pouring the hearth question??

[Neil2]

"This is because the perlite/vermiculite mixture absorbs so much water when its being mixed and it takes days to dry out."

Vermiulite concrete and other concretes require much more water that was used in the original mixing to cure. Within 2 days or so, it has used up all the water included in the mixing and has only reached 20-30 % of its potential strength. Concrete should be provided with unlimited access to water when curing for at least 6 days, preferably longer. A practical limit to its strength is reached only after 21 days or so.

Concrete does not "dry" it cures. - if you allowed to become prematurely dry, you will halt the formation of the microscopic crystals that give concrete its strength. The formation of these crystals will not fully restart if you subsequently add moisture to the dried concrete.

If you are using plasitic, make sure to add water under the plasic from time to time.

Some of this might seem a bit pedantic, but I believe we should try to use the correct technical terminology to avoid confusion.