Exploring carbon sequestration potential through 3D concrete printing

ABSTRACT

As global CO₂ concentrations rise, there is a pressing need for sustainable alternatives in the construction sector as many countries are striving to attain net carbon neutrality. Integrating carbon capture and sequestration (CCS) technologies directly into 3D concrete printing offer a promising solution to reduce the carbon footprint in the construction sector. This paper investigates a novel printing technique involving the purging of pressurised CO₂ gas was demonstrated and the various process parameters were evaluated for its effectiveness in promoting carbon sequestration. Results show that the carbon-sequestrated sample has a 15% increase in carbon uptake as compared to the control sample. The method can be complementary to existing sequestration technologies, facilitating large-scale carbon sequestration without chamber size limitations. Nevertheless, further research and development are necessary to optimise the various printing parameters and achieve a more balanced and efficient integration of carbon capture and sequestration technologies with 3DCP.

KEYWORDS:

• Carbon sequestration
• carbon curing
• additive manufacturing
• 3D concrete printing
• thermogravimetric analysis

1. Introduction

Carbon Capture and Storage (CCS) is an approach that has been researched to address the rising levels of atmospheric CO₂ [1]. In recent years, research on CCS has significantly intensified as efforts to combat the ongoing rise in CO₂ levels have grown. Much of the focus in carbon storage research has been on geologic carbon storage (GCS), involving the sequestration of compressed CO₂ in subterranean geologic formations like saline aquifers, depleted oil and gas fields, and unmineable coal seams [2]. However, GCS presents various uncertainties and potential risks, including issues of permanence, the need for long-term monitoring, and verification [3]. These concerns arise from the possibility of CO₂ leakage, which can occur gradually or due to a significant system failure, potentially negating the benefits of the capture process. Furthermore, the buildup of pressure underground may result in minor seismic activity [4]. Alternatively, carbon capture, utilisation, and storage (CCUS) through carbon mineralisation can be employed to mitigate risks and ensure long-term carbon storage. According to Zhang et al., carbonate minerals offer an ideal choice for this method [5]. In 2019, the National Academies of Sciences Engineering Medicine (NASEM) reported that carbon mineralisation offers various potential benefits for both human health and the environment. These include reducing atmospheric CO₂ levels, enhancing air and water quality, improving soil fertility, and creating new economic opportunities [6]. In a 2019 report authored by Bobeck et al. [7] from the Centre for Climate and Energy Solutions (C2ES), it was noted that carbon utilisation has the potential to reduce CO₂ emissions in the global concrete and aggregates market by 1.4–3.6 billion tons, respectively. This reduction would be significant given the expected growth of concrete and aggregates by 2030, estimated to reach around 40 and 50 billion tons, respectively [7].

In the cementitious product industries, a technique that has been utilised since the nineteenth century involves the use of CO₂ gas curing chambers to accelerate the hydration process of cementitious materials. According to Hills et al. [8], James L. Rowland was one of the pioneers who employed this method. When CO₂ reacts with the alkaline compounds in concrete, such as portlandite and/or hydrated calcium silicate (C–S-H) gel, a chemical reaction is induced, resulting in the formation of calcium carbonate (CaCO₃) [9]. This process is known as carbonation. The carbonation reaction reduces the pH value of the concrete to about 8.5–10 [9–11]. At this low pH value, CO₂ reacts with calcium hydroxide (CH) and calcium silicate hydrate (C–S–H) to form calcite, which has low solubility and precipitates within the concrete pores, altering the concrete’s properties. While carbonation exposure can contribute to the corrosion of steel reinforcement, it is noteworthy that carbonation-induced corrosion typically proceeds at a significantly slower rate compared to chloride-induced corrosion [12]. This can be attributed to the chemical reactions initiated by carbonation, which leads to the formation of a protective layer of calcium carbonate (CaCO₃) on the surface of concrete, and can reduce chloride ingress and subsequently mitigate the corrosion of steel reinforcement [13,14]. Through carbonation, the concrete's properties are altered, reducing porosity, pore size distribution, permeability, and diffusion, while enhancing mechanical properties such as tensile and compressive strength, electrical resistivity, and reducing chloride penetration [11].

3D Concrete Printing (3DCP) is widely recognised as a sustainable solution due to its ability to manufacture optimised designs and minimise waste [15]. Recent studies by Nguyen et al. [16] and Barjuei et al. [17] have demonstrated the potential for further enhancement of the 3D printing process through the integration of machine learning and monitoring systems. These advancements have resulted in streamlined processes that reduce time and effort, while also improving print accuracy. By optimising the manufacturing process, these researchers have contributed to the overall quality and sustainability of 3D concrete printing. Yet despite the advantages of 3DCP, there still is room for further reduction of its carbon footprint. 3DCP mixtures, in order to satisfy printability requirements, tend to comprise of higher cement contents than those of traditional cast concrete. A common approach to reduce its carbon footprint would be the partial if not full replacement of OPC with greener alternatives, such as Supplementary Cementitious Materials (SCMs) for alkali-activated Geopolymer concrete [18,19]. Several SCM examples include industrial by-products such as silica fume, fly ash, and slag, which are capable of exhibiting similar if not improved durability and strength as compared to conventional concrete [18]. On the other hand, existing literature has shown that replacing sand with more sustainable aggregates or optimising the aggregates-to-paste ratio can make a significant contribution towards sustainable construction, paving the way for circularity in 3D concrete printing [20,21]. The choice of materials plays a crucial role in determining the environmental impact and overall sustainability of the printed structures [22]. By utilising sustainable materials, such as low-carbon cementitious binders, employing solid waste as aggregate replacement, or sequestrating carbon in the material, the advantages of 3D concrete printing can be further amplified.

Methods aimed at achieving carbon sequestration in cementitious products typically involve subjecting precast concrete, cast-in-place concrete, 3D printed concrete, and/or their aggregates to chamber curing under specific conditions of elevated temperature, pressure, humidity, and concentrated CO₂ [23–25]. To improve the CO₂ sequestration level, additional parameters such as CO₂ injection during the mixing phase and addition of carbonated cement paste into the mixture design have been explored [26–29]. Combining both carbonation mixing and curing techniques have demonstrated a significant increase in CO₂ uptake in paste samples, surpassing 10% compared to curing-only treatments [27]. However, despite the positive findings reported in some studies, concerns remain regarding the time and energy consumption associated with accelerated carbonation methods using pressurised camber or chamber curing. These factors may pose compatibility challenges when implementing automated technologies like 3D concrete printing (3DCP). Monte Carlo simulations conducted by Ravikumar et al., using published experimental datasets, have also projected a higher probability of increased CO₂ footprints with the utilisation of CO₂ in concrete curing or mixing [30]. Considering the unique characteristics of 3DCP structures, which can be constructed with a hollow lattice-designed cross-section [31], they inherently offer a larger surface area exposed to CO₂ compared to solid cast concrete. Moreover, adjustments in print parameters such as layer height or stand-off distance can further optimise the material’s compaction and porosity [32], which are crucial for achieving CO₂ ingress and mineralisation objectives. Therefore, the simultaneous introduction of CO₂ gas and steam while altering these parameters may prove to be a more efficient and complementary approach to 3DCP, potentially surpassing the limitations of post-chamber curing techniques.

This research aims to investigate the potential and effects of sequestering CO₂ directly into 3D printable concrete through the application of a novel printing technique, which seeks to function as a preliminary concept that encourages further development and integration of Carbon Capture and Sequestration (CCS) technologies with 3DCP. The applied methodology herein entails a direct purging of pressurised CO₂ gas and vaporised water across the surface of each filament layer via a custom-built gantry printhead system. Various configurations of the experimental setup will be analysed to evaluate its effectiveness in promoting carbon sequestration with cementitious material during its printing phase. Samples extracted from the bulk of printed concrete will be subjected to thermogravimetric analysis (TGA) tests to determine the presence and quantity of mineralised carbonates after a 28-day curing period. This corresponds to the degree of carbon uptake in samples associated with its respective print configuration. While the integration of CO₂ concept is patented by Saudi Aramco [33], to the best of authors’ knowledge, there is a lack of studies conducted to investigate the impacts of integrating CCS technologies with 3DCP. The purpose of this study is therefore to serve as an initiative for further academic exploration and research in this domain.

2. Experimental program

2.1. Material preparation and curing conditions

The raw materials used in the study were ordinary Portland cement (OPC, ASTM Type 1), silica Fume (Undensified, Elkem), superplasticiser and river sand. The same mixture will be used for all prints. For consistency of the experiments, mixing procedures for all concrete admixtures are required to adhere to the same set of guidelines. First, all powder elements (i.e. aggregates and cementitious powder) will be dry mixed at low speeds, via a force action mixer (SoRoTo 80L mixer), for 3 min before hydrating solutions such as water, and superplasticizers are gradually added. This is to ensure even dispersion and distribution of solid particles/powder, after which 10 min of high-speed mixing will be carried out until homogeneity of the mixture is achieved. Information on the percentage of each raw material used in this study cannot be disclosed due to the non-disclosure agreement with the collaborators. Nevertheless, the primary focus of this manuscript lies in examining the impact of the CO₂ sequestration process rather than on the specifics of the material composition. While it's true that different mixtures may exhibit unique characteristics, the underlying principles should remain consistent.

The samples were cured in lab conditions and were wrapped in a clear plastic to prevent water from escaping due to evaporation. The wrapped samples were exposed to lab conditions at 25°C and relative humidity of 60%. In this study, different abbreviation, as shown in Table 1, was given to the different printing process investigated to simplify the naming of the sample.

Table 1. Abbreviation for the different samples.

Abbreviation	Definition
CTRL	Control sample which serves as a benchmark for comparison with the other samples
CJ	Samples printed with a single CO2 jetting nozzle
CJET	Samples printed with a single CO2 jetting nozzle at elevated temperature
CJS	Samples printed with a single CO2 jetting nozzle coupled with steam

2.2. Printing parameters

A three-axis gantry printer and a progressive cavity pump are used as the printer setup in the study. The travel speed of the nozzle was set at 100 mm/s and the material flow rate was set 39 ml/s for all experiments to ensure consistency. The nozzle orifice was a square nozzle of 20 mm. The material is delivered from the material reservoir to the nozzle by a three-meter hose. The printing speed of the gantry is fixed at 100 mm/s for all mixtures. Figure 1(a) shows the different samples printed within the same batch of materials. Figure 1(b) shows the front and rear sections of the print path. It is differentiated into the front and rear sections which is important due to the placement of the CO₂ nozzle.

Figure 1. (a) 3D printed sample for TGA and mechanical analysis; (b) Top view of the printing path to differentiate the front and rear half of the sample; Schematic diagram for the print path with dimensions for (c) TGA and compressive strength test and (d) flexural strength test.

Figure 1(c,d) shows the schematic diagram of the print path for the samples used for TGA test, compressive strength test and flexural strength test. After the printing process, the printed blocks using the print path shown in Figure 1(c) were cut into smaller sections with the length of 50 and 1 mm for the compressive strength test and TGA test, respectively, using a diamond cutter (Struers, Secotom-60). The whole printing experiment took less than 30 mins after the cementitious mixture material were poured into the pump’s hopper.

All specimens have a solidity ratio (SR) in the range of 0.95–1.05. SR can be used to determine the consistency of the printed filament based on the printing parameters [34]. Two different printing parameters can have the same SR which means the amount of material deposited per unit length is the same. SR is defined by Equation (1). (1) $SR={\displaystyle \frac{1000Q}{A_n\times V_n}}$(1) where A_n is the area of the nozzle orifice, V_n is travel speed, and Q is the flow rate of the material.

2.3. CO₂ jetting (CJ) method

The CO₂ jetting method was carried out by directly jetting CO₂ on the top surface of the printed filament. By directly jetting CO₂ on the different filaments, it was hypothesised to increase the CO₂ uptake. This approach involves positioning a single CO₂ nozzle alongside the concrete nozzle, with the outlet of the CO₂ nozzle located directly above the concrete filament. This enables the injection of CO₂ into the filament, facilitating concrete sequestration as it prints. The carbon dioxide gas used in this research has a purity of 99.998%. Due to the proximity of the nozzle and the high flow of CO₂ blowing directly on the top surface of the filament, some of the material tended to be blown off. The pressure from the CO₂ gas cylinder is fixed at 3 bar. The pressure was set at 3 bar to ensure the highest amount of CO₂ was penetrating the cement filament during printing while ensuring that the pressure from the CO₂ nozzle outlet is not blowing too much filament material away. Figure 2 shows the schematic diagram on the side view of the printing setup for a single CO₂ nozzle attached printing the front and rear half of the sample. Due to the setup of the nozzle the CO₂ applied to the surface is different, for the front half of the sample, CO₂ is applied to the bottom of the layer, whereas for the rear half of the sample, CO₂ is applied to the top surface of the filament.

Figure 2. Schematic diagram on the side view of printing with concrete and CO₂ jetting nozzle for the (a) front half of the sample (b) rear half of the sample.

2.4. CO₂ jetting at elevated temperature (CJET) and CO₂ with steam printing (CJS)

It is being shown in existing literature that increasing the temperature can improve the carbon sequestration [35]. The presence of higher humidity at elevated temperature has shown potential to enhance the CO₂ uptake in the cementitious samples [36]. Therefore, additional changes to CJ printing method were examined. Figures 3 and 4 illustrate an alternative approach to carbon sequestration by introducing heat and steam, respectively. As shown in Figure 3, the CO₂ was passed through a pneumatic tube with a portion of the pneumatic tube submerged in a pot filled with boiling water. The end of the pneumatic hose is measured to be an estimated of 40°C. Similar to CJ method, the CJET method was carried out by directly jetting CO₂ on the top surface of the printed filament. The printing of the front and rear half of the sample is shown in Figure 2.

Figure 3. Schematic side view diagram of CJET printing method.

Figure 4. Schematic diagram on the side view of printing with concrete and CO₂ jetting with steam for the (a) front half of the sample (b) rear half of the sample.

Figure 4 shows the steam installed to elevate the temperature of the CO₂ during printing. The temperature of the steamer is fixed at approximately 100°C at the outlet of the steamer. Similar to the set up and working principle elaborated in Section 2.3 and in Figure 2, Figure 4 shows the schematic diagram for the printing setup of the CO₂ jetting nozzle and the steam nozzle. The non-rotating attachment can affect where the steam and CO₂ are applied to the printed filament.

2.5. Thermogravimetric analysis test

The mass of the specimens to be placed in the equipment must be between 10 and 20 mg. To obtain samples for analysis in the CJ method, the concrete specimens were chipped from the centre of the cross section using a hammer. The resulting debris from the chippings was then pulverised into fine samples. The test specimens will be heated over a range of temperature from 20°C – 950°C at a constant rate of 10°C/min in a nitrogen atmosphere. Presence, phase, and quantity of stable carbonates (i.e. CaCO3) precipitated in each sample is determined by its mass loss percentage across a temperature range from 520°C – 950°C and the temperature can be calculated using Equations (2)–(4) [31]: (2) $\text{\%}{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{o}}_3}{,}_{\mathrm{D}\mathrm{C}}\approx {\displaystyle \frac{{\mathrm{m}}_{{520}^{\circ }\mathrm{C}}-{\mathrm{m}}_{{720}^{\circ }\mathrm{C}}}{{\mathrm{m}}_{{20}^{\circ }\mathrm{C}}}\times 100\text{\%}}$(2) (3) $\text{\%}{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{o}}_3}{,}_{\mathrm{H}\mathrm{C}}\approx {\displaystyle \frac{{\mathrm{m}}_{{720}^{\circ }\mathrm{C}}-{\mathrm{m}}_{{950}^{\circ }\mathrm{C}}}{{\mathrm{m}}_{{20}^{\circ }\mathrm{C}}}\times 100\text{\%}}$(3) (4) $\text{\%}{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{o}}_3},\mathrm{N}\mathrm{e}\mathrm{t}=\text{\%}{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{o}}_3},\mathrm{D}\mathrm{C}+\text{\%}{\mathrm{m}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{o}}_3},\mathrm{H}\mathrm{C}$(4) where %m_CaCO3,_DC is the percentage mass loss of the poorly crystallised CaCO₃, %m_CaCO3,_HC is the percentage mass loss of the highly crystallised CaCO₃, and %m_CaCO3,_Net is the net percentage mass loss of crystallised CaCO₃.

2.6. Mechanical strength testing

The mechanical strength was carried out on 28 days of curing to investigate the effect of carbonation curing on the strength of the concrete. This test aims to obtain the strength characteristics of the different printing and curing conditions of the samples and therefore they were compared to the control sample. Kim et al. showed an increase in the strength when CO₂ is used for curing [36]. This is mainly due to the reduction in the pore size due to carbonation curing [36]. The procedure for testing its mechanical strength is in accordance with the ASTM standards C109 [37] (Figure 5).

Figure 5. Alternative approach for TGA sample preparation; (a) Sliced sample; (b) Sliced sample pulverised into fine powders.

3. Results and discussion

3.1. Evaluation of CO₂ jetting with different configurations

All carbon jetted samples underwent different CO₂ exposure for the front and back half of the sample. As shown in Figure 6, pressurised CO₂ were jetted on the bottom surface and top surface of the filament for the front and rear half of the sample, respectively. The green triangle illustrates the degree of CO₂ exposure, acknowledging that some material may have been displaced during printing, allowing for partial CO₂ penetration into the filament. In contrast, the surface exposure of CO₂ for the front half of the specimen was much more gentle. The red circle represents the location where samples were taken for the TGA test.

Figure 6. Schematic drawing of the sample cross-section for all carbon cured samples. The green lines represent the where the CO₂ was jetted during printing. The red circle represents where the sampling was taken for TGA test.

Calcium hydroxide (Ca(OH)₂) and calcium silicate hydrates (C–S–H) form when calcium silicates present in the cement are mixed with water, resulting in an exothermic process [38]. C–S-H is responsible for the durability and strength of cement and concrete [39]. When CO₂ interacts with Ca(OH)₂ and C–S–H, it leads to the formation of CaCO₃ [40]. In the TGA result, the mass loss between 520°C and 950°C can be proportional to the quantity of the CaCO₃ present in the sample [41,42]. This thermal decomposition profile was used to assess the effectiveness of the respective CO₂ jetting process in sequestering CO₂. Figure 7 shows the percentage content of the total precipitated CaCO₃ (wt.%) across various printing approaches.

Figure 7. (a) Percentage of mass loss by weight of sample from 520°C to 950°C for samples 3D printed using various carbon jetting methods. (b) Percentage of mass loss by weight of sample from 520°C to 720°C and 720°C to 950°C for samples 3D printed using various carbon jetting methods.

The formation of CaCO₃ can fill pores in meso and micro sizes which enhances the material's microstructure [43]. The crystallinity of CaCO₃ can be influenced by several factors, including the chemical composition of the cement, impurities, and additives [44]. The crystallinity of CaCO₃ can be categorised into poorly-crystalline 520°C–720°C and well-crystalline 720°C–950°C [45, 42, 46]. The result aligns with existing literature, where poorly crystalline values were found to be higher than well-crystalline values [45]. Poorly crystalline values are often found to be higher than well-crystalline values due to the nature of the crystallisation process. When CaCO₃ forms in a cementitious matrix, it is usually in a disordered or amorphous state, which is poorly crystalline. Especially in the early stages of CaCO₃ formation, the environmental conditions may favour the rapid nucleation and growth of poorly -crystalline forms. These forms are less stable and have higher reactivity compared to the more ordered, well-crystalline CaCO₃. As a result, poorly-crystalline CaCO₃ tend to be more abundant in certain conditions or environments.

As shown in Figure 2 and elaborated in Section 2.3 and 2.4, the CO₂ was jetted into the top surface of the filament of the CP sample at close proximity, leading to increased CO₂ sequestration efficiency. Among the samples, CJ and CJS exhibited the most promising results (See Figure 7). When comparing the CJ sample with the CTRL sample, there is a 15% increase in carbon uptake. Further enhancement in capture efficiency could be achieved by elevating the CO₂ concentration or prolonging the exposure duration [40]. From Figure 7, all samples printed with a carbon jetting show larger variance when compared to the control samples. One potential factor contributing to this variation could be the sampling technique, as shown in Figure 6. Alternative approaches such as obtaining a complete cross-section slice and employing a pulverisation method can potentially improve the consistency of results. It's worth mentioning that while efforts to reduce this variance can enhance experimental precision, it do not change the principles of the results in this study.

It should be emphasised that, in contrast to conventional carbon sequestration and capture techniques, the method proposed in this manuscript involves significantly shorter exposure durations and operates at much lower pressure. This may result in comparatively reduced carbon sequestration efficiency. However, this approach, when applied during printing has the potential to revolutionise carbon capture on a broader scale. It may be coupled with existing carbon sequestration technology to improve sequestration efficiency. Furthermore, through the use of this in-situ carbon sequestration, it can facilitate large-scale carbon sequestration that is not constrained by chamber size limitations.

The TGA result for samples produced under various printing parameters are shown in Figure 8. Each sample was replicated three times, and the averages are presented to illustrate the overall trend observed across the three trials. These TGA and DTG results obtained are in good agreement with existing literature [47–50]. In the TGA curves, all samples exhibit a sharp mass loss from the beginning of the experiment, corresponding to a derivative thermogravimetry (DTG) peak below 180°C. This can be attributed to the evaporation of structurally unbound water of ettringite and calcium silicate hydrate (C–S–H) [47].

Figure 8. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) result of the samples with different printing parameters; (a) combined results for comparison between TGA and DTG; (b) Thermogravimetric analysis (TGA) result; (c) derivative thermogravimetry (DTG) result; (d) zoomed in derivative thermogravimetry (DTG) result for temperature between 520°C and 950°C.

The next DTG peak occurs between 400°C and 450°C, which is associated with the dehydroxylation of portlandite [49]. As seen in Figure 8 (b and d), samples exposed to CO₂ demonstrate a much steeper decline in residual mass as temperature increase and exhibit higher DTG values compared to the control sample. This phenomenon may be associated with the presence of carbonation. The surface exposed to CO₂ may lead to an extensive network of CaCO₃ formations in the concrete microstructure on the surface which can cause a reduction in pore size and permeability [51]. This could potentially lead to increased water retention within the material, contributing to the higher weight loss during the evaporation phase. It is evident that a significant amount of Ca(OH)₂ remains unreacted with CO₂ and has not fully transformed to CaCO₃. The addition of CO₂ jetting could have carbonated a protective layer of CaCO₃ encapsulating the unreacted Ca(OH)₂ particles [52].

CaCO₃ decomposes in the temperature range between 520°C and 950°C yielding CaO and CO₂ [46]. As seen in Figure 8 (d), all CO₂ jetted sample exhibits a higher degree of weight loss, corresponding to higher amount of CaCO₃ decomposition [53]. Existing literature indicates that increasing the degree of carbonation in concrete requires prolonged exposure to CO₂ with high concentrations [54]. While the proposed method of printing does enhance carbon sequestration, prolonged duration with higher CO₂ concentration will likely yield improved results.

3.2. Mechanical strength evaluation

The incorporation of various carbon jetting methods with 3DCP was observed to have a negative effect on its flexural strength and compressive strength performance (See Figures 9 and 10). A possible explanation would be the decline effect that integrating carbon dioxide with 3DCP has on the compaction of printed concrete samples, particularly along its cross-sections. This is attributed to the purging effect it has on concrete filaments during 3DCP, as the fixed direction of pressurised CO₂ jetting would counteract with printing paths for about 50% of its total travelling trajectory. As mentioned earlier, this would lead to undesired material losses during its print process, causing better mixing, as well as large cavities and gaps to form along the horizontal directions, particularly at the layer interfaces. The presence of large cavities and gaps along horizontal planes of samples would result in poorer interlayer bonding strengths and irregular density of its cross-section, consequently reducing its flexural performance. As the CO₂ is being jetted onto the top surface of the filament, it may also lead to a drying effect on the filament’s surface. Previous studies have indicated that losing moisture can negatively impact how well layers adhere together, potentially affecting strength and plastic shrinkage [55, 56]. Moreover, there can be internal stress within the layers of 3D printed structures when there is a slight change in moisture content [57]. Further research on the moisture levels of the filament's surface can be conducted to better understand these effects, as it is not the primary focus of this study.

Figure 9. Flexural strength of control samples, CO₂ jetting (CJ) sample, CO₂ jetting at elevated temperature (CJET) samples and CO₂ with steam printing (CJS) samples.

Figure 10. Compressive strength of control samples, CO₂ jetting (CJ) sample, CO₂ jetting at elevated temperature (CJET) samples and CO₂ with steam printing (CJS) samples.

While the retention of water is observed in Figure 8, it doesn't appear to significantly enhance the hardened properties of the filament. This could suggest that although water retention occurs, it may not directly translate to improve strength gain. In fact, the reduction in pore size may have more detrimental impact on the interlayer bonding strength. It's plausible that the surface pore size reduces due to carbonation, the altered microstructure may influence the adhesive properties between adjacent layers. This phenomenon merits further investigation to elucidate its implications for overall material performance and structural stability in 3D-printed objects. Both Figures 9 and 10 show similar behaviour. The CJ and CJET samples were found to have lower mechanical strength than the control samples. Whereas for the CJS sample, it has a higher mechanical strength than both the CJ and CJET samples. For the CJS method sample, the purging effect is much stronger due to the combination of both the flow from the steam and the CO₂ this could have caused better mixing of the material extruded and the layer below, thus resulting in better overall strength when comparing to the other carbon jetting methods.

4. Conclusion

The production of concrete and its primary binding agent OPC is responsible for a significant amount of greenhouse gas emissions and energy consumption. As the global concentration of CO₂ continues to rise, there is a pressing need for sustainable alternatives in the construction sector. The development of 3DCP has gained interest as a more efficient and sustainable construction method. However, there is still room for reducing the carbon footprint of 3DCP. Integrating carbon capture and sequestration technologies directly into the 3DCP process presents a more complementary approach.

This study investigated the efficacy of in-situ carbon sequestration during 3D printing of cementitious materials using various carbon jetting methods. The results demonstrate that the interaction of CO₂ with calcium silicates in cement showed promising results in promoting the precipitation of CaCO₃ in the printed samples although poorly-crystalline forms were predominant. The printing approach and CO₂ jetting configuration influenced the carbon uptake, particularly CJ and CJS, exhibited promising results in terms of carbon sequestration efficiency, with a 15% increase observed in the CJ sample compared to the control. The exposure to CO₂ led to significant changes in mass loss for the TGA result, indicating the influence of carbonation on the concrete microstructure. Notably, a protective layer of CaCO₃ encapsulating unreacted Ca(OH)₂ particles was observed.

In terms of mechanical strength, the incorporation of carbon jetting methods may have affected negatively which may be attributed to the purging effect of pressurised CO₂, potentially leading to material losses and reduced interlayer bonding strength. This resulted in the formation of large cavities, gaps, and irregular density along the horizontal planes, leading to poorer interlayer bonding strengths and reduced flexural performance. The results also suggested that while water retention occurred, it did not directly translate to improved strength gain, and the altered microstructure may influence interlayer bonding.

In conclusion, while the integration of CO₂ with 3DCP has the potential for carbon sequestration, improvements are needed to address the limitations observed in mechanical strength performance. Further research and development are necessary to optimise the printing approach, CO₂ jetting configurations, and curing conditions to achieve a more balanced and efficient integration of carbon capture and sequestration technologies with 3DCP. This study serves as a preliminary exploration of this topic and encourages future academic and practical investigations in this field. The method can be complementary to existing sequestration technologies, facilitating large-scale carbon sequestration without chamber size limitations. By advancing the understanding and implementation of sustainable practices in the building and construction industry, we can contribute to mitigating the environmental impact of construction activities and move towards a more sustainable and climate-resilient future.

Credit authorship contribution statement

Tay Yi Wei Daniel and Phua Seng Liang Bryan: Writing original-draft, conceptualisation Methodology and Data curation. Lim Sean Gip: Writing – review & editing. Tan Ming Jen, Bandar Fadhel and Issam Amr: Writing – review & editing, Resources and Supervision.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Additional information

Funding

This research is supported by Saudi Aramco Technologies Company, and National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme for the Singapore Centre for 3D Printing.