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By Chris Bennett and Chris Flint Chatto

Concrete is a tremendously useful and flexible material: building foundations, roads, walkways, bridges, and other infrastructure utilize concrete for its strength, durability, and plasticity in formation. Concrete also has relatively low-embodied carbon per unit volume when compared to other building materials, but, because of its utility, modern construction uses a lot of it. When it comes to carbon impact, concrete is responsible for an estimated eight per cent of global carbon dioxide (CO2) emissions. In the world of commercial architecture, concrete can represent 50 per cent or more of overall global warming potential (GWP) of a building’s structure and envelope.

While some concrete applications have real alternatives with reduced impacts (e.g. sustainably harvested mass timber as a primary above-ground structural system), many other applications (e.g. foundational footings and slabs) do not. As well, even if the operational emissions associated with typical buildings constructed today will predominate over the decades, embodied emissions associated with materials and construction happen immediately and climate science shows the “time value” of early reductions is far more impactful. For these reasons, strategies to greatly reduce the GWP of concrete are a critical element to decarbonize individual building and infrastructure projects as well as global industry.

Concrete is a composite material that consists of cement, aggregates (rocks and sand), and water. The main culprit for CO2 emission during manufacturing is cement. While cement is typically a small portion of the overall mix, ranging from 20 to 35 per cent of the material, it is responsible for around 75 per cent of concrete’s GWP.   When mixed with water, cement undergoes an incredibly complex reaction which produces a variety of compounds depending on the original composition of the cement, the most important being calcium silicate hydrate (C-S-H, the “glue” that holds the other components of concrete together and make it useful).

Concrete is also a local material. The reality of producing and shipping wet mixes to project sites typically limits potential suppliers to within 84.5 km (50 miles), forcing projects and specifiers to accept the range of materials and practices available and understood by local suppliers. The suitability of any concrete carbon reduction strategy is impacted not only by project benchmarks but by weather and raw material availability in a given region.

While there is no one-size-fits-all concrete carbon reduction strategy, there are numerous strategies that can be successful across a wide variety of scenarios.

With the demand from public and private sectors to reduce carbon emissions, the construction industry responded in a number of fruitful ways to make a positive impact. One of the most familiar methods for construction professionals to reduce cement content and GWP is by utilizing supplementary cementitious materials (SCMs).

Ground-granulated blast-furnace slag (GGBS) comes from the production of iron. Slag is a liquid containing impurities from iron and coke from the blast furnace process capable of forming into a material with latent hydraulicity. This material can be used effectively in concrete. However, slag can bring negative side effects in certain scenarios.

Fly ash has become a common SCM thanks to its ability to resist sulphate attacks, as well as chloride ion penetration, but recently fly ash has become less available in the market as coal power plants, a primary source for fly ash, are phasing out.

A newer SCM in the market is ground-glass pozzolans. Recovered consumer glass bottles are ground to a powder, providing performance similar to slag, with high level strength at mixes up to 50 per cent replacement. ASTM standard 1866 addresses its use, but with limited suppliers now, its availability is currently limited regionally and will take time to scale.

It is also important to point out that SCMs can improve the sustainability of a concrete mix by replacing cement, not merely by adding it. One should never compare mixes by just looking at the SCM percentage; the content of cement is a far better benchmark.

Newer to the market are carbon-sequestering mixes. These products actively incorporate CO2 in their formulation though the specific techniques and carbon reduction impacts vary. New products on the market can directly inject CO2 into wet mix. The CO2, sourced and purified from the same captured emissions from power plants used in the beverage industry, chemically bonds the calcium oxide in the mix’s cement, creating additional strength and allowing for reductions in cement content.  While the cement and GWP reductions are small (typically three to five per cent), they are still significant, and one of the advantages of this process is it can accommodate most other additional strategies outlined in this article.

Another related strategy substitutes typical Portland cement for Portland cement products with higher limestone content.  The two most common products are Portland-limestone cement (PLC) and limestone calcined clay cement (LC3), with the former much more common in North America (Europe has more experience with the latter). ASTM C595 guides PLC usage and increases the allowed limestone quantity in a cement mix from five per cent to 15 per cent, but the impacts to concrete formulation and properties are minimal. PLC replaces typical cement in the same quantity with an end product that measures and perform the same. The increase in ground limestone has a comparable 10 per cent reduction in GWP reduction compared to ordinary Type I Portland cement.

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