Limestone Sculptures

I do not know but you could try hydrated lime (type s lime from the building stores) mixed with sand and a little water. I think fine masonry sand 2 portions to 2 portion lime. mix it dry and then add water until it is stiff like bread dough. Then put it in your mold. But it will take ages to cure. It cures wet or moist by absorbing CO2 from the atmosphere and turning into limestone.