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...

[deejayoh]

I think David has given the reason for this. Unless you're an expert at this, I'd suggest not straying far from the tried and true solutions. You're more likely to end up with problems vs finding some radical new solution.

[jonv]

Definitely not an expert... but a few experiments might be interesting.

[leetheldc]

Hi there I have used cement fondue and grog for my mix I liked the 3 part mix 3 being water and the fact I was filling my gaps with ground up firebrick ( the grog). it was a pain to use as it sets straight away,you get one chance with the brick and that is it but it holds like hell once you get the hang of it,but I think some fire clay in there might help it be more workable I used a suggested 6 grog to 1 fondue and water a very Yorkshire pudding batter consistency to be used rapidly there is pics in my link. After nearly 2 years the mortar is is rock solid although I have only partially fired my oven last year

[jonv]

That's good to hear

[Alomran]

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.

Any idea if the lime with fireclay and portland cement mixed could cause the lime to pop out or blast? Any comment from experts (people of experience orpast mistakes!!) on this issue?
I was told this by a fire cement supplier,

[Alomran]

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.

How about using crushed pumice in the mixture, it does NOT shrink and it withstand high temperature as well as being an insulant?? Any idea??

[Gulf]

Any idea if the lime with fireclay and portland cement mixed could cause the lime to pop out or blast? Any comment from experts (people of experience orpast mistakes!!) on this issue?
I was told this by a fire cement supplier,

I am not an expert. but I have had some discussions with a few. That wont happen with the homebrew.
I was talking to a Boral Brick salesman the other day about something similar. They would not warranty their firebrick for fireplaces if they were installed with regular mortar. He said that the mortar would swell under the heat and pop the corners off the face of the fire brick. We used to make type N masonry cement (50/50 Portland/hydrated lime) on the site for fire boxes, That was then mixed with the normal 3 to 1 sand to masonry cement to make the mortar. There was never any problem with mortar joints popping out. It took many years to where even a portion of the brick joints would need repointing. I think that your fire cement supplier is referring to experiences with todays masonry cement. I learned from a discussion with Tscarborough that the industry has gone to replacing a lot of the lime with crushed limestone and other proprietary additives to masonry cements. I think that is the source for the problems that your supplier is confusing with portland and hydrated lime.

How about using crushed pumice in the mixture, it does NOT shrink and it withstand high temperature as well as being an insulant?? Any idea??

You don't want an insulating mortar in the joints. You want those joints to be as good of a heat reservoir as the brick for heat retention.

[deejayoh]

You don't want an insulating mortar in the joints. You want those joints to be as good of a heat reservoir as the brick for heat retention.

Heat retention, and heat transfer. You want the heat to flow from brick to brick inside your oven as easily as possible in order to equalize the heat. Probably a good reason not to shoot for a "mortarless" oven

[Gulf]

Heat retention, and heat transfer. You want the heat to flow from brick to brick inside your oven as easily as possible in order to equalize the heat. Probably a good reason not to shoot for a "mortarless" oven

+2, Great point! .

[JayMez]

let me get dis straight. You put the hydrated lime in the coconut and slap the bricks togedder?

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.

I am planning to heavily taper my bricks as in Hendo's dome. If I use the 'recommended ratio for materials' do you think that will work out okay? I am just double checking here and seeing if anyone has any further suggestions, maybe I'm missing something.

[david s]

No lime. If you use lime in conjunction with calcium aluminate cement it acts as an accelerant. The stuff already goes off very quickly, if you add lime it will start to set as soon as you have mixed it up.

The clay in the mortar is not a cementious material. Because the particles are so fine it is considered an aggregate. It helps to make the mix more sticky which is useful when laying the bricks, but it also increases shrinkage and will create shrinkage cracks if the proportion is too high.

[dakzaag]

JayMez, if your joints are really tight, less than 1/8" then you can use a refractory cement such as sairset which is like peanut butter. As the joints expand over 1/8" then you need a refractory mortar like the home brew or Heat Stop.

I have no idea what you mean by the coconut, hopefully you were just messing around.

The home brew recipe is not easy to use if you have really tight joints ie. 1/16" If tapered arch brick are cut with a compound miter saw, then Sairset or similar is the best cement and it is usually pretty cheap. I use both on most builds and find that each works best with the appropriate sized joints. Alsey sells a product called flue set which is also a refractory mortar containing sand which makes it good for bigger joints. It also is waterproof once it cures.

I think the fondue is fine for an oven, problem is a lot of products get called fondue, so you need to read the fine print.

NO Lime will not explode in the oven.

Grog is a potters term regarding clay and clay sand mix.

A lot of these ovens could be constructed with clay, sand and lime and would work just fine, the Portland just gives an initial set and once fired, the sand and clay vitrify and become the bonding agent. Lime also seems to stay in the mix as a binder, but Portland will deteriorate. I actually cut the Portland to .5 and increase the Lime to 1.5 in the homebrew. I think it helps keep the mixture smooth and creamy vs the harsh Portland

[Gulf]

She put the lime in the coconut

Brother bought a coconut, he bought it for a dime
His sister had another one she paid it for the lime
She put the lime in the coconut, she drank 'em bot' up
She put the lime in the coconut, she drank 'em bot' up
She put the lime in the coconut, she drank 'em bot' up
She put the lime in the coconut, she call the doctor, woke 'I'm up
And said "doctor, ain't there nothin' I can take?"
I said "doctor, to relieve this belly ache"
I said "doctor, ain't there nothin' I can take?"
I said "doctor, to relieve this belly ache"
Now lemme get this straight
You put the lime in the coconut, you drank 'em bot' up
Put the lime in the coconut, you drank 'em bot' up
Put the lime in the coconut, you drank 'em bot'up
Put the…

[JayMez]

Thanks for those answers very informative. I was only kidding of course with the lime in the coconut.