Wood’s Good: Green Building Materials

Two of the most common building materials, steel and concrete, are mostly made in ways that create a huge amount of carbon dioxide emissions. A large amount of heat is needed to make these materials, and this is often supplied with fossil fuels. But this is only part of the reason for the large carbon intensity of steel and concrete. The chemical reactions that occur with these processes also directly emit carbon dioxide. These direct chemical releases of heat-trapping gases are called process emissions. Steel and concrete are needed to get to a 100% clean energy future, and there are many methods to make these materials in more sustainable ways. But using less is always better, and there are several materials that don’t have nearly the impact of these other industries.

The building plant

There’s this new building material that you’ve got to hear about, and it comes from trees! Okay, wood for construction is not exactly new, but there are some new processes that can make it as strong and safe as steel and concrete, with significant climate benefits as well. Mass timber is the term used for construction using engineered wood materials that can be built into skyscrapers, referred to as “tall wood” or “plyscrapers.”

Cross-laminated timber (CLT) is a particularly exciting material that may be on the verge of revolutionizing the building industry. Developed as part of an Austrian professor‘s Ph.D. research in 1994, CLT is constructed by placing planks of wood next to each other in one direction, then adding another layer facing at a right angle, then another along the first direction, and so on. Layers of 3-9 boards are glued together and pressed to make a remarkably strong material, which can even replace concrete. Even damaged wood can be used within CLT, including forests that have been damaged by bark beetles across North America and Europe.

CLT has several benefits in its construction. It can be cut on-demand, meaning not nearly as much room for material storage is needed on site. Construction times are much shorter than for steel and concrete buildings, and the building sites are quieter. Experiments tend to show that CLT is remarkably resistant to fire as well, with charring on the outer layers first.

The earliest and largest CLT structures are in Europe. Since building regulations have been adopted to allow CLT in the United States and Canada, the Pacific Northwest is taking the lead. CLT skyscrapers include an 18-story dorm on the University of British Columbia campus, which was constructed in only 70 days, 4 months less than a typical concrete and steel structure. A University of Washington professor of architecture built her residence out of CLT, making use of the unique angles that CLT panels can be cut into to capture light and create picturesque views. Founders Hall at UW is a five story, 85,000 square foot structure currently being constructed out of CLT.

There are other forms of wood that are useful in buildings in different ways, of course. Nail-laminated timber is boards nailed together in the same direction, glue-laminated timber sticks pieces together, and interlocking CLT uses no glues or fasteners. These have different properties that can make them more useful or cost-effective in different settings.

Trees as a building material are renewable, since new forests are constantly growing around the world. As trees grow, they take carbon dioxide from the atmosphere via photosynthesis, so wood as a material has the potential to remove the main heat-trapping gas from the air, in small quantities at least. A cubic meter of mass timber embodies a little less than a ton of carbon dioxide taken from the atmosphere. Typical home has around 50 cubic meters of CLT, so traps about three year’s worth of carbon footprint of an average American within its wood.

Steel

Steel is made of iron with a little carbon added, occasionally with some other metals melted in. Strong and inexpensive, it provides the basis for much of the infrastructure, buildings and transportation around the world. It’s also highly carbon intensive, with 2.6 Gton of CO₂ emitted from steel worldwide in 2019.

Steel demand has increased by a factor of 3 since 1970, doubled since 2000. In 2019, of the 1.8 billion tons of steel made, 400 million tons went to scrap before use, 200 million tons went to consumer goods appliances, and packing, 300 million tons went to mechanical and electrical equipment, 200 million tons went to vehicles, 300 million tons went to transportation or other infrastructure, and 500 million tons went to buildings. The average carbon dioxide emitted from producing a ton of steel is 1.4 tons.

Around 70% of steel is from iron ore, in a highly energy intensive process. The remaining 30% is made from recycled scrap, in a process that takes only one-eighth of the energy of production from ore. Recycled steel is made in electric arc furnaces so doesn’t require coal burning. Around 85% of steel is recycled already, with industrial equipment particularly high and structural reinforcement steel recycled less, only about 50%. On average, it takes 18.6 GJ (5167 kWh) of energy to make a ton of steel.

Electric arc furnaces, used since the late 1800s, are impressive heaters. They can reach temperatures of 1800^o C. The furnace has a retractable roof and graphite rods which are lowered into the metal as a current runs through them. An arc of high-energy plasma is created, melting the iron and separating out slag, a stony mass of residue. Arc furnaces require a huge amount of electricity (around 440 kWh per ton of steel), but can be switched on and off with ease, meaning production hours can be scheduled for when electricity is more available. Blast furnaces, the carbon-intensive method of making steel, need to be on essentially continuously because the shutoff process is difficult.

Steel can be made from direct-reduced iron within electric arc furnaces as well, in a process that can be almost completely decarbonized. Instead of mixing iron with coke (pure carbon from coal) in a process that both requires high temperature and releases CO2 as a process emission, one can use hydrogen. Provided the hydrogen is made by electrolysis with clean energy, the process emits just a small fraction of the carbon dioxide in normal steel, less than one-twentieth. It also can be done at lower temperature. This method of steelmaking is beginning to be used in Europe.

Steel is currently needed in large amounts to make wind turbines. Onshore 5 MW turbines use 900 tons of steel, and to generate a tera (2.5 TW nameplate capacity at 40% capacity factor) requires 450 million tons. This is a lot of energy, but the energy payback time of the steel is short.

Concrete

Concrete has a large footprint both because it takes a lot of heat to make and because the process emissions cause a very large direct release of carbon dioxide. Concrete is made of cement plus aggregate material, some combination of sand, stone and gravel. Almost all concrete used today uses a substance called Portland cement, itself made from Portland clinker, the ultimate source of most of the carbon emissions from concrete.

Portland clinker is made by heating calcium carbonate, either in the form of limestone, shells, or chalk, mixed with clay or other substance. It takes 3.4 GJ of energy to make a ton of Portland clinker, a little under 1000 kWh, for fuel to heat kilns to temperatures of 1400^o C. Even more carbon emissions are associated with the process emissions of making Portland clinker. In 2019, 2.3 Gton of carbon dioxide were released from cement, 0.8 Gton from energy and 1.5 Gton from the process emissions. Adding the emissions associated with mining and transportation of the materials gives a total of 2.8 Gton of carbon dioxide, equal to 8% of world emissions. A total of 4.1 billion tons of cement are used each year, and each ton of cement results in direct release of around 0.54 tons of carbon dioxide emissions.

There are alternatives to Portland clinker; any way to use less in concrete is useful. Geopolymer cement uses substances like fly ash from coal burning or slag from steel production to lower the embodied emissions to 10-20% of Portland clinker. Ferrock is another option, made from waste steel dust and glass. The final material is even harder than regular cement, and more flexible as well.

Although there are certainly technological process that will come in these areas, we already have all the technologies we need for 100% clean industry. We just need to get them implemented.