Bricks, Masonry, Asphalt and Concrete - How long does the stuff remain 'plastic'

I just tried doing something with plaster of Paris. I setting a fiberglass fixture and using the plaster as a 'bed'. The idea is to lay down a mound of the stuff (while still plastic) and then set the fiberglass unit down on top of it.

I mixed up 15 pounds of the 'Plaster of Paris'. It took 10 minutes to get it all 'wet' and completely mixed. Before I had it mixed, the stuff started to harden. Before I could walk to the room where I wanted to use the plaster, it was nothing but a lot of little rocks. I wasted the whole batch of plaster.

Did I make it too wet? Not wet enough? Did I take too long to mix it?

The other option is a bag of mortar. Would this stay 'plastic' longer? If so, what are the mixing rules to insure 15 minutes of 'plasticity'?

Thanks,

Mark

I don't understand what, where, or why you are setting a fixture. The bed under a bathtub is usually joint compound poured from a container. Mortar mix will stay plastic for a while. Plaster of paris sets up fast, but you already know that.

Tell more about what you are doing, so that we can make some appropriate recommendations.

I'm setting a fiberglass bathtub-shower-enclosure.

I bought the thing a month or two ago. The thing is sitting in it's correct place, but the plumbing isn't connected, yet. While at a hotel, I notice the hotel bathtub was identical to mine. I didn't like the flimsy feel of the enclosure, so I decided to do some reinforcing.

I've used some urethane foam ('great stuff') for the enclosure walls, but was trying Plaster of Paris for the underside of the tub.

The idea was to 'tilt' the tub enclosure, build up a foam matrix on the enclosure back, lay down a mound of plaster under the tub and then drop the tub back in place. The plaster hardened to fast, so that part of the plan failed. The foam seems to have worked fine. The enclosure walls are nice and rigid.

All this said, I'm still interested in knowing more about mortar/plaster hardening time. I've done a variety of stepping stones. Sometimes they harden properly, sometimes they turn out very fragile. I don't have a clue what controls the way thing harden.

Mark

Many times a 5 gallon bucket of joint compound is poured under a tub to support it. It is cheap, easy, and effective.

Hope this helps.