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Old 10-29-2009, 05:36 PM
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Default Getting started this weekend, question on location for oven and refractory mortar?

I have attached the location for my WFO build. I checked with the town and I am good in regard to any regulations they have. I will be close to some bushes but dont think that will be a problem.

I am having all my bricks, concrete, supplies, etc.. delivered this weekend. I did not get the refractory mortar yet. Just wanted to make sure I am getting the right product. I have found heatstop which comes in a 50lb bucket. It says on the bucket that it is a refractory mortar however is not waterproof. I know there is better stuff, but is this OK? Or, I am better off making a home brew?
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Old 10-29-2009, 07:03 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

If it's the wet pre-mixed mortar, stay away. It's made for patching and repairs on commercial boilers. It takes forever to dry, and as you say, it isn't waterproof.

You want the dry powder if you go with the heat-stop. It comes in fifty pound bags or ten pound tubs. The home brew mortar, although not a true refractory mortar, seems to work fine if you want to save some money.
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Old 10-29-2009, 07:19 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

Why does it matter if it is waterproof? The oven itself should be waterproof.

Why does it matter what the final cure time is? The initial set time is what matters.

Wet premixed is made for building fireplaces, kilns, and ovens. Key is minimal joint size.

Fire clay is just that: clay. It has no hydraulic or carbonaceous setting properties and should be considered to be nothing more than a chinking material. Many masons use it straight with no issues on indoor and outdoor fireplaces, but the key to proper use is absolute minimal joints.

What is called "homebrew" here is just mortar. Adding fireclay to cementious mortar does not make it refractory mortar. This is not to say that it won't work just fine, it will, but the key, again, is minimal joints.

See a trend?

Last edited by Tscarborough; 10-29-2009 at 07:22 PM.
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Old 10-29-2009, 08:11 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

The joint size was what convinced me to go with a refractory mortar. I did not have the skill to taper firebricks on all sides. Although I did nip the inside edges so they would fit without much of a gap on the inside, outside as an entirely different matter. I was unwilling to cut the bricks to achieve the 1/8 in gap required by the premixed stuff in the tub.

Joe
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Old 10-29-2009, 08:17 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

The face is what matters. On the backside it is more important to reduce shrinkage cracks than it is to have a high heat mortar. Direct flame exposure is what kills normal cementious mortar.
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Old 10-29-2009, 09:00 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

My understanding is that it is the thermal cycles that breaks down Portland cement. It cannot withstand the expansion and contraction of heat up and cool down cycles, and it begins to break apart.

On the non-waterproof mortar, I think the problem is that the binder is not waterproof, so that the mortar comes apart when it gets wet. That's OK for an indoor fireplace, but not for an outdoor oven. No matter how well you built your enclosure, your oven has an opening to the outside world.

James
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Last edited by james; 10-29-2009 at 09:05 PM.
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Old 10-30-2009, 04:57 AM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

The aggregate can't withstand the thermal cycles. More specifically, it is the difference in the thermal coefficient of expansion between the matrix (portland cement paste) and the aggregate. This is why you have to use a fine silica aggregate in heat rated mortar. The problem with that, however, is that using a uniform sized aggregate causes drying shrinkage. Mortar aggregates optimally will range in size so as to fill all available space with a coating of cement paste.
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Old 10-30-2009, 04:06 PM
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Default Re: Getting started this weekend, question on location for oven and refractory mortar

"My understanding is that it is the thermal cycles that breaks down Portland cement."

Correct. The crystal micro bonds start to break down if heated to over 600 degrees Fahrenheit and cooled down.
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