The Best Uses For Slag

It would be an understatement to claim slag gets a bad rep. After all, even the name itself has negative connotations, especially if we’re to ask our friends over the pond. However, despite serious environmental hazards, slag does have many uses often ignored by the general public.

In simple terms, slags are nothing more than byproducts of metal smelting or refining. Since the demand for metal isn’t slowing down, it means slag supply is also at an all-time high. To put it in perspective, for every tonne of steel you get between 200 and 400 kg of slag (depending on the type of furnace used).

So, what happens to all this waste, what are the environmental repercussions, and can this material be used for something else?

Let’s see!

All about that slag

During the smelting of steel or iron, limestone flux is introduced to the melt where it’s combined with infusible silica to form calcium silicate. The newly formed material then merges with other impurities and is removed from the melt. It’s dried and ground, which is just one of the ways to get slag.

Depending on the smelting process, there are different types of slag:

1. Ferrous (waste materials from processing steel and iron)
2. Ferroalloy (waste materials from ferroalloy production)
3. Non-ferrous (produced non-ferrous metals like copper or zinc)

Additionally, this material can also be classified depending on the processing conditions like blast or an electric arc furnace.

So what does slag usually consist of? In general, it’s made of calcium oxide, iron oxide, magnesium oxide, aluminum, and silicon oxide. There are trace elements such as phosphorus and manganese, as well as others that depend on the raw materials.

The environmental impact

This chemically complex waste usually ends up in dumps where there is a high risk of exposure to the elements. As a result, toxic elements might leach into the ground or the groundwater which can have severe effects on the environment.

Even fine slags meant for recycling or for use in other industries can get carried off by the wind, thus causing direct health risks. This can be particularly risky for communities in the vicinity of plants or disposal sites.

The environmental impact can be partially mitigated if steel and iron industry move toward a circular economy where all the waste gets recycled and reused. This is the main reason why discovering new applications for slag matters. 

Let’s see where this byproduct is most commonly used: 

1. The construction industry

Slag quickly found its purpose in the construction industry way back in the 19th century when it was commonly used to build roads and railroad ballast. Over time, it slowly became a mainstay in cement production, where it’s mostly used today.

For example, by combining Portland cement with granulated blast furnace slag, you get cement on steroids. Once this cement is used to create concrete, the final result is a material that’s more durable and less permeable. 

These qualities come in handy in the construction of bridges and coastal structures where the resistance to sulfates and chloride also increases corrosion resistance and slows down the deterioration of structures.

Wholesome fact: today, slag is still used for soil stabilization during the construction of roads and railroads as it was back in the old days.

2. Agriculture

Slag is used extensively as a fertilizer on farms and gardens in steel-producing areas, but it’s also highly valued worldwide as a fertilizer for rice and sugarcane production. 

Naturally, this only applies to slags that don’t have great amounts of heavy metal contaminants. 


Which types of slags are the most popular as fertilizers? It all depends on the part of the world. For instance, in Florida, it’s phosphorous slag, and in China and Japan, they prefer steel and blast furnace slag, respectively.

3. Neutralizing acid from acid mine drainage

While this byproduct can pollute the environment, surprisingly enough, it can also be used to reverse the effects and remove the harmful chemicals from water.

Hard rock mining poses many environmental challenges, with acid mine drainage one of the most dangerous there is. It occurs when air and water come in contact with minerals such as iron sulfide which causes a chemical reaction that creates sulphuric acid. 

We don’t know how familiar you are with chemistry, but sulphuric acid is a strong and corrosive mineral acid that can cause a lot of damage. Once it dissolves other metals, it can contaminate water, disrupt the aquatic flora and fauna and corrode structures such as bridges.

Remember how we mentioned slag consists of calcium? Well, all that calcium can neutralize the acid from the acid mine drainage. What’s even better, a hefty dose of it can even reduce acid buildup in the soil itself.

Most of the slag is outright discarded, so repurposing it to heal the environment is one of the best solutions we have right now. It would decrease the current waste footprint of iron manufacturing and would also offset the cost of restoring abandoned mine areas.

4. Removing phosphate from water

This compound is one of the key ingredients of most fertilizers used today, and it’s also an important nutrient for plants. However, when too much fertilizer is used it can cause a big problem as all the excess phosphate can end up seeping into the local bodies of water. 

Since phosphate is a nutrient, it can result in excessive blooming of algae or dead zones (areas of water with low oxygen levels caused by nutrient pollution).

We have to loop back to slag’s high calcium content, which doesn’t only neutralize acids, but can also absorb excess phosphate from the water. Relying more on slag might, in due time, eliminate the need to mine new materials used to treat water.

5. Emerging applications: Co2 capture

Co2 capture and storage sequestration (CSS) is a set of technologies that provide industrial and energy sources with a way of capturing Co2 emissions before they enter the atmosphere. For instance, Co2 might get compressed and injected underground into safe geological formations.

Since slag has a serious potential for carbonation due to a high calcium and magnesium content, its properties mean it’s suitable for Co2 capture. 

As it stands, the iron and steel industry requires a high energy input at a high Co2 emission level. Thus, it’s imperative that these industries find a way to reduce their emissions. 

One study has shown that with just an additional level of management, a heap of slag can absorb a significant amount of Co2. This means that, if done properly, a waste byproduct could offset the emissions from steel production.

According to the researchers, for every tonne of steel produced, steelmaking produces two tons of Co2. The resulting slag could offset 10% of those emissions.

What does the future have in store?

Slag is a necessary evil in the production of metals. Even though it can be harmful, it still has a lot of uses from construction all the way to water treatment. 

However, we are yet to see the full environmental impact of our actions. Will we ever be able to use up all the slag we store in landfills that are lying around near factories? While using this waste material to capture Co2 sounds promising, the results are still inconsistent due to variability between different types of slags used.

What does the future have in store? We believe it’s looking good. Even raising awareness about the positive impact of what is just a waste product is a step in the right direction. We’re anxious to see what slag might be used for in a couple of years.