Steel slag used to be a residue with little value. But used as aggregate in asphalt, crushed steel slag provides more stable and quieter roads that require less re-laying. NCC Roads has signed a contract with Ovako to purchase the entire steel slag production in Smedjebacken for ten years – an example of industrial symbiosis with environmental benefits.

There are many real-world examples of circular economy and industrial symbiosis at Advantage Environment. Circular economy can mean that one company’s waste becomes a feedstock for another, or that residual heat from an industrial process is utilized in other processes or in district heating systems. Such refined resource flows can lead to innovative business models, and in the circular economy, business advantage and resource efficiency work in tandem. When long-term collaborative ventures are established to share resources such as material, energy and water, it is often referred to as industrial symbiosis.

Krossad stålslagg fungerar bra som ballast i asfalt. Foto: NCC.

Crushed steel slag is an excellent aggregate in asphalt. Photo: NCC.

NCC Roads recently signed a contract with Ovako for the supply of steel slag, and will purchase the entire steel slag production from the Smedjebacken facility for ten years to come. ”Crushed slag has only seen limited use as aggregate in asphalt in Sweden. That is about to change, and we are now offering special, high-quality pavements”, says Roger Lundberg, Asphalt Marketing Manager, NCC Industry.

Desirable properties

”As an asphalt producer, we have a wish-list of properties we would like the product to have”, Roger Lundberg says. What if the asphalt could:

  • Last longer before needing to be replaced, making the road cost less.
  • Result in noise reduction and less disturbance from traffic for people in the vicinity.
  • Provide more friction and improved traffic safety.
  • Reduce the risk of material coming loose from the pavement and cause fewer stone chip incidents.
  • Be laid thinner, with less binding agent, and be more transportable.
  • Be used in applications with very high loads and tyre rotation.
  • Be more environment friendly.

”As it turns out, slag asphalt has most of the desired properties. When crushed rock is replaced with Electric Arc Furnace steel slag, performance improves and pavements become much more durable. It also contributes to reduced mining. The term slag asphalt may not sound inspiring, but the product surely is”, Roger says.

Byproduct from steel production

Slaggasfalt ger hållbara vägbeläggningar som reducerar buller. Foto: Håkan Lindgren.

Slag asphalt provides durable and quieter roads. Photo: Håkan Lindgren.

Ovako’s steel mill in Smedjebacken makes steel from scrap. The process includes electric arc furnace smelting and final treatment in ladle furnace. In the electric arc furnace, lime (CaO) and dolomite (Cao, MgO) are added as slag formers. Slag is a byproduct from both processes. The largest amount comes from the electric arc furnace – approximately 60 000 tonnes annually. The slag consists primarily of calcium oxide, crystalline and amorphous quartz, magnesium oxide and iron oxide. It also contains a number of other substances, e.g. various metals. According to leach tests the material is inert, which means that leaching is extremely limited. It is referred to as EAF slag in technical documentation.

EAF slag is used around the world in various applications. Because of the high abrasion resistance, pavements is the major application. Steel slag is yet to become established as an aggregate in asphalt in Sweden; an abundance of quarried rock is likely part of the explanation, but more importantly, authorities have long been hesitant whether to classify EAF slag as a waste or a byproduct. ”At last, the steel industry has gotten EAF slag registered as a manufactured product, which simplifies matters. The slag from Ovako’s steel mill in Smedjebacken now has a CE marking and is registered according to the REACH regulation for chemical substances”, Roger Lundberg says.

The Swedish National Road and Transport Research Institute (VTI) has conducted research which shows that slag asphalt is an especially appropriate choice for surfaces subjected to high loads, such as roundabouts, intersections and bus stops.

Part of the NCC Green Concept

”Asphalt with EAF slag as aggregate has excellent mechanical properties: stability, rigidity and endurance. When the granules have undergone several steps of crushing, they become cubically shaped with a rugged surface, providing stability to the pavement. The slag contains the right amount of free lime to make it resistant to water. Since the granule density is higher than in conventional asphalt, less binding agent is needed”, Roger Lundberg says.

”We laid slag asphalt in Borlänge ten years ago, on a large roundabout with a lot of traffic. It remains in very good condition, and will not need re-laying for many years still”.

To Roger Lundberg, slag asphalt is a timely product with many environmental advantages, fitting in well with NCC Roads’ portfolio of green products and services:

  • Slag asphalt can be produced with green methods (NCC Green Asphalt). In practice, this means reduced energy consumption and reduced emissions compared to conventional asphalt.
  • Using slag instead of crushed rock saves non-renewable resources.
  • Slag asphalt pavement requires less frequent re-laying, especially where traffic intensity is high, which saves money and reduces environmental impact.
  • Slag asphalt pavement is more quiet.
  • Slag asphalt pavement provides better friction and is resistant to studded tire wear. This also results in less particulate matter (PM10) pollution.

”Things that used to be classified as waste can now often be utilized as resources instead. Materials that would have ended up in landfills are finding their way to entirely new applications. The EAF slag is a great example of circular economy in practice, and of industrial symbiosis between companies” Roger Lundberg says.

The article was published in September 2016.