Heat resistant mortar

rudysmallfryOctober 25, 2005

Okay, I just spent 2 hours getting absoultely nowhere on this one at HD and Lowes. I am building a hearth pad for my wood stove that is coming on Friday. According to the instructions I have, I am supposed to use 3/4 inch plywood with 1x3 or 2x4 supports or backers. That part's done. The next step is to secure wonderboard onto the plywood using "heat resistant mortar". After that is regular thinset, tiles and grout. The reason I have heat resisitant mortar in quotes is because every store I have gone into looking for it looks at me like I have two heads. So now I ask the garden web class, what is heat resistant mortar?

Please don't tell me to check with my town. Their response to all of my wood stove questions so far has been, "just follow the manufacturer specs". I called the company that makes the flexibond products and was told that they don't rate their products for heat. I looked at the cement products in the masonry section, but none of their products mention heat either. Does anyone know what I should be using for the wonderboard step and where I can find it?

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As far as I know all mortar is heat resistance . it don't burn . You can screw the wonderboard in don't think that a few screws would transfer enough degrees to start a fire also your tile on top with the grout will act as a barrier Imho that is ..Im a retired mason and never heard of heat resistance mortar.. just screw it it to the plywood or use a non combustable adhesive. But sure someone else out there will tell you somthing else.. but thats what I would do... Read on here that if you put brick agaist a wood wall it would not be fireproof .. thats bs Ive built many fireplaces and they all worked fine and no fires .. but thats another story

    Bookmark   October 25, 2005 at 11:26PM
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You want to look for refractory mortar.

Yes, all mortar is heat resistant, but refractory mortar is a lot more durable than simple portland cement or other types of mortar.

You can actually buy the stuff in tubes that you can put in a caulking gun, or you can get it in tubs, bags that you mix yourself, etc.

Here's an example of the different types: http://heating-and-cooling.hardwarestore.com/32-163-heat-proof-cements-and-gaskets.aspx

If you have a stove or fireplace store in your area, chances are you can find it there, or even a heating contractor.

If not, try back at the big box home center,

    Bookmark   October 26, 2005 at 12:18AM
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refractory mortar is used on firebrick and the proper way to use is is mix it like a surly and dip the brick edge in and shove them on to one another .. that will not work on the wood and wonderboard.. Like I said just screw the wonderboard to the plywood.. When you build fireplaces they say to use refractory mortar but if you look in your fireplace you would find 99 and 9/10 are built with reg mortar mix.. Including the old Rumford fireplaces that last a 100 yrs.. with beams right in across the fire opening.. But if you want to I guess you can.. use the refractory mortar.. if it will make you have peace of mind.. that's the important thing.. I've built sewer disposal plants and that is the only time we used the refractory cement on the furnaces that burn the waste.. also on the old coal chimney's.. because of the high temperature. What I sugest is that when you grout the tile on top of the wonderboard use the sand mix grout with the additive in it that way the joints will hold up tight and not move on you.good luck with your project

    Bookmark   October 26, 2005 at 2:06PM
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You want to make sure you have at least 1" of non-combustible material over the plwood including the tile and mortar. If necessary put down two layers of wonderboard. You can mix "heat resistant" mortar yourself. Buy a bag of mortar clay (sometimes called fire clay) and mix that into the mortar mix. The bag will usually have a suggested ratio of cement, sand and clay. All mortar is "heat resistant", the addition of the clay makes this mix less prone to cracking and crumbling due to the temperature swings.

    Bookmark   October 26, 2005 at 2:16PM
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Sort of sounds like the company is covering itself for insurance purposes.

    Bookmark   October 26, 2005 at 8:08PM
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"refractory mortar is used on firebrick"

Yes, yes it is.

But there's more than one way to mix the mortar. You don't have to mix it to slurry consistency if you're not setting brick with it.

100 years ago fireplaces were constructed with the material that they had at the time. That doesn't necessarily mean it's the best choice for the application today.

And yes, a lot of fireplaces have been built recently using standard bricking mortar (against code in many areas, I understand).

That's very likely why a lot of fireboxes I see have badly degraded mortar.

    Bookmark   October 27, 2005 at 1:01AM
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I don't know much about this but was curious. I had a patio contractor build in a fire pit - I was shocked they used regular mortar and bricks and was scared to use it. I believe regular cement can crack strongly enough for bits to shoot out. Of course they didn't know what they were doing and it turned out it smoked too much to use anyway.
Back to the point.....my dad worked in a factory that made glass tv tubes so he used to install these big furnaces and always used fire brick. They let him bring the bad looking and used bricks home and he would use them for other stuff. I can't imagine him not using fire brick and fire motar in a heated situation. But a wood stove with some clearance from the actual bottom of the stove may not really need it. Guess it depends on how paranoid about fire you are, how much you plan to keep the stove cranked at max for long periods of time etc.

    Bookmark   October 27, 2005 at 1:45PM
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I was able to get refractory mortar where I bought my firebricks, at a masonry supply house, should be one in your yellow pages

    Bookmark   October 28, 2005 at 11:37PM
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Refractory mortar.

Here is a link that might be useful: Refractory mortar

    Bookmark   October 31, 2005 at 1:04AM
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I bought refractory cement for kiln repair. It exists. I think the local fireplace dealer sells it. I don't remember where I got mine, but I experiemented creating molds with it in my kiln- to 1500 degrees farenheit-it didn't crack and maintained form.

soon to be wood burner, glass artist and potter

    Bookmark   November 1, 2005 at 8:20AM
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100 years ago they likely had some awesome asbestos mixed in their mortar too.

    Bookmark   April 3, 2011 at 3:36PM
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Could this refractory mortar be used as a cast for smelting glass? Would it hold its shape in the heat or crumble apart. I want to make glass bricks and the mortar would stay and hold the shape and give a little extra strength to the brick so it wouldn't matter that I couldn't get the glass out of the mortar shape, I just don't want the mortar to crumble apart and have the glass spill everywhere.

    Bookmark   December 19, 2011 at 10:07PM
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Stovo made by Rudland, Fireplace mortar comes in 1 Qt. Tub is good up to 2000 Deg F. You should be able to find it at Ace Hardware and it runs about 59.00 USD for a 6 pac.

    Bookmark   November 8, 2012 at 8:46AM
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Im looking into refractory mortar too. I just bought a 50 year old house that has a fireplace and ALL the mortar around the firebricks is cracked, crumbled, spalded and just plain long gone. Bricks are sitting loosely in place with nothing to connect them. This house was built by rednecks and an architect was not involved in the design. Crayons and the back of a book of matches was probably used in place of blueprints. Im probably not any smarter than they were since Im the sucker who bought this dump. So please don't think Im being a snob. My point is, when you can use the best materials for the job, it is wise to do so. $60 for premixed refractory mortar vs $2000 for a professionally installed fireplace insert? Sounds like a deal. So, does anyone have any recommendations for selecting and using refractory mortar? I don't want to do a repair job that will last for only 40 years or less like the previous job did using plain ole everyday mortar. The big box stores do not carry fireplace repair stuff probably due to insurance liability risk. Should I try to clean the soot off of the old firebricks before resetting them? I have zero masonry skills but am eager to learn. So, please walk me through this.

    Bookmark   July 21, 2013 at 11:07AM
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I should clarify a bit. One of the issues I have been running into when looking for a refractory mortar is that they often are intended to be applied in mortar joints no thicker than 1/4 inch. Since my place was built with regular mortar and masonry techniques, the joints are 1/2 inch thick. Obviously, if I am to rebuild the fireplace using the same bricks, I will need to rebuild it with 1/2 thick mortar joints so that it all fits back together nice and neat without having to buy more bricks, cut them down.... So, know of a refractory mortar that can be applied in 1/2 inch thickness?

    Bookmark   July 21, 2013 at 11:37AM
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You can use Heatstop II (expensive) (dry, you have to add water). mix it in very small batches (it sets quickly - do NOT retemper) wet the firebox brick thoroughly and keep it wet just before strongly pressing it into the CLEANED out joints w/ a putty knife or similar. When you mix the mortar mix it STIFF and it will stay put in 3/8 to 1/2" gap, although not ideal.

Another option, powder fireclay, portland, and coarse sand. mix 1/1/1 by volume. won't last as long, but, will hold together for a while.

Large joints are not used in firebrick anywhere, be it industrial boilers or the home. This is because regardless the material, it will not expand and contract the same as the brick and will not adhere and last indefinitely.

The largest joint I'll use when building new is 1/16th and that's on reclaimed (rough) firebrick.

    Bookmark   July 24, 2013 at 4:39AM
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Berlin, thanks for your tips. I just found Heat Stop products on my own yesterday. Nice to know someone, yourself, endorsed them and that you say the joints "can be made" in 1/2 inch thicknesses by mixing it thick. When you recommend mixing in small batches, how quickly does the "stiffly mixed mortar" set? I do have some masonry skills so this is doable for me.

Another issue I have is that I will have to mail order Heat Stop, but I can get "Fireplace Patch" manufactured by Rutland in 1 pound quantities from a TSC nearby. So, I was wondering if I could reset all the bricks on a background of regular mortar, because many of the bricks in my fireplace are completely separated from all the mortar that once held them. Then, finish pointing the mortar joints with the Fireplace Patch so that the Fireplace Patch will be in 1/2 inch to 1 inch depth in the joints. That way the regular Type S mortar won't be in contact with any flames but the Fireplace Patch will. Do you foresee any problems with taking that approach? Or should I just get a 50# sack of Heat Stop?

One more question, would you, or anyone else, have any tips for cleaning the carbon from the fireplace bricks before I rebuild the firebox, or is carbon on the bricks a non-issue?

    Bookmark   August 24, 2013 at 10:39AM
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I would use just the heat stop II. It's not cheap, but it IS better. The type S will expand and turn to dust compromising the whole firebox even if grouted w/ heat stop on the face; it's not the flame, but the heat that kills it. Did you check your local masonry supply houses for heat stop II ? most of them carry it - $50- 100 per 50lbs.

a stiff mix kept cool and with periodic mixing (I've even used ice cubes and refrigerated water to slow the set) will be workable for 10-15 minutes; hot or warm water (or being out in the sun) will give you less than 5.

carbon is an issue, there will always be a hairline crack or two in a firebox especially one built w/ larger joints, however the carbon will keep much of the Heatstop from adhering properly to much of the brick. Start w/ a dilute solution of muritic acid and a stiff brush and then finish w/ a stiff brush w/ acetone (makes sure the muritic acid solution is thoroughly washed away and dry before scrubbing w/ acetone.)

    Bookmark   August 25, 2013 at 1:58AM
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Sorry for grave digging this thread but I am in the process of building an outdoor fireplace. I've done a lot of research and know what I'm doing now. Whenever you are dealing with extreme heat you always want to use fire brick and refractory mortar. Those 2 items will withstand temps well over 2000F. But you don't use the refractory mortar to adhere the firebrick backside to whatever its going to. You set the firebrick in regular mortar and then you smeer the refractory mortar into the gaps between each brick

    Bookmark   June 10, 2014 at 7:54AM
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No really, thanks for the gravedigging. I forgot all about this thread. I still haven't gotten around to doing my fireplace and have largely forgotten all this excellent information. Yet, I've since decided to not just repoint the firebox, but now am planning to rebuild it into a sort of Rumford design. I've dry stacked bricks to test it out and it seems that it will work great. One thing at a time though.

    Bookmark   September 6, 2014 at 12:04PM
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