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Article

Towards Industrial Implementation: Experimental Campaign Based on Variations in Temperature, Humidity, and CO2 Concentration in Forced Carbonation Reactions of Recycled Aggregates

by
David Bastos
1,
Catarina Brazão Farinha
2,*,
Cinthia Maia Pederneiras
2,3,*,
Rosário Veiga
2,
José Alexandre Bogas
3,
Ricardo Infante Gomes
2,3 and
António Santos Silva
2
1
c5Lab—Sustainable Construction Materials Association, Edifício Central Park, Rua Central Park 6, 2795-242 Linda-a-Velha, Portugal
2
LNEC—National Laboratory for Civil Engineering, Av. do Brasil 101, 1700-066 Lisbon, Portugal
3
CERIS—Civil Engineering Research and Innovation for Sustainability, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(9), 5005; https://doi.org/10.3390/app15095005
Submission received: 28 February 2025 / Revised: 21 April 2025 / Accepted: 28 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Development and Application of Innovative Construction Materials)
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Abstract

:
This research presents a sensitivity analysis of various parameters that affect the carbonation of recycled aggregates (RAs), namely CO2 concentration, temperature, and relative humidity. The range of parameter values is close to that found in cement plant chimneys with regard to the forced carbonation of RAs. With this purpose, the main characteristics of flue gas streams (CO2 concentration, temperature, and relative humidity) from two Portuguese cement plants were identified and used in this research. The results indicated that temperatures around 60 °C and CO2 concentrations around 25% accelerate the carbonation reaction and increase CO2 absorption in mixed recycled aggregates (MRAs). CO2 absorption consistently decreased as the relative humidity was reduced from 60% to 40%. The highest amount of CO2 captured was by a recycled concrete aggregate (RCA) in the conditions of 23 °C, 60% RH, and 25% CO2. Overall, the RAs were able to capture a significant amount of CO2, ranging from 52 to 348 kg of CO2 per tonne of cement paste, depending on the nature of the RA. These findings drawn from a parametric campaign provide valuable insights into the potential enforcement of carbonation for recycled aggregates under conditions that closely reflect those found in cement plants.

1. Introduction

The cement sector stands as a major contributor to CO2 emissions, largely due to the decarbonation of limestone in clinker production. According to the International Energy Agency (IEA) [1,2], the cement industry is the second largest CO2 emitter. To mitigate CO2 emissions, different strategies can be considered, such as the use of alternative fuels (like biomass derivates of wastes), the incorporation of different materials [2], and/or different manufacturing processes, as well as approaches to capture the released CO2 [3,4].
One of the methods of capturing CO2 is through the carbonation of lime-based and cement-based materials. Carbonation is a chemical process in which carbon dioxide (CO2) reacts with the calcium hydroxide present in cement-based materials and other cementitious materials, such as ettringite and calcium aluminates, resulting in the formation of calcium carbonate (CaCO3) [2,5]. The formation of calcium carbonate within the pores of the cement matrix increases density and reduces water absorption [6]. When water and CO2 are present, carbonic acid is produced. This acid lowers the pH of the cement, causing the release of Ca2+ ions from hydrated cement compounds [4]. Consequently, CaCO3 is formed [7]. The reaction with CO2 can also occur with other hydrated phases of cementitious materials, such as calcium silicate hydrate (C-S-H), ettringite, and calcium aluminates [7]. The carbonation of C-S-H results in the formation of calcium carbonate and amorphous silica gel [8,9]. Furthermore, anhydrous cement compounds like tricalcium silicate (alite) and dicalcium silicate (belite) can also undergo carbonation. This process leads to the formation of calcium carbonate and, in some cases, silica gel [5,9,10].
The carbonation of recycled aggregates (RAs) has the potential to enhance material properties and contribute to environmental sustainability. RAs from construction and demolition waste often have uncarbonated cementitious compounds. This makes them well suited for accelerated carbonation. During this process, the aggregates are strengthened by filling voids and microcracks with calcium carbonate [10,11,12]. In addition, greenhouse gas emissions are mitigated by sequestering CO2. By optimizing the carbonation process in recycled aggregates, their mechanical properties—such as their crushing index—can also be improved, making them more competitive compared to natural aggregates. Furthermore, incorporating CO2 curing into the production of RA supports the industry’s broader efforts to develop sustainable building practices and reduce the carbon footprint of construction activities [13].
This study investigated the CO2 capture of recycled aggregates under different carbonation conditions to understand how each reaction factor, such as relative humidity, temperature, and CO2 concentration, influences the amount of CO2 captured [6,14,15,16,17,18,19,20]. The test campaign was divided into three parts: variation in relative humidity; variation in temperature; and variation in CO2 concentration. The experimental conditions selected for this study aimed to approach the typical operating conditions encountered in Portuguese cement plants and were based on data provided by cement companies operating in Portugal. Notably, temperatures can vary significantly, with exhaust gases frequently reaching temperatures above 100 °C. The maximum temperature tested in the parametric campaign was 60 °C. If the temperature needs to be reduced, which can be achieved by “air flow”, the CO2 concentration will also be reduced. Also, in Portuguese cement plants, the concentration of flue gases in CO2 can range from 10 to 25%. For relative humidity (RH), the range of 40–70% was adopted to validate the ideal range defined in the literature and to examine how reducing the humidity content (for example, by increasing temperature) influences CO2 uptake.
There are some studies that have assessed the carbonation of RA; nevertheless, none have focused on its carbonation under the specific conditions of a cement plant chimney. The aim of this study is to simulate conditions closer to industrial ones and perform a sensitivity analysis of these more realistic results. With this approach to the experimental set-up using a real-world scenario, this study intends to enhance the viability and applicability of RA carbonation within the cement industry, while seeking to reduce the emission of greenhouse gases from the industry, promoting the recycling and reuse of construction and demolition waste (CDW) and the reduced consumption of natural resources [21].

2. Materials and Methodology

2.1. Materials

In this study, 6 recycled aggregates were selected and collected directly from different sources [14]: recycling plants, in situ demolitions, and concrete industry wastes (Table 1). Additionally, a reference concrete aggregate was also produced in the laboratory to be compared with the recycled aggregates. The composition of the control concrete aggregate (CA-L) is presented in Table 2. The binder used was the cement type CEM II/A-L 42.5 R. This cement is composed of 80% to 94% of clinker, in addition to limestone and other minor constituents [22]. CA-L comes from cubic concrete specimens of strength class C30/37. These specimens were kept under water to prevent natural carbonation between production and testing. At about three months of age, these cubes were dried at 40 °C until constant mass, and after that ground in a laboratory crusher to be analysed before and after carbonation.
The aggregates’ characterization is presented in the study by Bastos et al. [14]. Table 3 presents the results of the XRD analysis of the RAs. The three mixed recycled aggregates collected directly from recycling plants (MRA-RP1, MRA-RP2, and MRA-RP3) primarily reveal the presence of quartz, muscovite, microcline, albite, and calcite. No portlandite was detected within the limits of the XRD analysis, suggesting a low carbonation potential for these mixed recycled aggregates. In contrast, the XRD analysis of recycled concrete aggregates (RCAs) collected from recycling plants showed a notable presence of portlandite, attributed to the higher cement content (Table 4). Other significant mineral compounds identified in the RCA included calcite, quartz, microcline, ettringite, gypsum, muscovite, hydrocalumite, kaolinite, and albite. Overall, the RCA was predominantly composed of calcite and quartz, along with microcline and albite [23], which are not likely to carbonate. Also, the XRD analysis of CA-L revealed the existence of portlandite and other major mineralogical compounds, namely gypsum, quartz, calcite, and microcline.
Table 2. RA classification (taken from [14]).
Table 2. RA classification (taken from [14]).
WasteRc (%)Ru (%)Rb (%)Ra (%)Rg (%)X (%)Fl (%)Classification According to EN 933-11 [24]—[25] *
(as per the Results Obtained in Laboratory)
MRA-RP142.527.521.79.60.20.8Fl5-Rc declared Rcug70 Rb30- Ra10- Rg2- X1- Fl5-
MRA-RP261.526.79.20.20.02.4Fl5-Rc50 Rg2- Rcug70 Rb10- Ra1- X1- Fl5-
MRA-RP337.416.326.66.26.96.7Fl5-Rc declared Rg2- Rcug70 Rb 30- Ra1- X1- Fl5-
RCA-RP157.828.412.40.00.20.4Fl5-Rc50 Rg2- Rcug70 Rb30- Ra1- X1- Fl5-
RCA-RP289.010.60.40.00.00.0Fl5-Rc70 Rcug90 Rb10- Ra1- Rg2- X1- Fl5-
RCA-IW90.49.60.00.00.00.0Fl5-Rc90 Rcug90 Rb10- Ra1- Rg2- X1- Fl5-
CA-L90.79.30.00.00.00.0Fl5-Rc90 Rcug90 Rb10- Ra1- Rg2- X1- Fl5-
* Classification of RA 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, metals, plastic, rubber, non-floating wood, and gypsum plaster (X).
Table 3. Semi-quantitative XRD analysis of RA (adapted from [14]).
Table 3. Semi-quantitative XRD analysis of RA (adapted from [14]).
Identified MineralsMRA-RP1MRA-RP2MRA-RP3RCA-RP1RCA-RP2RCA-IWCA-L
Muscovite++++++n.d.
Ettringiten.d.n.d.++++n.d.
Gypsum+n.d.+++++
Kaolinite++++++n.d.
Microcline+++++++++++++
Albite++++++++++n.d.
Quartz+++++++++++++++++++++
Calcite+++++++++++++++++++++
Hydrocalumiten.d.n.d.n.d.+n.d.+n.d.
Portlanditen.d.n.d.n.d.++++++
Notation—not detected (n.d.); low/medium proportion (+); high proportion (++); predominant compound (+++).
Table 4. RA portlandite quantification from TG analysis.
Table 4. RA portlandite quantification from TG analysis.
RA Type Designation Portlandite Content (%) Cement Paste Content (%) Origin (Collected at)
Mixed recycled aggregate (MRA) MRA-RP1 0 5.7 Recycling plant
MRA-RP2 0 8.3 Recycling plant
MRA-RP3 0 5.0 Recycling plant
Recycled concrete aggregate (RCA) RCA-IW1 2.1 12.2 Concrete producer
RCA-RP1 0.1 7.8 Recycling plant
RCA-RP2 0.4 12.0 Recycling plant
Control concrete aggregate (CA) CA-L 0.6 12.2 Concrete produced in laboratory

2.2. Methodology

The first step consisted of the selection and collection of RAs, which were then dried at 60 °C until constant mass was reached. Next, the RAs were ground and sieved according to the European Standard EN 1015-1 [25], and only particles under 2 mm were subjected to forced carbonation. The RAs will be used in mortars in future applications. The samples were prepared with the same particle size distribution in order to obtain different aggregates with the same granulometric distribution, eliminating the granulometry factor [14]. Then, the samples went through an automatic separator for better homogenization, obtaining a representative sample for each period of carbonation selected. Forced carbonation was carried out in an ARALAB Fitoclima 300 EP climatic chamber with different exposure times to a 99.7% pure CO2 flow, varying the temperature, RH, and CO2 concentration between 23 and 60 °C, 40 and 60%, and 10 and 25%, respectively. Figure 1 presents the workflow used in this study, adapted from Bastos et al. [14]. The temperature range studied extended from room temperature (23 °C) to 60 °C, as 60 °C is the highest temperature that can be used without degrading RA [26]. The RH range was selected as it is a well-defined parameter for carbonation reactions (around 60%). The RH levels of 40% and 50% were chosen to evaluate the effect of reduced water availability on carbonation efficiency. The CO2 concentration was initially studied at 25%, representing high-purity CO2 streams typically used in controlled carbonation experiments. Subsequently, it was reduced to 10% to reflect the variability in CO2 concentrations in industrial flue gases, which often range between 10% and 25%. All these conditions are guaranteed by sensors placed inside the carbonation chamber.
The XRD analysis of the RA prior to the carbonation was performed with an AERIS Malvern Panalytical X-ray diffractometer, Almelo, the Netherlands, with 40 kV and 15 mA, using Copper Kα radiation (λ = 1.5406 Å). Diffractograms were recorded in the range of 5–85 º2θ at a step size of 0.20°/s. The XRD analysis was performed according to an internal procedure based on LNEC specification E 403:1993 [27], using the HighScore Plus 5.2 software from Malvern Panalytical and the Crystallography Open Database (COD) for the peak identification [28,29,30,31,32,33,34,35]. This analysis aimed to verify the mineralogy of the samples, particularly the type of cementitious compounds present.
The RAs were characterized through TGA before and after carbonation to quantify the amount of CO2 captured. TGA was performed using a TG-ATD SETARAM TGA92 and a HITACHI STA7200 thermal analysis apparatus simultaneously. The TGA tests were conducted from 25 °C to 1000 °C at a uniform rate of 10 °C min−1 under an argon atmosphere (3 L/h), using crushed RA samples with a particle size < 75 μm. One sample per condition was tested.
The carbonation experiments evaluated the CO2 uptake of RAs under various exposure periods. For the temperature variation tests, samples were exposed to CO2 gas for 0, 1, 2, 3, 5, 8, 12, and 24 h, allowing for the observation of both rapid initial carbonation and longer effects. In the tests focused on the variation in CO2 concentration and the RH, a different time series was employed: 0, 3, 5, 8, 12, 18, and 24 h. These intervals document the carbonation process from its beginning to the extended exposure, giving insights into the kinetics of CO2 absorption under different conditions.
The CO2 capture was determined as a function of the cement paste content (Equation (1)) of the RA. An average value of 14% was considered for the cement content in all RAs [14,36], except for CA-L, for which the composition is known (11% cement).
% CO 2 cement paste = ( kg of CO 2 capture per ton of aggregate Cement paste content ) / 1000

3. Influencing Parameters on CO2 Capture by RA

Studying the influence of carbonation parameters on RAs is essential for advancing sustainable construction practices and enhancing the quality of recycled materials. By examining how the impact of factors such as CO2 concentration, temperature, RH, and exposure time impact the carbonation process, the process can be optimized. This knowledge allows the industry to improve the physical and mechanical properties of these materials, potentially resulting in better overall performance in other applications. Additionally, optimized carbonation processes can also promote greater CO2 sequestration, supporting global efforts to reduce carbon emissions in the construction sector.

3.1. Effect of Temperature (23 °C, 40 °C, and 60 °C)

To assess the effect of increasing temperature on CO2 capture, MRA, RCA, and CA-L were submitted to forced CO2 carbonation in a climatic chamber with the following conditions: 23 °C, 40 °C, and 60 °C, with a constant RH of 60% and a constant CO2 concentration of 25%.
Figure 2 presents the maximum CO2 content captured by each RA in relation to the respective cement paste content at 23 °C, 40 °C, and 60 °C. The values ranged between 3.6 and 34.8%, with the highest value obtained in RCA IW1 at 23 °C.
The increase in temperature from 23 °C to 60 °C did not cause significant differences in CO2 capture. Interestingly, the capture values obtained at 40 °C were lower in all RAs. Taking into account that 60 °C is the temperature that is closest to the temperature of flue gases in Portuguese cement factories, these results are promising. The temperature increase from 23 °C to 60 °C generally led to a decrease in the time required to achieve maximum CO2 capture. The enhancement in the carbonation reaction speed seems to be dependent on the portlandite and cement paste content present in the RA (Table 4). The results of the MRA show no significant difference in CO2 capture when increasing the temperature from 23 °C to 60 °C. Concerning the RCA and CA-L, different behaviours were found with the increment of temperature. Reductions were found in those with higher initial CO2 capture potential, meaning that they were low-carbonated before the forced carbonation (RCA-IW and CA-L). This phenomenon could be attributed to the fact that the carbonation reaction occurred faster at higher temperatures. Consequently, the rapid formation of CaCO3 during the early stages of carbonation creates a protective coating that shields unreacted cement components. The carbonated barrier obstructs the pores’ structure, hindering the diffusion of CO2 into the cement matrix, ultimately leading to a lower CO2 capture [37,38]. Additionally, a higher amount of CO2 can lead to more water being produced from carbonation reactions, making the pores more saturated and hindering CO2 penetration [39]. The low values of CO2 capture with increases in carbonation temperature can also be resultant of greater amounts of Ca2+ ions leaching from the silicates and reduced CO2 solubility, leading to higher pH values of the water, causing a lower dissolution rate of Ca2+ [40]. In terms of maximum CO2 uptake, the effect of increasing the temperature of carbonation varied among the different RA types, causing a non-linear tendency in the results (Table 5). This could be due to the respective compositions of the RAs and their initial carbonation states. Moreover, some studies, such as those by Liu et al. [41] and Lu et al. [42], argue that the transmission coefficient and reaction coefficient of CO2 increase with rising temperatures. As a result, more CO2 is able to penetrate the material and react with the hydration products. Li et al. [43] and Li and Wu [44] claim that increasing the temperature from ambient levels to a threshold range of 60 to 80 °C enhances both the CO2 sequestration capacity and the carbonation speed of materials. However, beyond this threshold, a slight increase in temperature appears to have little effect or may even prevent mineral CO2 sequestration.

3.1.1. Mixed Recycled Aggregates (MRAs)

Figure 3 shows the weight loss due to decarbonation (400–500 °C) before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved. In general, it was concluded that the increase in temperature from 23 °C to 40 °C resulted in lower CO2 absorption by the mixed recycled aggregates. However, at 60 °C, two of the MRAs (MRA-RP2 and MRA-RP3) absorbed a quantity of CO2 higher than that recorded at 23 °C. For MRA-RP1, the maximum CO2 capture was 12.6%, 3.6%, and 10.8%, at 23 °C, 40 °C, and 60 °C, respectively. For MRA-RP2, the maximum was 22.5%, 10.0%, and 28.7%, respectively. Finally, for MRA-RP3, the maximum values of CO2 captured were 12.3%, 10.3%, and 14.4% at 23 °C, 40 °C, and 60 °C, respectively. For industrial implementation, it is important to check that CO2 capture remains viable at a temperature as high as 60 °C.
Furthermore, for the tested MRAs, it was found that the maximum CO2 uptake occurred faster as the temperature increased. For MRA-RP1, the maximum CO2 capture at 23 °C, 40 °C, and 60 °C occurred after 5 h, 3 h, and 2 h, respectively. For MRA-RP2, the respective maximum CO2 uptake was achieved at 5 h, 3 h, and 1 h, respectively. Finally, for MRA-RP3, the maximum CO2 capture occurred at 12 h, 8 h, and 2 h, respectively.

3.1.2. Recycled Concrete Aggregates (RCAs)

Figure 4 shows the weight loss of the RCAs due to decarbonation before and after forced carbonation at 23, 40, and 60 °C, considering the time at which the maximum CO2 uptake of RCA was reached.
The results of RCA-RP1 have some similarity with those of the MRA. Looking at the Rc, RCA-RP1 has an Rc value closer to that of MRA, explaining the similar uptake. At 23 °C, the highest amount of CO2 captured was 13.3% and occurred after 12 h; at 40 °C, it occurred after 8 h; however, the value decreased to 8%; at 60 °C, the maximum CO2 uptake was 11.9% after 3 h, lower than the value obtained at 23 °C. As per the MRA, the increase in temperature led to a faster carbonation; thus, less exposure time was required to achieve the maximum values of CO2 capture [45].
The RCA-RP2 and RCA-IW1 samples presented the highest content of Rc fractions of all RAs in this study, as indicated in Table 4. However, they showed different behaviours with increasing temperature. For RCA-RP2, the increase in temperature led to an increase in CO2 capture: 5.2%, 15.5%, and 25.0% at 23 °C, 40 °C, and 60 °C, respectively. The maximum CO2 capture at 23 °C, 40 °C, and 60 °C occurred at 12 h (34.8%), 5 h (20.4%), and 3 h (22.1%), respectively. As mentioned, this aggregate contains the highest amount of portlandite (Table 4), which is a consequence of its reduced exposure time to natural carbonation. Hence, RCA-IW1 has the highest CO2 uptake potential. In this case, the increase in the reaction kinetics due to the increase in temperature promotes a more favourable environment for the creation of superficial “carbonate barriers” [10,15]. These barriers, in turn, act by reducing the CO2 diffusion rate, leading to a lower CO2 capture. This phenomenon was negligible in all aggregates, except for RCA-IW1, because of its higher amount of portlandite. Therefore, the maximum carbonation temperature depends on the amount of available carbonatable compounds.
The results for RCA-RP1, RCA-RP2, and RCA-IW1 are coherent with the MRA results, where higher temperatures led to lower carbonation periods. The same effect was presented in the works of Drouet et al. [39] and Montes-Hernandez et al. [46].

3.1.3. Concrete Aggregate (CA-L)

Figure 5 presents the weight loss due to decarbonation before and after forced carbonation, considering the time of maximum CO2 uptake of CA-L at 23, 40, and 60 °C, with 60% RH and 25% CO2. The results show that the aggregate captured 16.1% of the CO2 at 23 °C. At 40 °C, the maximum content of CO2 captured is similar to the value at 23 °C (16.1%), but it was achieved in less time; thus, there was a reduction in the period of carbonation of 5 h to 3 h. At 60 °C, it took even less time (3 h) to achieve the maximum content of CO2 captured; however, in this last case, the maximum CO2 capture (9.3%) was lower than that obtained at the other two temperatures. These results show a pattern comparable to RCA-IW1, which is another RA characterized by lower natural carbonation and a high Rc fraction value. Consequently, the rationale behind the results obtained is the same. The low exposure of this RA to natural carbonation led to a high concentration of carbonation-prone compounds. Additionally, as discussed, the increase in reaction kinetics promotes a more favourable environment for the formation of “carbonate barriers” [37].

3.2. Effect of CO2 Concentration (10%, 18%, and 25%)

To evaluate the effect of CO2 concentration on CO2 capture, seven RAs were submitted to forced carbonation over time in a climatic chamber with the following exposure conditions: 23 °C, 60% RH, and 10–25% CO2. Figure 6 presents the values of the maximum CO2 content captured as a function of the cement paste content for each RA at 10%, 18%, and 25% CO2 concentrations. The captured CO2 content varied from 0.7% to 34.8%, with the highest capture rate of 34.8% observed in RCA-IW1 with a CO2 concentration of 25%. In general, the results indicate that reducing the CO2 concentration to 18% and 10% hinders the amount of CO2 captured. This suggests that higher concentrations of CO2 might facilitate CO2 capture due to increased interaction with the cement paste. The same tendency is reported by other authors [13,47,48].
The increase in CO2 concentration promoted more CO2 capture (Table 6). In the case of MRA, the reduction in CO2 concentration below 25% resulted in less CO2 captured and a longer carbonation period required to achieve maximum CO2 capture. The results for the MRA indicate that higher concentrations are more beneficial for carbonating lower-“quality” recycled aggregates. Higher concentrations will create higher driving forces (more molecules of CO2) for the carbonation reaction, achieving greater carbonation even in materials that are less susceptible to carbonation. Regarding RCA, in general, the same trend occurred. The greater difference in this capture is from 25% to 18% CO2 concentration. However, when the concentration is reduced from 18% to 10%, the CO2 capture does not decrease [38]. For CA-L, no significant difference in CO2 capture was noticed with a reduction in CO2 concentration from 25% to 10%. In conclusion, a gas concentration of 25% is generally the most favourable situation based on all the analysed examples.

3.2.1. Mixed Recycled Aggregates (MRAs)

For MRA-RP1, the maximum amounts of CO2 captured per cement paste were 1.8% at 10% [CO2] and 12.6% at 25% [CO2]. Interestingly, in the tests at 18% [CO2], carbonation was very small (0.7%). For MRA-RP2, the maximum CO2 uptake values were 7.5%, 10.0%, and 22.5%, at 10%, 18%, and 25% [CO2], respectively. Lastly, for MRA-RP3, the maximum values of CO2 captured were 10.3%, 2.1%, and 12.3% at 10%, 18%, and 25%, respectively. Therefore, it is concluded that increasing the CO2 concentration from 10% to 25% promoted a higher CO2 capture by the mixed recycled aggregates. When the CO2 concentration is increased within the reaction environment, it stimulates the progression of the reaction, resulting in a more accelerated and thorough conversion of CO2 into carbonate compounds. In this sense, lower concentrations lead to longer periods of carbonation. These results are consistent with those of Xu et al. [7], who also found that increasing CO2 concentration led to an increase in the rate and degree of carbonation. Figure 7 presents the variation in weight loss from calcium carbonate decarbonation in the period of maximum CO2 capture, before and after forced carbonation, indicating an increase in CO2 in the aggregates.

3.2.2. Recycled Concrete Aggregates (RCAs)

In general, the analysis of the results presented in Figure 6 confirm the previous conclusion that the reduction in CO2 concentration decreased the CO2 capture content per cement paste. The exception is RCA-RP2, where the maximum capture was obtained with 10% CO2 concentration.
The results of RCA-RP1 are similar at 10% and at 18%, with only a slight increase at 25%. At 10% CO2 concentration, the maximum amount of CO2 captured was 9.3%, while at 18% the maximum was 8%, and at 25% the maximum captured value increased to 13.3%. Similarly to prior outcomes for MRA, the reduction in CO2 concentration led to a lower content of CO2 captured by RCA-RP1. RCA-RP2 and RCA-IW1 are the samples with the highest content of Rc fraction; however, they presented different behaviours in these tests. For RCA-RP2, the decrease in CO2 concentration led to an increase in CO2 capture, from 5.2% at 25% to 9.5% at 10%. RCA-IW1 had an opposite effect to RCA-RP2 but similar behaviour to RCA-RP1, with the maximum content captured at 25% CO2 concentration (34.8%). This happened because the decrease in CO2 concentration led to less CO2 being captured during carbonation. This is due to the principle of chemical equilibrium and the driving force it creates for the carbonation reaction. During the carbonation process, which involves the reaction between CO2 and alkaline compounds in the sample matrix, the reaction seeks to establish a state of equilibrium. This equilibrium is defined by the concentrations of reactants and products in the reaction. When the concentration of one of the reactants, in this case CO2, is decreased, the equilibrium is disturbed, leading to a reduced formation of CaCO3, meaning that there is less blocking the path of the diffusing CO2 and slowing it down [7,38]. However, there are also studies that suggest that high CO2 concentrations can result in less CO2 capture due to the obstruction originating from the formation of denser minerals [7]. These results agree with previous results (MRA), where lower concentrations lead to longer carbonation periods. The unexpected carbonation behaviour observed in RCA-RP2 can be attributed to several factors inherent to the complex nature of RCA. The heterogeneity of RCA and possible variations in concrete sources have a significant role in this phenomenon [49,50]. Also, differences in initial carbonation states among the aggregates can greatly influence their susceptibility to further carbonation; the differences in weight loss before carbonation shown in Figure 8 are indicative of this [49]. Additionally, variations in moisture content across the samples can affect the carbonation process, as optimal moisture levels are crucial for efficient CO2 uptake [49]. Finally, differences in the age and degree of hydration of the original concrete sources may contribute to varying carbonation behaviours [51].

3.2.3. Concrete Aggregate (CA-L)

In this section, the results of the carbonation of CA-L at three different CO2 concentrations (10%, 18%, and 25%) are presented and discussed. Figure 9 presents the variation in weight loss from calcium carbonate decarbonation in the period of maximum CO2 capture, as well as before and after forced carbonation. The results indicate that there is no significant difference in CO2 capture when comparing the 10% and 25% CO2 concentrations. At 10% CO2 concentration, the aggregates were able to capture approximately 18.6% of the CO2 after a 12 h period. In contrast, at 18% CO2 concentration, an anomalous result was obtained, as only 5.1% of the CO2 was captured. This result appears inconsistent with the remaining data and should be considered as an outlier. Lastly, at 25% CO2 concentration, CA-L captured 16.9% of the CO2 after 24 h. These results suggest that there may not be a significant advantage in using higher CO2 concentrations (e.g., 25%) over lower ones (e.g., 10%) for carbonation in the case of CA-L. Some reports in the literature also present similar results: the work of Cui et al. [6] reported that carbonation depth is higher when concrete samples are exposed to low concentrations of CO2 and have a lower carbonation depth when the concentration is higher. This effect can be explained due to the fact that in higher-CO2-concentration environments, the formation of a fast and highly dense network microstructure with low pore connectivity occurs due to the formation of calcium carbonate, hindering further CO2 diffusion [52]. However, it is important to bear in mind the unique nature of CA-L that has not previously been subjected to natural exposure, which might lead to different conclusions compared to other recycled materials like MRA and RCA.

3.3. Effect of Relative Humidity (40%, 50%, and 60%)

To evaluate CO2 capture under different relative humidity environments, the recycled aggregates (MRA, RCA, and CA-L) were submitted to forced CO2 carbonation at various exposure times in a climatic chamber with the following conditions: 60 °C, 25% CO2 with 40%, 50%, and 60% RH. Figure 10 displays the results obtained for the tested RA. Looking at the general trend of the results obtained, it is possible to conclude that the decrease in relative humidity to values below 60% impaired CO2 capture by the RAs studied. This result is in accordance to other works in the literature [53,54,55]. The percentages of CO2 captured per cement paste range from 2.5% to 34.8%, with the highest capture rate (34.8%) observed in RCA IW at 60% relative humidity.
The results indicate that, when the relative humidity drops below 60%, there is a negative impact on CO2 capture, regardless of the amount of cement paste available in the RA, with the exception of two cases (MRA-RP3 and RCA-RP2).
As demonstrated in the research of X, the utilisation of RH values that fall below the optimal range is likely to yield substandard water content, which in turn exerts a substantial influence on the reaction between CO₂ and cementitious materials in RA. However, at levels that exceed the optimal value, the RA microstructure is susceptible to saturation because of excess water, which leads to pore clogging and subsequent inhibition of carbonation.
As demonstrated in the work of Elsalamary et al. [55], the use of RH below the optimal value will result in poor water content, which has a crucial impact on the reaction between CO2 and cementitious minerals in RA. Also, at levels that exceed the optimal value, the RA microstructure is susceptible to saturation due to excess water. This saturation leads to pore clogging, which can inhibit carbonation. In conclusion, maintaining humidity levels around 60% is essential for effective CO2 capture, regardless of the amount of cement paste in the recycled aggregates. These results are consistent with the range defined in the literature for “optimal” relative humidity to achieve a higher carbonation rate [7]. In general, reducing relative humidity resulted in less CO2 absorption in MRA, RCA, and CA-L (Table 7). The decreased water supply hindered the efficiency of this reaction, leading to reduced CO2 capture. Interestingly, not all cases showed an increase in the time taken for maximum carbonation to occur. In some instances, the period of carbonation remained the same, suggesting that, while lower humidity affected the CO2 uptake of RA, it did not universally prolong the reaction time. The observed decrease in CO2 uptake with reduced humidity is in accordance with the literature regarding cementitious material carbonation.

3.3.1. Mixed Recycled Concrete Aggregates (MRAs)

The results for MRA-RP1 and MRA-RP2 (Figure 11) show a reduction in captured CO2 with decreasing RH. For MRA-RP1, the maximum CO2 captured was 11.9%, 4.0%, and 2.7%, at 60%, 50%, and 40%, respectively. For MRA-RP2, the maximum was 22.0%, 7.7%, and 2.7% at 60%, 50%, and 40%, respectively. Lastly, for MRA-RP3, the maximum values of CO2 captured were 12.3%, 24.6%, and 16.4% at 60%, 50%, and 40%, respectively. In the last RA studied (MRA-RP3), the value of CO2 uptake achieved at 50% RH can be considered as an outlier, considering the other values obtained for MRA-RP3 in Section 3.1 and Section 3.2. It is possible that sample contamination occurred during sampling processing. Based on the results of the carbonation of MRAs, reductions in RH hinder the amount of CO2 captured. As previously mentioned, the carbonation reaction requires the presence of water. When RH content is reduced, less water will be available for the reaction to occur and less calcium will be dissolved from the samples to react with the provided CO2, which, in turn, only reacts if dissolved in the liquid phase [53]. In addition, the reduction in water content can also cause a reduction in the rate at which CO2 is captured and converted into calcium carbonate. This trend was found in the samples MRA-RP1 and MRA-RP2. In MRA-RP1, at 60% RH, the maximum CO2 uptake occurred after 5 h of forced carbonation, while when reducing the RH to 50% and 40%, the time increased to 18 h, indicating that the carbonation rate decreases at lower RH values.

3.3.2. Recycled Concrete Aggregates (RCAs)

The results of the three RCAs (RCA-RP1, RCA-RP2, and RCA-IW1) studied are presented in Figure 12. In RCA-RP1 and RCA-IW1, the decrease in RH led to a decrease in the CO2 uptake content, while in RCA-RP2, the CO2 uptake values were very similar in the three different conditions. In RCA-RP1 and RCA-IW1, the highest value of CO2 uptake was registered at 60% RH after 12 h of forced carbonation (12.9% and 34.8%). With the reduction in RH to 50% and 40%, the CO2 uptake value of both aggregates also decreased: to 4.0% and 4.7% for RCA-RP1, and to 18.7% and 11.9% for RCA-IW1.
The predominant effect of the variation in RH identified in the carbonation of RCA was a decrease in CO2 uptake. As previously mentioned, water content is essential for the carbonation reaction, since CO2 is first dissolved (solvation) in water, forming carbonic acid. Then, the carbonic acid is dissociated by reacting with calcium cations, forming calcium carbonate. However, a decrease in the water content will lead to incomplete reactions, leaving some unreacted carbonatable minerals in the RAs [56], impacting the efficiency of CO2 capture and the formation of calcium carbonate minerals (lower CO2 uptake). In RCA-RP2, the CO2 uptake remained similar at 60%, 50%, and 40% RH (5.2%, 6.9%, and 4.3%, respectively), indicating that the reduction had less impact in this aggregate.

3.3.3. Concrete Aggregate (CA-L)

The results of the carbonation of CA-L at different RH values (60% and 50%) are presented in Figure 13. Decreasing RH had a notable impact on CO2 uptake, resulting in a decrease from 16.9% (at 60% RH) to 10.1% (at 50% RH). This observation suggests that, while lower humidity conditions may have restricted the availability of water, a critical component for the carbonation reaction, the overall kinetics of the process were not significantly affected. This nuanced relationship between humidity, CO2 capture, and carbonation period highlights the complex interactions of environmental factors that influence mineral carbonation reactions.

4. Conclusions

In conclusion, the investigation into the effects of varying the parameters of the carbonation reaction, such as temperature, CO2 concentration, and relative humidity, on the carbonation of different types of RAs has led to several findings:
-
Firstly, higher carbonation temperatures, mainly at 60 °C, lead to higher CO2 uptake in shorter periods of exposure for most MRAs. Increasing the temperature shows promise for industrial applications, especially for RAs with higher initial CO2 absorption potential.
-
Secondly, the CO2 concentration plays a significant role in the carbonation process. Environments richer in CO2, especially at 25%, promote higher CO2 uptake. As the concentration of CO2 in the environment increased, both types of aggregates demonstrated a greater ability to sequestrate CO2. This finding indicates that higher CO2 levels during carbonation treatments may improve the carbon capture potential of both RCA and MRA.
-
Third, a reduction in carbonation relative humidity below 60% consistently results in reduced CO2 absorption for most RAs studied (MRA, RCA, and CA-L). Lower humidity limits water availability, which is crucial for the carbonation reaction, leading to a decrease in CO2 uptake. However, not all cases exhibit a prolonged reaction time. In conclusion, this study offers valuable insights for the industrial application of forced carbonation in RAs.
The optimal conditions for maximum CO2 sequestration were determined to be a temperature of 23 °C, an RH of 60%, and a CO2 concentration of 25%, achieved by the RA RCA-IW1. Under these conditions, the optimal duration of the carbonation process was 12 h to achieve maximum capture.
For industrial applications, this study identifies 60 °C, 60% relative humidity, and a CO2 concentration of 25% as the most appropriate operating conditions. Although these are not the conditions under which the highest CO2 capture was achieved, they resulted in notable capture values within a shorter carbonation time: between 1 and 3 h.
RAs exhibited a significant CO2 capture potential, ranging from 52 to 348 kg of CO2 per tonne of cement paste. The findings from this research have considerable potential in terms of industrial applications in the cement industry. By identifying the ideal conditions for CO2 sequestration in real-life RAs, industries can enhance the CO2 uptake efficiency of the process.
The optimized carbonation process can assist the cement and construction industries in achieving carbon neutrality targets by 2050, in line with global environmental regulations and sustainability goals. Additionally, this process adds value to construction and demolition waste, supporting a circular economy. The clear identification of these optimal parameters and the effects of their variation can help the transition of this technology from laboratory scale to industrial scale, and can also lead to reduced operational costs. If this process can be applied in the industry, it can help mitigate the environmental impacts of cement production and increase the likelihood of achieving the carbon neutrality targets by 2050.

Author Contributions

Conceptualization, D.B., C.B.F., C.M.P., R.V. and A.S.S.; Methodology, D.B., C.B.F., C.M.P., R.V. and R.I.G.; Formal analysis, C.B.F., C.M.P., R.V. and A.S.S.; Investigation, D.B.; Writing—original draft, D.B.; Writing—review & editing, C.B.F., C.M.P., R.V., J.A.B., R.I.G.s and A.S.S.; Supervision, C.B.F., C.M.P., R.V., J.A.B. and A.S.S.; Project administration, R.V. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Interface Mission [Notice No. 01/C05-i02/2022].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Acknowledgments

This work was supported by c5Lab—Sustainable Construction Materials Association [CENTRO-04-3559-FSE-000096; LISBOA-05-3559-FSE-000008; 01/C05-i02/2022], the National Laboratory for Civil Engineering (LNEC), CERIS—Civil Engineering Research and Innovation for Sustainability (UIDB/04378/2020) and CERENA [FCT-UIDB/04028/2025 and FCT-UIDP/04028/2025].

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

The following abbreviations are used in this manuscript:
CO2Carbon dioxide
IEAInternational Energy Agency
CaCO3Calcium carbonate
C-S-HCalcium silicate hydrate
RARecycled aggregate
RHRelative humidity
CDWConstruction and demolition waste
CA-LControl concrete aggregate
MRA-RP1Mixed recycled aggregate from recycling plant 1
MRA-RP2Mixed recycled aggregate from recycling plant 2
MRA-RP3Mixed recycled aggregate from recycling plant 3
RCA-RP1Recycled concrete aggregate from recycling plant 1
RCA-RP2Recycled concrete aggregate from recycling plant 4
RCA-IW1Recycled concrete aggregate from concrete producer
RCARecycled concrete aggregate
RcCementitious material content

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Figure 1. Workflow methodology diagram (adapted from Bastos et al. [14]).
Figure 1. Workflow methodology diagram (adapted from Bastos et al. [14]).
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Figure 2. Maximum CO2 uptake content captured by each RA at 23 °C, 40 °C, and 60 °C, in relation to the respective cement paste content (relative humidity of 60% and 25% of CO2).
Figure 2. Maximum CO2 uptake content captured by each RA at 23 °C, 40 °C, and 60 °C, in relation to the respective cement paste content (relative humidity of 60% and 25% of CO2).
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Figure 3. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for MRAs at 23, 40, and 60 °C.
Figure 3. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for MRAs at 23, 40, and 60 °C.
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Figure 4. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each RCA at 23, 40, and 60 °C.
Figure 4. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each RCA at 23, 40, and 60 °C.
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Figure 5. Weight loss due to decarbonation, before forced carbonation and after; the maximum CO2 uptake for CA-L was measured at temperatures of 23, 40, and 60 °C.
Figure 5. Weight loss due to decarbonation, before forced carbonation and after; the maximum CO2 uptake for CA-L was measured at temperatures of 23, 40, and 60 °C.
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Figure 6. Maximum CO2 uptake content captured by each RA at 10%, 18%, and 25% CO2 concentration in relation to respective cement paste contents.
Figure 6. Maximum CO2 uptake content captured by each RA at 10%, 18%, and 25% CO2 concentration in relation to respective cement paste contents.
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Figure 7. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each MRA at 25, 18, and 10%.
Figure 7. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each MRA at 25, 18, and 10%.
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Figure 8. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each RCA at 25, 18, and 10%.
Figure 8. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for each RCA at 25, 18, and 10%.
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Applsci 15 05005 g009
Figure 9. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for CA-L at 25, 18, and 10%.
Figure 9. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for CA-L at 25, 18, and 10%.
Applsci 15 05005 g009
Applsci 15 05005 g010
Figure 10. Maximum CO2 uptake content captured by each RA at 60%, 50%, and 40% RH in relation to respective cement paste contents.
Figure 10. Maximum CO2 uptake content captured by each RA at 60%, 50%, and 40% RH in relation to respective cement paste contents.
Applsci 15 05005 g010
Applsci 15 05005 g011
Figure 11. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for MRAs at 60, 50, and 40%.
Figure 11. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for MRAs at 60, 50, and 40%.
Applsci 15 05005 g011
Applsci 15 05005 g012
Figure 12. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for RCAs at 60, 50, and 40%.
Figure 12. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for RCAs at 60, 50, and 40%.
Applsci 15 05005 g012
Applsci 15 05005 g013
Figure 13. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for CA-L at 60 and 50%.
Figure 13. Weight loss due to decarbonation before and after forced carbonation, considering the time at which maximum CO2 uptake was achieved for CA-L at 60 and 50%.
Applsci 15 05005 g013
Table 1. Recycled aggregates studied and origin (taken from [14]).
Table 1. Recycled aggregates studied and origin (taken from [14]).
Aggregate Type Designation Origin (Collected at) Location
Mixed recycled aggregate (MRA) MRA-RP1 Recycling plant 1 Pero Pinheiro, Portugal
MRA-RP2 Recycling plant 2 Figueira da Foz, Portugal
MRA-RP3 Recycling plant 3 Seixal, Portugal
Recycled concrete aggregate (RCA) RCA-IW1 Concrete producer Alhanda, Portugal
RCA-RP1 Recycling plant 1 Pero Pinheiro, Portugal
RCA-RP2 Recycling plant 4 Bucelas, Portugal
Control concrete aggregate (CA) CA-L Concrete produced in laboratory Lisbon, Portugal
Table 5. Maximum CO2 capture by weight of cement paste in RAs and carbonation time at varying temperatures.
Table 5. Maximum CO2 capture by weight of cement paste in RAs and carbonation time at varying temperatures.
CO2 Maximum Uptake (%)/Carbonation Period (h)
MRA-RP1MRA-RP2MRA-RP3RCA-RP1RCA-RP2RCA-IW1CA-L
Temperature23 °C12.6%/5 h22.5%/5 h12.3%/12 h13.3%/12 h5.2%/5 h34.8%/12 h16.9%/5 h
40 °C3.6%/3 h10.0%/3 h10.3%/8 h8.0%/8 h15.5%/3 h20.4%/5 h16.1%/3 h
60 °C10.8%/2 h28.7%/1 h14.4%/2 h11.9%/3 h25.0%/1 h22.1%/3 h9.3%/3 h
Table 6. Maximum CO2 capture by weight of cement paste in RA and carbonation time at varying CO2 concentrations.
Table 6. Maximum CO2 capture by weight of cement paste in RA and carbonation time at varying CO2 concentrations.
CO2 Maximum Uptake (%)/ Carbonation Period (h)
MRA-RP1MRA-RP2MRA-RP3RCA-RP1RCA-RP2RCA-IW1CA-L
[CO2]10%1.8%/10 h7.5%/12 h10.3%/24 h9.3%/5 h9.5%/12 h14.4%/5 h18.6%/12 h
18%-10.0%/5 h2.1%/24 h8.0%/5 h3.4%/18 h13.6%/8 h5.1%/12 h
25%12.6%/5 h22.5%/5 h12.3%/12 h13.3%/12 h5.2%/12 h34.8%/12 h16.9%/5 h
Table 7. Maximum CO2 capture by weight of cement paste in RA and carbonation time at varying relative humidities.
Table 7. Maximum CO2 capture by weight of cement paste in RA and carbonation time at varying relative humidities.
CO2 Maximum Uptake (%)/ Carbonation Period (h)
MRA-RP1MRA-RP2MRA-RP3RCA-RP1RCA-RP2RCA-IW1CA-L
RH40%4.7%/18 h2.7%/12 h16.4%/12 h7.6%/12 h4.3%/3 h11.9%/3 h-
50%4.0%/18 h7.7%/12 h24.6%/12 h10.0%/5 h6.9%/3 h18.7%/8 h10.1%/5 h
60%11.9%/5 h22.0%/5 h12.3%/5 h12.9%/12 h5.2%/12 h34.8%/12 h16.9%/5 h
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Bastos, D.; Brazão Farinha, C.; Pederneiras, C.M.; Veiga, R.; Bogas, J.A.; Infante Gomes, R.; Santos Silva, A. Towards Industrial Implementation: Experimental Campaign Based on Variations in Temperature, Humidity, and CO2 Concentration in Forced Carbonation Reactions of Recycled Aggregates. Appl. Sci. 2025, 15, 5005. https://doi.org/10.3390/app15095005

AMA Style

Bastos D, Brazão Farinha C, Pederneiras CM, Veiga R, Bogas JA, Infante Gomes R, Santos Silva A. Towards Industrial Implementation: Experimental Campaign Based on Variations in Temperature, Humidity, and CO2 Concentration in Forced Carbonation Reactions of Recycled Aggregates. Applied Sciences. 2025; 15(9):5005. https://doi.org/10.3390/app15095005

Chicago/Turabian Style

Bastos, David, Catarina Brazão Farinha, Cinthia Maia Pederneiras, Rosário Veiga, José Alexandre Bogas, Ricardo Infante Gomes, and António Santos Silva. 2025. "Towards Industrial Implementation: Experimental Campaign Based on Variations in Temperature, Humidity, and CO2 Concentration in Forced Carbonation Reactions of Recycled Aggregates" Applied Sciences 15, no. 9: 5005. https://doi.org/10.3390/app15095005

APA Style

Bastos, D., Brazão Farinha, C., Pederneiras, C. M., Veiga, R., Bogas, J. A., Infante Gomes, R., & Santos Silva, A. (2025). Towards Industrial Implementation: Experimental Campaign Based on Variations in Temperature, Humidity, and CO2 Concentration in Forced Carbonation Reactions of Recycled Aggregates. Applied Sciences, 15(9), 5005. https://doi.org/10.3390/app15095005

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