Rheological parameters and building time of 3D printing sulphoaluminate cement paste modified by retarder and diatomite

Construction and Building Materials 234 (2020) 117391

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Rheological parameters and building time of 3D printing sulphoaluminate cement paste modified by retarder and diatomite Mingxu Chen a,b,1, Laibo Li a,b,1, Jiaao Wang a, Yongbo Huang a,b, Shoude Wang b, Piqi Zhao b, Lingchao Lu a,b,⇑, Xin Cheng b,⇑ a b

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The utilization of retarder and

diatomite develops a 3DP SAC paste with controllable properties.
The elasticity modulus was applied to evaluate the structural deformation.
The diatomite was applied to increase the static yield stress and elasticity modulus.

a r t i c l e

i n f o

Article history: Received 8 June 2019 Received in revised form 20 September 2019 Accepted 25 October 2019

Keywords: 3D printing Boric acid Sodium gluconate Sulphoaluminate cement Rheological properties Building time

a b s t r a c t Recent advances have proposed new requirements on the controlling of the hydration, the rheology, and the structural build-up of 3D printing (3DP) sulphoaluminate cement (SAC) paste. In this study, the boric acid (BA), the sodium gluconate (SG), and the diatomite were used to control these behaviors of 3DP SAC paste, and the printed structures could be well built based on the extrusion system. The results show that the building time and printability can be improved by the introduction of BA and SG into SAC paste, but the excessive dosage will cause the structural failure. To decrease the structural deformation of 3DP SAC paste, the diatomite was applied to increase the static yield stress and elasticity modulus. Meanwhile, a paste sample with the structural deformation less than 10% can be achieved when the static and dynamic yield stress was controlled in the range of 590–895 Pa, and 509–722 Pa, respectively. In conclusion, the 3D printed structures and rheology could be well controlled by the introduction of retarders and diotomite in the 3DP SAC paste. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding authors at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China (Lingchao Lu). E-mail address: [email protected] (L. Lu). 1 Mingxu Chen and Laibo Li contributed equally. https://doi.org/10.1016/j.conbuildmat.2019.117391 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

3DP could be used as the rapid prototyping to build the objects with complex structures [1,2] through the manner of layer by layer [3,4]. In this case, 3DP technology will not be limited by

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M. Chen et al. / Construction and Building Materials 234 (2020) 117391

the traditional manufacturing technique, and it can be easily applied in the different area [5–7]. However, the application of 3DP building materials is recent [8]. 3DP building materials technology integrates the computer technology, numerical control technique, flow theory and forming technology [9,10]. The forming technology of 3DP building materials is similar to fused deposition modeling (FDM) and the structural style is free. However, the application of 3DP cementitious materials is limited by the uncontrollable setting time and flowability, the unstable 3D structures, and the low mechanical strength [11–13]. Le et al. proposed three basic requirements for 3DP fresh cementitious paste: 1) acceptable extrudability from nozzles to printed paste continuously; 2) sufficient buildability characteristics to enable the paste to support further layers without collapse; 3) enough strength to offset the inherently weaker [14,15]. Furthermore, to satisfy the requirements of 3DP, the setting time, particle size, mix design and admixture selection of cement paste should be suitably optimized. Buswell et al. offered a roadmap to inspire and guide the future research efforts about the fresh and hardened paste, mortar, and concrete materials [16]. 3DP technology may change the development direction of construction industry and it will enhance the efficiency of construction and premixed concrete industries obviously. Many research works for 3DP cement-based materials have been conducted to investigate the structural build-up and rheological properties. For instance, Marchon et al. and Ketel et al. also studied the hydration and rheology of 3DP paste through controlling the admixtures, aiming to offer a quantitative basis for assessment of 3D structure [17,18]. Reiter et al. reported the role of early structural build-up in 3DP concrete and it revealed that the range and type of yield stress are the main requirements for successful structural build-up [19]. Lecompte and Roussel et al. also provide an accurate description of the increase in yield stress in the first resting time, and then the yield stress rapidly increases as exponential [20,21]. There exists two yield stresses, including static and dynamic yield stress, corresponding to flocculation state of cement paste, which affects the 3D structure build-up most. For 3DP paste, static yield stress is related to the buildability, and the dynamic yield stress is related to the printability and pumpability of cement paste. Although many studies have built the 3D structure successfully in many methods and improved the properties of 3DP cementbased materials, few of them are focus on the effects of building time on the structural build-up. Building time is a time period during which the cement paste is extrudable. Consider the continuity and quality of 3DP materials, the building time is of great importance to enable the 3D structures to construct successfully, which is positively correlated with the setting time. For instance, under the short building time, the cement paste will be extruded difficultly from the 3D printer due to the rapid setting, which will cause nozzle blocking and discontinuous printing [22]. However, the long building time of cement paste could lead to a large deformation because of the increase in gravity. Rapid hardening cement, such as SAC and aluminate cement, presents a short setting time, which is difficult to be used in 3DP because of the uncontrollable rheological parameters and short building time. Therefore, a proper building time is indispensable to the improvement of printability and the flexible design of preset model, which will not be limited by the rapid cement hydration. Rapid setting and poor printability of SAC paste make it difficult to control the 3D structures and extrude from 3DP system [23], and the factors influencing of 3D structures are still fuzzy. In this study, BA and SG were introduced to control the building time and rheology of 3DP SAC paste. Diatomite in the 3DP SAC paste was used to improve 3D structure and the static yield stress. Structural parameters and the relationship between building time and static yield

stress are also studied to reveal factors influencing of 3D structural build-up. 2. Materials and methods 2.1. Raw materials SAC (42.5 grade, Zhonglian Company) was used as the cementitious materials in the 3DP paste. Diatomite (Macklin) was used to optimize the 3D structure and rheological properties of 3DP paste. The chemical components of SAC and diatomite are shown in Table 1. Particle size distribution of the SAC and diatomite was measured by a particle size analyzer (BECKMAN COULTER’s LS13320, Beckman Coulter, USA), as shown in Fig. 1. Hydroxypropyl methyl cellulose (HPMC, Heda Company) with a high viscosity of 75000 mPas was used as viscosity modifier agent (VMA) to improve extrudability and buildability [24]. Water reducing agent (WRA, polycarboxylate, Shandong Acadamy of Building Research) with a water reducing rate of 32% was used to improve the flowability and extrudability of 3DP paste. BA and SG (analytical reagent, aladdin) were used to control the building time and rheological behaviors of 3DP paste. 2.2. Experimental procedures The mix proportions of 3DP SAC paste is presented in Table 2 and the preparation procedure is as follows. (1) Firstly, SAC was mixed with diatomite and HPMC for 30 min to obtain a uniform powder mixture by using a V-shape blender; (2) Then, WRA, BA and SG were dissolved in the water and mixed with the above mixture for 3 min in the blender. The water to cement (w/c) ratio was set as 0.35; (3) Next, the fresh paste was placed inside the loading container, and it was vibrated evenly to eliminate air voids. Then, the container was installed on the 3D printer to start the printing process. (4) Finally, printed samples were cured in the curing chamber for 24 h with the relative temperature of 20 and the humidity of 95%. 2.3. 3DPextrusion system Fig. 2 exhibits the structures of 3D printer (Pottery artist, Dianfeng company, CHN) used in this study. As shown in Fig. 2a, 3D printer contains the XYZ axles, the platform, the air pump, the loading container, the extrusion, and the operating systems. The largest printing size is 0.8 m (Length)  0.8 m (Width)  0.8 m (Height), and the printing speed and loading speed are set as 10 and 15 mm/s, respectively. 3DP SAC paste was extruded continuously by the method of screw mixing (Fig. 2b) under the air pump pressure of 0.3 MPa. Fig. 2 c and d show the printed specimen and the different types of extrusion nozzles. 2.4. Testing methods 2.4.1. Rheological properties The rheological properties for 3DP paste were tested by a rotational rheometer (kinexus lab+, malvern, UK), including the static and dynamic yield stress, the plastic viscosity, and the thixotropy. For the measurement of static yield stress, the test process of 3DP paste can be divided into three stages (Fig. 3 a). Firstly, cement paste was loaded in the rotational rheometer and measured from 0 to 2 min with a high constant shear rate, which enables the paste

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M. Chen et al. / Construction and Building Materials 234 (2020) 117391 Table 1 Chemical component of SAC and diatomite (% by mass). Materials

CaO

Al2O3

SO3

SiO2

Fe2O3

TiO2

K2O

MgO

Others

SAC Diatomite

49.50 0.46

20.17 3.87

14.91 —

8.51 93.23

1.97 0.54

1.57 0.11

0.90 —

0.77 0.29

0.73 1.50

Based on the Bingham model, the plastic viscosity and dynamic yield stress are obtained by calculating the shear rate and shear stress of the down curves, as shown in Fig. 4 b. The calculation range of shear rate was in the stabilization stage of 50–150 s1. The relationship can be expressed by Eq. (1):

s ¼ s0 þ lc

Fig. 1. Particle size distribution of the SAC and diatomite.

to stir evenly. Secondly, rotational rheometer rested for 20 min. Thirdly, rotational rheometer measured the shear stress for 2 min under the low constant shear rate of 0.1 s1. Static yield stress is related to the flocculation state before the microstructure is broken down, and this information of structural build-up is useful for the buildability assessment. The cement paste was sheared under a low shear rate, and the shear stress increases to a peak value and then decreases gradually, as shown in Fig. 4 a. This peak shear stress value represents the static yield stress. For the case of dynamic yield stress and plastic visocsity, the test process can be divided into four stages (Fig. 3 b). Firstly, cement paste was loaded in the rotational rheometer and measured from 0 to 2 min with a high constant shear rate. Secondly, rotational rheometer rested for 2 min. Thirdly, rotational rheometer measured the shear stress for 2 min under the dynamic shear rate of 0 to 200 s1. Finally, the shear rate decreased from 200 to 0 s1. Dynamic yield stress and plastic viscosity are basic parameters to describe the flowability when the microstructures have been broken. For 3DP cement paste, these two behaviors related to the pumpability and printability.

ð1Þ

where s and s0 represent the shear stress and yield stress, respectively; l is the plastic viscosity; c is the shear rate. Thixtropy is defined as the decrease in viscosity (stress) under shear, followed by an increase upon removal of shear [25]. From another point of view, thixtropy is related to shear stress decay from the peak value to equilibrium value under a constant shear rate [26], as shown in Fig. 4 a. Shear stress is the interaction between any shear planes in the interior objects when it was subjected to deform and the thixotropy are extremely relevant with it. For instance, Ouyang et al. [27] used the structure parameters obtained from the shear stress curves to study the thixotropy under the constant shear rate of 100 s1. In this paper, to explain the thixtropy of 3DP paste with the addition of retarder, the structure parameter (Sthix ) was calculated. Under the constant shear rate (100 s1), the structure parameter indicates the relationship between static and dynamic yield stress [27]. High structure parameter represents the better thixotropy. It can be expressed by Eq. (2):

Sthix ¼ ðs0  se Þ=se

ð2Þ

where Sthix , s0 and se represent the structure parameter, peak value and equilibrium value, respectively. 2.4.2. Building time Building time is a time period during which the cement paste is extrudable. In this study, the building time was measured by using the nozzle of 0.5 mm. For a 3DP specimen, experimental building time should be higher than theoretical building time to ensure the build-up of 3D structures. Theoretical building time can be expressed by Eq. (3):

t¼

hlw

v pr 2

þ t1

ð3Þ

Table 2 Mix proportion of raw materials (%). Raw materials

Specimens-BA

Specimens-SG

Specimens-diatomite

Silanol groups

SAC WRA HPMC BA

100 0.30 0.40 0–0.25

100 0.30 0.40 —

100 0.30 0.40 0–0.25

— — —

SG

—

0–0.25

0–0.25

Diatomite

—

—

0–8

Water

35

35

35

—

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M. Chen et al. / Construction and Building Materials 234 (2020) 117391

Fig. 2. Structures of 3D printer: (a) Macrograph of 3D printer; (b) Extrusion device; (c) Printing specimen; (d) Extrusion nozzles.

Fig. 3. Test process of rotational rheometer: (a) constant shear rate and (b) dynamic shear rate.

Fig. 4. Typical shear stress curves under the (a) constant and (b) dynamic shear rate.

where l, w and h represent the dimension of designed model; v represents the printing speed; r is the radius of nozzle; t 1 is the time that no paste is extruded from nozzle during printing and t 1 is about 90 s in this study. It should be noted that the building time in this study need to be higher than 6 min because of the preset model.

2.4.3. Initial setting time The initial setting time in this study was measured by a manual Vicat apparatus according to the GB/T 1346-201 [28], and it was obtained when the falling penetration depth of Vicat needle reached 36 ± 1 mm.

M. Chen et al. / Construction and Building Materials 234 (2020) 117391

2.4.4. Structural deformation The structural deformation of specimens was calculated from the average deformation degree over time after printing. In this paper, the structural deformation of a specimen refers to the positive strain and could be calculated by Eq. (4).

D ¼

  jl  l0 j jw  w0 j jh0  hj  100% þ þ 3l0 3w0 3h0

ð4Þ

where D is the structural deformation; l0 , w0 , and h0 represent the sizes of model; l, w and h represent the largest sizes of specimens at a time after printing. 2.4.5. Compressvie strength The compressive strength of printed samples was tested by using a universal testing machine (CDT1305-2, MTS, USA) and its loading speed and testing range is 0.3 kN/s and 10–300 kN, respectively. Besides, the test samples for the compressive strength were obtained by the precision cutting machine and the size is 20  20  20 mm. 2.4.6. Hydration heat The hydration heat flow and cumulative heat curves were obtained by the conduction calorimeter (TAM Air C80, Thermometric, Sweden) under the constant temperature of 25 °C. The test system will record the value of heat flow and cumulative heat automatically until the required time. 3. Results and discussion 3.1. Rheological properties Rheology refers to the deformation and flowability of materials under the external forces, and it reveals the relationship between the paste deformation and rheological parameters [29,30]. 3DP materials require the low dynamic yield stress and good fluidity before printing, and the high static yield stress and viscosity after printing. Hence, rheology can be used to evaluate the correlations among the hydration, the deformation, and the flowability for 3DP cement paste [31]. 3.1.1. Structure parameter Since the yield stress and viscosity of 3DP paste requires a great change during the printing process, excellent thixotropy is beneficial to the 3D structural build-up. For the case of constant shear rate, the shear stress reaches the peak value at the beginning,

Fig. 5. Structure parameter of SAC paste with different dosages of BA and SG.

5

which is caused by van der Waals interaction force and AFt bridge. Then it goes down and becomes stable due to the breakdown of the flocculent structure. The thixotropy is caused by the breakdown and rebuilding of 3D structures [32]. The structure parameter of SAC paste with different BA and SG dosages calculated based on Eq. (3) is shown in Fig. 5. It can be seen that the structure parameter decreases gradually with the increase in BA and SG dosages, which is adverse to the improvement of thixotropy. Furthermore, the Sthix decreases more slowly in the first few dosage of BA and SG, and then decreases linearly. This phenomenon is caused by the instability of water film generated from BA and SG in 3DP paste [33]. Under the low dosages of retarder, the instability of water film can be damaged easily, which causes more free water between the particles, and the flocculation network forms quickly at this time [34]. Therefore, the dosage of retarder in the SAC paste should be as low as possible to ensure the enough thixotropy for 3DP paste. However, in this case, it is adverse to the improvement of building time. Besides, it should be noted that thixotropic parameter of SAC paste with diatomite is basically unaffected, which indicates that the contents of diatomite unrelated to the thixotropy. The relationship between Sthix and retarder dosage can be expressed by Eq. (5).

Sthix ¼ S0  expðkxÞ þ a

ð5Þ

where S0 represents the initial structure parameter which is the ability of initial 3D structure forming; x is the dosages of retarder; k is a parameter which is related to the dosages and types of retarder; a is the ability of structure rebuilding rate. From Figs. 5, 3DP paste with BA presents a better ability of initial 3D structure rebuilding than that with SG. Furthermore, the dosage of retarder should be as small as possible to ensure a relatively good thixotropy. Therefore, the parameters of S0 , k and a can be used to quantitatively characterize the structure forming and rebuilding process of 3DP SAC paste [27]. 3.1.2. Rheological parameters Yield stress is a critical shear stress value, which describes the minimum force required to flow like a liquid. The microstructure in the cement paste exists the intermolecular forces before it flows like a liquid, which is regarded as the yield stress. There exists two yield stresses (static and dynamic) corresponding to flocculation state and 3D structural build-up in the 3DP SAC paste. At rest, the undisturbed microstructure caused by the intermolecular forces and flocculation, such as AFt bridge and AH3 gel, can result in a higher yield stress, which is defined as static yield stress. When the paste starts to flow, the microstructure structure was broken, resulting in a lower shear stress, and this shear stress was defined as dynamic yield stress. Furthermore, the plastic viscosity manifests the ability in resisting the paste deformation once the dynamic yield stress is exceeded. Static yield stress is related to the breakage of microstructure and this information of structural build-up is useful for the buildability assessment. Fig. 6 shows the static yield stress of SAC paste with different dosages of BA and SG under a low shear rate. It can be seen that the static yield stress of 3DP SAC paste with BA decreases gradually. The reason is that BO33 , as shown in Table 2, will react with Ca2+ (C4A3$) to produce protective layer on the surface of cement particles. This protective layer will prevent the cement hydration which causes more free water exist in the cement paste. These two reasons can decrease the internal friction between particles, resulting in the reduction of static yield stress. It also can be seen that the static yield stress of 3DP SAC paste decreases significantly with the increase in SG dosage. The reason is that AOH and ACOOH groups (Table 2) from the SG can be adsorbed by hydrogen bonds on the surface of cement particles to generate the water film, which will strengthen the relative

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M. Chen et al. / Construction and Building Materials 234 (2020) 117391

Fig. 6. Static yield stress of SAC paste with different dosages of BA and SG.

motion between the particles [35]. Besides, ACOOH group in the SG will react with Ca2+ to generate the stable compound and it can prevent the water from contacting the cement particles. The above reasons cause more free water between the particles to decrease the static yield stress. Besides, the static yield stress of 3DP SAC paste with BA decreases more slowly than that with SG at the first few dosages. This phenomenon may be caused by the retardation efficiency of BA and SG. In the molecular structure, SG possesses the abundant CAOH and ACOOH group, which is easier to form the water film on the surface of cement particles than BA. For BA, the retardation is mainly caused by the generation of complex compound (calcium borate), which can be damaged easily in the low dosages. Therefore, these seasons caused the static yield stress of 3DP SAC paste with BA decreases more slowly at the first few dosages. Although the retarder in the 3DP SAC paste can effectively improve the printable properties, it is adverse to the build-up of 3D structures because of the reduction of static yield stress. Meanwhile, the introduction of BA and SG is also in contraction with the high early strength which is the basic requirement for 3DP cementbased materials. During the 3DP process, the paste must be sufficiently flowability for extrusion system but sufficiently stiffness for the structure mechanical stability. Perrot et al. suggested the structural build-up of the concrete to solve this paradox, which ensures both, sufficient flowability during extrusion and stability after deposit [36]. In this case, diatomite was used to improve

the static yield stress and structure stability of 3DP SAC paste. Fig. 7 shows the static yield stress of SAC paste with different contents of diatomite. It can be seen that the static yield stress increases gradually with the increasing content of diatomite. The reason is that the diatomite presents a large specific surface area and abundant Si-OH groups (Table 2) on the surface, which causes the high water absorption. This character will increase the internal fraction of cement particles and require a high shear stress to make the paste flow, resulting in a high static yield stress, which is beneficial to the structure build-up. Different from constant shear rate protocol, rheology measurement for dynamic shear rate protocol indicates the deformation and flow properties of the fluid in a changing state. For the printing process of cement paste, the dynamic rheological parameters, including dynamic yield stress and plastic viscosity, are basic parameters to describe the flowability. These two behaviors related to the pumpability and printability. In this study, the dynamic yield stress and plastic viscosity for the pseudoplastic fluid was obtained by the fitting of Bingham model, as shown in Fig. 8. It can be seen that the dynamic yield stress and plastic viscosity of 3DP SAC paste decreases gradually with the increasing dosages of BA (Fig. 8 a). However, the reduction of dynamic yield stress is small when the dosage of BA is less than 0.10%, thereafter it decreases significantly. The reason is that the complex compound caused by BO33 is unstable, which can be easily damaged under the high dynamic shear rate. Furthermore, when the dosage of the BA in the range of 0–0.25%, the dynamic yield stress and plastic viscosity decreases from 585 to 427 Pa, and from 2.432 to 2.068 Pas, respectively. For the case of 3DP paste with SG, as shown in Fig. 8 b, the dynamic yield stress and plastic viscosity decreases gradually with the dosages of SG increases. When the dosage of SG is in the range of 0–0.25%, the dynamic yield stress and plastic viscosity of 3DP SAC paste decreases about from 585– 443 Pa, and from 2.432–2.094 Pas, respectively. As a whole, the 3DP paste with BA exhibits a higher dynamic yield stress and plastic viscosity than that with SG. Besides, both of BA and SG can decrease the dynamic yield stress and plastic viscosity of the 3DP SAC paste, which indicates that the addition of BA and SG is beneficial to the pumpability and printability. However, this phenomenon is adverse to the structure build-up for 3DP. Fig. 9 shows the plastic viscosity and dynamic yield stress of 3DP SAC paste with different contents of diatomite. As shown in Fig. 9 a and b, the plastic viscosity and dynamic yield stress increases dramatically with the increasing contents of diatomite. It also can be seen that the plastic viscosity and dynamic yield stress of 3DP SAC paste decreases obviously with the dosages of

Fig. 7. Static yield stress of SAC paste with different contents of diatomite: (a) SG and (b) BA.

M. Chen et al. / Construction and Building Materials 234 (2020) 117391

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Fig. 8. Plastic viscosity and dynamic yield stress of SAC paste with different dosages of (a) BA and (b) SG.

BA increases. Fig. 9 c and d shows the plastic viscosity and dynamic yield stress of 3DP SAC paste with diatomite under the different dosages of SG. It can be seen that the plastic viscosity and dynamic yield stress of 3DP SAC paste with BA and diatomite presents the similar tendency to that with SG and diatomite. 3.2. 3DP building time The particularity of 3DP technology requires the cementitious materials to present a rapid hydration rate (i.e. rapid setting time) which is beneficial to the stacking of cement paste. However, in

consideration of the continuity and quality of 3DP specimens, a proper building time is of great importance to enable the 3D structures to have enough buildability. Building time is also positively correlated with the initial setting time, which means the 3DP paste begin to lose liquidity at this time. In this study, BA and SG are introduced in the 3DP SAC paste to control the initial setting time and building time. The initial setting time of 3DP SAC paste with BA and SG is shown in Table 3, and it presents obvious retardation effect for the SAC paste. It also can be seen that the setting time increases slightly as the BA dosage increases to 0.15% while the reverse phenomenon is true for SG which presents significantly

Fig. 9. Plastic viscosity and dynamic yield stress of 3DP SAC paste with different contents of diatomite: (a) (b) BA and (c) (d) SG.

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Table 3 Initial setting time of SAC paste with different BA and SG dosages. Retarder dosages (%)

BA

SG

0 0.05 0.10 0.15 0.20 0.25

22 24 33 47 81 112

22 29 40 61 92 107

retardation. It should be noted that the initial setting time of SAC paste with the addition of diatomite is basically unaffected. Fig. 10 shows the heat flow and cumulative heat of 3DP SAC paste with different dosages of BA and SG. It can be seen that the heat flow and cumulative heat of 3DP SAC paste with BA is similar to that with SG and it decreases visibly as the dosage of BA and SG increases. The first exothermic peak may be caused by the dissolution of solid powders and the followed exothermic peak is due to the generation of AFt (C3A3C$H32; C = CaO, A = Al2O3, $ = SO3, H = H2O), which decreases and delays gradually with the dosage of BA and SG increases. Moreover, the addition of BA decreases the third exothermic peak significantly, which is the hydration of C4A3$ (C4A3$ + 2C$ + 38H ? C3A3C$H32 + 2AH3). It also can be observed that the second and third exothermic peak coincide when the dosage of SG reaches 0.20%. The fourth exothermic peak is almost disappear with the increasing dosage of BA and SG, which may be due to the hydration of C4A3$ to produce the AFm (C3AC $H12; C4A3$ + 18H ? C3AC$H12 + 2AH3) [37–39], and it may be speculated that the addition of BA inhibits the formation of AFm. For the preparation process of 3DP cement-based materials, the building time affect the printing quality most. For instance, 3DP cement paste cannot be printed completely under the short building time, and it may present a large deformation when the building time is too long. Fig. 11 exhibits the building time of 3DP SAC paste

2% diatomite

4% diatomite

BA

SG

BA

SG

20 24 32 46 79 113

20 29 39 62 90 103

20 23 31 45 80 111

20 28 38 60 92 100

Fig. 11. Building time of 3DP SAC paste with different BA and SG dosages.

with different dosages of BA and SG. It can be seen that 3DP paste with SG presents a larger building time than that with BA, and this tendency is similar to initial setting time. Moreover, when the dosage increases to 0.25%, the building time of 3DP SAC with BA and SG paste can be prolonged to 85 and 90 min, respectively. Therefore, the addition of BA and SG in 3DP SAC paste is beneficial to increase building time and may control the structures better. Fig. 12 shows the building time of 3DP SAC paste with diatomite under different dosages of BA and SG. It can be seen that the building time decreases gradually with the increasing contents of diatomite. As shown in Fig. 12 a, the specimens cannot be completely printed because of the short building time when the content of diatomite reaches 6%. However, the building time will increase with the dosage of BA increases. From Fig. 12 b, it can be seen that the 3DP SAC paste with SG presents a similar tendency to that with BA when the contents of diatomite increases from 0% to 8%. It should be noted that the building time begins to be lower than the required theoretical building time when the contents of diatomite is higher than 6%, and the paste presents a poor extrudability at this time. To meet the requirement of continuity for 3DP, the dosage of retarder need to increase greatly in the SAC paste. Although the addition of diatomite is adverse to the improvement of building time, the increasing static yield stress can make the retarder with large dosage apply in 3DP SAC paste. By this stage, the building time can be prolonged.

3.3. Structural build-up

Fig. 10. Heat flow and cumulative heat of 3DP SAC with different dosages of (a) BA and (b) SG.

A better structure of 3DP SAC paste requires a low deformation which is depend on the 3DP process and the behaviors of paste. Two 3DP methods (Fig. 13) were adopted in this study to ensure the structural stability of 3DP SAC paste. Based on our previous results, these two printing paths are conducive to improving the

M. Chen et al. / Construction and Building Materials 234 (2020) 117391

Fig. 12. Building time of 3DP SAC paste with diatomite under different (a) BA and (b) SG dosages.

Fig. 13. Printing path and the model of 3DP.

Fig. 14. Macrographs of 3DP SAC paste with the different dosages of BA, SG and diatomite.

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mechanical properties of 3DP paste [40]. Fig. 14 shows macrographs of 3DP SAC paste with different dosages of the BA, the SG, and the diatomite. It can be seen that the samples collapse gradually with the increasing dosages of BA and SG. Furthermore, the addition of diatomite can improve the 3D structures significantly. However, when the content of diatomite increases to 8%, it presents poor extrudability and uneven structure caused by the high dynamic yield stress and plastic viscosity. It is worth noting that the successful deposition of consecutive layers in a 3D printing process leads to an increment of gravity-induced stresses, and the structure construction is affected by the rising speed, the printing velocity, the contour length, and the layer dimensions. Thus, the section of the layer is directly linked to the rheological requirements of 3D printed concrete, such as the initial yield stress value of the materials [41]. Fig. 15 exhibits the deformation rate and structural deformation of 3DP SAC paste over time under the different dosages of BA and SG. In this study, the anticipated structural deformation is lower than 10%. It can be seen that the deformation rates of 3DP paste with 0–0.15% dosage of BA and 0–0.10% dosage of SG is almost unchanged when the rest time is larger than 2 min after printing. The reason is that the hydration rate of 3DP SAC paste is rapid, and it can remain the structures of 3DP paste under a short rest time. However, as the dosages of BA and SG increases to 0.25%, the rest time lengthens gradually because of the delayed coagulation. In addition, as the dosage of BA and SG increases from 0% to 0.25%, the structural deformation of 3DP paste increases dramatically from 3.05% to 149.78% and 125.60%, respectively. Therefore, considering the stability of 3D structure, the increasing dosages of BA and SG in the 3DP SAC paste are adverse to the build-up of 3D structures. Furthermore, the structural deformation of 3DP paste with 0–0.15% dosage of BA and 0–0.10% dosage of SG are less than 10%, which meets the requirement of structure build-up. In this case, an excellent 3D printed structure can be achieved by the introduction of BA and SG. At this time, the building time can only prolong to 39 and 35 min, respectively. Furthermore, based on the results of rheology and structural deformation, the static and dynamic yield stress of 3DP SAC paste should be higher than 590 Pa and 509 Pa, respectively. The structural deformation of 3DP SAC paste with diatomite under different dosages of BA and SG is shown in Fig. 16. It can be seen that the structural deformation decreases significantly with the increasing contents of diatomite, which is in favor of the build-up of 3D structures. Therefore, to satisfy the anticipated requirement of structural deformation (<10%), high contents of diatomite is necessary. However, it should be noted that high con-

tents of diatomite can cause the 3DP SAC paste to extrude difficultly from the extrusion nozzle because of the high dynamic yield stress. The ultimate dosages of BA in 3DP SAC paste were 0.10%, 0.15%, 0.20%, and 0.25% when the different contents of diatomite was 0, 2%, 4%, and 6%, respectively. It should be noted that the highest dosage of BA can reach to 0.30% when the content of diatomite is 8%. For the case of SG, the ultimate dosages were 0.05%, 0.10%, 0.15%, 0.20%, and 0.25%, respectively. At this time, the building time can be prolonged to the range of 27–70 min and 25–63 min, respectively. Therefore, the introduction of diatomite prolong the building time significantly due to the increasing dosages of retarder. Furthermore, the structural deformation of 3DP SAC paste with different dosages of retarder is higher than the control sample at the same content of diatomite. The reason is that the control sample has a short setting time, and the rapid hydration will resist the deformation during printing. Based on the requirements of structural deformation and extrudability, the content of diatomite should be less than 6%. At this case, to print the 3DP SAC paste continuously, the plastic viscosity and dynamic yield stress should be lower than 2.901 Pas and 722 Pa, respectively. Furthermore, the static yield stress should be lower than 895 Pa. Recent studies also proposed the shear modulus to evaluate the structural build-up of paste through the oscillation shear protocol [21]. The frequency from 0.1 Hz to 100 Hz is commonly used for cement pastes to test the shear modulus in the linear viscoelastic region. The elastic modulus (G0 ) represents the ability of a material to store elastic the energy during deformation, and the viscous modulus (G00 ) represents the ability of a material to dissipate the energy during deformation. The complex modulus (G* = G0 + G00 i) refers to the amount of energy that a material can resist deformation. From Fig. 17, the elastic modulus of 3DP SAC paste with retarders and diatomite is much larger than the viscous modulus, indicating that the 3DP SAC paste presents a solid-like behavior. Moreover, the addition of BA and SG decreases the elastic and complex shear modulus, which is adverse to the structural build-up. However, the elastic and complex shear modulus of 3DP SAC paste increases significantly when the diatomite was added, and this result is similar to structure deformation. Therefore, the oscillation shear protocol can be used as a tool to probe the structural buildup in the 3DP cement paste. 3.4. Correlation of building time and static yield stress From the printing principle of 3DP system, the structural buildup is affected significantly by static yield stress and building time.

Fig. 15. Deformation rate and structural deformation of 3DP SAC paste with different dosages of (a) BA and (b) SG.

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Fig. 16. Structural deformation of 3DP SAC paste with diatomite under different dosages of BA and SG.

Fig. 17. Shear modulus of 3DP SAC paste with retarder and diatomite.

The building time presents crucial role for 3D structures when the static yield stress are relatively small. However, when the static yield stress is relatively large, the building time is insignificant for 3D structures. Therefore, the relationship between building time and rheological parameters was investigated in this study. Based on the results of structural deformation, the maximum static yield stress represents the maximum value under the highest dosages of BA and SG in the 3DP SAC paste. Fig. 18 shows the poly-

nomial fitting lines of maximum static yield stress and building time for 3DP paste. It can be seen that the variation of static yield stress increases significantly and then remain unchanged with the increase of building time. The reason is that cement hydration in the lower half of the sample structures is rapidly under the short building time, and it can withstand part of the gravity during printing. At this time, the static yield stress for 3DP samples is low. However, when the building time is long enough, the cement hydration is insignificant for the build-up of structures. High static yield stress are beneficial to improve deformation rate and build the excellent 3D structures. These results means 3D structure is supported mostly by the static yield stress rather than cement hydration when the building time is long enough. Therefore, high content of diatomite is beneficial to the build-up of 3D structures and prolongs the building time.

3.5. Compressive strength

Fig. 18. Polynomial fit lines of maximum static yield stress and building time for 3DP SAC paste.

The mechanical strength is an important index to evaluate the practicability and quality of 3DP cement-based materials. In general, the utilization of retarders in the cement-based materials positively influence for the long-term strength while the reverse phenomenon is true for the early strength. Fig. 19 shows the compressive strength of 3DP SAC paste with different contents of diatomite cured at 1 day. It can be seen that the compressive strength of 3DP SAC paste decreases gradually with the increasing dosages of BA and SG. The variation of compressive strength can be proved by the heat flow and cumulate heat of 3DP SAC paste with different dosages of BA and SG (Fig. 10). However, for the constant

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M. Chen et al. / Construction and Building Materials 234 (2020) 117391

Fig. 19. Compressive strength of 3DP SAC paste with different contents of diatomite cured at 1 day: (a) BA and (b) SG.

dosage of retarder, a small amount of diatomite can improve the compressive strength effectively. Besides, the maximum compressive strength of 3DP SAC paste increases from 37.2 to 41.5 MPa when the content of diatomite increases from 0% to 4%. Then, it decreases gradually with the increasing content of diatomite. The reason is that the particle size of diatomite is much smaller than that of SAC, which is in favor of the improvement of compressive strength. However, the compressive strength will decrease gradually due to the poor printability which can cause many large pores in the paste when the content of diatomite is higher than 6%. Another reason is that the amount of cementitious material decreases with the increase in diatomite content.

4. Conclusions In this study, retarders were used to control the printability and rheological properties, and the diatomite was used to improve the structural deformation and elasticity modulus. Furthermore, the relationship between rheology and building time was also studied to reveal factors influencing of 3D structural build-up. The main conclusions are highlighted as below: (1) BA and SG can prolong the building time and improve the printability of 3DP SAC paste significantly, but it can decrease the static yield stress, which is adverse to the structural build-up. (2) A paste sample with the structural deformation less than 10% can be achieved when the static and dynamic yield stress was controlled in the range of 590–895 Pa, and 509– 722 Pa, respectively. (3) The correlation between static yield stress and building time revealed that 3D printed structure is supported mostly by the static yield stress when the building time is long enough. (4) 3D structures could be well constructed and flexibly controlled by the introduction of retarders and diotomite in the 3DP SAC paste.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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