# Question About Manufacturing Fly Ash Bricks



## emccu (Nov 1, 2012)

Hey,

We are into manufacturing FAB here. But unfortunately, we are not getting the strength above 100kg/cm2

WE are using the following composition.

1. FA: 55%
2. LIME: 14% (70% purity, hydrated)
3. Sand: 29%
4: Cement: 2%

Curing it for 20 days. But when we strike two bricks together for more than 3 times, it gets broken from the center and the strength we are getting is around 100kg/cm2

What can be done to increase the strength of bricks? Any help would be appreciated.


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## Tscarborough (Feb 25, 2006)

Remove the lime and change the gradation of the sands as a starting point.


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## concretemasonry (Dec 1, 2006)

Just a guess without knowing your molding/manufacturing process, I would guess you have very poor compaction if the brick is breaking in half near the center. "Sand lime" brick are inherently weak and do take much longer to cure.

Also, if the conditions are not correct, the time may not be sufficient.


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## emccu (Nov 1, 2012)

We have a compression type machine which puts a compression load of around 35tonnes for 3 seconds while moulding the brick. From sand I mean the river bottom sand, we also tried Stone Dust from Stone Crusher Unit, that too did not help.

We tried curing upto a month, that too did'nt help much. Should we change the composition? If so, what should be the ideal composition?


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## emccu (Nov 1, 2012)

Tscarborough said:


> Remove the lime and change the gradation of the sands as a starting point.


Ok, then what would be the binder I should replace the lime with?


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## fjn (Aug 17, 2011)

*Fab*

Would not a good starting place for your answers be from the maker of the molding machine ???


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## emccu (Nov 1, 2012)

I indeed asked them, they said "we are not expert in helping you getting strong bricks, you have to look around.."


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## concretemasonry (Dec 1, 2006)

It may be that you are starting out fresh with the wrong equipment.

Where did your first mix design come from?

Lime takes time and some moisture. Proper compaction increases the density and makes it easier for the lime/aggregate mixture cure. Like all reactions, heat does wonders to speed up a reaction to get close to a final strength. Lime will never give you high compressive strengths, but that is not needed for typical applications.

The trick is to make the brick strong enough to palletize/stack the brick and be able to handle.

With the right equipment, controls and curing, making brick is simple. We made concrete brick and sand-lime brick for ourselves and all our competitors, since they hated to handle them in the plant and yard.


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## Tscarborough (Feb 25, 2006)

Portland cement and flyash are both binders, you may have to increase the PC, or use a double hydrated lime. The gradation of the aggregate is going to play a big part, with that high an amount of flyash, you will need no more fines.


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## emccu (Nov 1, 2012)

Alright. I will double check the company claims about compaction. One thing I want to know, can we make the brick, without including/mixing the sand/stone dust/ aggregate?

I mean only Fly Ash, Lime and Cement?


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## Tscarborough (Feb 25, 2006)

No. The definition of "concrete" is a matrix of graded aggregates coated with a binder. Without graded aggregate, it will be weaker, not stronger.


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## Tscarborough (Feb 25, 2006)

What moisture content are you running?


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## emccu (Nov 1, 2012)

dWe use enough water to properly moist the mixture. Too much water case the water to bleed upon compaction, and less causes bricks to break while shifting in the stockyard.


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## Tscarborough (Feb 25, 2006)

Obviously, but that is still a wide range of moisture. Truthfully, your best bet is to get a local expert to visit your location and work with you to achieve the strengths you want.


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## emccu (Nov 1, 2012)

I agree to that opinion. But what I want to know is the composition of the materials. Is that a correct or say, good composition to mix?

I will check two things, one is the compation and second is the Aggregate.


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## Tscarborough (Feb 25, 2006)

I have never heard of a mix design remotely similar to the one you describe used for anything, and can tell you that it is not close to the mix designs that I have seen for either CMU or concrete brick produced in a vibratory compaction plant.


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## emccu (Nov 1, 2012)

Can you suggest something? Keeping in mind the cost aspect, i would like to keep the cement on the lower side and fly ash on the higher side. What would you suggest?


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## Tscarborough (Feb 25, 2006)

Considering that the flyash is usually expressed as a (low) percentage of the PC, I really can't.


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## dom-mas (Nov 26, 2011)

Well i'm no expert by any means in making brick but my limited knowledge tells me that your flyash is WAY to high and your aggregate is WAY to low. Maybe try flipping the aggregate and flyash percentages. I can't see an aggregate level lower than 50% working out to well. Aggregate is usually pretty darned cheap and it's about the strongest part of that mix except maybe the PC which is such a tiny portion.


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## Tscarborough (Feb 25, 2006)

More like 95 percent aggregate by volume.


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## Tscarborough (Feb 25, 2006)

Concrete brick are made very differently than clay brick.

That video is funny. They are cooked in killens? 500x5# =1/4 ton?

More modern brick lines are quite a bit different than that.


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