via Sustainable Strides at NIKE, Inc. – News – Harvard Business School.

The Australasian (iron and steel) Slag Association was formed in 1990, with a model similar to the US National Slag Association. The ASA represents producers, processors, marketers, customers and suppliers of iron and steel slags across Australia, New Zealand, Indonesia and Malaysia.

The ASA conducted a benchmarking study in 2010 into the energy use associated with EAF S, potential end-use applications and attitudes and issues impacting on the effective utilisation of EAF S in Victoria. Key details to emerge from this report were:

• Limited volumes were effectively utilised at the commencement of the program;
• EAF S was poorly understood; moreover, Vic Roads held negative perceptions initially. EAF S were not addressed or specifically allowed for in key material specifications;
• There were insufficient projects and case studies demonstrating the environmental and performance characteristics of EAF S to provide confidence to Vic Roads;
• Local government were identified to be technically risk-averse to new products, they default to Vic Roads specifications and approved products;
• For the period, 3.4 million tons of iron and steel slag products were produced within Australasia (Australia and New Zealand) Of this 80% was effectively utilised (sold or reused for some beneficial use)
• 20% was used in cementitious applications (high value add, i.e. more than A$100/ton)
• 48% was used in non-cementitious or road construction applications (medium value add, i.e. A$10-100/ton)
• 12% was used in general civil or fill applications (low value add, i.e. less than A$10/ton)

For decades the principal use of slag in the US was as rail track ballast. AS quantities grew new applications were sought, and one that proved immediately valuable was in the building of military roads in WWI. By 1918, the year the US National Association of Slag was founded, the annual production of slag had grown to 20 million tons.

The Association was formed with the objective of promoting the use of slag, and today millions of tons of slag aggregates are used in the country. The Association and its members are constantly researching new applications for slag.

Slag has been successfully used in various projects, including the Detroit Metro Airport parking garage constructed in 2000-01; “white-topping” thin concrete road overlays in Southeast Michigan; the I-70 through Colorado’s Glenwood Canyon, where the asphalt mix had to withstand harsh conditions of weather and traffic; the Chicagoland Speedway, where slag was specified because it does not shine, giving the track skid-resistance and grip that are so important when drivers are travelling at over 300 km/h; in farming as a liming agent for adjusting the pH balance in the soil.

The Federal Highway Administration, Transportation Research Board and the various state departments of transport publish technical guidance reports on the use of slag as a road aggregate.

According to the US Geological Survey, data on US slag production are unavailable, but it is estimated to have been in the range of 18 to 25 million tons in 2013. Domestic US sales in the same period amounted to an estimated 17 million tons, valued at about US$290 million (fob plant). Blast furnace slag accounted for about 45% of the tonnage sold and had a value of about US$225 million; nearly 85% of this value was from sales of granulated slag. BOS and EAF slag accounted for the remainder.

Slag was processed by nearly 30 companies servicing active iron and/or steel facilities or reprocessing old slag piles at about 120 sites in 32 states; included in this tally are a number of facilities that grind and sell ground granulated blast furnace slag (GGBFS) based on imported unground feed. Prices per ton ranged widely in 2013 from a few cents for some steel slags at a few locations to about US$100 for some GGBFS

Air-cooled iron slag and steel slag are mainly used aggregates in concrete (air-cooled iron slag only), asphaltic paving, fill, and road bases; both slag types also are used as a feed for cement kilns. Almost all GGBFS is used as a partial substitute for Portland cement in concrete mixes or in blended cements.

Pelletized slag is generally used for lightweight aggregate but can be ground into material similar to GGBFS. Owing to their low unit values, most slag types can be shipped by truck only over short distances, but rail and waterborne transportation can be longer. The much higher unit value of GGBFS allows this slag to be shipped economically over longer distances.

Slag is commonly returned to the blast furnaces as ferrous and flux feed, but data on these returns are incomplete. Entrained metal, particularly in steel slag, is routinely recovered during slag processing for return to the furnaces, but data on metal returns are unavailable.

Recent data indicate that GBFS (mostly unground) is the dominant type of ferrous slag imported, but official import data include significant tonnages of non-slag materials (such as fly-ash and silica fume) and slags or other residues of various metallurgical industries (such as copper slag). Based on official data, the principal country sources for GBFS were Canada (40%), Japan (40%), Italy (7%) and South Africa (7%).

The availability of blast furnace slag is becoming problematic because of the decline in the number of active blast furnaces in recent years, the lack of construction of new furnaces, and the depletion of old slag piles. Recent draft regulations to restrict emissions (especially of mercury) from US cement plants and to potentially reclassify fly-ash as a hazardous waste for disposal purposes have the potential to limit the supply of these cementitious materials to the market and could lead to an increase in demand for GGBFS.

Although world slag data are unavailable, the USGS estimates that world iron slag output in 2013 was of the order of 260-320 million tons and steel slag about 170-250 million tons, based on typical ratios of slag to crude iron and steel output.

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:

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