The sponge effect and carbon emission mitigation potentials of the global cement cycle

Global cement cycle in 2014

Figure 1 illustrates the 2014 global cement cycle and the associated net CO₂ emissions balance (see “Methods”). Driven by the expansion and turnover of in-use stocks, 4.2 Gt of cement and 0.2 Gt of cement kiln dust (CKD) were produced in 2014. Cement stocks in 2014 amounts to ~75 Gt in total, nearly equally split between residential, non-residential, and civil engineering sectors with ~25 Gt each. The longevity of cement stocks means that only 0.5 Gt of demolition waste was generated in 2014. The challenges faced in recycling cement-based products lead to nearly all (99.1%) demolition waste being buried in landfills, or as part of backfills and aggregates in road base (see Supplementary Table 1). We calculate that the global cement cycle gave rise to 3.0 Gt of CO₂ emissions and 0.6 Gt of CO₂ uptake in 2014, offering a net balance of 2.4 Gt CO₂ emissions. Of the total CO₂ emissions released from cement production and upstream processes in 2014, 58.4% were released from carbonate calcination, 32.9% from fuel combustion, and 8.6% from indirect emissions for electricity generation. Our result indicates that most of the CO₂ uptake (~80%) in 2014 occurred in buildings and infrastructures (in-use stocks), with CKD, construction waste, and demolition waste, together, contributing only ~20% to the total CO₂ uptake.

Decarbonization storylines and scenario narratives

To understand how the cycle depicted in Fig. 1 could develop in the future, we used a top-down stock-flow approach driven by data on cement production, trade, sectoral use, and lifetime¹, to estimate the historical and contemporary cement stocks. We observed that the per capita cement stocks in all ten regions have increased since 1930 (see Supplementary Figs. 1–10). Global average cement stocks per capita reached 10.2 tonnes per capita in 2014, with industrialized and transitioning regions ranging from 12.7 to 23.7 tonnes per capita, developing regions ranging from 2.7 to 7.5 tonnes per capita, and several mature economies approaching 35 tonnes per capita. However, regional cement stocks are not equally distributed across sectors. Post-industrial regions (especially the Commonwealth of Independent States; CIS) typically have higher levels of per capita cement stocks in the civil engineering sector. In contrast, China has a lower level of per capita cement stocks in the civil engineering sector, but a considerably higher level in buildings. We speculated that these variations could be explained by multiple factors, such as the development stage, patterns of urban expansion, architectural specification, as well as availability and choice of construction materials¹. Earlier studies have shown a saturation phenomenon for per capita in-use stock development of bulk materials, such as iron^25,26 and copper²⁷ in industrialized countries, but not for aluminum, due to its relatively short history of use²⁸. Likewise, the development patterns of per capita cement stocks generally comply with an S-shaped curve, and saturation is evident in several highly developed countries¹. The saturation of per capita cement stocks implies that the growth rate of buildings and infrastructures in use (where cement stocks reside) will decrease marginally and eventually reach a plateau, as services provided by cement stocks become saturated^{17,29,30,31,32}. Furthermore, as evidenced in several highly developed economies¹, decreasing trends of per capita cement stocks have become manifest, reflecting that material efficiency strategies have come to play a significant role in these economies. We therefore envisage three scenario storylines with varying levels of cement stocks similar to the Resource Efficiency-Climate Change Nexus (RECC) scenario modeling framework³³, which is built upon the Shared Socioeconomic Pathway (SSP) scenarios and the Low Energy Demand (LED) scenario³⁴; the first scenario storyline (S1–3) is characterized by a low cement stock level, the second scenario storyline (S4–6) by a medium cement stock level, and the third scenario storyline (S7–9) by a high cement stock level. The saturation level of per capita cement stocks is regarded as a tangible indicator for various human needs in mature societies, including shelter, transport networks, factories, offices, as well as commercial, educational, healthcare, and governmental facilities. It is the level of service provided by per capita cement stocks that are expected to saturate, not just the quantity of material involved; the two are linked by the material intensity of the in-use product stocks. Concurrent with the development of cement stocks, demand for cement will slow down, decline, and ultimately stabilize, given that the dynamics of cement stocks, to a large degree, determine the demolition rate and reconstruction rate for cement-related materials, according to the mass-balance principle^21,35.

In light of the observed historical patterns of cement stocks and the essential role of in-use stock dynamics to the cement cycle, we simulate the future cement cycle in ten regions using a stock-driven approach¹⁷ based on the historical patterns of per capita cement stocks identified in our previous work¹, three storyline-consistent target values of per capita cement stocks (i.e., saturation levels), and a moderately growing population obtained from the medium scenario of United Nations World Population Prospects³⁶. We deem the level of in-use cement stocks as an explicit physical representation of service provision to society, thereby constructing nine stock-driven scenarios (created from three saturation levels and three saturation times) to explore the evolution of cement-related materials until 2100 due to the longevity of buildings and infrastructures. Our scenarios build upon three key assumptions: first, per capita cement stocks in the ten regions follow a development path that is consistent with S-shaped curves or inverted S-shaped curves toward a global convergence of per capita cement stocks, and therefore, regions or end-use sectors that have a per capita cement stock below the saturation level will see a continuing growth, while those with a per capita cement stock over the saturation level will see a decline (see Supplementary Fig. 11); second, the formulated pathways of per capita cement stocks do not entail abrupt changes in resulting cement demand, and therefore, the development pathways of per capita cement stocks in a few regions or end-use sectors are adjusted to smoothen the trends in cement demand; third, technological development for optimizing cement use in buildings and infrastructure proceeds, but without fundamental breakthroughs (e.g., new materials that replace cement to a full extent), because cement is a ubiquitous, relatively cheap building material of good workability.

In all of the nine scenarios, we parameterize two boundary conditions, saturation level and saturation time, to reflect the varying patterns of cement stocks and varying levels of future demand-side material efficiency in different regions. By considering a range of saturation levels, we cover both a range of service levels provided by the in-use cement stocks and a range of material efficiencies in their delivery. The saturation time reflects the speed of stock growth (parameterized by the time when the per capita cement stocks reach 98% of the saturation level). Given the regional heterogeneity of socioeconomic and geographic circumstances, we set varying saturation levels and times for different regions to fit the historical development of per capita cement stocks (see Supplementary Table 2). A modified Gompertz model is used for simulating the growth curves of per capita stocks based on assumed saturation levels and times (see Supplementary Note 2.2).

Under the nine stock-driven scenarios, we further characterize the sponge effect and its resulting net CO₂ emissions balance for the cement cycle and explore future decarbonization pathways. This includes both demand-side mitigation options to increase material efficiency, reflected in the chosen saturation levels for in-use stocks, and supply-side mitigation options, represented by changes in the CO₂ emissions intensity of cement production. We extract five supply-side CO₂ emissions mitigation measures from the global cement technology roadmap^4,9 (see Table 1): thermal efficiency (E-M1), electric efficiency (E-M2), alternative fuel (E-M3), clinker substitution (E-M4), and carbon capture and storage (E-M5). Each measure represents an effort beyond what would occur under a no-action scenario; therefore, the remaining CO₂ balance is quantified by subtracting the CO₂ emissions reduction potentials of the five measures (when they are rolled out simultaneously) from the no-action scenario. The CO₂ uptake is explicitly simulated in a physicochemical carbonation model⁵ by applying Fick’s diffusion law (see “Methods”).

Decarbonization pathways of global cement cycle

The gradual rise and then saturation of in-use stocks lead to cyclical variations in global cement demand over the next decades (see Supplementary Figs. 12–22), while the global demolition waste generation continues to rise due to the delay between demand and demolition caused by the longevity of in-use cement stocks (see Supplementary Figs. 23–33). Our estimates of cement demand in the year 2050 (4.3–6.7 Gt yr⁻¹) are more wide-ranging than those estimated by the International Energy Agency technology roadmap for the global cement industry (4.7–5.1 Gt yr⁻¹)^2,9.

Figure 2a shows CO₂ emissions under the no-action scenario and the effects of the mitigation measures. In 2050, the no-action CO₂ emissions under low-, medium-, and high-saturation levels reach 3.0–3.4 Gt yr⁻¹, 3.4–4.0 Gt yr⁻¹, and 3.8–4.7 Gt yr⁻¹, respectively. In parallel, the CO₂ uptake (effects of U-M4 subtracted, the same hereinafter) rises to 0.9–1.0 Gt yr⁻¹ (low-saturation levels), 1.0–1.1 Gt yr⁻¹ (medium-saturation levels), and 1.1–1.3 Gt yr⁻¹ (high-saturation levels) by 2050. The no-action CO₂ emissions balance (when CO₂ uptake is considered) in 2050 increases to 2.1–2.3 Gt yr⁻¹ (low-saturation levels), 2.4–2.9 Gt yr⁻¹ (medium-saturation levels), and 2.7–3.4 Gt yr⁻¹ (high-saturation levels), respectively. By 2100, the balance is at slightly lower levels, ranging from 1.5 Gt yr⁻¹ to 3.1 Gt yr⁻¹.

By implementing a full portfolio of mitigation measures, CO₂ uptake begins to overtake the remaining CO₂ emissions from cement production by the late 2090s, bending the net CO₂ emissions balance below zero (Fig. 2a). However, in the medium term, the 2050 net CO₂ emissions balance of the global cement cycle will reach 1.0–1.2 Gt yr⁻¹ (low-saturation levels), 1.2–1.5 Gt yr⁻¹ (medium-saturation levels), and 1.4–1.8 Gt yr⁻¹ (high-saturation levels), respectively. Of the nine stock-driven scenarios, none generates a trajectory of net CO₂ emissions balance that follows, or is below, the 1.5 °C-consistent pathway, meaning excessive CO₂ is emitted along all trajectories. If the cement industry is to contribute to the 1.5 °C limit in proportion with other industrial sectors, achieving the CO₂ emissions reduction target by employing mitigation measures in the production stage alone is extremely challenging, because net CO₂ emissions balance largely hinges on in-use stock dynamics, and concomitant demand and demolition.

Long-term accounting for CO₂ uptake along the cement cycle, which could be regarded as passive CO₂ sequestration, greatly changes the net CO₂ emissions balance of the global cement cycle. Across the stock-driven scenarios, the cumulative CO₂ uptake from 2015 to 2100 amounts to 81.1–117.2 Gt (Fig. 2b). These values correspond to roughly 30% of the no-action CO₂ emissions arising from the global cement cycle over the same period. All decarbonization pathways are characterized by widespread deployment of CCS technologies (E-M5) in the production stage. From 2015 to 2100, cumulative CO₂ emissions mitigated by CCS technologies, which could be regarded as active CO₂ sequestration, are 56.7–94.2 Gt, accounting for ~25% of no-action CO₂ emissions from cement production (Fig. 2b). We therefore conclude that deep decarbonization of the global cement cycle calls for both passive CO₂ sequestration and active CO₂ sequestration, but also that these measures are likely not enough to reach the 1.5 °C climate goal—more innovative or drastic approaches are needed.

Regional disparities of decarbonization potential

Figure 3 shows that the regional patterns of the sponge effect shift along with the stock dynamics and population trends, resulting in varying cumulative no-action CO₂ emissions and mitigation strategies. The population boom and gradual rise of in-use stocks are major factors that drive CO₂ emissions in emerging regions, as massive improvements in the provision of shelters and infrastructures in these regions take place. For example, Africa’s no-action cumulative CO₂ emissions from 2015 to 2100 are 53.9–108.5 Gt. Although China’s per capita cement stocks had already peaked in 2014, its cumulative no-action CO₂ emissions during 2015–2100 will still reach 61.7–75.6 Gt, due to the shorter lifetimes of in-use cement stocks in China. Meanwhile, the cumulative no-action CO₂ emissions that will occur in industrialized regions (NA, EU, CIS, and DAO regions altogether) from 2015 to 2100 are lower, ranging from 22.0 to 47.9 Gt. Compared with other regions, the active CO₂ sequestration (E-M5) plays a more dominant role in emerging regions (e.g., ~30% in both Africa and India). This indicates that CCS implementation should take place in the emerging regions where new demand for cement and production facilities increases rapidly. However, CCS is still at the demonstration stage, and their large-scale market deployment is hindered by high estimated costs³⁷, which is a significant issue for investment constrained emerging economies, suggesting that effective policies, intensified research to reduce CCS costs, and/or international financial support for CCS in cement production are urgently needed. Active CO₂ sequestration by CCS can be further utilized (carbon capture and utilization; CCU) as a feedstock to produce chemicals and fuels; however, the development of CCU technologies is still in its infancy and limited to the laboratory scale³⁷.

Source: Ecology - nature.com