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  #201  
Old 05-17-2011, 07:56 AM
Journeyman
 
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Default Re: It is begun

Being the guru of all things concrete and masonry tscarborough, do you have any idea how much heat can be applied to part of a (thinish) concrete slab before it spalls or cracks? Your grill/countertop proximity is similar to one of my kitchen tops which is next to my 130,000btu commercial wok burner gets too hot to touch if I'm using the wok for any period of time. It's been fine over the six years it's been in use, but I figure some day it's going to object.
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  #202  
Old 05-17-2011, 08:30 AM
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Default Re: It is begun

500 sustained degrees will cause the portland cement matrix to disintegrate. Direct flame will cause spalling, sometimes explosively. I have burned the grill quite a few times and measured the heat in that area, and it remains below 400 degrees.
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  #203  
Old 05-17-2011, 01:19 PM
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Default Re: It is begun

Tom,

Now you got me scared. The joints facing the inside of my oven are approximately 3/16" and made with 3-1-1-1 homebrew mortar. With this recipe, the portland comprises 16% of the mix. Exposed to direct flame, how long will it take for this mortar to disintegrate and eventually fail?

John
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  #204  
Old 05-17-2011, 01:47 PM
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Default Re: It is begun

That's what the lime in the mix is for. If the Portland cement fails the lime takes over as it can handle the heat.
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  #205  
Old 05-17-2011, 04:14 PM
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Default Re: It is begun

Below 20% the portland acts as a "gauging" agent, not the primary cement. It is used for a rapid enough set to allow the work to proceed. Lime is an excellent cement, but it does not have an initial set. It hardens over time by the process of absorbing carbon dioxide from the air, and thus something has to be added to give the initial set.

If it was readily available, brick dust is a better choice as a gauging material, and when I have it, that is what I prefer to use for lime mortars/plasters.
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  #206  
Old 05-17-2011, 05:47 PM
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Default Re: It is begun

Thanks for the explanation, Tom. Makes me feel a lot better since I opted for an 'old world brick' look to the inside of the dome rather than the 'mortarless stacked brick' look.

Two questions then:

Given portland's initial-set benefits and lime's cementious qualities, can the 3-1-1-1 proportions be tweaked to reduce portand's refractory weakness?

What is the difference between brick dust and fire (mortar) clay?

I am continually amazed at the amount of knowledge you have and readily share.

Thanks a million,

John
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  #207  
Old 05-17-2011, 06:22 PM
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Default Re: It is begun

As a rule, 10% is considered the max for gauging material. For plaster or anything similar I would not use more. For a compression structure, the mortar acts mainly to hold the units apart and won't fall out unless mechanically disturbed, so 20 to 25% Portland cement by volume of the cementious materials is not a problem.
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  #208  
Old 05-17-2011, 06:30 PM
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Default Re: It is begun

a far better thing to do than mess with the portland, lime, aggregate ratio is to add about .25 parts of a pozzolan to the mix. Pozzolans speed up the reaction process of the lime, extend the life of the portland cement and add initial hardness themselves. It is one of the many things I add to my concrete spec mixes for almost everything now.
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  #209  
Old 05-17-2011, 06:32 PM
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Default Re: It is begun

Pozzolans are a great gauging material if available.
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  #210  
Old 05-17-2011, 06:38 PM
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Default Re: It is begun

Wow. Pozzolans add initial hardness, speeds up lime action, extends the life of portland and keeps the wife satisfied. What else could a guy want?

More importantly, where can I get this miracle stuff?
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