Refractory Mortar Poll

[ThisOldGarageNJ]

Re: Refractory Mortar Poll

Wow Jamie...
Thats quite a statement.. Reading through it though does seem to make quite a bit of sense... Nice job on your research.. I did use the heat stop 50, the main reason being it removed so many variables.. I also buy pancake mix that is also Just Add Water...

Cheers
Mark

[cynon767]

Re: Refractory Mortar Poll

So I've been doing more research on this lately... If I were actually smart, I would have done the more thorough research before starting the oven, but better late than never.

There has been some debate as to the benefit of mixing a homebrew mortar with a higher clay content with regard to the portland. At issue is the fact that portland cement breaks down at high temps, which has important implications for the long-term stability of the dome. There has also been speculation as to the amount of lime required, and its effects on both wet and cured mortar.

Most of what I've been able to find out has led me to think that the lime may be a key component with regard to the effect on both the clay and the cement. This paper Quotes earlier studies showing that mortar with lime in high concentrations, equaling or exceeding that of portland cement, had increased flexibility and resilience to movement and cyclical changes like those of the heating cycles we put our ovens through. It mentioned brick industrial chimneys, which are subject to a great deal of flexing from wind stresses, of which many of the longest lasting used a mix of 5:2:1 of sand, lime, and cement. It also mentioned that a high lime content can help seal minor cracks, through a chemical reaction in the lime with airborne carbon dioxide which causes a crystalline limestone deposit to form.


Frances' mortar took the clay use to the extreme: just clay and sand. She basically counted on the heat of the oven to fire the mortar into little mortar-pocket shaped bricks. The outer surface of the bricks is not exposed to the direct heat of the fire, so they aren't going to be quite as hard as true brick; but the inner surface will likely fire quite solid. As long as the outer surface is weatherproofed, the oven should last quite a while. This is true of cob ovens as well, which are built entirely of a clay mixture.

Here's some more information about Fireclay... and here's the wikipedia entry on fireclay. What I'm reading is that in all likelihood, the firing of a clay-based mortar will indeed produce a relatively strong, temperature-resilient gap-filling wedge, helping prevent the large mortar gaps we have between rings from breaking down. It won't quite turn into brick, but will be fairly stable through the heat cycles of the oven.

The real kicker here, though, is that the harder and more solid we make the wedge-shaped clay "brick" between the rows, the less plastic the mortar joint. That's where I see a good relationship between clay and lime coming into play. The lime increases plasticity, not just in the wet mortar where it makes things stickier and more spreadable, but also in the flexibility of the cured mortar. Portland cement hardens up and sticks the bricks together, and provides excellent compressive strength, but breaks down when heated... especially when repeatedly, cyclically heated and cooled as in our ovens. Fire clay, which has high vitrification temperature, will never fully "fire", as the temps we're working with won't reach the threshold needed to turn the clay into ceramic; but the fact that the clay will remain porous and slightly plastic is actually beneficial in this case... the low temperature of our ovens (relative to pottery kilns, anyway) prevents it from becoming brittle. It will, in essence, remain a part of the mortar. It will still be susceptible to cracking during firing due to moisture inclusion if it gets wet; but when taken together with the lime's plasticity, the clay in the mortar will (theoretically, at least) take the place of the portland to provide compression resistance and give the dome long-term stability, while the lime will help prevent shrinkage cracking in the clay and will even help "heal" hairline cracks which do form.

Such is the hope, at least. The homebrew might not be as good as a truly refractory mortar; but all in all, it seems to be a pretty good balance of the properties we need: temperature-stable, good in compression, and flexible enough to withstand heating and cooling cycles. Whether or not you add extra clay or extra lime, it will likely perform quite well, and is certainly better than the airset sodium-silicate mortars previously mentioned.

[Dutchoven]

Re: Refractory Mortar Poll

Hi Phil,

I was told that my mortar would never be waterproof.
Doug

Doug is very likely correct because most of the airset mortars and not cementitious mortars but rather use sodium silicate or waterglass as the binder. It will not be water proof in the sense that if rainwater falls on it it will wash out and is therefore the reason why it should never be used on flue tile. In the oven it should be OK but I would fire low and slow for a few days. You could possibly burn an out side fire to coals and then shovel the coals into your oven and shut the door. That would reduce some of the shock and give you the low and slow heat you want.
Good luck!
Dutch

[Beagle]

Re: Refractory Mortar Poll

Hi Phil,

Your mortar has dried on the outside and feels solid, that is normal but it is not cured, that only happens when it is fired at a high temp. The high temp part comes AFTER the 10 hours of low temp. I was told that my mortar would never be waterproof. That is not a problem for me because the oven is enclosed.

They seemed to think that keeping the temperature at or just a little below 200 deg. for 10 hrs. would allow the moisture to be driven from the mortar. I don?t think the temp inside the oven has to stay exactly one temp; it can vary because there is so much mass in the bricks that the temp of the oven structure will change slowly. After that, I went with the oven-curing schedule that James recommends.

Doug

[ThisOldGarageNJ]

Re: Refractory Mortar Poll

Phil, If thats the case, low heat, then I agree you should use a small heater and close the door, Im sure if you go more than 10 hours you wont hurt any thing,,, you can always put refmix over the dome for and additional safety margin, though remember the strucure alone is self supporting I believe..

Good Luck and dont worry
Mark

[Lars]

Re: Refractory Mortar Poll

If Mudologist is thinking of using the particular mortar that Phil used in this thread, then hopefully he is building his oven indoors.

The plibrico mortar may be good for kilns, but the last choice for wood oven.

My 2 cents...

[Les]

Re: Refractory Mortar Poll

2000 years ago they used dirt and dung because thats all they had.

Dude - thats where James got the idea. If it worked then , why not now (not the dirt and dung, but the roll your own)? There are a couple of areas that I used the good stuff, for most of the build I don't think it's required. Hell - you could keep the brick to level with a river rock.

For what it's worth.

Les..

[Lars]

Re: Refractory Mortar Poll

Phil,
I am sure that your mortar has 'set', since you have been working on it for weeks, right?

Even though it has 'dried', it is still vulnerable to any moisture. Did they tell you at what temperature it would vitrify? ( become waterproof), harden?

Lars.

[philiph4@ameritech.net]

Re: Refractory Mortar Poll

I called the manufacturer for advice and they said that the mortar needed to be cured slowly at a temperature below 200 deg. for at least 10 hours,


Doug,

I lit my first fire yesterday with just newspaper. Are they saying to keep a low grade fire going for 10 hours or can I go several fires over several days? Anyway, the mortar that I used feels hard to the touch but it is definately not waterproof. Maybe I need a space heater to control the heat. Anyway thanks for the post it seems to make sense.

Phil

[Mudologist]

Re: Refractory Mortar Poll

I'm planning on using refmix on the next dome for several reasons. You're supposed to lay refractory brick with refractory mortar. They didn't invent it just to get us all for a couple hundred bucks. I' ve built domes out of homebrew and they've turned out well, but I think we all talk ourselves into short cut that is an unnessesary risk. 2000 years ago they used dirt and dung because thats all they had. If your going to build a cob oven great, but if your going build with modern firebrick buy the right mud. Buy the refmix because thats what the people who got you this far sell.

[Beagle]

Re: Refractory Mortar Poll

This is probably too late to help Phil; I used an air dry mortar for my oven, I called the manufacturer for advice and they said that the mortar needed to be cured slowly at a temperature below 200 deg. for at least 10 hours, they couldn?t give me an exact amount of time because they did not have any experience working with a small homebuilt oven. They thought, rightly so, that there would be a lot of variation in joint size and in thicknesses of the mortar. The reason is the mortar dries on the outside edges but retains moisture inside the joint. If it is heated above the boiling point steam will be produced and when that happens the pressure buildup in the joint will cause spalling.

Doug

[david s]

Re: Refractory Mortar Poll

Don't combine calcium aluminate cement with lime. The lime acts as an accelerant and the mix "goes off" too fast. I made this discovery when making a mix and the tech. data confirmed it.

[Sananguero]

Re: Refractory Mortar Poll

Hi all from the recipes on this page I came up with a 3:2:2:1 mix of mortar's sand, lime, fireclay, to portland ration and seems to be doing well although I am not quite done with the dome. Ive already used over 200lbs. of the stuff and wasted at least 50, this is my first shot bricklaying. I would have also added calcium aluminate, but was not able to acquire any.

[alec-pano]

Re: Refractory Mortar Poll

hey Folks, don't want to jump here at the end, but just saw this thread. I too have used a wet premix refractory mortar (SuperTiteBond from Clayburn). And has everyone else, have experienced a few drying cracks (I have not fired yet). I finished the dome yesterday and the outside of the mortar is hard (in some places REALLY hard), so everything seems OK.
I was curious on the firing schedule and if I should (could) fire right away. Phil, I was really curious what your experience was. How long did you wait, and did you fire the hell out if as Lars suggested? Any cracks?
I can confirm that I like using the stuff (56 canuck bucks for 55 lbs), very sticky, and clean up was pretty easy . Cheers.

[Lars]

Re: Refractory Mortar Poll

Phil,
Those are not exactly black and white options. You just need to judge for yourself. I developed several cracks while curing my oven. Right on the arch in front! You know what, they don't even open up anymore... and they closed up to the point where you would never notice them... they are hard to find.

You can start slow, and I guarantee that soon you will be saying 'this is ridiculously slow... it's going to burn like hell eventually, come on!' , so you will step it up a bit.

The thing is, you don't really have that much moisture in your build. You want extremely high temp on you mortar. As high as you can get it. The mortar can take it, the bricks can take it.... If a crack does open up, it may even close up when cooled and may or may not open up again upon subsequent firings.

Just start slow, and get a feel for it.

L.