Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D printing

Accepted Manuscript Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D printing Biranchi Panda, Cise Unluer, Ming Jen Tan PII:

S0958-9465(17)31081-8

DOI:

10.1016/j.cemconcomp.2018.10.002

Reference:

CECO 3151

To appear in:

Cement and Concrete Composites

Received Date: 28 November 2017 Revised Date:

27 September 2018

Accepted Date: 5 October 2018

Please cite this article as: B. Panda, C. Unluer, M.J. Tan, Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D printing, Cement and Concrete Composites (2018), doi: https://doi.org/10.1016/j.cemconcomp.2018.10.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D

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printing

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Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

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Biranchi Panda1, Cise Unluer2*, Ming Jen Tan1

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School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

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* Corresponding author. Tel.: +65 91964970, E-mail address: [email protected]

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Abstract:

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This study presents the development of fly ash-based geopolymer mixtures for 3D concrete

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printing. The influence of up to 10% ground granulated blast-furnace slag (GGBS) and silica

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fume (SF) inclusion within geopolymer blends cured under ambient conditions was investigated

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in terms of fresh and hardened properties. Evolution of yield stress and thixotropy of the

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mixtures at different resting times were evaluated. Mechanical performance of the 3D printed

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components was assessed via compressive strength measurements and compared with casted

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samples. SF demonstrated a significant influence on fresh properties (e.g. recovery of viscosity),

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whereas the use of GGBS led to higher early strength development within geopolymer systems.

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The feasibility of the 3D printing process, during which rheology was controlled, was evaluated

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by considering extrusion and shape retention parameters. The outcomes of this study led to the

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printing of a freeform 3D component, shedding light on the 3D printing of sustainable binder

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systems for various building components.

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Keywords: Rheology; geopolymers; thixotropy; 3D printing; performance

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1.

Introduction

3D printing of construction materials has the potential to disruptively change traditional building practices by the application of digital modelling and technologies to produce freeform building

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components. Recent developments in the printing of concrete materials started in the mid-1990s in California, USA, when Khoshnevis introduced a technique referred to as “contour crafting” (CC), where the cement mortars are deposited layer by layer following a 3-dimensional (3D) digital model [1]. CC can provide automation in traditional construction processes, while

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reducing excess material wastage and labour counts. Furthermore, the advantage of CC from an architectural perspective involves the design of freeform building components without the need

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of any additional formwork and tools. Figure 1 shows a schematic representation of extrusion-

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based concrete printing process, as described in [2].

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Figure 1. An example of typical concrete printing process

One of the major issues to be resolved in the implementation of 3D concrete printing is the

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development of a clear understanding of how the material can be fluid enough to flow through a hose pipe without clogging, meanwhile possessing a sufficient viscosity to maintain its shape after the printing process [3]. For this purpose, the developed concrete mix must be thixotropic in nature, for which it should demonstrate a high yield stress at rest and a low viscosity during flow. Keeping these challenges in mind, cementitious materials that can be easily extruded and retain their shape after extrusion have been developed by academic researchers and industry practitioners [3-6].

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A recent study [7], proposed a suitable mix design for printing high performance fiber reinforced concrete, whose binder component was composed of 70% Portland cement, 20% fly ash and 10% silica fume, with sand/binder and water/binder ratios of 3:2 and 0.26, respectively. This mix design was used to print a 3D bench via the use of a gantry-based concrete printing system. The

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hardened property of the printed section was later assessed by testing samples that were extracted from the printed block in three perpendicular directions. This was followed by studies on the characterization of the fresh and hardened properties of concrete mixes used for printing [4]. Perrot et al. [8] introduced a model for the structural built-up of cement-based materials used in

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the 3D printing process. This model was based on the comparison of vertical stress acting on a freshly deposited layer with the material critical stress, which can be useful in estimating the

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highest building rate prior to the printing process.

Most of the concrete mix designs proposed so far involve the use of ordinary Portland cement (OPC) as the main binder, with the addition of only 20-30% other supplementary cementitious materials (SCMs) such as fly ash (FA), silica fume (SF) and ground granulated blast-furnace slag (GGBS). As the production of OPC is associated with high energy demands and CO2 emissions,

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recent research initiatives focus on the development of mixes involving a higher volume of SCMs, thereby reducing the overall environmental impact. Geopolymers, developed as a part of these efforts, can not only reduce the dependence on OPC, but also potentially present mixes

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with lower environmental impacts that can be used in 3D printing [9-10].

Regardless of the binder component used, correct formulation, which leads to a printable mortar

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that exhibits extrudable and no-slump behaviour, is key in the success of the 3D printing process. Until now, a very limited amount of work has been reported on the effect of the mortar composition on the fresh properties of FA based geopolymers developed for the 3D printing process. Apart from the material viscosity and yield stress, another important parameter in the development of these printable materials is thixotropy. Thixotropy, within the binder context, refers to formulations that flocculate (i.e. demonstrate viscous behaviour) at rest, while revealing de-flocculation ability upon the application of shear forces.

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This study aims to investigate the effect of three key materials (i.e. FA, GGBS and SF) on both the fresh and hardened properties of geopolymer mixes, cured under ambient conditions, to be used in the 3D printing process. In terms of fresh properties, the yield stress, structural build up and breakdown of various mixes, whose composition was determined in line with a design of

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experiment (DOE) approach, were measured. In addition to these properties, thixotropy recovery was assessed by using the three-interval thixotropy test (3ITT). The mechanical performance of the prepared mixes was evaluated via their compressive strengths and compared with conventional mould casted samples. Overall, the findings of this study can be used in the

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components composed of alternative binder systems.

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determination of the right mix procedure for the production of sustainable 3D printed

2. Experimental program

2.1. Materials

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The primary raw material used in this study was class F fly ash (FA) obtained from HeBei BaiSiTe Technology Co. Ltd. (China). Ground granulated blast-furnace slag (GGBS), supplied by Engro Ltd. (Singapore), was used together with FA in the preparation of geopolymer mixes. Un-densified micro silica fume G940 (SF), purchased from Elkem Ltd. (Singapore), with a

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specific surface area (SSA) of 15000-30000 m2/kg, was also included together with FA (SSA = 200-285 m2/kg) and GGBS (SSA = 460-600 m2/kg) to increase cohesiveness of the mix. Table 1

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shows the chemical composition of FA, GGBS and SF, obtained by X-ray fluorescence (XRF). The alkaline activating solution was formulated by blending a commercial sodium silicate solution (32.4 wt.% SiO2, 13.5 wt.% Na2O and 54.1 wt.% H2O) with 45 wt.% NaOH solution to reach the desired modulus (MS = molar SiO2/Na2O ratio) of 1.8 [9]. Table 1. Oxide composition of FA, GGBS and SF

Element

FA

GGBS

SF

SiO2

49.15

29.65

98.37

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39.35

15.56

0.19

Fe2O3

3.48

0.35

0.08

CaO

2.94

39.38

0.35

TiO2

1.68

1.76

-

SO3

1.17

4.32

0.19

K2O

0.88

0.51

P2O5

0.43

0.012

MgO

0.40

7.54

Na2O

0.33

0.45

MnO

0.06

0.24

-

Cr2O3

0.04

-

-

SrO

0.03

ZrO2

0.03

ZnO

0.02

Rb2O

0.02

PbO

0.008

Ga2O3

0.005

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Al2O3

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0.28

0.04

0.15

0.20

0.01

-

-

-

0.04

-

-

-

-

-

-

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0.09

As can be seen in Figure 2, both FA and SF particles possessed a spherical morphology, whereas GGBS was made up of angular particles. X-ray diffraction (XRD) patterns of FA, GGBS and SF

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are shown in Figure 3. Broad humps at 18-28º 2Theta in FA, 25–35º 2Theta in GGBS and 18-22º 2Theta in SF indicated the amorphous phases within each material. In addition to the amorphous

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phases, some crystalline phases referring to the presence of quartz (SiO2), mullite (Al6Si2O13) and maghemite (Fe2O3) were seen in FA; whereas gypsum (CaSO4·2H2O) was detected in GGBS. In addition to their morphology and chemical compositions, the particle size distribution curves of these materials, measured by Malvern Mastersizer 3000, are shown in Figure 4. Amongst all the materials used in this study, SF had the smallest particle size, whereas FA and GGBS demonstrated relatively similar particle sizes, albeit possessing different physical shapes.

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Figure 2. FESEM images of (a) FA, (b) GGBS and (c) SF

Figure 3. XRD patterns of FA, GGBS and SF (Q = quarts, M = mullite, G = gypsum, Me =

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melilite, A = akermanite and C = cristobalite)

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Figure 4. Particle size distributions of FA, GGBS and SF

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2.2. Sample preparation and methodology

Ten mix designs involving different contents of FA, GGBS and SF were developed as a part of this study, in line with the findings of preliminary experiments and previous studies [3, 9]. The

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main phase, FA, was included at an amount ranging between 90% and 100%, whereas GGBS and SF contents varied between 0-10% (by mass) of the overall binder content, as shown in Table 2. The exact content of each material was determined via DOE, following a simplex-

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centroid design approach [11, 12]. The designed formulations were aimed to enable the hardening of the prepared samples under ambient conditions, providing a setting time and early age (i.e. 7-day) compressive strength of 30 minutes and 15-20 MPa, respectively. SF, which possesses a high surface area, was added at the same replacement ratio as GGBS to provide a ball-bearing effect in the blend. The solution/binder ratio was kept constant at 0.46 [12].

Table 2. Mix proportions of blends containing FA, GGBS and SF

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FA (%) GGBS (%) SF (%)

F100

100

0

0

F90G10

90

10

0

F90S10

90

0

10

F95G5

95

5

0

F95S5

95

0

F90G5S5

90

5

F93.3G3.3S3.3

93.3

3.33

F96.7G1.67S1.67

96.7

1.67

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Mix label

F91.7G6.67S1.67

91.7

6.67

1.67

F91.7G1.67S6.67

91.7

1.67

6.67

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3.33

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1.67

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2.2.1. Rheological measurements

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Rheology measurements of the prepared mortars were performed using a MCR 102 rheometer

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with a 4-blade vane geometry from Anton Par, Germany. The diameter of the vane was 30 mm, blade height was 40 mm and active length was 133 mm. Before the measurements, the pastes were prepared by first mixing FA with GGBS and/or SF, followed by gradually introducing the activator solution into the dry mix. The duration of the entire process involving the preparation

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of the geopolymer pastes was kept constant at around 3 minutes. The prepared pastes were then transferred to the rheometer cup, after which 1 minute of pre-shearing was done to create a uniform condition for all the samples. Stress growth test was performed to capture the evolution

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of yield stress (i.e. approximately every 10 minutes) at a constant shear rate of 0.01 s-1 for 180 seconds [13, 14], which was comparable to the undisturbed vane test measurement presented in earlier studies [15]. Considering that the growth of yield stress is an indicator of material stiffness, the yield stress test was conducted only once for each mixture due to limited material availability. To avoid any variations in sample analysis, extreme care was taken in the testing procedure. Nevertheless, the generation of more data points is important for the demonstration of experimental accuracy and repeatability.

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To obtain the thixotropy index of the geopolymer pastes, a protocol suggested in [13] was followed, as shown in Figure 5(a). First, a 2-minute pre-shear at 300 s-1 was used to create a uniform condition for all samples, followed by resting times of 1, 5, 10, 15 and 20 minutes. Then, the samples were sheared at a constant rate of 50 s-1 for 30 seconds. The data obtained in

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the third step was used to analyse the thixotropic behaviour of the geopolymer pastes. As shown in Equation 1, a dimensionless parameter (λ) was used as the thixotropy index, where are the initial and equilibrium shear stresses, respectively.

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λ=

and

(1)

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Additionally, a viscosity recovery test, as shown in Figure 5(b), was conducted in three intervals: (i) low shear rate of 0.01 s-1 for 60 seconds, (ii) high shear rate of 100 s-1 for 30 seconds and (iii) low shear rate of 0.01 s-1 for 60 seconds, to investigate the structural recovery behaviour of different geopolymer mixes after their extrusion from the nozzle. The shear rates and corresponding time intervals were intentionally selected to mimic the 3D printing process, as

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described in [16].

Figure 5. Rheology protocol to measure the (a) thixotropy index and (b) structural buildup/recovery of geopolymer pastes

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2.2.2. 3D printing

Mortar mixes involving one of the binder compositions investigated in this study (F90G5S5)

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were produced by using river sand with a fineness modulus of 2.75. A sand/binder ratio of 1.5 was used in the preparation of these mixes [17]. The motivation behind the selection of this particular mix design was explained in terms of the required fresh and hardened properties in the

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Results section.

The printing process involved the use of a 4-axis gantry concrete printer [18], operated at a speed

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of 80 mm/sec. The geopolymer mortar was extruded from a screw pump at a flow rate of 0.5 l/min. A 10 mm circular nozzle was attached to the extruder of the gantry printer with a cross section area ratio (i.e. nozzle/hose) of 0.16. Three samples were collected from the printed block of 400 x 60 mm (length x width) and tested for their compressive strength after 7 days.

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2.2.3. Isothermal calorimetry

The heat flow and cumulative heat evolved during the geopolymer reaction of each blend were studied at 26 oC by an I-Cal 8000 high precision calorimeter, in accordance with ASTM C1702-

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15a [19]. To prepare the paste samples, the dry powders were mixed with the activator solution, which was previously heated to 26 oC in order to produce mixes at the same temperature as the

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measurement temperature. The prepared mix was placed into the isothermal calorimeter channel within 30 seconds to measure the heat of reaction.

2.2.4. Compressive strength

The compressive strengths of the printed as well as the casted cubic samples with dimensions of 50 x 50 x 50 mm were measured under uniaxial loading according to BS EN 196-1:2016 [20]. The equipment used for this purpose was an ALPHA 3-2000A machine, operated at a loading 10

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rate of 0.6 MPa/sec. Before the test, all samples were cured under room temperature (25±2 oC) for 7 days. Three samples were tested for each set and their average and standard deviation was

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calculated and reported for each data point.

3. Results and Discussion

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3.1. Effect of mix design on the yield stress of geopolymer pastes

The structural build-up of cementitious materials is often characterized by the evolution of static

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yield stress with time. This property is crucial for many cement-based applications, especially 3D printing, since it influences the pump pressure required to initiate flow during the extrusion stage. In line with these requirements, the yield stress of the different blends of geopolymer pastes was assessed by analysing their shear stress responses. Figure 6 summarizes the development of the static yield stress at up to 30 minutes from the time of mixing. Like OPCbased mixes [21], the yield stress of geopolymers also increased with time, which could be

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associated with the physical interaction of the particles and the chemical activation of the binder. The use of SF along with FA in mixes F90S10 and F90G5S5 significantly increased the yield stress of the pastes. When compared to the F100 mix (control), which demonstrated a very low yield stress value of ~330 Pa, both mixes involving 5-10% replacements of FA with SF

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demonstrated yield stress values of almost double in value. This increase was associated with the improved particle packing in these mixes due to the finer particle size of SF [22, 23]. On the

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other hand, the 10% replacement of FA with GGBS in the F90G10 mix did not indicate a significant change in the initial yield stress when compared to F90S10. This lack of change could be due to low amount of GGBS present in the mix, as well as its bigger particle size when compared to the finer nature of SF. In the case of F90G5S5 and F90G10, there was a sudden increase in yield stress after 20 minutes of mixing, which could be due to the acceleration in the hardening of the mix via the activation of GGBS [24]. For the other two mixes, the rate of increase of yield stress over time was lower as the reactivity of fly ash based geopolymers without GGBS is lower in ambient conditions.

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Figure 6. Evolution of the yield stress of geopolymer pastes with time

The yield stress results of ten mixes containing up to 10% GGBS and SF, analysed by Minitab software (DOE Mixtures), are shown in Figure 7, where a contour plot of the yield stress is

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displayed. The maximum percentage of each material (i.e. FA, GGBS or SF) considered by the regression is placed at the corresponding corner while the minimum is positioned in the middle of the opposite side of the triangle. In line with this setup, the centre of the triangle represents a mixture in equal parts. As can be seen from the presented results, the addition of SF in the

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designed mixes led to an increase in the yield stress, indicated by the dark grey colour on the right-hand corner of the plot. On the other hand, the addition of GGBS did not change the colour

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gradient of the contour plot significantly, which confirmed its lack of influence on the yield response, as described earlier.

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Figure 7. Mixture contour plot showing the effect of FA, GGBS and SF contents on the initial yield response

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3.2. Effect of mix design on thixotropy and structure rebuilding

The term “thixotropy” refers to a reversible, isothermal, time-dependent decrease in viscosity when a fluid is subjected to increased shear stress or shear rate [25, 26]. As shown in Figure 8, thixotropic materials convert into a solution on the application of shear stress, while going back

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to a gel (semi-solid) state upon the removal of stress. This flocculation at rest and de-flocculation under flow behaviour plays a major role in concrete printing applications. Previous studies [33,

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34] have investigated the flocculation process and its growth with time, as the rate of flocculation can affect the buildability of printed structures. In line with this, this study included a measurement of the thixotropy index by mainly focusing on the flocculation phenomenon.

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Figure 8. Sol-gel transformation in thixotropy phenomenon

The structural evolution of a selected mix (F90G5S5) at different resting times is shown in

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Figure 9, where it was seen that the thixotropy index/parameter (λ) of the paste increased with resting time. This phenomenon was linked to both the physical interaction of particles and the hardening process. Unlike OPC-based cementitious mixes, geopolymer blends do not possess thixotropic behaviour due to the lack of colloidal interaction. A recent study [21, 35] revealed that the rheology of geopolymers is mainly allied with the viscous nature of the alkaline reagent rather than particle interaction. Furthermore, due to the polycondensation reaction, addition of

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any OPC-based additives in the geopolymer matrix does not work well in the improvement of thixotropic properties. Accordingly, these points were taken into consideration in the design of

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mixes presented in this study.

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Figure 9. Transient flow behaviour of the F90G5S5 paste at different resting times

Table 3 presents the evolution of λ for different geopolymer mixes at different resting times of 1, 5, 10 and 20 minutes. The premise for studying the thixotropic behaviour is that the defined λ

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should be reasonable for 3D concrete printing application. Within this context, a lower λ value refers to a less pronounced thixotropic behaviour. For all the four mixes, λ increased with resting time. However, for those mixes containing SF, λ value was a bit higher when compared to the control mix. The effect of SF can be explained in terms of its contribution to the increase in yield

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stress, which resulted in a higher

value, thus a higher λ [27]. This finding was in line with

those reported in [36], where the authors reported increased flocculation rates in the presence of

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SF particles due to their fine particle size and high surface area. On the other hand, the addition of GGBS did not seem to initially improve λ in the F90G10 mix, whereas its presence caused a rapid change in the λ value over time due to its accelerating nature in the prepared mixes.

Table 3. The relationship between λ and resting time

Resting time

Structural parameter (λ) F100

F90G10

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F90S10

F90G5S5

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0.24

0.26

0.35

0.34

5 min

0.34

0.92

0.38

0.39

10 min

0.58

1.23

0.46

0.81

20 min

0.73

1.42

0.66

0.86

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1 min

The results of structural recovery (build-up) test is shown in Figure 10. The three regions, as indicated on this figure, refer to the state of the material at rest before pumping/extrusion, the pumping stage and the condition of the material after being extruded out from the nozzle,

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respectively. Comparing the viscosity before and after printing, the addition of SF was found to have a notable influence on the recovery behaviour with respect to the control mix (F100) as well

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as the mixes containing GGBS at the same replacement level. The addition of 5% SF to the control mix (F100) led to the recovery of 13% of the initial viscosity, which was 5-7% higher than the recovery ability demonstrated by the F100 mix. This recovery level increased to 20% with an increase in the SF content from 5% to 10% of the binder content. On the other hand, the use of GGBS did not seem to improve the thixotropic property of the geopolymer blends, even at

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higher substitution rates.

Figure 10. Effect of FA, GGBS and SF contents on the viscosity recovery 16

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These abovementioned results can serve as an indication of the recovery ability of each mix design, where the extruded material is expected to retain its shape, owing to the recovery ability of its initial viscosity. Figure 11 shows the impact of thixotropy recovery on single layer

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extrusion of selected geopolymer mixes using a custom made 4-axis gantry printer. As observed earlier in Figure 10, the F100 mix demonstrated a very low ability in recovering its original viscosity. This was also evident from the large deformation (i.e. low shape retention factor (SRF) of 0.3) observed after its extrusion (Figure 11(a)). Alternatively, in line with the thixotropy

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results, the amount of deformation was limited within the F90G10 mix (Figure 11(b)) and especially the F90S10 mix (Figure 11(c)). The lack of deformation within the F90S10 mix was

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due to the higher yield stress and cohesiveness caused by the finer SF particles that improved the packing density of the mix [28, 29]. Alternatively, although the initial deformation of the F90G10 mix was noticeable, an increase in its stiffness was observed over time due to the rapid dissolution of GGBS, presenting a challenge in the extrusion process. Therefore, the contents of SF and GGBS within fly ash based geopolymers must be optimized for both easy extrusion and

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better shape retention properties.

Figure 11. Effect of mix design on the shape retention of the extruded filament width through a 30×15 nozzle for mixes (a) F100 (b) F90G10 and (c) F90S10

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3.3. Compressive strength and reaction kinetics of 3D printed geopolymers

The reaction kinetics and the mechanical properties of the two mixes (F90S10 and F90G5S5)

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that demonstrated a good recovery ability, were assessed and compared to the control mix (F100) prior to 3D concrete printing. The heat of reaction, obtained via isothermal calorimetry, was considered as an indicator of the overall hydration reaction within the prepared pastes [30]. As can be seen in Figure 12, a very low heat flow was observed in the F100 mix, while the addition

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of GGBS in mix F90G5S5 led to an obvious increase in the heat flow and cumulative heat. The contribution of GGBS to the polycondensation reaction was observed both in the higher intensity

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of the initial peak as well as the cumulative heat measured during the first 48 hours of reaction. This enhancement in the amount of heat was an indication of the geopolymer reaction in the presence of an amorphous phase, which dissolved in the alkaline reagent and released more heat than the F100 mix. These results were in line with the findings reported by earlier studies [31], highlighting that the use of GGBS within geopolymer mixes can reduce the setting time and provide denser microstructures, thus resulting in higher compressive strengths at early ages. On

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than the control mix.

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the other hand, the addition of SF in mix F90S10 led to a heat evolution that was slightly higher

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Figure 12. Isothermal calorimetry results showing (a) heat flow and (b) cumulative heat of mixes F100, F90S10 and F90G5S5

In line with these results, the F90G5S5 mix was chosen in the preparation of a printable mortar, which was extruded in a layer by layer manner to print a 400 mm length solid block with 6

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seconds delay time, as shown in Figure 13(a). 50 x 50 x 50 mm cubes were extracted from this block to measure their compressive strength, which was then compared with the same mix in casted form. According to Figure 13(b), printed samples demonstrated similar strengths (18.4 vs. 16.2 MPa) as casted samples under compression. This was due to the well-designed printing

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process that prevented the formation of voids during the extrusion of the mix, whereas the strength of the sample in the other two directions might differ according to the anisotropic nature of the 3D printing process [32]. It must be noted that for any sections obtained via 3D printing,

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strength is dependent on production parameters such as printing direction, quality of printing, extrusion pressure and bond strength between layers [37]. Therefore, it is important to consider these factors associated with the printing process and the properties of the raw materials used in the preparation of 3D printed components when comparing the results reported by different sources.

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Figure 13. F90G5S5 geopolymer mix used in the (a) preparation of a 3D printed section and (b) compressive strength measurement of the 3D printed section vs. casted samples

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4. Conclusions

This study aimed to investigate the effect of SCMs, namely GGBS and SF, on the fresh and hardened properties of geopolymer blends cured under ambient conditions, with the goal of designing mixes to be used in extrusion-based concrete printing applications. This was

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performed via the preparation of FA mixes containing GGBS and SF at a replacement ratio of up to 10%. The prepared mixes were characterised for their yield stress, viscosity and thixotropy,

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which are considered as key properties for the successful implementation of the 3D printing processes. The experimental results demonstrated the limited contribution of GGBS in improving the fresh property of the geopolymer pastes, whereas it had a notable influence on the early age compressive strength. Within these mixes, the inclusion of GGBS may have contributed to the development of a homogenous microstructure, as well as the production of a stronger 3dimensional network that can enhance the final strength of geopolymers due to its amorphous phases. Although GGBS did not provide a significant improvement in terms of rheology, it influenced the time dependent structural built up in the material by reducing the workable time of the paste. Therefore, its content in the mix should be carefully controlled, meanwhile 20

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maintaining an acceptable level of final strength. On the other hand, the inclusion of SF was effective in controlling the yield stress and viscosity of the blends in the fresh stage. These improvements were attributed to the high surface area and spherical shape of the SF particles, which enabled the smooth extrusion of the blend and portrayed a good recovery behaviour via

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the shape retention of the deposited filaments.

It must be noted that the suitability of any additives within geopolymer blends is dependent on several factors and cannot follow a one-size-fits-all approach due to the unique polycondensation

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reaction within geopolymer systems. Therefore, before generalizing the results of the present study, it is important to note that the suitability of the proposed mix design and test methods to

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characterize printable geopolymers at fresh and hardened stages are dependent on the chemical composition of the mixture and experimental parameters used for the assessment of each property.

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Acknowledgement

The authors would like to acknowledge Sembcorp Architects & Engineers Pte. Ltd. and National

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References

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Research Foundation (NRF), Singapore for their funding and support in this research project.

1. B. Khoshnevis, Automated construction by contour crafting—related robotics and information technologies. Autom. Constr. 13(1) (2004) 5-19. 2. Y. W. D. Tay, B. Panda, S. C. Paul, N. A. N. Mohamed, M. J. Tan, K. F. Leong, 3D printing trends in building and construction industry: a review. Virtual Phys. Prototyp. 1-16 (2017) 261276. 3. S. C. Paul, Y. W. D. Tay, B. Panda, M. J. Tan, Fresh and hardened properties of 3D printable cementitious materials for building and construction. Arch. Civil.Mech. Eng. 18(1) (2018) 311319. 21

ACCEPTED MANUSCRIPT

4. T. T. Le, S. A. Austin, S. Lim, R. A. Buswell, R. Law, A. G. Gibb, T. Thorpe, Hardened properties of high-performance printing concrete. Cem. Concr. Res. 42(3) (2012) 558-566. 5. G. Franchin, P. Scanferla, L. Zeffiro, H. Elsayed, A, Baliello, G. Giacomello, M. Pasetto, P. Colombo, Direct ink writing of geopolymeric inks. J. Eur. Ceram. Soc. 37(6) (2017) 2481-2489.

RI PT

6. B. Khoshnevis, X. Yuan, X. Yuan, B. Zahiri, B. Zahiri, B. Xia, Construction by contour crafting using sulphur concrete with planetary applications. Rapid Prototyp. J. 22(5) (2016) 848856.

7. T. T. Le, S. A. Austin, S. Lim, R. A. Buswell, A. G. Gibb, T. Thorpe, Mix design and fresh

SC

properties for high-performance printing concrete. Mater. Struct. 45(8) (2012) 1221-1232.

8. A. Perrot, D. Rangeard, A. Pierre, Structural built-up of cement-based materials used for 3D-

M AN U

printing extrusion techniques. Mater. Struct. 49(4) (2016) 1213-1220.

9. B. Panda, S. C. Paul, L. J. Hui, Y. W. D. Tay, M. J. Tan, Additive manufacturing of geopolymer for sustainable built environment. J. Clean. Prod. 167 (2017) 281-288. 10.

M. Xia, J. Sanjayan, Method of formulating geopolymer for 3D printing for construction

applications. Mater Design, 110 (2016) 382-390.

J. Cornell, Experiments with Mixtures, Third Ed., John Wiley & Sons, New York, 2002.

12.

C. Ruiz-Santaquiteria, J. Skibsted, A. Fernández-Jiménez, A. Palomo, Alkaline

TE D

11.

solution/binder ratio as a determining factor in the alkaline activation of aluminosilicates. Cem. Concr. Res. 42(9) (2012) 1242-1251. 13.

J. Ouyang, Y. Tan, D. J. Corr, S. P. Shah, The thixotropic behavior of fresh cement

14.

EP

asphalt emulsion paste. Constr. Build. Mater. 114 (2016) 906-912. B. Hasanzadeh, Testing and modeling of the thixotropic behavior of cementitous

15.

AC C

materials. PhD Thesis. University of Louisville. 2017. K.H. Khayat, A.F. Omran, S. Naji, P. Billberg, A. Yahia, Field-oriented test methods to

evaluate structural build-up at rest of flowable mortar and concrete. Mater. Struct. 45 (2012) 1547–1564. 16.

H. Li, Y. J. Tan, K. F. Leong, L. Li, 3D bioprinting of highly thixotropic

alginate/methylcellulose hydrogel with strong interface bonding. ACS Appl. Mater. Interfaces 9(23) (2017) 20086-20097. 17.

B. Panda, M. J. Tan, Experimental study on mix proportion and fresh properties of fly ash

based geopolymer for 3D concrete printing. Ceramics International 44 (2018) 10258–10265 22

ACCEPTED MANUSCRIPT

18.

B. Panda, S. C. Paul, N. A. N. Mohamed, Y. W. D. Tay, M. J. Tan, Measurement of

tensile bond strength of 3D printed geopolymer mortar. Measurement 113 (2018) 108-116. 19.

ASTM C1702-15a, Standard Test Method for Measurement of Heat of Hydration of

Hydraulic Cementitious Materials Using Isothermal Conduction Calorimetry, ASTM Committee

20.

RI PT

C01, West Conshohocken, PA 19428-2959, United States, (2015) p. 8.

BS EN 196-1:2016 Methods of Testing Cement – Part 1: Determination of Strength,

British Standards Institution. 21.

A. Favier, J. Hot, G. Habert, N. Roussel, J. B. D. E. de Lacaillerie, Flow properties of

SC

MK-based geopolymer pastes. A comparative study with standard Portland cement pastes. Soft Matter, 10(8) (2014) 1134-1141.

F. A. Memon, M. F. Nuruddin, N. Shafiq, Effect of silica fume on the fresh and hardened

M AN U

22.

properties of fly ash-based self-compacting geopolymer concrete. International Journal of Minerals, Metallurgy and Materials, 20(2) (2013) 205-213. 23.

D. Feys, Interactions between rheological properties and pumping of self-compacting

concrete, Doctoral dissertation, Ghent University.2009. 24.

S. Saha, C. Rajasekaran. Enhancement of the properties of fly ash based geopolymer

TE D

paste by incorporating ground granulated blast furnace slag. Construction and Building Materials. Constr. Build. Mater. 146 (2017) 615-620. 25.

N. Roussel, G. Ovarlez, S. Garrault, C. Brumaud. The origins of thixotropy of fresh

cement pastes. Cem. Concr. Res. 42(1) (2012) 148-157. Roussel, N. (Ed.). Understanding the rheology of concrete. Woodhead Publishing,

Cambridge. 2011.

T. M. Salem, Electrical conductivity and rheological properties of ordinary Portland

AC C

27.

EP

26.

cement–silica fume and calcium hydroxide–silica fume pastes. Cem. Concr. Res. 32(9) (2002) 1473-1481. 28.

J. Assaad, K. H. Khayat, H. Mesbah, Assessment of thixotropy of flowable and self-

consolidating concrete. Materials Journal, 100(2) (2003) 99-107. 29.

Aerosil thickening and thixotropy mechanism:

https://www.aerosil.com/product/aerosil/en/industries/resins/effects-applications/thickeningthixotropy/pages/default.aspx (accessed on 20 November 2017)

23

ACCEPTED MANUSCRIPT

30.

S. A. Bernal, J. L. Provis, V. Rose, R. M. De Gutierrez, Evolution of binder structure in

sodium silicate-activated slag-metakaolin blends. Cem. Concr. Compos. 33(1) (2011) 46-54. 31.

J. V. Temuujin, A. Van Riessen, R. Williams, Influence of calcium compounds on the

mechanical properties of fly ash geopolymer pastes. Journal of Hazardous Materials, 167(1)

32.

RI PT

(2009) 82-88.

B. Panda, S. C. Paul, M. J. Tan, Anisotropic mechanical performance of 3D printed fiber

reinforced sustainable construction material. Mater. Lett. 209 (2017) 146-149. 33.

Roussel, N. Rheological requirements for printable concretes. Cement and Concrete

34.

SC

Research (2018).

Reiter, L., Wangler, T., Roussel, N., & Flatt, R. J. The role of early age structural build-

35.

M AN U

up in digital fabrication with concrete. Cement and Concrete Research (2018). Roussel, N., Lemaître, A., Flatt, R.J. and Coussot, P., Steady state flow of cement

suspensions: a micromechanical state of the art. Cement and Concrete Research, 40(1) (2010) 77-84. 36.

Roussel, N. A thixotropy model for fresh fluid concretes: theory, validation and

applications. Cement and Concrete Research, 36(10) (2006) 1797-1806. Ma, G., Zhang, J., Wang, L., Li, Z. and Sun, J., Mechanical characterization of 3D-

TE D

37.

printed anisotropic cementitious material by the electromechanical transducer. Smart Materials

AC C

EP

and Structures. 27 (2018) 075036.

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