Ok, so here is a few real newb questions. I just got my two 90lb bags of portland cement and however many bags of vermiculite weighing next to nothing for my hearth pour this weekend. Having never worked with vermiculite (again why I am sticking to the newb end of the pool here) I was expecting some thing a bit more . . . substantial. It is just hard to believe that 180 lbs of cement mixed with fluff is going to create a 4" x 76" x 76" slab capable of supporting much of anything, particularly a one ton oven and a quarter-ton builder. Does anyone know the compression strength of this mix? How long can you work it ? cure time? Just point me in the right direction.