Enhanced CO₂ Sequestration in Recycled Aggregates: Exploring Novel Capture-Promoting Additives

Abstract

CO₂ emissions, a significant contributor to climate change, have spurred the exploration of sustainable solutions. One putative solution involves using recycled aggregates (RAs) from construction and demolition waste (CDW) to substitute natural sand in construction materials. This not only extends the life cycle of the waste but also reduces the use of natural resources. The potential to capture CO₂ in RAs presents a promising route to mitigate the environmental impact of the construction industry and contribute to its much anticipated decarbonization. This research takes a unique approach by investigating the incorporation of an amine-based additive—specifically 2-amino-2-methyl-1,3-propanediol (AMPD)—to enhance CO₂ capture into a real-case RA from recycling plants, transforming CDW with low carbon-capture potential into a highly reactive CO₂ capture material. Through TG analysis, FTIR-ATR and the combination of both (TG-FTIR), we were able to validate the use of RA materials as a support medium and quantify the CO₂ capture potential (12%) of the AMPD amine; a dual valorization was achieved: new value was added to low-quality CDW and we enhanced CO₂ sequestration, offering hope for a more sustainable future.

1. Introduction

The fight against climate change, namely through the mitigation of and reductions in carbon dioxide (CO₂) emissions, plays a leading role in the research of today. It is known that the construction sector, responsible for a significant part of CO₂ emissions at a global level, also generates large amounts of waste. This underscores the urgent need for the sector to reduce its environmental impact, both in terms of its decarbonization and through a reduction in CO₂ emissions and generated waste (construction and demolition waste—CDW) [1].

CDW is the result of the construction, rehabilitation, alteration, conservation and demolition of buildings, representing about one-third of all waste produced in the European Union [2] by volume. The processes involved in the recovery and treatment of CDW are generally similar across recycling facilities, with some variation which can significantly impact the final properties of the resulting RAs. Upon arrival at the recycling plant, CDW is visually inspected and classified as either clean or contaminated (waste containing impurities such as plastics, wood and cardboard). These materials undergo mechanical sorting to separate out non-mineral components. Following this screening, various waste fractions are set aside for recovery, while certain portions that are unsuitable for further processing are stored separately in designated areas. Some recycling plants are equipped with specialized machinery, such as hydraulic clamps, to reduce the size of the incoming waste [1,3].

Since CDW is available in large quantities with low acquisition costs, its incorporation in construction products (mortars and concrete) in the form of recycled aggregates (RAs) positively impacts end-of-life management and reduces the exploitation of natural aggregates [4,5,6]. However, RAs have different characteristics compared to natural aggregates (NAs) [1]. For example, RAs typically exhibit higher porosities, leading to increased water absorption and lower particle densities. This can negatively impact the performance of cement-based materials that incorporate RAs. To address this issue, several studies have proposed various treatment methods to enhance the characteristics of RAs and improve the performance of new materials [7,8,9]. One effective method is densification through forced carbonation, which reduces porosity, decreases water absorption and increases particle density, ultimately enhancing material performance [10,11]. However, RAs from CDW already undergo some degree of natural carbonation, which may limit their potential for CO₂ capture.

On the other hand, concrete, the most used building material in the world, is responsible for over 8% of total CO₂ emissions [12], essentially attributed to cement production. Indeed, about 650–800 kg of CO₂ per tonne of cement is emitted during its manufacturing process, over 50% being related to the decarbonation of limestone at high temperatures [13,14]. Cement-based materials can reabsorb part of this CO₂ through carbonation [15,16], a chemical reaction with CO₂, forming calcium carbonate. However, on average, the carbonation uptake during their lifetime is less than 18% of concrete-related CO₂ emissions and still far from the maximum carbonation potential [17]. Hence, improving the carbonation potential of these materials is possible.

Several technologies have been explored to force the carbon capture of cement-based materials, CCUS (Carbon Capture, Utilization and Storage), and reduce their carbon footprint [18]. Of this, forced and accelerated carbonation, as a pre-treatment of RAs, is a promising technology that causes positive effects on the characteristics of RAs compared with those of non-carbonated ones [1]. Their properties are improved and, consequently, so are those of the construction materials in which they are incorporated [7,18,19,20,21]. However, CO₂ absorption by RAs is limited, and some techniques can enhance this uptake.

Amine scrubbing is an up-and-coming CCUS technology, mainly using an aqueous solution of a primary amine [22]. Amines are organic compounds derived from ammonia, characterized by a functional group containing a nitrogen atom. Amines can be classified into three types according to the number of hydrogen atoms bonded to the nitrogen atom (N-H bonds). Primary amines have their nitrogen atom covalently attached to two hydrogen atoms. On the other hand, secondary amines have one hydrogen atom covalently bonded, and tertiary amines have only non-hydrogen atoms bonded to the nitrogen atom. In terms of reactivity, primary amines are typically the most reactive, followed by secondary and, finally, tertiary amines. However, this rule has some exceptions [23].

As such, primary amines appear to be the most attractive in the amine scrubbing process due to their higher CO₂ absorption capacity. However, other factors that need to be considered during solvent selection, such as the energy costs associated with the regeneration of the solvent solution, favor tertiary amines [8].

Primary and secondary amines react with CO₂, forming a carbamate or bicarbonate reaction product (depending on the stability constant of the carbamate formation). Tertiary amines cannot form a carbonate because their amino group is saturated and unable to make an additional N-C bond. Equation 1 presents the reversible reaction of a generic amine with CO₂ originating from the zwitterion compound.

$${CO}_2+RN{H}_2\leftrightarrow RNHCO{O}^{-}+RN{H}_3^{+}$$

(1)

Researchers have explored an alternative amine source for amine scrubbing processes [24]. However, although some of these amines presented better CO₂ absorption rates or lower regeneration energy costs, none have yet shown better cost/benefits than monoethanolamine (MEA).

Some researchers have attempted to use amine mixtures to improve the cost/benefits of using a single amine source for the aqueous solution [25,26]. These mixtures can comprise primary amines with primary, secondary, or tertiary amines. However, from the results found in the literature, data for critical parameters to consider when adopting these novel sources, such as the regeneration energy demand, the environmental impact and the corrosion effects of the amine mixtures, are still lacking. This lack of information hinders using these amines on an industrial scale [8].

Although using aqueous amine solutions is the most promising CCUS technique [27], the need to find a better alternative is growing [28]. Most aqueous amine solutions threaten to increase amine emissions to the atmosphere, with the amine being released through vapor and aerosol emissions during solvent regeneration [29].

This study aims to assess the CO₂ capture potential of a recycled aggregate impregnated with a commercial primary amine to maximize recycling construction materials CO₂ capture and storage (CCS). The capture efficiency of the RA was analyzed using TG analysis, FTIR-ATR and a combination of both methods (TGA-FTIR). Unlike most studies, which focus on carbonation using laboratory-made waste and on traditional methods of the potential natural carbonation of RAs, this article aims to develop and validate a novel method for improving the CO₂ capture capacity of RA directly obtained from a treatment and recycling plant by functionalizing it with a commercial primary amine.

This strategy presents a promising method for converting non-homogeneous CDW into carbon-negative construction material while promoting circular economy practices. By capitalizing on the porous structure of RAs, the study enhances sequestration through amine functionalization, thereby increasing CO₂ affinity. This approach not only demonstrates a viable pathway for CDW valorization but also aligns with circular economy principles and supports future research into replacing commercial amines with industrial waste amines from various sources.

2. Materials and Methods

2.1. Materials

The selection of the recycled aggregate for this study was based on the results of the carbonation of 7 different recycle aggregates (RAs) used in the study of Bastos et al. [30]. The selection of the mixed recycled aggregate designated MRA-RP for this work was because it is a representative recycled aggregate with a low CO₂ capture capacity, high porosity and a dominant presence in waste streams, making it an ideal candidate for amine treatment. This waste, previously characterized by Infante Gomes et al., referred to as CDW-A [31], as W1 [32] and by Bastos et al. as MRA-RP1 [30], corresponds to a mixture of particles with dimensions below 2 mm with a particle density of 2424 kg/m³, Figure 1. The selection of this RA was based on several key criteria. Firstly, it exhibited the lowest carbonation potential among all the studied recycled aggregates (RAs), presenting an opportunity to significantly enhance its CO₂ capture capacity [30]. Secondly, as demonstrated in previous studies [30,31], this RA showed higher water absorption (7.1%) compared to several other RAs. High water absorption can be advantageous for the effective impregnation of amine solutions. Lastly, MRA-RP was considered the least valuable material overall among the available options;

Table 1. MRA-RP classification (adapted from [30]).

Waste	Rc (%)	Ru (%)	Rb (%)	Ra (%)	Rg (%)	X (%)	Fl (%)	Classification According to EN 933-11 [33] and EN 13242 [34]* (as per Results Obtained in Laboratory)
MRA-RP	42.5	27.5	21.7	9.6	0.2	0.8	Fl5-	Rc declared Rcug70 Rb30- Ra10- Rg2- X1- Fl5-

* Classification of CDW-A constituents: concrete, concrete products, mortar and concrete masonry units (Rc); unbound aggregates, natural stone and hydraulically bound aggregates (Ru); clay masonry units (Rb); bituminous materials (Ra); glass (Rg); other materials, including cohesive elements, metals, plastic, rubber, non-floating wood and gypsum plaster (X).

presents the composition of MRA-RP. These combined characteristics made MRA-RP an ideal candidate for investigating potential improvements in CO₂ sequestration through the incorporation of capture-promoting compounds.

A commercial primary amine (AMPD), 2-amino-2-methyl-1,3-propanediol, C₄H₁₁NO₂, with ≥99% purity from Sigma-Aldrich was selected for impregnation into the selected recycled aggregate (RA) for CO₂ sequestration due to its favorable properties. The AMPD amine exhibits a high water solubility, which facilitated its effective impregnation into the porous structure of the RA. The amine was used to prepare an aqueous solution with a 20% AMPD concentration that was applied to a mixed recycled aggregate designated MRA-RP.

After the RA selection and collection, the recycled aggregate was dried at 60 °C, ground and sieved up to 2 mm. Due to MRA-RP’s heterogeneity, the samples were split into homogeneous fractions using a RETSCH DR 1000 sample divider. After the homogenization, a pre-characterization of MRA-RP was carried out through FTIR-ATR, TGA and TGA-FTIR. The MRA-RP was then submerged in an aqueous solution of the AMPD amine (20% concentration) for three days.

2.2. Carbonation Conditions

Once the amine impregnation was completed, the samples were dried at 40 °C until a constant mass was achieved and then placed in an airtight ARALAB Fitoclima 300 EP climatic chamber, where they were exposed to the following conditions for twelve hours: 23 °C, 60% RH and 25% CO₂ concentration. The adopted CO₂ concentration was within the ideal range reported in the literature to maximize carbonation (20–30%) and was also near the usual CO₂ concentration in the flue gas of cement plants [30]. The CO₂ used presented a 99.7% purity. Afterwards, the samples were again characterized through FTIR-ATR, TGA and TGA-FTIR to analyze the amount of carbamate formed and CO₂ uptake by the amine-treated MRA.

2.3. Characterization Methods

2.3.1. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed for the RA plus amine samples, amine samples and the RA using a SETARAM TGA92 simultaneous DTA-TGA thermal analyzer and a HITACHI STA7200 apparatus, with an argon atmosphere (3 L/h) and at a heating rate of 10 °C/min from 25 to 900 °C.

2.3.2. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (FTIR-ATR)

The Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed in a PerkinElmer FTIR Spectrometer Model Spectrum 2, coupled to an Attenuated Total Reflectance (ATR) unit from PerkinElmer with an individual diamond crystal.

2.3.3. TG-FTIR

The TG-FTIR analysis was performed using an interface comprising a REDShift Evolved gas analyzer with a flux of 150 mL/min, as in Figure 2. The temperatures of the interface, transfer line and cell line were 280 °C, 280 °C and 250 °C, respectively. The interface line was coupled to a PerkinElmer FTIR Spectrometer Model Spectrum 2 and a HITACHI STA7200. The latter used a 200 mL/min nitrogen atmosphere at a 10 °C/min heating rate from 25 to 900 °C. The analysis was carried out using alumina crucibles.

3. Results and Discussion

3.1. FTIR-ATR Characterization

A qualitative analysis was performed at different steps of the experiment using FTIR-ATR characterization. Figure 3 presents the spectra of the AMPD amine, MRA-RP (non-carbonated), MRA-RP with AMPD (non-carbonated) and MRA-RP with AMPD (carbonated). To assess if the amine was successfully impregnated, FTIR-ATR analysis was performed on the MRA-RP with and without AMPD impregnation. By analyzing both spectrums, the absorption bands of N-H and C-N detected in the samples impregnated with AMPD could mainly be attributed to the presence of the amine in the RA, the remainder being due to the existence of moisture [35,36,37,38,39]:

• N-H stretching vibrations (3400–3200 cm⁻¹)—this band represent the “free” stretching modes of the N-H bond.
• N-H bending (scissoring) vibration (1650–1580 cm⁻¹).
• N-H wagging 909–666 cm⁻¹, usually assigned to N-H wagging.
• C-N stretching vibrations band (1100–1000 cm⁻¹)—C-N stretching coupled with the stretching of adjacent bonds in the molecule.

After confirming that the impregnation step was successful, the MRA containing the amine was exposed to CO₂ gas. The exposure conditions were selected based on previous studies using this RA that had determined a specific set of conditions for maximum CO₂ capture using the aggregate [30]. Observing Figure 3, the spectra of the RA with the amine after being carbonated, it is possible to identify the following peaks [36,40,41,42,43]:

• 3387 cm⁻¹—carbamate formation, a product of the reaction of the CO₂ with the impregnated amine.
• 2128 cm⁻¹—assigned to the N-H combinations of secondary ammonium ion (NH₂⁺) bands.
• 1400–1300 cm⁻¹—the absorption of the carbamate.
• 1574 cm⁻¹—the asymmetric stretching mode of CO₂ and symmetric ${\mathrm{N}\mathrm{H}}_3^{+}$ deformation.

The FTIR-ATR analysis clearly showed that AMPD was successfully impregnated into the MRA-RP, as indicated by the distinct N-H and C-N absorption bands. When exposed to CO₂ gas under optimal conditions, the formation of carbamate and other related bonds confirmed that the impregnated amine could efficiently capture CO₂. However, to quantify the amount of CO₂ captured, a mass change analysis was still needed.

3.2. TGA

After the validation of a successful impregnation and CO₂ sequestration step, a quantity analysis was perform using TGA after the impregnation step both before and after CO₂ exposure. The results of TGA of the RA with the amine when non-carbonated and carbonated are presented in Figure 2. From the TG/DTG curves of the non-carbonated MRA with AMPD, a more significant weight loss between 100 and 200 °C can be found. This weight loss is likely to be essentially due to the vaporization of AMPD once its boiling point is reached at 150–152 °C, according to Sigma-Aldrich product specification. This significant weight loss is observed in all the samples studied, confirming that the AMPD was successfully impregnated in the MRA. Therefore, the use of AMPD as a supporting medium in RAs is validated. Weight loss after 600 °C is also visible due to the presence of carbonate compounds in the recycled aggregate [30].

The TG/DTG results after carbonation show two distinct weight losses visible in the temperature region of amine vaporization. The first peak in this region suggests the presence of carbamate. The weight loss between 200 and 300 °C is attributed to amine vaporization, and after 600 °C it is due to CO₂ release from the decarbonation of calcium carbonate, which is higher than the in non-carbonated sample. Due to the proximity of the regions of dissociation of carbamate and amine vaporization (1 and 2 of Figure 4), it was necessary to accurately identify each respective region in the DTG spectra.

The TGA showed that the AMPD amine was successfully impregnated into the MRA-RP and effectively captured CO₂. The distinct weight loss patterns in the TG/DTG curves, especially between 100–200 °C and after 600 °C, confirmed the presence of AMPD and carbonate compounds in the recycled aggregates. After carbonation, the analysis highlighted the formation of carbamate and the release of CO₂ from calcium carbonate decarbonation. These results demonstrate the potential of AMPD-impregnated MRA-RP as a promising material for enhancing CO₂ capture, contributing to advancements in sustainable environmental technologies.

3.3. TG-FTIR Analysis

To accurately correlate the events related to the thermal degradation of the amine-impregnated RAs before and after CO₂ exposure with the corresponding bands in the TGA, a specific analysis was performed using the TGA apparatus coupled with FTIR analysis, as shown in Figure 2. This method allows for the acquisition of multiple FTIR spectra per minute of the evolved gases from the TGA. Using this hyphenated system makes it possible to accurately determine the composition of the gases being released during the TG analysis. This information is crucial to evaluate the amount of CO₂ captured, carbamate dissociation and amine vaporization. Figure 5 presents the spectrum of pure AMPD, necessary to determine the range of weight loss corresponding to the amine used in this study. Amine vaporization occurred between 90 and 220 °C.

Before the analyzing the result of the impregnation process, it was necessary to analyze MRA-RP’s thermal behavior. The TG-FTIR results can be found in Figure 6. The results only display the release of CO₂ due to the decomposition of the calcium carbonate present in the sample. The observed bands correspond to CO₂ (2500–2300 cm⁻¹, 750 cm⁻¹) [44,45] released in the weight loss occurring at temperatures above 500 °C [30,46].

After analyzing AMPD and MRA-RP individually, TG-FTIR analysis was performed on the result of the impregnation process, MRA-RP with the AMPD amine. The results presented in Figure 7 support the thesis that the impregnation of AMPD into MRA-RP was successful, in accordance with previous analysis. The resulting spectrum is mainly the merging of the individual spectra of the amine and the RA, with it being possible to identify the two main regions previously mentioned: amine vaporization and CO₂ release due to calcite decarbonation. It is also clear that a small signal appears at 600–700 s, which, according to the FTIR spectra, corresponds to CO₂. This may be due to the high reactivity of the amine, which adsorbed CO₂ during the impregnation process.

With the mixed RA successfully functionalized with the amine, it was subjected to a CO₂-rich environment. Figure 8 and Figure 9 present the TGA-FTIR results for the MRA impregnated with AMPD after carbonation. The regions previously identified associated with amine vaporization (a) and calcite decarbonation (c) are still visible. However, the appearance of a new region is also visible, a large band between 600 and 700, in the temperature range of 90–120 °C, with a wavelength around 2400 cm⁻¹, which corresponds to the CO₂ molecule’s wavelength [44]. However, due to the temperature at which the event occurs, the idea that the released CO₂ is from the decarbonation of calcium carbonate can be excluded [30]. It can also be hypothesized that such CO₂ can only result from the thermal decomposition of the previously formed carbamate during the carbonation process. In fact, it is well documented that the reaction of carbon dioxide with an amine is reversible [47,48,49]. Correlating these results with the results obtained from the FTIR-ATR analysis, both provide evidence of chemical bonds associated with the carbamate compound. Also, the TGA-FTIR analysis of the carbonated sample clearly shows that the release of CO₂ from carbamate decomposition occurs before amine vaporization. Therefore, it is possible to estimate the temperature range of the CO₂ captured by the amine. That range is 74–125 °C, which corresponds to the CO₂ resulting from the decomposition of carbamate, formed upon the amine reaction with CO₂ during the carbonation process. On the other hand, between 125 and 243 °C, the amine vaporizes, while above 500 °C, the CO₂ release is due to the decarbonation of calcium carbonate (Figure 9).

Having assigned the ranges of temperature related to CO₂ release from carbamate decomposition and calcium carbonate decarbonation, one can estimate the efficiency of the process. Based on the values of the mass of each component of the MRA impregnated with AMPD and carbonated, it was estimated that the reaction yield was around 66% with a capture of 12%, meaning 0.23 kg CO₂/kg amine was achieved from an initial potential capture of 0.41 kg CO₂/kg amine, a high value given the aggregate composition and degree of carbonation.

Figure 10 illustrates the CO₂ gas release over time during TG-FTIR analysis, comparing the carbonated and non-carbonated AMPD-impregnated RA samples. The peaks at 2360 cm⁻¹, assigned to carbon dioxide [50], were observed in the evolved gases during TGA and FTIR both pre- and post-carbonation. In the non-carbonated sample, two distinct peaks appear. The low-intensity peak is likely from the amine’s reaction with atmospheric CO₂ during preparation, and the broader peak is from CO₂ release due to calcium carbonate decarbonation in the RA. Post-carbonation, both peaks increase significantly. The reaction between CO₂ and the impregnated amine under high CO₂ concentrations (25%) results in a sharp, intense first CO₂ peak. Additionally, the increase in the intensity of the second peak suggests that CO₂ also reacts with cementitious materials in the RA, such as C-S-H, AFt and portlandite, forming additional calcium carbonate.

The TG-FTIR analysis successfully linked the thermal degradation events of the amine-impregnated RA before and after CO₂ exposure with the corresponding bands in the TGA. This method allows us to precisely determine the composition of the gases released during the TG analysis, which is crucial for evaluating CO₂ capture, carbamate dissociation and amine vaporization. The results confirmed that AMPD was successfully impregnated into MRA-RP, as shown by the distinct weight loss patterns and the presence of CO₂ release due to calcite decarbonation. After carbonation, the analysis revealed additional CO₂ release from carbamate decomposition, highlighting the reversible nature of the amine–CO₂ reaction. The process was found to be quite efficient, with a reaction yield of 66% and a CO₂ capture rate of 12%, demonstrating the high potential of AMPD-impregnated MRA-RP for CO₂ sequestration. These findings underscore the effectiveness of the TG-FTIR method in analyzing the thermal behavior and gas composition of amine-impregnated RAs, contributing to advancements in sustainable CO₂ capture technologies.

4. Conclusions

This study investigated the interaction between carbon dioxide (CO₂) and a primary amine, 2-amino-2-methyl-1,3-propanediol (AMPD), when supported within a mixed recycled aggregate (MRA-RP) matrix derived from construction and demolition waste (CDW). The main objective of this research was to assess the potential of AMPD-functionalized MRA-RP for capturing CO₂. Using Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance (FTIR-ATR), it was possible to identify the functionalization of the AMPD amine within the MRA-RP matrix. The characteristic peaks corresponding to amine functional groups confirmed the presence of AMPD on the surface of the recycled aggregates.

Thermogravimetric analysis (TGA) revealed a significant weight loss in the amine-impregnated RA sample after exposure to a high concentration of CO₂, indicating that a chemical reaction had occurred. TGA-FTIR analysis of this weight loss indicated the dissociation of carbamate, a compound formed from the reaction between CO₂ and the amine groups. The distinct weight loss observed after carbonation strongly revealed successful sequestration, demonstrating the reactivity of the amine groups in the MRA-RP matrix with CO₂. Quantitatively, the results showed that impregnating AMPD into MRA-RP enhanced the CO₂ capture capacity by 12% compared to the non-impregnated RA. This increase is significant, particularly considering that MRAs typically have low CO₂ capture potential due to their heterogeneous composition. The capacity of this new material to capture additional CO₂ is promising and indicates potential future applications of RAs in sustainable construction and carbon capture technologies.

In short, from this study, the following conclusions can be drawn:

• It is possible to increase the value of an aggregate with a low potential for CO₂ sequestration by using an amine as an additive.
• This study studies highlight the use of several techniques to quantify the capture of CO₂ and amine impregnation.
• The use of TG-FTIR was crucial in evaluating and confirming the mass changes corresponding to CO₂ uptake/release by AMPD.

Impregnating AMPD into MRA-RP increased the CO₂ capture capacity by 12% compared to the non-impregnated RA. This research highlights the development and validation of a novel approach to enhance the CO₂ capture capacity of RAs by functionalizing them with an amine-based additive. By functionalizing RAs with amines, the ability to chemically bind CO₂ through carbamate formation increases significantly, unlike in traditional methods that rely solely on the natural carbonation potential of RAs. Using RAs from CDW to capture CO₂ provides significant environmental and economic benefits. Environmentally, it reduces the demand for natural resources and diverts waste from landfills. Economically, the use of RAs can reduce construction costs, support compliance with regulatory policies and promote the circular economy. Hence, increasing RA reactivity towards CO₂ can simultaneously address waste management and greenhouse gas mitigation issues. Future research should focus on optimizing the functionalization process of waste with amines and studying exposure to non-pure CO₂ streams, such as flue gas from clinker production. Also, future works should study the validation of other types of amines such as amines from industrial wastes.