You may have a case against the supplier; however, in my experience, 16 oz of retarder & 16 oz of mid-range, should not cause concrete to delay its set until 15 hours - not at 96 degrees ambient at least - not even with a relatively high percentage of fly ash or slag (8 to 12 hours would be feasible). The fact that the slab dusted typically means one of two things... 1. the concrete set there too long and continued to bleed instead of setting up. or 2. the mix was too wet when placed (that could be the fault of the supplier or the finisher) Unfortunately, it is difficult to prove/disprove why concrete did not set up quickly enough. Wiss, Janney, Elstner can help determine this petrographically, and possibly with the use of chemical extractions for actual admixture dosage, but these tests are usually reserved for high-end projects where their costs can be justified. The questions I would ask are "What type of cement was used?" "Was fly ash or slag used and how much?" "What was the design slump and what slump was the concrete placed at?" "Was the full slab still soft at 15 hours, or did it just have a wet surface?" I hope this helps - Good luck!