1. Introduction

In the global context of combating climate change, the construction industry has come under intense scrutiny due to its substantial carbon footprint. The construction and operation of buildings account for more than one-third of global energy consumption and CO₂ emissions^[1-3]. Notably, cement, being the core constituent of concrete, represents a primary emission source; its production contributes approximately 7% to 8% of total global anthropogenic CO₂ emissions^[4-7]. This stark reality highlights the urgency of deep decarbonization within the building sector. Consequently, any effective climate action must confront this challenge by seeking disruptive technologies that fundamentally transform material production and construction methodologies.

As a pivotal technological strategy for realizing the vision of global carbon neutrality, Carbon Capture, Utilization, and Storage (CCUS) aims to physically or chemically capture carbon dioxide and either convert it into high-value-added products or permanently sequester it geologically^[8,9]. In particular, the carbon sequestration pathway based on mineralization principles is capable of transforming gaseous greenhouse gases into thermodynamically stable solid carbonates. This mechanism provides a robust theoretical foundation for the low-carbon reshaping of traditional building materials^[10-12]. Deeply integrating this technological system into the fields of intelligent construction and novel building materials not only aligns with the industry trend driven by the synergy of smart construction and green development but also enables concrete, traditionally a major carbon source, to potentially be transformed into a carbon sink carrier possessing active carbon absorption capabilities. This transformation opens a new technological dimension for the sustainable development of the construction industry at a macro level, while simultaneously providing new opportunities to address the material performance challenges currently facing additive manufacturing^[13-15].

In recent years, concrete additive manufacturing has garnered increasing attention as a pivotal technological pathway for transforming traditional construction methodologies. Among the various techniques, 3D printed concrete (3DPC) is regarded as a critical approach for revolutionizing conventional construction practices due to its formwork-free nature, high material utilization efficiency, geometric freedom, and digital construction potential^[16-19]. Therefore, 3DPC, as the representative technology of concrete additive manufacturing, forms the primary focus of this review. However, the practical engineering application of this technology remains constrained by critical scientific and technical challenges, including the dual requirements of rapid shaping and stable load-bearing capacity imposed on the material during printing, the significant weak interlayer interfaces and mechanical anisotropy resulting from layer-by-layer stacking, and the impact of environmental variations on printing stability and quality consistency^[20-23]. Collectively, these factors restrict the widespread adoption of 3DPC in structural applications requiring higher safety grades and larger component dimensions^[24,25].

To address the aforementioned technical bottlenecks, the utilization of carbon mineralization curing technology for performance regulation of cementitious systems has emerged as a significant research direction. The reaction of CO₂ with hydration products and unhydrated clinker generates stable carbonates, which significantly enhances early-age strength and accelerates the structural build-up process. This mechanism precisely satisfies the stringent requirements of 3DPC for rapid shaping and stable load-bearing while simultaneously endowing the concrete system with carbon sequestration potential. Crucially, the layer-by-layer stacking process of 3DPC results in concentrated porosity at the interlayer interfaces, while the formwork-free characteristic results in a substantially larger surface area exposed to the atmosphere^[26-28]. Such a structural configuration provides ideal conditions for efficient CO₂ penetration and rapid reaction, thereby offering a distinct process-material coupling advantage in accelerated carbonation compared to traditionally cast concrete.

Current research on the integration of CCUS process systems within 3DPC has established several distinct technical routes. These encompass utilizing CO₂ during the fresh state to regulate rheological behavior for rapid structural stabilization, achieving layer-by-layer synchronous carbon sequestration during the printing process, and implementing post-printing accelerated carbonation to enhance microstructure and durability performance. These strategies have demonstrated positive effects in enhancing early-age strength, strengthening interlayer bonding, improving printing precision, and increasing carbon sequestration efficiency. Nevertheless, significant scientific challenges persist in this field, involving the control of carbonation depth and uniformity, the coordination between carbonation and hydration reactions, the reaction window and microstructural evolution mechanisms at the interface layer, and the performance stability and durability of large-scale printed components under real-world environmental conditions. Achieving the engineering application and standardization of this technological system necessitates the further establishment of cross-scale mechanistic models, precise process control technologies, and structural service performance evaluation systems.

In light of this background, this review systematically summarizes the latest research progress on CCUS processes in concrete additive manufacturing. It analyzes the subject from the perspectives of material reaction mechanisms, process coupling strategies, microstructural regulation, and macro-performance evolution, while also discussing the critical challenges and potential development directions facing the field. This work aims to provide a reference for constructing high-performance, low-carbon, and even carbon-negative concrete additive manufacturing systems, thereby offering technical support for the future development of intelligent construction and sustainable infrastructure.

2. Bibliometric Statistical Analysis

2.1 Data retrieval and screening

To systematically delineate the current research landscape of CCUS technology within concrete additive manufacturing, this review is grounded in the core research question: “How to integrate CCUS technology into concrete additive manufacturing?”. The formulation of this question established a definitive direction for the subsequent systematic literature retrieval and screening. Guided by this research objective, the keyword string was constructed as follows: (“3D print*” OR “3DCP” OR “additive manufactur*” OR “digital fabricat*” OR “additive construction”) AND (“concrete*” OR “cement*” OR “mortar*” OR “geopolymer*”) AND (“CO₂ mineraliz*” OR “carbon dioxide mineraliz*” OR “CO₂ sequestrat*” OR “carbon sequestrat*” OR “CO₂ curing” OR “carbonation”). Employing this string, a literature search was conducted within the Web of Science Core Collection database, yielding an initial corpus of 82 relevant documents. As illustrated in the literature screening flowchart in Figure 1, these documents underwent a multi-stage evaluation. Initially, non-English literature was excluded, and document types were restricted to journal articles and reviews, resulting in 81 remaining papers. Subsequently, through a meticulous examination of titles, abstracts, and full-text contents to assess their close alignment with the core research question, 30 non-pertinent documents were further excluded. Ultimately, 51 highly relevant papers were selected to serve as the analytical foundation for this review.

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Figure 1. Flowchart of literature screening.

2.2 Analytical tools

To conduct a systematic bibliometric analysis of this research domain, VOSviewer software (version 1.6.20) was employed as the core analytical instrument, being a widely recognized tool for constructing and visualizing bibliometric networks^[29-31]. Specifically, the robust text mining and keyword co-occurrence analysis functionalities of the software were leveraged to extract high-frequency terms from the titles, abstracts, and keywords of the 51 selected core documents. The software initially constructs a co-occurrence matrix, which is subsequently transformed into a distance-based visualization map via the “VOS mapping technique”. This spatial arrangement ensures that keywords sharing tighter conceptual correlations are positioned in closer proximity, while clustering algorithms are applied to identify distinct thematic communities. To facilitate an accurate interpretation of the keyword co-occurrence map presented in Section 2.3, clarifying its visual syntax is essential. Within the visualization, the node size is directly proportional to the occurrence frequency of the term, and the thickness of the connecting lines signifies the co-occurrence strength. Furthermore, the distance between nodes reflects the intensity of the relationship, while distinct color clusters represent specific “knowledge structures”^[31-33].

2.3 Results

An analysis of the distribution of publication years, as illustrated in Figure 2a, clearly demonstrates an accelerating growth trend in research concerning CCUS technologies within concrete additive manufacturing. The period from 2019 to 2021 can be regarded as an initial exploratory phase, characterized by an average annual publication volume of approximately two papers. Since 2022, publication output has increased significantly, with the three-year span from 2022 to 2024 yielding a total of 25 papers and an average of over eight articles per year. It is particularly noteworthy that the statistics for 2025, covering only up to November, have already reached 21 papers, a figure that far exceeds the annual total of any preceding year. This upward trajectory indicates that academic interest in the synergy between CCUS technology and additive manufacturing processes is rising rapidly. This surge is primarily attributed to the urgent global demand for carbon neutrality and the consequent emphasis on low-carbon construction technologies. The potential of this synergy lies in its capacity not only to utilize and sequester CO₂ as a resource but also to simultaneously resolve technical bottlenecks in concrete additive manufacturing related to early-age strength and interlayer bonding performance.

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Figure 2. (a) Number of papers obtained from literature selection criteria; (b) Number of publications by country/region.

In terms of national and regional contributions, as depicted in Figure 2b, research output exhibits a high degree of geographical concentration. China makes a particularly prominent contribution to this field, having published 25 papers, which accounts for 49.0% of the total. This dominance profoundly reflects the high priority and substantial investment dedicated to this interdisciplinary frontier by China, driven by the dual national strategies of Intelligent Construction and Carbon Neutrality. The USA serves as the second-largest contributor, publishing 6 papers (11.8%), followed by India at 7.8%. Furthermore, the active engagement of nations such as the UAE, Switzerland, Australia, and Singapore indicates that this research direction is not confined to specific regions but has instead emerged as a global topic of common interest among both developed and major emerging economies. This underscores a broad consensus regarding the immense potential of this field in driving the future of sustainable construction.

To further elucidate the research hotspots, knowledge structure, and thematic evolution of this field, this study employed bibliometric methods to conduct a co-occurrence analysis and visualization of keywords from the selected literature using VOSviewer software, as shown in Figure 3, Figure 4, and Figure 5. The keyword co-occurrence network map in Figure 3 delineates the research hotspots into three primary thematic clusters. The red cluster represents fundamental research in materials science, centered on “concrete”, “strength”, and “Portland cement”, while encompassing terms such as “fly ash”, “C-S-H”, and “accelerated carbonation”. This composition reflects that researchers are devoting efforts to exploring the mechanisms by which accelerated carbonation influences the chemical reactions and strength development of 3DPC cementitious systems. The green cluster focuses on the process and application dimensions, with “performance”, “3D concrete printing”, and “rheology” as the core, and includes terms like “fresh properties”, “sustainability”, and “additive manufacturing”. This indicates that this direction concentrates on how the introduction of CO₂ regulates the printability of the material and ultimately achieves sustainable construction performance goals. The blue cluster constitutes the community most closely aligned with the theme of this review. Characterized by the dual cores of “carbonation” and “3D printing”, it tightly connects mechanistic keywords such as “CO₂”, “sequestration”, and “carbon capture and storage” with process and outcome-related keywords like “extrusion” and “mechanical-properties”. This suggests that this thematic community represents the research pathway of applying CO₂ curing as a functional technology in situ during the printing process to enhance performance. Situated at the center of the map between these clusters, keywords such as “microstructure” and “strength” serve as critical bridges connecting the red material cluster and the blue mechanism cluster, while the term “rheology” connects the green process cluster, highlighting that maintaining material printability while introducing CO₂ remains a key challenge facing this interdisciplinary field.

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Figure 3. Keyword co-occurrence network visualization using VOSviewer.

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Figure 4. Overlay visualization showing the temporal evolution of keywords using VOSviewer.

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Figure 5. Density visualization indicating research hotspots using VOSviewer.

The keyword co-occurrence overlay visualization in Figure 4 intuitively illustrates the evolutionary trends of the research frontiers. Foundational terms such as “hydration” and “3D printing” appear in earlier blue or green hues, indicating they constitute the research basis of the field. Conversely, keywords directly related to the theme of this review, particularly “accelerated carbonation”, “CO₂ sequestration”, and specific carbonation products like “vaterite” and “calcium carbonate”, all appear in the most recent yellow hues. This distinct temporal evolutionary path indicates that research in this domain is rapidly shifting from the exploration of basic concrete additive manufacturing processes toward utilizing advanced carbon capture and mineralization technologies to synergistically enhance material performance and carbon sequestration efficiency. This implies that the deep integration of CCUS technology and concrete additive manufacturing is a development direction with immense potential. The density visualization map in Figure 5 further corroborates this assessment. High-density hotspot regions in the map are clearly concentrated on keywords including “carbonation”, “3D printing”, “concrete”, “microstructure”, “strength”, and “performance”. This demonstrates that the central focus of current research lies in exploring the integration of carbonation technology with concrete additive manufacturing processes, with the primary research objectives being the regulation of microstructure and the optimization of macro-performance and strength, which aligns closely with the theme of this review.

3. CO2 Curing Mechanism

The carbon curing of 3DPC fundamentally leverages the carbon mineralization process within CCUS technology as a reaction driving force, promoting the rapid formation of a solidified matrix with substantial strength in the cementitious material system within an extremely short timeframe. This mechanism not only achieves mineralized carbon sequestration but also effectively addresses the stringent requirements imposed by the 3D printing process on the early-age properties of concrete. This carbon sequestration process involves complex multiphase chemical reactions, where the gaseous carbon source dissolves into the pore solution to form carbonic acid (H₂CO₃), significantly lowering the pH value and providing the necessary chemical environment for subsequent ionic reactions^[34]. In this acidic environment, Ca(OH)₂ is rapidly consumed, precipitating solid-phase calcium carbonate (CaCO₃)^[35-37]. These products, initially appearing as metastable vaterite or aragonite, subsequently transform into stable calcite^[38-40], filling the interstitial spaces between particles to constitute the initial consolidation network. As illustrated in Figure 6, this microstructural evolution is highly dependent on environmental humidity. Optimal humidity facilitates deep CO₂ penetration and induces efficient pore filling by crystals to achieve matrix densification. Conversely, the micro-cracking features presented in the figure serve as a visual warning regarding the risk of expansive damage caused by excessive carbonation^[41]. This further corroborates the necessity of precisely regulating the reaction window in the 3D printing process to balance strength enhancement with structural integrity. Simultaneously, H₂CO₃ attacks the calcium silicate hydrate (C-S-H) gel, prompting the leaching of calcium ions to combine with carbonate ions and form CaCO₃. Consequently, the silicate chains within the C-S-H structure re-polymerize, transforming into silica gel with a lower Ca/Si ratio^[42-45].

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Figure 6. Schematic diagram of the carbonation mechanism and microstructural evolution model of concrete under different humidity environments. Republished with permission from^[41].

However, for 3D printing, a process that pursues instantaneous solidification, the most decisive mechanism is the direct carbon mineralization pathway. In this process, the carbon utilization aspect of CCUS technology involves the direct reaction of CO₂ with the surfaces of unhydrated tricalcium silicate (C₃S) and dicalcium silicate (C₂S) clinker mineral particles, generating CaCO₃ and early-age C-S-H in situ^[46-48]. In traditionally water-cured concrete, this is a relatively slow, late-stage process^[49-51]; however, within the high-concentration CO₂ environments employed in 3DPC, it forms a competitive or even synergistic relationship with early hydration reactions. This process forms a dense product shell composed of CaCO₃ and early-age C-S-H directly on the surface of cement particles, rapidly constraining the particles to establish an initial skeletal network. This imparts the quasi-instantaneous buildability necessary for 3DPC to withstand its own weight and subsequent layer loads.

4. Carbon Curing Processes in Concrete Additive Manufacturing

4.1 Fresh state carbon mineralization

To address the buildability challenges inherent to 3DPC, in situ rheological control has emerged as a critical technological pathway. Among the various approaches, carbon mineralization in the fresh state exhibits immense potential. This strategy utilizes CO₂ as a chemical trigger, injecting it into the fresh mortar via secondary mixing prior to printing to achieve an on-demand transformation of material properties.

Li et al.^[52] utilized the apparatus illustrated in Figure 7a to continuously inject CO₂ gas at a flow rate of 10 L/min into mortar that had undergone 4 minutes of conventional mixing. With increasing CO₂ mixing duration, the flowability of the mortar declined significantly, and the setting time was drastically shortened. Following 240 seconds of CO₂ mixing, the fluidity of the 3DPC decreased by 13% compared to the control group, while the final setting time was reduced by 40%. The injected CO₂ reacted rapidly with calcium ions and hydration products within the paste, generating solid-phase CaCO₃ and carboaluminates, as shown in Figure 7b,c. This process altered the surface morphology of the cement particles and induced flocculation, thereby causing the static yield stress of the 3DPC to escalate sharply within a short period. This rapid evolution of rheological properties induced by CO₂ mixing resulted in a significant enhancement in the buildability of the 3DPC. The uncarbonated control group collapsed upon reaching a printing height of 11 layers, whereas the 3DPC treated with secondary CO₂ mixing stably exceeded 50 layers without observable deformation.

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Figure 7. (a) Schematic diagram of the experimental setup. Republished with permission from^[52]; (b)-(c) Micro-morphology of the carbonated group immediately after mixing (0 h) and after 1 h, illustrating the in-situ generation and packing densification process of granular calcium carbonate and carboaluminates. Republished with permission from^[52]; (d) Schematic diagram of the process flow. Republished with permission from^[53]; (e) Quantitative phase analysis showing a significant increase in calcite generation with extending carbonation time. Republished with permission from^[53].

In a different approach, Qian et al.^[53] proposed a stepwise mixing pre-carbonation process, as depicted in Figure 7d. In this method, 50% of the cement was initially mixed with water to prepare a base slurry, which was subjected to carbonation pretreatment for a specific duration via a micro-bubble diffuser, followed by the addition of the remaining cement to complete the final mixing. This process flow substantially intensified the carbon capture capability of the material. Quantitative phase analysis results (Figure 7e) revealed that after 40 minutes of treatment, the calcite content within the paste surged by 301% compared to the uncarbonated control. This result substantiates that the staged introduction of CO₂ effectively drives the conversion of gas into stable sub-micron carbonate particles, thereby significantly elevating the mineralization and sequestration capacity of the material. Consequently, this provides a crucial theoretical and practical foundation for additive manufacturing technologies like 3DPC that necessitate precise rheological regulation.

Building on this foundation, the introduction of supplementary cementitious materials (SCMs) into the material system can yield significant synergistic enhancements when combined with CO₂ mixing. The micro- and sub-micron scale particles of silica fume^[54] provide abundant active sites for the heterogeneous nucleation of carbonate products, thereby substantially accelerating the carbon mineralization reaction process. As illustrated in Figure 8a,b,c, with the silica fume dosage increasing from 0% to 5%, there is a significant enrichment of granular carbonation products, such as mono-carboaluminate, generated on the surface of the paste particles. Driven by the synergy between CO₂ and silica fume, the buildability of 3DPC is further amplified; the maximum number of printable layers surged from 13 layers using CO₂ alone to over 33 layers, far surpassing the efficacy achievable by solitary CO₂ mixing. The elucidation of this synergistic mechanism offers a novel technical pathway for optimizing the performance of 3DPC. However, the efficacy of this technology is highly sensitive to the CO₂ injection dosage. An insufficient dosage fails to achieve the anticipated carbon mineralization modification, whereas an excessive dosage exerts a detrimental impact on the flowability of the paste (Figure 8d,e). Furthermore, the fresh state carbon mineralization technology demonstrates superior carbon capture capability^[54]. Research indicates that a mere 240-second CO₂ mixing process can mineralize and sequester a quantity of CO₂ equivalent to 0.86% of the cement mass. Facilitated by the synergistic promotion of 2% silica fume, the carbon sequestration rate can be elevated to a peak of 1.16%, representing a significant increment of 34.9% (Figure 8f). This signifies that CO₂ can serve not merely as a functional admixture to optimize printing performance but also be permanently sequestered within the building material, thereby offering an innovative pathway for constructing low-carbon or even carbon-negative 3DPC systems. However, rheological regulation solely during the mixing phase represents only the initial control step. To adapt to the layer-by-layer construction characteristics of 3D printing, carbon sequestration technology must extend from global mixture regulation to real-time intervention during material deposition.

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Figure 8. (a) Carbonated group without silica fume; (b) Carbonated group with 2% silica fume; (c) Carbonated group with 5% silica fume; Flowability changes of mortars with different silica fume dosages (0%, 1%, 2%, 5%) under different CO₂ mixing times (d) and flow rates (e); (f) DTG curves of paste groups after carbonation. Republished with permission from^[54]. DTG: thermogravimetric analysis.

4.2 Synchronous carbon sequestration during printing

The layer-by-layer construction characteristic of 3D printing technology offers unique temporal windows and spatial advantages for CCUS technology. Compared to the fresh state carbon mineralization strategy, synchronous carbon sequestration technology during printing is capable of fully exploiting the transient state of material extrusion and deposition by triggering an immediate carbonation reaction through the real-time introduction of CO₂. This process not only maximizes carbon sequestration efficiency at the critical moment of high material reactivity but also enhances interlayer bonding and structural stability through rapidly generated carbonation products. Based on the contact method between CO₂ and the printing material, this process is primarily categorized into two types, namely post-extrusion surface carbonation and pre-extrusion carbonation.

The post-extrusion surface carbonation process involves mounting nozzles alongside the printhead to spray CO₂ gas directly onto the surface of the freshly deposited 3DPC filaments. Early research by Tay et al.^[55] revealed that spraying pressurized CO₂ gas onto the surface of each layer during the printing process increased the carbon uptake of the 3DPC by 15% compared to control samples. However, the “blowing effect” of high-pressure CO₂ gas caused physical damage to the moist concrete filaments, leading to material loss and the formation of interlayer defects, thereby reducing mechanical strength. To address this issue, researchers explored a synergistic CO₂ and steam spraying process^[55], wherein synchronous steam injection not only promoted carbonation through hydrothermal conditions but also mitigated the drying and bonding issues caused by gas impact. Subsequently, addressing the issue of uneven coverage associated with single-jet spraying, Tay et al.^[56] compared unidirectional spraying (Figure 9a,b) with a bidirectional spraying process (Figure 9c,d); the bidirectional design effectively eliminated gas coverage blind spots on the rear of the filaments. Test results from Figure 9e further confirmed that, compared to the control and unidirectional groups, the bidirectional spraying system significantly enhanced calcium carbonate generation under ambient curing, ultimately resulting in a CaCO₃ content approximately 24% higher than that of the control group. Despite these advancements, this technology still struggles to completely avoid increased porosity and interfacial delamination induced by gas impact.

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Figure 9. (a)-(b) Schematic diagram of the CS setup and coverage area^[56]; (c)-(d) Schematic diagram of the DCS setup and coverage area^[56]; (e) Comparison of carbon uptake among the CTRL, CS, and DCS groups^[56]; (f) Schematic diagram of the two-step extrusion system^[57]; (g) Comparison of volumetric carbon uptake among the CTRL, CE, and CSE groups^[57]; (h) Schematic of the in-situ carbonation strategy^[58]; (i) Customized four-pronged diffusion nozzle and printing equipment^[58]. CS: unidirectional CO₂ spraying; DCS: bidirectional CO₂ spraying; CTRL: control; CE: CO₂-only exposure; CSE: steam-CO₂ synergistic exposure.

To overcome the potential damage to material structural integrity caused by post-extrusion surface carbonation, the pre-extrusion carbonation process was proposed. Lim et al.^[57] developed an innovative two-step extrusion system, as shown in Figure 9f, which integrates a secondary mixer at the printhead. This system directly injects and forcibly mixes CO₂ and steam into the flowing material at the moment the 3DPC is extruded. This approach aims to achieve more uniform volumetric carbonation rather than merely a surface reaction. Research has found that this technology achieves positive synergistic enhancements in both carbon capture and mechanical performance. As evidenced in Figure 9g, compared to traditional accelerated carbonation in a closed chamber, the volumetric carbon uptake increased by 38.2%. Simultaneously, the in situ generated nano-scale CaCO₃ particles accelerated early hydration, thereby exerting a positive influence on the final performance of the material.

Furthermore, synchronous carbon sequestration during printing exerts a significant positive influence on the rheological properties and printability of the material. On one hand, the pre-extrusion injection research by Lim et al.^[57] demonstrates that the injection of CO₂ enhances the thixotropy of the paste. Simultaneously, the rapid early-age carbonation reaction endows the material with set-on-demand characteristics, significantly elevating the structural build-up rate and increasing the printable height by 50% compared to the control group. On the other hand, Ralston et al.^[58] proposed an in situ carbonation strategy, as illustrated in Figure 9h, and designed a specialized CO₂ injection device, as shown in Figure 9i. This device achieves rapid stiffening of the material through multi-point gas injection. Their post-extrusion spraying research further confirms that employing the in situ CO₂ spraying process similarly ameliorates early-age deformation and shape stability of the printed components, thereby improving the structural fidelity of the printing.

Synchronous carbon sequestration during printing represents a highly promising in situ curing technology; however, its efficacy is heavily contingent upon the specific technical pathway employed. Post-extrusion surface carbonation demonstrates significant effectiveness in promoting surface carbon sequestration, yet the physical impingement of high-pressure gas on the printed filaments requires cautious management. This method yields maximum advantage when combined with porous structural designs and non-hydraulic cementitious materials. In contrast, pre-extrusion carbonation, by internalizing CO₂ within the material, achieves the synchronous enhancement of carbon sequestration, rheological regulation, printability, and mechanical performance, thereby providing a more promising technical route for developing high-performance, low-carbon 3DPC. Although synchronous carbon sequestration improves the immediate stability of printed filaments, it primarily acts on the fresh material. To further densify the microstructure and maximize the ultimate mechanical performance of the hardened structure, post-printing carbonation curing is an indispensable subsequent process.

4.3 Post-printing carbonation curing

Post-printing carbonation curing currently stands as the most widely implemented carbonation process in 3DPC. This process typically involves placing the components into a specific CO₂ environment for accelerated curing after the printing construction is completed. This approach aims not only to enhance the mechanical properties of the components, particularly early-age strength and the weak interlayer bonding, through carbonation reaction products^[59-61], but also serves as an effective CO₂ sequestration method that contributes to reducing the carbon footprint of the material^[62,63].

The efficacy of the post-printing process relies heavily on the precise regulation of the curing regime, specifically the synergistic matching of the pre-curing, carbonation, and subsequent curing phases. The primary critical step in the post-treatment process is pre-curing or pre-drying. Fresh 3DPC materials possess a high moisture content immediately after printing, and excessive water can block pore channels, thereby impeding the transport and diffusion of CO₂ gas^[64]. Consequently, it is imperative to achieve an “optimal moisture content” for the component through pre-curing (Figure 10a). Research indicates that the pre-curing duration is closely correlated with optimal mechanical performance and CO₂ uptake. A study by Han et al.^[64] focusing on ordinary Portland cement (OPC)-based 3DPC revealed that with the extension of pre-curing time, both CO₂ uptake and carbonation depth exhibited a trend of rapid initial growth followed by stabilization (Figure 10b,c). Among the tested durations, a pre-curing time of 10 to 17.5 hours, corresponding to a water loss rate of approximately 20%, was identified as the ideal choice for achieving optimal early strength. For other binder systems, such as γ-C₂S-based^[59] and magnesium slag-based^[65] printing materials, the optimal pre-drying water-to-solid ratio (w/s) was determined to be around 0.08. However, excessive pre-curing is not beneficial. Over-drying results in a lack of sufficient liquid water within the pores to dissolve CO₂ and leach calcium ions, thereby inhibiting the progress of the carbonation reaction. Simultaneously, excessive moisture loss exerts adverse effects on subsequent hydration reactions, potentially leading to a reduction in late-age strength^[64].

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Figure 10. (a) Variation of water loss percentage of 3DPC specimens with drying time. Republished with permission from^[64]; (b) Variation of CO₂ uptake with pre-curing time and carbonation curing time, showing the existence of an optimal window. Republished with permission from^[64]; (c) Promoting effect of pre-curing time on carbonation depth. Republished with permission from^[64]; (d) Schematic of the “carbonation shell” effect, illustrating the blocking mechanism of the external dense carbonation layer against internal moisture transport and reaction. Republished with permission from^[60]; (e) Evolutionary comparison of carbonation depths between printed strips and interlayer regions, confirming faster interlayer penetration. Republished with permission from^[60]; (f) Influence of printed strip width on overall carbonation percentage, showing the highest carbonation efficiency for the 10 mm narrow strip. Republished with permission from^[60]. 3DPC: 3D printed concrete.

Upon reaching the optimal moisture content, the components are transferred to a carbonation chamber for accelerated curing. Process parameters during this phase, such as CO₂ concentration, pressure, and curing duration, play a decisive role in the final performance^[64,66]. Notably, excessively prolonged carbonation times may have detrimental effects on mechanical properties. Although extending the carbonation duration typically increases total CO₂ uptake, multiple studies indicate the existence of an optimal duration. For OPC-based materials, a carbonation time of 12 to 16 hours maximizes early strength, whereas carbonation exceeding 22 hours leads to a decline in late-age strength compared to standard curing or results in no significant improvement^[64]. This phenomenon is attributed to the formation of a dense “carbonation shell” caused by over-carbonation, which hinders the infiltration of water into the component interior during the subsequent curing stage, thereby inhibiting the long-term hydration reaction of the unhydrated cement core (Figure 10d)^[60]. Furthermore, in specific binder systems such as calcium sulphoaluminate cement-based engineered cementitious composite, the carbonation process may decompose hydration products that are critical to performance, such as ettringite, leading to a decrease in flexural performance^[62]. Therefore, the post-treatment process must balance the instantaneous gain from the carbonation reaction with long-term hydration development to achieve an optimal solution for full-age performance. From the perspective of process mechanisms, the reinforcement provided by post-printing curing stems from the filling and densification of the microstructure by carbonation products^[59,61,64]. The inherent defect of 3DPC lies in its interlayer interface, a region possessing higher porosity than the dense printed strip interior. The unique advantage of the post-printing CO₂ curing process lies in its “targeted repair” capability. Wang et al.^[60] explicitly pointed out, by comparing the carbonation depths of the strip and the interlayer (Figure 10e), that during the initial stage of carbonation, the CO₂ penetration depth in the porous interlayer region is significantly higher than that inside the printed strip. This implies that the carbonation reaction occurs preferentially at the weakest links. Additionally, Figure 10f further quantifies the size effect, indicating that reducing the printed strip width can significantly enhance the overall carbonation rate. The newly generated carbonation products combine with hydration products migrating to the interface, forming effective “bridging” and “interlocking” microstructures within the interlayer pores^[67], thereby substantially elevating the overall bonding performance.

In summary, these three process pathways present distinct technical trade-offs. Fresh state carbon mineralization effectively regulates rheology to improve buildability, but excessive carbonation can compromise material extrudability. Synchronous carbon sequestration enables real-time performance enhancement, although high-pressure gas injection risks physical damage to the printed filaments. Post-printing carbonation curing provides the most significant improvement in mechanical properties and interlayer bonding, yet its effectiveness is limited by carbonation depth and the necessity of specific pre-curing regimes. Therefore, selecting the appropriate strategy tailored to specific engineering scenarios is essential to balance printability, early-age stability, and long-term performance.

5. Mechanical Performance Regulation Mechanisms via CO2 Curing in 3DPC

5.1 Ordinary cementitious materials

Systems utilizing OPC as the core binder constitute the foundation of concrete additive manufacturing applications. The layer-by-layer deposition process inherent to this technology introduces interlayer interfaces, which represent intrinsic weak links within the structure and result in significant mechanical anisotropy^[68-72]. CCUS technology provides an effective pathway to ameliorate these performance deficiencies through microstructural regulation.

The enhancement of mechanical properties in 3DPC by this technology is particularly profound at early ages. An appropriate carbonation curing regime can achieve a compressive strength increment of up to 39.9% at 3 days (Figure 11a)^[64]. This strengthening effect relies heavily on the optimization of curing process parameters, implying the existence of an optimal curing window. Both insufficient and excessive durations of pre-curing and carbonation are detrimental to strength development. Ideal pre-curing aims to achieve an optimal water loss rate to facilitate CO₂ diffusion. Conversely, excessive carbonation, such as that exceeding 22 hours, may lead to strength reduction due to the excessive consumption of hydration precursors and the hindrance of late-age hydration^[64]. As the curing age increases, the enhancing effect of CO₂ curing on late-age strength diminishes.

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Figure 11. (a) Influence of carbonation duration and pre-curing time on early-age compressive strength^[64]; (b) Evolution of pore volume and size distribution pre- and post-carbonation. Republished with permission from^[64]; (c) Interlayer bond strength under various curing regimes (standard, air, steam, carbonation). Republished with permission from^[61]; (d) Dense micro-morphology post-carbonation curing. Republished with permission from^[61]; (e) Macropore defects under air curing. Republished with permission from^[61]; (f) Loose structure under steam curing. Republished with permission from^[61]; (g) Bond strength enhancement ratio of adjacent and stacked layers via carbonation. Republished with permission from^[60]; (h) Anisotropic distribution of splitting tensile strength enhancement ratio. Republished with permission from^[60]; (i) Anisotropic distribution of compressive strength enhancement ratio. Republished with permission from^[60].

Specifically, the inherently loose and porous nature of the 3DPC interlayer transition zone provides ideal permeation channels for CO₂ gas, enabling carbonation products to effectively fill mesopores and macropores smaller than 200 nm, thereby significantly reducing porosity and the volume fraction of harmful pores (Figure 11b)^[64]. This improvement in pore structure is directly reflected in the comparison of macroscopic interlayer performance. Li et al.^[61] found that compared to standard curing, air curing, and steam curing, the interlayer bond strength after carbonation curing exhibited significant advantages at all ages (Figure 11c). Carbonation curing forms a dense skeletal and shell structure around interfacial pores (Figure 11d), whereas samples under air curing (Figure 11e) and steam curing (Figure 11f) retain numerous macropore defects and loose structures at the interface due to poor hydration environments, which explains the degradation of their interlayer strength.

Research by Wang et al.^[60] further quantified the targeted enhancement effect of carbonation curing on weak links. At an age of 3 days, carbonation curing increased the bond strength of stacked and adjacent layers by 81.9% and 77.1%, respectively (Figure 11g). It is noteworthy that the inherent mechanical anisotropy of 3DPC persists after carbon mineralization. When the X-direction denotes the longitudinal printing direction, the Y-direction represents the lateral direction perpendicular to the printing path, and the Z-direction corresponds to the vertical layer-stacking direction, the Y-direction typically possesses the highest absolute strength influenced by the stacked structure of printed strips. However, the strengthening efficiency of carbon mineralization is most pronounced in the X-direction. Data indicate that at 3 days, the splitting tensile strength in the X-direction increased by up to 97.4% (Figure 11h), and the compressive strength improvement rate in the same direction reached 39.9% (Figure 11i). This unique “anisotropic strengthening” is attributed to the fact that X-direction specimens possess more CO₂-exposed surfaces under the test configuration, and their interlayer interface distribution is more conducive to the deep penetration of the carbonation medium, thereby obtaining the maximum degree of early-stage strengthening effect.

In summary, carbon mineralization serves as an effective strategy for regulating the mechanical properties of OPC-based 3DPC. By generating calcium carbonate fillers in situ within the material, particularly at the loose interlayer interfaces, this technology significantly enhances early compressive, tensile, and interlayer bond strengths. This targeted reinforcement of weak links holds positive significance for ameliorating the anisotropy issues of the material.

5.2 Supplementary cementitious materials

SCMs play a pivotal role in the carbon sequestration systems of 3DPC, exerting a significant influence on the development of mechanical properties during the carbonation process through their unique chemical compositions and reactivity.

As a highly reactive SCM, silica fume exhibits a unique synergistic strengthening effect on the mechanical properties of 3DPC during the carbon sequestration process. Research by Hao et al.^[54] revealed that the incorporation of silica fume could significantly amplify the beneficial effects of fresh state carbon mineralization technology on mechanical properties. This is primarily attributed to silica fume particles serving as nucleation sites that promote the formation of CaCO₃ and monocarboaluminate during the mixing process, thereby improving the early strength development of 3DPC. The study indicates that under identical carbonation conditions, the compressive strength of specimens containing silica fume is significantly higher than that of the control group without silica fume, a trend that is particularly pronounced at early ages. Furthermore, silica fume not only functions within OPC systems but also plays a critical physical role in certain novel cementitious material systems. In a printable paste system based on γ-C₂S developed by Zhong et al.^[59], where strength development relies on the carbonation reaction of calcium silicates, silica fume was utilized as an additive to enhance the consistency and particle packing density of the paste. It was found that this γ-C₂S-based 3D printed material exhibited excellent mechanical properties under carbonation curing conditions, achieving a compressive strength of up to 109.3 MPa and an interlayer bond strength of 4.9 MPa.

The impact of high-volume SCM replacement strategies on the mechanical properties of 3DPC exhibits complex variation patterns. Research by Srinivas et al.^[73] indicates that when OPC is replaced by high volumes, specifically 60% to 75%, of SCMs such as silica fume, limestone powder, and rice husk ash, mechanical performance is compromised to a certain extent; compressive strength decreases by up to 14%, and flexural strength decreases by 36%. Nevertheless, the mechanical properties of these high-volume SCM systems are significantly enhanced through carbonation treatment. It was observed that while carbonation curing enhances material strength by facilitating CaCO₃ formation and refining the pore structure, this increment is insufficient to fully offset the mechanical performance deficit resulting from high-volume replacement relative to pure cement systems. This phenomenon is primarily attributed to the lower CO₂ buffering capacity of high-volume SCM systems, combined with a carbonation mechanism that relies more heavily on the decalcification of C-S-H gel. Such a process may induce pore coarsening, thereby limiting the ultimate improvement in mechanical properties.

The mechanisms by which different types of SCMs influence mechanical properties during the carbonation process exhibit significant differences. A review by Von Greve-Dierfeld et al.^[74] demonstrates that in cementitious materials containing SCMs such as slag, fly ash, silica fume, metakaolin, or limestone, the decalcification and recrystallization of C-S-H gel during carbonation alter the pore structure and compactness of the material, consequently affecting compressive and flexural strengths. In slag-containing cement pastes^[75], a substantial portion of calcium remains sequestered within the unhydrated fraction of the slag during the carbonation process. This distinct reaction mechanism causes slag-containing systems to exhibit mechanical property development patterns that differ from those of OPC during carbon curing. Research by Saillio et al.^[75] further confirms that the presence of diverse SCMs significantly alters carbonation depth and the distribution of carbonation products, thereby influencing the final mechanical properties.

5.3 Fine aggregates

Substituting natural river sand with recycled fine aggregate (RFA) in 3DPC is a key measure to promote the circular economy in the construction industry^[76-78]. However, RFA possesses defects such as high water absorption, low strength, and high angularity due to the porous old cement mortar adhered to its surface, which severely debilitates the workability of 3DPC as well as its mechanical properties and durability of the hardened body^[79-81]. To address this issue, carbonation pretreatment has been proven to be an efficient and environmentally friendly modification method. Its mechanism lies in the dissolution of CO₂ into the pore water on the RFA surface to form carbonic acid, that reacts with Ca(OH)₂ and C-S-H gel in the attached mortar to generate dense calcite crystals which fill surface micro-cracks and pores, thereby reducing water absorption and enhancing compactness and strength^[82,83]. To further intensify this effect, Ding et al.^[82] innovatively adopted a composite method of “saturated calcium hydroxide solution pre-soaking followed by re-carbonation”. By introducing an additional calcium source, it greatly promoted the depth and extent of the carbonation reaction, and experimental results indicate that this method can reduce the water absorption of RFA by up to 53.89% (Figure 12a), laying a solid material foundation for the subsequent performance enhancement of 3DPC. While this multi-step process increases preparation complexity, the utilization of inexpensive calcium hydroxide maintains economic feasibility. Considering that it significantly reduces water absorption to meet the stringent rheological requirements of 3D printing, this approach represents a viable strategy for upgrading recycled aggregates.

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Figure 12. (a) Improvement effect of pre-soaking carbonation on RFA water absorption. Republished with permission from^[82]; (b) Compressive strength of 3DPC in different directions at 100% CRFA replacement ratio. Republished with permission from^[83]; (c) Influence of different aggregates on micro-cracks at the interlayer interface. Republished with permission from^[84]; (d) Effect of aggregate carbonation duration on interlayer shear strength. Republished with permission from^[85]; (e) Effect of aggregate carbonation duration on intralayer shear strength. Republished with permission from^[85]; (f) Comparison of nano-indentation modulus distribution in the interfacial region after 0 and 7 days of carbonation. Republished with permission from^[85]; (g) Micro-morphology of hydration products in the CRFA interfacial transition zone. Republished with permission from^[83]; (h) Influence of CRFA content on mechanical anisotropy coefficients of 3DPC. Republished with permission from^[82]. RFA: recycled fine aggregate; 3DPC: 3D printed concrete; CRFA: carbonated recycled fine aggregate.

The application of carbonated recycled fine aggregate (CRFA) in 3DPC can significantly enhance the mechanical properties after hardening. Sun et al.^[83] found that when natural sand was replaced by 100% CRFA, the 28-day Z-axis compressive strength of 3DPC increased by 68.5% compared to the control group using natural sand (Figure 12b). In addition, carbonation pretreatment not only improved the aggregate itself but also strengthened the weakest interlayer interface in the 3D printed structure. Research by Liu et al.^[84] showed that incorporating CRFA could effectively refine the micro-cracks between printed layers; the crack width in the untreated group was about 15–20 μm, while the crack width in the carbonation-treated group was significantly reduced (Figure 12c), thereby enhancing the interlayer bonding force. Luo et al.^[85] conducted an in-depth exploration from the perspective of carbonation process parameters and found that extending the carbonation time of RFA could continuously improve the interfacial bonding performance of the printed body. Under optimal process conditions, the interlayer shear strength (Figure 12d) and intralayer shear strength (Figure 12e) of the printed specimens at 28 days increased by 21.5% and 18.7%, respectively. Using nano-indentation technology, it was further discovered that carbonation increased the indentation modulus of the interface region while reducing the interface width and porosity, which intuitively proved the densification and strengthening of the interface zone (Figure 12f). Its mechanism lies in the fact that the CaCO₃ generated on the CRFA surface not only repairs aggregate defects but also acts as crystal nuclei during cement hydration, promoting the generation of hydration products and optimizing the interfacial transition zone, thereby enhancing the overall load-bearing capacity of the printed structure (Figure 12g)^[83-85].

In addition, the incorporation of CRFA exhibits positive effects on the early-age performance and mechanical anisotropy of 3DPC. Liu et al.^[84] utilized digital image correlation technology to systematically evaluate the influence of carbonation treatment on the plastic shrinkage of printing mortar. They found that employing CRFA, or introducing CO₂ during mixing and curing, could significantly reduce the water loss rate of the mortar and mitigate the generation of plastic shrinkage cracks, thus ensuring the early integrity and long-term durability of the printed components. Regarding mechanical behavior, the inherent layer-by-layer stacking characteristic of 3D printing leads to significant mechanical anisotropy. Research has found that although carbonation treatment cannot completely eliminate this anisotropy, it can effectively ameliorate it (Figure 12h)^[82]. Printed specimens typically exhibit better toughness when loaded parallel to the printing direction but are prone to brittle failure when loaded vertically. The incorporation of CRFA, by strengthening the interlayer interface, enhances the vertical load-bearing capacity of 3DPC and reduced the performance difference between different directions^[84]. Furthermore, the application of carbon sequestration technology is not limited to the recycling of waste concrete. Research by Butkute et al.^[86] extended this technology to the treatment of municipal solid waste incineration bottom ash. By mixing it with cementitious materials for granulation and subsequent carbonation, they successfully prepared artificial aggregates suitable for 3D printing, providing new insights for the resource utilization of industrial solid waste and carbon utilization and storage.

5.4 Coarse aggregates

The introduction of coarse aggregate into 3DPC is a key step in advancing it from mortar systems to concrete applications, yet it faces challenges such as nozzle blockage, aggregate segregation, and aggravated mechanical anisotropy, and potential durability issues^[87-90]. When using recycled coarse aggregate (RCA), these issues become increasingly prominent due to its larger size and more significant defects and pore structure^[91-93]. Therefore, the modification of RCA is a prerequisite for realizing 3D printed RCA concrete. Tong et al.^[94] adopted a composite modification method involving “carbonation-nano-SiO₂ solution soaking”. This method exerted a synergistic strengthening effect. On one hand, accelerated carbonation generated CaCO₃ on the surface and within the internal pores of RCA, filling defects and enhancing compactness. On the other hand, nano-SiO₂ particles, with their extremely high specific surface area and pozzolanic activity, filled tinier pores and reacted with Ca(OH)₂ in the alkaline environment to generate additional C-S-H gel, further strengthening the RCA surface and the interfacial transition zone where it bonds with the new cement mortar.

This deeply modified carbonated recycled aggregate (CRCA) brought fundamental improvements to the mechanical properties of 3DPC, particularly regarding anisotropy. Systematic research by Tong et al.^[94] indicated that while the mechanical properties of printed bodies showed a declining trend with increasing RCA replacement rates, performance fully recovered after carbonation modification. Crucially, under the limiting condition of 100% RCA replacement, compared to printed bodies using unmodified RCA, the compressive strength of specimens incorporating CRCA increased by 24.71%, 19.38%, and 23.26% in the X, Y, and Z directions, respectively. Similarly, their flexural strength and splitting tensile strength showed significant improvements in all directions. This series of data forcefully proves that carbonation modification of RCA not only enhances the absolute strength of the material but, more importantly, significantly narrows the gap in mechanical properties between different directions. This effectively alleviates the anisotropy problem of 3D printed structures, which is vital for the safety and reliability of printed load-bearing components.

From the perspective of microstructural evolution, this improvement in macro-performance is fully explained. Through X-ray computed tomography technology, researchers were able to non-destructively observe the internal pore structure of the printed body. The results showed that the incorporation of CRCA could significantly optimize the internal pore defects of 3D printed RCA concrete, reduce the overall porosity, and improve the pore size distribution^[94]. The strengthening mechanism is twofold. The physical properties of the modified RCA itself (such as reduced water absorption and decreased crushing index) provided a high-quality skeleton for the printed body, and its surface became dense and rough due to carbonation and nanomaterial filling, which improved physical interlocking and chemical bonding with the new cement matrix. This formed a robust and continuous interfacial transition zone, enhancing the stress transfer efficiency and resistance to deformation and failure of the printed structure. Although the quantity of research on carbonated coarse aggregates in 3DPC is currently lower than that on fine aggregates, existing results have demonstrated that pretreatment of RCA via CCUS technology is an effective technical pathway for preparing high-performance, low-anisotropy 3D printed recycled concrete, offering possibilities for the cyclic utilization of construction waste in the field of intelligent construction.

6. Technical Challenges and Limiting Factors

The integration of concrete additive manufacturing with CCUS technologies faces fundamental challenges rooted in materials science and process control in practice. At its core, the rapid carbonation mechanism exacerbates the intrinsic rheological paradox within the concrete additive manufacturing process. Specifically, the process necessitates high flowability during the pumping and extrusion phases to ensure smooth transport, yet demands the immediate acquisition of high yield stress post-extrusion to resist self-weight and subsequent layer loads, thereby maintaining geometric stability^[95-97]. Although the carbon mineralization process generates rigid CaCO₃ via rapid carbonation reactions, significantly enhancing early-age strength and shape retention of deposited layers, this vigorous surface chemical reaction drastically curtails the open time window available for forming effective interlayer bonds. The rapidly formed dense calcium carbonate layer physically obstructs interlayer interdiffusion and chemical bonding, predisposing the interface to the formation of weak zones. This phenomenon, coupled with the distinct pore defect distribution inherent to the printing process^[98], serves as the primary driver of significant mechanical anisotropy within the structure.

This extreme emphasis on early-age performance is clearly corroborated by the bibliometric analysis presented in Section 2.3. The keyword density visualization (Figure 5) reveals that “strength” and “performance” constitute absolute research hotspots, exhibiting tight linkages with “carbonation” and “3D printing”. Conversely, nodes related to “durability” are situated at the periphery of the co-occurrence network (Figure 3) and display weak connectivity with core process nodes. This underscores a critical challenge in current research. Specifically, the research focus is disproportionately concentrated on achieving instantaneous early-strength enhancement via carbonation, whereas the potential implications of carbon sequestration processes for long-term service performance and durability remain a largely overlooked domain and represent a significant knowledge gap.

Current preliminary research suggests that carbonation processes primarily enhance the durability of 3DPC by densifying the microstructure, forming dense protective layers, and repairing interlayer defects. Regarding resistance to erosion and permeability, the synchronous carbon sequestration during the printing process generates a dense calcium carbonate protective layer in situ on the surface of printed strips, which aids in reducing gas permeability and hindering chloride ion ingress^[55]. Both fresh state carbon mineralization and post-printing carbonation curing fill the matrix pores via calcium carbonate precipitation, thereby reducing the volume of harmful pores and refining the pore size distribution^[52,64]. For the weak interlayer regions, carbonation curing effectively ameliorates the loose interfacial structures resulting from drying or steam curing environments^[61]. In particular, the introduction of a C₂S-based CO₂-activated interlayer enhancer can facilitate the in-situ mineralization of calcium carbonate and silica gel, effectively filling interlayer voids and lowering porosity, thus significantly elevating interlayer compactness and impermeability^[67]. However, carbonation exerts potential adverse effects on shrinkage and expansion behavior and long-term performance. On one hand, while densifying the matrix, carbonation may induce a slight increase in drying shrinkage^[52]; furthermore, in engineered cementitious composites that rely on ettringite to generate micro-expansion for shrinkage compensation, carbonation can decompose ettringite, thereby weakening its expansive performance and increasing the risk of cracking^[62]. On the other hand, excessive carbonation curing can form a dense “carbonation shell” on the surface, obstructing moisture migration to the interior and consequently inhibiting the subsequent hydration reaction of unhydrated cement, which is unfavorable for the sustained development of late-age performance^[64].

The aforementioned contradictions at the material level directly translate into complexity in process control and uncertainty in mechanical performance. To ensure structural stability while preventing the formation of weak interlayer planes, material deposition and carbon sequestration processes must achieve precise spatiotemporal synchronization at the millisecond level, rendering the process window extremely narrow. Any deviation in control may lead to over-carbonation. In this scenario, excessive CO₂ exposure prematurely consumes active cementitious components, such as C₃S and C₂S, which are essential for long-term strength development, and strips away necessary moisture. This inhibits subsequent hydration reactions, ultimately resulting in components that, despite possessing high early strength, exhibit stagnant or even regressive long-term mechanical property growth. Furthermore, carbonation curing is fundamentally a surface-to-interior diffusion-controlled process, which inevitably results in non-uniformity of curing effects, exhibiting pronounced size dependency, particularly in large-scale components. This heterogeneity creates a rigid carbonated outer shell enclosing an insufficiently hydrated inner core. Such a configuration not only causes uneven distribution of internal mechanical properties but may also induce stress concentrations due to differential properties, posing a potential threat to the overall load-bearing capacity and long-term durability of the structure.

7. Development Trends and Technical Outlook

To address the aforementioned challenges, future development trends are propelling this technology toward maturity through systematic synergistic innovation in materials, processes, and design. The bibliometric analysis map in Section 2.3 (Figure 3) provides a clear roadmap for these trends. In terms of materials, although nodes such as “geopolymer” have appeared in the map, their volume remains small, and they are situated at the periphery of the network. This presages a shift in research focus from the current red cluster centered on “portland cement” toward exploring the integration of low-carbon cementitious materials with carbon mineralization technologies.

Current research demonstrates a clear technological evolution in 3D printed low-carbon cementitious materials. Regarding the resource utilization of construction and industrial wastes, materials such as recycled clay brick powder^[99,100] and incineration bottom ash^[101] have been successfully modified to reduce embodied carbon without compromising printability. Subsequently, research has advanced toward the precise design of alternative binder systems tailored for 3D printing. This encompasses not only limestone calcined clay cement optimized for extrudability^[102,103], but also geopolymer concrete modified for large-scale applications^[104], as well as one-part geopolymers designed to further minimize carbon footprints^[105,106]. Moreover, reactive magnesium oxide-based materials^[107] exhibit unique dual advantages: they serve as activators to construct cementless systems for optimizing printing rheology while simultaneously utilizing carbonation reactions to actively sequester CO₂, thereby densifying the structure and enhancing strength. Building on these innovations, future efforts are expected to further synergize these innovative binder matrices with functional fillers possessing special microstructures, such as diatomite^[108,109]. The hierarchical pore structure of diatomite can not only improve the rheology and internal curing effects of the material, but also serve as rapid channels for CO₂ permeation, facilitating more uniform and deeper carbonation to fundamentally resolve the issue of non-uniform curing.

Regarding process control, intelligent closed-loop control is an inevitable trajectory. Although the term “intelligent” has not yet become a hotspot in the current map, the “digital fabrication” node in the green cluster already underscores the significance of process control. Future efforts must integrate multi-modal sensor arrays encompassing vision, thermal imaging, and pressure, and employ artificial intelligence algorithms to analyze real-time data, thereby achieving dynamic modulation of process parameters. This intelligent closed-loop system is pivotal for coping with material and environmental variations, transcending the narrow process window, and ensuring the stability of printing quality. To further validate the long-term reliability of printed components, future research must prioritize quantifying critical durability indicators, particularly chloride ingress resistance to verify the efficacy of microstructure densification, and long-term shrinkage and expansion behavior to assess the risks of cracking.

These technological advancements will ultimately be maximized through an integrated design-material-process approach. The geometric freedom endowed by concrete additive manufacturing makes it possible to design and manufacture structures with complex topological morphologies, such as triply periodic minimal surfaces^[110] or helical surfaces^[111]. Such structures, while substantially reducing material consumption, leverage their exceptionally high specific surface area to transform the carbon sequestration process from a slow, diffusion-dominated process into an efficient, convection-driven process, significantly elevating carbon capture efficiency and economic viability. By synergizing low-carbon materials, efficient intelligent curing processes, and extreme structural optimization design, future buildings are expected to achieve carbon-negative emissions, transitioning from being primary carbon sources to long-term carbon sinks. From a broader perspective, the combination of this technology with in-situ resource utilization provides key technical support for extraterrestrial construction in extreme environments, such as the Moon, by utilizing local CO₂-rich atmospheres, serving as a fundamental cornerstone for sustainable deep space exploration.

8. Conclusions

This review has systematically summarized the process pathways, regulation mechanisms, and development frontiers of CCUS technologies in concrete additive manufacturing. Research indicates that integrating CCUS technology with 3D printing processes is not only an effective means to enhance the performance of printed components and resolve technical bottlenecks, but also provides an innovative pathway for achieving carbon sequestration and carbon-negative construction within the building industry. Based on the analysis presented herein, the main conclusions are summarized as follows:

(1) Bibliometric analysis demonstrates an accelerating growth trend in this field, with research focus shifting from basic printing processes to synergistic optimization via carbon sequestration. While current studies concentrate on “strength” and “performance”, “durability” remains peripheral, highlighting a significant knowledge gap in long-term serviceability research.

(2) Three technical routes are established. Fresh state carbon mineralization regulates rheology to enhance buildability. Synchronous carbon sequestration favors pre-extrusion injection over post-extrusion spraying to avoid physical damage and ensure uniformity. Post-printing carbonation curing relies on pre-drying to control the optimal moisture content for effective CO₂ diffusion.

(3) Carbonation curing enhances early-age properties and ameliorates anisotropy by generating calcium carbonate to fill pores, specifically achieving a targeted enhancement of weak interlayer interfaces. Experimental results show that at 3 days, curing increased stacked layer bond strength by 81.9% and X-direction splitting tensile strength by 97.4%.

(4) Supplementary cementitious materials and recycled aggregates exhibit synergistic effects. Silica fume provides nucleation sites to accelerate mineralization. Carbonation pretreatment modifies RFA by forming dense calcite crystals; notably, using 100% carbonated RFA increased 28-day Z-axis compressive strength by 68.5% compared to natural sand controls.

(5) The field faces bottlenecks regarding curing uniformity and narrow process windows. Future directions include intelligent closed-loop control for dynamic parameter management and integrated structure-material-process design, employing functional fillers like diatomite to achieve efficient carbon sequestration.

Authors contribution

Liu L: Investigation, writing-original draft.

Liu H: Methodology, writing-original draft, funding acquisition.

Ma R: Data curation, visualization.

Wang Z: Conceptualization, analysis.

Wang Z: Supervision.

Liu C: Conceptualization, project administration, supervision, funding acquisition.

Conflicts of interest

Chao Liu is an Editorial Board Member of Journal of Building Design and Environment. Huawei Liu is a Youth Editorial Board Member of Journal of Building Design and Environment. The other authors declare no conflicts of interest.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

Not applicable.

Funding

This study was supported by the National Natural Science Foundation of China (Grant No. 52178251, Grant No. 62276207), the National Natural Science Foundation of China Youth Fund (Grant No. 52408289), the Technology Innovation Guidance Program of Shaanxi Province (Grant No. 2023GXLH-049), Shaanxi Provincial Natural Science Basic Research Program (Grant No. 2025JC-YBMS-550), Qinchuangyuan’s “Scientist and Engineer” Team Building of Shaanxi Province (Grant No. 2023KXJ-242), and the Special Research Program for Local Service of Shaanxi Province (Grant No. 23JC047).

Copyright

© The Author(s) 2026.