Introduction

Building structures are an example where great environmental and economic benefits can be obtained by optimizing the shape of structural elements according to the specific load scenario for each building. Structures are, in general, the most material-intensive layer in buildings, representing up to 75% of their total weight [1]. Due to the types of materials commonly used in buildings today, the higher the mass concentration, the higher the embodied carbon emissions [2]. An effective strategy used to reduce these emissions is to optimize the shape of structural elements to reduce their mass. Generally, this is achieved by subtracting material where it is not structurally necessary and adopting curved shapes that better accommodate the flow of internal stresses in the structure.

Another area where customizing the shape of building elements can bring significant benefits is the possibility of incorporating additional functionalities. An example of this is the possibility of improving the acoustic insulation of a building element without adding insulating elements but optimizing its shape [3]. Another example is the optimization of perforation percentage, pattern, and shape of perforated solar façades for improving interior daylight conditions and thermal performance [4].

Although the benefits of customizing the shape of constructive concrete elements can bring multiple benefits due to their complex and often unique shapes, their fabrication still presents challenges with conventional formwork strategies. The formwork used for the fabrication of geometrically complex elements is typically single-use and made of timber or milled foam [5]. The fabrication of custom formwork is often time-consuming, and the costs of these can reach up to 70% of the total cost of the structure [6]. To the above, it should be added that as they are usually single-use formwork, they end up being disposed of after use and transformed into waste.

3D printing (3DP) formwork systems can be a more economical and environmentally friendly alternative to conventional formwork systems for shaping geometrically complex concrete elements. An advantage of using 3DP formwork is the possibility to print highly complex geometrical molds with almost no waste and use a minimum amount of material. Another advantage is the possibility of including additional functionalities with subsystems, such as alignment details and installation ducts. Additional functionality can be integrated by bespoke formwork systems when they are 3D-printed with porous materials. Foam 3D printing (F3DP) enables the automated and waste-free production of geometrically complex stay-in-place functional formwork that can improve the thermal and acoustic properties of the resulting composite building element while reducing the amount of concrete used and the resulting total weight.

This paper presents a novel approach for creating bespoke foam components with F3DP that can be used as functional stay-in-place formwork for material-efficient and lightweight concrete composite elements. After the state-of-the-art of innovative formwork studies and F3DP, the proposed material and robotic fabrication system are described along with two experimental case studies. The first study investigates the use of F3DP with sustainable mineral foam for fabricating brick-sized modules, which can be assembled into an architectural-scale structure as a lightweight facade application [7]. This paper extends the scope of this research and highlights F3DP with mineral foam as a promising formwork technique for concrete casting to produce highly sustainable mineral composite building elements. A second study uses the same fabrication configuration but investigates the use of F3DP for fabricating the formwork for a compression-only funicular floor concrete structure [8,9,10].

The paper concludes the challenges and findings of both studies and elaborates on the implementation of F3DP for lost formwork on large-scale building elements and future improvements in the fabrication technology.

State of the art

Innovative formwork studies

In recent years, innovative concrete formwork systems have been developed that could be a more economical and sustainable alternative to traditional formwork systems for the fabrication of geometrically complex constructive elements. These techniques can be distinguished by being removable, or stay-in-place lost formwork solutions.

The first group includes studies that use 3DP of complex molds from sand, thermoplastics, or clay [11]. Binderjet sand 3DP was used to produce the molds for a lightweight ribbed slab that was executed with sprayed and cast concrete [12]. Ultra-thin thermoplastic 3DP was used simultaneously with fast-curing set-on-demand concrete to produce a geometrically complex reinforced concrete column [13]. In a similar study, the thin thermoplastic formwork was dissolved with water after the conventional concrete had cured sufficiently [14].

The second group includes studies that use textile formwork, concrete 3DP, and thermoplastics 3DP as stay-in-place formwork. Popescu proposes a stay-in-place formwork system based on the use of knitted fabrics [5]. In this system, the fabric is hung and stretched using a rigid falsework. Once in position, the fabric is stiffened with a cement mortar coating and once cured, this coating is used as formwork for the subsequent pouring of the concrete.

A series of columns of complex shapes and different geometrical features could be fabricated using concrete 3DP [15]. The base is cast, and the outer shell of each of these columns is 3D printed as stay-in-place formwork. The cores are then filled with steel reinforcement and concrete to form a monolithic column. 3DP of thermoplastics was used to integrate additional functionality into an arched funicular slab [16]. The printed shells create vent-shaped voids within a larger volume of concrete and activate the thermal mass for cooling and heating purposes.

Foam 3D printing

For creating interior voids in lightweight and material-optimized concrete slabs, standardized volumetric elements such as plastic forms or insulated concrete forms can be used (Fig. 1). Hollow-core slabs are commonly used for standardized, high-volume construction applications, for example, in multi-story residential buildings. In contrast, for non-standard applications, custom-cut foam elements are most commonly used as formwork [8]. They are assembled from modular foam blocks that can be machined with subtractive processes such as milling and hot-wire cutting. Here, significant productivity advancements were achieved by using robotic hot-wire cutting that can save up to 80% machine time when compared to conventional CNC milling [17]. However, these application methods of foams for bespoke formwork use materials with high embodied carbon, such as Polystyrene, and are very wasteful through the resulting chipping and offcuts.

Fig. 1
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Schematic section of a conventional weight-optimized slab, using stay-in-place elements to create interior voids

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In contrast, F3DP introduces the use of porous construction materials for the automated, waste-free materialization of complex shapes [18]. This enables the fabrication of stay-in-place formwork for custom geometrically optimized building elements that benefit from the excellent heat and sound insulating properties of foams, particularly for wall and slab elements. Thus, F3DP makes novel composite structures possible that are lightweight, easier to transport and handle, use less material, and are produced with a higher degree of labor efficiency and work safety.

Several studies investigated F3DP for lost formwork. Expanding polyurethane foam was used to create the stay-in-place formwork for a dome structure [19] and a single-story residential building [20]. Other researchers printed cement foams on existing masonry walls to improve their insulation performance [21]. Furthermore, foamed concrete mix designs and production techniques were investigated for large-scale 3DP [22]. More sustainable cement-free mineral foams were developed recently, and it was demonstrated that they could be used for 3DP [7, 23, 24].

Material and fabrication system

Mineral foam

Foam materials in construction can be distinguished by their inorganic or organic resourcing and their natural or synthetic processing. Today, synthetic organic foams such as polystyrene (EPS, XPS) and polyurethane (PU) constitute more than 41% of the European building thermal insulation market [25]. Although they dominate the industry due to their low cost, outstanding thermal performance, and durability, their petrochemical resourcing and inherent flammability render them unsuitable as future construction materials. In contrast, mineral foams provide a sustainable alternative, and novel cement-free solutions have been developed recently [23].

One such novel formulation is used in this research and is developed by FenX AG [26]. For producing the mineral foam, industrial waste-based fly ash particles are mixed with water and modifiers. The resulting suspension is then vigorously foamed using a conventional household mixer. After five minutes of mixing, the foam is filled into portable printing cartridges. Eventually, foams with wet densities ranging between 500 and 550 kg/m3 are obtained. After printing, the elements are dried at room temperature for a minimum of 48 h. After that, they are hardened through sintering in a ceramic kiln. The sintering process consists of two steps: a burnout period (450 °C for two h) and a sintering period (1100 °C for three h). The elements are then cooled down to room temperature and achieve a dry density of 350 kg/m3.

Robotic fabrication

The robotic F3DP uses a stationary custom-made syringe extruder (Fig. 2a), which was developed in previous studies investigating the extrusion of mineral foams. The extruder was developed independently from any kinematic system without payload or movement constraints. For both studies in this paper, an ABB IRB 120 is used as a kinematic system for F3DP. This 6-axis robotic arm has a maximum reach of 580 mm and a payload of 3 kg. A 400 × 400 mm platform is mounted to the robot flange as a moving print bed. Here, the fifth robot axis is oriented downward to maximize the payload performance (Fig. 2b). On the platform, 300 × 300 × 5 mm sintering plates are placed for transport and furnace processing. They consist of recrystallized silicon carbide (R-SiC), which makes them particularly durable and lightweight. Conventional cordierite sinter plates would have exceeded the robot payload limitations during the printing process.

Fig. 2
figure 2

a dual syringe extruder with replaceable cartridges and nozzles. b robotic fabrication setup with (1) ABB IRB 120, (2) print bed end-effector, and (3) stationary syringe extruder

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The syringe extruder was specifically designed for the precise large-scale extrusion of semi-fluid material that can be batch-processed. The development was influenced by numerous examples that are present in the research literature on paste extrusion [27, 28]. The extruder is designed as a large-scale double-chamber syringe that is actuated by two independent linear drives. This allows the precise flow rate control and operation of one or both chambers. Each of the chambers can be filled with cartridges measuring 90 mm in diameter and 500 mm in height, each with a volume of 3.2 L. The linear drives are controlled with an Arduino Uno microcontroller. All extruder operations, such as start, stop, speed, and homing, can be executed with a GUI application written in Java. The extruder works independently from the robot controller, and no signals are exchanged between them.

Toolpaths are generated parametrically in the CAD software Rhinoceros3D through the Grasshopper Plugin and the use of the COMPAS framework core library [29]. Print geometries can be selected, and different toolpath solutions computed and visualized using a custom Grasshopper Python script (Fig. 3). Robot target frames are then parsed and exported as a JSON file. The robot movement is programmed using the COMPAS framework and the COMPAS_RRC extension. During fabrication, a python script is responsible for establishing the online connection to the robot controller, parsing the JSON robot targets, sending them to the controller, and handling feedback.

Fig. 3
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Parametric toolpath generation and visualization in Rhino3D Grasshopper using the COMPAS framework

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Initial exploration

The first phase of print experiments aimed to identify the suitable print resolution that optimizes the extrusion quality and the resulting surface smoothness of the specimen (Fig. 4, first row). To achieve this, different layer widths were tested, which is the result of the interplay of nozzle size and print parameters such as extruder flow rate, robot speed, and layer height. The first nozzle tested measured 10 mm in diameter and was used with a 7 mm layer height, a flow rate of 3 ml/s, and a robot speed of 50 mm/s. The resulting layer measured 15 mm in width and featured a rough surface quality due to over-extrusion. The second nozzle diameter measured 12 mm and was used with an 8 mm layer height, a flow rate of 1 ml/s, and a robot speed of 10 mm/s. Here, the results showed an 18 mm layer width and fewer disturbances of the material microstructure with overall improved filament quality and a smoother surface of the print specimen. The third nozzle diameter measured 16 mm and was used with a 10 mm layer height, a flow rate of 1 ml/s, and a robot speed of 9 mm/s. Those parameters resulted in a 25 mm layer width and the best printing quality. The low robot speed reduced the vibrations and dragging of the extruded filament. Although low robot speeds affect the fabrication efficiency, this could be neglected because of the low volume of the print specimen in this phase of the study.

Fig. 4
figure 4

Prototyping evolution. Top row: improving surface quality. Middle row: maximizing print height. Bottom row: reducing cracks

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The second phase of experiments aimed to maximize the printing height of the specimen by minimizing the elastic buckling of extruded filament. The methods used for this objective were adaptive wall thicknesses, wall corrugations, and bracing structures. First, double-wall structures were tested with overlapping print paths. This resulted in plastic deformation and tapered wall cross-sections. Second, corrugated print paths were explored that improved the built-up with a maximum of 8 consecutive layers without plastic deformation. However, elastic buckling occurred and led to print failures through partial collapses. Third, cellular bracing patterns with intersecting paths were tested (Fig. 4, center row). Those patterns consisted of triangular and square space-filling aggregations and proved to be the most successful approach for maximizing the print height.

The third phase of experiments focused on reducing the stresses acting on the foam filament throughout the printing process and leading to cracks during the drying period. Different cellular print patterns and the impact of asymmetric layer loading occurring in print geometries with stepped height were explored. First, square-based patterns were tested with an element consisting of one half with three layers (30 mm) and the other half with six layers (60 mm) height. After drying, cracks occurred in the sharp filament turns and in transitions between three and six stacked layers because of the asymmetric loading of the fresh layers. Second, circle-based cellular patterns were investigated that feature intersecting and overlapping horizontal layers with a vertical build-up of six consecutive layers (Fig. 4, bottom row). The result showed that cracks only occurred in areas with layer overlap of less than 40% of layer width. Third, the circle-based cellular patterns were optimized for sufficient layer overlap of 50% layer width and resulted in a reliable reduction of cracks in the foam elements after the sintering procedure.

Case study 1: facade panel

Design

After the findings of the initial exploration with the F3DP setup, a surface-like assembly of interlocking cellular foam modules was chosen as the design approach for this case study. The goal was to assemble printed foam elements and use ultra-high performance fiber-reinforced concrete (UHPFRC) as structural filler in between them. Consequently, the printed foam acts as lightweight stay-in-place formwork for the UHPFRC while providing additional functionality through geometric features. This application resembles a lightweight composite panel element that can be used for a second skin facade for controlling sunlight and ventilation. Using F3DP enables the geometric customization of the foam elements that can be optimized for controlling the porosity and depth of the panel. This material-efficient and waste-free fabrication make custom-made panels feasible that can be tailored for a specific building location.

The design steps for the case study demonstrator are depicted in Fig. 5. The size of the facade panel was defined by the available fabrication time of 4 weeks. During this period, 24 foam elements could be produced, including drying and sintering procedures. Circle-based patterns were chosen for the design of the foam elements. First, circle locations were randomly distributed within the dimensions of the facade panel measuring 500 × 1000 mm. Second, circles were generated with parametrically varying radii in relationship to their distance to an arbitrary attractor curve. Simultaneously, a circle packing routine moved their locations dynamically for an even distribution. This step can be used to program the permeability of the facade panel for light and ventilation control. Third, the same attractor curve was used to differentiate the print heights of the circles ranging from 3 to 6 layers. Lastly, the circles were bundled into clusters to create individual foam elements while ensuring that they fit into the fabrication space of the print bed. This last step further refined the location of circles for an optimized print path overlap which was observed during experiments in the initial exploration phase. Circles of the same cluster packed together with a distance of 12.5 mm and circles of different clusters 30 mm.

Fig. 5
figure 5

Design steps of final demonstrator: a boundary condition, b circle packing, c module clustering, and d) toolpath bundling

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Fabrication

A total of 22 cellular foam elements were produced during the fabrication phase of 4 weeks for this case study (Fig. 6a). The module sizes ranged from 60 to 280 mm in diameter and 30 to 60 mm in height, which corresponds to 940 mm (3 layers height) to 5060 mm (6 layers height) print path length and 0.2 to 1.2 L of print material. Printing times ranged from 8 to 15 min, with an average of 6 min per foam element. In total, a volume of 12.4 L mineral foam was printed. After sintering, the elements were assembled inside a boundary frame made from 30 × 30 mm steel L-profiles with reinforcement pins (Fig. 6b). The frame served as perimeter formwork for casting UHPFRC between the printed foam elements and could be left in place as a finished edge with mounting details. The resulting composite facade panel shown in Fig. 7 weighs only 15.7 kg. That is comparable to commercially available lightweight ceramic facade cladding systems. Approximately 4.3 kg are printed mineral foam, 9.2 kg UHPC and 2.8 kg the steel frame.

Fig. 6
figure 6

Fabrication of the final demonstrator. a 3DP of one module. b assembly of sintered modules in steel frame before the casting of UHPC

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Fig. 7
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Full-scale demonstrator. a Overall impression and b, c closeups

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Case study 2: arched beam

Design

The second case study presented in this paper is a prototype module of a floor structure based on a rib-stiffened funicular shell, similar to precedence research projects [8]. The ribs can provide sufficient structural depth to cope with changing loading conditions, such as additional point loads. The proposed floor module is a one-way spanning structure designed for a four-meter span, which could be used in residential buildings. Figure 8 illustrates how the module works as a floor system as a series of unreinforced funicular arched beams arranged side by side [30].

Fig. 8
figure 8

3D visualization of the floor structure concept

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The arched shape of the beams is based on a parabolic arch. This shape allows the distribution of imposed loads to the supports in compression only. A post-tensioned steel tie solves the horizontal equilibrium and is inserted in a sleeve to prevent it from adhering to the concrete. The construction detail is similar to the steel ties used in conventional prestressed slab systems. This makes the use of embedded steel reinforcement unnecessary for the stability of the structure, thus reducing the amount of steel required and facilitating recycling. Prefabricated mineral foam elements made by F3DP are placed as lost formwork where there is no structural requirement to constrain the higher-strength material (C12/15 concrete) to places where it is needed to activate the arching action. This material differentiation is similar to that of functionally graded concrete beams, where lighter aggregates are distributed to lower the density of the concrete in places where it is not structurally necessary [31]. Moreover, the mineral foam elements used in the floor module prototype can improve thermal and acoustic insulation. Although the proposed formwork system produced with F3DP is more suitable for the fabrication of non-standard geometry vaulted floor structures [9], the floor module prototype in this first study resembles a simple arched beam for better understanding.

Currently, the technology used allows elements up to 8 layers high to be printed before experiencing considerable deformations in the lower layers due to the excessive weight of the upper layers. For this reason, the scale of the prototype is reduced to a length of 60 cm when it is originally two meters. It is estimated that the reduction in structural mass and volume of concrete can be as much as 50% when comparing the arched beam with a comparable reinforced flat slab section.

Fabrication

Figure 8 shows the fabrication process of the prototype. The mineral foam components are 3D printed using a robotic arm and the stationary foam extruder (Fig. 9a). A total of eight of these foam blocks were printed. Each block has a different height, ranging from 20 to 80 mm. The printed foam blocks differ in size, with a diameter ranging from 140 to 235 mm and resulting print path lengths ranging from 490 to 6090 mm. The printed foam volume ranged from 0.1 to 1.2 L, and the printing time from 1 to 11 min (considering a robot speed of 9 mm/s). The side faces of the foam component are tapered to prevent them from deformations while being in a fresh material state (Fig. 9b). Once sintered, the foam blocks are arranged in the intended layout inside the timber formwork and fixed to the wall with nails (Fig. 9c). After that, concrete is poured into the mold. Figure 9d shows how the sintered mineral foam and the concrete interact, resulting in uniform and robust bonding between materials. Figure 10 shows the finished prototype. A mirror image was used in the figure to illustrate the full shape of the arch. A circular saw was used to slice and remove one side face and make the internal foam blocks visible.

Fig. 9
figure 9

Stay-in-place formwork system made of mineral foam. a 3D printing of the mineral foam components, b resulting mineral foam components drying at room temperature, c mineral foam components fixed to timber formwork, and d mineral foam components embedded in concrete

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Fig. 10
figure 10

 A 60-centimeter-long prototype was fabricated to test the stay-in-place formwork system made of mineral foam. The prototype comprises only half of the span. A mirror is used to illustrate the complete arch form (indicated by the blue line)

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Conclusions & outlook

Large-scale 3DP with mineral foam for freeform construction is a novel approach with promising applications for sustainable, resource-efficient, and innovative building elements. For the first time, this research shows how F3DP can be used for custom concrete formwork that exploits the geometric freedom of the fabrication method. Thus, geometrically-complex non-standard concrete parts become feasible through automated waste-free manufacturing with positive effects on productivity and affordability. Additionally, the lost formwork adds functionality such as acoustic and thermal insulation. Such concrete elements are more material-efficient and lightweight than monolithic parts and, in turn, require less logistical effort. These savings are comparable to hollow-core slabs but can now be achieved for non-standard geometries.

The two demonstrators presented in this study prove the validity of the fabrication system for material-efficient, lightweight concrete composite elements. The combination of the strength of cast concrete and the low mass of mineral foam results in significant material and weight savings. The first case study is a 1 × 0.5 m facade shading panel that weighs 12.9 kg (without the metal frame) and uses 4.3 kg (12.4 L) printed mineral foam and 9.2 kg cast UHPC. Hence, it is comparable to other lightweight facade cladding systems. Compared to a custom facade shading panel with identical geometry using concrete 3DP (26.04 kg printed concrete with 2100 kg/m3), this is a weight reduction of more than 63%. The second case study is a 60 × 14 × 14 cm half-arched beam that weighs 19 kg using 1.6 kg of printed mineral foam and 17.4 kg of concrete. In comparison with a conventional monolithic concrete beam (2500 kg/m3, 28.8 kg), this saves 40% of concrete and reduces the total weight by 34%. Applied on an entire building floor with the dimensions of 20 × 10 × 0.2 m, this approach would save 16 m3 of concrete and reduce the weight by 34 t. However, in contrast to mass-fabricated lightweight elements such as hollow-core slabs, the present study is likely more suitable for low-volume, high-customization applications, where the complex formwork would otherwise be cost-prohibitive.

The use of F3DP formwork for the fabrication of bespoke concrete elements could help to greatly reduce their environmental footprint compared to the use of traditional non-reusable formwork systems. One of the areas where the use of these formwork systems presents environmental benefits is at the end-of-life stage. Because both concrete and foam are of mineral composition, when combined, they form a mineral composite. This makes recycling much easier than concrete, which is combined with plastics or metals. Consequently, the mineral composite structures can be recycled without separation and reintegrated as ground aggregate into the material cycle. The use of F3DP elements as lost formwork allows for the quasi-elimination of formwork-related waste and for the placement of structural materials of higher mass and environmental footprint only where they are functionally needed. The latter, combined with the low density of the mineral foam, would greatly reduce the overall mass of a building. This reduction in the overall weight of a building would also allow for a reduction in the size of the foundations and, consequently, a further reduction in the environmental impact of the building. Another environmental benefit of weight saving through the use of mineral foam can be the reduction of the environmental impact of logistical processes related to the construction process, such as transportation and the use of heavy machinery on the construction site. The low density of the mineral foam also allows the use of these lost formworks as thermal insulation and thus avoids the impact of additional insulation elements.

However, there are various opportunities to improve the current limitations of the material system and fabrication process toward sustainable and lean construction. In particular, the energy-intensive consolidation process of sintering in a ceramic kiln increases the embodied energy of the material system significantly. This can be improved with hardening strategies similar to concrete printing. Furthermore, the drying and sintering processes increase the sensitivity for cracking. Both steps are time demanding and slow down the fabrication speed. Improvements in the print setup could include the transfer of the ink in a continuous manner from foaming to printing instead of filling a cartridge.

Similarly, a moving printing plate could be detrimental to shape retention, which eventually limits the printable height. Of course, the future development of an integrated extruder tool head within a mechanical system, e.g., the industrial robot, would increase the scalability and robustness of the fabrication process. Moreover, the freshly printed mineral foam is a compressible fluid, and deviations can occur during the consecutive buildup of layers during F3DP. Machine sensing such as optical monitoring and 3D scanning, can help to implement feedback-based process control and improve the accuracy of the F3DP process [32].

Towards a new monolithic

The decarbonization of the building industry is one of the most pressing challenges today. Using less material for construction and improving the performance of building elements are imperative steps toward this goal. The significance of F3DP with mineral foams for stay-in-place functional formwork in bespoke resource-efficient lightweight structures could be shown in this research. Specifically, bespoke facade elements and funicular floor structures that are designed for a specific load case and building context are presented. The empirical exploration of the design space with the current material and F3DP fabrication setup is an important contribution to future research.

The presented case studies are the first small prototypes of formwork solutions for concrete structures. The proposed large-scale extrusion technique is beneficial for the scalability of the F3DP process with higher throughput than other 3DP solutions, such as FDM and binder jetting. Another scaling factor is the energy-intensive consolidation strategy of the sintered mineral foam, which can be replaced with alkaline hardeners [33]. Comparable studies indicate that the proposed robotic F3DP process can increase productivity compared to conventional formwork manufacturing when complex structures are built [6]. More studies are required to validate the findings in this paper, particularly comparing production efficiency and tolerances with existing techniques.

Beyond that, mineral foams exhibit a very high thermal resistance and can be used with F3DP for freeform insulating building elements. The open-cell foam structure allows them to be used as acoustic absorbers as well. These properties are particularly beneficial for building components such as slabs and facade elements that are investigated in this study. With the continuation of this research and the aforementioned improvements in scale and accuracy, entire construction systems can be envisioned for non-structural and load-bearing applications. The case studies presented in this paper are modular bespoke elements and give an outlook on building-scale solutions (Fig. 11).

Fig. 11
figure 11

Visualization of a building-scale application of the facade shading system developed in case study 1

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For this technological stage in the research, sintered mineral foams were used as stay-in-place functional formwork, and UHPC was used as a filler. Considering the high cement content in this concrete, a more sustainable option would be a cement-free material. Ideally, the structural filler can be produced by the same material composition as the lightweight infill but of higher density. This opens a new chapter for future sustainable construction with F3DP of functionally graded mono-material building elements made entirely from mineral foams - a new kind of monolithic.