Potential applications of steel and metallurgical slag

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The steel and metallurgical industries produce not only metal and alloys, but also a by-product that has been successfully used in many construction and agriculture applications. Using slag instead of natural materials is a sustainable alternative with high durability in several applications. The use of slag is ecologically sound and economically smart.

Use of slag in South Africa appears to have been in fits and starts. There appears to be widespread experience and acceptance in other international markets, as will be demonstrated in the next section. In this section the focus is to highlight the various applications that have been identified.

The use of slag aggregates from iron and steel production in construction dates back to the Romans, who used crushed slag from the crude iron production of that time to build their roads. Nowadays, slag is still used to build roads. However, slag aggregates are now widely used in all kinds of civil works, as will be seen from the table below:

1. General applications of slag aggregates:

Different grain sizes of slag aggregate as used in various

applications

2. Asphalt

The inherent properties of of iron and steel slag make it an ideal aggregate for base and surfacing asphalt products (bitumen bound application of slag aggregate.) However, as they all exhibit different characteristics the properties have to be matched to the requirements of the specific material type. The characteristics are assessed in accordance with the requirements of the relevant national standards.

Steel slag asphalt used on heavily trafficked road         Close up of porous asphalt – safe and quiet

 

3. Unbound applications

Unbound use includes a wide variety of applications in civil engineering, road and hydraulic works:

Application Description GBS ABS Steel Slag
Unpaved roads Durable surface layer for trafficked areas x
Constructional layer (base, sub-base and sub-grade layers) Bearing layer to support the pavement (asphalt, concrete or blocks) and to conduct and divide traffic load to the sub-soil. x x
Non-constructional layer (embankment and fill) Non-bearing layer or body. x x x
Armourstone Very coarse aggregate for filter layers, ballast layers, erosion control, etc. in hydraulic works. x
Gabions Caged coarse aggregate for retaining walls, erosion control, noise absorbing walls, etc. x x
Railway ballast Coarse aggregate layer upon which railway sleepers are laid. x x
Roofing Coarse aggregate layer to ballast the roof or fine aggregate on bituminous sheets x x

Construction of an unbound base layer

Gabion, used e.g. for retaining walls, erosion control and noise absorbing walls

4. Hydraulic bound and semi-bound applications

A special advantage of slag mixtures resides in their hardening carbonic and/or hydraulic reactions without using a binder like cement or bitumen. This causes an increase in their load-bearing capacity, determined e.g. by an increase of CBR value or compressive strength.

5. Waste-water treatment

This is seemingly a newer and still developing application. Removal of phosphorus in wastewater treatment has been a problem in different countries all over the world. The Japanese pioneered this use; other countries have also carried out studies into this area. The efficiency of slag for this purpose has been established and well proven through several scientific studies.

6. Cement/Concrete application

Today, the main by-product of hot metal production in blast furnaces is granulated blast furnace slag. After grinding to cement fineness (GGBS = ground granulated blast furnace slag) it is used as a main constituent of cement or as a separate concrete addition.

7. Agriculture

The use of fertilisers and liming materials produced from blast furnace and steel slags has a long tradition dating more than a hundred years. They contain elements with useful properties for plant nutrition and soil quality.

Calcium and magnesium in slag have a better solubility than that of magnesium carbonate in natural limestone and dolomite. Both elements serve as plant nutrients and stabilisers for soil aggregates and their basicity increase or sustains soil pH.

8. Slag Sand

“Slag sand” is produced after fine wet processing of electric arc furnace slag from stainless steel production (EAF S) and secondary metallurgical slag (SECS), but all other slag types could be used. This high tech method of processing slag allows for utilisation of slag that could not be valorised with traditional processing.

The mineral aggregate is calcium-silicate material with approximately 50% of the particles below 75 um and is used in a number of commercial applications:

  • Agriculture – pH adjustment and plant available silicon;
  • Acid mine drainage prevention, treatment and remediation;
  • Soil stabilisation and road base reclamation;
  • Road base and sub-base
  • General construction engineered fill, embankment, and backfill
  • Sludge solidifying and stabilisation
  • Hazardous waste stabilisation
  • Flowable fill and excavation backfill
  • Cement and concrete
  • Asphalt

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