A Comprehensive Review on Effects of Material Composition, Mix Design, and Mixing Regimes on Rheology of 3D-Printed Geopolymer Concrete

Pasupathy K., Ramakrishnan S., Sanjayan J.

The utilization of waste materials in the new emerging 3D concrete printing enables a sustainable pathway to reduce environmental-related issues. This study investigates alkali-activated brick waste powder as the binder for developing 3D printable geopolymer mixes. The brick waste was used as a partial replacement to fly ash in a geopolymer binder. The effect of brick waste content on the fresh properties of printable mixes, such as flow, setting time and rheological properties were investigated. Besides, the hardened properties of 3D printed brick waste geopolymer were evaluated with the varying brick waste content in the mix. The test results demonstrated that the fresh properties of 3D printable mixes were improved with the brick waste content in the mix. Compared to the control mix, the mixes containing brick waste displayed high yield strength and apparent viscosity at an early age. On the contrary, the compressive strength and interlayer strength properties of 3D printed concrete specimens were decreased with the high brick waste content; however, the incorporation of brick waste for up to 10% has enhanced the hardened properties. Finally, the sustainability assessment of brick waste geopolymer studied with embodied energy and carbon emission calculations reveals the proposed geopolymer concrete could reduce the embodied energy and carbon emission by up to 60–80%, compared to OPC concrete.

Ranjbar N., Kuenzel C., Gundlach C., Kempen P., Mehrali M.

This study investigates the role of halloysite nanotube as a mineral-based thixotropic admixture to 3D printable geopolymer mortar. The first part of this paper focuses on the fundamental characterization of the thermal evolution of halloysite at 30–1000 °C. In the second part, we show how the calcination and concentration of halloysite influence the fresh and hardened properties of 3D-printable geopolymer mortar. It was found that regardless of thermal treatment, using only 1–2 wt% halloysite can significantly increase the rheological properties and buildability of the mortars without compromising their mechanical strength. However, the setting time of geopolymer only accelerated when highly reactive dehydroxylated halloysite was used. Compared with mold-cast specimens, the mechanical properties of 3D-printed specimens were lower at early ages due to their higher surface dehydration; however, the gap became closer over time.

Muthukrishnan S., Ramakrishnan S., Sanjayan J.

In-line activation techniques in concrete 3D printing enhance the buildability of fresh concrete without influencing its pumpability, thus enabling fast construction rates with long pumping durations. While the in-line activation of cementitious mixes with set-accelerators is a promising approach, the set-accelerators fail in geopolymer system. Therefore, this study investigates a new approach of attaining on-demand setting in geopolymer by alkali activation of binder slurry at the print head. The primary advantage of the proposed method is that the mix remains non-activated until it reaches the print head, thus facilitating a long pumping duration. It was found that the mix design parameters (i.e., precursors to activator ratio, fly ash to slag ratio etc.) and operating conditions (mixing duration of precursor slurry, stage wise activation etc.,) are critical in attaining the desired fresh concrete properties in geopolymer concrete. This is demonstrated by the fact that the optimum mix design chosen with 20 wt% of fly ash as precursor and the activator dosage of 35 wt% of binder showed a yield strength growth rate from ∼500 Pa before activation to ∼70 kPa in 20 min after activation. Moreover, this study provides a new insight into the potential benefits of polycarboxylate based superplasticiser, which was widely reported as an incompatible admixture for silicate activated geopolymer concrete.

Chen Y., Zhang Y., Xie Y., Zhang Z., Banthia N.

Extrusion-based 3D-printed concrete (3DPC) structures are reported to hold mechanical anisotropy behaviors and weak transport properties compared with cast concrete. Fundamental insights into the pore structure discrepancy between printed and cast concrete are essential to the performance prediction and improvement strategy for 3DPC. This study analyzes the pore structure alternations in 3D-printed geopolymer concrete (3DPGC) with cast ones as the reference. Several pore characteristics, i.e., pore volume, distribution, specific surface area (SSA), shape and connectivity are investigated via X-ray CT and MIP. The results demonstrate that a larger porosity, coarser pore size distribution and higher pore SSA exist in 3DPGC compared with CGC. The coarser pore size distribution respectively lies in large voids (>0.2 mm) and small pores (

Demiral N.C., Ozkan Ekinci M., Sahin O., Ilcan H., Kul A., Yildirim G., Sahmaran M.

This work aims at evaluating the anisotropy (direction-dependency) in terms of mechanical performance and bonding properties of entirely construction and demolition waste (CDW)-based geopolymer mortars fabricated by 3D-additive manufacturing (3D-AM) technique. In the study, a combination of hollow brick (HB), red clay brick (RCB), roof tile (RT), concrete waste (CW) and glass waste (GW) obtained from various demolition sites and different combinations of alkaline activators including sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH) 2 ) were used for geopolymerization . CW was also used as fine aggregate in geopolymer mortar production. Specimens were subjected to ambient curing conditions until testing ages. Direction-dependent mechanical performance of printed specimens was evaluated at 7-, 28- and 90-day via compressive strength test in three different loading directions of perpendicular, parallel, and lateral to the printing path and flexural strength test in two different loading directions of perpendicular and lateral to the printing path. Moreover, bond strength between the consecutive printed layers were tested through direct and splitting tensile strength tests at the end of 7-, 28- and 90-day ambient curing and used to compare the directional performance of tested mixtures. In addition, compressive and flexural strength test results of printed specimens were compared with those of conventional mold-casted specimens. Results showed that alkaline activator content affects the mechanical properties considerably. According to compressive and flexural strength test results, 3D-printed geopolymer mortar specimens have anisotropic behavior and the bond performance between consecutive layers is one of the main influencing parameters for the anisotropic behavior of 3D-printed structures. However, perpendicular-loaded 3D-printed specimens showed similar or slightly better performance compared to the mold-casted ones, indicating that the bond zone had little influence on the performance of specimens loaded in perpendicular loading direction. This study pointed out that the anisotropic performance of printed structures can be diminished with the enhanced bond adhesion between consecutive layers and the adhesion can be improved by optimizing the rheological properties and matrix performance of the mixtures.

Chougan M., Hamidreza Ghaffar S., Nematollahi B., Sikora P., Dorn T., Stephan D., Albar A., Al-Kheetan M.J.

• Natural and calcined Halloysite clay additives were characterised. • Halloysite clay additives were incorporated to modify AAMs' printing properties. • Enhanced compatibility of calcined Halloysite clay additives in the AAMs was verified. • Buildability and mechanical properties were improved in CH-modified AAMs. This study investigates the effects of natural and calcined halloysite clay minerals (“NH” and “CH”, respectively) on the performance of 3D printed alkali-activated materials (AAMs). Halloysite clay minerals are selected as they are low-cost and abundantly available. At first, different characterisation techniques were employed to characterise the NH and CH additives. Mechanical performance, extrusion window, and shape stability of several AAM formulations containing various dosages (0.5 wt.% to 5 wt.%) of the NH and CH additives were evaluated. The best-performing mixtures in terms of fresh and hardened properties namely, NH-1.5 and CH-1.5 mixtures (containing 1.5 wt.% of NH and CH additives, respectively) were then selected for 3D printing. The results showed that the CH-1.5 mixture exhibited enhanced shape stability, buildability, and mechanical properties as compared to the control mixture. The flexural and compressive strengths of 3D printed CH-1.5 samples were 88% and 40%, respectively higher than those of the printed control samples. Using the CH-1.5 mixture, a twisted column with an intricate shape was printed to verify the suitability of the developed CH-modified AAM for the construction of complex structures. This study establishes the use of halloysite clay minerals as low-cost additives for enhancing the mechanical properties and printing performance of AAMs.

Ma G., Yan Y., Zhang M., Sanjayan J.

A new type of 3D-printable ‘one-part’ geopolymer was synthesized with fly ash (FA), granulated blast furnace slag (GBFS), steel slag (SS) and flue gas desulfurization gypsum (FGD). The effects of SS content (0–40%) on the rheological properties, 3D-printability, mechanical anisotropy and reaction kinetics of geopolymer were investigated. The yield stress and plastic viscosity monotonically decreased with the increasing SS content. Contrarily, the geopolymer with 10% of SS presented better extrudability, buildability and mechanical strength than those with 0, 20%, 30% and 40% of SS. This was mainly attributed to the conflicting influence of SS on geopolymerization, of which the OH − produced by hydration of SS raised the alkalinity of the reaction system and accelerated the dissolution of SiO 4 4− and AlO 4 5− , while the low reactivity prohibited the following polymerization process. Furthermore, the 3D-printed geopolymer presented more compact microstructure and less mechanical anisotropy thanks to the crosslinking of morphologically complementary products, including N(C)-A-S-H, C–S–H, AFt and CH, formed via synergistic reaction of FA-GBFS-SS-FGD system.

Elsayed H., Gobbin F., Picicco M., Italiano A., Colombo P.

A large volume binder jetting printer was employed to fabricate prism-shaped geopolymer components. A two-parts system was used, comprising: 1) a highly alkaline solution, which was jetted on a powder bed containing aggregates (sand particles), and 2) a reactive solid component (pure metakaolin or metakaolin plus fast setting cement), present in the powder bed. In order to be able to generate appropriate, defect-free powder layers and suitably discharge the powders from the hopper feeding the layer forming system, a granulation approach was employed. The jetted alkaline solution effectively reacted with the metakaolin powders present in the granules, forming a geopolymer. Printed geopolymer parts, fabricated with the addition of 30 wt% metakaolin, possessed a compressive strength of ~20 MPa, even with ~30 vol% of residual porosity, and no significant variation in the compressive strength was observed after leaving the printed parts submerged in water for 1 week. • Binder jetting was employed using a large-scale printer to fabricate 3D geopolymer. • The granulation of the metakaolin allowed for the defect-free powder layers. • Selectively jetting of alkaline liquid on metakaolin led to Eco-friendly geopolymer. • 3D printed geopolymer parts possess compressive strength ~20 MPa at ~30% porosity. • The produced parts are competitive with structural concrete for varied applications.

Kondepudi K., Subramaniam K.V., Nematollahi B., Bong S.H., Sanjayan J.

The rheological behavior of concrete mixtures made with alkali-activated fly ash-slag (AAFS) binder paste is investigated for varying aggregate content. The static yield stress, plastic viscosity and thixotropy of the mixtures are evaluated and related with the performance in 3D Concrete Printing (3DCP). The thixotropy of the concrete mixture is primarily influenced by AAFS binder paste composition and does not change with the aggregate content. Rheology control of the AAFS binder paste is achieved using additives like nano-clay, which enhance the buildup of internal structure and provide thixotropy in the concrete mixtures. The paste content, the surface area of aggregate, and the packing of solids in a concrete mixture influence its yield stress. The static yield stress increases sensitively in relation to the proportion of the surface area of the solids to the paste volume content in the concrete mixture. The minimum requirement of the paste content from packing of solids in the mixture is given by the Fuller-Thompson curve. Excess paste content in proportion to the demand determined from the surface area of aggregate and packing of solids in the concrete mixture produces a decrease in the static yield stress. For producing AAFS concrete mixtures suitable for 3DCP, the paste content in the concrete mixture should meet the requirement of paste demand, which depends on the aggregate content and packing of solids.

Liu S., Lu B., Li H., Pan Z., Jiang J., Qian S.

3D printing construction techniques are believed to have potential sustainability benefits, including improved resource efficiency, increased construction productivity, and construction of complex geometries without supporting structures. 3D printable concrete materials, when introducing industrial wastes such as fly ash, silica fume, and slag, may also bring additional sustainability benefits. These advantages need to be verified quantitatively. This study investigated the environmental impact of 3D printable concrete materials using industrial wastes compared with the conventional ones via life cycle assessment (LCA). Two types of concrete materials applied in concrete casting or 3D printing were compared, that is, cement-based concrete and geopolymer concrete. The results indicate that using waste materials as cement replacement could bring environmental benefits; however, such environmental benefits might be diminished with increasing activator content in geopolymer concrete for 3D concrete printing. Based on the material-level LCA results, this study further conducted an LCA study at the component level, which investigated the life-cycle environmental impact of concrete components of different shapes constructed by Contour Crafting method. Results show that the potential environmental benefit of 3D concrete printing increases with the level of building complexity while decreases with the reuse times of formwork, which leads to the conclusion that 3D concrete printing method is more desirable for constructing non-repetitive freeform concrete structures.

Chen Y., Jia L., Liu C., Zhang Z., Ma L., Chen C., Banthia N., Zhang Y.

The mechanical anisotropy of 3D-printing concrete is a crucial factor limiting the application of this technology. This study focuses on the evolutions of mechanical anisotropy and interlayer pore structure in the 3D-printed alkali-activated materials (3DPAAMs) with different precursors combinations of grounded granulated blast-furnace slag (GGBFS) and fly ash (FA), aiming to elucidate the relationship of mechanical anisotropy evolution and precursor selections. The pores generated by the printing process were quantitatively characterized with X-ray computed tomography (X-CT) tests using image processing methods . Results show that the mechanical anisotropy coefficient ( I Mechanical ) of 3DPAAMs mitigates with the curing age since the formed reaction products fill the pores generated by the printing process. With the curing age extension from 3 to 28 days, the I Mechanical of GGBFS-based 3DPAAMs decreased 51.1%, 60.9%, and 71.1% for compressive, flexural and split tensile strengths , respectively. Over-high FA incorporation (50 and 75%) yields lower mechanical strengths and aggravated anisotropy. Compared with GGBFS-based 3DPAAMs, the addition of 75% FA increases the I Mechanical at 28 days by 274%, 236% and 274% for compressive, flexural and split tensile strengths, respectively. The low activation reactivity of FA and the higher thixotropy of FA-incorporated mixtures coarsen the interlayer pore structure, contributing to a higher mechanical anisotropy. • Mechanical anisotropy and interlayer pore structure were quantitatively characterized. • The pores generated by the printing process were characterized. • The mechanical anisotropy of 3DPAAMs mitigates with the extension of curing age due to the formation of reaction products. • High-dosage FA incorporation aggravates mechanical anisotropy due to the weak activation reactivity and high thixotropy.

Bong S.H., Nematollahi B., Xia M., Ghaffar S.H., Pan J., Dai J.

• Mineral wollastonite microfiber is a low-cost and sustainable reinforcement for 3DCP. • At 10% replacement of wollastonite, the static yield stress and thixotropy were enhanced. • At 10% replacement of wollastonite, the flexural strength was enhanced. • At 10% replacement of wollastonite, the compressive strength was not changed. Integration of reinforcement in the 3D concrete printing (3DCP) process is a major challenge. As a possible solution, the addition of short synthetic/metallic fibers directly to a fresh mixture before extrusion has been investigated in previous studies. However, the use of natural/inorganic microfibers such as wollastonite as reinforcement for 3DCP has received less attention. Wollastonite is substantially cheaper and more environmentally friendly than synthetic/metallic fibers. To fill this knowledge gap, this study reports a systematic approach to enhance the flexural strength of a 3D-printed geopolymer by the addition of wollastonite microfiber. The effect of different replacement levels of wollastonite (0, 5, 10, 15, 20, and 30% by weight of sand) on setting time and mechanical properties of several mixtures were evaluated to identify the optimum wollastonite content. The printing performances, rheological properties, and mechanical strengths of the optimum mixture were then evaluated and compared with the control mixture (without wollastonite). The results showed that at 10% replacement level, the static yield stress and thixotropy property of the mixture were enhanced, which is desirable for the superior printability of the mixture. In addition, the flexural strength of the mixture incorporating 10% wollastonite was superior to the control mixture, whereas the compressive strength was not changed. The use of mineral wollastonite microfibers as a low-cost and environmentally friendly reinforcement for 3DCP is experimentally established in this study.

Ziejewska C., Marczyk J., Korniejenko K., Bednarz S., Sroczyk P., Łach M., Mikuła J., Figiela B., Szechyńska-Hebda M., Hebda M.

In recent years, 3D concrete printing technology has been developing dynamically. Intensive research is still being carried out on the composition of the materials dedicated to innovative 3D printing solutions. Here, for the first time, concrete-geopolymer hybrids produced with 3D printing technology and dedicated environmentally friendly building construction are presented. The concrete-geopolymer hybrids consisting of 95% concrete and 5% geopolymer based on fly ash or metakaolin were compared to standard concrete. Moreover, 3D printed samples were compared with the samples of the same composition but prepared by the conventional method of casting into molds. The phase composition, water leachability, compressive, and flexural strength in the parallel and perpendicular directions to the printing direction, and fire resistance followed by compressive strength were evaluated. Concrete-geopolymer hybrids were shown to contain a lower content of hazardous compounds in leaches than concrete samples. The concentration of toxic metals did not exceed the limit values indicated in the Council Decision 2003/33/EC; therefore, the materials were classified as environmentally neutral. The different forms of Si/Al in fly ash and metakaolin resulted in the various potentials for geopolymerization processes, and finally influenced the densification of the hybrids and the potential for immobilization of toxic elements. Although the compressive strength of concrete was approximately 40% higher for cast samples than for 3D printed ones, for the hybrids, the trend was the opposite. The addition of fly ash to concrete resulted in a 20% higher compressive strength compared to an analogous hybrid containing the addition of metakaolin. The compressive strength was 7–10% higher provided the samples were tested in the parallel direction to the Z-axis of the printout. The sample compressive strength of 24–43 MPa decreased to 8–19 MPa after the fire resistance tests as a result of moisture evaporation, weight loss, thermal deformation, and crack development. Importantly, the residual compressive strength of the hybrid samples was 1.5- to 2- fold higher than the concrete samples. Therefore, it can be concluded that the addition of geopolymer to the concrete improved the fire resistance of the samples.

Chen Y., Liu C., Cao R., Chen C., Mechtcherine V., Zhang Y.

This research aims to illuminate the effect of precursor nature on the rheology of alkali-activated materials (AAM) for providing some understanding for the precursor selection and mixture design of 3D-printing AAM (3DPAAM). Binary AAM pastes were prepared with several precursors, including ground granulated blast-furnace slag (GGBFS), fly ash (FA) and silica fume (SF). Various 3D-printing-related rheological properties were characterized, including static yield stress, thixotropy and viscosity recovery evolutions. Results show that the replacements of GGBFS with FA or SF increase the static yield stress of AAM systems within 30 min after deposition. The FA or SF incorporation affects the structural build-up rates in the flocculation and polycondensation stages as well as their intersection times. The structure build-up rate in the flocculation stage is governed by the inter-particle distance of suspension, while the counterpart in the polycondensation stage depends on the reactivity of the precursor. The effects of FA or SF incorporation on the thixotropy and viscosity recovery are dependent not only on the adopted dosage but also on the rest time. 25 wt% FA or 10 wt% SF is the appropriate incorporated dosage in 3DPAAM to improve the 3D-printing-related rheology. • Experimental tests address the impacts of precursor combinations on the 3D-printing-related rheology properties. • FA or SF addition affects structural build-up rates and intersection times. • The driving forces of structural build-up rates in different stages are different. • The effects of FA or SF incorporation on the thixotropy and viscosity recovery depend on dosage and rest time.

Muthukrishnan S., Ramakrishnan S., Sanjayan J.

The concept of set-on-demand in concrete with an intervention at the print head could be used to accelerate the concrete's yield strength development after placement without significantly affecting the pumpability and extrudability . This study demonstrates the proof-of-concept of the set-on-demand approach in geopolymer concrete, where the initial mixing of reactive components (i.e., activators + binder) is conducted at the print head. The study first analyses the feasibility of the proposed concept by assessing the yield strength development rates of fresh concrete for varying mix design parameters. With a suitable mix design is chosen, a modular in-line dynamic mixing equipment was utilised to assess the effect of various mixing regimes and operating parameters (shearing histories) on the yield strength development rates. The results demonstrated that the in-line mixing process of reactive components for a short duration (i.e. 45 secs) would be sufficient to achieve a yield strength development rate of 800 Pa/s after placement, making it suitable for the print-head mixing process. The rapid setting of geopolymer concrete was utilised to print out-of-plane structures (simply supported beams) as a proof of concept .