Curing grout/concrete

[bcreps]

How would one properly cure concrete covered styrofoam for either backgrounds or water features. I wanted to do a few layers of runny grout or concrete, then use nontoxic acrylic paints then seal it in several layers of polyurethane.

What do I do to cure it properly (ive read different things)?

Is that good enough for ponds/waterfalls?

Should I do a coat of polyurethane before the concrete?