The path to zero: Concrete’s role in decarbonizing the built environment

December 11, 2020

By Adam Auer

Photo © Ed White. Photo courtesy HDR[1]
Photo © Ed White. Photo courtesy HDR

When it comes to strategies for addressing climate change, it is unsurprising buildings are taking centre stage. Globally, the construction, operation, and decommissioning of buildings and infrastructure account for approximately 40 per cent of all anthropogenic greenhouse gas (GHG) emissions. Three-quarters of these emissions are associated with the operation (heating, cooling, lighting, etc.) of existing buildings, while the remaining 25 per cent is attributed to ‘upfront,’ embodied emissions associated with manufacturing of building materials as well as emissions from construction and maintenance.

Until recently, the predominant focus of addressing the climate impacts of buildings has been to improve operational energy efficiency, largely because these emissions have made up the lion’s share of a facility’s GHG emissions over its full life cycle. Driven in part by advances in operational efficiency as well as the coming of age of life-cycle assessment data analysis and tools, the embodied emissions of building materials have now come under scrutiny.

The emerging interest in embodied emissions has put concrete in the spotlight as:

This raises important questions. What role does/could concrete play in decarbonizing the built environment? Can concrete compare favourably to other building materials in a net-zero carbon world? What does concrete’s path to carbon neutrality look like?

Heating, cooling, and powering buildings

For years, the drive toward climate-friendly buildings has been fixated on tighter and better insulated building envelopes to reduce heating and cooling energy loads. Operational efficiency is of particular importance in temperate regions like Canada where operational energy demand can drive over 90 per cent of a building’s GHG emissions over its lifespan, and doubly important where electricity is generated using GHG-intensive fossil sources like coal.

As our ability and willingness to reduce the heating and cooling demands of facilities improve, so, too, does the potential for net-zero building operations. Whereas ‘electrification’ and low-carbon sources like hydrogen were once GHG-mitigation strategies confined to the auto sector, they are now also seen as a viable pathway for net-zero buildings operations.

When it comes to operational emissions from buildings, concrete can have a significant impact. The material’s natural imperviousness to air and water can help achieve a tight building envelope. Arguably, concrete’s most important property when it comes to reducing operational GHG emissions is its thermal mass. Studies suggest strategic integration of thermal mass into a building’s energy management system can help reduce as well as shift heating and cooling loads and lead to significantly less GHGs as well as operational costs (see “The Thermal Mass Advantage”).

Thermal mass can also drive climate resilience or what some call “passive survivability” by leveraging a building’s structural concrete as a giant thermal battery, enhancing the building’s ability to maintain critical life-support conditions if services such as power and heating fuel are lost in extreme heat or cold.

Embodied carbon

Without abandoning a commitment to pushing the boundaries of operational efficiency, leading voices in the architectural and developer community are turning their attention to the other significant source of GHG emissions from construction—in particular, those associated with building materials.

Together, the production of the two most common structural materials—steel and concrete—are estimated to account for more than 11 per cent of global GHG emissions. If this number is contextualized at the building scale, its significance is more obvious. Of all the embodied emissions in a typical multistorey building, close to 50 per cent can be attributed to the concrete used in the building’s structural elements.

This has led some to look for alternatives (e.g. wood), but only to find that cutting embodied carbon is not straightforward and there are no silver bullets (see “Mind the Carbon Gap”).

The cement and concrete industry’s path to carbon neutrality. Images courtesy Cement Association of Canada[2]
The cement and concrete industry’s path to carbon neutrality.
Images courtesy Cement Association of Canada

On this note, there are now several important initiatives to help provide greater transparency, consistency, and robustness in accounting for embodied carbon. In Canada, one of the most promising and significant will be the National Research Council’s (NRC’s) Low Carbon Assets through Lifecycle Assessment (LCA2) initiative. LCA2 aims to establish a definitive regionalized life-cycle data[3] inventory for the country, including filling gaps in how LCA accounts for biogenic carbon from wood products as well as the carbon dioxide (CO2) that is naturally absorbed by concrete over its life (recent research suggests concrete can absorb over 25 per cent of the CO2 process emissions associated with its cement over its full life cycle).

The timing makes this all the more important, with Architecture 2030 estimating that between now and 2050, embodied carbon could represent as much as 50 per cent of the total life-cycle GHG emissions from new, energy-efficient structures. Importantly, and in contrast to operational emissions, these upfront emissions are released entirely at the beginning of a building’s life.

As the spotlight has begun to focus on upfront carbon, building material manufacturing sectors are taking note, and none are underestimating what a low-carbon transition entails for their respective sectors. For steel producers, it could mean a wholesale conversion to electrification and recycled content. For wood, it likely means a full retreat from logging the intact boreal and coastal rainforests and more rigorous, transparent accounting to support carbon-optimized forestry practices.

For cement and concrete, the challenges are significant, but the pathway is clear.

Decarbonizing concrete starts with cement

Cement is the active ingredient in concrete and while it makes up only 10 to 15 per cent of concrete’s recipe, it is responsible for 80 per cent or more of concrete’s GHG emissions. Decarbonizing concrete is largely a matter of decarbonizing the cement manufacturing process.

Canada’s fleet of cement facilities are relatively modern, taking advantage of the latest energy-efficient technologies (e.g. preheaters and pre-calciners, waste heat recovery systems, etc.) and pollution abatement equipment. However, when it comes to reducing GHG emissions, a number of opportunities are being actively pursued.

Cement is made by superheating limestone in a long rotary kiln to make an intermediary product called clinker. Clinker is mixed with ground limestone and gypsum to create Portland cement, and then added to sand, gravel, water, and sometimes other additives to make concrete[4].

The most obvious sources of GHGs are the combustion emissions coming from fossil fuels (a mixture of coal, petcoke, and natural gas) used to heat the kiln. The combustion accounts for about a third of cement GHG emissions. These can be reduced through fuel substitution and carbon-capture technologies.

Less commonly known sources of GHGs from cement are the carbon emissions coming from the chemical reaction taking place in the kiln. When limestone is super-heated to 1450 C (2642 F), it releases its carbon as CO2, making available the chemical bonds required to make clinker. These account for two-thirds of cement GHG emissions. They can only be reduced at the facility with carbon-capture technologies or at the product level (i.e. in concrete) by minimizing the amount of clinker used in the cement and/or replacing cement with supplementary cementitious materials in concrete to make low-carbon cement formulations.

Low-carbon fuels

The easiest and most economical way to reduce cement manufacturing emissions is by substituting a portion of fossil fuels used in the kiln with lower carbon alternatives (some facilities have achieved 100 per cent substitution. However, a mix of fossil and non-fossil fuels is more typical). Among the best alternatives are waste-derived fuels, such as construction and demolition waste (which is mostly wood), rail ties, biosolids, as well as unrecyclable tires, tire fluff, and non-recyclable plastics. In fact, because cement kilns have high temperatures and fuels linger in them for a long time, they are ideal end-uses for hard-to-manage wastes. Importantly, these fuels reduce GHG emissions (from ~15 per cent for tires to 100 per cent for biogenic fuels like waste wood) and have no impact on other air emissions (some alternative fuels also reduce nitrogen oxides [NOx] emissions).

Canada’s fuel substitution rate has historically been quite low (between seven and 10 per cent) compared to Europe, where the average is over 40 per cent and often much higher. A major impediment to leveraging the benefits of fuel substitution in Canada has been the difficulty in obtaining provincial permits to use non-traditional fuels; provincial permitting systems for industrial fuel are simply out of sync with waste management policies that dictate how potential waste-derived fuels must be handled. However, as addressing climate change becomes a priority along with an increased emphasis on ‘circular economy’ solutions to waste management, the cement sector is confident it can reach substitution levels meeting or exceeding best practices—with the potential to reduce the carbon intensity of Canadian cement by over 20 per cent. In fact, Canadian cement facilities have invested over $100 million in low-carbon fuels in the past several years.

Low-carbon cements

The second significant near-term opportunity to reduce the carbon intensity of cement is the transition to Portland limestone cement (PLC). It substitutes some of the carbon-intense clinker in regular cement with finely ground unprocessed limestone, offering a type that is a functional and cost-equivalent substitute for regular cement, but with up to 10 per cent fewer GHG emissions. It is the industry’s ambition to make PLC the default cement across Canada, reducing emissions by almost 1 million tonnes annually.

MIND THE CARBON GAP
Carbon is complex, and discussions about the embodied carbon and carbon emission implications of building materials are too important to get wrong.

All materials have a role to play in creating more sustainable communities and we need to rely on science to ensure our plans to cut carbon deliver those results.

The “Emission Omissions: Carbon accounting gaps in the built environment,” a new study by the International Institute for Sustainable Development (IISD) found gaps in how carbon is being measured in building products such as steel, concrete, and wood.*

They concluded that because life-cycle assessment (LCA) models do not account for biogenic carbon losses, they effectively omit up to 72 per cent of a wood product’s carbon footprint, enough to erode wood’s perceived carbon advantage.**

According to this study, these gaps could be serious enough to misdirect efforts to reduce carbon in the built environment and change the outcome of carbon comparisons from “advantage wood” to “advantage concrete”.

The IISD study concluded LCAs are still the best approach to measure carbon emissions in buildings but that more data, transparency, and robust standards are needed, especially with respect to unaccounted biogenic carbon.

* Visit www.iisd.org/library/emission-omissions[5].

**  Biogenic carbon dioxide emissions are defined as emissions from a stationary source directly resulting from the combustion or decomposition of biologically based materials other than fossil fuels.

THE THERMAL MASS ADVANTAGE
By Alex Janusz and Kim Pressnail

Figure 1: Percentage reduction in electricity costs using load shifting  and 2018 Ontario time-of-use electricity prices, relative  to operating at 23 C [73 F], for various building retrofits.[6]
Figure 1: Percentage reduction in electricity costs using load shifting and 2018 Ontario time-of-use electricity prices, relative to operating at 23 C [73 F], for various building retrofits.
In the search for more responsible building designs minimizing life-cycle energy use and associated carbon emissions, designers sometimes overlook the advantages of thermal mass. It is easy to do so because thermal mass is a hidden resource. For some, the effects can be difficult to estimate, and therefore, challenging to incorporate into building rating systems such as Leadership in Energy and Environmental Design (LEED).

Yet, studies in Canada and Europe—and several documented examples—have shown thermal mass can have a significant impact on operational energy. Depending on the location and orientation and design of a building, utilizing thermal mass can reduce heating energy use by between four and eight percent compared to a lightly constructed building with low thermal mass.*

Thermal mass leads to energy savings over the life of the building that are significant especially compared to embodied energy since the ratio of operational energy to embodied energy in many buildings designed in Canada today is often three or four to one. This means saving operational energy has a multiplier of three or four compared to saving embodied energy. Couple these savings with improvements in thermal comfort and you have two strong reasons why one should consider designing with thermal mass in mind.

Thermal mass comes in many forms, but it is a feature of structures constructed with reinforced concrete floor slabs and envelope and demising walls built using some combination of masonry or concrete materials.

In one study, it was found the amount of reinforced concrete required for the ‘structure’ provided sufficient thermal mass to manage solar heat gains and to minimize energy-use.** Increasing the quantity of thermal mass in floor slabs beyond the structural needs consistently reduced heating and cooling loads, but the energy savings were a diminishing return. The study also showed floor slab thermal performance was most influenced during the heating season by the intensity and distribution of solar gains transmitted through exterior windows. This means thermal mass performance is most sensitive to window properties, size, and orientation.

A second study, carried out by the authors, examined ways in which thermal mass could be used to reduce energy costs and greenhouse gas (GHG) emissions in electrically heated buildings in Ontario. This study examined how thermal mass could be used to do something called “load shifting.”

Load shifting occurs when heating demands are postponed to periods when electricity is cheaper and the electricity used is less GHG intensive. At night, time-of-use charges are at a minimum and most of the grid-supplied electricity comes from hydro and nuclear power sources that are almost carbon-free.

To the extent that heating loads can be shifted to nighttime, electrical cost and carbon savings accrue, even though the actual heating energy use goes up slightly as slabs and walls are ‘pre-heated.’

The authors also examined ways in which the effect of peak shifting using thermal mass could be maximized in a 1970’s multi-unit residential building. It was found that by coupling the heat distribution system with the floor slab through a radiant floor system, thermal mass benefits could be maximized for various retrofit options. Exposure of the thermal mass elements led to greater savings as well (Figure 1).

The right-most plot in the images in Figure 1 depicts the base or reference case where no retrofit measures were carried out to the building ventilation system or to the envelope. It is clear from Figure 1 load shifting can be a very effective operational strategy for utilizing thermal mass to reduce electricity costs and GHG emissions during the heating season. It is also clear exposed floor slabs with a radiant floor heating system are an effective way to achieve these savings.

Not only can shifting heating demand reduce electricity heating costs and GHG emissions, it can also reduce the stress on the electrical grid.

In many places, peak electricity demand is driven by weather. For example, in Ontario, peak electricity demand typically occurs during hot summer days, or during exceptionally cold winter nights. At these times, electricity is added to the grid by operating natural gas-fired generators or purchasing electricity from other providers who have higher carbon emission rates. Using thermal mass to shift heating or cooling demand in buildings would reduce stress on the grid when it matters most and limit carbon emissions.

Canada lags behind Europe in taking advantage of thermal mass but that is starting to change. Free tools for performing detailed building simulation, such as Google Sketch Up, Open Studio, and Energy Plus, are making it easier for designers to consider and evaluate the effects of thermal mass.

As building codes evolve, so do designs. As Canadians move toward low-carbon energy sources for heating and cooling buildings, demand for electricity will necessarily increase. With this increase, there will be a concerted effort to save energy, as it usually cheaper to save a kWh of electricity than to produce and distribute it. This will inevitably lead to energy conservation and carbon reduction methods, including the use of thermal mass and radiant floor heating systems.

* Consult the 2009 paper, “The influence of the external walls thermal inertia on the energy performance of well insulated buildings,” by N. Aste, A. Angelotti, and M. Buzzetti.

** Read Adam DiPlacido, “A Parametric Analysis of the Thermal Performance of Concrete Floor Slabs in Cold Climates”, M.A.Sc. thesis, University of Toronto, 2014.

[7]Alex Janusz is an energy and sustainability analyst at RDH Building Science Inc., where he conducts energy modelling and performs building science research as a member of the energy and sustainability team. He has a passion for sustainable design and a keen interest in building science. Janusz completed M.A.Sc. at the University of Toronto where he studied ways of using thermal mass to reduce operational energy costs and greenhouse gas emissions in Canadian buildings. Janusz can be reached at ajanusz@rdh.com[8].

[9]Kim D. Pressnail is an associate professor of civil and mineral engineering at the University of Toronto. He has been teaching building science within the Faculty of Applied Science and Engineering since 1990. His research interests include the design and construction of low-energy buildings and, in particular, utilizing thermal mass to reduce carbon footprint. Pressnail can be reached at pressna@ecf.utoronto.ca[10].

Implementation of low-carbon strategies.[11]
Implementation of low-carbon strategies.

Importantly, PLC is complementary to other carbon-reducing strategies, such as using supplementary cementitious materials (SCMs) like fly ash and slag. Together, PLC and SCMs can achieve carbon-intensity reductions of 30 to 40 per cent.

All cement producers in Canada are now able to manufacture PLC and it has been fully recognized in the Canadian Standards Association (CSA) cement standards and building code. While there are no technical barriers to the adoption of PLC, its market penetration has been hindered by resistance to change, a function of the inherent conservatism in the construction community.

Carbon capture, utilization, and storage

One of the most exciting developments in the cement and concrete industry in recent years has been the explosion of carbon-capture technologies.

Carbon capture offers a complete solution to cement GHG emissions, with the potential to capture virtually 100 per cent of the emissions from both the cement industrial and combustion processes. Captured emissions can then be stored underground or used to make other products like CO2-neutral synthetic fuels and aggregates. The latter can replace virgin aggregates used in concrete.

For many years carbon capture has been viewed as a kind of ‘Hail Mary’ pass for the coal-fired utility sector. In fact, SaskPower’s Boundary Dam coal-fired power plant in Saskatchewan is among the most well-known full-scale carbon capture systems in the world. However, the long-term success of carbon capture hinges, to a great degree, on its commercial application in the cement sector. This is, in part, because cement kilns produce a high concentration stream of CO2 making it efficient to capture. Additionally, the captured carbon dioxide can be used as a material input in the concrete manufacturing process.

Several cement facilities in Canada are well advanced in the implementation of carbon capture systems. For example, a cement manufacturer has undertaken a $3 million advanced feasibility study for full-scale carbon capture system at their Edmonton, Alta., cement facility. Another manufacturer recently completed the installation of a flu gas pre-treatment system at their Richmond, B.C., cement facility, paving the way for a full carbon capture system.

Examples of carbon capture, utilization, and storage (CCUS) technologies include:

Most promising, if the industry is successful in its goal of manufacturing carbon-neutral cement, and if carbon-capture technologies are combined with other low-carbon materials, such as biomass fuels or carbonated aggregates, concrete could transform into a carbon-negative building material.

Conclusion

For anyone concerned about climate change, it is self-evident building practices need to change—material manufacturers, specifiers, architects, developers, and engineers need to work together at each stage of the construction process to lower embodied carbon while producing ultra-efficient, durable, and climate-resilient buildings and infrastructures.

[12]Adam Auer is vice-president, environment and sustainability, at the Cement Association of Canada (CAC). Auer has almost 20 years of experience as a sustainability professional. In his role at CAC, he works with government, industry, environmental, and other civil society groups to promote and enhance concrete’s contribution to sustainability, with an emphasis on life-cycle approaches to climate change mitigation and adaptation. Auer holds a master’s degree in environmental studies from York University and a bachelor’s degree in ecology from the University of British Columbia. Auer can be reached at aauer@cement.ca[13].

Endnotes:
  1. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/BCChildrensBCWomensHosp_TeckAcuteCareCtr_PP1001_lg-1.jpg
  2. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/concretes-path-to-carbon-neutrality-graphic-august-2020_003-font-outlined.ai_.jpg
  3. life-cycle data: http://www.sciencedirect.com/science/article/pii/S0008884619301929#bb0020
  4. to make concrete: http://rediscoverconcrete.com/en/sustainability/how-cement-concrete-are-made.html
  5. www.iisd.org/library/emission-omissions: http://www.iisd.org/library/emission-omissions
  6. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/Figure-1-1.jpg
  7. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/Alex-Janusz-Sq.jpg
  8. ajanusz@rdh.com: mailto:ajanusz@rdh.com
  9. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/Kim-Pressnail-Photo-2.jpg
  10. pressna@ecf.utoronto.ca: mailto:pressna@ecf.utoronto.ca
  11. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/Putting-low-carbon-strategies-to-work-illustration_D_Vertical.jpg
  12. [Image]: https://www.constructioncanada.net/wp-content/uploads/2020/11/Adam-Auer.jpg
  13. aauer@cement.ca: mailto:aauer@cement.ca

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