Steel slag is helping the concrete and cement industry reduce its carbon emissions, but as the steel industry also works to reduce its carbon footprint there may be less steel slag available in the future. 

The built environment is the single largest contributor to CO2 emissions. As the steel, concrete and cement industries are big players in the construction industry, jointly and separately, these sectors are working on many initiatives to reduce their carbon emissions. One project they are working on together involves using steel slag in the manufacture of cement and concrete, a method proven to cut emissions from cement and concrete production.

In this instance, steel slag, or what is officially termed ground granulated blast furnace slag (GGBFS) is used as a replacement for clinker. “GGBFS is what we call a clinker substitute,” explains Claude Loréa, Cement, Innovation and ESG Director at the Global Cement and Concrete Association. “Clinker substitutes – or supplementary cementitious materials (SCMs) – are a wide range of both naturally occurring and industrial by-product materials that can be used to replace a proportion of the clinker in Portland cement. As clinker is the element within cement responsible for the majority of its carbon emissions, reducing the clinker content (also known as the clinker factor) of cement has the benefit of lowering its environmental impact.” 

Steel slag is not a new cement ingredient

The use of steel slag in cement is not a new phenomenon. Loréa says: “Its first commercial use dates back over 150 years to 1865, when a lime-slag cement was commercially produced in Germany. By 1901, it was being used in Portland cement to make Eisenportlandzement with a maximum 30% GGBFS content; this was followed in 1907 by Hochofenzement , with a GGBFS content of up to 85%. 

Since then, it has been an important supplementary cementitious material in the production of cement with several key advantages: it can be handled and transported in the same way as clinker and can be used at higher substitution rates than other supplementary cementitious materials (with, for example, only small limitations on its use in European concrete standards). It is also widely available around the world for use in cement manufacture.”  

GGBFS also has a number of beneficial results in the manufacture of concrete, adds Loréa. GGBFS concrete shows lower heat of hydration, reducing the risk of cracking, and continues to gain strength over a longer period of time, resulting in higher ultimate strengths. In addition, GGBFS reduces the risk of damage caused by alkali-silica reactivity, while providing higher resistance to chemically-aggressive environments. As a result, GGBFS concrete is likely to last longer and require less maintenance over its lifetime – both important benefits in terms of the economic, social and environmental sustainability of the buildings it is used to construct.” 

At the cement or concrete plant, ground granulated blast furnace slag, fly ash, ground limestone and other materials can be added to deliver concretes with reduced CO2 emissions but still the required or even enhanced performance.

Steel slag availability is destined to drop

As the steel industry works to reduce its own emissions, however, the amount of steel slag available to the cement and concrete industry will reduce and Loréa says that the industry is already looking into alternatives.

“Availability of suitable materials around the world varies now, and will do into the future, because, for example, fly ash comes from coal-fired power stations and ground granulated blast furnace slag from the steel industry’s blast furnaces and these industries are also transitioning,” she says. “In coming decades, there will be increased use of ground limestone and the introduction of calcined clays to both compensate for the reduced supply of fly ash and ground granulated blast furnace slag and further reduce the clinker binder ratio. Calcined clays rely on clay deposits that are geographically spread and sufficiently abundant to meet projected demand.”

Data from the Global Cement and Concrete Association, states that, on average, globally, the clinker binder factor is currently 0.63. It is projected to reduce to 0.58 and 0.52 by 2030 and 2050 respectively. For the time being, however, steel slag remains an important component in reducing CO2 cement, although Loréa points out, it still needs to gain acceptance amongst some cement buyers and its availability is limited.

Whilst the availability of materials can be a limitation on clinker binder ratio, client acceptance is a current barrier in fully exploiting this lever in some developed and emerging economies. Regional and even country variations are also inevitable due to differing material availability and market requirements,” she says.

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