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: