#11  
Old 05-15-2014, 08:23 PM
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Default Re: limestone vs. brick construction

I would like to hear from stonecutter, I understood that lime was added to a Portland cement mortar to improve workability and offered some self healing of shrinkage and other small cracks via the lime dissolving and recrystallising there seems to be some idea that it makes the mortar flexible there is no such thing as flexible cement to my knowledge
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Old 05-15-2014, 08:40 PM
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Default Re: limestone vs. brick construction

As I said, lime is not limestone. Lime is added to mortar for several reasons, and in fact lime mortar with no portland cement is often a better choice than a portland cement/lime mortar (PCL).

Mortar is not "flexible" per se, it has a modulus of elasticity (MOE) measured by flexural strength. "Strong" or "hard" mortars made with a high ratio of portland cement and no lime have a low MOE, while lime mortars and PCL mortars have a higher MOE. High lime mortars also are capable of autogenous healing, while portland cement does not have this property.
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Old 05-15-2014, 08:54 PM
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Default Re: limestone vs. brick construction

Note also that autogenous healing takes place on a microscopic scale, not a macroscopic one. That is, if you can see a crack, it ain't gonna heal itself, no matter what the composition of the mortar is.
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Old 05-15-2014, 09:30 PM
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Default Re: limestone vs. brick construction

I'm not sure I can add much to what's already been said. The most important parts of this discussion are these:

Limestone is not a suitable material to build an oven with.

Limestone is not the same as type S or NHL used for mortar.

Tom and I could probably turn this into a 20 page thread about lime and limestone. In the end, all the info is out there, and let the builder use discretion.
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Old 05-15-2014, 09:40 PM
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Default Re: limestone vs. brick construction

Quote:
Originally Posted by Toomulla View Post
I would like to hear from stonecutter, I understood that lime was added to a Portland cement mortar to improve workability and offered some self healing of shrinkage and other small cracks via the lime dissolving and recrystallising there seems to be some idea that it makes the mortar flexible there is no such thing as flexible cement to my knowledge
I will touch on this a bit, and say that numerous study's have shown lime mortar to be superior to opc based mortar in many ways. When opc is added to lime, the mortar has a higher compressive strength, but comes at a cost... Breathability and tensile strength...that being relative to masonry. There is a ton of information out there about opc and lime mortar...it's not really that critical to small scale projects like what you see on the forum.

The main focus should be on the material used for building, with what type of mortar a close second...and there is a lot of forgiveness in that area
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Last edited by stonecutter; 05-15-2014 at 09:43 PM.
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Old 05-15-2014, 11:26 PM
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Default Re: limestone vs. brick construction

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Originally Posted by Tscarborough View Post
Limestone is not lime.
Obviously. However, nonhydraulic lime mortars set by carbonation, i.e. turning back into calcium carbonate (the principal constituent of limestone).

Lime mortar - Wikipedia, the free encyclopedia

So the question still remains - why is this calcium carbonate good when it's providing long term strength to the mortar, bad when it's the masonry unit?

Last edited by wotavidone; 05-15-2014 at 11:29 PM.
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Old 05-16-2014, 05:15 AM
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Default Re: limestone vs. brick construction

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Originally Posted by wotavidone View Post
Obviously. However, nonhydraulic lime mortars set by carbonation, i.e. turning back into calcium carbonate (the principal constituent of limestone).

Lime mortar - Wikipedia, the free encyclopedia

So the question still remains - why is this calcium carbonate good when it's providing long term strength to the mortar, bad when it's the masonry unit?
Show me one example where limestone was or is being used as a liner for a furnace, wood stove, wood oven...or even a fire box in a fireplace. How about limestone burnt in a kiln, ground down then moulded into units used in the previously mentioned applications? You won't. It will spall, delaminate, and degrade with every cycle.

It's weak in that state, and it contains other minerals that react poorly to thermal shock...feldspar and quartz to name a couple. And Lime in mortar is never the only binding component when used in thermal applications ( ovens, fireplaces)....other pozzolins are added, like clay or crushed brick. These add strength and durability to the mortar.
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Old 05-16-2014, 07:43 AM
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Default Re: limestone vs. brick construction

Quite frankly, I do not know what purpose the lime serves in home made refractory cement other than as a plasticizer.
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Old 05-16-2014, 03:25 PM
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Default Re: limestone vs. brick construction

Thanks to all for the responses I agree with all the recent comments, I would say we have established that limestone is not a suitable material for long term success ( if that is all you have got and you don't care how long it lasts go for it) have we also established that the Idea that mortar is flexible or self repairs is incorrect and that we rely on gravity friction and adhesion to keep the components in place with the mortar suction and adhesion assisting while laying the bricks only particularly as the chemical reactions rely on moisture and we strive for a moisture free environment once the dome is complete
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Old 05-16-2014, 05:00 PM
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Default Re: limestone vs. brick construction

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Originally Posted by Toomulla View Post
Thanks to all for the responses I agree with all the recent comments, I would say we have established that limestone is not a suitable material for long term success ( if that is all you have got and you don't care how long it lasts go for it) have we also established that the Idea that mortar is flexible or self repairs is incorrect and that we rely on gravity friction and adhesion to keep the components in place with the mortar suction and adhesion assisting while laying the bricks only particularly as the chemical reactions rely on moisture and we strive for a moisture free environment once the dome is complete
I have a lot of unanswered questions, myself.

For example, I question whether lime is only to provide plasticity.

Think Brick Australia represents the brick manufacturers of Australia.
They have an Industry Reference Guide.

http://www.thinkbrick.com.au/system/...pdf?1394520197

Part 2 deals with mortars and references Australian Standard 3700
Two interesting things jump out at me when I skim it.
1) Type M2 mortar is the preferred mortar for fireplaces and barbecues. It consists of 1 part cement, 2 parts lime, 9 parts sand.
2) AS3700 does not permit the use of fireclay as an admixture, which might explain why it's so hard to get at an ordinary hardware store, rather than just the relative scarcity of fireclay deposits in Australia.

It's commonly acknowledged that Portland cement decomposes at temperatures easily achievable in a pizza oven or incinerator. So we'd better hope that the mortar specified by Standards Australia and the industry representative body is using the lime for something other than plasticity while wet, eh? Though the reference guide does extol the virtues of lime for this purpose.

Which brings me back to my original point.
That is, we use lime, which is limestone burnt and slaked to form calcium hydroxide, as at least one of the binders in mortar. Everything I have ever seen on this says that this lime hardens by carbonation. Which is essentially turning it back to limestone. At that point we say the lime component is contributing to the strength of the mortar, and lose no sleep over its durability at pizza oven temps.

Given that using limestone as a masonry unit in a fireplace or oven is considered a bit of a no-no, I'm a bit bemused by the somewhat passionate replies I get for mentioning this apparent contradiction between using lime in mortar (where we expect it to turn back to limestone) and using limestone.
Anyway, all I originally suggested was that since the OP had access to these limestone blocks, and was planning to spend short period of time building a dry stacked oven as an "inexpensive first step towards something more permanent", little would be lost if he gave it a go and it didn't work.

I wasn't suggesting building a fireplace in anyone's house, or pressing burnt lime into bricks or any such nonsense, and I do know the difference between limestone and lime.
Ah, the joys of trying to have a reasoned discussion with the denizens of the internet.

By the way Toomulla, we only strive for a moisture free environment in our oven after we have given the initial moisture time to chemically react with the binders we use, whatever they might be. Concretes and mortars that are allowed to dry out before these reactions take place will be weak.

Last edited by wotavidone; 05-16-2014 at 05:18 PM.
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