A growing Japanese steel production and a corresponding increase in slag output has forced Japan to develop new alternative ways of using slag Global Slag Magazine gives an overview of several environmentally friendly slag applications.
The growth of steel production in Japan over the last half century has been accompanied by a corresponding increase in slag output from the nation's blast furnaces and electric arc furnaces. Disposing of the growing volume of slag has become a major task for the steel industry which has been forced to develop inventive uses for slag to try and solve the problem.
New uses for Japanese slag
Leading Japanese steel makers continue to invest significant resources in developing new uses for slag that are both beneficial as well as environmentally friendly. The steel industry's difficulties disposing of slag are recognised by the government which requires regional governments to buy processed slag and other recycled waste products to support Japan's growing recycling programme.
In 2004 Japan produced 34.7Mt of slag as a bi-product of steel production. Blast furnace slag accounted for 91% of the total tonnage while the remaining 9% was electric arc furnace slag. Five Japanese steel makers operate blast furnaces. Nippon Steel produced 12.6Mt of blast furnace slag, accounting for 36% of total domestic slag production, followed by JFE Steel which produced 11.9Mt of blast furnace slag, accounting for 34% of Japan's total slag output. Three other steel producers operating blast furnaces – Sumitomo Metal Industry, Kobe Steel and Nisshin Steel produced a further 26% of slag output combined, while more than 20 steel companies operating electric arc furnaces account for the other 9% share of Japan's slag production.
JFE Steel Corporation
JFE Steel Corporation is one steel manufacturer at the forefront of developing new uses for blast furnace slag. A leading member of Nippon Slag Association, JFE Steel is part of JFE Holdings Inc, a joint holding company established on 1 April 2003, by Kawasaki Steel Corporation and NKK Corporation following the two steel companies‚ decision to merge their steel making operations. JFE Holdings is the world's third largest steel company after Arcelor and Nippon Steel, and consists of five companies including JFE Steel and JFE Engineering. Following the merger of Kawasaki Steel and NKK, JFE Steel operates facilities that include nine blast furnaces at various sites across Japan.
"Some 99% of our slag is a bi-product of the steel we make. We are not dumping it. This is important. All 99% will be processed into slag products," explained Dr Tatsuhito Takahashi, general manager of JFE Steel Corporation's Technology Planning Department. "We have got JIS (Japan Industrial Standard) standard rating for our slag products. In Japan selling 99% is a good market. Under the law we can sell our slag products to a lot of different customers but we cannot sell slag for dumping."
A short history of slag
The main use of blast furnace slag in Japan during the early years of the 20th century as the fledgling steel industry began to develop was for landfill on steel companies‚ its own premises being used as a foundation on which to expand their steel plants. From the late 1960s a sizeable share of slag began to be used in road construction to form road base. The next major development was the growing use of slag to make slag cement, which remains an important use of slag today.
"An estimated 85% to 95% of steel industry slag is used on construction projects for central and prefectural governments, while an estimated 70% to 80% of blast furnace slag from iron making is used to blend with cement," Dr Tatsuhito said, "Some companies make blast furnace powder for cement production and export the powder, for example, to Southeast Asian countries. Their economies are growing so they need a lot of cement."
JIS standards for Portland blast furnace slag cement (PBFSC) Type A permit the slag content of Type A cement to vary from over 5% to a maximum of 30%. PBFSC Type B cement has a slag content of 30% to 60%, while PBFSC Type C cement has a slag content ranging from 60% to 70% slag. Of the various types, PBFSC Type B cement is the most popular slag cement currently used in Japan.
According to industry figures, about 14Mt of PBFSC cement was used in Japan in 2003, while CO2 emissions were estimated to be reduced by about 4Mt. As part of efforts to support development in the use of processed slag, the government has gradually introduced a system of certification for PBFSC cement along with other construction materials containing processed slag. Certification attests to the quality of the material and its environmentally friendly contents.
"In 2001 we got certification for our blast furnace slag cement, then in 2002 certification for blast furnace slag cement fine aggregate," Dr Tatsuhito said, "The same year sub-road material were certified – asphalt and rock materials. In 2003 grinding slag for civil works especially harbour works was certified. In 2004 certification was awarded to steel making slag used for ground improvement in harbour floors with sand piling for compaction. In 2005, slag aggregate for slag cement (electric furnace oxidising slag) was added to the list of certified products. So now there are eight certified items; in other words 74% of our products are certified as ecologically friendly. The government also buys slag products, but not at a fixed price. A market price is paid."
Slag and the Kyoto Protocol
Slag cement use recently has received a boost as a result of the Japanese government's declared commitment to achieve environmental pollution emission targets agreed during the 1997 Kyoto protocol negotiations, including a reduction in CO2 emission. The manufacturing process for slag cement produces 38% less CO2 emission than the process used for Portland cement production due to less limestone being burned for slag cement production than that required for Portland cement. Under a revised plan to reduce CO2 emission introduced by the government in 2005, the use of blended cement including slag cement is recommended as one course of action that Japan needs to take to reduce the present level of CO2 emission.
Support for the production and consumption of slag cement and other certified slag-based materials has been provided by the government's longwinded titled law concerning the Promotion of Procurement of Eco-Friendly Goods and Services by the State and other Entities (Law on Promoting Green Purchasing). Introduced in 2000, the law is intended to encourage the use of a wide range of environmentally friendly materials and products of which processed slag products are just one group. Regional governments throughout Japan have been instructed to buy environmentally friendly materials as part of their official procurement programme and report on their purchases each year.
"Green is a good image for Japanese people. Some 74% of slag products in Japan are green products," Dr Tatsuhito noted, "We tried to get green product certification. It was tough work but we got it. The government will buy green products. That's the new rule. Prefectural governments should use these products. Every year the government reports on how many green products were used during the year."
Slag applications
In 2004, some 25.3Mt of slag was used in Japan, some to make products that eventually were exported. Cement production used 15.5Mt, equivalent to 61% of total slag use. Some 5.5Mt, almost 22% of total slag use was for cement production intended for export. Highway construction used a further 4.2Mt equivalent to 16.5% of total slag use while 2.7Mt or almost 11% was used as aggregate. The remaining tonnage was employed for various uses including civil engineering purposes which used 1.8Mt of the total and soil improvement which consumed 493,000mt or 2% of the combined total.
As a member of Nippon Slag Association, JFE Steel also cooperates with other steel companies in researching, developing and commercialising new uses for processed slag products. After attending the Euroslag 2005 conference in Norway and presenting a position paper on Japan's slag product development programme Germany, JFE Steel and other Japanese steel maker members of Nippon Slag Association decided to prepare a more detailed slag industry position paper to present to central and prefecture government authorities in Japan.
Completed in April 2006, the Japanese version of the position paper is intended to inform government officials about all aspects of slag and the steel industry's efforts to find beneficial uses for its disposal. The position paper describes the production of slag as a bi-product of iron and steel making along with the history of slag use in Japan and rising slag output levels. Details also are provided of slag transport arrangements as a hazardous cargo due to its alkaline content. According to Dr Tatsuhito, the paper highlights the usefulness of slag in certain applications such as marine and coastal structures, car parks and airport runways. Various sections of the paper emphasise the related environmental benefits that can be obtained at the same time as supporting Japan's steel industry by using a natural bi-product.
Efforts by the Ministry for Economic Trade and Industry (METI) to promote slag use are covered in detail. Chapters are devoted to explaining the big project use of slag for car parking areas at Chubu International Airport in the third city of Nagoya and the runway of the New Kansai International Airport in Osaka. Information also is given on the usefulness of slag in marine protection including government guidelines for using slag in harbour construction and improvement projects, also the use of slag for coastal rehabilitation schemes.
JFE Steel has been involved in the development of various new uses for slag with some of the developments having been started by R&D departments and technology centres under Kawasaki Steel Corporation and NKK Corporation before the two steel makers merged in 2003.
Heat island phenomenon
One recent development is the use of slag to construct car park surfaces and pedestrian pavements in major cities as part of efforts to tackle the 'heat island phenomenon' found around the world whereby ambient air temperatures in the centre of large cities are higher than in smaller towns and cities.
Historical records show that the temperature in Tokyo has risen by 3 degrees Centigrade since 1900 while the temperature increase in small and medium size cities across Japan has been 1°C over the past 100 years. The reasons for the higher increase in temperature in Tokyo are thought to include heat exhaust from airconditioning units, rising vehicle exhaust, other energy consumption and the growing area of artificial pavement and buildings that prevent cooling moisture being released from the earth's surface.
To help reduce the heat island phenomenon in Tokyo and other large cities, JFE Steel has developed a water retaining pavement made of slag that traps rainfall which then cools the atmosphere as it evaporates. The water retaining pavement consists of open-graded asphalt concrete pavement with water retaining material filled into voids in the pavement. JFE Steel has developed water retaining material containing ground granulated blast furnace slag which has been used for many roads in Japan. Dr Tatsuhito explained that the water retaining material is mixed with water to form a slurry which is then used to fill purpose made voids in the open-graded asphalt concrete pavement. After being left to age for about three hours, the water retaining material solidifies in the pavement voids and the water retaining pavement is complete. According to Dr Tatsuhito the porosity of the water retaining pavement is about 70% with the average diameter of the pores being 1.9μm. Water is absorbed into the pores by capillary action and then released as vapour once the rainfall has ended.
JFE Steel has experimented with various densities of water retaining pavement to develop the optimum cooling pavement design. Research has shown that filling pavement voids with 40% to 75% of water retaining material is to be recommended for reasons of economic efficiency and hydraulic permeability. Hydraulic permeability is reduced with increased filling of water retaining material. Once filling with water retaining material reaches 80% density, the final pavement lacks water permeability.
The performance of various water retaining pavements has been measured at various sites around Japan. JFE Steel research teams sprayed water on the pavement at the rate of five litres per square metre and then measured the pavement temperature and that of the soil 20mm below ground surface. Using adjacent test sites of conventional dense graded asphalt concrete and water retaining pavement, the research team discovered that the highest temperature the conventional dense graded asphalt concrete pavement reached was 52°C while the high temperature for the water retaining pavement was 40°C, almost the same as the underlying soil.
The cooling effect of water retaining pavement can last for up to one week after rainfall if water is fully absorbed into the pavement. After one week the temperature difference between conventional dense graded asphalt concrete pavement and water retaining pavement is about 6°C to 8°C. The heat transferred to a person walking over the different types of pavement is calculated to be 30% less when walking on water retaining compared to conventional pavement.
"There is a lot of rainfall in Japan. Tokyo and neighbouring Kanagawa prefecture have been using this water retaining pavement for about two years," Dr Tatsuhito explained, "The volume of water retaining material used in pavement construction is increasing. The material must be used to build new roads as the surface of old roads is contaminated. Water retaining pavement can be used for roads, pedestrian pavements and parking lots. The difference for the road surface temperature is 6°C to 8°C compared with conventional pavement. This idea would be very good for hot countries. In Japan, traditionally we throw water on the ground around our homes when it is hot and the ground cools as the water evaporates. This method of pavement construction uses the same mechanism."
Artificial coral reefs
Another use for blast furnace slag that JFE Steel has developed is to make carbonated marine blocks to restore damaged coastal environments by creating artificial coral reefs and improving the sea bed in inshore waters. The company decided to develop the marine blocks following discussions with marine biologists including Professor Mineo Okamoto at Tokyo University of Marine Science and Technology, and staff at Kyushu University in southern Japan who are involved in a project to rehabilitate coral reefs near Okinawa Island and elsewhere in Japan's southern seas.
Japanese coral reefs suffered severe bleaching in the summer of 1998 and again in the summer of 2001. In 1998 some 90% of the corals around Okinawa Island died due to the effects of the bleaching with the remaining coral being damaged again in 2001. Sekisei lagoon, Japan's largest coral reef, which lies 450km southeast of Okinawa Island, also was damaged by the 1998 and 2001 summer bleachings though not as severely as the corals off Okinawa. The task facing marine biologists was to replace the damaged coral to avoid the serious threat of eventual coral extinction on the affected reefs and nearby smaller reefs which rely on the large reefs for coral larvae. After the idea of transplanting coral fragments was rejected as too difficult to apply to Japanese coral reefs, the team of marine biologists decided to experiment obtaining coral larvae through mass spawning and raise them in situ for future transplantation to coral reefs decimated by bleaching.
In experimenting with different materials used for stone settlement sticks to which coral larvae could attach themselves and grow, the marine biologists asked JFE Steel to supply marine blocks made of slag which were tested along with blocks made of pottery stone and limestone.
The company decided to use slag particles less than 3mm diameter, Dr Tatsuhito explained, compacting them in a mould to which 20% CO2 exhaust gas saturated with water vapour was introduced. The reaction time was 48 hours during which the mould pressure was kept slightly higher than atmospheric pressure. The marine blocks were then left in the open air for one year after production to cure, after which no cracks or surface destruction was discovered on inspection. The main carbonation product was calcite while a small amount of aragonite also was detected in all marine block samples.
The first field test of industrial size marine blocks took place at Setoda-cho near Hiroshima. JFE Steel placed 15 industrial size marine blocks measuring 1m by 1m by 50cm in a 2.5m high, 7m wide pyramid wall with the blocks being spaced 50cm apart to allow the sea water to flow through and around the pyramid wall. Less than a year after the blocks were put in place, a diving inspection a large variety of plants living on the blocks and many species of fish living around them, in comparison to adjacent Portland cement blocks to which no plant had become attached.
"After seven months the slag blocks were covered in algae while the cement blocks used in the same tests remained bare, probably because of their alkaline content. The reaction between the slag block and the CO2 makes calcium carbonate which is the same material that sea shells and sea coral are made from," Dr Tatsuhito said, noting that the slag block's porosity is another factor encouraging coral and other marine plants to attach themselves and grow.
After having shown the suitability of slag blocks to form artifical coral reefs, the research project has moved on to test the suitability of different materials for coral larvae to attach themselves to which can later be moved to other locations that require coral restoration. Slag is one of several materials being used to make discs that are placed in small piles on the slag blocks near to where existing coral will spawn. Once coral is attached to the small discs, these can be transferred to other sites for coral to regenerate. "We are trying to recover the coral seas. JFC Steel co-works with Professor Mineo Okamoto who uses slag, ceramic and neutral material discs on the slag blocks for corals to hide underneath. We are testing these cogs from northern Japan to Okinawa in the south," Dr Tatsuhito explained. "The slag blocks are very stable every time I have checked them. Coral eggs hatch once a year in May. At that time I go scuba diving with Professor Mineo Okamoto to put the slag discs on the blocks for coral larvae to settle on."
