FOOTPRINT+ 2024: Concrete Developments

An abundant solution?

One of the presentations I attended during FOOTPRINT+ was titled “Concrete: Developments in the supply chain”, which focused on the problem of constrained resource GGBS and potential solutions in limestone fines and calcined clays.

Will Arnold gave an excellent whistle-stop explanation of a recent report, which assessed global GGBS resources and usage.

The Problem

Ground Granulated Blast Furnace Slag (GGBS) is a by-product of the iron and steel industry, obtained from blast furnaces as they process iron ore into iron. It is rapidly cooled with water and then ground to produce a granular product. GGBS is used as a supplementary cementitious material (SCM) in concrete, enhancing the durability and strength of concrete structures.

GGBS is considered a constrained resource because its production is limited to the output of the iron and steel industry. About 90% of all iron slag produced is already processed into GGBS, leaving little room for increased production. There are no significant stockpiles that can be used to increase supply and the global production of clinker (highly energy-intensive and CO2-emitting product used to make ordinary Portland cement) significantly outstrips that of GGBS.

The report recommends that GGBS should continue to be used in concrete where it is technically required, for instance, in applications requiring enhanced durability or specific performance characteristics like temperature and crack control. However, it should not be relied upon as a primary method for reducing environmental impact due to its limited supply. Increasing local use of GGBS is unlikely to reduce global emissions because it would simply shift the usage from one region to another - its a zero-sum.

The Solution(s)

Both limestone fines and calcined clays offer significant potential as alternative SCMs, either individually or in combination. Their effective use could lead to more sustainable and lower-carbon concrete, but their adoption will require overcoming technical, regulatory, and supply chain challenges. As the cement and concrete industries evolve, these materials stand out as critical components of a broader strategy aimed at reducing environmental impacts while maintaining or enhancing the performance characteristics of concrete.

Limestone fines are fine particles obtained from the process of crushing limestone and are often considered a by-product of quarry operations. With limestone being one of the most abundant minerals on the planet, the fines are widely available and relatively cost effective due to the low level of processing. Adding these fines to cement can partially replace clinker, however they do not significantly contribute to the strength of concrete, especially at later stages, which can be a limitation. There are also limits to the amount that can be included in cement, which is a current barrier to more widespread adoption.

Calcined clays are produced by the thermal treatment of kaolinitic clays (650°C to 800°C) which partially calcines the clay minerals, making them reactive in cement. They contribute to the mechanical properties and durability of concrete, enhancing resistance to sulphate attack and alkali-silica reaction. Production of calcined clays involves lower temperatures compared to clinker production, meaning significant reductions in CO2 emissions, plus the technology can make use of lower quality clays that might be unsuitable for other industrial uses, thus reducing waste. Not all clay deposits are suitable for calcining, so the availability of kaolinitic clays with the right mineral composition is a key factor. Currently, their use requires specific knowledge and adjustments in mix design, which can be a barrier in regions without the technical expertise or infrastructure to support its use, however Gareth Wake explained that many calcined clays and mix specifications are within the code definitions already, so there is nothing stopping their use right now.

So in summary…

• Use GGBS only when it is required for technical reasons (not to reduce emissions)

• Send demand signals to the cement and concrete supply chains that projects are keen to explore and use limestone fines and/or calcined clays in mixes to help improve the environmental performance of concrete.