That is an interesting use for piss, really anything that forms crystals to encase another particle can be called concrete, it is great when they find uses for it.
For the 12,000 psi mix you are looking to be in the 20% range for the water cement ratio, which is definitely doable with some superplasticizer. the standard mixes will fall between the 40%-60% range for workability but that will give you your 2500-4000 psi mixes with 3"-7" slumps. You can add some air entrainment to that to get a higher slump but too much will hurt strength. The Super Ps are the real way to go without the addition of other range pozzolans. One of my old graduate students made a flyash mix with plasticizer for eccentrically loaded column research and he made some 4x8 compression cylinders to have that data. He did his 3, 7, 28, and 56 day tests then left the rest for another year. His 56 day numbers were around the 12,000-15,000 psi range. The ones we tested the year later were closer to 30.000 psi, as fly ash continues to cure for a much longer duration than portland. It was funny, everytime one broke it sounded like a shotgun going off, and the machine ended up walking a few inches when the sudden unloading of 400,000 lbs was happening.