So I'm pretty sure that silicone latex would work since it's flexible enough to be pulled off your concrete, but Plaster of Paris wouldn't work because the connection is too tight and the plaster is too rigid.
So what else can you use to make molds with? The latex is too expensive, IMO.
Can you re-use greensand molds? I was hoping to make pavers, garden edging, and some stuff like that. And does anyone know if you can slip cast concrete if you make it watery enough?