You can get some quickcrete bags and spread it dry over wetened ground, mist with a hose, smooth with trowel and the next day you have a concrete floor. Not strong enough for floor supports and that kind of shit but 3/4 inch you wont know it's not 4". With open earth inside i've been told radon is a possibility but not sure how much truth there is to that. If anything a smooth floor that can be painted and then washed is such an awesome benefit.