Refractory mortars

[MikeD55]

So I’m gonna go against the grain here. I know most DIY’ers use Homebew but I’ve decided I want to use a dedicated refractory mortar. I know HeatStop is the most popular but I’d like to look at what else is out there. I’ve seen Ankona, Sairbond, FlueSet. There’s also a couple of European products like Hafner HaftHeat (contains reinforcing fibers) and Ortner Hafmortal that I’ve heard great things about. Would like to hear from anyone using these products or similar.

[MikeD55]

No one wants to take a stab at this?

[david s]

Many refractory mortars are designed for far higher service temperatures like those seen in kilns and furnaces reaching double the temperature range to that which we fire. Some actually derive all of their strength once they’ve been sintered so would be totally useless because we don’t go that high. Some rely on calcium aluminate cement that produces a mortar with extremely limited working time. The same applies to some of the sodium silicate based mortars. Contact the manufacturer. The problem with that is that you will be dealing with sales staff who either don’t have the required knowledge or who are only interested in making a sale, so you may not get appropriate information.

[Giovanni Rossi]

I don't have a lot of experience but I can second David's comment on being cautious of working time with these products. The kit I used came with a 2-part system. It was a proprietary refractory mortar and Ciment Fondu mixed 3:1. I'm not the fastest worker, but even after days of working with the stuff, I could only mix enough to lay 5 bricks at a time. I finally cried uncle and just premixed small batches for the work I wanted to finish each day. From the posts I've reviewed here, it certainly seems the home brew works and is more workable.

[UtahBeehiver]

Many times commercial refractory mortars recommend small mortar joints, ie 1/16" although I have seen some builder using HeatStop substantially exceeding the recommended joint thickness.

[MikeD55]

Most of the ones I’ve seen say 1/16-1/8” although a few go to 1/4”. I’d like to do compound cuts so I think refractory will be a good option. I know it’s more expensive than homebrew but as a part of the final cost it’s only fractionally more. HeatStop seems to be the most popular on this forum but there are plenty of others. Just don’t know what other people may have tried.

[david s]

I’m not 100% sure, but I think the reason the manufacturers recommend thin mortar joints is the possibility of steam spalling caused by excess moisture trapped in the middle of the joint. Just as for castable refractory which already contains the burnout fibres which reduce the risk of steam spalling, they can also be added to any mortar batch, but must be dispersed well.

[UtahBeehiver]

Even if you do compound cuts, the back side of the joint will be fairly thick (over 1/2") IMHO, I did "full" side compound cuts for my build, it looks cool but it takes much effort that I could of used elsewhere on the build. JR Pizza came up with a hybrid solution by only trimming the conflicting area around the top of the brick joint and only about 1" or so back. Look at his build. Also be careful with commercial mortar as some are heat set, air set is what is needed.

[david s]

I’m not 100% sure, but I think the reason the manufacturers recommend thin mortar joints is the possibility of steam spalling caused by excess moisture trapped in the middle of the joint. Just as for castable refractory which already contains the burnout fibres which reduce the risk of steam spalling, they can also be added to any mortar batch, but must be dispersed well.

[MikeD55]

Utah: I’m looking to get into this as a second career of sorts. I want to challenge myself. I’m building this for a friend. I like the idea of the compound cuts ala Alex Chernov “stovemaster” or Jon Santiago. Their joints are very tight. 1/16” or less inside and out. Jon Santiago shoots for 1/20”.

David: I thought they recommended tight joints because of risk of shrinkage cracking. Hafner in Austria makes a refractory mortar with fibers in it that’s good for joints 1mm-60mm. They advertise it as shrink free and crack free.

[david s]

You could be correct there. One of the components of most refractory mortars, homebrew included, is clay, both fired and unfired. The very fine particles of unfired clay produce shrinkage and can cause problems with shrinkage cracking for thick joints. Once unfired clay reaches 573C it becomes sintered (permanent and won’t return to mud). This gives the mortar a lot of strength and the clay becomes harder the higher the temperature goes. This is why the specs often say “cures between 600-800C.”. At the service temperatures we use it is doubtful if any unfired clay components will achieve this condition. So below sintered temperature the clay should be considered an extremely fine aggregate.

Regarding tight joints Daniel Rhodes, the author of Kilns (considered the kiln builders bible) has this to say.
”In kiln building it is much better to have a loose structure than a tight one. For this reason the amateur may have a slight advantage over the professional mason, because his bricklaying is apt to be somewhat loose and not tightly locked together. I have seen kilns made by professional bricklayers which in use suffered severe cracking and swelling due to the overly tight and precise workmanship and the lack of expansion joints.” Kilns experience around double the temperature and thermal expansion to that of an oven, but the same principles apply.

Cutting every brick on all sides also increases labour as well as increasing brick waste and number of total bricks required. This can add considerable cost because fire bricks are expensive.
My own feeling is that because of these reasons the firebricks are better cut on two sides only and the large triangular joints filled with the triangular offcuts and homebrew. However I’d love to see you forge ahead with your plan and prove me wrong (or right).

[MikeD55]

Regarding the refractory mortar, the reason I bring it up is because it seems these products are made specifically for this purpose. A couple come to mind. HeatStop and Ankona here in the states. In Europe there’s Hafner HaftHeat and Ortner Haftmortal. Both are used extensively in masonry heaters, fireplaces and fire pits. All are dry, air setting, hydraulic mortars. None of these applications experience the temperatures seen in kilns. I’m sure there’s many more. I know home brew is used by 95% of the guys here but the thing I question about homebrew is why use a product that Portland cement burns off at 600*? If that’s the case and I do go the homebrew route, then maybe it would be a calcium aluminate “Fondu” cement would be better.

As for compound cuts there are numerous builders on this site that have at least partially if not fully gone this route. Not to mention pros like Alex Chernov and Jon Santiago who build these ovens for a living every day. As for expansion these ovens again won’t nearly see the temps that are seen in kilns and furnaces (2000*F ++). Based on the expansion of firebrick of .0000021”/*F at 1000*F a 42” ID oven will expand 0.29” over its longest inner circumference. Sounds like a lot. But when you divide it over say 60 bricks that’s about .005” per brick joint. Not a lot in my estimation and well within the tolerance of error for cuts. Please check my math. I could be wrong. Now I’m not a an expert. Full disclosure. But I’m try to approach this in a scientific way. I like to ask a lot of questions. The biggest one being “why”. I don’t like to do things just because everyone does it that way. Sometimes different is good.

Will I be creating a lot more so for myself? Absolutely. But I love a challenge…..

[david s]

Whatever you settle on, do not attempt to replace Portland cement with calcium aluminate cement in the homebrew. The combination of lime with calcium aluminate cement acts as an accelerant, making the already severely limited working time pretty much unworkable. There used to be a recipe (on this site) for replacing the Portland with CAC. I’ve complained about its inclusion a few times. Most castable refractory tech sheets warn against mixing it with even a trace of either lime or Portland. Also, being very temperature dependant it’s working time can be extended with chilled water. I always use chilled water in the mix during hot months.
The theory with the homebrew mortar (which explains why it’s so rich, is that if the Portland fails and it doesn’t happen suddenly, it’s gradual, the lime which begins to fail at 500C (as does CAC) takes over. Over 500 C the sintering of the clay then becomes the cementious material.

[MikeD55]

David. Yes agreed regarding the lime. It’s interesting to note the firebrick company uses some type of Fondu cement for their mortar but only discloses it as a two part. Type A and type B. For their render they call it an air set mortar and recommend one teaspoon (5.7g) per 25kg of cement. That’s a VERY small amount. 0.00028%. Would love to know what exactly they use.

[david s]

David. Yes agreed regarding the lime. It’s interesting to note the firebrick company uses some type of Fondu cement for their mortar but only discloses it as a two part. Type A and type B. For their render they call it an air set mortar and recommend one teaspoon (5.7g) per 25kg of cement. That’s a VERY small amount. 0.00028%. Would love to know what exactly they use.

From memory we’ve had a few builders report failure with airset mortars. If it says “cures between 600-800C”, that could be the reason. Utah or Gulf may recall more detail.