Rheological Properties of Cement-Based 3D Printable Ink containing Sustainable Materials – An Overview

Additive manufacturing, also called as 3D printing, has been emerging tremendously in various arena include mechanical, aerospace, medical, food, architecture, and construction sectors. Moreover, it has been considered as one of the primary drivers of the industry’s digitalization. 3D Concrete Printing (3DCP) is a novel technology that integrates the digital technologies with the discoveries of new material science and enables the free-form construction without using formwork [1]. The materials, in the 3DCP process, are deposited layer by layer to create a structure using a three-dimensional computer model [2]. As compared to the conventional construction methods, the application of the 3DCP technique in the construction industry could provide significant benefits including cost savings by eliminating the formwork, geometric freedom, and speed of construction [2]. Moreover, it reduces accident rates at construction sites resulting in higher construction safety, creating high-tech and technology-based jobs, operating consistently, and reducing onsite construction time, and cost. The key potential of 3D concrete printing technology is the ability to prototype ideas rapidly. With the use of an appropriate material deposition method, the chance of errors can be reduced and promotes sustainability in construction by reducing formwork waste. Moreover, the increased architectural flexibility results in the development of more complex structural and aesthetic designs [1]. Fig. 1 portrays the potential advantages of concrete 3D printing in the construction industry.

Fig. 1: Advantages of concrete 3D printing

Concrete is the most widely used construction material which consumes a large amount of natural resources. In total Global Anthropogenic CO₂ emissions, the production of concrete contributes about 8-9% of CO₂ emissions [3]. As there is a significant impact in industrialization and urbanization, the need for new construction demands more natural resources, which leads to the depletion of natural resources. To overcome this, supplementary cementitious materials have been extensively used in the construction industry to reduce the carbon foot prints. The replacement of cement with supplementary cementitious material not only gives sustainability but also improves the rheological properties [4], mechanical and durability properties of the materials. For instance, if such supplementary cementitious material is included in emerging technology like 3DCP technology, it can create a new era in the sustainable construction, reduction in disposal of waste materials, minimizing the depletion of natural resources and has a positive impact on the properties include rheology and early strength which are essential characteristics of printability properties of Concrete 3D printing. It also addresses the potential utilization of the industrial waste in concrete 3D printing in reducing the carbon footprint.

Sustainable Materials in Concrete 3D Printing Technology

Fig. 2 depicts the amount of generation of solid wastes in India. Incorporating sustainable supplementary cementitious material (SCM) in concrete 3D printable material gives significant results in terms of properties [5]. Based on the literature review, it can be seen that some of SCMs include fly ash and iron ore tailing are abundantly available in India.  Fig. 3 shows the usage of various supplementary cementitious material used in 3D Concrete printing. 

                                                           Fig. 2: Amount of generation of solid waste in India [6]

Fig. 3: Supplementary cementitious materials which are commonly used in 3D Concrete printing [7]

The by-product fly ash obtained from coal-based industries is one of the major problems in most of the countries. Fly ash is categorized as class C and class F based on the presence of CaO content. Fly ash is one of the widely used supplementary cementitious materials in concrete 3D printing. The impact of fly ash in terms of the rheological properties of the concrete varies with respect to the class of fly ash.   The use of class F reduces the plastic viscosity of concrete as compared to class C fly ash [8]. Similarly, the addition of Ground Granulated Blast furnace slag (GGBS) decreases the plastic viscosity of the concrete. The presence of limestone powder in the 3DCP  affects the rheological properties like yield stress and viscosity based on fineness and surface roughness of limestone powder [8]. Replacement of cement with a small amount of limestone could accelerate the hydration at an early age. Subsequently, the results of Vance et. al. [9] have shown with the replacement of limestone by 5% of OPC, the yield stress decreases by 20% however no reduction in plastic viscosity in cement pastes.  Ma et al. [10] have studied the incorporation of copper ore tailings by replacing sand from 0% to 50%. Findings have shown that the mix prepared with 30% replacement of fine aggregate gives the optimal mixture which leads to a significant impact on the mechanical properties and required buildability. However, the replacement of copper ore tailings with the fine aggregate in the concrete 3D printable material increased the flowability of the printable materials. Moreover, they found that replacement of copper ore tailings not more than 40% gives a positive impact in terms of the strength properties of 3D printable concrete. Guowei et al. [11] have studied the inter-layer bonding behavior of blended OPC and calcium sulfoaluminate cement, which is a major problem in normal OPC mortar based concrete 3D printing. The findings depict that the mortar mix with OPC and calcium sulfoaluminate cement could enhance the interlayer bonding between the concrete 3D printings. It was also found that the silica fume, in geopolymer mixtures, plays a major role in enhancing the rheological properties include yield stress, viscosity, thixotropy. The replacement of the binder with 5-10% of silica fume doubles the yield stress and extrudes the material smoothly without any cracks. Moreover, this inclusion improves the required thixotropic behavior of 3D printable geopolymer concrete [12]. The addition of 10% of silica fume into the 3D printable mixture increases the fresh properties of the mixture  [13]. Moreover, the buildability of the 3D printed material doubles when 2% of silica fume is added to the mixture without changing any other factors. Zhang et al. [14] have concluded that the addition of silica fume shows quick pozzolanic reactions and improves the microstructure due to high surface area. It also increases the plastic viscosity as well as dynamic viscosity [8]. Researchers incorporated silica fume into 3DCP enhances yield stress and buildability, however it increases the demand of water for the mixture. The water demand can be further addressed by adding superplasticizers. Li et al. [15] have studied the incorporation of iron ore tailings and copper ore tailings as a cementitious material in concrete 3D printing. The key results show that the optimal mixture can be obtained at 40% iron ore tailings, 10% copper ore tailings, 19% fly ash, 30% belite cement and 1% naphthalene-sulfonated formaldehyde condensates (FDN) water-reducing agent. It also found that the compounding effect of iron ore tailings to copper ore tailings is at a 4:1 ratio. This mixture has excellent workability and controlled setting time [15]. Subsequently, the results of [16] also reveals that the addition of 25% of IOT into the concrete gives better results than the normal concrete.

Apart from this, some nanomaterials include nano silica and nano clay are used in the preparation of printable concrete. Fig. 4 shows the particle size and specific surface area of various constituents of concrete. The addition of nano materials may act as the filler material in the concrete which thereby densifies the microstructure of concrete. Therefore, the porosity of the hardened material can be reduced.

Fig 4:   Particle size and specific surface area of various constituents of concrete [17]

The addition of nano clay into the cementitious material could enhance the rate of hydration, degree of flocculation and gives a strong bond between the cementitious mixture. It also increases the viscosity and yield stress due to the water absorption ability of the nano clay which reduces the free water [18]. Quanji et al. [19] pointed out that the inclusion of nano clay into the concrete 3D printable material could significantly affect the rheological properties. Several researchers [20, 21]  have investigated the addition of nano clay into the cementitious material and they found that incorporation of nano clay up to 3% could significantly increase the yield stress, thixotropy, and viscosity of the cementitious material. Subsequently, Kazemian et al. [13] have pointed out that the addition of nano clay into the concrete 3D printing could enhance the shape-stability of the layers. Moreover, it could enhance the mixture robustness and also the increase in nano clay content increases the yield stress [22]. Panda et al. [23] have studied the effect of adding attapulgite clay in different percentages in 3D printable materials along with a significant volume of fly ash. The results have shown that the inclusion of nano clay significantly improved the thixotropic property of high volume fly ash mixes resulting in increasing its suitability for concrete printing applications. Moreover, the substantial rise in apparent viscosity, degree of flocculation, and yield stress enhances the buildability of printable mix.

The addition of nano silica into the cementitious material enhances the overall quality of the concrete in terms of strength, porosity, and durability properties. Nano silica with a high specific surface area affects the hydration kinematics thereby enhancing the microstructure and pozzolanic activity [24]. Senff et al. [25] revealed that the incorporation of high specific surface area minerals into cementitious material increases the requirement of water to maintain the required workability. It also increases the viscosity and yield stress of the mixture due to increased friction between particles as well as dense packing of the nano-silica in the cementitious mixture [25,26].

Conclusions

Due to the modernization and advancement in technology, the need for new construction rises, therefore, increasing the demand for natural resources worldwide that leads to depletion of the natural resources. In order to overcome the depletion of natural resources and reducing the carbon print, the alternate cementitious materials have been extensively used in the construction industry that too especially in 3DCP in recent years. From the short review, it can be seen that by adding industrial wastes in cement mortar could significantly increase the rheological properties of concrete 3D printable material. The dosage of the material and chemical composition might influence the pumpability, buildability and extrudability of cementitious printable ink.  The particle size of supplementary cementitious materials (SCMs) could significantly affect the buildability and extrudability of concrete in 3D printing. Moreover, the ease of extrudability may increase because of reduction in particle size and shape, however the buildability may decrease. To enhance the extrudability and buildability properties, a proper combinations of suitable SCM materials and chemical admixtures is required. More experimental investigation is required especially in the use of tailings in developing a 3D printable ink. New approaches in developing the high volume supplementary cementitious materials may lead to much more sustainable concrete 3D printable material. In addition to that, the effect of the incorporation of various SCMs need to be further investigated.

References

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