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

[jonv]

Hi folks,

I'm debating what sort of mortar to use. There appear to be rather limited options here in the UK - most are premixed sorts that I understand from previous posts may not be best. So, I'm considering either the homebrew mix or bags of 'refrax' mortar.

Anyone have any experience with this stuff? Is it any good and roughly how far might I expect a 10kg bag to go?

thanks,

JV

[deejayoh]

I think most builders go with homebrew. Works great, and costs a lot less. 10 kilos of refrax will probably only go a couple of courses. It's kind of amazing how much mortar you use building and oven.

[jonv]

On my pretty big oven (1.2M) that's going to be a lot of bags. Is there any sense in which refrax is 'better' than the homebrew?

I'm planning on using a pile of side arch bricks to form the dome, so there should only be thin mortar joints.

[deejayoh]

I used one bag of a US product called Heatstop 50 - and moved to homebrew when I realized it didn't go very far. I could not even begin to give any sense of how the change in mortar made a bit of difference to anything but my pocketbook. The oven went up, and stayed up. Homebrew was, if anything, easier to work than the HS50. Now that the oven is done and up 4 years - I have zero regrets about not spending the money for refractory mortar.

[jonv]

I know from following the forum for quite a while that the homebrew mortar has been used for many ovens and seems to work well. I just have a little concern that the integrity of the joints might be affected when heated and the portland breaks down. I sort of understand that it's fine but somehow can't quite trust it. My bad no doubt.

Edit: I really know little about cement so it is probably in my head...

[deejayoh]

I think most builders worry before they start. Couple of thoughts 1) probably hundreds of ovens built on this site using homebrew. I haven't seen any failures claimed due to it's use in the years n the 6 (what!) years I have hung around here. And 2) the mortar in a dome holds the bricks apart, not together. Even in a hot environment Portland works fine in a thin joint mortar joint. Wouldn't work to cast the entire oven, but that is a not the use case.

[jonv]

I happily concede that the homebrew has done a good job for a lot of folk I just want to be sure that I'm putting the best stuff I can between my bricks - I'm only doing this once, I hope! I had read that a true refractory mortar is the ideal thing to use. If it is in any sense better, I'd probably want to use it... So, cost aside, what is really the best for thin joints?

Edit: just reread the official plans - think that must have been where I saw it mentioned before - are they wrong to suggest that a refractory mortar is the "best solution"?

[RandyJ]

I don't think you will have any problems with the home brew. One thing for fine grout lines is to use a very fine screened sand. The stuff I had was like sugar. All the same size grain and no small rocks. That will help you a lot. Otherwise some of the best builds on the site are done with home brew. I think you would be very happy with it.

Randy

[jonv]

OK, so I'm building a pretty big oven - 1.2M, approx 47" - but hopefully with fairly small joints. Any guesses as to how much fire clay I need to get for the homebrew?

[UtahBeehiver]

I used 1.5 - 50 lb bags fire clay for a 42" using the 3:1:1:1 ratio (sand, lime, fireclay, cement). You may have thin front and possibly side joints but you will have fairly large 3/8" to 1/2" rear joints that take the bulk of the mortar. If you feel you need to use factory refractory then it is your choice.

[jonv]

Thanks, that gives me a good idea. I should have thin joints at the rear too because I can get a stack of side arch bricks which should give me almost the right taper to make the dome for the same price as flats - I'll then taper the sides so everything should fit pretty tight together. At least that's the plan.

[jonv]

So... I emailed the place that's providing me with the firebricks and calsil and they don't seem to have fireclay, but offered 'ball clay' instead. I looked it up and it appears to have quite a different composition. I'm guessing this is not a good substitute for real fireclay?

The chap did suggest that if mixing my own mortar I use ciment fondu, though. Any thoughts? It's not expensive, but I have no idea whether its any good or what to mix it with.

[OT-in-TN]

So... I emailed the place that's providing me with the firebricks and calsil and they don't seem to have fireclay, but offered 'ball clay' instead. I looked it up and it appears to have quite a different composition. I'm guessing this is not a good substitute for real fireclay?

The chap did suggest that if mixing my own mortar I use ciment fondu, though. Any thoughts? It's not expensive, but I have no idea whether its any good or what to mix it with.

The ciment fondu would be better than using portland cement.

[OT-in-TN]

CIMENT FONDU® is a refractory cement that can be used as the primary binder in mortars and concretes exposed to high temperatures

[jonv]

Sounds good, so what would one mix it with? Sand, fire clay, lime again?