Joining new perlite mix to Old.

[KJB]

I have run out of perlite part way through plastering my gym ball oven. Part finished now under wet towel and being allowed to cure slowly. The ball is mostly covered but in some places only about 1 inch instead of the 2 to 3 planned. I want to let the job to date harden off and then add more perlite mix when I have it.
I see people recommending to wet of and let dry off to moist before adding the new mix. Is this the solution or should I also add Portland or Lime into a wash to add strength to the join?

[david s]

You don't say what ratio of perlite you used, but because the stuff is so water absorbent it takes ages to dry, so you probably won't need to redampen it. The addition of perlite to any mix seriously weakens it, particularly if it's thin. The table gives strengths of different mixes. By contrast brick or dense castable are around 3000 psi. In addition lightweight aggregates like vermiculite or perlite reduce thermal mass, a desirable quality so the oven retains heat.

[KJB]

Thank you. It is a 5 to 1 mix and still well damp. Will just continue to build up without any wetting or slurry.

[david s]

5:1 is not very strong, be careful when you de-mould and move it or you'll break your casting