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Superior durability and cost performance |
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Iron and steel slag used in road construction is manufactured by crushing and mechanical stabilization of blast furnace slag and steelmaking slag for use as paving material. These two types of slag are used in base course materials that are produced from them individually or in a mixture, and steelmaking slag is used as an aggregate for asphalt mixtures.
Research into the use of iron and steel slag as a material for roads began from the 1960s, with the development of key technologies such as aging technology and particle size distribution technology. Consequently, design and work guidelines for these products were created and incorporated into the Manual for Asphalt Pavement. A JIS standard was formulated in 1979, and after multiple revisions became the current standard JIS A 5015.
The hydraulic and mechanically stabilized slag HMS-25, which is a typical iron and steel slag base course material, hardens over a long period of time. This hardening characteristic can be utilized in order to create a thinner pavement than when ordinary crushed stone (i.e., mechanically stabilized crushed stone) is used. The material is highly rated for its good workability, with advantages including the fact that the road can be opened to traffic immediately after work is finished, and that ongoing roadwork to compact the base course can continue even if it rains. In addition to use as a base course material, because of its excellent hardness and resistance to wear, steelmaking slag is also used as an aggregate for asphalt mixtures.
In 2002, base course material containing iron and steel slag and asphalt mixture containing iron and steel slag were designated as a designated procurement item under the Green Procurement Law, and are widely recognized as materials that can contribute to environmental preservation.
Aging as a solution to prevent yellow water leaching and expansion and degradation of steel slag |
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Blast furnace slag contains a small amount of sulfur. When sulfur contacts water, the water turns yellow and may emit an unpleasant odor. To prevent these problems, aging is performed. In this process, the sulfur is oxidized by reacting with air to form stable sulfate ions or is neutralized with carbon dioxide gas. Aging is carried out by piling the product in a holding yard immediately after crushing and sifting until it has stabilized.
Similarly, aging is performed for steelmaking slag as well. If the quick lime used in refining the steelmaking raw materials (such as pig iron and scrap) does not fully dissolve, it may remain in an unstable state inside the slag. This quick lime that has not fully dissolved is called "free lime," and when it reacts with water it can swell to twice its original volume. This results in a popout phenomenon in which the asphalt road surface is pressed from below and ruptures. To prevent this, aging is performed to cause the free lime to react with water in advance, changing it into non-reactive slaked lime and stabilizing its volume. Aging can be performed by piling the crushed product in a holding yard until it stabilizes, or by using the high temperature and moisture of steam or high-pressure steam to accelerate the reaction.
Comparison of iron and steel slag base course material and natural material |
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Example of work |
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Examples of use |
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Example of asphalt paving work
(Kobe City) |
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Himeji Higashi ramp on Sanyo Expressway |
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Highway 170, Osaka Loop Line |
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