Carbon Footprint and CO₂ Emissions in the Concrete-Polymer Composites Technology

Ecological footprint is the method developed to measure human demands on natural capital, i.e. “the quantity of nature it takes to support people or an economy” [1]. Carbon footprint is considered as the more narrow component of the ecological footprint, and is defined in ISO 14067 as “the sum of the greenhouse gases emitted and absorbed by a product, expressed as CO₂ equivalents and based on a life cycle assessment”. In other words, one can understand the carbon footprint as the total greenhouse gas (GHG) emissions caused directly and indirectly by an individual, society, organization, areas, products, services, etc., while carbon dioxide equivalent (CO₂e or CO₂eq or CO₂– e) is representing global warming potential (GWP) [2]. As different long-lived greenhouse gases (methane, nitrous oxide, halocarbons, sulphur hexafluoride, etc.) contribute to global warming to a different extent, the CO₂e enables to compare the emissions of gases on a common scale (it is calculated as GWP times mass of the other gas), taking into account how efficiently a given gas retains heat in the atmosphere and how long it stays in the atmosphere before it breaks down. For example, the average methane molecule, CH₄ stays in the atmosphere for around 12 years, much shorter than in case of CO₂ (estimated between 300 to 1,000 years or longer) yet it “captures” the heat more effectively – 1 tonne of methane released into the atmosphere would cause the same warming as ca. 27–30 tonnes of CO₂ [3]. Table 1 presents the most current values of lifetime and 100-year global warming potentials of three main greenhouse gases published in the Intergovernmental Panel on Climate Change report, IPCC AR6 (2021) [3] that are improvements upon the still commonly cited values from the older IPCC reports [4‐6] due to the changing concentration of CO₂ in the atmosphere over time.

Regardless of the GWP values adopted for the calculations (more or less recent), there are also various methods of calculating carbon footprint. For instance an organization's carbon footprint includes the emissions caused by all its activities, including the energy consumption of buildings and means of transport, while product's carbon footprint includes emissions caused by the extraction of the raw materials from which it was made, production, use and storage or recycling after use [7]. The common classifications assumes 3 main types of emissions (depending on the level of control exercised by the particular organization) as following [8]:

Calculating all types of emissions can be a complex task. Moreover, currently there is lack of consistency in methods for calculation and reporting carbon footprint so it is difficult to compare the published values of footprints.

Table 2 contains exemplary data on the amount of CO₂ released during the production (direct and indirect manufacturing effects) of selected building materials and products, including cements (as cements production sector is responsible for 5–7% of the world’s CO₂ emissions [9, 10]) and concrete, considered the most common building material.

As in the case of any construction material, the calculation of the carbon footprint of C-PC composites should cover the material life cycle assessment: (1) production phase followed by (2) the operation phase and (3) the post-use phase (including demolition and eventual recycling). Production phase can be additionally divided including into various sub-phases (see Table 3), depending on the qualitative composition, namely: the presence and content of cement and polymers.

In the case of polymer-modified concretes, PMC the amount of the polymer modifier is not significant (polymers are used mainly as the admixtures, so their content should not exceed 5% of cement mass), their life cycle is considered analogical to non-modified cement based concretes. Remaining C-PC (i.e. polymer impregnated concrete, PIC, polymer-cement concrete, PCC and polymer concrete, PC) life cycles are more divers and have a greater impact on the total CO₂ emissions and carbon footprint.

Table 4 presents the general shares of main components of C-PC composites, i.e. polymer binder, cement and aggregates. The table does not include mixing water PMC, PIC and PCC, nor a polymer admixture. CO₂e of concrete admixtures (including production and transport) is generally estimated as c.a. 220 kg CO₂/t [16]. In case of polymer plasticizers/superplasticizers it is estimated as 1,880 kg CO₂/t [11] but taking into consideration its small content in the concrete mix it can still be treated as marginal impact on total concrete emissions. As for mixing water (which is about 6–7% of the concretes total mass) CO₂e of tap water is estimated as c.a. 320 kg CO₂/t [17].

Cement. As mentioned earlier, cement production is considered to account for 5% of global CO₂ emissions. In the last decade that emissions have stabilized at around 4 billion tonnes per year, but is over 2.5 times more than it was at the beginning of the 21st century [21]. Takin into account the high emissions of cement production (up to 912 kg CO₂/t for CEM I [11]), cement content remain an important factor influencing total carbon footprint of C-PC composites, of which only PC is totally cement-free.

Aggregates. In PMC, PIC and PCC the aggregate constitutes about 70–85% of the mass (60–80% by volume) [18]. In the case of PC the mass content of mineral fillers is higher – c.a. 80 ÷ 90% [19], of which the traditional aggregate (sand, gravels) is about 60–80% and the remaining 20–40% is a microfiller [19, 20]. In [17] one can find general estimation of CO₂e of aggregates depending on their size, i.e. gravels (size > 2 mm): 4.32 kg CO₂/t, sand (fraction 0/2 mm): 10 kg CO₂/t, microsand: 110 kg CO₂/t. Considering that the finer the aggregate, the higher the CO₂ emissions, the use of microfillers in PC noticeably increases the carbon footprint of their aggregate blends.

Polymers. Table 5 summarizes exemplary data on the amount of CO₂ released during the production of selected polymers commonly used in construction, including the polymer modifiers, impregnates and binders or co-binders of C-PC.

C-PC carbon footprint – examples of estimation. The above individual data were used to estimate the carbon footprint of selected C-PC composites: ordinary concrete (OC), concrete modified with polycarboxylate superplasticizer (PMC), two polymer-cement concretes with various carboxylated SBR latex co-binders in amounts of 10% (PCC-10) and 20% (PCC-20) of dry polymer in relation to the cement mass and, finally, the polymer concrete (PC) with bisphenol-A based vinyl-ester resin. With the exception of PC, concretes were to present similar strength (tested acc. to EN 12390-3) and consistence (class S3, tested acc. to EN 12350-2). Table 6 presents compositions and compressive strength, CO₂e of components and summarized “material” CO₂ emissions (without later life cycle phases impact) of the analyzed composites.

Higher compressive strength is theoretically associated with higher carbon footprint, however it is difficult to explicitly determine the actual values. According to “Circular ecology” carbon footprint calculator (based on ICE database [11]) for the analyzed OC and PMC compositions the carbon footprint is respectively 158 and 152 kg CO₂/t. Meanwhile according to the empirical model based on concrete characteristic strength (CO₂= \(\delta \sqrt{{f}_{ck,cyl}}\) where δ = 46.5 [14] after [24]) for the analyzed concretes classified as C30/37 and C40/C50 the carbon footprint is almost twice as high – respectively 255 and 294 kg CO₂/t. Taking into account the data from the Table 6 determined for specific compositions and the most closely matched components CO₂e, in the case of OC and PMC the summarized emission was – respectively 176 and 168 kg CO₂/t, thus between the values estimated using the two abovementioned models. Moreover, these models lack the possibility to include the larger amounts of polymers used as co-binders.

Table 6 shows that in the case of PCC, where the cement content was not reduced, but polymer was added additionally, the total emission increased by 15–20% (26–33 kg CO₂/t) in comparison to reference concretes (with no SBR). Interestingly, the obtained results show how important the quantitative selection of PCC composition is – both in terms of strength and carbon footprint. A higher amount of polymer co-binder does not necessary determine the higher carbon footprint. In the case of PCC-20 where almost twice as much SBR latex was used, but less cement and much less water (the polymer binder acted as a superplasticizer) in comparison to PCC-10, practically identical carbon footprint was obtained, but a much higher strength.

In case of analyzed PC the carbon footprint is much higher because of the use of large amount of vinyl-ester. However taking into consideration its very high compressive strength (110 MPa after 14 days and over 125 MPa after few years [20]), as well as tensile strength (of c.a. 20 MPa) and excellent chemical resistance (including acid resistance) [20], the use of vinyl-ester resin in such an amount (11.8% by composite mass) is justified. Especially that small-sized elements with thin cross-sections are usually made of PC and the material consumption is not as high as in the case of ordinary concrete with/without admixtures or in case of PCC.

Calculating carbon footprint for OC can include CO₂ sequestration, i.e. concrete carbonation phenomenon taking place during the operational phase of the life cycle. As during carbonation reaction (CO₂ + Ca(OH)₂ → CaCO₃) carbon dioxide is incorporated into the near-surface layers of the hardened concrete, this can be considered a reverse emission and thus included in the total CO₂ emissions and ultimately lower the concrete carbon footprint [26]. One can expect that in PMC, such a phenomenon could also occur, though in the case of PCC, despite the significant cement content, the polymer forming a continuous phase should prevent the phenomenon of carbonation. Therefore, some studies show that even in case of PMC, using properly selected cement and admixture (e.g. polycarboxylic acid [27]) enables significant increase in the concrete anti-carbonation abilities. Therefore, what is considered an advantage in the context of the durability (tight microstructure, often completely isolated pores and therefore improved tightness), in the context of CO₂ sequestration works negatively. Off course benefits of increased durability are undeniable. Especially that the full use of the CO₂ sequestration potential is not possible even in OC due to the high risk of reinforcement corrosion and the limitation of the carbonation progress over time. The case study of CO₂ sequestration in the life cycle of concrete viaduct [26] showed that the structure was able to absorb only about 2% of total CO₂ emissions. For PIC or PCC it would be less and therefore it can be considered a marginal impact.

There is yet no data on the C-PC made of recycled or bio-based polymers, however there are published promising results on production of other pre-cast elements made of recycled polymers. The report on the sanitary pipes production [28] showed for example that the energy consumption during production of 3m long PVC, PE and PP pipes containing 80% recycled polymers was reduced by 65–74% and CO₂ emissions – by 60–71% in comparison to the fossil-based polymers elements production. In the context of PCC and PC concretes, the authors consider the use of recycled PET polymers, which undoubtedly have a lower carbon footprint, but are hardly available on the market and so far cannot be considered as materials for use in large-scale C-PC pre-cast production.

Meanwhile the use of very fine powdered waste materials or by-products as PCC fine aggregate [29] or PC microfiller is a solution easy to implement, yet may noticeably reduce the aggregate CO₂e. The authors tested the long-term durability of PC with fly ashes – the high degree (up to 79%) of substitution the quartz powder with the fly ash enabled to reduced the composites carbon footprint even by c.a. 20 kg CO₂/t [20].