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What is Slag?

Mary McMahon
Updated May 17, 2024
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Slag is a byproduct of metal smelting, and hundreds of tons of it are produced every year all over the world in the process of refining metals and making alloys. Like other industrial byproducts, slag actually has many uses, and rarely goes to waste. It appears in concrete, aggregate road materials, as ballast, and is sometimes used as a component of phosphate fertilizer. In appearance, slag looks like a loose collection of aggregate, with lumps of varying sizes. It is also sometimes referred to as cinder, in a reference to its sometimes dark and crumbly appearance.

This substance is produced during the smelting process in several ways. Firstly, slag represents undesired impurities in the metals, which float to the top during the smelting process. Secondly, metals start to oxidize as they are smelted, and slag forms a protective crust of oxides on the top of the metal being smelted, protecting the liquid metal underneath. When the metal is smelted to satisfaction, the slag is skimmed from the top and disposed of in a slag heap to age. Aging material is an important part of the process, as it needs to be exposed to the weather and allowed to break down slightly before it can be used.

Common components of slag include the oxides of silicon, aluminum, and magnesium, as well as sulfur, which is always present. It also contains phosphorous, calcium, ash, remnants of flux materials such as limestone, and remainders of chemical reactions between the metal and the furnace lining. Other compounds found in this material depend on the type of smelting. Non-ferrous smelting, used to refine copper, lead, and similar metals, produces highly ferrous slag, as iron is an undesired element. Ferrous smelting, such as that used to produce steel, creates non ferrous slag, as all the iron is used in the smelting process.

After slag is allowed to age, it can be sold for use in other industrial processes. One of the most common is as a constituent of concrete and cement. Slag works very well as a loose aggregate and can be ground to produce a more even grain. It is also mixed with materials for making roadways, used as ballast on trains and large trucks, and is also applied as phosphate fertilizer. When used as fertilizer, the slag is ground very fine before being spread, and it slowly time releases nutrients because it takes a long time to break down.

AboutMechanics is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AboutMechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By skniazi722 — On May 27, 2014

How many types of slag are there? What is the chemical analysis of slag? What is it according to result.

Sio2 25 (- + 5)%

Al2o3 7(- + 5)%

Cao 25 (- + 5)%

Fe2o3 6 (- + 5)%

Mgo 0-3%

K2O 1-4%

Na2O 0-1%

TiO2 0-2%

MnO2 80.07PPm

SO3 0-1%


Cl 0.0002

By yy22kk — On Mar 24, 2014

I put some slag in a part of my back yard where the soil stays moist. I put it there to drive on the part of the yard. Since I put the slag there, my office near the yard has a strong fishy odor when it gets hot outside. Also, I am having some allergic like reactions when I am in the office (mucus build up, burning in chest and stomach). I now see a brown slimy residue coming out of the soil area. It is very slippery. I am thinking about removing the slag but not sure if I can do it without an environmental specialist.

By anon340071 — On Jun 29, 2013

What is the difference between basic and acidic slag?

By anon311693 — On Jan 03, 2013

Industrial slag is toxic and it should not be used in a residential environment unless it is sealed beneath an impermeable barrier such as concrete or asphalt. It commonly tests positive for both lead and arsenic which are toxic metals that can leech into the surrounding environment.

In many states, heavy concentrations of slag can be considered to be a "recognized environmental condition" and a due cause for environmental remediation.

By anon288229 — On Aug 29, 2012

A new product called Lemeltic Flux has been released onto the market by a company called Lemeltic Technology. It is used for melting and casting precious metals and the base metal brass. The amazing fact about this flux is that no slag is produced and the flux is non-toxic, a world first!

By anon87679 — On May 31, 2010

you should not be using slag. it is very dangerous and highly flammable. it can cause a fire of up to 4000'

By anon83326 — On May 10, 2010

Careful. Some slag can be classified as hazardous waste due to high metal levels (total and soluble). I would not want it on my property. I have seen regulatory agencies require costly remediation at sites were slag was found to contain high lead levels, etc.

By anon78207 — On Apr 17, 2010

where is the purpose of slag blended cement? i want to know in what type of soil this type of cement is used.

By anon75408 — On Apr 06, 2010

Can anyone tell me if using slag instead of gravel is safe? Does it cause lead build up in the soil that can seep into your well water?

By anon69405 — On Mar 08, 2010

What are the components of ood slag?

By snowbored — On Oct 12, 2009

Thanks wisegeek.

By anon43260 — On Aug 27, 2009

"What is mean by acidic and basic slag?"

That depends on the operation through which it is produced. suppose if it's a gasification slag, surely it contains acidic components. that is called an acidic slag

By anon39630 — On Aug 03, 2009

I found some slag from an old steel plant that I brought home to use as lawn art. Is slag toxic in any way?

By pradigan — On Jun 18, 2009

Can I use slag material as a backfill behind timber tie retaining walls? I am building 3 foot high walls made of treated 6x6 timbers ties. I have designed the deadman/headers to go back into the "stable triangle" behind the wall. I have 15-20 yards of slag from an old foundation. I am afraid of using it as backfill for the new walls. It seems as though it is very unstable and will not suffice to hold the deadmen in place. Can anyone offer advice???

By khalid — On Jun 16, 2009

What is mean by acidic and basic slag?

By vijendra — On May 03, 2008

in some stage slag is not fluid, why?, after adding sufficient amount of lime, even then foaminess in slag not come, arcing not cover, and this is look like just like dead body. pl suggest me what is problem

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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