Concrete: how much is too much?


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I see comments that concrete is deadly to fish. But also comments suggesting putting pumps on top of bricks (made of concrete, presumably). So are just a few blocks of concrete in a pond totally safe?

The main reason I ask is I'll be adding a lotus this spring and it seems I built the pond without a big enough, shallow enough area for one, so I will probably have to elevate the pot a bit. Any worries with using a couple of concrete blocks to accomplish this?
 
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Meyer Jordan

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I see comments that concrete is deadly to fish. But also comments suggesting putting pumps on top of bricks (made of concrete, presumably). So are just a few blocks of concrete in a pond totally safe?

The main reason I ask is I'll be adding a lotus this spring and it seems I built the pond without a big enough, shallow enough area for one, so I will probably have to elevate the pot a bit. Any worries with using a couple of concrete blocks to accomplish this?
Should create no negative effects in a 2500 gallon pond.
 
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@Waterbug good to see you. I'm relatively new here and remember seeing many of your older posts when I was doing my research and saying "I wish that guy was still around!"
 
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sissy

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The only reason I don't like concrete is you are locked on size with concrete if later you want to make it bigger .Seems that part is recurring theme here ,WISH I HAD GONE BIGGER
 

cas

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I had to double check the date of this thread when I saw a post from you @Waterbug . Thought you had left this forum - Glad you are back. I have read a lot of older posts and have really appreciated your insight on things.
 
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It is concrete dust, and calcified minerals in sand, or concrete that has not cured that is deadly. Totally cured concrete is super stable and will not leach, especially 100% Portland cement, that's the good stuff. You can always paint it with epoxy for added longevity, but then plant roots can't grip to it as well. I like to make it rough and porous beyond the veil. Pretty up top of course.
 
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Only because I find concrete fascinating...technically concrete never fully cures, the chemicals of concern remain in the concrete. That scares the heck out of many people unfortunately because of the "leaching" worry. The good news is there's another property of concrete that makes it so useful and that is that its surface reacts with CO2 in the water or air, kind of like rust but CO2 rather than O2. This reaction makes the concrete less porous at the surface which makes it more waterproof and also keeps the uncured worrisome chemicals sealed inside for the most part.

Acid in water and air slowly breaks down the "rust" layer which allows more CO2 react with the surface so the surface remains sealed. You can find lots of people promoting the use of acid on new ponds to "neutralize" the worrisome chemicals but the opposite is true. They're just burning off the protection layer exposing the very chemicals they're worried about.

Endlessly fascinating stuff at least to me.
 
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I understand the concern, concrete is predominately a calcium oxide-hydroxide/calcium carbonate conglomerate with a good dose of silicates added in. In the pond the calcium oxide-hydroxide are going to cause the pH to be higher, but it has very low solubility and is quickly converted to carbonate. If you were to make your pond out of unsealed concrete, there would be pH issues, but a few blocks to space plants it is unlikely.

Besides, if Meyer says something, it's a pretty good bet there's published scientific articles to back it up.
 
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Concrete has a ph of 8.3 and is not deadly to fish unless you drop a concrete block on your fish. It's no different than adding baking soda to a pond to raise the ph. There may be impurities and additives in the concrete powder and I've heard that there are additives in plaster of Paris that causes concern, but we have never had a problem with solid concrete or plaster of Paris brickettes.
 

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