Pond foam to seal space under water?

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I have a small space under some bricks and concrete that my intake and return pipes pass under that I would like to create a watertight seal for a water feature I am half way done with.

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I do not want to be a permanent blockage so I do not want to use concrete or mortar to fill that space. Ideally it will be something I can easily removed so if I need to service the pipes they would not be constrained.

Is pond foam the product to use here or will pond foam break down with constant submersion and sun exposure?

How is the pond foam different from normal construction expansion foam from a hardware store?
 
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The foam products are all the same - pond foam is gray to black just to make it easier to conceal. Remember though it is not water tight - it will divert water but not prevent all water from encroaching. So if no water entering is your goal, consider using a small piece of rubber liner as a backer and then foaming the liner into place. That will be at least 90% better I would guess.

As for easy to remove - indeed. You can easily cut or hack it out.
 
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Sorry to say but not even concrete and mortar will seal around those pipes for long. The reason it WILL FAIL is expansion and contraction. The pvc will not expand the same as concrete and it will leak eventually or even within an hour you never know.
Waterfall foam absolutely not. The best picky mouse seal I can give you is try the waterfall foam then coat it with a good coat of caulking but more like your applying a complete coating.not just a bead. I would cut the tube open and use a putty knife and ice cubes. Yes ice cubes the caulking will not stick to the ice.
 
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I haven't tried the foam yet.

But I don't think I need 100% watertight. I experimented today with some old socks and torn tee shirts rolled up into plastic bags to plug up the space, I didn't try too hard, but did enough to allow the area to be plugged enough to get the water level to rise over the brick dam and spill over for about an hour.

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The foam products are all the same
I wonder how true this is. I was trying to decide which foam product to use and got very confused.

TOTALPOND pond foam sealant according to Amazon's product specifications requires the surfaces to be completely dry before application, takes 12-24 hours to fully cures, but need to wait at least 48 hours before getting it wet. I wonder once it fully cures, what is still going on from that time say 24 hours to 48 hours that it can't get wet. Yet strangely enough, I also read all the Q&A questions, where in one of the answers TotalPond support said if the cavity is greater than 3 inches, moisten the surface before application. Huh? So it needs to be completely dry, but you should moisten it if the cavity is greater than 3 inches?

GREATSTUFF Pond Foam from Ace Hardware says it can be applied to damp surfaces and it cures in 8 hours.

AQUASCAPE waterfall foam sealant in a video says once applied, ready to run water in 15 minutes.

I wonder if these are indeed formulated differently waiting 15 minutes vs 48 hours is a huge difference. I am in south Florida, it rains every day, I can't really keep the spot bone dry and even if I drain everything and wipe the surfaces down, the bricks and concrete absorb water into the material so it will never be dry completely.

Or could be just the same stuff and up to different manufacturer's marketing gurus to put up different product specs and some are more aggressively worded than others?
 

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