Research on flexural mechanical properties and mechanism of green ecological coir fiber foamed concrete (CFFC)

Abstract

To explore the effect and mechanism of coir fiber on the performance of foamed concrete, the flexural performance test, pore characteristics and microstructure test of coir fiber foamed concrete with different content were carried out. First, Image-Pro Plus (image processing software) was used to study the pore morphology, porosity, average pore diameter, and pore roundness of CFFC with various fibers dosage (0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%) by binarization processing method. Then, a total of eighteen specimens, divided into six groups, were used to investigate the effect of CF dosage on flexural strength, toughness, energy absorption, and failure patterns of FC through a three-point flexural test. Furthermore, the microscopic properties of coir fiber foamed concrete (CFFC) were observed by scanning electron microscope (SEM) and energy dispersive X-ray detector (XRD) to explain the influence mechanism of CF on FC flexural properties. According to the research, CF can affect the pore characteristics of CFFC and improve its flexural performance. When CF content is 1.5–2.0%, the porosity, diameter and roundness of CFFC have lower values of 68.6%, 1.96 mm and 1.29. After the fiber dosage reaches 1.5%, the CFFC failure mode changed to plastic damage, the flexural strength increased from 0.33 to 0.73 MPa, and the toughness energy absorption value was increased from 0.05 to 1.4 J. The optimum dosage of coir fiber is 2.0% for improving the flexural mechanical properties of FC. CF affects the process of hydration reaction of CFFC, but does not change the type of hydration product. However, the flexural performance of FC would decrease with excessive dosage of CF (> 2.0%) due to accelerating the formation of Ca(OH)₂. CFFC can solve problems such as brittleness and easy cracking existing in traditional foamed concrete, and it can be used in the field of pavement engineering, foundation backfill and lightweight wall structure with CF dosage of 15–2.0%.

Introduction

Foamed concrete (FC), known for its environmental friendliness and low energy consumption, is a lightweight and porous material used in backfilling foundations, lightweight partition structures, and collision avoidance^1–3. However, the disadvantages like poor flexural strength, brittleness, and quick cracking lead to poor integrity and durability, restricting the application of it. Currently, more studies have focused on the performance and preparation of foamed concrete in an attempt to improve its strength, reduce shrinkage and increase crack resistance⁴. The methods of preparing foamed concrete usually include chemical foaming method and physical foaming method. The foamed concrete prepared by physical foaming method with plant protein-based foaming agent has better mechanical and crack resistance properties⁵. In addition, some scholars have investigated that admixtures such as silica fume, fly ash and mineral powder can replace part of the cement, thus slowing down the hydration heat evolution rates, reducing the shrinkage of foamed concrete and improving its mechanical properties^6,7. However, problems such as low strength, brittleness and easy cracking are still seen in the foamed concrete prepared by existing technology.

Researches have shown that adding fibers into FC could improve flexural strength, toughness and energy consumption performance in recent years⁴. The research on the flexural mechanical properties of fiber FC had mainly focused on inorganic non-metallic fibers (basalt fiber⁸, glass fiber^9,10 and organic synthetic fibers (polypropylene fiber (PP)^11–14, and polyvinyl alcohol fiber (PVA)^15,16, carbon fiber¹⁷. Gencel⁸ et al. investigated the performance of silica fume basalt fiber-containing foamed concrete under freeze–thaw cycles and developed a durable concrete with compressive strength of up to 46 MPa. Amran et al.¹⁸ used fly ash, silica fume and polypropylene fibers as raw materials for the preparation of foamed concrete with compressive strength of more than 20 MPa, and the drying shrinkage rate was significantly reduced. However, they prepared foamed concrete with coarse aggregates, which leads to a density close to 2000 kg/m³ and loses the advantage of light weight. The foamed concrete they prepared was unanalyzed at pore structure and microstructure level, and the influence of specific conditions¹⁹ was not considered, thus it is difficult to fully reveal the the mechanical properties of fiber foamed concrete. More importantly, the above fibers are generally high energy-consuming and the preparation process is complicated, causing waste of resources. Therefore, finding an eco-friendly material with excellent strength performance is valuable to significantly improve these drawbacks of FC.

Coir fiber (CF), as a natural plant fiber, has the advantages of being renewable, abundantly available, and low in energy consumption. To compare with other plant fibers (such as hemp fiber²⁰, kenaf fiber²¹, and sisal fiber²², CF has stronger toughness, higher tensile strength and elastic modulus. CF has a tensile strength more than 150.0 MPa and an elastic modulus more than 5.0 GPa from the test in previous research²³. Previous studies were conducted to investigate the effect of CF with different types, shapes and admixtures on the mechanical properties and durability of ordinary concrete, and the results showed that CF can improve the properties of concrete²⁴. Wang²⁵ analyzed the dynamic performance of CF reinforced concrete under different drop heights. The study showed that CF reinforced concrete has a higher dynamic enhancement factor than ordinary concrete, and the failure mode plasticity is obvious. Mydin²⁶ found that CF concrete's compressive and shear strength is improved compared with ordinary concrete specimens with a CF dosage of 2.0%. In addition, some researches explored the influence of CF on the compressive, flexural, anti-impact properties of cement-based composites^27,28 and the influence of coir fiber length and dosage on the flexural performance of magnesium phosphate cement^29,30. The studies mentioned above showed that CF can improve the properties of ordinary concrete, but the impact of CF on FC is unclear. Furthermore, only Zhang³¹ and Raj³² explored the influence of CF dosage on the compressive strength and dynamic mechanical properties of FC in recently reports. The flexural performance also is an important property index, which is the key to evaluate the energy absorption toughness of coir fiber foamed concrete (CFFC). However, the research on the flexural properties and microcosmic mechanism of CFFC has not been reported yet. Therefore, it not only needs to study the compressive and impact properties that the influence of CF on FC, but also needs to pay more attention to the flexural performance urgently.

On the basis of the existing foamed concrete preparation technology, considering that the problems such as brittleness, easy cracking and poor energy absorption still exist in current foamed concrete, this study aims to replace the traditional fibers which are of high energy consumption, complex preparation process and poor mechanical properties with green ecological natural plant fiber—coir fiber to improve the flexural properties of foamed concrete, so as to achieve the goals of ecological green and economic savings. The research results of CFFC are expected to address the brittleness and susceptibility to cracking existed in traditional foamed concrete. Firstly, it was analyzed that the effect of CF dosage on pore characteristics (porosity, average pore diameter and roundness) of FC by graphic processing software (Image-pro plus, IPP). Then, a series of three-point flexural tests were carried out to study the influence of CF dosage on the flexural properties (failure mode, flexural strength, load–displacement curve, flexural toughness, etc.) of FC. In addition, with the help of modern microscopic techniques (scanning electron microscopy, SEM) and energy dispersive X-ray detector (XRD), this research explained the effect mechanism of CF on the properties of FC from microscopic.

Experimental program

Test program and specimen parameters

In this work, a total of eighteen specimens were divided into six groups according to CF volume dosage of 0, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% to investigate the effect of CF dosage on the flexural properties and pore characteristics. Three-point flexural test was carried out to study the flexural strength, load–deflection (L-D) curves, flexural toughness, and failure patterns of CFFC. The pore structure morphology, porosity, average pore diameter, and pore roundness were analyzed to prove the relationship between the pore structure parameters and the mechanical properties of CFFC. Moreover, scanning electron microscope (SEM) was adopted to analyze the microstructure of coir fiber foamed concrete for clarifying the macroscopic properties.

There are six group specimens to research flexural properties and pore characteristics of CFFC, and the parameters of specimen were shown in Table 1. The dimension of the specimen was 40 mm × 40 mm × 160 mm, as shown in Fig. 1a. The form of “Cement slurry-Foam-Coir fiber dosage-specimen number” numbered each specimen. For example, in the C-F-F0-1, the specimen number was 1, and the CF dosage was 0%. It should be noted that specimens of the pore characteristics and microcosmic test were made by slicing failure specimens of flexural test.

Table 1.

The parameters of specimen.

Number	Cement quantity (g)	Fiber quantity (g)	Fiber length (mm)	Foam dosage (mL)
C-F-F0-1	125	0	20	175
C-F-F0-2
C-F-F0-3
C-F-F0.5-1	125	1.25	20	175
C-F-F0.5-2
C-F-F0.5-3
C-F-F1.0-1	125	2.5	20	175
C-F-F1.0-2
C-F-F1.0-3
C-F-F1.5-1	125	3.75	20	175
C-F-F1.5-2
C-F-F1.5-3
C-F-F2.0-1	125	5	20	175
C-F-F2.0-2
C-F-F2.0-3
C-F-F2.5-1	125	6.25	20	175
C-F-F2.5-2
C-F-F2.5-3

In the table, C-F-F represents the Cement slurry-Foam-Coir fiber, and the ratio of cement, water, and foam is 1:0.5:2 for each specimen. 0, 0.5, 1.0, 1.5, 2.0, and 2.5 represent the volume ratio of Coir fiber to the specimen, which respectively is 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, -1, -2, -3 are a serial number of specimens.

Figure 1.

Specimen dimension and test equipment: (a) specimen dimension, (b) test equipment.

Materials performance

The FC was composed of Portland cement P.C 32.5R, Coco-amide propyl Betaine (CAB-35), Hydroxypropyl methylcellulose (HPMC), Nano-silicon dioxide (NS), and a proportion of water. Raw materials and the detail parameters of materials are shown in Table 2. In order to understand the CF’s characterization, the SEM morphology of the coir fiber was observed, as shown in Fig. 2. As seen in the microscopic morphology of coir fiber, dense cellulose of fiber cross-section and layered cellulose stacking phenomenon from fiber longitudinal cross-section are the reasons for the higher strength of coir fiber.

Table 2.

The detail parameters of materials.

Material	Parameters	Value
P.C32.5R	Clinker + gypsum (%)	50–80
	Slag + fly ash + limestone (%)	20–50
	MgO (%)	≤ 6.0
	SO3 (%)	≤ 3.5
	Cl− (%)	≤ 0.06
	28-days Compressive strength (MPa)	≥ 32.5
CAB-35	Betaine (%)	30 ± 2
	NaCl (%)	0–6
	Dissociative amine (%)	0–0.5
HPMC	Molecular content (g·mol−1)	200,000
	POOH (%)	11.75
	OCH3 (%)	26.5
NS	diameter (nm)	15 ± 5
	Ratio of mass to surface area (m2/g)	250 ± 30
	Purity percentage (%)	99.8
Coir fiber	diameter (um)	150–350
	Length (mm)	20
	Cellulose (%)	33.7
	Lignin (%)	25.1
	Hemicellulose (%)	18.3
	Pectin (%)	22.9
	Density (g/cm3)	1.20
	Elasticity modulus (GPa)	3.86–5.6
	Tensile strength (MPa)	128–157

Figure 2.

SEM morphology of the coir fiber.

Specimen preparation

The preparation of foamed concrete³³ is divided into foam preparation and cement slurry preparation. Section one: Mix the NS into the water to form a uniform stirred dispersion solution. Then, slowly mix the HPMC into the solution, and stir for 1 min with a foaming machine at low speed to fully dissolve the HPMC. Subsequently, the foaming agent is stirred and foamed at low and high speed for 1 min respectively. Section two: pour water into cement as a ratio of water to cement being 1:0.5 and mix at low speed for 2 min to form uniform slurry. After completing the above preparations, pour the foam twice the volume into the cement slurry and stir for 1 min to achieve the density of 520 kg/m³. It should be noted that the foam was prepared with CAB-35, HPMC, NS, and water, with a ratio of 1: 0.05: 0.2: 7.5. Table 3 shows the basic mix ratio of foamed concrete.

Table 3.

The basic mix ratio of foamed concrete specimens.

Cement slurry		Foam				Density (kg/m3)	28-days compressive strength (MPa)
P.C32.5R (g)	Water (g)	CAB-35 (g)	HPMC (g)	NS (g)	W ater(g)
1000	500	10	0.5	2	75	520	0.80

Finally, mix the CF into the foamed concrete. Adding progress needs to control the mixing time within 1 min because it is so long that it would affect the performance of the foamed concrete. To prevent the fibers from agglomerating, the fibers should be dispersed before adding into concrete mixture. Then, the required dosage of coir fibers is divided into three equal parts. The fibers are mixed into the foamed concrete in three times, and the interval between fiber dispersion is 20 s. It should be noted that the mortar mixer is kept in rotated at a constant low speed during the process of adding fibers. The parameters of CF were shown in Table 2. The entire operation of the preparation process is shown in Fig. 3.

Figure 3.

Specimen preparation process.

SEM and XRD samples were prepared using post-bending test specimens, with reference to ASTM C1726-16³⁴, which specifies the method for testing the microscopic morphology of hardened concrete. Firstly, the CFFC specimens were breaked into pieces and, fragments with fibers (5 mm × 5 mm) were selected and soaked in anhydrous ethanol for 3 days. Then, the fragments were baked in a 60 °C drying oven for 48 h to ensure complete drying. Finally, the partially dried fragments were placed on the carrier stage of the SEM scanning electron microscope to affix the conductive adhesive and gold spraying (180 s), and then the samples in vacuum condition could be observed. In addition, XRD test powders were prepared by grinding the dried fragments, and the powder particle size was ground to less than 75 um.

Test method

Pore characteristic test

The strength of CFFC is not only determined by the strength of the matrix material but also depends on the characteristics of the pore structure^35,36. This experiment used Image-Pro Plus (image processing software) to study the pore structure morphology, porosity, average pore diameter, and pore roundness of CFFC by binarization processing method.

Three-point flexural test

Three-point flexural test carry out according to code ASTM C1609/C1609M-10^29,37. As shown in Fig. 1b, three-point flexural test was conducted by an MTS-E45.305 electric servo testing machine with a load sensor. The machine has a range of 300 kN and a precision error of ± 0.5%. The displacement speed of the load cell is 1 mm/min as a loading control method. Then, when the residual load capacity is only 10% of the peak load known at the preliminary experiment, it is commonly regarded that the specimen is destroyed. Further, this work obtained the microscopic morphology and crystal structure characteristics of the three-point flexural test specimens through a scanning electron microscope (SEM).

Microscopic properties test

Three-point flexural test carry out according to code ASTM C1723-16 .The SEM (LEO1530VP) is a precision instrument that uses the interaction between electrons and substances to obtain the measured material’s microscopic morphology and crystal structure characteristics and perform qualitative or semi-qualitative analysis of the chemical composition. The XRD (PW3040/60) is a qualitative analysis instrument based on the diffraction of X-rays and materials to analyze the composition and crystallization degree of materials through diffraction patterns. This experiment researched the micro characteristics of CFFC through a scanning electron microscope (SEM) and energy dispersive X-ray detector (XRD).

Pore characteristics

Figure 4 shows the pore morphology of specimens before the processing of binarization. It can be obviously found that the number of large pores in the specimen without CF (CFFC 0-1) was larger than that in specimen with higher fiber dosage (CFFC 2.0-3). However, when the fiber dosage is excessive, the number of large pores in the specimen (CFFC2.5-1) was lager than that in CFFC 2.0-3.

Figure 4.

The pore morphology of specimens.

Porosity is the ratio of the pore volume to the apparent volume of CFFC, which can be transformed into a relationship between absolute and apparent density, as shown in Eq. (1). The roundness value is a parameter that characterizes the geometric shape of CFFC pores, which indicates the degree of pore deviation from the spherical shape, as calculated using Eq. (2). The size of pores and average pore diameter are common indicators for analyzing the pore characteristics of CFFC. The average pore diameter is obtained by the weighted average and calculated using the Eq. (3).

$$\epsilon =1-\frac{{\rho }_d}{{\rho }_s}$$ 1

$$S=\frac{P^2}{4\pi A}$$ 2

$${\text{R}}_{\text{n}}=0.75\times {\text{R}}_1+1.25\times {\text{R}}_2+1.75\times {\text{R}}_3+\cdots +4.75\times {\text{R}}_9+5\times {\text{R}}_{10}$$ 3

where ε is porosity, ρ_d is apparent density (kg/m³), and ρ_s is absolute density (kg/m³); S is roundness value, P is pore perimeter (mm), and A is pore area (mm²); R is average pore diameter (mm), and R_n is pore diameter of the ‘n’, n = 1, 2, 3…

Binary pore morphology of different fiber dosage was shown in Fig. 5. According to the data of binarization, the curves of porosity, average pore diameter and roundness value at different fiber dosage were plotted in Fig. 6. As shown in Fig. 6, the porosity generally decreased after fibers were incorporated. When no fiber was added, the porosity of the specimen had a maximum value of 78%. Then, the porosity went down to 68.6 with CF dosage of 2.0%. After incorporating CF, the pore diameter of CFFC was smaller than that without fiber. Moreover, when fiber dosage increased from zero to 2.5%, the pore diameter went down from 2.28 to 1.89 mm and became uniform with the increase in CF. However, the evolution law of roundness value was different from porosity and average pore diameter. With an increase in the amount of CF, the roundness value, evaluated through binary data, decreased firstly and then increased. When the fiber dosage is not exceeded 1.5%, the roundness value decreased from 1.57 to 1.22. However, when the fiber exceeded 1.5% of the volume threshold, the roundness value became larger again. The roundness value backed to 1.41, as seen in CFFC-2.5-1.

Figure 5.

Binary pore morphology of specimens.

Figure 6.

Parameter curve of pore structure characteristics.

The above phenomenon indicates that CF can improve the pore structure, but the improvement of effect is weakened with excessive CF and leads to deterioration of the pore structure. The reason is that CF punctured the thin air bubbles, and there is an evident phenomenon of foam broking. Therefore, with CF dosage increasing, the porosity and pore diameter decrease. Also, the smaller the foam is, the smaller its roundness is, and the better degree of dispersion is. However, the roundness value increases again with excessive CF.

Three-point flexural property

Failure pattern

Figure 7 shows the flexural failure morphology of specimens with different CF dosages. The C-F-F0-1 showed a visible crack running up and down in the middle span. The crack rapidly developed from the tensile area to the compression area in the middle span, and the deflection of the mid-span and the width of the crack remained small until the specimen failure, as shown in Fig. 7. After adding CF, the characteristic of brittle failure was alleviated. The crack width of C-F-F0.5-1 was broader than that of C-F-F0-1, and the crack direction of C-F-F0.5-1 inclined to some degree. As the fiber dosage grew, the deflection of mid-span, crack inclination, and crack width were generally increased during the process of failure. The above phenomenon indicates that the flexural failure pattern of FC is affected by CF, and CFFC still has some bearing capacity after cracking.

Figure 7.

Flexural specimens failure morphology.

There were two failure modes of three-point flexural properties with different CF dosages, namely, brittle failure mode and plastic failure mode, as shown in Fig. 8. It could be found that the concrete matrix on both sides of the crack had apparent fiber bridging effects, and the plastic failure mode was obvious. When cracks appear at the peak load, fibers can transfer a part of load on the substrates and inhibit the development of cracking. With the number of fibers in the bridging state increasing, CFFC has the higher flexural bearing capacity and more plastic deformation with the increase of CF, and the plastic of CFFC is enhanced. Therefore, when specimens without CF, the failure form is a brittle failure, and when specimen with more fibers, it transformed to plastic failure.

Figure 8.

Failure mode schematic diagrams. (a) Brittle failure mode; (b) plastic failure mode.

Load–deflection curves

Figure 9 shows the curve configuration and load–deflection curves of specimens. The curve configuration could be divided into three stages in Fig. 9a, namely, the elastic stage (O-A), the elastic–plastic stage (A-B), and the unstable failure stage (B-C-D-E). In the elastic stage (O-A), when the load did not reach the point of the elastic limit, the load–deflection showed an excellent linear, and the deformation was coordinated between fiber and concrete, and fibers and concrete matrix borne load jointly. In the elastic–plastic stage (AB), after surpassing point A, the linear limit, the load–deflection curve gradually changed to non-linear. Under flexural load, the deflection increased more quickly than bearing capacity. Therefore, the curve slope decreased gradually. When the flexural load reached the bearing capacity limit, prominent cracks was formed gradually with the extension of micro-cracks in the CFFC matrix at the failure stage (B-C-D-E). When it came to the point of C, more and more fibers crossed the cracks between matrices, and CF transferred the bonding stress to both sides of the cracking, as shown in Fig. 8b.

Figure 9.

Load–deflection curves and configuration. (a) Curve configuration; (b) load–deflection curves.

The flexural stiffness, calculated by the slope of O-A at the load–deflection curve (Eq. (4)), was shown in Fig. 10. Furthermore, the flexural strength of CFFC was calculated by load–deflection curves. According to ASTM C293M-10, this paper used a simple beam with center-point loading to analyze the flexural properties of CFFC and calculated the flexural strength by Eq. (5).

$$K=\frac{P_{\text{p}}}{{\delta }_{\text{p}}}$$ 4

$$f=\frac{3Fl}{2b{h}^2}$$ 5

where K is secant stiffness (kN/mm), P_p is peak load (kN), and δ_p is the displacement at peak load (mm), f is the flexural strength (MPa), F is the maximum load value (N), l is the distance between two supports (mm), b is the section width of the specimen (mm), h is the section height of the specimen (mm).

Figure 10.

Flexural capacity and flexural stiffness.

Figure 10 shows the flexural capacity and flexural stiffness of specimens with six group dosages of CF. With the addition of the CF, the flexural capacity of CFFC increased significantly, and it was 310 N with 0.5% CF dosage. Furthermore, it continued to rise with the increase in CF dosage. When CF dosage was 2.0%, the bearing capacity had a maximum value of 472 N. However, when CF was 2.5%, bearing capacity decreased to 350 N. Also, the law of increasing and decreasing stiffness is the same as flexural capacity. When fiber dosage was 2.0%, the flexural stiffness reached 1830 N/m, as shown in Fig. 10. Figure 11 shows the average flexural strength and strength growth rate of CFFC at 28 days of age for different CF dosages. It could be observed that flexural strength increased first and then decreased with the excessive dosage. When the fiber dosage was 2.0%, the flexural strength reached 0.73 MPa, 2.21 times the specimen without fiber. However, when the dosage was 2.5%, exceeding the threshold, the flexural strength was reduced to 0.55 MPa, only 1.67 times the primary specimen.

Figure 11.

Flexural strength and enhance the coefficient.

Above data indicate that a proper dosage of CF can improve the flexural capacity, stiffness, and strength of CFFC, while excessive fiber can weaken the enhanced effect. One reason is that CF can promote the performance of CFFC matrix, which can be verified by the declining law of pore diameter, porosity and roundness value as described in “Pore Characteristics”section. In addition, the more important reason is that fibers have a restraint effect on the CFFC matrix. As shown in Fig. 12a, fibers are disorderly distributed in the CFFC matrix to form a combined loading system, and the integrity of CFFC is reinforced with increased CF dosage. Therefore, the more obvious the three-dimensional mesh structure is, The greater the flexural capacity, stiffness and strength are. However, the CF also increased the number of defects and weak interfaces, as shown in the analysis of pore characteristics (“Pore Characteristics”section) and microstructure (Fig. 12b and "Microscopic morphology" section). Therefore, flexural performance degrades when the fiber dosage exceeds 2.0%.

Figure 12.

Microstructure and schematic of CFFC. (a) Three-dimensional mesh structure schematic, (b) microscopic morphology.

Energy absorption capacity

According to the ASTM C1018-97 standard, this paper adopted the toughness test to evaluate the flexural toughness of CF reinforced concrete and initial cracking strength. The toughness is a value of energy-absorbing, equal to the integration area between the load–displacement curve and the X-axis in this section. Equation (6) shows the calculation formula of energy absorption value.

$$G_n={\int }_0^{\frac{n+1}{2}\delta }P\left(\delta \right)d\left(\delta \right)$$ 6

where G_n represents toughness (J), δ is mid-span deflection at the initial crack (mm), P(δ) is flexural load value (kN), and n is the toughness index (n ≥ 1).

The ASTM C1018 standard uses the index of toughness (I_n) to evaluate the toughness properties of fiber-reinforced concrete matrix composites to distinguish the contributions of matrices and coir fibers to flexural toughness. The test used the deflection of the first main crack as an evaluation point (δ), and the corresponding deflections of the index I₀, I₅, I₁₀, and I₂₀ are δ, 3δ, 5.5δ, 10.5δ, which correspond to the integration area under the curve. The calculation formula and schematic of toughness calculation are shown in Eq. (7) and Fig. 13a.

$${\text{I}}_n={\int }_0^{\frac{n+1}{2}\delta }P\left(\delta \right)d\left(\delta \right)/{\int }_0^{\delta }P\left(\delta \right)d\left(\delta \right)$$ 7

where n represents toughness index,$\delta$ is mid-span deflection at initial cracking (mm), and P($\delta$) is flexural load (kN).

Figure 13.

Flexural toughness and schematic of toughness calculation. (a) Schematic of toughness calculation; (b) Flexural toughness and energy abosorbing.

Figure 13b shows flexural energy absorption and toughness, when the fiber dosage did not reach 2.0%, the energy-absorbing value of specimen increased with the increase of fiber dosage under flexural stress. When the fiber dosage reached 2.0%, the energy-absorbing value was 1.4 J. However, the value was only 0.95 J with 2.5% CF dosage. Moreover, I5, I10, and I20 generally showed a rising trend with the increased of CF dosage. It should be noted that the increments of toughness index are gentler at 2.5% CF dosage than at 2%.

The above results showed that a proper amount of CF could improve the flexural energy absorption and toughness of CFFC, which increased firstly and then decreased with high CF. The main reason is that when the prominent crack emerges, fibers connect matrix on both sides of the crack and transfer the load from the cracked zone to the non-cracked zone. This phenomenon is called the bridging of fibers. The more fibers there were in the matrix, the larger the number of cross-sectional fibers was, and the effect of bridging was better, as shown in Fig. 8 (in section “Failure Pattern”). Therefore, flexural energy absorption and toughness increased firstly. However, excessive CF led to increased interface defects: a case in point is that the roundness value increased again with excessive CF and fibers gathered. As a result, the energy-absorbing value of CFFC with 2.5% CF dosage decreased, and the improvement effect of fibers on toughness reduced.

SEM and XRD analysis

Microscopic morphology

The microscopic morphology of specimens with different CF dosages was observed through SEM, as shown in Fig. 14. The pore diameter was significantly larger without CF, indicating that the pore was ruptured severely. After adding CF, block-shaped crystals (Ca(OH)₂ and calcium-silicate-hydrate) were embedded in the matrix, and the degree of pore rupture was reduced. The thickness of the pore wall tends to increase. When the CF dosage was further increased to 1.5%, the phenomenon of pore wall fracture and the formation of interconnected pores intensified. Additionally, from the high-magnification micrographs, it was observed that fibers were easily pulled out under loading with a lower fiber dosage (0.5%), and adhered concrete particles were fewer. With the increase of CF (such as 1.0% and 1.5%), more fractured concrete particles were found adhering to the pulled-out fibers. However, when the fiber dosage exceeded the optimal range (nearly 2.0%), the number of concrete particles adhering to the pulled-out fibers decreased again.

Figure 14.

Microstructure and morphology of specimens with different CF dosages. (a) 0%, (b) 0.5%, (c) 1.0%, (d) 1.5%, (e) 2.0%, (f) 2.5%.

The reasons are as follows: When coir fiber foamed concrete is prepared, fibers puncture the larger bubbles so that the voids are filled by the cement paste. Therefore, the pore walls in the matrix are thicker and the pore diameter is more uniform. When the fiber dosage is lower, the bond between the fiber and the concrete matrix is well working, and the fiber bear massive load. However, the concrete matrix does not reach its maximum bond strength with fewer fibers, resulting in bond slip and fiber pull-out. As the increase of fiber dosage, the combined bonding force between the fibers and the matrix increases, causing the concrete matrix to fractured after reaching its peak bond stress. Adhering to the fibers, the fractured matrix is pulled out. However, the number of pores and interconnected pores in the matrix increases with excessive CF, which leads to a decrease in the bond effectiveness between the fibers and the matrix. The result is that bond failure is appear under low stress, and fibers are pulled-out with fewer fractured particles adhering to the fibers. Additionally, excessive fibers tend to absorb more water, which may cause greater shrinkage during the hydration of the surrounding cement. Analysis indicates that the observed microstructure morphology is consistent with the rule that the flexural load-bearing capacity with the pattern of initially increase and then decrease.

XRD analysis

The XRD spectra of CFFC with different CF dosages were shown in Fig. 15. It can be observed that there are four main phases present: hydrated calcium silicate (C-S-H gel), calcium carbonate crystal (Ca(CO₃)₂), calcium hydroxide crystal (Ca(OH)₂), and unreacted silicon dioxide crystal (SiO₂). The C-S-H gel has crystallinity and amorphous forms, with the highest diffraction peak intensity occurring around 2θ = 30°. The C-S-H gel is the primary source of strength in CFFC, featuring the highest proportion among all source materials, accounting for approximately 50–70%. At peak around 2θ = 30°, when fiber dosage is increased from 0 to 2.5%, the intensity value of C-S-H decreases from 2260 a.u. to 890 a.u.. It indicates that C-S-H gel exhibits more amorphous forms and less crystallinity. Meanwhile, at 2θ = 18°, the peak intensity increased from 730 a.u. to 2050 a.u. with the increase of CF from 0 to 2.5%. It suggests that the intensity and area of Ca(OH)₂ diffraction peak increase with the increase of the amount of CF. It also declares an increase in crystallinity and content of Ca(OH)₂, which promotes the aggregation of Ca(OH)₂ into larger sizes under experimental conditions. Fibers did not alter the types of hydration products in foamed concrete, the XRD analysis result is similar to the effect of coir fiber on magnesium phosphate cement²⁹. However, the addition of CF changed the hydration process of foamed concrete. Therefore, with an increase in fiber dosage, the improvement of the flexural performance of CFFC was primarily influenced by the enhanced effects of fiber bridging and confinement. However, C-S-H gel decreased and Ca(OH)₂ increased with the increase of fibers, relatively, which leaded to a transition from the dominant influence of fiber-related effects to the dominant influence of deteriorating matrix properties, resulting in a decrease in performance of CFFC.

Figure 15.

XRD analysis of CFFC with different CF dosages.

Conclusions

In this study, the effect of CF on the flexural mechanical properties of foamed concrete was investigated by three-point bending test, and then the pore characteristics of CFFC were analyzed though Image-Pro Plus (image processing software). Further, the microscopic properties of CFFC were observed by means of scanning electron microscopy (SEM) and X-ray detector (XRD), so as to explain the mechanism of CF effect on the flexural properties of FC. Ultimately, the optimum amount of CF was determined to solve the problem of brittleness and easy cracking existed in conventional foamed concrete. The following conclusions were drawn from the study:

1. The eco-green CF can replace the traditional synthetic fiber with complicated process and high energy consumption and improve the mechanical properties of CF. When the fiber dosage is from 0% to 2.0%, the strength of CFFC increases improved from 0.33 to 0.73 MPa.

2. CF can improve the plasticity and toughness energy absorption properties of FC. When the fiber dosage increases from 0% to 2.0%, the failure mode of CFFC changed from brittle failure to plastic failure, and the toughness energy absorption value increases from 0.05 to 1.4 J.

3. CF can improve the pore characteristics of CFFC. When CF content is 1.5–2.0%, the porosity, diameter and roundness of CFFC have lower values of 68.6%,1.96 mm and 1.29 respectively.

4. The excessive CF leads to severe pore wall rupture and an increase in the number of weak interfaces. Although CF does not alter the types of hydration products in FC, it accelerates the crystallization rate of Ca(OH)₂ and reduces the content of C-S-H. The optimum dosage of CF is 2.0% to improve the flexural mechanical properties of FC.

5. CFFC can solve the problems such as brittleness and easy cracking existed in traditional foamed concrete, and it can be used in the field of pavement engineering, foundation backfill and lightweight wall structure with CF dosage of 15–2.0%.

In this study, the research of coir fiber on the performance of foamed concrete is limited to the fiber dosage. Therefore, the research on mechanical properties and mechanism of foamed concrete under the effect of others factors (such as length and treatment method of fiber) also should be focused in the subsequent work. In addition, researches on the long-term performance and engineering applications of CFFC should be carried out.

Author contributions

The testing, writing and diagram drawing of this study were done independently by Chuan Wu. Thank you very much!

Data availability

All the required data is provided within the manuscript. Feel free to contact the corresponding author at any time: Chuan Wu (chuanchuan_wu@163.com).

Competing interests

The authors declare no comepting interests.