I uses plaster of paris for silicon mold.there's no problem when making brick,cobble stone or paving slab,but when making manhole,drainage (both from dioramadebris),the plaster tend to crack.is there anything stronger than plaster fof paris, or should i add something,mix together with plaster ?

Hydrocal/ Hydrocal 30. Just like plaster but much stronger

Adding some white glue to the mix will make it stronger. Not tried yet but they say a few drops of lemon juice avoids cracks.

Yes, adding white glue hardens the plaster, I made a stone wall by that way, it's very hard and doesn't brake easy.

Cheers!
V

thanks!