3D Printing in Sustainable Buildings: Systematic Review and Applications in the United Arab Emirates

: Three-dimensional printing (3DP) has been rapidly evolving to be one of the leading technology fields in the coming decades. However, as in the early years of new technologies, 3DP suffers from technical limitations and various implications. This study focuses on the applications of 3DP in the construction industry, emphasizing its environmental, financial, and social sustainability aspects. A systematic bibliographic analysis was carried out for the relevant publications which increased by 25-fold from 2014 to 2021. The 3D-printed construction projects in the United Arab Emir-ates (UAE) are presented to demonstrate the early 3DP technology penetration into its booming construction sector. The UAE case is of particular interest from a sustainability perspective due to the environmental footprint of the ample construction activities in the country, combined with the ambitious strategic plans to achieve sustainable development goals. This critical analysis shows that, despite the limited literature, 3DP could potentially be a sustainable alternative with up to 49% less environmental footprint and 78% more cost-effectiveness compared to conventional construction techniques. As the social sustainability aspect was the least addressed, this study discusses relevant social impact indicators and systematic assessment criteria. It is evident that 3DP is already reshaping the future of the built environment, especially in cases where 3DP is advantageous, such as customized designs, quick project delivery, and sustainability-oriented constructions.


Introduction
The three-dimensional printing (3DP), or additive manufacturing (AM), technology was invented over four decades ago and was originally utilized to create prototypes [1].Three-dimensional printing is defined as the process of adding layers on top of each other to print a 3D object designed on computer software.The materials used for printing can be metal, concrete, plastic, ceramic, etc. Three-dimensional printing has become part of everyday practices in the last few years and has solidly emerged in various industries and applications.Recently, 3DP has been utilized to construct buildings; today, it is a reality that a 400 ft 2 house can be built in 24 h, as completed by Apis Cor in Russia.Designers now have the freedom to be creative and design complex shapes that are impractical to build using traditional construction techniques.The integration of 3DP into the construction industry has caused a revolution in productivity, primarily due to the reduced time of the manufacturing and fabrication process.Construction wastes are also reduced as materials can be easily controlled and modified during the manufacturing process.However, the technology still suffers from technical limitations and sustainability issues [2].
Building affordable, smart, and eco-friendly structures is considered the ultimate goal of the modern construction industry.The construction sector is yet considered one of the most resource-consuming and environmentally deteriorating industries [3].The International Energy Agency (IEA) reported 9.7 Gt of CO2 emissions from the construction sector; that is approximately 40% of the global carbon emissions [4].Moreover, the global construction market was reported to have a gross domestic product (GDP) of USD 10.7 trillion, which is expected to increase by USD 4.5 trillion by 2030 [5].The industry has significant social impacts worldwide, for example, increasing the global employment rate by 1.7% between the years 2010 and 2019 [6].Recently, a few review articles explored the sustainability aspects of 3DP in the construction sector.For example, [6] discussed the socioeconomic and environmental aspects of 3DP, [7] covered the performance of the 3DP technology and its environmental impact, and [8] reviewed the socioeconomic and environmental burdens of 3D-printed concrete structures.Moreover, fewer studies have performed a life cycle assessment (LCA) for 3D-printed structures [9].
The United Arab Emirates (UAE) represents a noteworthy case for reflecting on the sustainability of the 3DP technology.The implementation of several large-scale 3Dprinted structures in the UAE has triggered a debate over the sustainability of the technology compared to conventional construction techniques.The World Bank classified the UAE as one of the top 10% countries for carbon emissions from construction and manufacturing activities [10].To mitigate those impacts, local initiatives and action plans were implemented to reduce and mitigate the effects of the construction industry through unconventional techniques such as 3DP.In addition to its environmental implications, the construction industry has been the driving force of the economy in the UAE with USD 5 billion investments in 2015 and numerous prominent landmark projects, particularly in Dubai, its most populous city [11].Moreover, the construction projects were reported to have a 15% contribution to the UAE's GDP in 2017 [12].Furthermore, the social sustainability aspects are of great importance as the UAE is located in the Middle East region, which has unique heritage features and rich cultural values.In addition, the expanding construction industry in the UAE has played a major socioeconomic role by providing 1.64 million job opportunities [13].
The main goal of this study is to assess the sustainability aspects of the 3DP technology and how they affect the architecture and built environments.This review begins with the evolution and applications of 3DP technology in construction, followed by the sustainability assessments of 3D-printed structures.Additionally, due to the limited studies that evaluated 3DP under the social aspect of sustainability, this study discusses wellestablished social indicators and systematic assessment criteria to be considered in future studies.The study focuses on the UAE as a rich case study highlighting its 3DP projects and sustainability profile.The critical analysis is concluded by highlighting the research gaps and future research trends relevant to the sustainability of 3DP in construction.

Review Framework
A review of the literature was conducted in this work to provide a comprehensive analysis of the studies with a particular focus on 3DP in construction, architecture, and the built environment.Figure 1 shows the framework used for planning, conducting, and documenting the present review.An extensive search was performed using Google Scholar, Scopus, Science Direct, and Web of Science multidisciplinary databases with the selected keywords "3D printing technology", "Construction industry", "Additive manufacturing", and "Architecture".The search was narrowed to the time range between 2014 and 2021 (inclusive), and search refinement methods using Boolean operators, parentheses, quotations, etc., were applied.The selection of primary studies included journals, conferences, and review articles.Moreover, out-of-scope papers were excluded based on an initial screening of the title and abstract and further filtered by reading the full text.The final selected primary studies (35 papers) were used to synthesize conclusions and are further discussed in the present work.

Review Statistics
Figure 2 shows the total number of publications that discuss 3DP in the construction industry and architecture on a yearly basis from 2014 to 2021.The overall trend from various database sources is generally increasing, with Science Direct attaining the highest number of publications.The number of publications based on geographic location is summarized in Figure 3. Europe, Asia, and North America have had the most publications in the field of 3DP.Nevertheless, interest has increased in the Middle East, where the number of publications increased by 25-fold from 2014 to 2021.
The current study covers several aspects of 3DP in the realm of architecture.Figure 4 presents the general structure of the current review.The history and evolution of 3DP are first discussed, followed by the potential to integrate 3DP into architecture.Moreover, the three sustainability aspects of 3DP (i.e., environmental, economic, and social) are thoroughly analyzed.Large-scale 3D-printed construction projects in the UAE are presented, and finally, the research gaps and future outlooks of sustainability in 3DP are explored.

Evolution of 3DP in Construction
In the early 2000s, 3DP technology was being prepared for use in the construction industry, and 3D printers were available for printing building materials [14]; refer to the selected projects outlined in Table 1 and shown in Figure 5.In 2006, the University of Southern California printed a wall that was considered its first 3DP application in construction.Later in 2014, a Dutch firm built the first 3D-printed canal house in Amsterdam.This project used a scaled-up 3D printer that was able to build a block of 2 m by 2 m area and 3 m height.Towards environmental sustainability, bio-based printing materials including molten bioplastic, a mix of 80% plant oil reinforced with microfibers, were the material used to print the building blocks.The blocks were then connected to form the four-story house whose designs upheld the traditions of Amsterdam [15].

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The consideration of the social aspect in 3DP.

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The smart applications designed to control the house.

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The larger dining areas created to fit the Chinese cultural needs.

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The first project to claim its sustainability for 100 years.

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The green roof created to give a sense of nature and help residents feel relaxed and calm.[22] In 2015, Winsun, one of the pioneer Chinese 3DP companies, constructed the world's tallest 3D-printed building: a five-story apartment block [18].What distinguishes this building is that different materials were used-a mixture of glass fiber, steel, cement, hardening agents, and recycled construction waste-and large sections were printed separately and connected on-site [23].Unlike the several houses printed in China at that time, this five-story apartment building was fully printed.Later, in 2015, 3DP started to gain a reputation worldwide, reaching the hotel industry, where the first 3D-printed hotel rooms were constructed in the Philippines.The 130 m 2 hotel extension was completed in 100 h using local materials.After the completion of this project, the Philippine government decided to adopt 3DP projects for affordable housing for low-income families.In 2017, Apis Cor printed a 37 m 2 curved shape house in Russia.This was when the technology started to deliver complex shapes in the construction industry.This building was built as a single unit; no transportation and assembly costs were incurred since all components were built on-site.The residential house design was adapted to the environmental conditions of Russia.The house was printed at very low-temperature conditions; geopolymer concrete was the construction material used, and its carbon emissions were reported to be negligible [24].
In 2018, the first 3D-printed project reflecting the bond between innovative technology and nature was launched.The building was printed by a group of students at Washington University in St. Louis and was focused on reducing emissions and material waste.The life cycle impacts of the building were assessed, specific production steps were eliminated, and lighter products were produced.For example, the surface mold could be used up to 100 times, unlike typical wooden molds, which can be used only twice; this has also reduced construction costs.This project particularly considered several social aspects of sustainability.The team developed an application to control all house appliances, cameras, and door systems for a better quality of life.Additionally, the project reflected the Chinese culture, in which dining rooms are the most important living areas; therefore, they were designed to be large to enable family and social gatherings [25].Later, 3DP was demonstrated in a national museum (Madrid), where a replica of the San Pedro de las Dueñas Arch was reproduced with 3DP.This model is exposed to the public, which allows for the preservation of the original [26].
More recently, the Prvok floating house was printed in the Czech Republic and was projected to last for 100 years.Its new features, such as a green roof and its ability to float, were distinct from other projects.A robotic arm was programmed to print the house in 32-48 h, including the internal partitions [22].About 17 tons of concrete mixtures were used after being enriched with nano-polypropylene fibers and other substances to improve the material quality and speed up the drying process [27].Finally, looking into the future of 3DP, Foster + Partners has been working on a project, coined the Mars Habitat, in which robots will be sent to the surface of Mars to build 3D-printed shelters for astronauts, as shown in Figure 6 [28].Overall, based on the aforementioned chronological demonstration of the 3DP evolution in construction, it is clear that solid steps are being taken toward the improvement and development of 3DP technology.Materials, printing techniques, and building structures have all evolved in one way or another.Geopolymer concrete and recycled materials are being used instead of plastic to reduce the overall environmental footprint.Curved walls and complex shapes have been introduced to maximize the competitiveness of the technology.The development and integration of multiple sustainability concepts, in design and/or materials, marked a new stage in the evolution of 3DP and revealed a potential competitive aspect against traditional construction techniques.It is evident that 3DP is already reshaping the future of architecture and the built environment, especially in cases where 3DP is advantageous, such as customized designs, quick project delivery, and sustainability-oriented constructions.

Sustainability of 3D Printing
Sustainability is one of the critical concepts in architecture continuously pursuing the balance between the environmental, economic, and social aspects.The following sections discuss the relevant literature covering the three main pillars of sustainability.

Environmental Footprint
The construction industry is one of the most hazardous industries in the world.Conventional techniques exploit natural resources, generate non-recyclable construction waste, consume large amounts of energy, and threaten the occupational health and safety of workers.As a solution to these problems, the 3DP technology has been introduced as an eco-friendly construction technique.In general, according to the reviewed literature, it is agreed that 3DP significantly reduces harmful impacts on the environment.A reduction in greenhouse gas (GHG) emissions is achieved due to the limited use of construction materials, elimination of waste, reduced energy consumption, and greater production efficiency [29].In addition, the materials used for printing could be recycled and used as a 3D printing mortar with high tensile properties [30] in mixed cement.For example, polylactic acid (PLA) is a frequently used 3DP material that can be recycled and reused because it is biodegradable [24].The environmental aspect of 3DP has positively impacted society; for example, 3D printers produce a low level of noise and improve air quality [31,32].From a geometric design perspective, curved walls can reduce the space needed in buildings [33], so less material is used, although the speed of the printer is rather reduced.
In several studies, environmental impact assessments have been performed to determine the environmental performance of the 3DP technology.References [34][35][36] quantified the environmental impacts of different 3DP applications in construction.As shown in Table 2, the reviewed studies agreed that 3DP is generally eco-friendly as GHG emissions, materials, energy consumption, and waste are all reduced.Moreover, [9] presents a comparative eco-efficiency analysis between a 3D-printed house and a conventionally built counterpart.The results revealed the sustainability of 3DP over the conventional construction technique.The measured global warming potential was found to be 1150 kg CO2 eq for the conventionally constructed house and 608.55 kg CO2 eq for the 3D-printed household.Moreover, non-carcinogenic toxicity impacts were measured in the same study, and the findings revealed the relatively higher value of 675.10 kg 1,4-DCB for the conventional house compared to 11.9 kg 1,4-DCB for the 3D-printed house.

Economic Feasibility
The profitability of the 3DP technology remains questionable, as suggested by [37].However, based on the most recent literature, 3DP could be more cost-effective in construction projects than conventional techniques, saving up to 78% of the construction capital costs [9].A key point of consideration is the high productivity of 3DP; robotic technology has the capability of working round-the-clock, reducing construction time and improving productivity compared to conventional technologies.Winsun reported a 50-70% reduction in the overall construction time [2].Moreover, [38] reported cost savings of 60% in building costs when 3DP was used instead of traditional techniques.In terms of building materials, 3D printers can use a variety of materials, including ceramics, concrete or powdered forms of polymers, metals, nylons, and sand [39], which provides flexibility in the selection and procurement of materials.Moreover, the materials used in construction 3D printers are consumed only as needed.Consequently, there is less consumption of materials; the costs of materials, i.e., tools, machines, molds, etc., are reduced; and less waste is generated-30-60% of construction waste is reduced [40].The overall fixed cost of a construction company will significantly drop if the company applies 3DP techniques [19].
Similar to the environmental aspect, several economic assessment studies were conducted for 3DP in the literature, accounting for the costs of fuel, materials, software, hardware, and waste management [41][42][43].Table 3 presents the findings of selected financial feasibility studies.It can be noticed that manpower cost was not one of the indicators in those studies; however, it is important to consider its consequent reduction in overall project costs.In many cases, construction costs may not be entirely reduced as thought to be; in on-site printing, the transportation and installation costs of the printer place additional financial burdens on the construction companies and consequently affect the cost-effectiveness of the technology.Moreover, international and local suppliers of compatible building materials in such a growing industry remain limited, leading to costly materials that contribute to the overall project costs.

Reference
Input Variable Output Main Conclusion [9] Electricity consumption cost; material cost

Present value (USD)
 Capital cost reduction of 78% for the 3D-printed house compared to conventional construction method.[41] Part design; process parameters Cost (USD)

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Designs that require accuracy are more expensive as they require a lot of material.[42] Software; hardware costs; process parameters; material consumption cost EI * (Pts)  Time and cost of construction are affected by the orientation of the produced parts.[43] Process parameters; energy, material fuel consumption costs.machine and waste management costs

Unit cost (USD)
 A significant reduction in overall cost and cost per part using the AM technology.
* EI: eco-efficiency index, incorporating environmental and financial aspects, reported in points per USD.

Socio-Cultural Impacts
Social impact is defined as the impact on human beings of any private or public actions that change people's way of living [43].The replacement of labor by 3D printers partially eliminates the involvement of humans, leading to fewer injuries and fatalities [44].The reduced labor force will further lead to lower labor costs in construction; according to statistics presented by [40], a 50-80% reduction in labor costs can be achieved.Countries that suffer from a chronic labor shortage, such as Oman, can address this issue by adopting 3DP technology.In addition, one of the social benefits of 3DP is that it potentially leads to cleaner and quieter work sites for the surrounding neighborhood population [45].On the other hand, though less human intervention in 3DP construction is an advantage in terms of reduced costs, jobs will be at high risk, threatening community employment and social stability, particularly for the low-income labor population [24].Moreover, [44] highlighted that the lack of knowledge and skills of the construction workers for this new technology poses another limitation on the workforce involved.Furthermore, the negative occupational health impacts of 3DP arise mainly from the exposure of workers to toxic substances that can lead to eye irritation and allergies [46], as well as the production of lead and nickel as a result of the gases produced [47].
Determining the social impacts of 3DP is not a straightforward task, and hence, it was rarely discussed in the literature.The social impact assessment (SIA) is a systematic framework covering all human, cultural, demographic, and other impacts of technology on society [48].The SIA is an important tool that can identify how communities benefit from any technology, as the community can help reshape the way the development moves forward.The challenge is that no standardized indicators are set for researchers to follow, and each study adopts its own strategy to identify social impacts.For example, [49] used UNEP/SETAC methodological sheets for subcategories in social LCA.[46] reviewed the social impact of AM from a technical point of view.The results showed that customized products improve health and quality of life and achieve a simpler supply chain, which increases efficiency.[50] found that the only relevant study relating 3DP with social LCA is the study proposed in [46].Based on the reviewed literature, no further studies were published to assess the social impacts of 3DP applications in construction.

Social Indicators and Assessment Criteria
According to [51], checklists are not useful in SIA, and each situation should be thought of accordingly to undertake a proper scoping process.Instead, the study identified the following important aspects that should be considered when assessing the social impacts of 3DP:

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People's way of life: Can 3DP affect how people live, work, socialize, and interact with one another on a daily basis? People's culture: Can the adoption of 3DP affect the region's culture?This may include loss of heritage, loss of language, or change in cultural integrity. People's community: Can 3DP change the structure of a family or their social network? People's environment: Can 3DP have an impact on the quality of air and water that people consume?Does it affect the availability and quality of the food they eat?Can it affect their health and safety?Does it have any limitations in local climates? People's health and wellbeing: Can 3DP affect the mental, physical and social wellbeing of the people?Does it cause any spread of disease or infirmity? People's personal and property rights: Are people financially affected by the 3DP technology?Does it affect the government's housing policies? People's fears and aspirations: Does 3DP affect people's perceptions about their safety, fears about the future of their community, and aspirations for their future and the future of their children?
The overall framework of SIA must consider both negative and positive impacts.As shown in Figure 7, at the core of the SIA framework is "wellbeing", which is a broad notion that includes basic human needs, being in good mental and physical health, feeling connected to and part of the local community, and a general feeling of being satisfied with life [52].This framework encompasses all elements needed for a project; e.g., the people's way of life, health, wellbeing, fears, and aspirations are all combined under the "people" dimension.Table 4 summarizes the impact of 3DP on the different social aspects.The following sections discuss the social impacts of the 3DP technology with the aid of the social aspects presented in Table 4.

Aspect Positive Impact Negative Impact
Community  Customized products for consumers.

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Workers in construction sites lose their jobs.

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Intellectual property rights will be an issue and no solid steps are taken towards this issue.

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Low level of noise and air pollution.

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Reduced emitted gases and disposed waste.

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Dust particles in the air.

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Exposure of workers to toxic substances.

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Production of lead and nickel.

Community
The community-level aspects of social sustainability can be perceived through the following points:

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The community is greatly affected by the revolution in the customization of a product or a building whereby people can obtain their exact specifications and expected quality.This improves the relationship between the construction developers and the customer, leading to an increased level of satisfaction.

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According to the United States Department of Labor, one out of ten workers is injured annually in the construction industry [53].The 3DP technology can mitigate this issue as a smaller number of workers is needed; i.e., the number of construction workers who are exposed to the risk of injuries or death will dramatically decrease.In addition, based on the environmental impact assessment performed by [36], emissions from 3DP construction activities are reduced; i.e., workers are not exposed to harmful gases.Accordingly, the occupational health of workers is improved, and their life expectancy is increased.

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In terms of employment, although 3DP technology reduces labor requirements, new jobs are also created in different fields, e.g., specialists in construction material design, software developers, printer operators, and technicians [2]. Three-dimensional printers require minimal human intervention, yet skilled laborers are needed to manage and control them.[54] indicated the 3DP skilled workers are hard to come by.However, training workshops can aid in the acquisition and transfer of the necessary skills.

Culture and Religion
Religion and culture are two important factors that identify communities.The cultural impacts include "all impacts (changes) on the culture or cultures in an affected region, including loss of language, loss of cultural heritage, or a change in the integrity of a culture (the ability of the culture to persist)" [51].Incorporating the local architecture in the interior walls and exterior structures of 3D-printed buildings can effectively reflect the culture of a country for citizens and tourists and help to preserve the local heritage for younger and future generations.

Personal and Property Rights
This social aspect discusses whether people are financially impacted or experience any personal disadvantages.[33] analyzed how 3D printers can be used to build affordable houses in New Mexico.It was found that local communities will not be negatively affected by 3DP.Clearly, the investors and owners of 3DP companies are expected to be financially benefited in every positive way, as the overall cost of 3D-printed structure was shown to be cost-effective from cradle to grave.[46] indicated that 3DP results in a simplified supply chain, which leads to a reduction in cost due to a reduced workforce.
The 3DP technology affirms the availability of 3D printers to large-scale businesses as well as small and medium enterprises in the future.This means that products will be easily customized, leading to a higher rate of production, and the local community will be more satisfied.As a result, the export and import economies will change; countries will not have to import products, which will also affect other countries' trade balances.
An important point to mention is that intellectual property rights will be an issue that must be dealt with; everybody will be able to reprint or replicate a product and use it.[20] recommended suggestions for protecting intellectual property in the AM world, e.g., converting files to STL extension, which changes the structure of the data but not the design.

Summary
Overall, as the 3DP technology continues to evolve and roll out, laborers will develop their education and skills, experience less risk, and have a safer work environment.The public will have a better quality of life with a higher level of customer satisfaction and reduced construction noise and air pollution.Societies will be able to preserve their heritage and culture and materialize their beliefs and unique identities in their built environment.With time, communities will have a positive attitude and trust toward the technology, which will push the development of 3DP even further.

3D Printing Applications in the UAE
As a representative of developing and emerging countries with a booming construction sector, the evolution of the 3DP applications in the United Arab Emirates (UAE) is further discussed herein.The UAE case is of particular interest from a sustainability perspective due to the environmental footprint of the ample construction activities in the country, combined with the ambitious strategic plans to achieve sustainable development goals.The UAE has planned a 2030 vision to ensure that 25% of Dubai's buildings are to be 3D printed by this date [45].

3D-Printed Offices in Dubai
The first 3D-printed offices in Dubai were completed in 2016 in collaboration with Winsun [45].The design structure and construction stages of the offices are presented in Figure 8.The total area of the project was 250 m 2 , consisting of four chambers open to each other to ease accessibility [55].The gantry-based 3D concrete printer used to build the project was 6.6 m tall, 10 m wide, and 40 m long.The structure was designed to adapt to the size of the printer.The printer was only capable of building a low-tensile-strength mixture.This was a challenging step as the concrete had to be supported by adding steel reinforcements between its layers.The material used was a special mix of concrete, glassfiber-reinforced gypsum, and fiber-reinforced plastic [45].A total of 48 h was needed to fully print the four chambers [55].However, it was also reported that the offices took 17 days to be printed and only 2 days to be assembled onsite [56].All interior designs, including furniture, were 3D printed [45].According to Winsun, using 3DP in this project saved up to 80% in construction costs and 60% in labor costs and accounted for a 60% reduction in solid waste [40].

3D-Printed House in Sharjah
A 3D-printed house was constructed in Sharjah with the support of CyBe and the American University of Sharjah (Figure 9).This house is part of the Sharjah Research Technology and Innovation (SRTI) Park initiative, which aims to build a series of 3D-printed houses in the area [57].The house consisted of two bedrooms, a kitchen, and a bathroom [57].It was reported that the costs were reduced, and architects had more design freedom and manufacturing flexibility [58].The building preserves Sharjah's culture and heritage, and the materials used in the house were specified to suit the hot climate of the UAE.

3D-Printed Building in Dubai
The Guinness Book of World Records recorded a 3D-printed building in Dubai as the largest two-story 3D-printed building worldwide.The building, shown in Figure 10, is 9.5 m tall with a total area of 640 m 2 .Curves and complex shapes were printed to examine the capabilities of the 3DP technology.Large spaces were built to be used as offices or rooms.Apis Cor executed the entire project directly on-site [59].The walls were 3D printed using a local gypsum-based mixture, based on a concrete foundation, and then reinforced with typical construction materials.Efficient insulation systems were installed to reduce energy consumption [60].Only three workers were hired, and waste was reduced by 60% compared to typical construction techniques.In total, the construction costs were reduced by 60% [61].

3D-Printed Guest Suites
As shown in Figure 11, 3D-printed concrete guest suites were recently proposed for Emirati homes in Abu-Dhabi, the capital of UAE.The Middle East Architecture Network (MEAN) has prepared three different designs for those suites: the Capsule Pod, for which a new construction method will be designed; the Fluid Space, which includes a separately 3D-printed shell and roof; and the Folded Walls, which uses smooth-textured trapezoidal walls.Each guest suite will have a kitchen, a washroom, and a large accommodation area [62].The MEAN project team will attempt to address many of the 3DP challenges while envisioning new spatial narratives for local guest houses [62,63].

Other 3DP Projects
In 2017, the UAE reproduced one of Syria's cultural landmarks in Dubai to attract tourists.The replica of Syria's historic 1800-year-old Palmyra Arch of Triumph was 3D printed in Dubai [64].More recently, Dubai's Roads and Transport Authority (RTA) announced its plans to implement the 3DP technology in multiple municipal projects such as pedestrian bridges, bus stops, and marine transport stations [65,66].

Sustainability of 3D-Printed Projects in the UAE
In the 3D-printed offices of Dubai, it was stated that hard cutoffs were achieved by Winsun in terms of construction and labor costs and solid waste production.The 3Dprinted house in SRTI park was built in two weeks, and local materials were used, both of which have led to a cheaper and more sustainable construction.Similarly, the 3Dprinted two-story building in Dubai used local materials to boost the local economy, and only three workers were needed, which significantly reduced the labor cost, and the construction costs decreased by 60% [65].
A single systematic study discussed the environmental and economic impacts of 3Dprinted projects in the UAE.[9] presents a comparative eco-efficiency study between the 3D-printed house in SRTI Park and a similar conventionally built single-story house.The scope of the study included the construction stage and the operation stage, particularly the energy consumption.LCA and lifecycle costing (LCC) systematic methods were utilized for environmental and economic impact evaluation, respectively.The findings revealed that the 3D printing scenario was found to be the most eco-efficient, whereas the conventional scenario had approximately 70% higher environmental deterioration impacts.Similarly, the present worth analysis revealed that the conventionally built house incurred double the capital costs of the 3D-printed house.Such results were attributed to the demand for additional material acquisition and manufacturing in the conventional scenario (i.e., steel reinforcement and formwork).
From a social perspective, the UAE has strong attachments to certain religious designs.The UAE strongly values its Islamic heritage and Arabic culture; hence, the adoption of the 3DP technology in the UAE will facilitate the incorporation of such designs at a reasonable cost.With an emphasis on the technology's potential in rapid prototyping and the advantages of having 3D-printed cities in a very short time, this technology might not be the best choice for UAE nationals considering the Arab culture of personalization and privacy.The 3D-printed house in the SRTI park is a key example of a heritage building whose walls and structure clearly reflect the culture of the UAE.To date, the social impacts have not been studied for any of the UAE 3DP projects.

Research Gaps towards Sustainable Development
Sustainability is embraced by empowering mindful technologies such as the 3DP.Through the years, 3DP has developed from printing prototype walls to full-scale buildings.Such a frontier technology should align with the United Nations Sustainable Development Goals (SDGs).One of the SDGs is to have intelligent urban planning that creates safe, resilient, and affordable cities with green and culturally inspiring living conditions.Three-dimensional printing can effectively deliver sustainable houses and cities at a reasonable cost and a smaller environmental footprint, with positive social impacts.However, developing and undeveloped countries may not be able to widely implement 3DP due to limitations such as the high machinery cost, lack of technological base, and limited local capacities.
Industries and global organizations are upgrading and adopting sustainable technologies.It is agreed that the 3DP technology reduces the overall environmental footprint due to the efficient utilization of eco-friendly materials, e.g., geopolymer concrete and recycled materials.However, not all recycled materials can be utilized as their durability and mechanical properties are ultimately affected [67].This could affect the lifetime of the printed buildings and products, particularly under severe circumstances, e.g., acute fluctuations in weather conditions and aggressive environments, and the product may be damaged.Hence, more research studies should be implemented on a variety of materials to ensure their mechanical and physical properties remain unchanged, taking into consideration their relative environmental footprint, cost-effectiveness, and social benefits.There is limited literature covering the environmental, financial, and social impacts of 3DP in construction.Most of the published work was focused on identifying the environmental impacts, whereas only a few papers considered the economic and social aspects.Conceptually, adequate sustainability practices require an even-preferably combined-consideration of all sustainability pillars.Future research should target the combined assessment of the three sustainability aspects, e.g., through socio-eco-efficiency studies.Research should be performed on a variety of large-scale applications and under a wide range of operating conditions to have solid evidence that 3DP is the future of sustainable construction.

Conclusions
This paper presented a systematic literature review including a bibliographic analysis of the 3DP applications in the construction and architecture of the built environment.The critical analysis covered the three aspects of sustainability to assess the potential and impacts of the technology.It was found that the existing research sufficiently covered the environmental aspects; however, there was a lack of studies that addressed the socioeconomic impacts.The study showed the sustainability consideration in the 3DP process within material selections, using bio-based materials with microfibers, and recycled construction waste, in addition to the reduction in CO2 emissions released from construction, and the construction waste and cost were reduced by 60% in many cases compared to typical construction techniques.
This study presented an assessment framework based on multiple social indicators of the 3DP technology.The UAE was presented as a case study for a developing economy with a booming construction industry.The 3DP projects in the UAE were highlighted, and the sustainability profile of those applications was critically analyzed.

Figure 1 .
Figure 1.Framework of the review phase execution.

Figure 2 .
Figure 2. Number of relevant publications between 2014 and 2021.

Figure 3 .
Figure 3. Relevant publications based on geographic location.

Figure 4 .
Figure 4. Schematic diagram of the research framework.

Figure 5 .
Figure 5. Selected 3D-printed construction projects: (a) Offices of the Future in the UAE and (b) curved shape house in Russia [19].

Figure 7 .
Figure 7. Schematic of the social analysis framework.

Figure 9 .
Figure 9.A 3D-printed house in Sharjah Research, Technology and Innovation Park.

Table 1 .
Summary of selected major 3D-printed construction projects.

Table 2 .
Summary of the environmental assessment of the 3DP technology.
* EI: eco-efficiency index, incorporating environmental and financial aspects, reported in points per USD.

Table 3 .
Summary of the economics of 3DP technology in the literature.

Table 4 .
Summary of the social impacts of 3DP technology.