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homegrown kiln- to calcine or not to....castable formulation

updated thu 13 dec 07

 

Robert on fri 7 dec 07


To clarify and repost:
Glad clayart is back up. Woohoo. What I'm looking for is the
scientific basis for the mechanical strength of castables. This
material has zero fusion at 2400. The mechanical strength is provided
by the calcium alumina cement as opposed to ceramic fusion. So in a
general or specific way I need to know the correlation between
mechanical strength and a) aggregate particle size including fines b)
proportion of CaAl cement and c) proportion of calcined materials.
Technical research /data (acers publications perhaps??) or a general
reference/understanding would be greatly appreciated. Original post
follows.
TIA
Robert
> Robert wrote:
> Hi All,
> Been off list a while. Last November I walked away from 10 years of
> accumulated equipment and am starting again from scratch. Over the
> summer I researched a local deposit of high alumina (30% average) flint
> fireclay which is primarily kaolinite in its mineralogy. Hank has been
> kind enough to fire a couple of test bricks for me in determining a
> suitable castable recipe. The basic recipe is 4pts flint fireclay,2
> pts. saw dust to 1 pt calcium alumina cement (70% alumina) The deposit
> is quite large and has boulder size pieces down to gravel and fines. In
> my first set of tests I screened two different sizes of fire clay
> 1/2"-1/4" and 1/4" to fines and used them in equal parts for one set and
> 1/4" to fines exclusively in another set. The test came out looking
> very much like commercial IFB's with some spalling/shrinkage cracks and
> zero fusion at cone 10. The tests with the larger aggregate were
> obviously more crumbly. In addition my saw dust was a fairly coarse fir
> end grain shaving which seemed to contribute to a lack of mechanical
> strength. I have since dug enough material to build a 30 cu ft kiln and
> just received word from Hank today that the second set has been fired.
> For this test I used only 1/4" to fines and I varied the proportion of
> saw dust as well as using sifted table saw sawdust with a more
> vermicular shape. I have yet to see the tests to assess them but have
> been considering over the months the possible need to calcine all or a
> portion of the kaolinite. The kiln I am constructing will be fire to
> cone ten. Cast as separate largish blocks. The brick will be air set
> only and with a 7" wall thickness the temperature differential between
> the hot face and exterior will be significant.
> So the question is: what advantage, in terms of mechanical strength,
> would be gained by calcining all or a portion of the kaolinite. It
> would not be a simple thing to calcine the volume of material I'm
> dealing with (about 2 cu. yds.) Or, alternatively, what other additions
> might you suggest to increase the mechanical strength of the castable.
> I am trying to use primarily local materials but this kiln will not be
> purist in that sense. Any other thoughts or comments would also be
> appreciated. So... TIA
> Robert
>
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>
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Michael Wendt on sat 8 dec 07


Robert,
Google the word "metakaolin"
The strongest concretes and mortars
contain specific proportions of this
form of kaolin. Ashgrove Cement
has shown an interest in Helmer
Kaolin for this purpose. Penn State's
materials prof Michael Grutzeck
p>
has done extensive work in this area.
Basically, you heat Helmer Kaolin just
hot enough to drive off the water and
hold briefly, then cool. Mixing this with
your cement produces 3 to 4 times
stronger chemical bond than standard
concretes and mortars. Use preshrunk
aggregates like grog and Kyanite for the
larger material.
If you have an electric kiln with a computer
controller you can write a program to fire
Helmer Metakaolin by holding at 1125 F
for 1/2 hour, no hotter as sintering will
occur and produce non-reactive clinkers.
Additions as small as 8-12% of the total
mix produce large gains in strength.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
Robert asked:
To clarify and repost:
Glad clayart is back up. Woohoo. What I'm looking for
is the
scientific basis for the mechanical strength of
castables. This
material has zero fusion at 2400. The mechanical
strength is provided
by the calcium alumina cement as opposed to ceramic
fusion. So in a
general or specific way I need to know the correlation
between
mechanical strength and a) aggregate particle size
including fines b)
proportion of CaAl cement and c) proportion of calcined
materials.
Technical research /data (acers publications perhaps??)
or a general
reference/understanding would be greatly appreciated.
Original post
follows.
TIA
Robert

Robert on sat 8 dec 07


The material is flint fireclay. It's primary mineral is kaolinite.
The analysis is:
SiO2 51.2
Al203 30.4
Fe2O3 3.6
Ti02 2.4
Ca,K Na are all under 0.2
LOI @ 700C 11.5
LOI @ 950C 11.8

The question regarding calcining would be if the driving off of the
chemically combined water would impact the mechanical strength of the
aggregate negatively. And if this would have an adverse effect on the
aggregate bond of the (CaAl) cement. In addition, since, in practice
the exterior sections of the kiln wall may not reach sufficient
temperature to drive off the chem combined water, will it compromise the
integrity of the individual section in terms of differential volume or
differential thermal expansion. The interior will reach cone 10 but the
exterior perhaps only 350F. I won't be firing these bricks except as
part of the kiln wall. So I see the potential for weakness developing in
the transiton from hot face to cold. I may be overthinking this but I
would like to understand the principles to maximize the strength of the
castable.

As to metakaolin ... I think this is related to geopolymers no? I've
not had time to study them as much as I would like. Very intriguing for
low fuel input cements. Faster setting, harder setting and better
thermal expansion properties than portland.

thanks for the replies so far..
Robert

Ivor and Olive Lewis on sun 9 dec 07


Dear Robert,
Must be more than twenty years ago that I made a small kiln using a =
ramming mix to line a 50 gallon oil drum. I followed the recipe given by =
Jack Troy in his book on salt glaze ceramics. I still believe this is =
good information. His use of Portland Cement as a binder ensures that =
the exterior of a rammed or cast wall had mechanical strength. I got =
good service from my experimental kiln though the steel shell corroded =
before the refractory collapsed.
I think the high level of Iron oxide in your raw fireclay may be a =
problem. In a reducing atmosphere this will enter into a solid state =
reaction with such free silica as might be released during a calcining =
operation, or even at a later date, to form Fayalite. This has a melting =
point of 1205 deg C +/- ?(2200 F) which would set the upper service =
temperature for your Kiln well below you intended firing point of cone =
10.
Though I would not dispute tha advantages of calcining Kaolin to get =
Meta-kaolin, I would be wary of introducing any ingredient that might =
flux the refractories. Good mechanical strength can be secured by solid =
state sintering to a temperature that would induce grain growth and =
recrystallisation. This will lie somewhere between the Tamman Point and =
the fusion point of the most easily melted ingredient.=20
I wish you success with this venture.
Best regards,
Ivor Lewis.
Redhill,
South Australia.

Robert on mon 10 dec 07


Ivor and Olive Lewis wrote:
> Must be more than twenty years ago that I made a small kiln using a ramming mix to line a 50 gallon oil drum. I followed the recipe given by Jack Troy in his book on salt glaze ceramics. I still believe this is good information. His use of Portland Cement as a binder ensures that the exterior of a rammed or cast wall had mechanical strength. I got good service from my experimental kiln though the steel shell corroded before the refractory collapsed.
> I think the high level of Iron oxide in your raw fireclay may be a problem.
Thanks Ivor,
I'll look into the Jack Troy reference. The analysis I quoted was
actually a composite average. The actual analysis range from 0.28 to
3.0 in the quarry area. In addition the iron content is relatively
discrete and has been fairly excluded in our collection of materials.
The quarry site is about two acres and the whole deposit is close to
twenty. There is in excess of 20 million tons in the total ore body, so
it is not hard to be selective. The brick we have tested so far, at
cone 11, have burned almost pure white with occasional iron spots
typical of hard brick refractories. The material was quarried in the
40's and shipped to Portland for the manufacture of refractory bricks
and I have seen abundant evidence in the local vicinity that at least
one local brick maker was also producing brick using this material
prior to that time.

As to the Portland cement, I am using a high alumina (71%) calcium
alumina cement purchased from a refractory manufacturer in Portland. I
would prefer to use a 30%-50% alumina cement for cost but have not been
able to find a local supplier on the NW coast. My understanding is that
Portland cement has a tendency to flux excessively at high temperature
due to the inclusion of a higher proportion of silica, sodium and
potassium from the raw stock shales. The refractory qualities of the
CaAl cement are controlled by a using very low percentage of silica
to produce calcium aluminates rather than aluminosilicates. As I
understand it, in their varying grades, these have been industry
standard in monolithic castable production for years.

I have realized that my previous question regarding calcining was not
the most accurate. The real question should be whether I should convert
all or a portion of the flint fireclay into grog. As Duff pointed out,
the mechanical strength of the aggregate will follow a u curve. My
firelcay aggregate portion of the mix is 1/4" to fines. Across the
thermal zone from hot face to kiln exterior I will theoretically have
aggregate in all positions on the curve. The weakest point will be
where the temerature has reached water smoking temperature but no fusion
has taken place. The strongest, over time, will be the hot face. The
question becomes, is the strength of the weakest section, in the
midzone, with 1/4" calcined (naturally, by firing cycle/position) flint
fireclay sufficient to overcome the differential thermal expansion and
contraction of typical repeated cone 10 firings. Curious if anyone
knows if castable manufacturers use grog for their aggregate or some
other raw stock. Using grog would balance the strength of the
aggregate over the whole thermal zone, though it would seem the hot face
would increase in strength through repeated firing cycles due to mullite
formation.
I appreciate the discussion. It is definitely helping to clarify the
issues.

Take care,
Robert

Hank Murrow on mon 10 dec 07


On Dec 10, 2007, at 10:41 AM, Robert wrote in part:

> I have realized that my previous question regarding calcining was not
> the most accurate. The real question should be whether I should
> convert
> all or a portion of the flint fireclay into grog. As Duff pointed
> out,
> the mechanical strength of the aggregate will follow a u curve. My
> firelcay aggregate portion of the mix is 1/4" to fines. Across the
> thermal zone from hot face to kiln exterior I will theoretically have
> aggregate in all positions on the curve. The weakest point will be
> where the temerature has reached water smoking temperature but no
> fusion
> has taken place. The strongest, over time, will be the hot face. The
> question becomes, is the strength of the weakest section, in the
> midzone, with 1/4" calcined (naturally, by firing cycle/position)
> flint
> fireclay sufficient to overcome the differential thermal expansion and
> contraction of typical repeated cone 10 firings. Curious if anyone
> knows if castable manufacturers use grog for their aggregate or some
> other raw stock. Using grog would balance the strength of the
> aggregate over the whole thermal zone, though it would seem the hot
> face
> would increase in strength through repeated firing cycles due to
> mullite
> formation.

Perhaps if the kiln were insulated effectively for the first two or
three fires, the brick might reach a temperature high enough in the
'weak' zone to provide strength.

Cheers,

Hank.......... who wonders still when you want the sample bricks I
fired for you.

Michael Wendt on tue 11 dec 07


Robert,
Hank is right. The way to cast the kiln is
thin wall backed on the outside with
Kaowool. The result is a kiln that is
sound, strong, economical and does not
shed Kaowool onto the work.
Cover the outside with sheet metal just for
protection of the Kaowool. Insulating the
outside and making the walls much thinner
will cure the refractory liner fully and
result in much better fuel economy than
any other scheme.
Regard,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com

Duff bogen on wed 12 dec 07


Robert
Ivor's experience say Portland cement can work.
As you say "Portland cement has a tendency to flux excessively at high temperature" but that speaks to finding a balance- what's an excessive amount of P.C. to use.
"as the girl in the story says- not too hot, not too cold but just right."
Guys on the other hand can get stuck on gnarly cone 47 refractories {Harrrr! say it with cajones} and overlook lower tech options.

Duff
Robert wrote:
Ivor and Olive Lewis wrote:
Jack Troy...His use of Portland Cement as a binder ensures that the exterior of a rammed or cast wall had mechanical strength. I got good service from my experimental kiln though the steel shell corroded before the refractory collapsed.
Portland cement has a tendency to flux excessively at high temperature

______________________________________________________________________________
Clayart members may send postings to: clayart@lsv.ceramics.org

You may look at the archives for the list, post messages, or change your
subscription settings here: http://www.acers.org/cic/clayart/

Moderator of the list is Mel Jacobson who may be reached at melpots2@visi.com




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