Effect of Alkalized Straw Fibers on the Properties of Three Dimensional Printed Cementitious Composite

Abstract

The use of plant fibers instead of commercial fibers in building materials is environmentally sustainable. Here in this work, the alkalized straw fibers were introduced into the 3D printed cementitious composite to investigate their effect on the properties of the printed composite. The flowability, buildability, and mechanical strengths of the printed composite were tested, and the internal pore distribution was analyzed by X-ray computed tomography. In addition, the straw fibers and printed specimens were tested by X-ray diffraction and scanning electron microscopy. Results showed that, with the increase of straw-fiber content, the flowability of the composites gradually decreased, and the buildability and mechanical strengths first decreased and then increased. Alkaline treatment can improve the crystallinity of cellulose in straw fibers, reduce water absorption, and enhance the bonding performance with the mortar interface. Therefore, compared with the natural-straw-fiber group, the flowability, buildability, and mechanical properties of the alkalized-straw-fiber composites were improved. Specifically, when the content of alkalized straw fiber was 0.4%, the 28 days flexural strength of the fiber alkalized group increased by about 12.6% compared with the natural-straw-fiber group. Microscopically, alkaline straw fibers have better energy absorption and load transfer capabilities inside the composite material, enhancing the toughness of the specimen. Overall, the incorporation of alkalized straw fibers into 3D printed cementitious composites showed better printing, mechanical, and environmental benefits.

Introduction

As one of the major crop species in the world, rice feeds a large number of people. However, how to deal with the large amount of straw fibers produced is a difficult problem, and the main treatment method is incineration, but this will bring serious environmental pollution problems and is not consistent with the global trend of reducing carbon emissions.^1–3

According to research, burning 3.24 Mt of rice straw will release 3.82 Mt of carbon dioxide gas and 301 Gg of carbon monoxide, as well as 29.5 Gg PM10 and 27 Gg PM2.5.⁴ The use of straw fibers in the construction field is an effective treatment method, and the incorporation of straw fibers can improve the thermal insulation performance of mortar^5,6; it was found that the thermal conductivity of mortar composites made of 10% straw fibers was about 73.7% lower than that of the undoped group.⁷

Further, adding an appropriate amount of straw fibers can also improve the mechanical properties of cement-based composite specimens.^8,9 Xie et al. found that the 28 days flexural strength of mortar composite specimens with 8% straw fibers was 24.3% higher than that of undoped specimens.¹⁰ Nevertheless, substances such as lignin and hemicellulose in natural straw fibers hinder the hydration of cement-based materials.^11–14

According to previous studies, the cumulative 72-h heat release of mortar composite specimens with 2% straw fiber was only about 41.7% of the undoped group.¹⁵ This is because, in an alkaline environment, lignin and hemicellulose dissolve to form sugar-acid substances, which react with calcium ions in the system, thus inhibiting the hydration reaction.¹⁶ In addition, the internal porosity of the specimen will increase after the fibers dry and shrink.¹⁷

Therefore, fiber modification is necessary. The modification of straw fibers can be completed by spraying a modifier on the surface of straw fibers or soaking straw fibers in modifier solution.^18,19 NaOH solution is often used as a modifier for straw fibers; the alkaline treatment of straw fibers immersed in the NaOH solution can remove lignin, hemicellulose, and other substances on the fiber surface and improve the fiber strength.¹¹

The 28 days compressive strength and flexural strength of the cementitious composite specimens contained 9% (wt.) straw fibers and were 8% and 27.4% higher than those of the control group, respectively.¹⁶ Nguyen et al. found that the 28 days flexural strength of the mortar composite material specimen mixed with 3% alkalized rice straw fiber is significantly higher than that of the fiber non-alkalized group, which is about 4.91 MPa.²⁰

Temperature and alkalization time are main factors affecting the results of fiber alkalization. For example, straw fibers soaked in 3.8% NaOH solution for 1 h at 90°C can obtain better mechanical properties than those soaked in 20°C for 12 h.¹⁵ However, as a green building material, cementitious composites containing straw fibers are mostly used in the field of traditional casting methods and are less considered to be applied in the field of 3D printing, although the fibers have the potential benefits to bridge the defects such as cracks and voids in the printed materials.

Three dimensional printing technology is a rapid automated manufacturing technology. Through the computer input of model instructions, 3D printers can manufacture physical models automatically.^21,22 Three dimensional printing technology has the advantages of diversified manufacturing forms, high manufacturing efficiency and precision, and also labor-and-material savings.^23–25 With the development of printing materials, 3D printing is currently widely used in construction, industry, medical, and other fields.^26–31

However, 3D printing in construction and building engineering has higher requirements for material properties, such as reasonable flowability, condensation properties, buildability, mechanical properties, etc.³² In literature, many works had been done to improve the printability of the printing mortar. For instance, polycarboxylate superplasticizer was often used to enhance the fluidity of cement-based materials.³³ The incorporation of retarders such as sodium gluconate (SG) and tartaric acid can significantly prolong the setting time of cement-based materials.^34–36

By adding hydroxypropyl methyl fibers, the thixotropy of cement-based materials can be improved, making the material easy to be extruded in the printing process, and has a higher static yield stress. These are conducive to improving the buildability of the material.³⁷ In addition, various commercial fibers were introduced in 3D printing cementitious materials to improve the mechanical properties of printed specimens.

The incorporation of basalt fibers generated more C-S-H (hydrated calcium silicate) in the hydration process, which can improve the mechanical properties of the printed specimens.³⁸ By adding 1% glass fibers, the 28 days flexural strength of 3D printed mortar specimens can be increased by 18%, due to the ability of glass fiber to absorb load energy and transfer load stress in printed samples.³⁹ In addition, the incorporation of steel fibers can improve the elastic modulus, compressive, flexural, and tensile strengths of the 3D printed mortar specimens.^40,41

The bonding between polyethylene fibers (PE) and the mortar interface can inhibit crack propagation, the addition of PE transforms the compressive failure mode of mortar composite materials from brittle failure to plastic failure, and adding 1% PE can increase the 28 days compressive strength of 3D printed specimens from 22.1 MPa to 28.6 MPa, with an increase rate of 29.4%.⁴²

In comparison with commercial fibers, plant fibers show advantages of lower production cost and higher environmental friendliness.⁴³ However, a few studies had been conducted to investigate the printing and mechanical properties of 3D printed cementitious composites containing straw fibers.

To fill the gap, this work introduced straw fiber in 3D printed materials and paid more attention to the effect of alkaline treatment on the printed straw-fiber cementitious composites. The flowability, buildability, mechanical strengths, and microstructures of the 3D printed composites of alkalized straw fibers and natural straw fibers were compared to analyze the optimal rice straw-fiber content. This study may provide knowledge for the application of plant fibers in 3D printed building materials.

Materials and Methods

Alkaline treatment of straw fibers

Straw was produced in the suburbs of Suqian City; later, the straw was dried and cut by machine to a straw fiber with a length of 10–20 mm. The density of straw fiber is 0.53 g/cm³, and the tensile strength of a single fiber is 22.3 MPa. The straw fiber was soaked in a 4% sodium hydroxide solution for 2 h at 60°C, then rinsed to neutral pH with tap water, and dried for 48 h at 35°C in an air blast drying oven.

The water absorption rates of natural straw fiber and alkaline-treated straw fiber were 195.2% and 153.6%, respectively. Natural straw fibers were labeled SF0, and alkaline-treated straw fibers were labeled SF1. Figure 1a and b show the images of straw fibers. The chemical composition of the two straw fibers is shown in Table 1, where it can be found the fibers contain cellulose, which made the fibers more easily biodegradable and helped to reduce the environmental impact produced by the cement mortar. Further, the straw fiber, due to its water-soluble nature, could improve the crack resistance of cement mortar, reducing the occurrence and expansion of cracks, which is beneficial for enhancing the durability of the cement mortar.

FIG. 1.

(a) Natural straw fiber; (b) Alkaline-treated straw fiber; (c) Grain size distribution of the river sand; (d) 3D printer; (e) Printing in progress.

Table 1.

Chemical Composition of Natural Straw Fiber and Alkaline-Treated Straw Fiber (wt%)

Rice straw	Water soluble	Cellulose	Hemicellulose	Lignin	Pectin	Others
SF0	18.96	32.46	22.64	16.43	4.37	5.14
SF1	12.35	48.13	15.78	14.17	2.56	7.01

Materials

Ordinary Portland cement (P.O 42.5) was used as the binder and its chemical composition is shown in Table 2. Natural river sand with a particle size of less than 1.18 mm and a water absorption rate of 1.2% was used as the fine aggregate, and the particle size distribution of the sand is shown in Figure 1c. Superplasticizer (powder, bulk density 0.75 g/cm³) was used to regulate the flowability of materials, SG was used to prolong the setting time of the materials, and hydroxypropyl methyl cellulose was used to improve the thixotropy of the materials.

Table 2.

Chemical Composition of Ordinary Portland Cement (%)

CaO	SiO2	Al2O3	Fe2O3	SO3	MgO	K2O	TiO2	NaCl	P2O5	SrO	MnO	ZnO	ZrO2	Cr2O3
54.66	23.31	9.71	3.745	3.73	2.11	1.14	0.51	0.16	0.16	0.15	0.10	0.07	0.02	0.01

Through the preliminary experiment, the water-cement ratio was determined to be 0.34. The proportions of the mixture are shown in Table 3. 3DP-SF0 and 3DP-SF1 represent 3D printed specimens from the natural fiber group and the alkaline fiber group, respectively.

Table 3.

Mix Proportion of Straw-Modified Cement Composites

Label	Cement (g)	Sand (g)	Water (g)	SP (g)	HPMC (g)	SG (g)	Straw fiber (g)	SF ratio (wt%)	SF ratio (vol%)
3DP-SF0	1000	1000	340	1.2	0.6	1	0	0	0
3DP-SF0-0.2%	1000	1000	340	1.2	0.6	1	2	0.09	0.38
3DP-SF0-0.4%	1000	1000	340	1.2	0.6	1	4	0.17	0.77
3DP-SF0-0.6%	1000	1000	340	1.2	0.6	1	6	0.26	1.15
3DP-SF0-0.8%	1000	1000	340	1.2	0.6	1	8	0.34	1.53
3DP-SF1-0.2%	1000	1000	340	1.2	0.6	1	2	0.09	0.38
3DP-SF1-0.4%	1000	1000	340	1.2	0.6	1	4	0.17	0.77
3DP-SF1-0.6%	1000	1000	340	1.2	0.6	1	6	0.26	1.15
3DP-SF1-0.8%	1000	1000	340	1.2	0.6	1	8	0.34	1.53

HPMC, hydroxypropyl methyl cellulose; SP, superplasticizer; SG, sodium gluconate.

Material mixing and printing procedures

The powder material was first mixed in a mixer at a speed of 100 revolutions per minute for 1 min. Then, water was added to the mixer and the mix continued for 1 min. Afterward, the straw fiber was evenly spread into the mixer and the mixing continued for 3 min. Finally, the mixed materials were ready for 3D printing. The printing accuracy of the gantry printer used in this study is 0.1 cm. In the printing process, the extrusion speed and moving speed of the nozzle were 50 mm/s and 1 r/s, respectively.

The printer is shown in Figure 1d. The size of a single printing filament was 200 × 20 × 10 mm, and the single layer was set at 200 × 120 × 10 mm. The lifting height of each layer of the printer was 10 mm, and each specimen was printed five layers. After printing, the specimen would be placed in a curing room with a temperature of 20°C and a humidity of 95%.

Experimental methods

Flowability test

Appropriate material flowability is conducive to 3D printing.^44,45 According to the standard GB/T 2419-2005, the electric jump table experiment was carried out on the mixed material. The mixed material was filled in two conical molds with a diameter of 70 mm, 100 mm, and a height of 60 mm, and compacted with a ram. After the mold was lifted vertically, the material was retained on the jump table. The electric jump table was turned on, and the two spreading diameters of the material were measured after the electric jumping table jumped 25 times. The average value of the spreading diameters was calculated.⁴⁶ Each group of materials was tested three times, and the average value was taken as the final value.

Buildability assessment

Poor buildability of composite may lead to damage and collapse of 3D printed specimens. Good buildability means that with the increased number of printed layers, the printed specimen will not produce excessive deformation, showing a high yield strength.^47,48 One way to evaluate the buildability of 3D printed materials is to measure the maximum number of stacked layers and height of the specimen.⁴⁹ Another way is to measure the horizontal distance and vertical distance from the top surface of the molded specimen to the bottom layer, and evaluate the buildability of the material by the ratio $tan\theta$ of the two distances,⁵⁰ as shown in Figure 2. The $tan\theta$ is inversely proportional to the buildability of the composite.⁵¹ The tanθ values of different groups were taken as the average of the measurement results of three specimens.

FIG. 2.

The buildability test method.

Flexural strength and compressive strength tests

When the curing time of the specimen reached 7 and 28 days, the specimen was cut into 40 × 40 × 160 mm⁵² by a cutting machine. According to the standard GB/T 17671-2021, the flexural strength test and the compressive strength test were carried out. In the flexural strength test, the span length of the test block was 100 mm, the loading speed was 50 ± 10 N/s, the effective compressive area of the compressive strength test was 40 × 40 mm, and the loading speed was 2400 ± 200 N/s. The average value of three specimens in each group was recorded as the flexural strength and compressive strength, with a precision of 0.1 MPa.

Taking the type of straw fiber and the content of straw fiber as variables, the influence of two-way analysis of variance (ANOVA) on the mechanical properties of the specimens at 7 and 28 days ages was analyzed. ANOVA was performed at 95% confidence level.⁵³

X-ray computed tomography

Two specimens, 3DP-SF0 and 3DP-SF1-0.6%, were scanned by X-ray computed tomography (X-CT). The specimens cured for 28 days were cut into 40 × 40 × 40 mm cubes, and the specimens after the compression test were also scanned by X-CT. The volume resolution of the CT images was 0.1 × 0.1 × 0.1 mm³, and 400 images were obtained. The digital structure of the printed specimen was reconstructed in three dimensions using the software Avizo, and the manual threshold segmentation was performed to separate the internal pores from the material and calculate the porosity and cracks.^50,54,55

X-ray diffraction test

X-ray diffraction (XRD, Bruker D8 Advance) was used to characterize the crystal structure of cellulose in fiber SF1 of the alkali group and fiber SF0 of the control group. The operating voltage of the diffractometer was 40 kV, the current was 30 mA, and the scanning range was 5°–90°.

Scanning electron microscope test

Specimens in groups 3DP-SF0-0.4% and 3DP-SF1-0.4% after curing for 28 days were selected, and their microstructure images were captured by scanning electron microscopy (SEM, SU8220; Hitachi). The surface change of straw-fiber alkaline treatment and its effect on 3D printed specimens were analyzed. This article does not include any research involving humans or animals.

Results and Discussion

Printability

The flowability of the printed composite with different contents of straw fibers is shown in Figure 3a. Results indicated that the flowability decreased with the increase in fiber content. The flowability of the 3DP-SF1 group was higher than that of the 3DP-SF0 group, mainly because the alkalized treatment could close the internal pores of the straw fiber and reduce the water absorption of the straw fiber, and increase the free water content involved in the hydration reaction in the system.¹⁵ When the straw-fiber content was less than 0.4%, the flowability of the composite was greater than 175 mm.

FIG. 3.

(a) Flowability of different straw fiber content; (b) Effect of straw fiber content on buildability; (c) Bad extrudability and (d) bad buildability of the printing with higher fiber contents; and (e) Fiber entanglement around the screw.

As shown in Figure 1e, at this time, the material can be uniformly extruded and there is no obvious gap between the printed monofilaments, which is beneficial to the strength development of the printed specimen. With the fiber content increases, the flowability of the printed composite decreased due to the water absorption of the fiber; when the fiber content was 0.8%, the flowability loss rates of 3DP-SF1-0.8% and 3DP-SF0-0.8% were 13.3% and 15.9% of that of the control group, respectively. This resulted in discontinuous discharge from the printed nozzle.⁷

As shown in Figure 3b, with the increase in rice-straw-fiber content, the tanθ of printed specimens in the fiber-alkalized group and the natural fiber group increased first and then decreased. When the straw-fiber content was 0.2%, the buildability of the composite decreased, and the tanθ values of 3DP-SF0-0.2% and 3DP-SF1-0.2% specimens were 0.32 and 0.29, respectively.

The reason is that the incorporation of straw fiber increases the internal voids of the printed composite, leading to a decrease in the load bearing capacity of the composite and larger lateral deformation at the bottom after the upper layers have been printed and increased in number.

When the fiber content increased to 0.4%, the tanθ values of the 3DP-SF0 group and the 3DP-SF1 group decreased significantly. Although the increase in the content of straw fiber leads to the increase in internal porosity of the composite material and reduces the bearing capacity of the material, the increase in the content of fiber also leads to a decrease in material fluidity, which inhibits the flow of the printing material into the surroundings to a certain extent.

In addition, the removal of impurities such as lignin and hemicellulose from straw fiber by alkaline treatment can improve the bearing capacity of straw fiber. Therefore, the tan $\theta$ value of the 3DP-SF1-0.4% group was higher than that of the 3DP-SF0-0.4% group.¹⁶ When the fiber content reached or exceeded 0.6%, the tanθ decreased significantly, because the mortar was difficult to be extruded from the printer nozzle as shown in Figure 3c and d.

This phenomenon was related to the decrease in the flowability of the composite material and the incomplete structure of the specimen caused by the discontinuous extrusion process of the material. At this time, the tanθ cannot reflect the true buildability of the composite.

In this experiment, the printability, especially the extrudability, highly affected the quality and also the following tested strengths of the printed specimens. Generally, when the fiber content increased, the extrusion of the mortar became difficult due to the fiber twinning around the screw next to the nozzle, as shown in Figure 3e, which resulted in poor extrudability and printing quality of the filament that gradually became thinner, coarser, and full of defect pores. This also reduced the open time to print the mortar of higher-fiber-content.

The solution to this problem is washing the screw more frequently in the printing, although this may increase labor consumption. On the other hand, other or new types of printer nozzle such as those powered by compressive propulsion may circumvent this problem. This can be researched in the future for fiber-reinforced 3D mortar printing.

Mechanical properties of composites

Flexural strength

The flexural strength of the 3D printed straw-fiber composites is shown in Figure 4a; under the same fiber content and specimen curing age conditions, the flexural strength of 3DP-SF1 is better than that of 3DP-SF0. This is because impurities such as lignin and hemicellulose in natural straw fiber hinder the cement hydration process.¹⁵ Alkaline treatment can remove the impurities on the surface of the straw fiber and improve the crystallinity of the fiber, which improves the mechanical properties of straw fibers.^56,57

FIG. 4.

(a) Flexural strength; (b) Compressive strength; (c) The overall schematic diagram of flexural failure; and (d) Interface diagram of specimen flexural failure.

The alkaline treatment increases the surface roughness of the straw fibers and enhances the interface bonding between the straw fibers and mortar.^58,59 When the fiber content was 0.2%, the flexural strength of 3DP-SF1-0.2% and 3DP-SF0-0.2% decreased at 7 and 28 days compared with the control group, mainly because the fiber content was less and could only provide a weak energy absorption,^60,61 and the incorporation of fibers increased the internal porosity of the specimen, resulting in a decrease in mechanical properties.

This situation was improved when the fiber content reached 0.4%; the composite extrusion 3D printing method enables straw fibers to be arranged in the specimen along the printing direction, which can improve the flexural strength of the composite.⁶² Therefore, the flexural strength value of the 3DP-SF0-0.4% group specimens at 28 days of age is 17.5 MPa, which is higher than the flexural strength value of the 3DP-SF0-0.2% group specimens at 16.3 MPa. This is different from the flexural strength of composite specimens made by traditional cast methods.

According to the research of Shang et al.,¹⁵ when the content of the study is 0%, 0.2%, and 0.4%, the flexural strength of the cast straw-fiber composite specimens at 28 days is about 6.3, 6.15, and 6.08 MPa, respectively.¹⁵ It is obvious that the flexural strength of the specimens gradually decreases with the increase of fiber content. The main reason for the decrease is that the increase in fiber content leads to the increase in water absorption, resulting in insufficient hydration inside the specimen and higher internal porosity.

In addition, the arrangement of straw fibers in the casting specimen is different from that of the 3D printed specimen. Generally, the fibers in the casting specimen cannot absorb or transmit the vertical load well, resulting in a decrease in the flexural strength.

However, the flexural strength of the 3D printed specimens incorporated with straw fiber at the age of 7 days was lower than that of the undoped specimens, which might be due to inadequate hydration reaction and the incomplete connection between fiber and mortar. When the curing age of the specimen reached 28 days, sufficient hydration reaction caused the interface between the straw fiber and mortar to bond tightly, reducing the internal voids of the specimen, and the straw fiber fully exerted its energy absorption effect, which was conducive to improving the bending performance of the specimen.⁶³

The flexural strength of the 3DP-SF0-0.4% group is 10.2% lower than that of the 3DP-SF0 group, and the flexural strength of 3DP-SF1-0.4% group is about 1% higher than that of the 3DP-SF0 group. The failed specimen is shown in Figure 4c and d. When the content of straw fiber reached or exceeded 0.6%, the significant decrease in the flowability of composite materials led to a decrease in the extrudability, increasing the probability of blockage at the printer nozzle, which can also result in incomplete printed specimens, as shown in Figure 3d and e.

With a continuous increase of straw-fiber content, the flowability of the printed composite decreases, and the discontinuous discharge during the printing process leads to the incomplete structure of the specimen, which will reduce the mechanical properties of the specimen. In addition, the effect of fiber absorption energy and transfer load can no longer offset the negative effects of insufficient hydration and high porosity caused by higher fiber content and fiber agglomeration.¹⁶ Therefore, when the fiber content reaches or exceeds 0.6%, the flexural strength of the specimen will decrease.

Compressive strength

The compressive strength of the 3D printed straw-fiber composite is shown in Figure 4b. Similar to the flexural strength, the compressive strength of the 3DP-SF1 group was greater than that of the 3DP-SF0 group under the same straw-fiber content and curing age conditions; this is related to the fact that alkaline treatment can reduce the water absorption rate of straw fibers, remove impurities such as lignin and pectin that affect cement hydration, improve the mechanical properties of straw fiber, and enhance its bonding effect with the composite.^11,16

When the amount of straw fiber reached 0.2%, the compressive strength of the natural straw-fiber group and the alkaline-treated straw-fiber group for 7 and 28 days was lower than that of the control group. The reason is that when the fiber content in the specimen was small, the fibers only absorbed a limited amount of load energy, but the internal porosity of the specimen increased more than that of the control group, resulting in a decrease in compressive strength.

When the straw-fiber content was 0.4%, the compressive strength of 7 and 28 days in the 3DP-SF0-0.4% group and the 3DP-SF1-0.4% group reached the maximum, but it was still reduced by about 10.3%, 5.6% and 10.2%, and 4.9% compared with the control group, respectively. This may be because the distribution of straw fibers along the printing direction has no significant impact on the compression performance of the printed specimen, and cannot offset the negative effects of the increased porosity caused by the incorporation of straw fibers.⁶⁴

As the content of straw fiber continues to increase, the compressive strength of 3D printed cementitious composites shows a significant downward trend. This is similar to the law that the compressive strength of the casting straw-fiber composite specimen changes with the increase of fiber content. Shang et al. found that for the cast-in-place straw-fiber composite specimen, when the dimension content is 0%, 0.4%, 0.8%, and 1.2%, the compressive strength is about 33.0, 27.0, 28.7, and 25.3 MPa, respectively.¹⁵

With the increase of fiber content, the compressive strength of the specimens first decreased then increased, and then continued to decrease. With increasing fiber content, the water absorption of fibers led to a decrease in the flowability of the material, resulting in bad extrudability and buildability, as shown in Figure 3c and d, where the partitioned printing filaments and defects in the printing material are the main reasons for the decrease in compressive performance of the specimen.

ANOVA of mechanical properties

To have a clearer understanding of the impact of the two variable factors (fiber type and fiber content) on the flexural and compressive properties of 7 and 28 days printed specimens in this study, we performed the ANOVA in this section. In the ANOVA table, SS, DF, and MS represent the square sum, degree of freedom, and mean square of each factor, respectively. The F value is the ratio of SS to MS. The greater the F value, the greater the influence of the factor on the whole.

According to the F value query table, the p-value can be obtained. At the 95% confidence level, whether the p-value is <0.05 is the criterion for judging the degree of influence of the variable.⁶⁵ It can be seen in Table 4 that for the flexural strength and compressive strength of the specimens at 7 days age, the p-value of straw-fiber type is greater than 0.05 and the p-value of straw-fiber content is less than 0.05.

Table 4.

Two-Way Analysis-Of-Variance Results for the Specimen Properties

Response variables	Factors	SS	DF	MS	F ratio	p-value
7d flexural strength ($R_f$)	Fiber content	2.916	1	2.916	6.22412	0.067137
	Fiber type	15.27	4	3.8175	8.148346	0.033233
	Error	1.874	4	0.4685
7d compressive strength ($R_c$)	Fiber content	9.025	1	9.025	6.539855	0.062817
	Fiber type	490.756	4	122.689	88.90507	0.000368
	Error	5.52	4	1.38
28d flexural strength ($R_f$)	Fiber content	8.836	1	8.836	9.340381	0.037798
	Fiber type	42.92	4	10.73	11.34249	0.018629
	Error	3.784	4	0.946
28d compressive strength ($R_c$)	Fiber content	13.225	1	13.225	12.50591	0.024092
	Fiber type	878.246	4	219.5615	207.6232	<0.001
	Error	1.0575	4	1.0575

Therefore, it is considered that the content of straw fiber has a great influence on the mechanical properties of the specimens at 7 days age, and whether the fiber is alkalized has little effect on it. This may be due to the incomplete hydration reaction inside the specimen, the loose connection between the fiber and the mortar, and the performance difference of the fibers.

As the curing age increases to 28 days, the internal hydration of the specimen is sufficient, and the fiber is tightly connected to the surrounding mortar. At this time, the fiber type can significantly affect the mechanical properties of the specimen. In the analysis of the flexural and compressive properties of the 28 days age specimen, the fiber type and the fiber content p-values are <0.05; it can be proved that the fiber type and fiber content are the main factors affecting the flexural strength and compressive strength of the specimens.

X-ray diffraction

Figure 5 shows the XRD patterns of natural straw fibers (SF0) and alkalized straw fibers (SF1). And the maximum diffraction peaks of SF0 and SF1 correspond to 21.96° and 22.34°, respectively. After the alkaline treatment of straw fibers, the cellulose peak in the crystalline area was intense, because alkaline treatment can remove impurities such as lignin and hemicellulose in straw fiber and improve the crystallinity of cellulose.^13,66

FIG. 5.

(a, b) XRD patterns of straw fibers. XRD, X-ray diffraction.

Cellulose is a macromolecular polysaccharide substance with good mechanical properties and can be tightly combined with the mortar interface.⁴³ Therefore, alkaline treatment can improve the mechanical properties of straw fiber and also strengthen the adhesion between straw fiber and mortar.

Scanning electron microscopy

Figure 6 shows the SEM image of straw fiber and 3D printed straw-fiber cementitious composite with 28 days curing age. Comparing Figure 6a and b, it can be seen that the alkaline treatment removed most of the hemicellulose, lignin, and pectin components of the straw fiber, and mainly retained the cellulose structure.⁶⁷ From Figure 6c and d, it can be seen that the 3DP-SF0-0.4% group fibers have obvious pores at the interface with mortar bonding, whereas the 3DP-SF1-0.4% fibers shown in Figure 6e and f were tightly bound to mortar.

FIG. 6.

SEM diagram of straw fiber and 3D printed straw-fiber cementitious composite: (a) SF0; (b) SF1; (c, d) 3DP-SF0-0.4%; (e, f) 3DP-SF1-0.4%.

It was shown that the straw fiber could strengthen its bond with mortar after alkaline treatment.⁶⁸ Comparing Figure 6c and e, it can be seen that the incorporation of primary straw fibers has a negative effect on the cement hydration process, and the alkaline treatment of straw fiber can eliminate this negative impact.

X-ray computed tomography

Analysis of 3D printed composite

The volume of the scanned specimens was cropped into a 30 × 30 × 30 mm³ cube ROI using the software Avizo, and manual threshold segmentation was performed to separate the internal pores of the specimen from the matrix.^69,70 It can be seen from Figure 7a and b that the addition of 0.6% alkalized straw fiber increased the number of internal pores of the 3D printed composite, which was confirmed by the obvious increase in the pore volume fraction of Figure 7c and d.

FIG. 7.

Pore area distribution of 3D printed straw-fiber cementitious composites: (a) 3DP-SF0; (b) 3DP-SF1-0.6%. Pore volume fraction of 3D printed straw-fiber composite: (c) 3DP-SF0; (d) 3DP-SF1-0.6%.

The total porosity of the 3DP-SF0 group and the 3DP-SF1-0.6% group calculated by Avizo was 0.99% and 2.08%, respectively. The pore ratio of pore surface area greater than 0.5 mm² was 4.95% and 6.11%, respectively, showing that the ratio of large pores also increased. This is mainly due to the incorporation of excessive straw fibers absorbing a large amount of free water in the system, resulting in an insufficient hydration reaction, and the cluster phenomenon of fibers was also the reason for the increase in porosity.

Compression failure analysis of 3D printed composite specimens

The volume of the scanned image was cropped into a 35 × 35 × 35 mm cube ROI by Avizo, and the manual threshold segmentation was performed to separate the cracks generated after the compression failure of the specimen from the matrix.⁷¹ Figure 8a and b are the reconstruction of the specimen after compression failure and its corresponding layer slices.

FIG. 8.

Avizo-reconstructed X-CT structures of specimens after compression failure: (a) 3DP-SF0; (b) 3DP-SF1-0.6%. Crack display of specimen after compression failure: (c) 3DP-SF0; (d) 3DP-SF1-0.6%. X-CT, X-ray computed tomography.

It can be seen that the specimens from 3DP-SF1-0.6% group produced a larger crack distribution after compression failure. Figure 8c and d show the crack distribution of the two groups of specimens after compression failure. It can be seen that the 3DP-SF1-0.6% group has stronger crack connectivity than the 3DP-SF0 group, due to the alkaline treatment and higher fiber contents in the 3DP-SF1-0.6% group.

Environmental benefits

As the straw fibers are usually burned after the crop harvest, this would generate lots of carbon dioxide, carbon monoxide, PM10, and PM2.5,⁴ thus ruining the environment. On the contrary, printing the straw fibers into the cementitious composites instead of burning the straw can reduce the emissions of carbon and harmful particles as shown in Table 5, where a building wall with a volume of 10 × 3.5 × 0.4 m³ was used in the estimation.

Table 5.

Environmental Benefits of Printing Straw-Fiber-Reinforced Cementitious Building Wall Instead of Burning the Straw

Wall size (m3)	Fiber content (wt%)	Fiber weight (kg)	Carbon dioxide emission reduction (kg)	Carbon monoxide emission reduction (kg)	PM10 emission reduction (kg)	PM2.5 emission reduction (kg)
10 × 3.5 × 0.4	0.17	23.8	28.1	2.21	0.22	0.20

Largely, the wall would consume 14 tons of concrete. If the concrete was printed with a straw-fiber content of 0.17 wt%, the emission reductions of carbon dioxide, carbon monoxide, PM10, and PM2.5 were 28.1, 2.21, 0.22, 0.20 kg, respectively, when compared with the situation of burning the straw fibers. From this calculation, it can be found the application of straw fiber in the construction materials is beneficial to environmental protection.

Limitations and perspectives

In this work, the experiments were carefully conducted, though limitations and drawbacks still existed. For example, the anisotropy⁷² of the printed specimens was not considered in the strength tests, which can be studied in future. Further, the stress-strain relationships,^60,61 especially those indicating the mechanical behavior after the peak load, are also important and can be obtained by more advanced testing machines.

In addition, other physical properties,⁵⁰ such as thermal conductivity and noise resistance of the printed cementitious composite, can be also investigated in future studies, as these properties are also advantages when using the present printed material in construction. Moreover, recycled materials,⁴⁶ such as recycled binders and aggregates, can be also used in the present printed fiber-reinforced concrete to reduce the carbon footprint, although the mix design should be also researched together to get better printability and properties in the future studies.

Conclusions

In this study, the flowability, buildability, mechanical properties, and microstructure of 3D printed cementitious composites with different contents of alkalized and natural straw fibers were evaluated. Based on the results, the following conclusions can be drawn:

• The alkaline treatment of straw fiber can shrink its surface pores and reduce its water absorption, allowing more free water in the system to participate in the cement hydration reaction. The alkaline treatment can also remove lignin, hemicellulose, and other impurities on the fiber surface and improve the crystallinity of cellulose, which is beneficial to the bonding of the straw fiber and mortar interface.

• Under the same straw-fiber content, the compressive and flexural strength of the alkalization group were higher than those of the non-alkalization group, which was related to better mechanical properties of the alkalized straw fiber and the good bonding with the mortar interface.

• When the straw-fiber content was small, the fibers did not have an obvious positive effect on bearing the load but led to an increase in the porosity of the specimen, which decreased the mechanical properties of the specimen. However, excessive incorporation of straw fiber led to a decrease in the fluidity of the composite material, and the extrusion was discontinuous during the 3D printing process, resulting in an incomplete structure of the printed specimen and reduced mechanical properties.

• Specifically, when the content of straw fiber was 0.4%, it was more suitable for 3D printing of cement-based composites. For this mix, the 28 days flexural strength of the specimens was 1% higher than that of the specimens without fibers, but the compressive strength was reduced by 4.9%. Compared with the natural fiber group, the 28 days flexural strength and compressive strength of the fiber alkalization group increased by 12.6% and 6%, respectively.

• The incorporation of alkalized straw fiber significantly increased the porosity of the composite specimen. The extrusion method of 3D printing made the straw fiber directionally arranged and distributed inside the specimen, which retarded the crack propagation during the strength test. It can be seen that the printed straw-fiber cementitious composite showed prospects as a building material.

Authors' Contribution

A.C.: Conceptualization, Methodology, Software, Formal analysis, Investigation, Visualization, Data curation, and Writing—original draft. P.D.: Methodology, Formal analysis, and Investigation. Q.L.: Funding acquisition, Writing—review & editing, and Supervision.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the National Natural Science Foundation of China (51908075).