Shinagawa refractory Mortar Cracking problem

[kiwigirl]

I purchased Shinagawa refractory mortar. It's an Australasian company and the product is in New Zealand. It came in a brown bag and I was surprised it didn't have instructions but I assume you just add water. I made it up quite runny at first and as soon as it was on the brick it started showing cracks in it. I then made up different consistencies to see if it made any difference but it didn't. All of my mortar cracked straight away. It's been very frustrating for me as I find the mixing of cement and mortar is very technical and I'm a girl so never did this stuff in cooking class. The guy that served me this stuff wasn't helpful. I even mentioned before I put it in car it didn't have any instructions on package and he gave me a blank look as it I was stupid. I just assume you add water and you're suppose to know this stuff.

[Johnny the oven man]

Originally Posted by kiwigirl

I purchased Shinagawa refractory mortar. It's an Australasian company and the product is in New Zealand. It came in a brown bag and I was surprised it didn't have instructions but I assume you just add water. I made it up quite runny at first and as soon as it was on the brick it started showing cracks in it. I then made up different consistencies to see if it made any difference but it didn't. All of my mortar cracked straight away. It's been very frustrating for me as I find the mixing of cement and mortar is very technical and I'm a girl so never did this stuff in cooking class. The guy that served me this stuff wasn't helpful. I even mentioned before I put it in car it didn't have any instructions on package and he gave me a blank look as it I was stupid. I just assume you add water and you're suppose to know this stuff.

Sounds like too much water added, mortar should have enough "guts" to stay on the brick and actually hold it in place.

[Tscarborough]

When mortar dries, it shrinks to an amount almost equal to the amount of water added. Therefore, the more water added, the more shrinkage cracking there will be. This is not a problem with cementious mortars which have additives to address the issue, but it can be a problem with refractory cements, which have fine aggregate and do not as a rule have the shrinkage reducing agents.

The mortar should be somewhere between a 70% hydration dough and cookie dough.

[kiwigirl]

Oh I didn't have a dough consistency. Does this mean my mortar is stuffed? I placed the bricks in place so my shape was formed and then I simply threw the mortar in places to make sure it wouldn't collapsed. The bricks are supporting each other the way I've placed them so the mortar was just to make sure it was stable. Now as I said lots of cracking. O.k seems to be o.k but what will happen when I fire it up. Will it simply crumble? the mortar that is. I used old bricks from a chimney so they are fire bricks. I will put a insulating layer over it so will that give it strength as well? this pizza oven has been quite a challenge. I didn't really realise how technical the mixes or layer were going to be. Is it possible to not have the insulating layer if using fire bricks for a home oven? is it really necessary?

[morrishuckeba]

Did you mix sand with your cement?
1 part cement
1 part lime
1 part fire clay
3 parts sand

[kiwigirl]

It's Mortar not cement. Different product.

[nissanneill]

Hi Kiwigirl,
morrishuckeba has the formula for the 'poor man's mortar' rather than using the expensive refractory cements which seem to have inherent cracking characteristics (as many other members have reported similar problems to you).
My oven is over 2 years old, built with secondhand firebricks and has no internal cracks (only ones outside the insulation layers in the water proofing render).
You bricks from an old chimney, may not be firebricks, but only fired clay bricks! Firebricks are expensive and contain special ingredients that handle the thermal shock of reaching high temperatures better than what the average house, or for that matter chimney brick will reach.
If I were you, I would mix up a batch of 'poor man's mortar' and put it on the outside in all those wedge shape gaps between the bricks and then put on your insulation layers, eg. vermiculite cement and/or superwool blankets.
You will need this if you want your oven to retain it's heat for other baking, eg bread or meats where you use the residual heat for cooking rather than having a fire within your oven A well insulated sealing door is also necessary.
Good luck.

Cheers,

Neill

[kiwigirl]

Thank Neil. My mortar went on the outside in the places you mention. I places my bricks first then I tried to put the mortar on the outside joins to keep them stable. What do I do now I've put all this cracked mortar on. Will it disintergrate with heat and collapse? my bricks all join so the inside of this oven will show no mortar only the outside. Do you think I should take all the cracked mortar off and put the poor mans mortar on? I hate this part of the oven. Bloody messy. I think I'll stick to baking cakes it's easier.
Thanks in advance for help.

[kiwigirl]

Or could I just put the poormans mortar over the cracked stuff???????

[Tscarborough]

If you are not using mortar (mortar is basically anything with a binder and an aggregate used to hold things apart, BTW) between the bricks, you are engaging in a process call surface bonding. It can work, but the requirements are slightly different as to the makeup of the coating. Technically what you would want to use is a product like Insul-cast or castable refractory, but in the real world, the poor man's mix would work fine. The only thing it is lacking is fiber, but short of SS needles the fiber is going to lose structural integrity anyway.

Regardless, the idea is still to use as little water as possible and still have a workable mix.