The construction industry is one of the largest in the world, and cement and concrete are literally the building blocks of its success. Evolving from prehistoric caves to today’s towering skyscrapers, concrete structures have and will continue to be vital components of modern civilization, providing long-lasting, reliable support for buildings, roads, bridges, tunnels and dams. So much so that concrete is the most consumed material on Earth, second only to water, while the steel used to reinforce it is by far the most commonly used metal. But this doesn’t come without high environmental costs: concrete accounts for 8% of global CO2 emissions, much of which come from the extraction and transportation of aggregate materials such as sand, gravel and crushed stone.
A big part of building healthier cities is making concrete more sustainable, but this comes with many challenges. Stopping concrete production is simply not possible, and replacing it entirely is more of a long-term approach that still seems out of reach. So, today it is rather about finding ways to decrease the environmental effects of the material without compromising its high performance. But where to look? Recent research suggests that one solution may be found in recycled industrial waste, particularly in a by-product of steel-making referred to as steel slag. Below, we will unveil its promising potential in the architecture and construction industry.
Replacing natural aggregates with industrial waste
Aggregates are one of the most crucial components of concrete; about three-quarters of its volume is composed of fine and coarse aggregates bonded together with cement. This means that all the fundamental properties of concrete –durability, mechanical strength, serviceability, etc.– are highly linked with the quality and type of aggregate. Used tirelessly for decades, most of the natural aggregates that are traditionally used for concrete mixes are scarce, show problems related to durability, and their mining has caused immense amounts of pollution. As this 2021 study suggests, “The uncontrolled use of natural resources has brought serious environmental problems for many years. For instance, sand and gravel taken unrestrainedly from stream beds has caused environmental damages.” Replacing natural aggregate thus becomes an alternative solution to tackle sustainability.
Although its large-scale architectural potential is still being studied and tested, steel slag could have a promising future in the construction industry as a partial substitute for cement or aggregate. Several investigations have explored this throughout the years, and many researchers and engineers have already found it possible to use steel slag as a suitable concrete aggregate. Considered one of the most encouraging waste materials, steel slag is an unavoidable by-product of the production of steel. It is produced in large quantities during the separation of the molten steel from impurities in steel-making furnaces. The slag occurs as a molten liquid melt and is a complex solution of silicates and oxides that solidifies upon cooling, resulting in a glassy, granular material that is then crushed to the appropriate size for use as an aggregate in concrete (or further processed to produce other products).
Slag is both plentiful and potentially valuable. “The global steel-making industry produces over 130 million tons of steel slag every year,” says water engineer Biplob Pramanik of Melbourne’s RMIT University. According to data from the World Steel Association, 15.7 million tons of this steel slag emerge as a by-product in Europe alone, of which 11.5 million tons are recycled. The massive amount of steel slag produced by steel mills around the world is a major motivator for recycling the material, even more so knowing that it would otherwise be discarded.
Improved strength, durability and environmental sustainability
Using steel slag in concrete ultimately reduces the amount of waste generated by the steel industry. Industrial waste is highly contaminating, so giving it a new value is a great way to contribute to the circular economy. At the same time, steel slag concrete reduces the need for virgin aggregate materials, preventing the extraction of limited, natural resources and reducing CO2 emissions while maintaining concrete’s outstanding physical qualities.
Because it is not possible to stop cement and concrete production, it would be best to turn towards recycling industrial wastes, which is another environmental problem, as an alternative solution to the current problem. – Gencel et al., 2021
Another reason that steel slag concrete has gathered a lot of attention in recent years is that it provides a series of benefits beyond its environmental sustainability. When the concrete is placed and cured, the steel slag particles become an integral part of the concrete matrix, acting as a filler and helping to improve the density of the concrete –which in turn improves its strength and durability.
Because steel slag is a very dense and hard material, it is resistant to weathering and tear and can withstand more load and pressure than traditional aggregates such as gravel and crushed limestone. In fact, Australian scientists made new concrete with steel waste and the results were 17% stronger. Others found that samples with steel slag aggregate had significantly higher compressive strength than samples with crushed limestone, and that the volume of permeable pores decreased with the increase in steel slag rate, which grants better workability and makes it easier to place and finish. It also has a lower coefficient of thermal expansion than traditional aggregates, meaning it can reduce shrinkage cracking in concrete structures.
Steel slag can provide great benefits both in terms of environment, economy, and sustainable development. It can replace portland cement at replacement ratios up to 45% with significant compressive strength values. – Gencel et al., 2021
A disruptive material with extraordinary potential
Due to its mechanical strength, porosity, wear resistance and water absorption, steel slag is already being used as a fine and coarse aggregate for asphalt road mixes, as well as a raw material for clinker, as ballast for railways, and as a filling material in various excavations. Thanks to advanced research and testing, its use in concrete is increasingly recognized for its environmental and physical properties, which likely means it could soon expand to other uses. For instance, as a suitable material for high-strength concrete structures like bridge decks and skyscrapers, or for any other building that would traditionally be built in regular concrete and could use the extra durability and sustainability. Engineers, architects and designers can take advantage of these promising qualities and put the material to good use as they aim for healthier, better buildings.
It is important to note, however, that steel slag concrete is not as well-studied as traditional concrete, and the way it is developed today still has some limitations –among them, the potential for expansion due to the presence of metallic iron in the slag and other reactions with concrete components that may be undesirable in the long-term. But even as a disruptive material, steel slag concrete holds extraordinary potential. And as the world only gets warmer, innovative materials of this nature will always be worth exploring.
