There is heavy lime buildup in the water trap of my toilet bowl. Does
anyone know the chemistry?

The toilet has a four gallon flush tank and I am reluctant to pull a
flush for a simple pee. My solution is to fill a one gallon bucket
with water and flush that. There will be a trace of yellow urine
remaining but it is dilute enough not to smell or look gross. I am
the only occupant so toilet traffic isn't that heavy. The trouble of
course is that after about six months the lime buildup is bad enough
that I have to use a teaspoon (curved surface) to scrape off the lime,
followed by steel wool and a scouring pad. What is there in diluted
urine that will cause the lime to to precipitate out of solution and
stick to the porcelain?