Re: Sculpting with cement
Ditto what AD says about the salt. But a little clay's not a problem; most mortar mixes have some fireclay in them to impart plasticity. Sharp sand is best for building up directly; round sand flows better for casting. A little gypsum plaster will make the concrete "kick" faster, which can be helpful at times. I didn't know Hydrocal or Hydrostone contained Portland cement; if they do it can't be very much, since they act a lot more like plaster than concrete. USG's Gardenstone has quite a bit, so it lasts better outdoors.