Effect of nano clay and PCE on the buildability of ultra-fine dredged sand-based 3D printing materials

The use of ultra-fine dredged sand instead of natural sand in construction 3D printing materials can significantly reduce the cost. However, ultra-fine dredged sand has fine particles and high angular morphology, which can hinder the buildability and continuous printability of construction 3D printing materials. The addition of polycarboxylate superplasticizer (PCE) can effectively solve this problem. Considering that the change of PCE (content of 0, 0.1%, 0.2%, 0.3%) content has a great influence on the printing performance of mortar, in order to make up for this deficiency, nano clay (content of 0,1%) is added to the mortar. The experimental results showed that the addition of nano clay can significantly reduce the negative effects of PCE on the yield stress and apparent viscosity of 3D printing materials (3DPM). When the content of PCE is 0.2%, the addition of 1% NAC could increase the static yield stress and viscosity growth rate of 3DPM by 111.8% and 115.3% respectively. In addition, unconfined compressive strength test, isothermal calorimetry, Mercury invasion porosity method and thermogravimetric analysis were used to characterize the hardening properties of 3DPM. The results of heat of hydration showed that the addition of nano clay reduced the hydration exothermic peak of 3DPM, but increased the total heat release. The results of pore structure analysis showed that the addition of nano clay reduced the macropore (>1000 nm) of 3DPM from 19.31% to 18.82%. Thermogravimetric analysis showed that the addition of nano clay increased the hydration products. Therefore, the compressive strength of 3DPM was kept within an acceptable range. Finally, the laboratory’s printing results indicated that the 3DPM can print up to 20 layers continuously.


Introduction
Compared with traditional use of concrete in construction, construction 3D printing has advantages of fast construction rate, diverse architectural appearance, labor saving, simple construction technology and so on. However, due to the need of continuous printing and the lack of formwork, construction 3D printing technology puts forward high requirements for the buidability and thixotropy of materials [1][2][3][4][5][6][7]. Moreover, long-distance pipeline transportation also puts forward certain requirements on the flowability and printing time of the printing materials [8][9][10]. To achieve these printing performance, printing paste is often mixed with various additives, which makes the materials more expensive and hinders the promotion of construction 3D printing [11]. Therefore, on the premise that the printing materials meets the printable performance, solving the problem of high cost of printing materials is one of the key issues concerned by many research teams around the world. To ensure the buildability and mechanical properties of printing material, a certain amount of portland cement and admixtures is indispensable in the design of printing materials. Therefore, replacing the aggregate in the printing material with other waste materials is one of the important ways to reduce the cost of printing materials. Moreover, the replacement of aggregate in 3D printing materials with other waste materials can also solve the problems of natural sand depletion and environmental pollution. Wu et al [12] replaced different dosages of natural sand with recycled coarse aggregate, and evaluate the rheological parameters as well as buildability of these printing materials. It was found that with the passage of time, recycled coarse aggregate would increase the yield stress of the printing materials. Yue et al [13] used spherical arc furnace slag instead of natural sand to improve the flowability of the printing material. Not only does the utilization of spherical electric arc furnace slag can reduce the use of superplasticizer, but also increase the buildability and mechanical properties of printing materials.
As the waste material produced in the process of regulation and maintenance of the Yangtze River waterway, dredged sand has little application value due to the small fineness modulus, low smoothness and high angularity and uneven particle gradation distribution. In recent years, dredged sand is usually thrown into the deep reaches of the river or used for land reclamation, which would cause damage to the sensitive environment [14,15]. Therefore, there is an urgent need to find a more cost-effective way to deal with the increasing number of waste dredged sand. The utilization of dredged sand as building materials is a new method to consume dredged sand [16,17]. In this paper, 3D printing materials are prepared with dredged sand as the main raw material replacing natural sand. On the one hand, it can make extensive use of dredged sand to avoid secondary pollution to the environment caused by waterway dredging projects, and on the other hand, ultra-fine dredged sand can completely replace natural sand to reduce the cost of construction 3D printing materials by about 20%. The construction 3D printing materials are extruded through the nozzle. If the particle size of the aggregate is too large, the nozzle may be blocked. This problem can be avoided by using ultra-fine dredged sand as aggregate. In addition, due to the fine particles of ultra-fine dredged sand, the surface of the prepared construction 3D printing materials is smoother [18]. In conclusion, the rational utilization of dredged sand has good ecological benefits and broad application prospects. However, until now, there is little research on how to solve the problem of poor buildability of printing materials caused by the small size of dredged sand.
Due to the high angular particle morphology of ultra-fine dredged sand, it will affect the flowability of mortar. In order to ensure that the flowability of mortar meets the requirements of printing, more water consumption is needed. This is disadvantageous to the mechanical properties of construction 3D printing materials. It is necessary to add superplasticizer to reduce the water-cement ratio of 3D printing materials and improve the dense structure. Studies have shown that Polycarboxylate superplasticizer (PCE) can improve the dispersibility of cement in fresh mortar. It destroys the flocculation structure of the slurry and releases the wrapped mixed water, thus increasing the fluidity of the fresh mortar, which is beneficial to the pumpability of construction 3D printing [19]. However, as peer literature, the higher the sand fineness, the larger the specific surface area, the greater was PCE admixture adsorption/consumption [20]. This caused the steric hindrance effect of PCE be extremely obvious, which makes mortar extremely sensitive to water. It has a negative effect on the buildability of construction 3D printing materials. To solve this problem, the worldwide research teams have been trying. Qian et al [21] controlled the fluidity and structural build-up of cement paste in fresh state by adjusting the amount of PCE and water-cement ratio, but it cannot improve the structural build-up rate of concrete 3D printing. Marcelo et al [22] used the setting accelerators to increasethe structuration rate of cement paste to make up for the retarding and repulsive effect caused by the addition of superplasticizer. However, it is found a large number of setting accelerators are needed to achieve proper constructability, which is harmful to the long-term compressive strength of concrete 3D printing. Moreover, other studies have also shown that nano clay (NAC) can accelerate thixotropic rebuilding rate of cements [23,24]. Biranchi Panda et al [25] found that adding appropriate amount of nano clay to the printing materials can improve the viscosity recovery rate and improve the structural accumulation of the high content fly ash mixture designed for 3D printing under different static time and shear rate. Gao et al [26] researched that the introduction of nano clay reduced the attachment of PCE on the appearance of cement particles. However, his study is limited, and whether this phenomenon is conducive to 3D printing of buildings needs further research.
In this study, the 3D printing materials (3DPM) with dredged sand replacing 100% natural sand is developed. In order to further reduce the cost of materials, ground granulated blast furnace slag and fly ash partially replace Ordinary Portland cement as cementitious materials. NAC and PCE are used together to adjust the properties of 3DPM. The adsorption of NAC to PCE is used to reduce the effect of PCE on the rheological properties of 3DPM, so as to improve the printability of 3DPM. To study the effect of NAC and PCE on 3DPM with ultra-fine dredged sand, the rheological properties of the different printing materials are tested by brookfield rheometer (R/SP-SST). This research tests the buildability of printing materials by examining the stability of a fresh sample of a cylinder [27]. A WHINE-200N compression machine is used to test the mechanical performance of the printing materials. To verify the reaction mechanisms of the printing materials, isothermal calorimetry, Mercury invasion porosity and thermogravimetric (TG) analysis are also used in this study. Finally, the application of dredged sand in printing materials has been verified through the printing of various formulations in the laboratory. , purchased from Conch Company, ground granulated blast furnace slag (GGBS), purchased from Yonggang Company, and fly ash (FA), purchased from Sinopce Yangzi Petrochemical Company, were used as binders in this study. The particle gradation analysis of OPC, GGBS and FA are shown in figure 2. The chemical composition of various cementitious materials used in the test are shown in table 1. Ultra-fine dredged sand (DS) was used to replace natural sand as aggregate in this study, which is taken from Changjiang Nanjing River Waterway Engineering Bureau. The maximum particle size of DS is 0.32 mm, and the particle morphology of DS is shown in figure 1. It can be seen from figure 1 that DS has low roundness and high angularity, which will lead to low fluidity of fresh printing materials in practical application. Meanwhile, the particle size distribution and chemical composition of DS are also shown in figure 2 and table 1, respectively and the x-ray diffraction (XRD) analysis of DS is shown in figure 2.
In addition to the cementitious materials and aggregate, nano clay (NAC), purchased from Pastoral Group, sodium desulfurization ash (SD), purchased from Yonggang Group from Jiangsu, and Polycarboxylate superplasticizer (PCE) were also used in the study to adjust the properties of 3D printing material. Table 1 shows the the chemical composition of NAC and SD. The x-ray diffraction (XRD) analysis of NAC is also shown in figure 2. It can be seen from the diagram that the mineral composition of NAC contains palygorskite, which is viscous and plastic. Therefore, NAC can improve the thixotropy and the structural build-up of the printing materials. Characteristics of PCE are presented in table 2. Figure 3 shows the particle size distribution of Ds, OPC,GGBS and FA.

Mix proportion
All the 3D printing materials mixing ratios used in this experiment are shown in table 3. In order to reduce the cost of architectural 3D printing materials, the cementitious materials used in the experiment were all constituted of 50% OPC, 40% FA and 10% GGBS. According to the literature [28,29], the doping amount of SD and NAC were all fixed at 1% to promote the hydration of geopolymer materials. Meanwhile, the formula NP2  and NP3 without NAC were selected as the control group. The content of PCE varied according to 0,0.1,0.2,0.3% of the mass of the cementitious material. For the sake of comparison, the initial fluidity of all fresh mortars is kept at 190 mm (as shown in the figure 4) by adjusting the water-binder ratio.
In the mixing process, the dry printing materials were first mixed evenly with 2 min, and then the 1 min were mixed at a low speed of 90% water in a mortar mill. Then, NAC and PCE were mixed with 5% water and added to the mortar mill. Finally, all the mixtures are evenly stirred 6 min in the mortar grinder (slow stirring 3 min, fast stirring 3 min).

Experimental methodology 2.2.1. Buildability test
In this experiment, the buildability of the printing materials is tested by measuring the remaining height of the collapsed concrete cylinder under gravity [27]. According to previous research [30], the height and diameter of the cylinder die used in this test were both 80 mm. The buildability testing steps were as follows: first of all, the fresh printing materials were added to the mold in three layers, and each addition was extruded 15 times with a tamping rod; Secondly, fill the edge of the mold with more 3DPM and tamp it as before. Excess 3DPM is removed from the mold. Finally, the mold was lifted vertically to test the remaining height of the cylinder sample under the action of gravity. If the sample is not deformed after demoulding, it shows good buildability.

Rheological properties
In this study, the rheological behaviors of 3D printing materials with different formulations were measured under different shearing mechanism in a Brookfield rheometer (R/SP-SST) with a 4-blade vane (figure 5). After preparing the fresh slurry according to the above requirements, add it to the testing instrument. The constant shear rate of 50 s −1 is applied to the mortar for 20 s, and then the mortar is deformed at the constant shear rate of 0.2 s −1 for 120 s after standing for 30 s [31].
The maximum value of the curve obtained in this experiment was considered to be static yield stress [32]. In addition, in order to study the ability of printing materials to overcome their own gravity to keep the shape unchanged, the apparent viscosity was also tested [25]. The testing mechanism of apparent viscosity was as follows: increased the shear rate from 0 to 50 s −1 in the first 30 s, and reduced the shear rate from 50 to 0 s −1 in the next 30 s, referring to the literature [29].

Mercury intrusion porosimetry test and TG-DTG test
The pore distribution of different specimen was tested through a GT-60 Mercury intrusion porosimeter (Kanta, USA). Before the test, each sample were all firstly dried at 60 ± 2°C and then crushed into small partilces. For each different formulation, 2 g sample with 28d curing age was taken. Meanwhile, thermogravimetric/ thermogravimetric differential analysis (TG/DTG) in this study was carried on by thermogravimetric analyzer manufactured by HENVEN, Beijing. The experiment involved heating the sample in an air atmosphere from room temperature to 50°C at a rate of 5°C per minute and holding it for 10 min. The sample was then further heated from 50°C to 1000°C at a rate of 10°C.

3D concrete printing
The 3D printer in this study was purchased from Jian Yan Company, and the print outlet of the printer was the circular nozzle of 20 mm. For AP2 and NP2 formula, the long strip structure with horizontal length of 200 mm was printed under the condition that the extrusion speed was 1.4 s −1 and the moving speed was 35 mm s −1 . The study assessed the printability of various specimens based on two factors: the continuous printable height of printed products and their ability to maintain shape.

Results and discussion
3.1. Buildability test From the buildability experimental results shown in figure 6, it can be seen that the shape retention of 3DPM mixed with NAC is much better than that of 3DPM without NAC, which manifest that the addition of NAC was beneficial to the improvement of yield stress of 3DPM. Moreover, it can also be seen from figure 6 that on the basis of the addition of NAC, the variation of PCE in 3DPM had little effect on the shape retention of the printing paste cylinder. Table 4 shows the slump of 3DPM with different formulations. When the volume of PCE increased from 0.2% to 0.3%, compared with 3DPM without NAC, the addition of NAC reduced the slump of 3DPM from 17 mm-5 mm. From this trend, it can be concluded that the addition of NAC can improve the buildability of printing paste with high content of PCE. And this phenomenon may be attributed to that the adsorption of NAC to PCE weakens the effect of PCE into highly dispersed cement particles.

Static yield stress and apparent viscosity
During the process of 3D printing, the printed filament needs to withstand the weight of the accumulating printed layer. Therefore, 3D printing material (3DPM) must have sufficient static yield stress to avoid the deformation and collapse of printing construction [32]. Figure 7 shows the rheological behavior of 3DPM with different formulations at constant shear rates, and the rheological curve shows a trend of rising at first and then decreasing. According to previous studies reported [33,34], the vertex of the rheological curve is considered to be the static yield stress of 3DPM. As can be seen from figure 7, the static yield stress of 3DPM is closely related to the content of PCE and NAC. When the content of NAC was fixed at 1%, the static yield stress of 3DPM increased at first and then decreased with the increase of PCE content. While the content of PCE increased to 0.2%, the static yield stress of 3DPM reached the maximum, reaching 1831.03 Pa. When the amount of PCE was below 0.2%, the increase of the PCE content decreased the water-binder ratio of 3DPM. Meanwhile, due to the short distance between particles, cement particles have the interaction of van der Waals force. In addition, the existence of ions adsorbed on the surface of the particles will also produce electrostatic forces. It promoted the flocculation of 3DPM [35]. However, when the amount of PCE was too much, the cementitious particles of 3DPM were covered by PCE polymer. The steric hindrance and electrostatic repulsion caused by PCE polymer dispersed the slurry particles and hindered the flocculation of paste [36]. Obviously, it can be known that the yield stress of 3DPM was affected by the flocculation effect [37], Therefore, the static yield stress of 3DPM with change of PCE amount show this trend, and the results of this section confirm the above analysis. It can also be seen from figure 7 that when the amount of PCE is fixed at 0.2%, the yield stress of 3DPM with NAC was 1831.03 Pa, which was much higher than that of 3DPM without NAC. When the content of PCE increased from 0.2% to 0.3%, the yield stress of 3DPM mixed with NAC decreased only by 577.37 Pa, while that of 3DPM without NAC decreased from 864.68 Pa to 0 Pa. The results showed that the addition of NAC as thixotropic agent is helpful to the rapid construction of the early structure of fresh slurry. This phenomenon could be attributed to that the increased content of PCE dispersed the cementitious particles, which decreased the bridging bond between the cementitious material and the static yield stress of 3DPM. However, the addition of NAC provided thixotropic rigid bonding, forming a high thixotropic slurry system, which made up for the poor flocculation effect caused by the high amount of PCE [38,39]. It shows that NAC has good compatibility with PCE. The experimental results of this section are consistent with that mentioned above section. The viscosity of the fresh paste indicates the ability of 3DPM to resist deformation after accumulating. When the viscosity of 3DPM is too low, it showed that 3DPM cannot keep its shape after extrusion and has low supporting capacity [40]. Figure 8 shows the change of apparent viscosity of 3DPM with different formulations when the shear rate is increased from 0 to 50 s −1 . When the content of NAC is fixed at 1%, the change trend of apparent viscosity is the same as that of static yield stress with the increase of PCE content, and when the content  of PCE is 0.2%, the apparent viscosity of 3DPM is also the highest. Moreover, when the content of PCE is constant, the apparent viscosity of the 3DPM with NAC is much higher than that of 3DPM without NAC. When the content of PCE increased from 0.2% to 0.3%, the decrease rate of apparent viscosity of 3DPM with NAC was much lower than that of the slurry without NAC. The experimental results further demonstrate the accuracy of the above conclusions, indicating that NAC has good compatibility with PCE, and the addition of NAC can adjust the yield stress and apparent viscosity of 3DPM containing PCE, so as to meet the requirements of rheological properties of construction 3D printing materials.

Compressive strength
Compressive strength has always been regarded as the traditional quality parameter of concrete [41], and for construction 3D printing, it is also very important that 3DPM has high compressive strength to support the continuous accumulation of printed product [42]. The compressive strength of 3DPM with different formulas are shown in figure 9. In aspect of 3DPM mixed with NAC, the compressive strength of 3DPM increased gradually with the increase of the amount of PCE and the decrease of water-cement ratio. This phenomenon could be attributed to that the decrease of the water-cement ratio reduced the free water in the paste, which reduced the evaporation of free water during the slurry condensation and hardening. Accordingly, there were fewer pores in the specimen with higher amount of PCE, which resulted in higher compressive strength [43]. It  can also be found from figure 9 that the compressive strength of 3DPM mixed with NAC is lower than that of 3DPM without NAC. The reason for this phenomenon maybe because that the increase of water-cement ratio caused by the addition of NAC increased the internal pores of the specimen. Although the addition of 1% NAC reduced the compressive strength of 3DPM from 61.1 Mpa to 48.3 Mpa, it is acceptable in industrial application on the basis of low content of NAC [29]. Moreover, considering that the addition of 3DPM improving the thixotropy of 3DPM significantly, to make the printing process run smoothly, NAC is an indispensable thixotropic agent in this experiment.

Heat of hydration
In order to study the effect of NAC and PCE on the hydration of 3DPM, the hydration heat of the mixture with different amount of PCE and NAC was tested. Figures 10(a) and (b) show the hydration heat and cumulative heat release of 3DPM with different formulations, respectively. It can be observed that the hydration of cement is a strong exothermic reaction [44], which includes four stages: pre-induction period, induction period, acceleration/deceleration period and stable period. Considering the effect of energy level replacement and temperature on the exotherm of hydration, the first dissolution peak in the early stage of induction is removed from the diagram [45]. It can be seen from figure 10(a) that with the increase of PCE content, the hydration exothermic peak is delayed and the exothermic peak decreases. This shows that the addition of PCE affects the induction period and acceleration period of hydration at the same time. Because PCE contains a large number of carboxyl groups in its molecular structure, it significantly complexes the dissolved Ca 2+ , from the slurry particles and reduces the concentration of Ca 2+ in the solution [46]. Moreover, as shown in figure 10(b), the cumulative heat release of 3DPM decreased with the increase of PCE content, which confirms that the addition of PCE has a significant inhibitory effect on the early hydration of the slurry [47].
Meanwhile, when the content of PCE is constant, compared with the 3DPM without NAC, it can be found that although the intensity of exothermic peak of 3DPM decreased slightly, the exothermic peak moved to early satge and the cumulative heat release increased. Accordingly, it can be concluded that the addition of NAC promoted the hydration of 3DPM, and this phenomenon could be attributed that the adsorption of NAC to PCE reduced the amount of superplasticizer cting on cement paste, thus shortening the induction period of hydration and resulting in an increase in cumulative heat release [48]. Generally speaking, the addition of NAC is beneficial to the construction of Printing material structure, which could improve the buildability of 3DPM and increase the number of printing layers.

TG analysis
As shown in figure 11, the samples (AP0, AP2 and NP2) with curing age of 28d were analyzed by TG/DTG. As previous studies reported, the weight loss of the samples in the temperature range of 50°C-150°C is mainly caused by the dehydration of ettringite, gypsum and C-S-H. The weight loss of the samples in the temperature range of 420°C-530°C is mainly due to the decomposition of Ca(OH) 2 . The weight loss of the samples in the temperature range of 600°C-1000°C is mainly due to the decomposition of carbonate [19]. It can be seen from table 5 that in the case of the sample with NAC, the mass loss of the samples with PCE is slightly smaller than that of the samples without PCE at the temperature of 50°C-150°C. This may indicate that the increase of free water involved in the reaction leads to the increase of C-S-H. In the case of the samples with same amount of PCE, the mass loss of the samples with NAC is higher than that without NAC at 50°C-150°C. This phenomenon confirmed that the addition of NAC promoted the hydration reaction and increased the hydration products through the adsorption of PCE and its own pozzolanic reaction. This is consistent with the results of heat of hydration. Therefore, it is necessary to add NAC into the 3DPM containing PCE to promote the hydration of the slurry and to meet the engineering properties of the field application of construction 3D printing.  3.6. Pore structure analysis The pore size distribution and cumulative intrusive pore volume of 3DPM with curing age of 28d are shown in figures 12 and 13, respectively. As peer literature [49], the pore size of mortar can be divided into micropore (<10 nm), mesopore (10-50 nm), capillary pore (50 nm-1 μm) and macropore (>1 μm). According to the pore size distribution curve shown in figure 12, compared with AP0, the addition of PCE into 3DPM made the overall pore size distribution around 20 nm shift to the left, which may be due to the fact that the addition of PCE destroyed the flocculation structure of cementitious particles [50] and released flocculating water wrapped by cement particles, making it free water to participate in hydration reaction. Although the addition of PCE inhibited the early hydration of 3DPM, the increase of free water involved in the reaction led to the increase of C-S-H in the later hydration stage. Thus the gel pore distribution of 3DPM with PCE became more evenly. This phenomenon shows that the addition of PCE could optimize the pore structure of mortar to a certain extent, and improve the compactness of mortar. Moreover, compared with the slurry NP2 without NAC, it can be found that there are more micropores and mesopores in AP2. It may be attributed to that the addition of NAC optimized the gradation of 3DPM and produced extra gel through pozzolantic reaction, thus decreasing the volume of pore. From the cumulative intrusive pore volume shown in figure 13 and the total porosity shown in table 6, it can be seen that the addition of PCE reduced the cumulative pore volume and optimized the pore structure of 3DPM with NAC. This is mainly due to the fact that the addition of PCE reduced the water-cement ratio of 3DPM, thus  reducing the pores caused by the volatilization of free water. Meanwhile, it can also be seen from table 6 that the total porosity of AP2 is higher than that of NP2. This is mainly due to the low reactivity of NAC and its adsorption of a large amount of free water, which decreased the hrdration of cementitious material and increased the watercement ratio of 3DPM, thus playing negative role in the pore development of 3DPM [51]. Although the high water absorption of NAC led to the increase of cumulative intrusive pore volume of AP2, the volume of capillary pore of AP2 are less than those of NP2. It is worth mentioning that the addition of NAC reduces the macropore (>1000 nm) of 3DPM from 19.31% to 18.82%. This shows that although the total porosity of AP2 increased, the extra gel and gradation optimization produced by the additon of NAC made up for the drawback caused by water absorption and optimizes the pore structure.

3D concrete printing
For construction 3D printing, the layer number of the printed products that can be printed continuously are the key factors measuring the quality of printing materials. The maximum number of continuous printing layers of printed material determines the interval time in the printing process, which affects the printing efficiency of field application [13]. The industrial printing process is shown in figure 14.
The purpose of this section is to evaluate the printability of 3DPM. In the actual 3D printing process, under the condition of constant moving speed and extrusion speed, whether the printing strip is continuous and whether it can keep its shape unchanged after accumulation is an important criterion to evaluate the printability of 3DPM. In the case of 3DPM mixed with PCE, due to the cementitious material of 3DPM was highly dispersed by PCE, the yield stress and viscosity of 3DPM were so low that the shape of the print strip cannot be guaranteed, so it is impossible to print successfully in the laboratory. When NAC is added, the printing layer number of 3DPM was improved greatly. This could be attributed to that (1) The adsorption of NAC to PCE weakens the dispersion ability of PCE in 3DPM [38]; (2) the water absorption capacity of NAC reduced the free water in fresh paste and improved the thixotropy of 3DPM, which makes the printing material have higher structural stability (the ability to keep its own shape unchanged) [52]. Figure 15 shows the laboratory printing test results of different formulations of 3DPM at a printing speed of 30 mm s −1 . On the basis of constant amount of PCE, the number of continuous printable layers of 3DPM doped with NAC is higher than that without NAC. The maximum printable layers of the printed material are 20 and 10 layers, respectively. Therefore, the adsorption of NAC to PCE can effectively improve the printability of 3DPM.  As shown in figure 16, industrial construction is only carried on AP2. According to the printing performance of the AP2, it can be seen that the printing filament of AP2 has good lubrication layer and solid undeformed section, which is helpful in the industrial construction. Printed products can be used in styling flower beds ( figure 17).

Conclusions
The purpose of this paper is to study the influence of NAC on the freshness and hardening properties of ultrafine dredged sand-based construction 3D printing materials in the presence of PCE. The experimental results show that the addition of NAC can significantly improve the printing performance of construction 3D printing materials in the presence of PCE. Moreover, although the addition of PCE has negative influence on the printing properties of 3DPM, the hardening properties of 3DPM can meet the needs of practical engineering applications.
The specific experimental conclusions are as follows: (1) The addition of NAC can make up for the poor bonding performance of 3DPM caused by the addition of PCE. When the content of PCE is 0.2%, the addition of 1% NAC could increase the static yield stress and viscosity growth rate of 3DPM by 111.8% and 115.3% respectively, which is much higher than those of 3DPM without NAC. Moreover, from the results of buildability experiment, it can be seen more intuitively that 3DPM with 1% NAC has better ability to overcome its own gravity without deformation.
(2) On the basis of constant NAC amount, when the content of PCE was increased from 0% to 0.3%, the compressive strength of 3DPM increased gradually. Although the addition of NAC decreased the compressive strength, when the content of NAC is 1% and the content of PCE is 0.2%, the 28-day compressive strength of the hardened specimen is 46.11 MPa, which is enough to meet the needs of engineering applications.
(3) The addition of NAC can significantly inhibit the effect of PCE on the early hydration and increase the cumulative heat release of 3DPM, which helps to improve the bonding properties of 3DPM and is beneficial to the engineering performance of fresh printing paste. The addition of NAC reduces the macropore (>1000 nm) of 3DPM from 19.31% to 18.82%, which optimizes the pore structure.
(4) From the laboratory printing results, it can be seen that the addition of NAC can significantly improve the printability of 3DPM, and compared with 3DPM without PCE, the number of continuous printing layers is doubled. Figure 17. Styling flower beds.