Heat resistant mortar - hydraulic or ceramic drying??

[Frances]

I have been looking in the forum, but didn't find any mention of this:
When I went to get my firbricks, the guy asked me if I wanted hydraulic drying mortar (I gather this is cement based and hardens as it cures) or ceramic drying mortar (which reaches its final strength through heat and only after the oven is fired). The Pompei plans obviously use cement based, but will the other stuff work too?

The thing is, this guy gave me an I-was-building-ovens-when-you-were-still-wearing-diapers look and I ended up with the ceramic sort. He said it was better, because it expands and contracts with the rest of the oven when heating up, so you don't get cracking others have mentioned. He also assured me that it was easy to build with. So what is the down side? Has this been discussed on the forum before? What have I bought???

Frances

[DrakeRemoray]

Is one type premixed in a tub and the other a dry powder to be mixed with water?

[nissanneill]

One has to ask.
Does it really make much, if any difference, provided that they both do the job as required?
The only factors which might sway the consumer is:
• price
• workability
• freshness (or useby date whether premixed or powdered)
• accessibility/availabilty
• characteristics (maximum recommended thickness, etc).

Neill

[Hendo]

Frances,

I wonder if we're getting tied up with nomenclature, across cultures? Perhaps the 'hydraulic' drying is an air-set mortar and the 'ceramic' a heat-set mortar. I'm just guessing here, but it may explain it.

My brickie said most industry kilns etc use a heat-set mortar, the bricks laid on a form (like a gigantic Alan Scott barrel oven), and when the forms are removed, the bricks are quite stable as they are self-supporting. When the kiln is ramped up, the mortar sets/cures/hardens thoroughly.

For ease of assembly, my supplier recommended I use an air-set mortar, as it sets up more quickly and you can get on with the next brick sooner.

Here, air-set mortars are generally available both wet or dry, and are available in a range of alumina contents, and heat-set mortars mostly dry, with higher alumina contents (hence their use in kilns etc).

Hope this helps.

Paul.

[Frances]

The stuff I bought is not premixed, and I can add 20% of cement to it for areas which need to hold especially well (like the top of the dome, I guess - but any other recommendations are welcome).

Yep, heat-set and air-set, that sounds about right :-) (and I thought I was good at translations...) Good to know I can use either one, too.

Workability, no idea, but I'll find out when I get that far. The price is good (about 10 USD for 25 kg). And once its mixed, without cement obviously, you can continue working with it for hours. He said I need pressure joints of a couple of mm max, but only on the inner surfaces. Cos there's no way I'll get small joints on the outside!

Thanks for the help,
Frances

[maver]

Frances, a few thoughts on workability and holding. If you have mason's lime available, it really helps to make the mortar plastic and sticky. It is supposed to hold up with thermal cycling better than portland cement as well. The tradeoff is it does not harden as much as portland and not as quickly (but for those upper dome bricks stickiness may be more important than a quick bond). If you don't have to modify your mortar for stickiness or workability then don't - I'm sure mortar you have will have better performance as is than as modified.

[Frances]

I have FINALLY (Hooray!!) stared work on my dome today - pictures will follow. The mortar is reall good to work with. It must have lime in it, because its really sticky stuff.

I have two more questions though:
As its heat-dry mortar, am I right in thinking that it doesn't have to be kept wet? Should I cover it if the sun is shining on it?
And seeing as it doesn't form a chemical reaction with the water, would it be ok to wash my tools in water afterwards? Rubbing them down with newspaper sure doesn't work...

Thanks for all your help.
Frances

[Frances]

One last thought: if rain gets on my half finished dome, will it collapse? I have this horrible picture in my head of the tarp blowing off and all the mortar melting away in the night...

[Frances]

Hello? Anybody?

I've been reading past threads on similar subjects, but didn't find any that give tips on working with heat dry mortar. Cement based mortar I'm used to, but this is a bit different. Isn't it?

I mean obviously I'm going ahead with building the dome, but it would be nice to know what I have to watch out for sooner rather than later.

Frances

[aureole]

I've got a very similar question myself: how did you get on with the air-drying mortar?

According to the link below and some other stuff I've read, "hydralic" means the more tradition portland cement-like action: it needs water to set.

Build Masonry Fireboxes

I've been offer the stuff that comes wet in a tub and dries out - but this, apparently, isn't good for exterior builds or big gaps.