About 40% of global greenhouse gas emissions are attributed to the building sector. Three-quarters of this 40% are associated with building use (heating, cooling, lighting, etc.). The other one-quarter (about 10% of global emissions) is associated with building materials and construction [1].
Now 10% may not sound like much, but it adds up. If we were to assume these proportions for the US, it would be like saying that building materials and construction are responsible for emitting 1.7 metrics tons of CO2 per US resident per year [2]. That’s a lot of CO2, and a lot of potential for improvement.
Let's look at how three of the most common materials used to build structures— concrete, steel, and wood— contribute to this emissions total.
1) CONCRETE
I am going to start off by saying that contrary to popular belief, concrete is not another word for cement [3].
Concrete is the hardened, blocky material used for structures, while cement is a powder-like substance that with the addition of water binds the many ingredients in a concrete mix together. These ingredients include aggregates like rocks and sand, and "secondary cementitious materials" that also contribute to determining the exact characteristics of the concrete.
Concrete is commonly known to have a very high carbon footprint, and this is primarily due to its cement content. To produce cement, raw materials like limestone, shale, clay, sand, and iron ore are heated to over 2700 degrees Fahrenheit in a giant kiln. As a result, 1 ton of cement produces approximately 1 ton of carbon dioxide. Put this number together with the 88.5 million metric tons of cement produced in the US annually, and it becomes clear why this is a problem [4].
Perhaps lesser known is the notion that we are quickly depleting our stores of the rough-surfaced, water-eroded natural sand crucial to modern concrete. We are increasingly resorting to mining methods that are at once more energy-intensive and destructive to ocean and river ecosystems. To put it plainly, concrete production is currently unsustainable in more ways than one [5].
Is the construction sector doing anything about this? In reading up on this subject, I was surprised to find out that concrete is frequently recycled. Unfortunately, "recycling" in this case refers to recycling of aggregates rather than cement, and because new cement is still needed for every new construction project, the “recycling” does little to reduce the carbon footprint of concrete [6]. Cement cannot be reused because its hydration process cannot be reversed and repeated, and due to the brittle nature of concrete, it is equally difficult to reuse entire concrete blocks.
Making concrete more environmentally sustainable is a multi-faceted challenge. Developing concrete mixes with low cement content is currently a hot topic for research, but in the meantime it seems that there’s little that can be done with concrete waste beyond recycling the aggregates, whether or not this offsets the overall emissions associated with the material to any significant degree.
2) STEEL
Steel (an alloy of iron and carbon) is another material that is associated with high emissions. A large part of this is due to the process required to extract iron from iron ore.
[7]
When steel is recycled at the end of its useful life, it becomes possible to skip the extraction step. And recycling has become common practice— currently, about 98% of structural steel is recycled in the US. Thanks in part to this, the steel industry has reduced emissions by 36% in the last 30 years [8].
However, recycling steel requires melting down and remolding the material at temperatures of over 2700 degrees Fahrenheit, and is therefore energy intensive in its own right. And because the vast majority of US energy does NOT come from renewable energy sources, the high energy requirement unfortunately equates to high emissions.
On the bright side, steel has greater potential for reuse than concrete, and making the shift from recycling to reuse has the potential to drastically reduce associated emissions [9]. Even better, this thought is already being recognized by key players such as the American Institute of Steel Construction (AISC).
3) WOOD
And then there is wood. Wood is currently very controversial.
On the one hand, the act of making (growing) wood is a natural process that accumulates carbon and stands in stark contrast to the high-energy, very much human-engineered manufacturing process behind cement and steel production. However, things get more complicated when attempting to look at the full “lifecycle” impacts of wood.
The controversy stems in part from missing data. But what is known is that:
Corporate tree plantations are significantly less resilient to climate change (more susceptible to wildfire damage, for example) than natural forests, in addition to being a significant cause of ecosystem degradation.
Mature trees absorb carbon at a greater rate than young trees do. Because the lumber industry tends to cut trees while they are young, there is concern that scaling up the lumber industry would mean losing out on more and more of these important carbon sinks.
The equipment used by loggers is generally not run on renewable energy.
Both cutting wood and overturning soil (as is commonly done when cutting wood) release a substantial amount of carbon to the atmosphere [10].
It is also worth noting that despite the effort required to procure mass timber, over half of wood waste from the construction sector is still being landfilled. Part of this has to do with the fact that wood found in buildings is often coated in paint and adhesives, nailed to other components, or otherwise difficult to separate out [11].
There is clearly much room for improvement here through better forestry practices (reforestation, long rotations, etc.); through the recycling (or "downcycling") of wood into products as varied as furniture, laminate panels, mulch, wood pellets, and animal bedding; and perhaps even through the reuse of select structural components [12].
So, among concrete, steel, and wood, which is the most environmentally sustainable building material?
The frustrating answer is that it depends. It depends on the assumptions made in lifecycle assessments, the metrics used (for example, does comparing "tons of CO2 per tons of [building material]" make sense when different materials are used in different quantities to carry the same load?), the equipment involved and the fuel sources used to power that equipment, and so on.
Given the urgency of addressing climate change, and with the understanding that concrete, steel, and wood will continue to be the top building materials used in the US for the time being, I think that it is more important to first focus on intuitive next steps than to endlessly debate over which is the "best" or "worst" material. For concrete, steel, and wood suppliers, this means improving upon current practices (funding R&D on low-cement concrete, implementing more sustainable forestry techniques, etc.) with a renewed sense of urgency. For general contractors, this might mean mean carefully tracking the details of procurement and waste management decisions for every project to aid in a reasonably accurate lifecycle analysis. And for the building sector as a whole, it might mean better prioritizing building longevity, flexible use of building spaces, and material reuse.
FOOTNOTES
[1] Interestingly, about 40% of US greenhouse gas emissions are also said to be attributed to the building sector. However, I was unable to find a breakdown of how much of this 40% is attributed to building use and how much to materials and construction for the US.
[2] This rough calculation assumes that the majority of the said greenhouse gases are comprised of CO2, that 5.1 billion metric tons of CO2 are emitted by the US per year, that 11% of these emissions are due to building materials and construction, and that the current US population is 330 million.
[3] Many thanks to Cornell University's esteemed Professor Kenneth C. Hover.
[7] Image from https://www.thechemicalengineer.com/news/recycling-carbon-dioxide-in-steel-production/
[9] I have yet to come across a publicly-accessible study comparing the environmental impacts of steel reuse vs. recycling. If you see something of the sort, please send it my way!
[11] https://www.epa.gov/sites/production/files/2021-01/documents/2018_ff_fact_sheet_dec_2020_fnl_508.pdf