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TLDR: Does urine make concrete hygroscopic enough to absorb moisture from the air so much that a spot of humidity is visible? Are uric acid crystals moisture-absorbent? (Urea apparently is according to Wikipedia)

Here is the story:

In my parents' house, there is a mystery that we were unable to solve. During the renovation, certain spots on the cement floor appeared wet (used to be covered by glued carpet). Even worse, some spots appeared to evolve and sometimes grow. We assumed the worst. Since this was the ground floor (there is no basement) we thought groundwater might be infiltrating the house. One particularly intriguing spot was near the hinge side of an interior door. The door is in a wall of cored brick next to an interior concrete pillar. The spot on the floor was next to this pillar.

Several investigations have been made:

  • A dig outside the house to check the depth at which water could be found. It was deep enough.
  • Checking if there is hydro-isolation on the foundation. There was.
  • Checking on the outside of the base of the house for humidity. It was dry.
  • Checking the heating pipes buried in the floor. They were nowhere near and eventually the in-floor pipes were abandoned.

All of the people who investigated (engineers, constructors) were mystified as the spot kept evolving even though we properly ventilated the area for a significant time. Even more puzzling, the concrete pillar was dry. Eventually, believing there might be a spring under the house (and that the pillar was ok due to the hydro-isolation on the foundation) we broke the first layers of the cement floor. Underneath the first layer, there was a layer of extruded polystyrene thermal insulation. Underneath that, a hydro-isolation layer. And underneath that the concrete slab of the floor. All were dry. At this point, all other people just gave up and some even suggested we call an exorcist.

However, this meant a return to one of the first theories about these spots. In the house used to live several cats and unfortunately, there have been plenty of times when some of them peed on the carpet floor. The spot near the pillar was one of the preferred places. This was one of the reasons for completely removing the glued carpet. This theory was initially rejected because the house was ventilated for more than 2 months with the carpet removed and the cats long gone. During this time and more the spots evolved and grew. We believed that since the spot grew noticeably (we traced the edge) and since it didn't smell it could not be because of the cats and it had to be from below (of above, but the roof and the ceiling were ok so we eliminated that possibility).

Now with layers below also dry, the only possible source of humidity left was the air. So, does (cat) urine make concrete hygroscopic enough to produce a visible wet spot?

I have searched Google and Wikipedia about the moisture absorbant properties of uric acid but I have not found anything relevant. According to Wikipedia, urea is hygroscopic and highly soluble in water. I decided to ask this here since it is about the physical properties of a chemical compound.

The closest I have found is this:

https://pubs.acs.org/doi/10.1021/ja00836a027

What makes these particles hygroscopic?

Salt + Sodium Bicarbonate releasing water in carpet

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  • $\begingroup$ Urine is not only urea and water, but there are some colored compounds present, too, e.g. from bile which sometimes is recorded as billirubin. Concrete is porous, so by capillary action water may ascend and travel; water may partially evaporate (thus increasing the concentration of the dyed compounds) till it is visible to the eye. If aiming for repair, perhaps SE DIY/homeimprovement (diy.stackexchange.com) is more suitable, then ChemSE. $\endgroup$
    – Buttonwood
    Jan 25 at 17:31
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Very interesting question - and thorough. But I would like to get proof that the spot is due to moisture. #1 Does the spot grow and shrink dependent on the relative humidity at that position? #2 If the spot is misted with a water sprayer, is it sharply defined? #3 If the spot is gently warmed with a space heater, does it disappear? #4 What color is the concrete? Very light (low iron), medium gray (medium iron), or dark, or brown?

If multiple cat pees have been located there, it's possible that attempts at cleaning or deodorizing have also been applied to the carpet, and have soaked thru and into the concrete. At this point, so much could have happened to the cement compounds that you and your parents should consider yourself fortunate that the deviation from perfection is only visual and not oderiferous! I have a spot on an outside patio that is dribbled upon from an aluminum gutter, only occasionally, when it rains (the end cap on the gutter has a tiny leak). The spot is about 4 inches in diameter, and is only visible at certain times (usually high humidity, like when rain has almost completely evaporated). Without a microscopic examination, I can't see anything unusual, but I suspect that some of the $Ca(OH)_2$ in the concrete has been dissolved, or perhaps carbonated more, or both, and the water in the pores at that spot is slower to evaporate.

If the discoloration is obnoxious, you might try painting (or dusting) that area with a non-paint paste of calcium carbonate in water. It would be harmless to the concrete, but provide some lightening. But then the area might be too white...

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  • $\begingroup$ Concrete exposed to air has tendency to catch humidity, regardless of additional exposure (e.g., youtube.com/watch?v=17wnf7ehcZw). $\endgroup$
    – Buttonwood
    Jan 25 at 17:24
  • $\begingroup$ The Youtube video is woeful. I learned that concrete is acidic and corrosive to steel, and a battleship in water is supported indefinitely, whereas a battleship placed on concrete (dry land) will fall down ("pancake") because its sides are not supported. I do believe concrete exposed to water will be somewhat wet, but I did not learn this from the video. $\endgroup$ Jan 26 at 19:43

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