I swear, I would never put one piece of concrete in a tank, unless I know precisely what is inside the black box :)
So to clarify for all us non build types.

Cement and concrete - different things

“Raw cement” will be fine for an aquarium (use at your own risk though)
 

alnitak

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France - Essonne
use at your own risk though
That's it :)

Besides leaching nice chemicals, the main issue with cement (and concrete, which is almost the same) will be the pH. Very alkaline, and it will last for a long long time. Maybe it is not an issue on reef tansks, but in fresh water it could upset everything.
 

dw1305

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nr Bath
Last edited:

Simon Cole

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25 Dec 2018
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Buckingham
Having read these comments I would have to agree, the impact on water quality will be somewhere between no impact at all and fully saturated like Lake Tanganyika.
 

Ehcosbie

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24 Mar 2021
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45
Location
Cornwall
So to clarify for all us non build types.

Cement and concrete - different things

“Raw cement” will be fine for an aquarium (use at your own risk though)
Concrete is made with aggregates, water and cement. Raw cement is a powder that crystallises when hydrated with water. If these crystals form around aggregate, concrete is made. Otherwise you would have a brittle solid that has low structural integrity when compared with concrete.
 
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