Steel slag can be used to reduce carbon emissions in the concrete and cement industry, but the amount available in the future is gradually decreasing as the steel industry implements various measures to reduce its carbon footprint.
The main source of carbon dioxide emissions is the built environment. Steel, concrete and cement are all important players in the construction industry, and as such, these industries are taking a variety of measures, together and separately, to reduce carbon emissions. One such collaborative project involves the use of steel slag in cement and concrete production, which has been proven to reduce carbon emissions during the production process.
In this context, steel slag, formally known as ground granulated blast furnace slag (GGBFS), can be used as a substitute for clinker. GGBFS is what we call a clinker substitute,” says Claude Loréa, Director of Global Cement and Concrete Association Cement, Innovation and ESG. A clinker substitute, or supplementary cementitious material (SCM), is a natural and industrial by-product material that can be used in a wide range of applications, including replacing part of the clinker in Portland cement. Since clinker production in cement production generates a large amount of carbon emissions, reducing the clinker content of cement (also known as the clinker factor) also helps to reduce its adverse environmental impact.”
Steel slag is not a new cement component
The use of steel slag in cement is not a new thing. Loréa said: “The first commercial application of steel slag can be traced back more than 150 years, in 1865, when a lime slag cement was commercially produced in Germany. By 1901, it was used in Portland cement to produce iron portland cement with up to 30% blast furnace slag content, and in 1907 blast furnace slag cement was produced with up to 85% blast furnace slag content.
“Since then, blast furnace slag powder has been an important supplementary cementitious material in cement production, with the following key advantages: it can be handled and transported like clinker, and has a higher replacement rate than other supplementary cementitious materials (for example, its use is subject to very few restrictions in European concrete standards). In addition, blast furnace slag powder is also widely used in cement production around the world.”
Loréa adds: “Blast furnace slag also offers many advantages in concrete production. Concrete made with blast furnace slag has a lower rate of exothermic hydration, which reduces the risk of cracking and allows it to maintain its strength for longer, thus achieving a higher ultimate strength. In addition, blast furnace slag reduces the risk of damage caused by alkali-silica reaction and is more resistant to chemical attack. As a result, blast-furnace slag concrete lasts longer and requires less maintenance over its life cycle, which significantly improves the economic, social and environmental sustainability of buildings if it is used as a construction material.”
In cement or concrete plants, concrete can be produced by adding ground blast-furnace slag, pulverised fuel ash, ground limestone and other materials, which not only reduces carbon dioxide emissions but also maintains the same or better performance.
Available steel slag is gradually decreasing
However, the amount of steel slag available to the cement and concrete industry will gradually decrease as the steel industry also strives to reduce its carbon emissions. The industry is already looking for alternatives, says Loréa.
She says: “Not only is the availability of suitable materials varying around the world now and in the future, for example, fly ash comes from coal-fired power stations and blast furnace slag from the blast furnaces of the steel industry, which is also looking for a transformation. In the coming decades, the use of finely ground limestone will increase and calcined clay will be introduced to compensate for the reduced supply of fly ash and blast furnace slag and to further reduce the proportion of clinker binders. Calcined clay comes from clay deposits, which are widely distributed geographically and abundant in reserves, and can meet the projected demand.
Data from the World Cement and Concrete Association shows that the global average clinker-to-binder ratio is currently 0.63 and is expected to fall to 0.58 by 2030 and 0.52 by 2050. However, as things stand, steel slag remains an important factor in reducing CO2 emissions in the cement industry, although it still needs to be accepted by some cement buyers and its supply is limited, Loréa points out.
She says: “While the availability of the material may limit the proportion of clinker binders, customer acceptance remains the current barrier to fully exploiting this approach in some developed and emerging economies. Regional and even national differences remain inevitable due to insufficient material supply and varying market requirements.”