US Cement Standards: Accelerating the green transition

It is no secret that the cement industry accounts for approximately 7% of the entire world’s CO₂ emissions. If the cement industry was a country, it would be the third largest CO₂ emitter in the world, with up to 2.8 billion t, surpassed only by China and the US.¹ At the same time, cement is very much needed. Growing populations, accelerating urbanisation coupled with the impacts of climate change are creating an increasing demand for safe, affordable housing and infrastructure. Thus, the cement industry has a unique opportunity to make an impact.

Limestone calcination is the problem

The CO₂ in cement production results from three sources; the calcination process (56%) which releases the embodied CO₂ in the limestone, the combustion of fuels (37%), and from the power consumption (7%) used to run the plant. In the US, according to the PCA (Portland Cement Association), a strong culture of innovation has led to a 35% reduction in the amount of energy to produce cement over the last 47 years. Similarly, company-driven improvements have made alternative fuels an increasingly larger share of the total energy consumed by cement plants across the country. However, one area that has not gotten as much attention is the impact of the calcination process.

Why not use less clinker?

In virtually every other country, finished cements typically have clinker factors of 70 to 80%. Meaning, 20 to 30% of the mix is limestone or other available additives such as, fly ash, slag, calcined clay, and natural pozzolans. However, in the US, plants generally follow the ASTM C-150 Standard (American Society for Testing and Materials) which defines a limit of 5% limestone, which in practice often translates to 3%.

Given this reality, consider the possibility that if US practices were more comparable to global peers, and finished cement would have an average clinker factor of 75%. A rough calculation suggests this would reduce CO₂ emissions by approximately 20%, given domestic production of 88.5 million tpy in 2019 and assuming 0.85 CO₂/t. These savings would be the equivalent of taking over 3 million cars off the road or the energy used in over 1.7 million homes.

To be fair, it is not quite as bad as described. The difference is not as dramatic if the entire concrete value chain is considered. The ready-mix industry blends their products to a much more diverse standard, often introducing various additives and even recycled concrete to accommodate the needs of their local markets and specific applications. This helps.

A performance-based standard

The dominant standard, ASTM C-150 can be thought of as a ‘recipe’ or a ‘prescriptive specification’, describing the exact composition. This has served the construction industry well for decades, reliably delivering quality concrete. Thus, resistance to change is inevitable. For example, the current, rather conservative limit of 5% limestone is fairly new and required over 20 years of deliberation to finalise. Further advancements would likely also face a rather complex and time consuming process.

There could be a better way. ASTM C-1157 Standard (Performance Specification for Hydraulic Cement) specifies the performance requirements for finished cement without limitations to the ingredients. Just like ASTM C-150, it is for general use. Therefore, in theory, producers can use innovation as a competitive advantage. They can create a mix that meets or exceeds the needed strength and quality requirements, and at the same time minimise cost and the environmental footprint. Using additives strategically sounds perfect in theory, but yet is not widely practiced. Why? It is certainly not a question of technical competence or desire, rather it is result of external obstacles. First, the availability of suitable additives can be challenging. Fly ash supplies are dwindling due to the down-sizing of the coal fired power plant industry. Similarly, domestic steel supply is not large enough to ensure the guaranteed availability of slag. Calcined clay is a real possibility; however, the raw materials are not ideal in all areas of the country and the technology is not yet widespread.

The other, perhaps more formidable obstacle is market forces. As explained before, ASTM C-150 has long been the expectation. Many concrete suppliers tend to prefer or possess a single silo. A plant that only has one silo is likely going to want it filled with C-150, as it can be sold into the vast majority of applications, maximising market potential. If the downstream demand for ASTM C-1157 cement increased, eventually the business case for adding a second silo would become attractive. But, in most markets, this is not yet the case. Architects and engineers are extremely comfortable specifying concrete made with C-150 cement. A performance-based standard is new, unproven and requires typically conservative engineers and architects to think differently. As such, there is too much friction, it is akin to taking on a risk without a direct reward.

References

1. https://www.theguardian.com/cities/2019/feb/25/concrete-the-most-destructive-material-on-earth

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