Maybe I'm confused, or reading too much into this, but from what I gather, you paid 55-60% of the total BEFORE any work was done, and you paid it, part or whole, in CASH????? Do you have any proof that you paid the contractor the $6K? Did the fact that the contractor wanted this much money down throw a warning flag? Did the fact that he wanted cash throw a warning flag???
Personally, I'd take a free consultation with an attorney before I paid anyone anymore money. Regardless of decisions you may or may not have made, the contractor needs to make the project whole and correct the problems to your satisfaction before you shell out anymore dough. Once you make final payment, don't expect any co-operation from anybody, they're guaranteed to "cut-an-run".....
Also, you REALLY NEED to understand your state specific mechanic lien laws before making a payment. Here in WI, you'll get a warning shot in the form of an "Intent to Lien" via mail from the ready mix supplier, likely between Jan. 5-10, if you haven't already received one of course. From that point, the supplier can't put the physical lien in place on your property for at least 90 additional days. So, this would give you quite a bit more time to figure this out. Be educated on how the system works, and don't let anyone scare you into paying before you're satisfied.........