Slag In Concrete 1

[BSVBD]

The Concrete Quality Control Lab Coordinator is requesting to include ground, granulated blast furnace slag (slag/ggbfs) in the concrete mix.

From past project concrete mix submittals I observe that when slag is used, fly ash is not, and vice-versa.

Resources claim that both fly ash and / or slag have similar benefits to include improved workability, higher compressive strength, etc...

So what does it REALLY do for the concrete mixes?

Is one preferred over the other? Why?

THEN... the concrete supplier asks, "If I can use slag in the mixes, what is the maximum percent replacement allowed?

How much concrete chemistry am I expected to understand?

Shouldn't the vendor furnish a lasting concrete product able to satisfy the specified compressive strength without asking us how to make it?

Thank you!

 

[JAE]

Maybe these free ACI videos might help.
We usually limit fly ash to maximum 25% of the total cementitious materials.

https://www.concrete.org/education/freewebsessions/completelisting/coursepreviews.aspx?id=51688734

https://www.concrete.org/education/freewebsessions/completelisting/coursepreviews.aspx?id=51686894

and the ACI document on the topic of slag cement:

https://www.concrete.org/store/productdetail.aspx?ItemID=23317&Format=DOWNLOAD&Language=English

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[racookpe1978]

If slag is used, how is its rusting, discoloration controlled? (From a non-concrete engineer.)

 

[TehMightyEngineer]

Fly ash and slag can be used together if I recall but the ratios are critical to ensure a consistent, workable mix. You want more fly ash than slag. Typically you use either fly ash or slag in a mix for simplicity by my understanding.

Slag can work up to a point and then it becomes "sticky". We've had some of our slag mixes literally get stuck in our buckets and refuse to pour without vibrating them out of the bucket.

Slag can increase initial and final set time significantly at greater than 20% of cementous materials.

Slag will increase your early and 28 day strengths vs regular Portland cement.

I'd keep slag to around 20% of your cementous materials in the mix but if the mix is workable enough for the contractor then going as high as 30% wouldn't be a problem by my understanding.

There are additives that can "fix" a "sticky" mix. They can be added after mixing into the truck with no change in properties other than reduced "sticking" of the mix.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JLNJ]

Despite its name, the slab does not contain iron. So no rusting.

If you read the GGBFS Wiki page you'd think the stuff is quite magical.

 

[Ron]

GGBFS can be a good additive to concrete, as long as you understand the properties. Same with fly ash.

Both will retard strength gain. The old rules of thumb for portland cement concrete of 70% strength at 7 days will not apply. For GGBFS you'll see 45 to 50% in 7 days depending on the percentage. For fly ash, sometimes even slower, again depending on percentage.

The advantages of both fly ash and GGBFS are that both will decrease the alkali available for reactivity with certain aggregates, such as chalcedony chert and other known alkali-silica reactive aggregates. The disadvantage of reducing alkalinity in the cement paste is that both carbonation and reduced passivity may increase rebar corrosion potential.

A properly designed concrete mix with either or both can work well as long as you match them to the application. Durability of the concrete is not necessarily increased with either. Neither of them should be used to compensate for poor mix designing or poor concrete placement and control.

 

[cvg]

flyash and slag reduces heat of hydration. this is useful in mass concrete or with hot temperatures. however the strength gain is slower so that might be a reason to reduce the amount of flyash or slag in normal structures.

https://link.springer.com/article/10.1007/s10973-017-6914-z