G'day James, and thanks mate.

[jeff]

I'm a mug wrt websites, so advance apologies for breaches of protocol.
First up, biggest thanks to James for both this site and his patience. (He knows, believe me, He knows.)
Fellers, I'm near Bundaberg, Queensland, Africa.
(Kidding eh. More like - ahh, doesn matter.....)
Questions first, attempted humour later - maybe.
So, here's the situation . I'm building a trial Pompeii oven, essentially as a learning experience. It's smaller than recommended, but such is life.

Q 1:- If I mix sawdust/wood shavings in the cement/vermiculite concrete, will tannins/sugars from the wood impact on the lime/cement chemistry?
Q2:- There are BIG gaps between my floor bricks: so do I fill 'em with a stiff fireclay mortar, or pretend to ignore them? I noticed in the FB e-book that a herringbone floor appeared to have substancial, filled joints. Please enlighten.
Reasons for Q1 are twofold.
Q1(a).Sawdust is cheaper than vermiculite. When mixed, it will eventually ash, so improving heat retention. So why have I not seen it endorsed?
Q1(b)Probably self-evident. I realise that sugar (in minute proportions) ,inhibits the setting of concrete. With Aussie hardwood timbers, I don't see this as a problem.
Comments please.

OK, so here's the very, very sick attempt at humour: You folk can actually SURVIVE in places where your pool freezes over? And it's only Fall in your zone?
Man, us dikwits would perish in weather like that!! More power to you.
Thanks again James.
Your regular guests are also SO giving: pure Gold.
Jeff.

[cvdukes]

Re: G'day James, and thanks mate.

Q 1:- If I mix sawdust/wood shavings in the cement/vermiculite concrete, will tannins/sugars from the wood impact on the lime/cement chemistry?
Reasons for Q1 are twofold.
Q1(a).Sawdust is cheaper than vermiculite. When mixed, it will eventually ash, so improving heat retention. So why have I not seen it endorsed?
Q1(b)Probably self-evident. I realise that sugar (in minute proportions) ,inhibits the setting of concrete. With Aussie hardwood timbers,

Sawdust is routinely mixed with mortar at fairly high porportions for Cordwood Masonry construction (a cheap building method ... but not for WFOs). The sawdust does not substantially affect the strenght of the mortar when used that way but slows the setting time. The cordwood theory is that the sawdust in the mortar better adapts to the wood as it shrinks.

As for the sawdust "ashing", there could be a problem with creating too large of a void inside of the concrete if using large sawdust .. (ash being considerably smaller than the unburnt sawdust). So, it becomes a problem of getting the porportions right...too much sawdust in the mix that will ash away removes aggregate and creates voids. Voids are compressable and lowers insulation quality (unlike vermiculite or perlite that are essentially non-compressible). Concrete depends on a mixture of an aggregate (sand, gravel, vermiculite, sawdust,. etc.) and a binder (the lime or portland) for strength. If all the aggregate is removed (say, by burning out the sawdust), the residual binder has nothing to bind...and concrete made entirely of binder (i.e pure portland) is very weak.

[asudavew]

Re: G'day James, and thanks mate.

This is an interesting topic.

I think this is a related question....

Instead of perlite or vermiculite in the outer insulation layer.. could leftover ash work?

This may save some moola for people!

[cvdukes]

Re: G'day James, and thanks mate.

Ash is used in "cinder blocks" as an aggregate substitute. The blocks weigh about 40% less than regular concrete blocks but have really no great insulating qualities. They seem very conductive of heat and cold, perhaps not quite as much as regular concrete, but not something that is readily discernible. I'm sure there is a heat transfer coefficient readily available somewhere on-line that would show it to be a poor option..

For cost savings, fiberglass or rock wool batting is viable around the dome depending on how the exterior is finished (just need to build an outer shell so as not to compress the batting). The temperatures outside WFOs are not high enough to remelt the fiberglass fibers. As unbelievable as it may seem, even chopped cellulose (ground newspaper!) treated with a flame retardant could work as long as air is excluded (remember there are 3 things needed to sustain a flame: heat, fuel and oxygen... around the dome there is heat & the cellulose would be a fuel so significant leaks of air in the outer shell could be spectacular). For under the oven floor, it needs to be both insulating and non-compressible (so that rules out fiberglass etc).
Of course, I got lucky on finding a vermiculite mine near enough to me that was willing to part with a large quantity at wholesale costs so vermiculite has ended up being my cheapest option....

[jeff]

Re: G'day James, and thanks mate.

Gentlemen,
Thank you for your feedback. I'm happy enough to run a trial mix: possibly 1:1:5:5, cement, lime,sawdust, vermiculite.
Gently mixed, I reckon it could be a cheaper, and better, insulator than one without sawdust.
Fellers, I'm not sure where to start: I assume that , in your country, same as here,the Ready Mix concrete drivers carry a kilo (2lb)of sugar to retard concrete setting in case of a truck breakdown.
So how might wood fragments impact on insulating concrete
setting rates?
Thanks eh.
Jeff.