Durham’s Rock-Hard Putty is a popular home repair product in the US, converting into a cream with water and curing after several minutes to a rock-hard filler; it consists of 70–80% plaster, 5–15% talc, 5–9% dextrin, <1% quartz, and <1% ochre. As noted in Derctuo, alabaster costs US$8 per tonne and calcines to plaster at 100°–150°. Presumably the dextrin is a thickener and the talc is to make it more thixotropic, though maybe it has some chemical effect.
I can’t find dextrin locally but food-grade maltodextrin costs US$5/kg at retail (AR$600) and carboxymethylcellulose is about US$9/kg (AR$1400).