Sustainable Development Approach for 3D Concrete Printing

Tha amount of publication upon 3D concrete printing increase very fast over last years. The research teams focuses on analysis the early age properties of 3D printable mix, which are crucial for fulfill the requirements of printing process [1‐6]. In other side the research team try to improve hardened and durability properties of 3D printed mixes [7‐11]. Nevertheless mentioned works evaluate the properties of 3D printed structures.. It should be noted that initially, 3D printing relied on high-performance concretes as the primary material choice [12, 13], characterized by a significant proportion of cement that detrimentally affects the environment [14, 15]. Nevertheless the increased presence of binders in the printing mixture has a favorable impact on its rheological properties, which is a crucial parameter in the process [1]. The production processes involved in the cement industry result in substantial amounts of CO₂ being emitted, making it a major contributor to anthropogenic CO₂ emissions. This connection arises from the unique nature of the production processes, which generate large volumes of CO₂ [16]. In 2019, in cement and concrete industry it was approximated that about 10% of global CO₂ emissions associated with energy use, transportation, production, and demolition activities were attributable to factors such as fuel combustion, power consumption, and carbonate decomposition [17, 18]. According to IPCC report [19], cement industry itself plays a vital role as a major contributor to anthropogenic carbon dioxide (CO₂) emissions, accounting for approximately 7% of global emissions.

The mentioned environmental problem do not limit to cement industry, the additional factor is related to the aggregate. The 3D printed concrete usually need to used only fine aggregate due to pumping process. The mentioned type of aggregate is also on the brink of depletion. It should be noted that aggregate is the most widely used material in concrete in terms of volume [20]. The excavation of this type of material has led to irreversible environmental changes [21].

Nowadays, sustainability factors are becoming increasingly important for the entire civil engineering sector. This notion compels the industry and research community to offer sustainable solutions for 3D printing materials. Existing research has, in many instances, tackled these aforementioned issues [22‐26]. However, there is still a need to draw conclusions and explore new research directions. This paper provides a brief summary of the sustainable approach for 3D printed concrete, encompassing both technology and material aspects. The presented work sets a new course for future research. The paper focuses exclusively on the extrusion-type printing process of cementitious materials.

Finding a suitable mix and machines that meet the requirements of 3D concrete printing is one of the most challenging problems in this technology. Many research teams consider various aspects related to these problems [27]. These aspects can be related to technology issues, such as the pumping system and robot controls, or to materials issues associated with the appropriate mix design. It should be noted that both technological and materials aspects are closely interconnected. A mix that is suitable for printing with one type of pump may be impossible to print with another type of pump [28]. The example of successfully printing process of cementitious mix is presented on Fig. 1.

The parameter that describes the transportation of fresh concrete from the pump to the extrusion nozzle is defined as pumpability/deliverability [12]. One of the main issues concerning these properties is the possible segregation within the hose. This can occur due to an inappropriate mixture composition (e.g., very high viscosity) or insufficient mixing before the printing process begins. Another problem highlighted by research teams is the potential segregation of concrete particles within the hose [9]. Additionally, the friction between the mixture and the hose wall can increase viscosity, making pumping impossible [29]. To ensure adequate pumpability, many research teams use mixtures with a high cement content, which reduces the friction between the mixture and the hose walls. Several research teams have attempted to determine the pumpability parameter for their specific equipment.

Extrudability and printability are parameters that determine the ability to extrude a mixture through the printing nozzle while maintaining the desired shape (without significant deformations) and an acceptable level of path discontinuity [9]. The extruded mixture must not exhibit structural damage, should not be segregated, and its consistency should remain consistent during construction. In addition mentioned parameters can be defined as the ability to arrange the mixture-nozzle system to print a path of desired quality with appropriate rheological properties [9]. For example to measure the quality of extrudability, the shape retention factor (SRF) was introduced by Panda et al. [30]. It is an indicator that assesses the disparity between the printed path and the printing nozzle. The SRF is calculated by dividing the width of the printed layer by the width of the nozzle used. When the SRF value increases, it indicates unsatisfactory shape retention of the mixture. A value of one for SRF signifies that the printed mixture maintains the shape corresponding to the size of the nozzle.

In addition to mentioned parameters, some research teams [27] defined print quality as well. This property evaluate the defects of printed layers, which can also influence the mechanical and durability properties.

The duration of proper workability, which allows for printing, is known as open time [12]. It refers to the period during which a concrete mixture can be pumped, meets quality requirements, and is capable of achieving adequate adhesion to the surface it is being placed on. Open time is particularly crucial for geopolymer or other alkali-activated materials, as their open time is typically very limited.

For mainly a materials point of view, it is important to design a mixture that has the ability to carry loads and maintain the same shape under its own weight and the weight of subsequent layers without excessive deformation. These requirements allow for the printing of high and complex structures. To evaluate these crucial properties, many testing methods have been proposed, starting with basic rheological parameters such as slump flow, yield stress, or structurization rate [1]. These parameters can be evaluated using traditional rheological methods. A better way to evaluate the “green” (non-hardened) mechanical properties of the mix is uniaxial unconfined compression testing (UUCT) or similar tests. There are two ways to perform the test: (1) at constant intervals [2, 15], and (2) at a constant displacement rate [4, 31]. This type of testing was initially adopted by Perrot et al. [2], and later modified by Wolfs et al. [4], who used a triaxial compression test. Casagrande et al. [5] also performed various assumptions by examining different test setups, including casting procedures, different displacement rates, etc. However, the UUCT test does not accurately evaluate the ability to carry loads [15]. Buildability or shape retention during printing is one of the best ways to examine the mentioned aspects. This research procedure involves direct printing tests, usually until collapse, to evaluate the real load-carrying capacity of the printed mixes. Different research teams print different structures for the purpose of this test and analyze various aspects such as changing the height of layers [32], complete collapse [15], and the type of collapse [33, 34]. It should be noted that these assumptions can be influenced by stability aspects [33]. There are also other indirect techniques to evaluate the green properties of the mixture, including penetration tests [35] or ram extrusion tests [36]. However, due to the limitations of this study, the topic will not be further expanded.

The sustainable aspects mentioned in point 1 can be categorized into three main groups: aggregate composition, binder composition and technological aspects. Table 1 presents these aspects along with potential solutions related to 3D printing technology. These groups play a crucial role in the sustainable development of 3D printing technology. In the case of binder composition, a high amount of binder is typically used for 3D printed concrete [52], but this aspect needs to be addressed and changed. As for aggregate composition, it should be noted that the use of coarse aggregate is limited in 3D concrete printing. This limitation necessitates the use of high-quality quartz aggregate, which leads to the depletion of limited natural resources. Technological aspects should provide additional advantages for 3D printing technology, thereby improving the overall construction process. In the following sections of the study, each of these groups will be reviewed in detail.

The potential solution for aggregate composition could involve the use of alternative and recycled aggregates. Recycled sand shows promise for 3D printed concrete, as it often exhibits higher water absorption, which can enhance the load-bearing capacity of the printed structure [38]. However, research indicates that the origin of the aggregate plays a crucial role in the final performance of the mixture. For instance, recycled aggregate obtained from crushing and screening 100% waste concrete [38] has a positive impact on the green strength of concrete. On the other hand, aggregates derived from post-abrasive wear of steel and engineering plastics (spent garnet) have a negative influence on green properties [6]. A comparison of these effects is presented in Fig. 2.

The artificial / plastic recycled aggregate (PET) was successfully used in 3D printing technology by Skibicki et al. [40]. The research proves that this type of material does not influence the rheological properties of the mixture, and the modified reference mix with the mentioned aggregate meets all the printing requirements. However, PET granules hinder the hardened properties. Additionally, exposure to freeze-thaw cycles and high temperatures significantly reduces the material's properties, up to 82.4% and 68.8%, respectively. The research concludes that adding up to 10% by volume results in a reasonable reduction in durability properties.

It should be noted that recycled glass aggregates [39], tyre waste, and recycled rubber aggregate [41, 53] do not significantly influence the rheological properties. However, data on the utilization of these materials in 3D printing of cementitious materials is limited. Evaluation of the hardened and durability properties is needed.

Promising results for buildability evaluation were obtained by adding 10–15% of copper slag [37]. This procedure led to an increase in buildability, although a higher amount of slag significantly deteriorates the print quality.

The potential solution for binder composition in 3D printed mixes has been evaluated in many research papers [42, 54]. For this brief study, specific assumptions were chosen for review. Changing the binder can be favorable for the environment. Cement production results in the production of 0.91 kg of CO₂ for every 1 kg of cement [55], but the use of supplementary cementitious materials helps limit CO₂ production. Silica fume and fly ash have a minimal environmental footprint [22] due to their waste nature. However, the availability of silica fume and especially fly ash is becoming limited according to EU policy. In relation to 3D printing technology, silica fume improves buildability [42], although there is a lack of research on high amounts of silica fume addition. Fly ash, on the other hand, reduces buildability but can act as a retarder, especially when used with alternative binders such as Calcium Sulfoaluminate Cement (CSA) or Calcined Clay Cement (LC3) [56]. Both materials have positive effects on long-term mechanical properties.

Another assumption to reduce CO₂ emissions is the use of alternative binders such as Magnesium Cement (MG), Calcium Sulfoaluminate Cement (CSA), and Calcined Clay Cement (LC3), which have approximately 45% [57], 35% [45] and 73% [55] lower CO2 production than ordinary cement, respectively. The MG [58] does not contribute the properties requires for 3D printing, but existing literature data only analyze pastes, and further research is needed. The CSA and LC3 cement [36, 47, 48] greatly improve green strength properties but significantly reduce the open time due to their fast hardening process. Both materials require efficient retardation to control the hardening process.

A sustainable and promising idea for 3D printing materials is the use of inert microfillers as partial replacements for aggregate [31, 44]. In cited research, successful replacement of up to 30% by volume was achieved. Figure 3 present the influence of limestone powder (LP) to binder (B) ratio on the green strength of 3D printed mixes. The presented comparison is based on findings from the West Pomeranian University of Technology [15, 31]. Five mixes with different amounts of binder were compared (in each case, the binder consists of 70% cement, 20% fly ash, and 10% silica fume). The results clearly indicate that the green strength increases with an increase in the amount of limestone powder in the mix. This phenomenon allows for a possible reduction in the binder content within the mix. Additionally, it should be noted that the effectiveness of limestone powder, in most cases, increases with the binder amount. Nevertheless, the presented research shows that it is possible to reduce the amount of binder and achieve the same green strength. In summary, this approach helps maintain similar green strength and rheological properties required for printing, even for mixes with reduced binder. However, it may slightly reduce the hardened properties [44]. Additionally, due to the higher amount of binder, some cement remains unhydrated, and partial replacement by inert microfillers does not significantly impact the hardened mechanical properties [31].

Binder composition holds great promise in preserving the natural environment. However, due to the limitations of this study, only the main findings have been highlighted.

Different varieties of 3D printers are currently employed, such as Cartesian robots, robotic manipulators, and Delta robots [51]. Numerous items have already been successfully manufactured utilizing this technology, serving both as demonstrations and for practical applications [51].

Taking into account that the expenses related to formwork can make up 35–54% of the overall costs involved in constructing a concrete structure [59] the utilization of additive manufacturing yields significant benefits. The greatest reduction in construction costs and completion time occurs with elements of complex geometry (e.g., curved walls, arches, etc.). In such situations, traditional methods often lead to significant cost and time increases, whereas in 3D printing technology, the shape of the printed structure does not have a major impact on the overall cost of the project. This innovative approach not only enables the creation of structures without the need for formwork, but it also leads to a reduction in total production time, costs, and labor. Additionally, this technology enhances the safety of construction site workers, minimizes waste generation, and utilizes raw materials with a low level of embodied energy [59].

Some studies have analyzed the mentioned problems using a quantitative approach [49, 50, 60]. Weng et al. [50] conducted a comparison between the construction processes of precast concrete bathrooms using 3D concrete printing technology and traditional methods. The results indicate that certain costs were significantly reduced with 3D printing technology (e.g., a 47% reduction in material costs), while electricity costs increased by over 25 times. However, when considering the overall cost, the comparison demonstrates that 3D concrete printing technology leads to a construction cost reduction of over 25%. Furthermore, the analysis of CO₂ emissions reveals that this technology achieves a reduction of over 85%. Similar results were obtained by Mohammad et al. [49], providing evidence that 3D concrete printing can reduce CO₂ emissions by over 50% when compared to four different construction scenarios involving concrete walls, two using traditional methods and two utilizing 3D printing technology.

Based on the own research and experiences with 3D printing, and the international research achievements presented in this brief study, the following conclusion remarks can be made:

To sum up, there are promising approaches for sustainable 3D printing materials. However, most of these solutions require further research to fully evaluate all the necessary properties. It should be noted that, particularly for alternative binders, the commercial usage of these materials is still uncertain due to their short open time. Nonetheless, the rapid development of 3D printing research holds the potential for new solutions that meet both printing and sustainability requirements.