Old 08-25-2013, 01:36 PM
david s's Avatar
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Location: Townsville, Nth Queensland,Australia
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Default Re: Mortar Joints

Originally Posted by yorkshireknight View Post
Thanks for those tips David, the weather is hot above 30 degrees every day, possibly not as hot as it gets 'down under'.

Yesterday I mixed only sand and the fire cement ( possibly as you say with calcium aluminate content, however it does not state on the bag what is contained within only that's its resistant up to 1000 deg C ) I also mixed the cement only with water.
This morning I tried the drop test and the mixture with sand broke,( although before dropping it on a pile of stones it was quite stable) the cement only with water held until the 3rd drop.

As I mentioned before it does state on the bag NOT to be mixed with other building materials.

Today I mixed 4 very, very small batches of the cement and water and did my header course, it was easy to apply and the gaps are all less than 8mm at the back.I have covered it with a tarapaulin to try keep it going off to quick.

Cant post photos yet as my battery on the camera needs charging.

I will endeavour to seek out some fireclay which I believe to be different from the stuff I have, I think its made from Chamonix ?? have you ever heard of this? Maybe it will work better for the home brew mix.
It sounds like your fire cement already contains high temperature aggregate added to it i.e. calcium aluminate cement and grog. In which case if you add more sand you will end up with a weaker mortar.That would also explain why they state not to add other materials to it. Usually if the word cement is used it refers to the active ingredient. It should really be labelled high temp refractory mortar to avoid confusion.

Don't know what Chamonix is apart from skiing in France, but perhaps it is chamotte which is fired and crushed clay also known as grog. This is not really what you want, it will act more as an aggregate. The clay content in the home brew mortar is just to give it some stickiness. Any powdered clay should do.Try to get some bricklayers clay. Sand is quite ok to use in the mortar for the temps we fire to. Sand at high temps ( getting around 1000C can turn to glass, if in the presences of certain fluxes.

Last edited by david s; 08-25-2013 at 08:45 PM.
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Old 08-26-2013, 02:06 AM
yorkshireknight's Avatar
Join Date: Feb 2013
Location: Veliko Tarnovo. Bulgaria
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Default Re: Mortar Joints

David once again your expertise on this subject is of excellent help.

I have just returned from visiting several suppliers of building materials, some were helpful, some not.
The mixture I have is what they call a glue which is cement based, so I would take it that there is sand present, they recommend it for the purpose I need, I asked them about the max 8mm gap and they said don't worry if it goes over that (well they would wouldn't they, I suppose)
The mixture I tested the other day is solid unlike the mixture with the sand, thereby agreeing with your idea that the weak is too lean.
I also enquired about the Chamonix/ shamot and got puzzled looks, so yep it must be the skiing area in France.
I will endeavour to use this 'glue' with just added water throughout as so far its stuck good, the price is equivalent to about 10 for 25kgs so not a great wallet breaker.

What would happen if I had a gap say of 25mm (3x the recommended) what would be the detriment?

Once again many thanks for the invaluable information

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Old 08-26-2013, 02:59 AM
david s's Avatar
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Default Re: Mortar Joints

If you are worried about too big a gap jamb a wedge of brick in there, you should have plenty once you start cutting.
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