The role of concrete in life cycle greenhouse gas emissions of US buildings and pavements
Table of Contents
Jeremy Gregory, Hessam AzariJafari, Ehsan Vahidi, Fengdi Guo, Franz-Josef Ulm, and Randolph Kirchain
a Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
b Materials Research Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139
PNAS September 14, 2021 118 (37) e2021936118; https://doi.org/10.1073/pnas.2021936118
Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved August 4, 2021 (received for review October 20, 2020)
Significance
Changes to concrete production as well as in building and pavements systems—the largest consumers of concrete—can lead to more than 50% reductions in associated GHG emissions by 2050. Over this period, the operational phase of newly constructed buildings and pavements still generates most GHG emissions unless the electrical grid, heating, and transportation are decarbonized aggressively. Meeting decarbonization targets will require lowering the GHG emissions of concrete production as well as innovative uses to lower building and vehicle fuel consumption. Owing to their low abatement costs, several concrete solutions should be prioritized in climate change policies. More than one-third of the embodied impacts of building and pavement construction can be offset by implementing concrete solutions.
Keywords
greenhouse gas emissions; buildings; pavements; concrete; life cycle assessment
Abstract
Concrete is a critical component of deep decarbonization efforts because of both the scale of the industry and because of how its use impacts the building, transportation, and industrial sectors. We use a bottom-up model of current and future building and pavement stocks and construction in the United States to contextualize the role of concrete in greenhouse gas (GHG) reductions strategies under projected and ambitious scenarios, including embodied and use phases of the structures’ life cycle. We show that projected improvements in the building sector result in a reduction of 49% of GHG emissions in 2050 relative to 2016 levels, whereas ambitious improvements result in a 57% reduction in 2050, which is 22.5 Gt cumulative saving. The pavements sector shows a larger difference between the two scenarios with a 14% reduction of GHG emissions for projected improvements and a 65% reduction under the ambitious scenario, which is ∼1.35 Gt. This reduction occurs despite the fact that concrete usage in 2050 in the ambitious scenario is over three times that of the projected scenario because of the ways in which concrete lowers use phase emissions. Over 70% of future emissions from new construction are from the use phase.
Concrete is the most extensively used building material in the world because it possesses a unique combination of attributes—strength, versatility, and durability—for a relatively low cost using raw materials found all over the world. It is used in nearly every element of our built environment including buildings, pavements, bridges, and water and energy systems. This ubiquity in infrastructure has also made concrete use tightly linked to achieving societal sustainability goals. Thacker et al. (1) found that infrastructure, which makes extensive use of concrete, either directly or indirectly influences the attainment of every United Nations Sustainable Development Goal.
On a weight-normalized basis, concrete has a lower carbon and energy footprint than nearly all materials used in the built environment (2). Nevertheless, the cement and concrete sectors are deservedly under scrutiny regarding their environmental footprint because of the sheer scale of production (3). Greenhouse gas (GHG) emissions from the production of cement (the primary driver of GHG emissions for concrete) account for a little over 1% of the total US GHG emissions footprint (4). Thus, the challenge of sustainable development is manifest in microcosm in the use of concrete: accomplishing societal goals while minimizing environmental impacts.
There is no question that we need to reduce the emissions associated with cement and concrete production. However, the mitigation solutions for products made with concrete extends beyond the cement and concrete production value chains. Materials dictate the modes of manufacture and constrain the operational performance of the products into which they are fashioned (5). Concrete is a prime example of this phenomenon. Forming the backbone of large, complex, long-lived systems, changes in the use of concrete can positively or negatively impact the in-use performance and GHG emissions of these systems for decades.
In this systems context, we seek to evaluate the cost and effectiveness of a range of strategies for reducing the GHG footprint of two important systems—buildings and pavements—including both changes in cement and concrete production and changes in system design, maintenance, and operations. Using this comprehensive model, we also evaluate the relative contribution of embodied and operational emissions as these systems undergo significant change and explore whether GHG emissions reductions are possible in these systems even if there is increased use of concrete. Mapping these changes for buildings and pavements is challenging, because the impact of system structure is influenced by local context, the role of extant stock and its evolution, and the long timeframe that needs to be considered. To overcome these challenges, we develop and apply spatially and temporally heterogenous, life cycle models of the buildings and pavements systems. We limit our analysis to the United States because of the extent of data required for modeling. In the United States, these systems account for over 60% of apparent cement usage according to data from the industry and the building, transportation, and industry sectors account for 90% of all GHG emissions (4). As such, changes in the structural components of these systems can provide influential leverage in meeting climate targets.
Literature Review
There is an extensive literature on approaches to mitigate the embodied emissions of cement and concrete production (2, 3, 6⇓⇓–9). Most mitigation approaches involve making cement with lower GHG emissions or making concrete with less cement. Concrete GHG emissions can be lowered through the use of cement substitutes such as low-carbon cements [blended cements or alternative cement binders (10)] or supplementary cementitious materials (such as fly ash or granulated blast furnace slag) (3) or through active use of captured carbon to produce synthetic limestone aggregates or cure concrete (11).
While embodied emissions of concrete are important, life cycle assessments of infrastructure systems built using concrete have shown that in most cases they are much smaller than emissions that occur during the use or operational phase of the structure. In most buildings, the energy consumption over the building’s life dominates the total life cycle environmental impact, representing 80 to 90% in many cases (12⇓⇓–15). Similarly, in most pavements, the excess fuel consumption in vehicles caused by pavement–vehicle interaction (excess energy dissipation due to pavement roughness or deflection) is much larger than the embodied impacts of paving materials. Depending on the context (i.e., the traffic and location), roughness and deflection-induced excess fuel consumption contribute 23–78% of the life cycle GHG emissions of pavements (16). Including the reflectivity impact of pavements on the climate in the use phase can increase the use phase contribution to ∼90% of the total GHG impacts (17). Thus, the ways in which we design and maintain structures that use concrete can have much larger impacts than the impact of materials. All of these studies have explored existing buildings or pavements operated under conditions that exist today. Significant changes are expected in the carbon intensity of energy used for the operation of buildings and transportation systems. While this is expected to increase the importance of embodied emissions to future mitigation efforts, the literature does not provide quantification of this trend.
Although several studies have evaluated the whole life cycle impacts of pavements (16, 18, 19) and buildings (20, 21), proposed solutions usually fall into a few categories. Analyses of pavement systems focus on material flows (22, 23) or optimizing budgets and treatment schedules to minimize vehicle fuel consumption and the associated life cycle GHG emissions (24, 25). Similarly, building system analyses concentrate on material quantities or energy consumption (26–30). Thus, there is a disconnect between analyses of embodied GHG reductions for concrete that focus on materials and do not put those reductions in the context of the full life cycle for the structures in which they are used, nor the system of buildings and pavements.
We contextualize the role of concrete in greenhouse gas reductions in the US building and pavement sectors. This includes the potential impacts and costs of reducing the embodied impacts of concrete along with changes in the design and maintenance of structures that use concrete throughout their entire life cycle. We explore whether total life cycle emissions can decrease even with increased usage of concrete due to functional requirements or opportunities to lower use phase emissions. To a limited extent, we examine other actions that can be used to lower GHG emissions in US building and pavement systems. This allows us to evaluate concrete GHG reductions in the context of the systems and frame those opportunities based on cost and GHG reduction potential.
The models applied in this work consider geographic heterogeneity (at a US state level) in the demographics of the current stock of buildings and pavements, local climate, prevailing construction codes, norms for system maintenance, and, in the case of pavements, available public budgets for infrastructure. Using this information, we identify a range of approaches that can be applied to bring emissions from these systems to less than 50% of current levels and how embodied and operational emissions reductions strategies compare.
Approach
System Attributes and Strategies for Scenarios.
Our analysis of building and pavement systems in the United States is based on attributes of those systems (e.g., material or energy use) and strategies that may be used to lower GHG emissions. We estimate GHG reduction potential for the strategies from 2016 to 2050 using two scenarios: projected and ambitious improvements.
Table 1. Attributes of concrete production and building and pavement systems for projected and ambitious GHG reduction strategies.
| Attribute definition | Projected improvement scenario | Ambitious improvement scenario |
|---|---|---|
| Concrete (for both buildings and pavements) | ||
| Alternative binders | 40% clinker replacement by 2050 | 50% clinker replacement by 2050 |
| Particle packing | No implementation | Improve the binder intensity |
| Design optimization | No implementation | 19% reduction in concrete consumption per unit area |
| Reuse of concrete elements | No implementation | 0.1-m3 reuse per cubic meter concrete |
| End-of-life carbon uptake | Base on the alternative binders’ scenario and 2-y spreading | Increasing the spreading period of 3y |
| CCS | 100% of the average tech by 2050 | 100% of the best performing tech by 2050 |
| CCU | No utilization | Use of industrial sources of alkalinity (/ˌælkə’linəti/) by 2050 |
| Building specific | ||
| Energy codes (appliances, lighting, HVAC, insulation) | 100% IECC 2015 adoption in 2025. 100% energy efficient adoption in 2045 | 100% IECC 2015 adoption in 2025. 100% energy efficient adoption in 2035 |
| Electricity grid | Grid decarbonization following the US EIA projection until 2050 | Grid decarbonization following the US EIA projection for New York state |
| Pavement specific | ||
| Asphalt(/‘æsfælt/) | 35% RAP and 100% WMA by 2050, no implementation of recycled binder | 50% RAP, 100% WMA, 50% GTR, and waste oil by 2050 |
| Smoothness | Equivalent to required budget for keeping surface roughness constant | 20% increase in the currently projected budget |
| Concrete overlay | Only used in regions where it is already in place | Inclusion of concrete overlay action for all regions |
| Reflectivity | Average network aged albedo values: concrete = 0.25; asphalt = 0.1 | Use reflective coating/binder to reach average network albedo = 0.3 |
| Stiffness | Current stiffness values existing in the national road network | Increase the stiffness to the 95th percentile of the current range |
| Vehicle fuel efficiency | According to the US Energy Outlook forecast | Same as the projected improvement scenario |
- CCS, carbon capture, sequestration; CCU, carbon capture, utilization; GTR, ground tire rubber; HVAC, heating, ventilation, and air conditioning; IECC, International Energy Conservation Code; RAP, recycled asphalt pavements; US EIA, United States Energy Information Administration; WMA, warm mix asphalt. More details are in SI Appendix, Table S1.
Table 1 summarizes the building and pavement system attributes and strategies under the two scenarios. Strategies are framed in terms of technical targets (e.g., percentage of renewables in the grid) and timing of adoption of those targets, which in some instances varies regionally. The “projected improvement scenario” is intended to reflect a future where current trends to improve system attributes will continue. For buildings, this includes continued decarbonization of the electrical grid and increases in energy efficiency requirements in building and appliance codes. These energy efficiency improvements include increased thermal insulation where concrete can play a role. For pavements, this includes continued improvement in vehicle fuel economy. For both systems, the projected improvement scenario evolves toward the use of net zero emissions concrete (portland cement–based and asphalt-based) through the use of lower-carbon constituents (including recycled content), carbon capture in cement production, and use of captured carbon to produce aggregates and cure concrete. The “ambitious improvement scenario” is intended to reflect a future where more aggressive actions to lower GHG emissions are taken. In all cases, ambitious strategies are limited to technologies that exist today, but have not been adopted at meaningful scale. Building ambitious strategies are similar to the projected actions but with earlier timing of adopting technical targets. Pavements ambitious strategies are primarily tied to an increase in funding for pavement maintenance and repair. This increase in funding is important because, unlike buildings, there are few current policies explicitly intended to improve infrastructure GHG emissions. As such, for the projected improvement scenario we assume that there is no change over time in the pavement system’s stiffness, reflectivity, or types of maintenance, rehabilitation, and reconstruction actions (referred to as MRR). The increase in available budget in the pavement ambitious strategy allows for more extensive application of all pavement-related improvements, so there is more of an interdependency among these ambitious strategies. Details on the technical targets and timing of pavement and building sector by region are in SI Appendix, section 2, including SI Appendix, Tables S1 and S2, along with justification for why the strategies and targets were chosen.
Modeling Approach and Data Sources
We used a bottom-up approach shown in Fig. 1 to model the characteristics of individual reference buildings and pavements and then scale up the results to regional networks and ultimately the entire country using temporally and spatially varying data. To capture spatial variation in building codes, construction practices, structural performance, climate, and energy demand, reference designs and practices were developed for climatic regions across the United States. For buildings, six reference designs (representing the two most common building materials used in residential [single and multifamily] and commercial buildings) were developed for each of the 14 climate zones described by the US Department of Energy (DOE). For pavements, several reference designs and operating schedules were estimated for four pavement types for each of the four climatic regions described by the US Department of Transportation. In both cases, states were assigned to their appropriate region and the prevalence of reference designs was modeled using state-level data. Several models were used to establish the associated material, operational energy, and other use phase requirements for each design over the course of the analysis period (2016–2050). The system boundaries of life cycle assessment include a cradle-to-grave scope and incorporates the emissions of materials and energy from the extraction of material until end of life (see Dataset S1 in the SI Appendix spreadsheet for the impacts from each component). A dynamic life cycle inventory was developed to capture the geographical and temporal aspects of the technologies and practices in different states to precisely estimate the material, operational energy, and other use phase requirements over the course of the analysis period (2016–2050). The buildings and pavements modeled in the analysis represent ∼60% of the total cement consumption in the United States (31). Details of the methodology along with the input data sources are described in SI Appendix, sections 3 and 4, for pavements and buildings, respectively.
Fig. 1. Summary of the bottom-up approach for investigating the life cycle GHG impact of buildings and pavements in the United States.
Limitations
Our analysis uses projected and ambitious scenarios that include a set of strategies for lowering US GHG reductions. The strategies are intended to represent major points of leverage but are by no means comprehensive. Indeed, other design-focused strategies could be considered including design for longer life, increased hazard resistance, smaller size, adaptability, and recycling or reuse. Furthermore, there are other actions that could be taken within the building sector such as retrofits of existing buildings, lowering embodied impacts of other building materials besides concrete, and increasing use of on-site renewables. Increasing recycling of all construction and demolition waste would also be beneficial. Thus, the results of this analysis should not be viewed as precise since we do not account for numerous sources of uncertainty in data and future trends, or comprehensive since we have not evaluated the potential of all GHG reduction strategies. However, the results still provide valuable insight on the potential of the strategies in both the projected and ambitious scenarios to mitigate GHG emissions in the building and pavement sectors.
Results
Opportunities for GHG Reductions
The original stated goal for the United States in the Paris Agreement was to lower the total anthropogenic GHG emissions from 6.5 Gt in 2017 to a range of 1.7–2.3 Gt in 2050 (32), a reduction of 65–75%. When electric power emissions are allocated to end-use sectors, buildings, transportation, and industry accounted for 2.0, 1.9, and 1.9 Gt of US GHG emissions in 2017, respectively (4). This represents 90% of the 6.5 Gt of US GHG emissions. Thus, achieving GHG reduction targets requires significant contributions from all three of these sectors, and the use of concrete impacts all of them.
Fig. 2 shows the historical and our modeled future GHG emissions for both the buildings and pavement sectors. Historical emissions increase in the 1980s and 1990s due to growth in building stock and the pavement network and vehicle-miles traveled. Emissions peak in the early 2000s and decrease due to building energy efficiency improvements, a lack of pavement network expansion, and vehicle fuel economy improvements.
Fig. 2. Built area and historical and future estimated GHG emissions in the (A) buildings and (B) pavements sectors. Historical emissions are before 2016. The 2016 levels are used as a reference for the reduction of future emissions. Projected and ambitious emissions reductions for the individual attributes listed in Table 1 are plotted (with the exception of buildings ambitious strategies, which are omitted for clarity), with the cumulative total projected and ambitious emissions reduction noted. The ambitious scenario GHG emissions are broken down by building and pavement types.
In the buildings sector, the projected improvement scenario results in a modeled GHG emission of ∼1 Gt in 2050, a reduction of 49% relative to 2016 levels. The ambitious improvements scenario results in a 57% reduction in 2050 down to ∼0.75 Gt GHG. In either scenario, modeled building sector emissions in 2050 are split evenly among residential (single and multifamily) and commercial (all other categories) buildings, with single-family buildings the largest individual category of buildings by far. Both scenarios project nearly identical use of concrete in the buildings sector, around 240 Mt (110 Mm3) in 2050. Energy consumption in the building stock plays a significant role in the sector, which is why the largest opportunities for GHG emission reductions shown in Fig. 2A (and the cumulative reduction quantities in Fig. 3A) derive from changes to the electricity grid (purple line), appliances (orange line), and lighting (green line). Changes in these three attributes make up over 85% of the 22.5 Gt of the cumulative GHG reductions projected over 34 y under the ambitious improvements scenario (Fig. 3A). In this analysis, heating, ventilation, and air conditioning (HVAC) improvements, enhanced thermal insulation, and concrete production only affect new construction, so opportunities for reduction are more limited. Although the ambitious scenario considers changes to important system attributes such as electricity grid decarbonization and more intensive adoption of energy efficiency codes, its 2050 results represent only an additional 10% reduction in GHG emissions from 2016 levels. Unfortunately, this change is not enough to reach the 65% reduction target. This suggests that the sector will have to look to other solutions or possibly much more aggressive changes to these attributes to reach the goal.
Fig. 3. Cumulative 2016–2050 GHG emission reductions under the ambitious scenario for (A) buildings and (B) pavements. Reductions for each category are broken down into projected and ambitious scenario contributions (ambitious builds off of projected). The 2016 level assumes that emissions do not change from 2016 levels over the entire 34-y period.
The pavements sector is estimated to have a 14% reduction of GHG emissions in 2050 relative to 2016 levels under the projected improvement scenario. Our modeling projects the use of 9.5 Mt (4 Mm3) of concrete in 2050. For the ambitious scenario, we project both a much more intensive use of concrete (28.2 Mt or 11.9 Mm3) and a much larger reduction of GHG emissions (Fig. 2B).
Supplementary Information for: The role of concrete in life cycle greenhouse gas emissions of US buildings and pavements
1. Overview
The research question of “How can the climate change performance of the US buildings and pavements be improved through the choice and intensity of materials” can be solely answered by a bottom-up approach. We developed and implemented a bottom-up approach to evaluate the current and future conditions for the building and pavement life cycles. It should be noted that different model components, such as building energy consumption, pavement-vehicle interaction (PVI), carbonation, and albedo, would not indicate within top-down material flow analysis. Moreover, we used the intensity of the material (focus on concrete) to understand the extent to which the total life cycle impact (both the operational and embodied emissions) can be mitigated. To this end, the use of materials must be tracked in time and in location, which requires a bottom-up approach. Similarly, speculative context-dependent solutions (i.e., the incremental changes to achieve the specified targets by 2050) relevant to the materials flows (e.g., improvement in insulation systems, and increased use of concrete overlays, increased budget which changes the amount of concrete used, and energy-efficient manufacturing of concrete products) cannot be incorporated in a bottom-up approach.
Material flows of construction materials used in different regions of the country require a sophisticated modeling framework. We first defined a representative temporal and geographical resolution at the state level to understand for what purpose and to what extent the materials will be used in the future. Although the newly constructed and demolished areas of buildings are available based on the US EIA projection statistics, the estimation and employment of representative reference buildings for each building type need precise elaboration. The building operational energy simulation representative of each building type and each climate zone was incorporated to understand the intensity of energy consumption in the buildings. For pavements, the situation is more complex as there is no projection on the rate of demolished and added materials. A flexible pavement management system is required to simultaneously track the surface condition of road segments based on the climate condition, road geometry, traffic, and local maintenance and repair practices, and assess the amount of materials required to fulfill these actions. To our best of knowledge, the total life cycle impacts of buildings and pavement can be consistently investigated only if a bottom-up approach is applied.
2. Details of projected and ambitious scenario assumptions
2.1.Mixture embodied impacts (buildings and pavements)
Decarbonization of cement and concrete value chains, similar to any other products and services, requires compensation measures, such as carbon capture and sequestration or utilization as there is no alternative way to approach net-zero mixtures [18]. For the cement value chain, the carbon capture and sequestration (CCS) technology has been recognized by a few cement plants in North America (both US and Canada). We assumed the target value of 23% CCS until 2050 for the portland cement (普通水泥，硅酸盐水泥) consumed in pavements and buildings as projected by the cement technology roadmap report (assumption for the projected scenario). Nevertheless, according to the CSI roadmap [1], up to 61% of the cement CO2 is feasible to be captured by 2050 using a more expensive but more effective technology of oxy-fuel CCS in the cement plants. This aggressive scenario was applied to the portland cement production and linearly adjusted until 2050.
Previous studies have shown the feasibility and significant benefits of carbon capture and utilization (CCU) technology that can be jointly used with the sequestered carbon in the cement. Among all the studied CCU technologies by the recent reports of the National Academy of Science [12][11], CO2 utilization through the production of carbonate-based construction materials is the closest to the application at a commercial scale compared to other technologies. Some of these approaches are already employed commercially, suggesting the near-term potential for increased adoption of mineral carbonation in the large and growing construction materials market. The combination of CCS and CCU technologies was stated as a solution to offset up to 85% of the cement CO2 footprint [19]. We considered leveraging the CCU technology developed for aggregate manufacturing in concrete (the same scenarios were applied to asphalt as well) as one of the viable emerging technologies proposed in the US as an overarching goal to compensate for the accumulation of GHG emissions in the atmosphere [20]. More than 70% of concrete mass consists of coarse and fine aggregates. This significant mass of materials allows authorities and suppliers to benefit the mixture aggregates as a significant
permanent carbon sink. Moreover, according to our analysis, a linear increase in synthetically
carbonated aggregates up to around 16-18% of total mass would result in net-zero GHG mixtures
(in combination with other scenarios) for concrete by 2050.
To analyze the feasibility of implementing the CCU technology for aggregate production, we
investigated the availability of alkalinity sources that are required to react with CO2 to form the
carbonate products to utilize the carbon. In this sense, four sources of desalination brine, air
pollution control residues from municipal solid waste (MSW) incineration, brucite, and
ultramafic mine tailings were included as ex-situ strategies for carbon mineralization [21]. These
alkalinity sources can be used for binder production as well, but they may need strict
performance specifications and the standardization process may need a long period of
investigation, possibly beyond the time horizon of this study. Therefore, although these sources
can be used for producing binder materials, the CCU aggregate technology may be a more viable
solution considering the performance requirements and quality control of the aggregates used in
concrete and asphalt assuming that the same shapes, mechanical and absorption properties of
natural aggregates can be manufactured. In this study, although the ultramafic rocks are
abundantly available in the US [22], [23], these alkalinity sources were excluded as they may be
cost-prohibitive given the significant depth where the rocks can be quarried in the US. Currently,
for slag and soil conditioning and refractories, more than 80% of the US industry demand is
imported from Norway [23].
In 2017, the US produced 10.91 Mm3 desalinated water per day, which corresponds to 11% of
the global production [21], [24]. The total produced desalination brine was around 5.3 Mm3 per
day [25]. This amount of brine was produced mostly near the coastlines, but the majority of
brines were produced inland. The produced brine consists of an average concentration of 2360
and 730 g/m3 Mg2+ and Ca2+, respectively [21], which can be an appropriate candidate for the
required alkalinity in the CCU process. Given the intensified desalination trend, the total amount
is predicted to be tripled by 2050 [26], [27]. Currently, the water desalination brine is usually
disposed of by dumping it back into the sea. This process requires an expensive pumping system
with a high susceptibility to damage the marine environment [28]. With the intensive growth in
the desalination industry (as estimated in Table S3 and elaborated in Section 2.1 of the SI
document), this alkalinity source seems abundantly available for carbon mineralization in the
US.
Another source of alkalinity, which is abundantly available in any urbanized area, is the air
pollution control residues (APCr) generated through MSW incineration. Globally, 11% of the 2
Gt of municipal solid waste is incinerated, and a further 4% is converted to APCr. When waste is
incinerated, the acidic gaseous emissions are contacted with calcium hydroxide for
neutralization, creating APCr that is often rich in Ca‐species. In 2017, 12.7% of MSW generated
in the US was disposed of through waste incineration with energy recovery [29]. Combustion
reduces waste on average 80% by mass, resulting in a significant amount of ash residue. Most of
the generated MSW ash has been landfilled in the US. Thus, the carbonation of APCr presents an
appropriate opportunity for synthetic aggregates production [21]. 12.7% of the total municipal
waste (264 Mt and 360 Mt in 2017 and 2050, respectively) is incinerated in the US. The
incineration can provide 33.5-45.7 Mt/year MSW and subsequently 1.34-1.83 Mt/year APCr
during the 2017-2050 analysis period.
13
Two sources of brucite and ultramafic mine tailings are abundantly available with extremely low
cost compared to other sources (average abatement cost of less than $50/t CO2). The ultramafic
mine tailings are a “low hanging fruit” material source for carbon mineralization [11] due to their
high surface area and porous structure and as a by-product. The production scale of ultramafic
mine tailings is about 200 Mt/year and is available in different regions. Also, among ultramafic
rocks, the extraction of Brucite has a lower cost (5 times lower cost compared to Olivine and
Serpentine). Although Brucite minerals are not as abundant as ultramafic mine tailings, their
mineralization capacity is quite larger (0.76 t CO2/t alkalinity for Brucite as opposed to 0.1 t
CO2/t alkalinity for tailings [11]), providing a larger carbon sink for CO2 mineralization.
For every ton of CO2 sequestered as an Mg-carbonate, 1.3 t of magnesium hydroxide is required,
which needs 8.2 GJ of electricity per t of CO2 (5.96 GJ/t magnesium hydroxide only for the
electrolysis process) [30]. As a conservative assumption and due to the lack of available data, the
same energy consumption values were assumed for the calcium-based alkalinity process but the
amount of CO2 capture per unit of Ca-based source was assumed proportional to the molar ratio
of CO2, and carbonates. Also, it was assumed that the required energy is supplied by renewable
sources (average of wind, photovoltaic, hydro, and geothermal plants) onsite. Otherwise, the
GHG emissions from the energy generation offset an extensive proportion of the capture CO2.
Considering the total alkalinity sources available and according to our analysis results, in the
ambitious scenario, we assumed that around 18% of the aggregates used for concrete buildings,
16% of the aggregates used for concrete pavement, and 25% of the aggregates used asphalt
production will be procured using the CCU technology in 2050. All these implementations were
assumed to evolve linearly in time. Table S3 shows the details of the assumption and availability
of alkalinity sources from the sources. Only less than half of the estimated mineralized CO2 (12.4
Mt) can attain the net-zero carbon mixture for the US mixtures used in buildings and pavements.
In other words, a fraction of produced desalination would sufficiently supply enough material to
satisfy the required alkalinity source for attaining net-zero mixtures for pavements and buildings
in the US.
The total quantity of concrete used in buildings and pavements in 2050 and under the ambitious
scenario were estimated as 102 and 12 Mm3
, respectively while the quantity of asphalt used for
US roads was estimated as 110 Mm3 in 2050. The state-level embodied emissions under different
scenarios are presented in Datasets S7-S10 for pavements and S11 and S12 for buildings. Also,
the bill of materials and GHG emissions associated with the building archetypes, for each
climate zone, frame, and user type, and code adoption, are provided in Datasets S14-S69.
