The Path to Achieving Net Zero: why GGBS is not THE solution

Understanding GGBS: What Is It and How Is It Made?

Ground granulated blast-furnace slag (GGBS) is a by-product of the steel and iron industry, created when limestone is added to molten iron ore in a furnace. As the ore melts, slag forms on the surface, and this slag can be removed, cooled, and then used as a replacement for cement in concrete.

The Appeal of GGBS in Sustainable Construction

For decades, GGBS has been valued in concrete for its mechanical benefits, including enhanced durability and greater resistance to chemical attack. Recently, however, GGBS has gained prominence as a lower-carbon alternative to traditional 100% cement concrete mixes. So, if we specify more GGBS in our concrete, we eliminate emissions tied to cement production and help prevent climate catastrophe…right?

…Not quite. GGBS is a by-product of blast oxygen furnaces (BOFs), which rely on coal derivatives as fuel. As BOFs are gradually phased out, the supply of GGBS is becoming limited as a result. This shrinking supply is unable to keep pace with growing global demand.

In practical terms, if “Project A” specifies a 100% GGBS mix, it may mean that “Project B” won’t be able to use any GGBS at all. With limited global availability, the embodied carbon savings GGBS can offer are restricted, leaving projects competing for slices of a finite supply. The limited, fully utilised global supply of GGBS is a well-documented area of discussion, and is outlined well within ‘the efficient use of GGBS in reducing global emissions’, a paper produced by a group of 13 industry experts.

Proposed Changes and Industry Reactions

In addition to supply challenges, the carbon factor of GGBS—its defining characteristic as a low-carbon material—is set to change. The steel industry is currently lobbying to nearly double the carbon factor of GGBS to account for a portion of the emissions generated in steel production. This shift is recognised by concrete suppliers including Holcim, Cemex, Hanson, Aggregate Industries, and Tarmac, and could significantly impact the perceived sustainability of GGBS in construction applications.

The Path Forward: Moving Beyond GGBS

So, from the perspective of sustainability consultants and design teams, what can be done to combat GGBS’s fall from grace? At early stage, the design can be challenged to reduce the sections and thicknesses of structural members, whether that’s through the omission of basements or transfer slabs, the optimisation of the structural grid, or a move away from concrete altogether. The key is to ask the ‘concrete’ question and begin conversations around it’s reduction from day one.

Significant carbon reductions can also be achieved by having early, collaborative discussions with engineers and contractors. By engaging the supply chain from the start, teams can secure the highest feasible percentage of alternative cementitious materials in their designs.

While GGBS will likely continue to be used in smaller amounts within concrete mixes for its mechanical benefits, the construction industry’s path to achieving Net Zero by 2050 requires moving beyond simply increasing GGBS content. To make impactful strides, this “copy-and-paste” approach must be critically examined and refined throughout the design and procurement stages.

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