Broken concrete is good aggregate for new concrete. The challenge is breaking it into smaller pieces. I have used it to pour a new walk; I just made sure no part of the broken stuff got any closer than about one inch from where the new surface would be. My walk has very low loads in service . Uses that have some stress like a pond would require the old concrete be broken to roughly 2" max size gravel.Also when some stress is involved , rebar and maybe screen are essential to give the concrete some strength. If the pond is shallow ,like 6 ", you don't have to have steel . But at a typical 3' depth , the water force on the bottom will be about 186 pounds per square foot ( over 3000 lb total on the bottom of your size). That is no problem where the concrete rests solidly on the soil but differential settling of the soil will leave pockets where the concrete must bridge a gap. Those are the spots you need the rebar to strengthen the concrete. Then there are pond considerations like : drain piping, filtering ( a location in the pond works well) , over flow ( from rain) , etc. Concrete must be aged and rinsed before fish can be put in. Unless you are interested in fish , I would discourage your pond idea ; maybe make a patio with the concrete.