Evaluating the use of recycled fine aggregates in 3D printing: a systematic review

Abstract

This study addresses environmental concerns related to construction and demolition waste (CDW), which constitutes 35–40% of global waste. It explores the potential use of recycled fine aggregates (RFA) from concrete demolition waste as a sustainable alternative to natural fine aggregates in 3D printed mortar (3DPM). By conducting a systematic literature review (SLR) and utilizing VOSviewer for bibliometric analysis, the research assesses the mechanical properties, flowability, extrudability, and buildability of RFAs in 3DPM. The analysis also highlights key trends in keywords and research distribution across different countries. The findings reveal that while RFAs in 3DPM slightly reduce compressive and flexural strengths compared to natural aggregates, they significantly contribute to environmental sustainability by reducing landfill waste and conserving natural resources. The study underscores the importance of further research to optimize RFA-based mixtures for wider application in 3D printing technologies.

Article highlights

Recycled fine aggregates (RFAs) from construction waste are suitable for eco-friendly 3D printing applications.

3DPM with RFAs has reduced mechanical strength but remains viable for structural use.

Optimizing RFA mix designs can enhance both sustainability and performance in 3D-printed concrete structures.

Introduction

The worldwide urbanization and industrialization resulted to significant concern over the global production of Construction and Demolition Waste (CDW) [1]. With the on-going expansion of building operations, the production of CDW, comprising of materials such as concrete, bricks, wood, metals, and glass, has reached record levels. CDW now accounts for around 35–40% of the total waste generated annually. This waste stream is among the largest on a worldwide scale, making a substantial contribution to environmental concerns such as the reduction of resources and excessive utilization of landfills [2]. Typical forms of CDW include Recycled concrete aggregates (RCA), Crushed brick aggregates (CBA), and Ceramic Aggregates (CA) [3]. The production of recycled aggregate concrete, a green concrete product that might help minimize the shortage of sand and gravel sources and the disposal of CDW, is a result of recycling CDW to replace natural resources [4].

Now a days, Construction industry is significantly transitioning away from traditional approaches and towards advanced ones as engineering practices develop toward greater automation and smartness [5]. In the process of providing a solution for construction and demolition waste storage, implementing recycled aggregate over natural aggregate greatly reduces the excessive use of natural resources [4, 6]. This study examined the mechanical performance of utilizing RFA to the 3DPM, the results obtained from relevant research show that the 3D printed mortar with RFA exhibited slightly lower compressive and flexural strengths than NFA [7].

The automated method of creating layer by layer is called 3D printing [7]. The process gets the name "additive manufacturing" as a result, and it is viewed as a key component of "Industries 4.0," or the fourth industrial revolution [8]. Figure 1 shows the advantages of 3D technology over conventional building methods make it a strong option for adoption in the construction industry in recent years. Although the term "additive manufacturing" gained a lot of application in the last ten years, the concept has long been utilized in the construction sector [9]. In industries where traditional building is too complex or difficult, shotcrete and slip forming techniques are frequently employed to create various kinds of structures that make the project's completion schedule unattainable [10].

Fig. 1 [Images not available. See PDF.]

Advantages of 3D printing

In general, 3D printing technology speeds up construction and reduces the need for labour leading to increases the productivity [11]. The 3D printing method will tend to take 25% less time than build a structure in contrast with conventional procedures [12]. This benefit may become even more significant in the event of armed conflicts or extreme weather (hurricanes, earthquakes, etc.), as 3D technology would enable quick reconstruction of damaged housing and infrastructure [13]. Also, CDW waste (zero waste manufacturing) can almost completely reduce waste generation using the application of this technology [14].

The ability to decrease the environmental impact by using less materials as compared to traditional (subtractive) building processes is another significant advantage for ensuring the sustainability of the construction industry [15, 16]. Through the application of the typological optimization concept in 3D printing, structural lightening is made feasible while keeping the MP of the target component and reducing the weight load on structures [17].

An innovative method 3D printing reduces the need for workers and formwork, while also boosting automation and industrialization [11]. As a result, 3D printing has inspired a lot of interest in industries like industrial manufacturing, biological medicine, and engineering building [18]. Looked at the performance specifications, testing parameters, and mixture of 3D printed mortar, all of which support evaluation and growing of the properties of 3D printed mortar, upgrading its properties both when it's fresh and when it's hardened [19]. The absolute practical usage of 3DPM is assessed by its hardened abilities that including interlayer adhesion strength, flexural strength, and hardened compressive strength [20]. The mortar mix and printing method have significant effects on these characteristics [21]. Due to the layer build-up in the vertical direction and the lack of external vibrations, 3DPM bulk exhibits distinct anisotropic characteristics in terms of strength and weak interlayer bonding when compared with a standard one [22].

This review explores the potential of Recycled Fine Aggregates (RFA) as a sustainable alternative to Natural Fine Aggregates (NFA) in construction [23, 24]. It compares the mechanical properties, flowability, extrudability, and buildability of RFA with conventional aggregates while addressing the challenges and limitations of using RFA. The review also highlights innovative strategies to improve RFA for 3D printing applications and advocates for eco-friendly construction by encouraging the recycling and reuse of Construction and Demolition Waste (CDW) [25, 26].

In the next section, we will discuss the materials and procedures employed in this study, which will serve as the basis for the subsequent analysis. Section 3 provides an analysis and discussion of the results of 3D printing, emphasizing significant discoveries and their consequences. Section 4 reviews the mechanical characteristics of materials from different studies, while Sect. 5 reviews their Durability. Section 6 of the paper presents a thorough bibliometric review that provides a full analysis of the area by providing context and identifying trends. Section 7 examines the uses and practical implementation of 3D concrete structures, followed by an evaluation of the economic implications in Sect. 8. Section 9 provides recommendations based on previous findings, while Sect. 10 concludes the article by summarizing the main discoveries and proposing paths for further research.

Method

This study was initiated and adopted a systematic literature review (SLR) as its method. For a thorough coverage of advancements in 3DPM, Fig. 2 shows SLR and analysis of relevant articles was followed in the methodology of this comprehensive research. Scopus database was analysed with the intent to gather a broad spectrum of scientific results, from book sections to reviews and conference proceedings. The inclusion criteria consisted of peer-reviewed journals and conference papers that were published during the last decade. These publications focused on the mechanical properties, printing properties and practical uses of RFA in the field of 3D printing. The exclusion criteria are research that lacked sufficient data or had techniques that were unclear. A grand total of around 150 articles were considered for the purpose of data and bibliometric analysis. They’re utilized various kinds of keywords combinations, such as "3D concrete printing," "Recycled fine aggregates," and "Eco-friendly 3D Printing in Construction" to retrieve the relevant publications. It begins with the selection of Scopus due to its comprehensive coverage of relevant literature, which is narrowed down through carefully chosen keywords to ensure relevance and manageability. The process yielded a significant pool of 2623 documents, highlighting the expansive research interest and on-going developments in these fields. This stage is critical as it sets the groundwork for subsequent analysis. Furthermore, the meticulous removal of duplicates and ineligible articles ensures the integrity and accuracy of the data, which is essential for any robust bibliometric study.

Fig. 2 [Images not available. See PDF.]

Research Methodology

The selected time frame was designed specifically on gathering the most recent advancements and trends in the field. The focus of the study was on investigating the use of RFAs generated from CDW in the context of 3D printing technologies. This study primarily focused around exploring the Mechanical Properties, Durability, Flowability, Buildability and Extrudability of RFA in 3DPM, studying how these characteristics affect 3D printing processes, conducting sustainability assessments, and developing new processing techniques. Criteria were used to exclude publications that did not align with the objectives of the research. Each of these steps, from data gathering to cleaning and analysis, not only underscores the systematic approach taken but also emphasizes the analytical rigor necessary to derive meaningful insights and identify gaps for future research in the innovative applications of 3D printing technologies in construction. In this study, Recycled fine aggregates, OPC & water were used to prepare the 3DPM. The RFA used was obtained by crushing CDW and sieving. Crushed CDW used to replace NFA and the detailed comparison of their physical properties [29].

Bibliometric analysis

Bibliometric analysis is a quantitative assessment of the literature that focuses on identifying links, patterns, and trends in published research on a specific topic [30]. By the examination of data such as authorship, citation rates, publication numbers, and keywords, it offers valuable insights about the evolution and significance of a particular research field. This approach helps in detailing the intellectual structure, tracking its evolution, and showcasing important works and scholars. By using database Scopus, bibliometric analysis makes use of co-citation, co-authorship, and keyword analysis methods to provide a thorough picture of the state of research and to inform future theoretical and practical advances.

A significant advance in bibliometric research has been the application of advanced digital tools and techniques. Some of the sources focus how to analyse bibliometric data using VOSviewer and RStudio Bibliometrix [31]. Application of Nano silica in the construction industry using Methodi Ordinatio [32]. 3D Printing of Concrete: A Systematic Review of Rheology, Mix Design, and Properties [33] it offers a thorough analysis of the features, mix design, and rheology of concrete that is suitable for 3D printing. Additive Manufacturing of Concrete in Construction: Potentials and Challenges of 3DCP [33] this paper highlights advances in technology and their applications to construction while analysing the advantages and disadvantages of 3DPC. It also discusses the limitations of the approaches' scalability, structural integrity, and material characteristics. 3D Printing of Sustainable Concrete: An Overview [34] in this research, sustainable concrete materials for 3DP is reviewed with an emphasis on industrial by-products and recycled resources. It emphasizes the advantages for the environment as well as difficulties with printability and mechanical qualities. Comprehensive analysis of the barriers to effective construction and demolition waste management via RStudio software [35].

Materials and dematerialization making the modern world [36] it examines the use of waste materials in construction, highlighting the advantages to both the environment and the economy in addition to the technical difficulties. It addresses the required technical advancements and evaluates the potential advantages of integrating waste into construction materials. Recent advances in sustainable concrete materials and structures the latest accomplishments in sustainable concrete materials are covered in this investigation, including advances in curing techniques, the use of supplementary cementitious components, and mix design innovations. It also looks at how sustainable concrete behaves mechanically in various environments. A review the utilization of Recycled Aggregates in the manufacturing of concrete is the subject of this review paper's latest improvements. It examines recycled aggregate concrete's durability, MP, and environmental effects and offers case studies of its successful applications. The above-mentioned papers with analysis are the references for Bibliometric analysis.

Data collection

The above graph, Fig. 3 shows the number of papers produced throughout time, taken from Science Direct (Total No. of publications over 1600) until 2023. The evolution of 3D printed constructions utilizing various materials developed over time can be analysed from the figure. An assessment of 3D printing Early in 2000, concrete printed with a 3D printer was made available for construction. Subsequently, robotic 3D printing systems specifically made for concrete construction were introduced in 2015, non-developed curved panels were released in 2020, and printing technologies advanced to provide sustainable materials and expand their use in construction projects. There is a steady phase from early 2000 to 2015 while are less publications, and a rapid phase from 2015 to 2023 while there are more publications and more effectively generated articles because of changes in generation.

Fig. 3 [Images not available. See PDF.]

Number of papers published on Recycled Fine Aggregates (RFA) from Construction & Demolition Waste (CDW) in the last 23 years from science

Results and discussion on 3D printing

Properties of 3D printing

The qualities of the material in both its fresh and hardened states, as well as the printing parameters (nozzle size, extrusion pressure, printing speed, element size and form, etc.), must be taken into account when choosing the material to be printed [37]. Figure 4 shows the generation of CDW and production of 3d printing structures by using RFA from of CDW. The qualities of cementitious materials that make them suitable for 3D printing are fluidity, buildability, sufficient setting time, minimal shrinkage and dimensional stability, and a certain amount of mechanical strength [38].

Fig. 4 [Images not available. See PDF.]

The process of production of 3D printing structures (3DPS)

Flowability

Studies consistently show that RFA from CDW have adequate flowability properties, similar to natural fine aggregates (NFAs), which are critical for maintaining printability and ensuring uniform material deposition in 3D printing applications [39]. Figure 5 shows the testing and computation of flowability by using flow table test. Efforts are being made to improve the flow behaviour and overall quality of printed concrete buildings by refining mix designs and rheological characteristics [40]. Found that controlling is critical for achieving consistent and reliable printing results. [22, 41] discovered that CO2 pre-treatment of recycled aggregates significantly enhanced flowability [42]. Similarly [20], concentrated on adjusting the mix design to improve the flowability of concrete with recycled sand [43], to ensure that the material could pass through the printing nozzle. Table 1 shows the results of flowability by using flow table test by RFA for different studies. The joint efforts in this research underline the importance of striking a compromise between keeping enough flowability and assuring the structural integrity and printability of the concrete mix [44].

Fig. 5 [Images not available. See PDF.]

Flow table test

Table 1. Flowability: type of RFA, percentages added (%), water/cement ratio, number of tamps, flow table index (%), reference

Type of RFA	Percentages added (%)	Water/cement ratio	Number of tamps	Flow table index (%)	References
Recycled powders from construction waste	0, 25, 50, 75, 100	0.4	25	140–180	Duan et al., 2020 [45]
Recycled fine aggregates and powders	0, 30, 50, 100	0.45	25	145–175	Singh et al., 2023 [46]
Recycled sand	0, 20, 40, 60	0.5	25	150–160	Wu et al., 2023 [47]
Recycled fine aggregate	0, 20, 50, 80	0.45	25	135–155	Zou et al., 2021 [40]
CO2 pre-treated recycled fine aggregates	0, 25, 50	0.42	25	140–165	Sun et al., 2022 [22]
Recycled sand	0, 10, 30, 50	0.48	25	140–160	Ding et al., 2021 [48]
Recycled fine aggregate (RFA)	0, 10, 20, 30, 40	0.4	25	145–170	Robayo-Salazar Ret al., 2024 [3]

The type of RFA used exerts a substantial impact on the flowability of the concrete mixture. The study conducted by [45] found that recycled powders generated from building waste have a wider range of flow table index (140–180%), suggesting exceptional flowability. The higher performance can be attributed to the smaller particle size of the recycled powders, which increases the density of the packing and decreases the internal friction within the mixture. On the other hand, RFA often show a lesser flowability, with flow table index values ranging from 135 to 170%. The research conducted by [47] has shown that recycled sand has a reasonable flowability range of 140–160%. This indicates that it is a dependable material for keeping sufficient workability in different building applications. These findings indicate that the use of finer aggregates in RFAs results in improved performance, particularly for applications that need smooth material flow, such as 3D printing.

An important finding from the data is the correlation between higher percentages of RFA replacement and a decrease in flowability. As the percentage of recycled aggregates grows, the flow table index often decreases, indicating the greater need for water and the coarser texture of these aggregates. For instance, [22] observed a decrease in the flow table index from 165% when using 25% CO2 pre-treated RFA to 140% when using 50% replacement. This trend highlights the difficulty of maintaining optimal workability while using substantial quantities of RFA, which is crucial for guaranteeing smooth extrusion and layer deposition in 3D printing. The results further suggests that when using greater replacement levels (75–100%), as shown in the tests conducted by (Duan et al. 2020) and (Singh et al. 2023), the flowability tends to be towards the lower end of the spectrum. This emphasizes the need of making exact mix design modifications to provide sufficient workability.

The water-to-cement W/C ratio is a crucial determinant of the flexibility of concrete mixtures. Reducing the W/C ratios, such as the 0.4 ratio employed by [3, 45], often leads to higher flow table index. This indicates that by making suitable modifications to the mixture design, lower W/C ratios can still provide satisfactory flowability. This discovery is especially significant when utilizing more refined recycled powders or when RFAs undergo pre-treatment to decrease water absorption, since these approaches can assist in preserving workability even with lower water content. In contrast, mixtures with greater W/C ratios, such as 0.48 as utilized by [48], exhibit a moderate level of flowability. Increasing the amount of water enhances the ability of the mixture to flow, but it can also weaken the strength and durability of the mixture. This emphasizes the need for a balanced approach when creating mixtures containing RFA.

Pre-treating RFAs, especially with CO2, has had a beneficial effect on flowability. In their study, [22] showed that pre-treating CO2 may increase the flow table index. The findings varied between 140 to 165% depending on the proportion of replacement. The observed enhancement in the mix's workability is most likely a result of the decreased porosity and water absorption of the RFAs after treatment. This discovery is crucial for situations when there is a need for a high content of RFA without compromising the capacity of the material to flow. It provides a realistic resolution to one of the primary difficulties linked to the utilization of RFAs in concrete. Control mixes with 0% RFA (Recycled Fine Aggregate) in all trials often exhibit greater flowability, as seen by the higher flow table index. This discovery strengthens the idea that substituting natural aggregates with RFA often decreases the ability of a material to be worked with, requiring careful changes in the mixture design to balance this impact. Utilizing superplasticizers, fine fillers, and other admixtures can efficiently preserve or improve the capacity of mixes with high RFA content to flow, hence enabling their efficient utilization in construction applications, such as 3D printing.

Extrudability

RFA from CDW exhibit the comparable extrusion performance to NFA, suggesting their applicability for 3D printing applications [48]. Figure 6 shows schematic diagram of extrusion-based technique for 3DPM. Optimization solutions target to improve the flow uniformity and printability of RFAs in order to provide smooth extrusion and structural integrity in printed concrete [24]. [40, 47] studied the mix design of mortars including RFA to achieve smooth and consistent extrusion [40, 49] especially addressed the issues of plastic shrinkage and cracking by modifying the mortar mix to optimize extrusion. [22] also investigated the application of CO2-pretreated RFA to improve mortar extrudability, which resulted in superior print quality. [48] Addressed extrudability concerns by ensuring that the addition of fibres and RS did not impair the concrete's ability to be extruded smoothly [22]. Table 2 shows that the results of extrudability of different studies for RFA.

Fig. 6 [Images not available. See PDF.]

Schematic Daigram of Extrusion-based technique

Table 2. Extrudability: Nozzle Speed, Nozzle Diameter, Thickness of Layer, Water/Binder Ratio, Reference

Nozzle speed (mm/s)	Nozzle diameter (mm)	Thickness of layer (mm)	Water/binder ratio	References
50–100	2 to 5	5 to 10	0.35–0.40	Alghamdi et al., 2019 [50]
20–60	1.5 to 3	1 to 5	0.40–0.45	Bai et al., 2023 [51]
30–80	2 to 4	5 to 12	0.35–0.40	Dey et al., 2022 [52]
40–70	2 to 5	5 to 10	0.30–0.35	Christen et al., 2022a [53]
60–120	3 to 6	1 to 5	0.35–0.40	Demiral et al., 2022 [54]
40–90	2.5 to 4	5 to 12	0.30–0.35	Zhang et al., 2023 [55]

The ability for extruding concrete is a crucial aspect of 3DP, especially when using RFA generated from CDW. The given parameters for nozzle velocity, nozzle size, layer height, and water/binder proportion provide valuable information for enhancing the extrusion process of these mixtures. For example, the speeds of the nozzle can vary from 20 to 120 mm/s. Studies, such as [54], have shown that higher speeds (60–120 mm/s) can improve production. However, choosing slower velocities within the range of 20–60 mm/s, as proposed by [51], might be advantageous for controlling RFAs. This is because RFAs possess an inconsistent form and exhibit increased water absorption, requiring more precise regulation throughout the extrusion process.

Both nozzle diameter and layer thickness are crucial factors to considered. For instance, [54] demonstrated that employing a nozzle with a wider diameter (3–6 mm) and a thinner layer thickness (1–5 mm) may effectively control the flow and minimise the possibility of clogging when utilising RFAs. This arrangement ensures a constant and uniform extrusion, which is essential for maintaining the structural strength of 3D printed parts. [53] showed that using nozzle sizes of 2–5 mm and layer thicknesses of 5–10 mm achieves a good compromise between flowability and buildability, which is important when dealing with RFAs.

The ratio of water to binder is a crucial component that influences the capacity to extrude. Reported ratios range from 0.30 to 0.45, with higher ratios (e.g., 0.40–0.45 according to [51] often resulting in improved flowability but potentially causing weaker bonding in the printed layers. In contrast, reducing the ratios (e.g., 0.30–0.35 according to [55] may enhance mechanical strength but might also provide difficulties in the extrusion process. Essentially, it is of utmost importance to optimise these parameters, especially when using RFA, to achieve the required stability between the ability to be extruded and the mechanical characteristics of 3DPC.

Buildability

Buildability involves selecting appropriate supplies and creating geometries that work together [56]. Complex forms and minute details are made possible by technology that is not feasible with traditional methods. Ceramics are among the materials available. Size is one factor to take into consideration; large-scale projects call for specialized tools and structural support [57]. According to the complexity and size, build time and cost vary, customization, less waste, and numerous possibilities for creative and effective building across a range of sectors are provided by 3DP [11]. The maximum layers are laid in the analysation of 3DPM is 17 layers mentioned in the Fig. 7 by using RFA from CDW [3]. Table 3 shows the results of buildability for different studies using RFA.

Fig. 7 [Images not available. See PDF.]

Printing of a hollow empty cylinder (17 layers) [3]

Table 3. Buildability: type of RFA, number of layers, water/binder ratio and reference

Type of RFA	Number of layers	Water/binder ratio	References
Recycled sand and PE fibres	–	–	Bai et al., 2023 [58]
Recycled sand	Up to 10 layers	0.40–0.50	Cui et al., 2022 [59]
Crushed concrete	changes	0.35–0.45	Nerella et al., 2020 [60]
Recycled fine aggregates from CDW	17 layers	0.30	Robayo-Salazar R et al., 2024 [3]
Recycled sand	–	–	Ding et al., 2021[61]
Crushed concrete	5–8 layers	0.35–0.50	Wolfs et al., 2018 [62]
CDW-based geo polymer mortars	–	–	Demiral et al., 2022 [63]
Fine recycled aggregates	Specific to build ability assessment	–	De Vlieger et al., 2023 [64]
Recycled sand	10–12 layers	0.38–0.42	Moini et al., 2022 [65]
Various cementitious composites	–	–	Hamidi et al., 2019 [66]
Crushed concrete	8–10 layers	0.37–0.48	Suiker et al., 2020 [67]
Recycled sand	12–15 layers	0.33–0.45	Panda and Tan, 2018 [68]
Various sustainable materials	–	–	Bhattacherjee et al., 2021 [69]

The type of RFA utilized significantly influences the maximum number of layers that may be successfully printed. According to the studies conducted by [59] 68, recycled sand may sustain a varying number of layers, namely between 10 and 15 layers, depending on the water-to-binder ratio (W/B ratio). This suggests that the use of recycled sand has a moderate influence on the ease of construction, allowing for the building of rising structures. On the other hand, the research conducted by [62] and [67] indicates that crushed concrete may accommodate a somewhat a smaller number of layers (5–10) compared to recycled sand. This suggests that although crushed concrete can be utilized efficiently in 3DP, it may not be able to sustain as many layers as recycled sand. The difference may arise from the coarser consistency and uneven form of crushed concrete, perhaps resulting in increased internal friction and reduced layer stability.

The study [3] Showed that RFA from CDW have good buildability. They were able to print up to 17 layers using a low water-to-binder W/B ratio of 0.30. These findings indicate that by using an appropriate combination of materials and closely monitoring the amount of water used, RFAs have the potential to exhibit superior workability, when it is treated with chemicals like pre-soaked lime water [63]. Is demonstrated that CDW-based geo-polymer mortars have a promising performance, indicating the possibility of utilizing recycled materials in 3D printing. However, in certain instances, particular information on layer numbers and W/B ratios was not supplied.

The ratio of water to binder is a critical parameter that significantly affects the capacity to construct. Lower water-to-binder ratios, such as 0.30 as reported by [3], often improve the ease of construction by minimizing the likelihood of structural failure and assuring that the printed layers can withstand added weight without substantial deformation. In contrast, investigations conducted by [59] and [62] have shown that greater water-to-binder (W/B) ratios, specifically ranging from 0.40 to 0.50, might potentially delay the construction process by raising the risk of layer deformation caused by excessive water content. The observed range of W/B ratios (0.30–0.50) indicates that obtaining optimal buildability necessitates a careful stability. Excessive water compromises the strength of the mixture, whereas insufficient water might result in inadequate extrusion and bonding between layers.

The buildability of a structure may be directly determined by the number of layers that can be printed before it becomes structurally unstable. Research indicates that the number of layers supported by different types of RFA, and the W/B ratio varies. Generally, recycled sand may support 10–15 layers, broken concrete allows for 5–10 layers, and recycled fine aggregates from CDW can achieve up to 17 layers. This suggests that the specific kind of RFA and the composition of the mixture used have a significant influence on the capacity to preserve the structural soundness while carrying out the printing process. The increased buildability found in RFAs derived from CDW can be attributable to enhanced particle packing and reduced water absorption, which contribute to the preservation of layer stability.

Cementitious material optimization based on these factors is a difficult undertaking that presents a challenge for material science and engineering since there are two obstacles in the mix design process that must be overcome to simultaneously satisfy the requirements of flowability, buildability, and mechanical strength [70, 71]. The first conflict results from the need of high mixing water content for creating a mix. Low water/cement ratios, however, simultaneously promote a mix's necessary characteristics, such as good buildability and adequate mechanical strength [60].

As soon as the mixing process is in different periods of time, the flowability index can be calculated to evaluate the gradual loss of vitality [72]. In fact, there can be a direct relationship between a cementitious material's working time, or open time, and the mix's setting time. The open period, however, typically shorter than the initial setting time; this is important because of the way flowability loss is impacted by it. Observations on the indirect yield stress computation in relation to this process, taking into account the potential to ascertain the load required for the mixture to begin passing through the nozzle [73]. Before scaling the process to a real printer, it is possible to determine the best variable configuration that maintains continuous printing by linking extrusion speed and pressure, depending on the diameter of the nozzle [74]. Similarly, using this extra technique allows one to visually evaluate the impact of the height between the nozzle and the printing surface, preventing the development of discontinuities (ruptures) caused by stresses during the mix's deposition [75].

Mechanical properties

Compressive strength

The CS of 3D-printed mortar, determining values that depend on printing parameters including printing speed and layer lapses [76, 77]. Together, this research indicates how important process variables are influencing how 3D-printed mortar behaves mechanically. Figure 8 shows schematic diagram of the computation of compressive strength in X, Y and Z-Directions.

Fig. 8 [Images not available. See PDF.]

Schematic diagram of loading of Compressive strength

Several studies have reported compressive strength values for 3DP with RFA from CDW. A review studied the key features of recycled powders, finding CS ranging from 30 to 60 MPa [45]. Investigated RFA and powders in concrete, reaching CS [46] ranging from 25 to 55 MPa. Similarly, assessed geopolymer mortars based on building and demolition waste, with CS [78] ranging from 25 to 50 MPa. Evaluated several activation methods for recycled concrete powder, demonstrating compressive strengths ranging from 30 to 55 MPa. These findings illustrate the range of compressive strengths achievable with recycled fine aggregates, as well as the possibility for their use in 3D printed concrete applications [79], which contributes to sustainable construction methods.

A rapid change in the 3D printed mortar's CS with various RFA replacement amounts. The CS of 3DPM decreases in each test direction as the replacement ratio of RFA increases [22]. The compressive strength drops by 9.4–29.2%, 12.5–33.1%, and 14.9–40.0% in the X, Y, and Z directions as the RFA replacement ratio increases from 0 to 100%, indicating a decrease in RFA performance compared to NFA [80, 81]. By putting more force on the newly mixed mortar to extrude a single filament, the piston removes trapped air gaps in both the longitudinal and lateral printing directions [82]. Figure 9 shows Results of Compressive Strength of 3DPM by using recycled fine aggregates for different studies (a) In X-Direction (b) In X, Y and Z Directions. The lightweight aggregates spread evenly across the sample's height as a result of the core of the sample being continuously dividing into thinner layers [83]. This decreases segregation and the formation of air gaps between the aggregate and matrix, increasing the cementitious matrix and interlayer connection [84].

Fig. 9 [Images not available. See PDF.]

Compressive Strength of 3DPM a In X-Direction [42, 47, 85–88] b In X, Y and Z Direction [22]

The development of compressive strength in 3D printed mortar after 7 and 28 days, as seen throughout various studies. After 7 days, some combinations have a significant initial strength of around 30 MPa, while others begin with a lesser strength of around 15 MPa. After 28 days, the compressive strength of all samples improves, with the most powerful mixes reaching a maximum of 60 MPa and the least powerful reaching roughly 35 MPa. The variation noticed in this case is a direct reflection of the effectiveness of the combination of mix designs and material characteristics utilised. Overall, the results highlight the differences in performance which arise from using different materials and procedures in 3D printed mortar.

Interlayer bond strength

The strength of the binding between successive layers of printed mortar material is referred to as the interlayer bond strength in 3DPM [89]. It is a crucial feature that establishes the general strength and structural integrity of 3D printed items [90]. A solid structure is created in 3D printing by depositing and fusing consecutive layers of mortar material [91]. Figure 10 shows the schematic diagram of computation of interlayer bond strength. Because it guarantees that the layers attach correctly and securely, the interlayer bond strength is essential to producing a sturdy and dependable printed item [92]. Many factors influence the IBS in mortar used in 3DP. These include the composition and characteristics of the mortar mixture, the printing parameters (temperature, speed, etc.), and the bonding mechanisms used in the printing process [93]. The use of strategies, such as surface preparation, appropriate adhesive or bonding material selection, curing condition optimization, and ensuring adequate [76] contact between the layers, can be used to increase the strength of the interlayer connection. By encouraging chemical or mechanical interlocking between the layers, the use of interlayer bonding agents or additives may additionally improve the connection between the layers [76].

Fig. 10 [Images not available. See PDF.]

Schematic diagram of loading Interlayer Bond Strength of 3D Printed Mortar

The Interlayer Bond Strength (IBS) of 3 Dimensional printed mortar (3DPM) with Recycle fine aggregates (RFA) from Construction and demolition waste (CDW). For example, [94] investigated the key features of recycled CDW powders and discovered that integrating these materials into mortar has a substantial impact on the bond strength between consecutive layers. Similarly, [95] investigated the mechanical anisotropy and bonding properties of 3D-printable geopolymer mortars based on CDW, emphasizing variations in interlayer bonding caused by varied processing circumstances and mix designs. Furthermore, [96] evaluated several activation methods for recycled concrete powder, demonstrating that the IBS of 3DPM can be improved by using proper processing procedures that maximize particle packing and hydration. Figure 11 shows the results of Interlayer bond strength of 3DPM (a) In X-Direction (b) In X and Y-Direction for different studies. These findings highlight the importance of material composition, processing parameters, and mix design in influencing the interlayer bond strength of 3DPM created with recycled fine aggregates from CDW, advancing the understanding and development of sustainable construction practices [96].

Fig. 11 [Images not available. See PDF.]

Interlayer bond strength of 3DPM a In X-Direction [88, 97], 93b In X and Y-Direction [22]

The Fig. 11a presents a comparison of the IBS after 28 days as investigated by the studies. This component has an essential part in determining the compressive strength of 3DPM. The study conducted by demonstrates the highest possible bond strength, slightly above 25 MPa, which suggests strong adhesion between layers and potentially greater compressive strength. The study conducted by Singh et al. in 2023 reported bond strength of around 22 MPa, which is a bit lower. In other study, it had lowest bond strength because of the surface of mortar is little smooth in texture. This emphasizes the significance of optimizing the combination of materials and adjusting the printing settings to improve the connection between layers and the overall strength of 3D printed applications.

The Fig. 11b displays the IBS in the X and Y directions at 7 and 28 days, as reported by studies. In the X-direction, the tensile strength of the bond improves from about 1.8 MPa at 7 days to around 2.6 MPa at 28 days, showing an important increase in strength with time. In the vertical direction, the bond strength exhibits a higher value overall, beginning at around 2.7 MPa after 7 days and gradually increasing to about 2.9 MPa after 28 days. These findings indicate that the layers in the Y-direction have a stronger connection between them compared to the layers in the X-direction. This is essential for enhancing the material's ability to withstand compression and maintain its structural stability when it is printed.

Flexural strength

In 3DPM, flexural strength is the material's resistance to bending or deformation under the influence of external forces. In the context of 3D printing, mortar is a specific mixture used in additive manufacturing procedures. Flexural strength is an important feature in 3D printed mortar because it affects the structural integrity and load-bearing capability of the printed products [98]. The printed mortar constructions have high flexural strength, which means they can sustain bending and twisting pressures without breaking or deforming [99]. The composition and ratio of the mortar mixture, the printing parameters, and the curing process are some of the elements that affect the flexural strength of 3D printed mortar [100]. Engineers and researchers can increase the overall performance and longevity of printed buildings as well as the flexural strength of 3DPM by controlling these aspects [101]. Figure 12 shows the schematic diagram of flexural strength of 3DPM in X, Y & Z-Directions. It is frequently evaluated by testing methods like four- or three-point bending tests [102].

Fig. 12 [Images not available. See PDF.]

Schematic diagram of loading Flexural Strength loading of 3DPM

Based on the analysis of the relevant articles, that focused on the flexural strength values of 3DPC using RFA from CDW. Ding et al. (2021) investigated the flexural strengths of 3D printed fibre-reinforced concrete with RS [103] and reported a flexural strength of around 6.5 MPa. Liu et al., 2023: Examined the flexural performance of 3D printed composite beams using engineered cementitious composites (ECC) and RFA concrete [61], attaining a flexural strength of 4.8 MPa. Wu et al. (2023) investigated 3D printed concrete with recycled sand, focusing on pore structures and mechanical properties [104], and discovered a flexural strength range of 5.2 MPa to 6.3 MPa. Investigated the rheology of mortar using RFA for 3DP. The study provides light on the material qualities critical for flexural strength enhancement [69]. Figure 13 shows the results of Flexural Strength for 3DPM (a) In X-Direction (b) In X, Y and Z Directions for the various studies. These studies demonstrate the potential for utilizing RFA from CDW in 3D printed concrete, in flexural strengths typically ranging from 3.4 to 7 Mpa.

Fig. 13 [Images not available. See PDF.]

Flexural Strength of 3DPM a In X-Direction [20, 55, 61, 93] b In X, Y and Z Direction [22]

The Fig. 13a) shows the flexural strength measures taken after a period of 28 days for several research investigations. The readings span from around 3.2 MPa to 4.2 MPa. Documented two almost identical measurements at roughly 3.4 MPa, but Liu et al., 2023, observed a slightly elevated result of around 3.7 MPa. In their study, showed that the flexural strength reached a peak value of around 4.2 MPa, suggesting improved performance compared to the other samples reported a value of around 3.6 MPa. This comparison emphasizes that Singh et al.'s mix design or material selection may provide improved flexural performance, which is essential for enhancing the overall durability and load-bearing capacity of the material.

The Fig. 13b displays the flexural strength measures taken at 7 and 28 days in three different directions (X, Y, and Z) for a study conducted by the studies. The results indicate that the flexural strength gradually improves over time in all directions, with the greatest strength recorded in the Y-Direction after 28 days, reaching around 7 MPa. At 28 days, the Z-Direction exhibits considerable strength, approaching 6 MPa. On the other hand, the X-Direction shows the least amount of resistance to bending, both after 7 and 28 days. This suggests that the material's performance varies depending on the direction.

Durability

A crucial part of building materials is durability, which ensures both structural integrity and long-term process. Durability of 3D printed mortar determines its suitability for various kinds of structures, including load-bearing and non-load bearing [105]. This study gives knowledge about the water absorption, chloride penetration, and abrasion resistance. Research has demonstrated that adding RFA to conventional concrete can result in satisfactory performance, although usually at a reduction of some mechanical property loss when compared to natural aggregates [106]. To reduce these impacts and improve durability, mix design must be properly treated and optimized. It discovered, for instance, that the quality of recycled aggregates is greatly impacted by the crushing process [107], which in turn influences how well they perform in concrete. Described different recycled aggregates and emphasized how much more absorbent of water and porous they were than NA, indicating possible durability issues that may be resolved by mix design and treatment [108].

The addition of 5% Ca(OH)₂ to alkali-activated materials (AAMs) has been found to reduce workability by accelerating the hydration process and the generation of calcium silicate hydrate (C–S–H), which causes a conversion of free water to bound water. This phenomenon is especially crucial in 3D printing applications, as maintaining a constant flowability is necessary to provide reliable and precise prints. To address the decrease in workability that occurs when employing RFA in these applications, various solutions can be applied. An effective strategy is to optimise the mix design by modifying the water-to-cement ratio or including superplasticizers. These additives can improve the flowability of the mixture without affecting its strength. In addition, the use of supplementary cementitious materials (SCMs) such as fly ash or ground granulated blast furnace slag (GGBS) may slow down the process of hydration, resulting in an increased amount of available water in the mixture and enhancing its workability. Pre-treating RFAs, such as by using CO2 pre-treatment, has been shown to improve flowability, reducing the negative impacts of adding Ca(OH)₂. At last, the use of viscosity-modifying agents (VMAs) and retarders can effectively preserve the workability of the mixture by decelerating the hydration process, thereby guaranteeing its suitability for printing throughout the whole process. By employing these tactics, the adverse influence of Ca(OH)₂ on the ease of working may be efficiently controlled, allowing for the successful use of RFAs in 3D printing.

Freeze–thaw resistance is crucial in areas with severe weather, since the repetitive process of freezing and thawing can result in substantial harm to concrete structures, particularly if the RFAs exhibit elevated levels of porosity and water absorption. It is essential to incorporate these features into the mix design to avoid internal cracking and degradation in the long run. In addition, the resistance to sulphate attack is another important factor in terms of durability; especially in situations where the concrete is exposed to soils or water that contain high levels of sulphates. Sulphates can result in the creation of expanding substances inside the concrete structure, which can lead to the development of cracks and a decrease in strength. The utilisation of RFAs, due to their possibly elevated amounts of impurities and uncertainty, might impact the concrete's susceptibility to sulphate attack.

Water absorption

Water absorption in RFAs is an important factor influencing the workability, strength, and durability of concrete in both its fresh and hardened condition [109]. Due to the adhering mortar and micro-cracks, RFAs typically absorb water at a higher rate than natural aggregates [94, 110]. The presence of mortar and micro cracks from the original concrete gives recycled aggregates a higher porosity than natural aggregates [80]. The source and nature of the CDW influence the water absorption characteristics. Table 4 shows the results of water absorption of different studies using RFA with specifications. For example, aggregates from old, high-quality concrete may have lower absorption than those from newer, lower-quality sources. Compared to natural fine aggregates, RFA often absorbs more water [111]. It can vary from 5 to 12 percentages or even more, based on the way the CDW gets processing. RFA made of CDW with a high water absorption rate can have an impact on the MP of 3DPM [112].

Table 4. Water absorption: Type of RFA, Maximum Grain size, Water Absorption (%), Type of Binder Used, Admixtures/Modifiers, Reference

Type of RFA	Maximum grain size mm	Water absorption (%)	Type of binder used	Admixtures/modifiers	References
CW	4.75	10 to 15	OPC	–	Duan et al., 2020 [113]
RFA	0.9	8 to 10	OPC	HPMC, SP	Ding et al., 2021 [61]
RFA	1.18	12 .1	OPC	SG, SP	Sun et al., 2022 [41]
CW	4.75	8 to 10	OPC	SP	Zou et al., 2021 [40]
CW	2.36	13.5	OPC	HPMC, SG NC, SP	Zhang et al., 2022 [93]
CW	1.18	13.5	OPC, FA, MS	SP	Bai et al., 2022 [93]
CW	1.18	13.7	OPC	HPMC, SG, NC, SP	Hao et al., 2022 [114]
CW	0.9	14.1	OPC, FA, MS	HPMC, NC, SP	Liu et al., 2023 [93]
CW	3	8.8	OPC, FA, MS	HPMC, SP	Wu et al., 2023 [20]

CW: Concrete waste, RFA: Recycled fine aggregates, FA: Fly Ash, MS: Micro Silica, HPMC: Hydroxypropylenemethylcellulose, NC: Nano clay, SG: Sodium gluconate, SP:Super plasticizer

The higher water absorption of RFA generated from CDW is mainly attributed to the natural characteristics of these recycled materials. An important issue is the increased porosity and the existence of microcracks in RFAs. The microstructural flaws tend to develop when the crushing and recycling process breaks down the original material into tiny bits. The process increases the surface area and creates additional empty spaces within the aggregates, resulting in an increased ability for absorbing water. For example, [93, 113] found that water absorption levels varied between 10 and 15%. This can be mostly assigned to the RFAs' high porosity and the existence of microcracks.

The presence of attached old mortar on the surfaces of the recycled aggregates is another significant component that contributes to the increased water absorption in RFAs. During the demolition process, the initial mortar that was utilized for bonding the aggregates frequently remains affixed to the recycled components. The porosity of this aged mortar is often larger than that of natural aggregates, resulting in a considerable improvement in the total water absorption capacity of the RFA. Research done by [40] and [93] has emphasized that the presence of residual mortar on RFA is a significant factor contributing to the increased water absorption levels seen in these materials.

In addition, the presence of high porosity, microcracks, and aged mortar contributes to the issue of water absorption, making RFAs less suitable for applications that need minimal water absorption. The investigations also observe that the utilization of different substances and agents, such as superplasticizers (SP), hydroxypropyl methylcellulose (HPMC), and silica fume (SG), can somewhat alleviate these impacts. Nevertheless, the inherent characteristics of RFAs derived from CDW continue to pose a difficulty, hence requiring more investigation into treatment techniques and mix design enhancements to enhance their effectiveness in building applications.

Freeze–thaw resistance

The Freeze–thaw resistance is an important issue in 3D printed concrete, particularly when utilising recycled fine aggregates (RFA) generated from construction and demolition waste. The issue develops because of the increased porosity and water absorption of RFAs, resulting in a greater amount of water being present inside the pores of the concrete. During the process of freeze–thaw cycles, the water within the concrete expands as it freezes. This expansion creates internal stresses that can result in the formation of cracks, spalling, and general deterioration of the concrete structure. To address this issue, air-entraining admixtures are frequently employed, as they generate tiny air bubbles inside the concrete. These air bubbles serve as relief zones, enabling water to expand without causing any harm. In addition, the use of supplementary Cementitious Materials (SCMs) such as fly ash, silica fume, or slag can decrease the permeability and increase the density of the concrete, thereby enhancing its resistance to freezing and thawing. Through careful mix design optimisation and the use of these ingredients, the durability of 3D printed concrete with Recycled Fine Aggregates could be greatly improved in freeze–thaw conditions.

Chloride penetration

The main factors that contribute to chloride penetration in concrete are diffusion, capillary suction, and permeation. These mechanisms enable chloride ions to enter the concrete matrix. The permeability of concrete is a major factor that greatly affects these processes. A high water-to-cement ratio may improve the permeability, resulting in a more porous structure. In addition, the existence of microcracks, frequently induced by drying shrinkage, thermal stresses, or mechanical loads, enhances the permeability of chloride ions. Utilizing RFA generated from CDW could increase the concrete's susceptibility to chloride penetration, mostly because of their naturally greater porosity. Environmental elements, particularly the presence of marine environments or dicing salts, expedite the penetration of chloride, thus raising the susceptibility of concrete buildings to corrosion and reducing their long-term resistance.

These studies specifically investigate the role of recycled fine aggregates (RFA) and the impact of environmental factors. [115] examined the influence of using recycled concrete aggregates on chloride penetration resistance. They found that the higher porosity of RFAs can make them more vulnerable to chloride ingress. [116] examined the diffusion processes in concrete containing RFA and discovered that chloride ions had a higher propensity to permeate through microcracks and holes within the RFA matrix [117]. Investigated the influence of marine settings on the penetration of chloride. They showed that the presence of saltwater speeds up the diffusion of chloride and results in quicker corrosion of reinforcing steel. In their study, Investigated the impact of capillary suction on chloride penetration, with a specific focus on recycled aggregate concrete. The findings revealed that the increased absorption capacity of recycled materials might exacerbate the ingress of chloride. [118] conducted a study on the impact of environmental exposure on the infiltration of chloride in concrete buildings. They highlighted that variables such as freeze–thaw cycles and wet-dry conditions might accelerate the rate of penetration.

Abrasion resistance

Evaluation the longevity of 3D printed mortar that contains recycled fine aggregates (RFA), abrasion resistance is a crucial factor to consider. This characteristic assesses the material's resistance to surface erosion and wear brought on by mechanical motion, such as walking or driving [119, 120]. Maintaining sufficient abrasion resistance for 3D printed mortar, which is being used more often in flooring and establishing applications, guarantees long-term performance and lowers maintenance requirements [121]. Studies have examined how recycled aggregates affect conventional concrete's abrasion resistance [122]. Studies have shown how important aggregate parameters and mix design are to reaching desired durability properties [71]. The findings of these studies highlight the importance of aggregate quality, surface treatment, and additional materials in enhancing abrasion resistance. It guarantees that 3D printed structures are appropriate for uses that requirement durability against wear, such pavement and flooring [123].

Discussion of bibliometric review

Keywords co-occurrence analysis

The key terms that form the basis of the entire network are 3D Printing and Additive Manufacturing shows in Fig. 14, which are represented as the largest nodes in the field due to their central role in the construction of objects layer by layer from digital models [124, 125]. An additional concept which is prominently featured is Concrete that suggests its essential role as the material of choice in many 3D printing construction applications [125]. Table 5 shows the keyword data for 3D printing around worldwide. The interrelation of these core concepts forms the basis of modern construction techniques involving digital fabrication [126].

Fig. 14 [Images not available. See PDF.]

Keywords co-occurrence analysis by VOSviewer

Table 5. Keyword data: Label, Occurrences, Average Publications per Year, Average Citations, and Average Normal Citations

Label	Total link strength	Occurrences	Avg. Pub. year	Avg. citations	Avg. Norm. citations
3d printing	524	582	2021.12	37	1.08
3d printing concrete	33	66	2021.71	23	1.37
Additive manufacturing	330	387	2021.02	47	1.16
Anisotropy	40	41	2022.44	27	2
Buildability	122	118	2021.66	24	1.35
Compressive Strength	55	59	2021.66	17	1.62
Concrete	276	191	2020.70	61	1.14
Construction	81	57	2020.68	34	0.80
Digital construction	35	38	2020.21	67	1.75
Digital fabrication	73	68	2020.86	35	0.94
Durability	53	40	2022.20	25	1.58
Extrusion	70	50	2020.72	50	1.33
Mechanical properties	106	104	2022	35	1.54
Microstructure	41	38	2022.31	23	1.22
Printability	71	61	2022.34	24	1.38
Reinforcement	67	56	2020.82	31	1.03
Rheology	214	194	2021.55	38	1.40
Sustainability	86	78	2022.01	27	2.10
Thixotropy	48	38	2020.73	72	1.34
Yield stress	49	43	2021.40	40	1.46

The green cluster of the network concentrates on terms related to practical applications of 3DP in the construction sector. This includes Extrusion, Reinforcement, Digital Construction, Digital Fabrication, and Construction. These terms draw priority to the complex the 3D printing process that includes everything from beginning with digital creation and manufacturing to the actual construction and reinforcement of structures.

The properties of the material that are so critical to the success of 3DPM revolve around in the blue cluster. Rheology, Buildability, Printability, Thixotropy, and Yield Stress are a few of the key terms in this cluster. As it deals with the flow and deformation behaviour of the mortar, which is crucial to maintaining the stability and form of the printed layers, rheology is very important. Buildability and printability, which reflect how precisely a structure can be put together and how properly the printed material is printed, are both crucial. The features of a material that define its responsiveness to external forces and its ability to recover its shape thixotropy and yield stress are essential for ensuring the durability of printed structures.

The red cluster, including terms like mechanical properties, compressive strength, durability, microstructure, anisotropy, and sustainability, addresses the performance characteristics and sustainability of 3DPS. Ensuring the finished product's structural integrity and load-bearing capability depends heavily on the MP and CS of the printed concrete. Since there are fewer published articles on RFA, the keyword "recycled fine aggregate" is not one of the keyword Co-occurrence analyses in Fig. 14.

The research trends identified through the VOSviewer keyword analysis, such as the focus on 3D printing, sustainability, and mechanical properties, closely correspond to the practical requirements of the construction industry, particularly in its search for inventive, effective, and eco-friendly building techniques. The prominence of terms such as "3D printing" and "concrete" signifies the increasing tendency of the industry towards using advanced technology to improve building procedures, while the emphasis on "mechanical properties" and "durability" addresses the sector's requirement for dependable and resilient materials. However, despite these connections, there could exist a gap in the transformation of research into flexible, practical applications in the real world, especially when it comes to the practicality and actual execution of digital manufacturing methods. This suggests that although there is progress in research related to industry requirements, there is still a difficulty in connecting creative research with its practical implementation on a wide scale in the construction industry.

Country co-occurrence analysis

The key countries, their relationships, and the important contributions they make to the growth of this latest technology are recognized in this paper [8, 32]. Country Co-Occurrence analysis for different countries in 3D printing shows in Fig. 15.The two most notable centres are China and the US, which have extensive research partnerships with many other nations. The United States is a major node that shows close connections to France, Germany, the United Kingdom, and China. In the same way, China's relationships with Germany, India, and the United States show strong networks of cooperation. Germany is also a major player, with strong ties to other major nations in the area of 3DPM research.

Fig. 15 [Images not available. See PDF.]

Country co-occurrence analysis by VOSviewer

The emerging players in the area, such as South Africa, Singapore, South Korea, India, and South Africa, are also highlighted in the graph. Moderately big nodes are shown for these countries, indicating their increasing research output and collaboration efforts. India's growing influence in the global research network is demonstrated by its strong ties to both China and the USA. Countries on the other edge of the network including Poland, Belgium, and the Russian Federation. Table 6 shows the Country data of 3d printing around worldwide. Their presence in the network gives action and potential for future growth indicating lesser contributions.

Table 6. Country data: label, occurrences, average publications per year, average citations, and average normal citations

Label	Documents	Citations	Norm. citations	Avg. Pub. year	Avg. citations	Avg. Norm. citations
Australia	235	13,918	491.19	2021.29	59	2.09
Belgium	94	2978	134.48	2021.52	31	1.43
China	586	11,778	763.99	2022.09	20	1.30
France	120	6057	130.79	2020.8	50	1.09
Germany	243	6253	214.79	2021.09	25	0.88
India	161	2227	183.08	2022.36	13	1.13
Italy	90	1448	83.41	2021.66	16	0.92
Netherlands	127	6931	181.28	2020.41	54	1.42
Poland	56	938	65.17	2021.76		1.16
Russian federation	89	541	34.84	2021.03	6	0.39
Singapore	102	6876	160.44	2020.30	67	1.57
South Africa	66	2279	106.1275	2021.34	34	1.6
South Korea	70	1020	70.80	2021.44	14	1.01
Switzerland	84	3442	83.92	2021.40	40	1
United kingdom	130	3917	188.12	2021.50	30	1.44
United states	336	14,327	381.26	2021.22	42	1.13

Applications and practical implementation of 3D concrete structure

Applications

The 3DPM is a promising technology with numerous potential applications in construction and other fields due to its flexibility, the economy, and speed. Affordable Housing, Disaster Relief Housing, Custom Homes, Bridges, Public Amenities, Retail and Office Spaces, Historical Building Restoration, Green Buildings, Vertical gardens and green walls, Prototyping & Experimentation, Building training facilities, Military and Defence Space exploration, Sculptures and Art projects, Interior Design Elements.

Practical implementation of 3D concrete structure [127]

Milestone project urbanization of houses in 3D (Netherlands)

This Project shown in Fig. 16 uses a specially formulated 3D-printed concrete mix with Portland cement, fine aggregates, and admixtures for optimal viscosity and setting time. For sustainability and thermal efficiency, insulated materials (such as mineral wool or foam) are mixed with recycled materials.

Fig. 16 [Images not available. See PDF.]

Milestone Project Urbanization of houses in 3D (Netherlands)

Castle printed in 3D

The materials are used for the 3D-printed castle Portland cement, fine aggregates, and particular admixtures such super plasticizers for flowability and retarders for controlled setting in the concrete mix. An illustration showing this innovative application is presented in Fig. 17.

Fig. 17 [Images not available. See PDF.]

Castle printed in 3D

3D printed TECLA habital project (Massa Lombarda, Italy)

A natural fibre and geopolymer-based clay combination was used in the TECLA Habitat project as depicted in Fig. 18 showing the complex shapes and forms of the concrete structure. Plasticizers were added to increase workability and stabilizers to maintain structural integrity were one of the changes. A large-scale 3D printer named Crane WASP was used in the study to create the habitat layer by layer.

Fig. 18 [Images not available. See PDF.]

3D printed TECLA habital project in 3D (Massa Lombarda, Italy)

Catilla La park bridge printed in 3D (Madrid, Spain)

A special concrete mix containing Portland cement, aggregates, and admixtures such as super plasticizers for improved flowability and silica fume for strength was utilized for the Castilla-La Mancha Park Bridge shown in Fig. 19. Large-scale 3D printing technology was used to build the bridge, extruding the concrete in precisely the right layers. Steel fibres were used as reinforcement to increase the structural stability.

Fig. 19 [Images not available. See PDF.]

Catilla La Park Bridge printed (Madrid Spain)

The Gaia house, a 3DP prototype (Italy)

The Gaia House is a 3D-printed prototype that was built in Italy in 2018 using a biodegradable mixture of straw, rice husks, and local soil. The structure provides an ecologically friendly and sustainable construction achieved with the use of a Crane WASP printer and printing process as shown in Fig. 20. The purpose of the house's design was to showcase the possibilities of locally and naturally produced materials in modern construction.

Fig. 20 [Images not available. See PDF.]

The Gaia House

3D printed IAAC Pavilion (Barcelona, Spain)

In order to improve flow and adhesion, plasticizers and concrete were combined to create a composite material that was utilized in the IAAC Pavilion in Barcelona. Large-scale robotic 3D printing technology was used to manufacture the structure, allowing complex and accurate designs as depicted in Fig. 21. This innovative approach showed how to create complex architectural structures by combining advanced 3D printing technology using sustainable materials.

Fig. 21 [Images not available. See PDF.]

3D printed IAAC Pavilion (Barcelona, Spain)

World’s largest (Apis Cor) (Dubai)

The concrete mixture used in the world's largest 3DPS, created by Apis Cor in Dubai, was constructed specially using 3D printing technology as represented in Fig. 22 showing its attractiveness and complex geometries that are difficult to achieve with traditional manufacturing methods. In order to ensure structural integrity and durability, the concrete was improved with a variety of admixtures to improve flowability, workability, and setting time. A robotic arm system was utilized by the 3D printing technology to accurately layer the concrete according with the digital model.

Fig. 22 [Images not available. See PDF.]

World’s largest 3D Printed House (Apis Cor) (Dubai)

TOVA

The first 3D-printed building made entirely of regionally produced natural materials was the TOVA project, which made use of a combination of straw, sand, and clay to achieve sustainability as presented in Fig. 23. Layer by layer, the material was deposited using a massive robotic arm powered by 3D printing technology, which ensured accuracy and uniformity. Natural fibres were added as reinforcement, and a tiny bit of lime were added to improve the binding qualities of the material.

Fig. 23 [Images not available. See PDF.]

TOVA

Bridge (Nijmegen, the Netherlands)

In Nijmegen, the Netherlands, a reinforced concrete mixture created especially for 3D printing was utilized for the pedestrian bridge. By precisely laying the concrete layers using a robotic arm, complex geometries can be produced without the use traditional formwork as shown in Fig. 24. The printed layers were reinforced with steel to improve structural durability and strength. This innovative construction technique lowered the amount of material waste and reduced the project's duration.

Fig. 24 [Images not available. See PDF.]

Bridge, Nijmegen

Two storey house (Beckum, Germany)

The two-story home in Beckum, Germany, used a particular mix of concrete that was enhanced for 3D printing and included additives to enhance the properties of flow and setting. A large-scale 3D concrete printer manufactured by PERI was the construction technology employed, and it carefully stacked the concrete in accordance with digital strategies as depicted in Fig. 25. To provide structural stability, steel bars placed both vertically and horizontally were used to strengthen the printed walls. This technique made construction more efficient while using less labour and waste.

Fig. 25 [Images not available. See PDF.]

Two storey house

House Zero (East Austin, USA, 2022)

House Zero in East Austin, USA, constructed in 2022, utilized a proprietary concrete blend called Lava Crete, specifically designed for 3D printing and shown in Fig. 26. ICON's Vulcan printer was used in the construction process to accurately layer Lava Crete to create the walls of the building. Additives were included in the concrete mix to improve the qualities of flow, bonding, and curing. This creative method allowed for quick, environmentally friendly construction that also minimized material waste and offered a great degree of design freedom.

Fig. 26 [Images not available. See PDF.]

House Zero, East Austin

Economic implications

When examining the economic consequences of using 3D printed mortar created from recycled fine aggregates (RFA) obtained from building and demolition waste, it is essential to consider three main factors: cost reduction, extra expenditures, and comparative cost-efficiency. The purpose of this discussion is to conduct a thorough examination of the impact of integrating recycled aggregates into 3D printing on project finances. An analysis of the possible financial benefits, such as reduced expenses for raw materials and lower fees for waste disposal, in addition to the costs related to establishing and maintaining the technology, provides a more distinct comprehension of the economic feasibility of this new approach.

A key advantage is cost savings, as recycled fine aggregates are often cheaper than new materials, resulting in savings on raw materials. In addition, the recycling of CDW helps in reducing waste disposal costs and reducing regulation costs. However, the use of 3D printing technology involves significant additional spending, such as the considerable initial expenditure for purchasing 3D printers, the related software, and staff training, along with ongoing expenses for ensuring quality control and maintenance. However, it is necessary to evaluate the cost-effectiveness of 3DPM with recycled aggregates compared to traditional methods, even with the costs involved. While the initial demands may be more, the long-term advantages, such as fewer workers and material waste, have the potential to pay for these costs, making 3D printing a financially feasible alternative for conventional building techniques.

The economic implications of using 3D printed mortar made from recycled fine aggregates (RFA) encompass significant potential for cost savings, additional expenses, and comparative cost-effectiveness. Cost savings arise primarily from the lower cost of recycled aggregates compared to virgin materials, as evidenced by the review of which highlights reduced raw material costs and the financial advantages of recycling construction and demolition waste. This reduction in material costs is complemented by lower waste disposal expenses, which further enhance the economic viability of utilizing recycled aggregates in 3D printing. However, the adoption of 3D printing technology introduces additional expenses, including the high initial investment required for 3D printers, related software, and the training of personnel. Note that these upfront costs, along with on-going expenses for quality control and process optimization, can significantly impact the overall expenditure. Despite these costs, comparative cost-effectiveness plays a crucial role in evaluating the technology’s financial viability. Provided a detailed analysis demonstrating that while the initial costs of 3D printing technology may be higher, the long-term benefits, such as reduced labour requirements and material waste can offset these expenses. By comparing these factors with traditional construction methods, it becomes apparent that 3D printing with recycled aggregates may present a financially advantageous alternative, particularly when considering the lifecycle savings and sustainability benefits.

Recommendations

To enhance the mechanical characteristics, specifically the compressive strength, and guarantee a high level of flowability in 3D printed concrete, it is essential to employ a careful methodology which involves several admixtures and modifications to the mix design. Incorporating supplementary cementitious ingredients like fly ash and micro silica is crucial for improving the mechanical properties of 3DPM. Fly ash, a substance with pozzolanic properties, enhances the long-term durability of the material and decreases the heat generated during the hydration process, resulting in a more manageable mixture. Micro silica, known for its tiny particles, occupies the empty spaces among cement particles, resulting in a more compact microstructure and enhanced compressive strength. When utilized in suitable ratios, these components have the potential to greatly improve the efficiency of 3D printed constructions.

Superplasticizers are crucial for attaining a high level of fluidity, ensuring seamless extrusion, and accomplishing accurate layer deposition in 3D printing. These additives decrease the amount of water while preserving the capacity to be worked with, which is crucial for attaining the correct texture and minimizing the chance of separation during the printing process. Modifying the water-to-binder ratio is equally crucial; a decreased ratio often enhances strength and durability but may decrease flowability. Consequently, fine fillers are employed to achieve a balanced mixture by filling in gaps and enhancing the overall cohesiveness of the mixture. This is particularly useful during the extrusion process. Pre-conditioning RFA using carbon dioxide (CO2) is a novel technique that has demonstrated a substantial enhancement in the flowability of mortar. Carbonation decreases the porosity and water absorption of RFAs, hence enhancing the mix's workability and performance in 3DP applications. This approach addresses a primary obstacle associated with the use of RFAs, which is their increased water requirement resulting from their porous characteristics. To enhance extrudability, including fibres and targeted additives into the mixture enhances cohesion, guaranteeing seamless extrusion and deposition of the material without any segregation or blockages. Ensuring precise and consistent extrusion requires careful adjustment of the nozzle's speed, diameter, and layer thickness, together with the proper configuration of extrusion pressure and speed. The balance between these parameters guarantees that the printed layers retain their form and structural soundness throughout the construction procedure. Optimizing the water/binder ratio and using stabilizing agents are crucial for enhancing buildability. These modifications aid in preserving the form of the printed layers by preventing sagging or deformation, so guaranteeing that the structure can support new layers without compromising its stability. An ideal blend composition, which includes the precise quantity of water and binder, guarantees that the substance solidifies and becomes rigid at the suitable pace to provide support for subsequent layers throughout the printing procedure. Enhancing durability, especially in the context of RFAs, can be achieved by formulating the mixture in a way that minimizes porosity and water absorption. One way to do this is by using admixtures like super plasticizers and water-reducing agents, which decrease the total water content of the combination while preserving its capacity to be worked with. The long-term durability of the 3D printed structure may be greatly improved by precisely modifying the mix design and treating RFAs to minimize their water absorption.

To investigate the performance declines found while employing recycled fine aggregates (RFAs) in comparison to natural aggregates, it is necessary to conduct thorough experiments to comprehend the impacts of various RFA replacement ratios. One might also consider using fibers or chemical admixtures to improve both the compressive and tensile strengths. The objective of these studies should be to create mix designs that are ideally balanced in order to enhance both buildability and structural integrity. The primary focus should be on minimizing distortion during the printing process. In addition, investigating new additives and treatments that decrease the porosity and water absorption of RFAs will improve the material's ability to withstand environmental factors like abrasion and chloride penetration. This will ultimately extend the lifespan and durability of the 3D printed structures.

Conclusions

The purpose of this review is to analyse the possibility, challenges, and possible future developments of using RFA in 3DP by combining the results from various studies. Several studies focus on the value of mix design and the fresh properties of the material when it comes to 3DP. The workability and mechanical characteristics of the printed concrete are affected by the addition of RFA. Optimal mix designs are required in order to ensure adequate flowability and structural stability during printing. When proper mix adjustments are made, inclusion of RFA can yield acceptable workability and buildability for 3D printing applications. RFA can affect the flowability of mortar due to high water absorption, due to this there is effect on buildability. The MP of 3DPC with RFA is crucial for its structural applications. Mortar using RFA can have compressive strengths that are like those that exist for conventional aggregates, while performance may be enhanced by adding fibers or chemical admixtures. The quality and treatment of the recycled aggregates influence the flexural properties and overall durability of concrete by using RFA. Through waste reduction and resource conservation, the usage of RFA from CDW significantly increases sustainability of buildings and development of new additives and treatments, and the environmental impact of using RFA in 3D printed construction. 3D printed mortar using RFA from CDW exhibits satisfactory durability for various applications, though some performance metrics are slightly lower than those using natural aggregates. Optimizing the processing of RFA, mix design, and additive use can significantly enhance durability. A workable and sustainable approach is to incorporate RFA from CDW into 3DPM. Although understanding of material properties, mix design, and mechanical performance has improved significantly, issues related to aggregate quality variations and long-term durability need additional research. Through further growth, RFA could become a widely used material in the construction industry, supporting more eco-friendly and effective methods of construction. A workable and environmentally friendly solution is to incorporate RFA from CDW into 3DPM.

Author contributions

VRP: Conceptualization, Methodology, Investigation, Formal analysis, Validation, Writing—Original draft. NG: Conceptualization, Methodology, Investigation, Formal analysis, Validation, Writing—Original draft. DR: Conceptualization, Methodology, Investigation, Validation, Writing—Original draft. GUA: Conceptualization, Methodology, Investigation, Writing—Original draft.

Funding

The authors declare they have no financial interests.

Data availability

The datasets used and/or analyses during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Abbreviations

RFA

Recycled fine aggregates

3DPM

3 Dimensional Printed Mortar

NFA

Natural fine aggregates

IBS

Interlayer Bond strength

3DPS

3 Dimensional Printed structures

RS

Recycled sand

CDW

Construction and Demolition waste

3DP

3D printing

CS

Compressive strength

MP

Mechanical properties

OPC

Ordinary Portland cement

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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