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Temp limitations of Portland cement

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  • Gulf
    replied
    I'm trying to settle a dispute once and for all regarding the use of a using just a regular Portland based mortar for the high temp conditions of encounter in a brick oven or the firebox of a masonry heater for that matter.
    My internet usage is limited at the moment. But, I will respond from memory. Sometime around the 1940's the formula for portland based mortar changed. Up until that time Type-N mortar was 1 part portland and 1 part hydrated lime. That formula + the sand was just fine for fireplace fireboxes. The hydrated lime in the original formula was quite caustic. The formula was changed to read "hydrated lime or the equivalent.". Many manufacturers replaced some of the hydrated lime with crushed limestone and some other proprietary additives. That change made the the mortar made less caustic to the hands and skin of brick layers and it still allowed a workable mortar for laying brick. However, prior to the change, firebox mortar joints only needed to be repointed after 20 years or so. (The addition of fireclay would have added even more years to that imo) The problem with the " or the equivalent" is that crushed limestone swells when heated. That swelling pops the faces off of the firebrick requiring brick replacement.
    I did not understand it at the time while tending my brick and stone laying uncles years ago. Sometimes, we would use a bag mix to lay the fire boxes. Sometimes, we would make the mortar from scratch using portland and hydrated lime. My guess is that it was due to the availability of the few bagged mortars by manufactures that still made the original formula at the time.

    Sorry for not having Pi R Square charts. But, hopefully this will aid you in your search.

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  • mefornaio
    started a topic Temp limitations of Portland cement

    Temp limitations of Portland cement

    Hey all,

    I'm trying to settle a dispute once and for all regarding the use of a using just a regular Portland based mortar for the high temp conditions of encounter in a brick oven or the firebox of a masonry heater for that matter.

    I don't want to give the wrong impression but I don't need a recipe, I don't need an alternative advice or anything other than hard published information on the heat tolerances of the mix.

    I'm talking type 1 Portland, hydrated lime, sand and water.

    My father was a mason his whole life who went to trade school after college way back in the late 40's then apprenticed under is brother in law who was a mastor stone and brick mason.

    I worked for him from the time I was 12 untill my late teens. We did literally thousands of fireplaces, fireboxes, etc. We have always use FIRECLAY in a high temp mortar with Portland it's just the way I was taught.

    It's obvious to me that that worked and even the home brew I see on here of 3:1:1:1 has withstood the test of time.

    But I have been unable to find a study or published information on the subject.

    Thank you for any help I'm really sick of debating this with people that are seeking advice then argue"well I saw on YouTube or on this oven building stir or that blog or wherever some hack is offering advice to get views or sell something.

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