Just wondering if anyone knows of any plaster additives to strengthen it so it's not so fragile...perhaps some type of adhesive?

Depends on the size of the part.
I have used pva glue as an additive for thin section otherwise it's best done the same way as concrete, use some kind of reinforcing material. A small mesh like insect screen might work, otherwise adding chopped fibreglass strands(without binders) could also work.

The other option is to use a stronger plaster, like hydrocal.

I've use PVA glue (white glue) to cast some wheels and they were solid as rock. I even had some hard times trying to engrave the tires tread pattern.

Olivier

Olivier, you have piqued my curiosity. Engraved treads? How did it turn out?

Hello - I'll second the comment on the use of hydrocal. It is pretty hard, yet quite fine, so you can capture any specific details in the mold you might have. I typically add a few shots of Vallejo paint while I'm mixing it. While I can't honestly tell if it strengthens the final product, it does help to tone down any cracks or chips that may occur during your build - which otherwise would be bright white.

I've built a few model building foundations with hydrocal - which were pretty big. To avoid cracking, I've added florists wire to/within the formwork - and just like rebar, it does a very good job holding the parts together. Depending on your level of patience, you can run the "rebar" ends long (outside of the form ends), or intentionally not make your pour complete in key places to represent weathering or destruction. While a bit tedious the results can be really nice. If you get really ambitious, you can make a grid lattice of rebar for a slab, and when it's hard, intentionally crack off a corner etc and pick around with a tool, and you'll have what really looks like a broken slab or floorplate - the tricky part is getting the lattice of "rebar" to be set in the center of the depth of the slab - and not to wind up poking above the finished surface, or accidentally falling to the bottom.

Hydrocal can be used to represent big slabs, pavement surfaces, or floorplates too, but I have found it to be a bit of a challenge to "float" or level - it's not easy to make large, uniformly flat surfaces. But, by integrating color (the paint you add to the mix), you can sand the surface flat and maintain a nice, uniform color. Imagine pouring pancake batter over the paved surface of your diorama! It can be nerve wracking! That said, it can look pretty good when you're done.

Good luck

Nick

I use Durham’s Rock Hard Water Putty.
Sometimes mixed and sometimes by itself (if its a small area as it semi pricey).

http://www.waterputty.com/creative.html

In the past, I've finished the plaster parts, sanded, etc., then used cheap Autoparts resin for fibre-glassing, & just dripped the yellow resin on the plaster, which absorbs it & hardens...

Good even after 30+ years....