vermiculite mix

[roost]

hi all finished covering my dome with vermiculite and cemt ent mix , reading all the posts it takes ages to dry ? what is my next stage , chicken wire over the dome the cement ovet the top and is this ok to complete while the vermiclite mix is drying ?

[downunderdave]

Jun 22, 2018 20:58:16 GMT roost said:

hi all finished covering my dome with vermiculite and cemt ent mix , reading all the posts it takes ages to dry ? what is my next stage , chicken wire over the dome the cement ovet the top and is this ok to complete while the vermiclite mix is drying ?

Because vermicrete requires so much water in the mix, about double the amount of normal concrete, after the hydration process ther is still a huge amount of free water left. As one litre of water creates about 1650 litres of steam it would seem prudent to eliminate as much as possible before rendering over it and locking it in. For this reason I like to leave it for at least a week, weather permitting, before doing any drying fires and rendering over it. Then to keep moisture in the outer layer for damp curing strength, wrap the whole thing in cling wrap for a week. This offsets the tendency for the dry vermicrete to suck moisture from the outer rendered layer. This is obviously not the only procedure that works, but i’ve Built many ovens using this method and it works well for me.

[roost]

hi again , so leave it for a week to dry
with the chicken wire over it for a week then
render over it before i remove ball , false door
etc ? , once the rendering is dry the make small
fiires inside the dome before cooking .. it will
be a long week waiting for this to dry lol

[roost]

forgot to ask also what mix did you use for
vermicullite ?

[oblertone]

It'll be an even longer week if you don't remove the ball first as no air can get to the inside of the dome to start the drying process ...

[downunderdave]

Vermicrete PHYSICAL PROPERTIES 2 copy.docx.zip (61.3 KB)

Did you use the vermiculite mix directly against the ball, or do you have a denser layer beneath it? If the inner dome is made with vermiculite in the mix it will not have much thermal mass to retain heat. This means the temperature during firing and cooking will drop off really fast if the fire dies down. In addition, any mix containing vermiculite will be significantly weaker depending on the proportion used (see attached table). This leaves the inside of the dome susceptible to knocks and abrasion damage. If you have used a dense material against the mould then you can ignore this advice. The general method is to have a dense inner layer for strength and thermal mass, over which a well insulative layer sits to retain heat.
You don't say what mix you used, but if you used normal cement then it requires at least a week of damp curing to attain reasonable strength. In fact 28 days damp curing is recommended to attain full strength, but a week is considered sufficient. If you used CAC cement it attains full strength in 24 hrs in which case you can proceed faster.

(there is an error in the table. The compressive strength for 5:1 vermicrete should read 175-225)