Fly ash vs Limestone powder 2

[KimWT]

Hi!

Last week I posted my question about fly ash market.
Most engineers can predict that fly ash supply will reduce eventually.

This time, I wonder limestone powder can be a substitute (supplementary cementitious materials) for fly ash.
This means calcined lime is used for cement and its 'raw' powder instead of fly ash will be added later.

Another question is a small concrete company can make trustworthy mix design.
(Right now, my company has been using fly ash with the ratio of 20% of concrete by weight.)
A small concrete company can do only compression strength and workability tests.
But, other physical and chemical data are needed to use this concrete for major structural concrete members.

Please share your knowledge and experience.

Thanks!

 

[concretemasonry]

Fly ash is really just a pozzolan that can bond with "left over" materials in concrete (mainly calcium). This is because the cement hydration process is not 100% efficient balanced reaction.

The complete reaction between the materials is slow at normal temperatures and there can still be a substantial amount of calcium unbonded after months or years (about 30%). There are processes using heat to accelerate the rate. One is an autoclave process where the concrete is subjected to 360F in a saturated atmosphere (150 psi), but is applicable to for smaller units that has large autoclaves (pressure vessels) that can be loaded and unloaded on a scheduled basis (4 to 6 hour cycle) and can achieve very high strengths (far beyond normal concrete) and uses all of the calcium during that shot cycle.

Fly ash has been used for centuries. The early fly ash was natural ash from volcanic activity, but ash from power plants is the main type used now.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BUGGAR]

The small batch plants I've worked with had contracts with labs for all the testing they couldn't do in-house. This isn't too unusual when you get into long term testing like concrete shrinkage tests requiring special equipment. Make sure the lab is friendly with the agencies requiring the testing.
Large contractors frequently set up batch plants in remote locations and have to meet government requirements so it's all doable. Sometimes the Contract even specifies the lab facilities that must be furnished by the Contractor.

 

[SlideRuleEra]

I wonder limestone powder can be a substitute (supplementary cementitious materials) for fly ash.

Yes.

The Use of Limestone in Portland Cement: A State-of-the-Art Review - Portland Cement Association, 2003

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[Ron]

To answer your direct question.....No. Limestone powder, depending on its source will be in the range of 75% calcium carbonate. This adds to the free calcium in the cement, and as concretemasonry noted, the hydration reaction does not even consume all the calcium that is presently in cement. Limestone powder will not have the same pozzolanic effect as fly ash and could be detrimental to the hydration process. It will likely also cause expansion and a higher tendency for delayed ettringite formation, which will be deleterious to the performance of the concrete in the long term.

 

[JedClampett]

If Fly Ash runs short, which I consider unlikely, there are other substitutes, like pulverized blast furnace slag. Powdered limestone is not a substitute.

 

[SlideRuleEra]

Reread the PCA article that I referenced and see now that the 5% limestone discussed in it has nothing to do with the question - I should have listened to concretemasonry and Ron.

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