Concrete Masonry Handbook for Architects, Engineers, Builders (5th ed)
by by Portland Cement Assn.
"I was in a bookstore today and came across the "Concrete Masonry Handbook". In the section on fire resistance, it stated that at 800 deg F concrete undergoes structural changes and temps. over this are to be avoided. It said that by 800F all the water has been driven off and that the cement paste starts to degrade.
Specifically, it mentions spalling first due to the coefficients of expansion, modulus of elasticity, and modulus of rupture all seem to come together to a point where any further insult on the structure causes a marked increase in failure potential. In the case of fire, you have spalling on the heated face along with considerable expansion while on the cooler face the matrix remained essentially intact with little expansion. The result is the cool side acting virtually like a lever.
The article also addressed specific aggregates and the fire worthiness. For instance, it stated that quartz crystals failed at 1025F. Lightweight aggregates and pozzolans such as vermicilite and perlite were, predictably, the best performers.
Therefore, in the context of our fireplace mortar concerns, it seems to me the issue of a mortar's fireworthiness is a function of the cement paste (Portland, lime, refractories, etc.), the aggregates (types of sand, impurities, pozzolans, & stones in the case of concretes), and the thickness of the assembly. The article in question made this specific point that all things considered, thicker walls perform better. Less thermal expansion leverage against the cooler face. With this in mind, it would appear there is a good case for laying firebox brick in the stretcher position as opposed to the shiner position.
Jim, your thoughts on this? I know, I still don't have a number for Portland specifically, but I'm working on it."
I appreciate your research and scholarship. If you ever get tired of sweeping, you should be a professor.
My main thought on all of that is: Firebrick is a refractory product made of vitrified fireclay. Firebrick contains no Portland cement, lime, sand or other aggregates. While there are certainly some impurities and certainly some fireclay is better than others, firebrick works as a refractory basically because it doesn't expand much with heat and therefore the internal stresses are minimized.
As for thicker being better, I've heard that some of the exotic European ceramic flue liners are quite thin. I don't know that I would argue that shinner fireboxes are better than stretcher boxes but I might argue that shiners are good enough, meet code and require half as much time, material and labor to build.
Thanks again for the reference. I'll have a look at the "Concrete Masonry Handbook".
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