Refrax mortar - any good and how far does it go?

[jonv]

So I've been recommended bags of ciment fondu and finely ground mullite with a suggested mix of 3:1, which sounds reasonable.

Really not expensive here so I'll try it on a couple of scraps and see how it does. Is there any reason to add clay to the mix? I'm thinking that it won't get anywhere near hot enough to fire the clay anyway and it sucks up a ton of water (needing more than is recommended for the fondu mix) and then shrinks a bit as it dries out...

[jonv]

Thanks for info. Learning more about all sorts of stuff I never thought I'd need to know

[david s]

Yes, for the temperatures we fire to any clay is suitable, although as clay imparts more shrinkage to the mix I'd be staying away from bentonite which has a very high rate of shrinkage due to its extremely small particle size. The clay makes the mortar more refractory (fire proof) and also makes the mix far stickier. The cheapest source of clay I can get is bricklayers clay, often referred as fire clay in the building industry. For a potter fire clay is an extremely refractory clay usually white in colour. Probably the cheapest powdered clay from a potter supplier would be ball clay.

[jonv]

So any clay, then, not necessarily fire clay.

[UtahBeehiver]

If you do a Google Search this is what you would of come up with.

http://pottery.about.com/od/potterygloassary/g/grog.htm

[jonv]

...and what the heck is the 'grog' I keep seeing mentioned in various formulae?

[jonv]

I've never really understood what exactly the fireclay does - can someone explain?

[deejayoh]

This is interesting - source not validated... But I do remember seeing the comment about the portland being burned away on this site before. David is resident expert on castable/refractory. I would take his advice.
FULL ANSWER

When using calcium aluminate cement, the recommended ratio for the materials is 10 parts sand, 3 parts calcium aluminate cement and 1.5 fire clay. If Portland cement is preferred, the recommended proportions for the mixture are 10 parts sand, 6 parts fire clay, 2 parts Portland cement and 3 parts lime. Portland cement helps the mixture to set but will be eventually burned off leaving lime, which is a type of calcium, to take its place.

[OT-in-TN]

Refractory Fire Clay Mortar
Sand Calcium Aluminate Cement Fire clay
10 parts 3 parts 1.5 parts

[OT-in-TN]

So could you make smaller batches and eliminate the lime?

[david s]

Do not use ciment fondue as a substitute for Portland cement in the home brew. Fondue (calcium aluminate cement) goes off far more quickly than Portland cement so you have to make it up in much smaller batches. Lime acts as an accelerant with this stuff which will result in even shorter working time. Use the homebrew as advised.

[jonv]

Good tip about the sand! Think I'll do a bit more research on the fondu.

[deejayoh]

I was going to suggest that you might try Ciment Fondu instead of portland - but I can't recall any builders that have blazed that path before.

+1 on the comment about getting fine sand. Makes a world of difference. Whatever you do, don't get "brickies sand", it has lots of large pebbles in it.

[jonv]

OK, thanks

[OT-in-TN]

3:1:1:1, 3 parts sand, one part ciment fondue, one part fire clay, and one part lime.