Fire cement

[david s]

Re: Fire cement

I also forgot to mention he strongly advised against builders sand as there are too many unknown particles that could shrink crack or blow

This may be true, but we are not firing our ovens to stoneware temperatures. You are building a WFO here, not a kiln.The Homebrew has proved time and again, for the temperature range we fire to, perfectly adequate. Calcium aluminate cement based mortars (fondu) are more difficult to work, they go off way faster and are considerably more expensive. Regarding grog it is basically sintered and crushed clay and there are a great variety of grogs available which vary in grain size and clay composition used. I think the cheapest grog with fairly fine grain size (no bigger than 60 mesh) is adequate.

[Niro]

Re: Fire cement

Does anyone know\used 'unibond' mortar and does it suitable for building an outdoor dome

[leetheldc]

Re: Fire cement

So this home brew would last me at least 20 years?

[david s]

Re: Fire cement

So this home brew would last me at least 20 years?

Yes. I've seen plenty of ovens in Europe, that would have used lime based mortars, that must have been 500 years old and looked perfectly usable. Likewise those in Pompeii over 2000 years.

[wotavidone]

Re: Fire cement

Its what gives the mix the stickness and workability that makes it easy to use for mugg bricklayers. Too much of a good thing is still to much so don't overdose it or it can lead to cracking.
Regards dave

I agree Dave, to the extent that on the oven I helped my mate build, I cut the proportion of clay back. I modified the homebrew to 3.5 sand:1 cement:1.5 lime:0.5 clay.

Reasoning was that the clay I was using seemed high shrinkage, so I just wanted to use enough to keep that lovely thixotropic effect.

I substituted more lime rather than cement - the ideal is to use enough cement for initial set, then lime for long term resistance to thermal degradation. However, many manuals warn of problems if the lime is more than 2 times the cement. So 1.5 parts lime.
And I pushed up the sand content a bit because it was brickies sand, so some of it was clay anyway, and I wanted to be sure I got a full quota of sand.

Don't forget to buy fresh lime. My mate couldn't see why we couldn't use my 2 year old part-used bag of lime when we built his oven, but I finally convinced him that spending $9 on a fresh bag was cheap insurance.

I keep looking for signs of my mortar disintegrating in my first oven, but the worst I have seen is separation from the brick in the outer arch the other day, when I got really ambitious with regard to heat. After 1.5 hours of raging fire using pine off-cuts, the dome temp was off-scale for my 550C infrared thermometer.
There was a large temperature differential on the uninsulated outer arch, and I now have a hairline crack between the wedge of mortar and one of the arch bricks. The arch still stands, and the mortar itself is still rock hard.

[stonecutter]

Re: Fire cement

Yes. I've seen plenty of ovens in Europe, that would have used lime based mortars, that must have been 500 years old and looked perfectly usable. Likewise those in Pompeii over 2000 years.

Important to point out though...not one of those mortars contain Portland.

[wotavidone]

Re: Fire cement

Important to point out though...not one of those mortars contain Portland.

Indeed.
In fact, if I lived in Europe where these things are easily obtainable, I would do some research and consider whether I might even just use sand and hydraulic lime. Never seen it in Oz, unfortunately.

[david s]

Re: Fire cement

Important to point out though...not one of those mortars contain Portland.

The ancient Roman technology was way better than most people imagine. The Colloseum and the Pantheon were both concrete structures. They may not have called it Portland but it was still cement.

[stonecutter]

Re: Fire cement

The ancient Roman technology was way better than most people imagine. The Colloseum and the Pantheon were both concrete structures. They may not have called it Portland but it was still cement.

Correct...but the pozzolans used were not like modern cement. I've done some extensive reading on roman engineering and concrete work...it's a shame a lot of their secrets have disappeared. Years ago there was a fascinating documentary on the dome construction of the Pantheon...still the largest unreinforced dome in the world

[stonecutter]

Re: Fire cement

Indeed.
In fact, if I lived in Europe where these things are easily obtainable, I would do some research and consider whether I might even just use sand and hydraulic lime. Never seen it in Oz, unfortunately.

Based on my experience working with and researching Lime use, I feel that a lime and sand only mortar wouldn't be a great mix for ovens. But, that doesn't mean it wouldn't function, just that I question long term durability to thermal cycling.

[Gulf]

Re: Fire cement

I have never (ever) layed a brick with lime mortar. I have though, reclaimed thousands of brick from old houses, fire places, and chimneys that were built with it. From what I have seen, it may be superior to portland and lime (what we call mortar today) for longevity. I don't know if the old folks ever repointed their fire places like were done for those built from the 50's thru the 70's. If they did, the evidence is hard to find. I have seen clay brick in fire boxes that was eaten about 1/2 way through. The lime mortar followed suit. I reclaimed the brick from one brick house (the exterior walls were 2 bricks thick with row locks on about 4 feet intervals). When I got down to grade, I found that the brick just widened out the width of one brick per course. That was about five courses down to the bottom of the footer. There was no concrete under the footer, just undesturbed clay based soil. There were no cracks in any of the brick walls. I don't remember the source, but I, also remember reading about lime mortar being more elastic for settling, earthquakes etc.

just sayin'

[stonecutter]

Re: Fire cement

I'm not going any further than saying I love working with lime, least this turn into a 100 page thread. Portland has a place, but my general feeling is lime has been pushed aside because it is faster to use, and takes less knowledge to work with....an unfortunate byproduct of modern construction. Masons have to make a living, so you do what you gotta do....this is not a shot at guys that haven't, can't or won't incorporate lime usage in their work.

[wotavidone]

Re: Fire cement

I'm not going any further than saying I love working with lime, least this turn into a 100 page thread. Portland has a place, but my general feeling is lime has been pushed aside because it is faster to use, and takes less knowledge to work with....an unfortunate byproduct of modern construction. Masons have to make a living, so you do what you gotta do....this is not a shot at guys that haven't, can't or won't incorporate lime usage in their work.

That's the way of the world, sadly.
I'm an old school assayer, and I loved doing it the old ways, but the fact is a machine run by a trainee can do it faster and cheaper than a qualified bloke with a balance and and a burette, so I'm not able to make a living the old fashioned way anymore.
Happily, old time lime work is making a little bit of a comeback. There was even a building industry course on restoring lime mortared stone buildings a couple of years ago here in South Oz.

[david s]

Re: Fire cement

the Pantheon...still the largest unreinforced dome in the world

And an absolute must see if you visit Rome. I stood in on a university lecture going on when we visited it.....enthralling.