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Mike Spear 03-19-2014 10:50 AM

Refractory Cement / Mortar
 
Hello all.

This is my first thread on the forum and I'd like to ask a couple of questions.

Now the weather's improving I'm about to resume work on my pizza oven using FB downloaded plans and recycled firebricks from a demolished bakery, great bricks! I built the insulated slab and herring boned the oven floor last Autumn but mothballed everything for the winter.

Q. Will my refractory cement / mortar air dry like normal cement or does it need to be fired to set hard? My firebrick floor mortar still seems a little tacky!

Q. The interface between the dome and the entrance arch seems to be crucial to the strength of the oven. Should one construct dome and arch simultaneously, arch first or dome?

Any help or advice gratefully accepted

Thankyou

cobblerdave 03-19-2014 06:24 PM

Re: Refractory Cement / Mortar
 
G'day
A1..... I would be checking with the manufacturer on that mortar. That's not right . I have heard of one member who used a air set material that didnt fair well over the winter months in the wet and cold. You'll find that a lot of members here use a home made refractory cement of 3 sand 1 clay 1 Portland cement 1 lime. You find too hearth floors are usually not mortared together or mortared to the dome to allow for movement.
A2 the transition between dome is something that takes a bit of effort to do well. You don't have any problems with strength both the dome shape and the arch are some of the strongest building methods.
All of the this is in the free plans still Ask any question.
Regards Dave

david s 03-19-2014 08:59 PM

Re: Refractory Cement / Mortar
 
Some refractory mortars are designed to harden with firing. Unfortunately the change occurs at a higher temperature than we fire to so they don't work too well. Use the home-brew instead.

Mike Spear 03-20-2014 12:19 AM

Re: Refractory Cement / Mortar
 
Cobbler Dave....

Thanks, I didn't plan to mortar my first course to the oven floor, just the joints behind.

I'll get back to my mortar supplier for manufacturers specs on the curing temperatures of their mortar. If this is too high I'll go with your recipe. If it doesn't cure I'd have an oven held together by gravity, not mortar!

Thanks for the advice

cobblerdave 03-20-2014 03:40 AM

Re: Refractory Cement / Mortar
 
G'day
Funny you should mention gravity ... The dome owes it strength to gravity.
Once you put that last brick into place it only as strong as mortar holding the bricks in place. The last brick goes in and now gravity takes over making it a super strong structure. That mortar now just holds the units ( bricks) in position.
And us oven builders come along and introduce fire and heat to this .... Heat goes up ... Uneven expansion ... You'll need more than gravity to keep this structure up..... and cooking
Love to see a pic of those bricks
Regards dave

Mike Spear 03-22-2014 12:08 PM

Re: Refractory Cement / Mortar
 
CobblerDave.....

Thanks for your last..... I 'phoned my supplier (Central Refractories, Falkirk, Scotland.) and the boss man there assured me the refractory mortar they sold me is air dry / set. I was doubtful because when I laid my herringbone firebrick oven floor I brushed dry mortar into the joints like a grout and then sprayed the oven floor with water. This stayed tacky for longer than I was happy with, but he said this was because the top layer of dry mortar was too wet a mix, while the dry mortar underneath remained dry!

I'll crack on with the build, using 10 mill. expanded polystyrene insulation as a former for the dome which will look like an eight point star from above. When the dome's done Ill break the polystyrene out from the door arch.

Any comments, hints or advice would be most welcome....

Awrabest.... Mike Spear

cobblerdave 03-22-2014 01:23 PM

Re: Refractory Cement / Mortar
 
G'day
So what temp does that mortar set at? Like Davids said usually at a higher temp than our ovens get to. And heating it to low will only put uneven stresses that with bring the structure down.
Regards dave

cobblerdave 03-22-2014 08:48 PM

Re: Refractory Cement / Mortar
 
Gday Mike
Further to the mortar you have. I have to admit I was a little thrown by the term Air dry mortar, so I set about a bit of investigation.
I went and goggled central refractories in Falkirk and checked out the mortar. At first it didn't tell me much . I took a copy of the ingredients and googled the local company that supplies here in Brisbane. Low and behold same ingredients in the refractory cement that I'm familiar with.
Not much difference but I did notice a higher percentage of Alumia and a higher Mpa in your local refractory cement.
The other difference was the term used "air set" cement. That was easy to understand. Air dried makes it sound like clay that will turn back to slop if rewet and not fired.
Sorry if I have caused any confusion, but from my end it looks like you have the right stuff.
Regards dave

cobblerdave 03-22-2014 10:51 PM

Re: Refractory Cement / Mortar
 
Quote:

Originally Posted by Mike Spear (Post 171083)
CobblerDave.....

Thanks for your last..... I 'phoned my supplier (Central Refractories, Falkirk, Scotland.) and the boss man there assured me the refractory mortar they sold me is air dry / set. I was doubtful because when I laid my herringbone firebrick oven floor I brushed dry mortar into the joints like a grout and then sprayed the oven floor with water. This stayed tacky for longer than I was happy with, but he said this was because the top layer of dry mortar was too wet a mix, while the dry mortar underneath remained dry!

I'll crack on with the build, using 10 mill. expanded polystyrene insulation as a former for the dome which will look like an eight point star from above. When the dome's done Ill break the polystyrene out from the door arch.

Any comments, hints or advice would be most welcome....

Awrabest.... Mike Spear

G'day Mike
I built with ply form which I burnt out later ( scary in the extreme)
The main disadvantage of building to a form is that you can't see what is going on in the inside of the oven and you can't clean and regrout as you go.
I'm pretty masonry impaired anyway but I was faced with gapes and uneven brickwork. Its still a dome and strong, holding up and cooking well nearly 4 yrs now.
If I was to build again I wouldn't bother again with a bricksaw unless one just fell in my lap. The bricks stick up there pretty well for 5 or 6 rows I'd use a 1/2 dome shape to orientate the bricks. Easy to get out of the way to check and clean. Simple sticks to hold the bricks till they grip after that.
The final layers a simple circle of ply held up with a block of wood and a car jack. Built a sand form on top, finish the last bit the, drop the jack and clean up.
There is no real right or wrong way as long as it achieves a dome and suits you
Regards dave

Mike Spear 03-23-2014 01:34 AM

Re: Refractory Cement / Mortar
 
Hi Dave.

Thanks for the advice, and many thanks for the confirmation research on google re the mortar. That's reassuring. The build is going to be one of those jobs you put off starting for fear of screwing up and then finish thinking "that wasn't so bad!"

I've posted a pic of where I'm at at the moment and will try and post more as the build progresses. The best tool I've found so far is this forum..... what a great place for advice!

Kindest regards..... Mike.


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