Study on the physicochemical properties of Chinese PVA fibers: Toward low-cost cement-based composites

Engineered Cementitious Composites (ECC) reinforced by Japanese Polyvinyl Alcohol (PVA) fiber showed excellent tensile ductility, which attracted much attention in the field of civil engineering in recent years, but the application was still limited by cost. There were quite a few kinds of low-priced PVA fibers in China, with a tremendous difference in physical and chemical properties. In this paper, a variety of typical PVA fibers produced in different regions in China were compared with the products imported from Japan to analyze the performance differences, especially the dispersion and enhancement effect in fiber reinforced cement matrix composites. Combining with the physical and chemical characteristics of each fiber, the relationship between the characteristics of fibers and the properties of composites was summarized. The experimental results illustrated the strength and interface characteristics of PVA fibers were the dominating influencing factors to the bending and tensile properties of PVA-ECC, while hardly affected the compressive strength. Insufficient intrinsic strength of fibers significantly decreased bending resistance and tensile strength. Excessive oil agent treated PVA fibers performed lower interfacial reaction with the cement matrix, meanwhile original PVA fiber with high hydrophilicity was not conducive to the dispersion and bridging function. The appropriate interface treatment approach could guarantee the fibers to achieve the sliding pull out bridge connection, and also meet the high slip strengthening effect, leading to the multi-seam cracking and strain hardening characteristics of ECC material. This work provided the preparation of low-cost cement-based composites, guiding the selection of fiber raw materials in ECC.


Introduction
To improve the weak tensile strength and shrinkage crack problem of concrete, fiber reinforced cement matrix composites were carried out by mixing the cement mortar with discontinuous short fiber or continuous long fiber.[3] Back in 1992, Victor. Li et al. [6][7][8][9] conducted a lot of research on PVA-ECC, and found that significant strain-hardening characteristics could be achieved by adding fiber within 2% volume fraction.In the process of direct tensile test, the characteristics of saturated multi-seam crack occurred and the crack width was less than 100 μm, finally, the ultimate tensile strain was more than 3%.Sahmaran et al. [10] investigated that the tensile strain of PVA-ECC increased with the improvement of the mass ratio of fly ash to cement.When the curing age was seven days, the ECC tensile strain could reach 4.54% under the high fly ash dosage, moreover the crack width was as low as 37 μm.In addition, Chinese professor Xu, professor Zhang and academician Sun et al. [11][12][13] also reported the ECC related research results.It was noteworthy that all these work used the same PVA fiber from Japan Kuraray company.The production of the fiber cost too much, as a result, PVA-ECC was still difficult to popularize in practical engineering applications.In order to realize the low cost of ECC, some Chinese scholars began to use cheap domestic PVA fiber to prepare ECC and study their properties.Wang et al. [14] determined the optimization direction of domestic PVA-ECC mix ratio based on the microscopic mechanical model.The results indicated that the generated composites were difficult to produce saturated multi-seam cracking or even multi-seam cracking by only replacing the fiber in the existing ECC typical mix ratio with domestic PVA fiber.Peng et al. [15] prepared ECC with domestic PVA fiber, and studied the influence of water cement ratio, the amount of fly ash and gel sand ratio on its uniaxial tensile performance.The results showed that when the addition of fly ash was 70%, the ultimate tensile strain of PVA-ECC could exceed 1%, and the crack width was less than 0.1 mm.Liu et al. [16] selected two kinds of domestic PVA fiber reinforced cement matrix composites, and found that the tensile strain of ECC prepared by C-PVA was up to 2.1% at 28d age, while the tensile strain of S-PVA fiber was only 0.75%.Obviously, the properties of ECC prepared by Chinese PVA fibers were uneven, because the the physicochemical properties of Chinese PVA fibers were quite different.Thus, this paper studied the physical and chemical characteristics of PVA fibers produced in different regions in China and compared with Japanese fiber.The performance differences of fibers were analyzed, helping to explore the relationship between the characteristics of fibers and the properties of ECC.

Chemicals and materials
Portland cement, fly ash and quartz sand were supplied by Guangdong Hongtu Mining Co., Ltd.Water reducing agent was provided by Shanghai Sanrui Polymer Material Co., Ltd.REC-15 PVA fiber (R-PVA) was purchased from Kuraray Co., Ltd., Japan.Four kinds of PVA fibers were purchased from China: J-PVA supplied by Jiaoshuai Polymer Material Co., LTD, Guangdong, S-PVA supplied by Sobute New Materials Co., Ltd, Jiangsu, C-PVA supplied by Chenqi Chemical Technology Co., LTD, Shanghai and Y-PVA supplied by Yingjia Industrial Development Co., Ltd, Shanghai were selected.

Experiment method
NaOH solution (1 mol/L) was prepared and heated to 80℃.Each kind of fiber was randomly selected into a group (5 g) and soaked in the alkali solution at constant temperature for 80℃, keeping 6 h.The macroscopic and fine morphology of fibers before and after alkali treatment were observed and the fiber diameter distribution was counted.The hydrophilic and alkaline resistance of each fiber were analyzed.Different kinds of fibers were prepared into PVA-ECC.According to the kinds of fibers, PVA-ECC was classified as R-ECC, C-ECC, Y-ECC, J-ECC and S-ECC.The mixing proportion of ECC was shown in Table 1.In the preparation process, the accurately weighed powder (cement, fly ash, quartz sand) was poured into the cement sand mixer for 2 min to achieve pre-dispersion.Then water and water reducing agent were added with quickly stir for 3 min.Finally, PVA fiber was added and stirred until uniform dispersion.The slurry was poured into the mold for vibration, then put into the standard curing box with (20 ± 1) ℃ and 95% humidity to the test age.

Characterization techniques
The fine morphology and diameter distribution of PVA fibers were observed using Carl Zeiss Microscopy GmbH optical microscope, and 100 samples were randomly selected for statistics.The hydrophobicity of PVA fibers was tested using the THETA Optical of KSV Instruments.After the PVA fibers were pressed into a flake, 5 microliter of deionized water were added to the material surface through the instrument and the contact angle was tested.The average contact angle of 5 points of each sample was taken as the contact angle of the sample.The oil content of PVA fiber was tested with the neutral soap liquid washing method, and the oil content of the sample was calculated according to formula (1).
Where: Q was the oil content of the sample, %. m1 was the mass of the sample before washing, g.W was the water content of the sample, %. m2 was the mass of the sample after washing, g.The PVA fiber alkaline resistance test was conducted according to the tensile strength before and after alkali treatment.The retention ratio of tensile strength (ηf)≥95% indicates good alkaline resistance.The calculation formula is as follows.ηf=σf,NaOH/σf (2) Where: σf,NaOH was the tensile strength after base treatment.σf was the initial tensile strength.The mechanical properties of PVA-ECC were tested using multi-use mechanical testing machine.The size of bending and compression test was 40 mm×40 mm×160 mm, the loading speed was 1.5 mm / min.The tensile specimen was dumbbell shaped, and the loading speed was 0.2 mm / min.The age period was set at 7 d and 28 d, and three specimens were made for each age period.The tensile section was observed using a field emission scanning electron microscope.

Physicochemical properties of the fibers
The cement slurry was highly alkaline during the process of cement hydration.It was necessary to study the alkaline resistance of PVA fibers.Figure 1 showed the photographs of PVA fibers before and after alkali treatment.According to the figure, R-PVA and Y-PVA were linear fiber filaments.J-PVA and S-PVA were linear fiber bundle with several flexibility filaments.C-PVA showed the highest flexibility, even the fiber bundles could be easily bent.After alkali treatment, all fibers showed yellowing and curling.R-PVA, Y-PVA and S-PVA changed slightly, the scattered fiber filaments became soft and bent.Significant changes occurred in J-PVA such as contraction and agglomeration.Carbonyl group was generated from dehydration and oxidation of alkali treated PVA molecules.The PVA fiber turned yellow when the carbonyl group and conjugated double bonds acted together.In addition, the internal stress generated during fiber production was constrained by the glassy state.Swelling phenomenon happened while PVA fibers soaked in hot alkali solution.The infiltration weakened the intermolecular force and intensified the thermal contraction of the fiber.Therefore, it can be preliminarily judged that C-PVA and J-PVA fiber molecular chains were highly dehydrated and had poor alkaline resistance, while R-PVA, Y-PVA and S-PVA showed relatively excellent alkaline resistance.
Figure 1.Photographs of PVA fibers before and after alkali treatment Further conducting quantitative analysis, Carl Zeiss Microscopy GmbH optical microscope was used to observe various fibers.Figure 2 showed the optical microscope photographs of J-PVA fibers, which had the most significant changes before and after alkali treatment.It was clearly visible that the fibers bent from a straight state and entangled with each other.The diameter of fiber could be measured more accurately by microscope.For each group of fiber samples, 100 samples were randomly selected to calculate their diameter distribution, as shown in Figure 3.The diameters of R-PVA, C-PVA and J-PVA single filament were concentrated between 30 and 35 μm, while those of Y-PVA and S-PVA were slightly larger between 40 and 45 μm.The fiber diameter size was ranked: C-PVA ≈ J-PVA <R-PVA <Y-PVA <S-PVA.After alkali treatment, most of the fibers swelling increased the diameter, among which the swelling of J-PVA was the most obvious, and the diameter increased from 31-33 μm to 44-47 μm, which was consistent with the morphological changes of J-PVA fibers shown in Figure 1(d     The hydrophobicity of PVA fibers before and after alkali treatment was analyzed by contact angle measurement.The contact angle was measured after compressing the fibers tightly into a sheet, in which R-PVA, C-PVA and J-PVA all showed obvious hydrophilicity before and after the alkali treatment.The droplets were directly infiltrated into the internal pores of the fiber sheet.The contact angle of Y-PVA was up to 140.2 and close to the superhydrophobic properties, as shown in Figure 4 (a).After alkali treatment, its hydrophobicity decreased and the contact angle decreased to 105.1 (Figure 4 (b)).S-PVA was changed from hydrophobic to hydrophilic after alkali treatment, as shown in Figure 4 (c) and (d).The oil content of fibers was plotted in Figure 5, and the oil content was ranked: Y-PVA> S-PVA> R-PVA> C-PVA> -PVA> J-PVA.Y-PVA and S-PVA had high oil content,were 1.87% and 1.55%, respectively.Oil content of R-PVA and C-PVA were 0.37% and 0.21%, respectively.There was almost nothing on the surface of J-PVA.PVA fibers contained a lot of hydroxyl groups on the surface, so they usually showed hydrophobicity, while Y-PVA and S-PVA were hydrophobic due to the high amount of oil coated on the fiber surface, but their hydrophobicity decreases after the action of alkali solution, and S-PVA even changed into hydrophilic characteristics.The tensile strength of each fiber was tested by the single fiber force instrument, and the results were shown in Table 2.The tensile strength of R-PVA imported from Japan was above 1600 MPa, while the four kinds of Chinese fibers did not exceed 1500 MPa, with the intensity range ranging from 1300 to 1400 MPa.The tensile strength of the fiber was ranked: R-PVA> S-PVA> J-PVA> Y-PVA> C-PVA.After alkali treatment, ηf of each fiber was higher than 95%, showing good alkaline resistance, except for J-PVA.The tensile strength of J-PVA decreased from 1438.6 MPa to 1323.5 MPa, mainly because of the lack of oil film protection on the fiber surface.The intensity attenuation was obvious under the action of high-temperature alkali solution.The increase in diameter and morphological structure changes of J-PVA were also the most significant of all the fibers.Although the surface hydrophobicity of S-PVA changed significantly after alkali treatment, the retention rate of tensile strength was still maintained at a high level, mainly due to the protection of the high oil content, which was also verified by almost no change in the fiber diameter.Interestingly, R-PVA and Y-PVA, the former had low oil content swelling much, but the retention rate of tensile strength was 96.9%, the latter had the highest oil content among the fibers, but the fiber still expanded, and the retention rate of tensile strength was less than the former.In a summary, the oil content is only one of the influencing factors for the alkaline resistance of PVA fiber.The type of surface coating oil and the differences in performance of fibers may affect the alkaline resistance.3 and plotted as bar charts in Figure 6.In the compression test, all PVA-ECC blocks had similar strength, reaching about 30 MPa and 45 MPa at 7d and 28d, respectively, because all the samples had the same mix design leading to the same cement matrix strength, meant that the incorporated fibers did not provide enhancement during the compression process.In the bending test, the strength of ECC mixed with different fibers varied significantly.R-ECC specimens had high bending strength, reaching 13.2 MPa and 13.7 MPa of 7d and 28d instar period,respectively.Y-ECC and S-ECC were slightly lower than R-ECC, about 11 MPa and 12 MPa of 7d and 28d instar period, respectively.C-ECC and J-ECC had the lowest strength, and the 28d bending strength was 8.5 MPa, which was only 62% that of R-ECC.  .SEM photographs of the PVA-ECC tensile sections Figure 7 showed the uniaxial tensile stress-strain curves for various typical PVA-ECC samples of the 28d instar period.The five typical samples shown in the figure all showed varying degrees of strain hardening and multi-seam cracking characteristics.R-ECC exhibited the highest tensile strength and elongation of tensile break, of 7.6 MPa and 2.55%, respectively.The ultimate tensile strength of S-ECC was close to that of R-ECC.However, the fracture elongation rate was only 2.25%.The maximum tensile strength of Y-ECC was slightly lower than the S-ECC, at around 6.5 MPa, and the strain-softening phenomenon occurred after reaching the maximum tensile strength.The final fracture elongation rate reaches about 2.5%.J-ECC partial brittle occurred at the initial crack.A typical curve with ECC fracture characteristics was plotted in Figure 8.Its tensile strength was comparable to that of the Y-ECC, but the fracture elongation rate was only 2%.C-ECC tensile strength was lowest of the five samples, only 5.7 MPa, while the break elongation was close to that of S-ECC.In order to find out the key factors that lead to the different performance of ECC prepared by various fibers, the microscopic morphology of the tensile section was deeply studied.The high resolution field emission scanning electron microscope was used, as shown in Figure 8.The fiber distribution was uniform on the R-ECC tensile section.The length of the fibers extending from the section varies.Some of the fibers showed tearing damage, others were smooth and almost undamaged.It was speculated that R-PVA adopted the strategy of functional gradient oil covering.So that some of the uncoated or less coated fibers had strong chemical bonds with the cement base, other fibers with higher oil content had lower interfacial interaction.Cooperation of these fibers played the effect of the fiber slip pulling bridge while also satisfying the higher slip reinforcement effect.Thus, the optimal uniaxial tensile performance gave ECC material stable multi-seam cracking and strain hardening characteristics.The morphology of the extracted fibers in C-ECC section was similar to that in R-ECC, but the mechanical properties was still poor, which was mainly caused by the insufficient strength of C-PVA fibers themselves, besides, the diameter and strength of C-PVA were the smallest of the five fibers.The fiber surface was smooth on the section of Y-ECC.The section also had a large number of hollow holes.This phenomenon coincided with the Y-PVA surface hydrophobic characteristics, high oil content made it highly dispersed in the cement matrix and realized the maximum slip out, played a significant role of fiber bridge.In all the Chinese PVA-ECC, the fracture elongation of Y-ECC was the highest, but low interfacial interaction caused tensile strain softening phenomenon in the late of uniaxial tensile.J-PVA fiber surface had almost no oil, a large number of side chain hydroxyl (-OH) directly bonded with the cement matrix, and was not conducive to fiber dispersion in the matrix.As shown in Figure 8 (d), the fibers on the J-ECC section were directly removed, broken and adhered to a large number of cement slurry.The high chemical binding energy limited the fiber bridging function, so the fracture elongation was lower than other PVA-ECC, and uneven dispersion led to direct brittle fracture of some specimens.S-ECC section characteristics were similar to R-ECC, S-PVA fiber had the largest diameter, the strength of a single fiber had the advantage, but reduced the number of fiber root and the fiber-matrix interface contact area per unit volume, weakened the fiber bridging.S-PVA had the highest tensile strength and the retention rate in Chinese fibers, but there was still a gap compared with R-PVA, resulting in the mechanical properties of S-ECC is slightly lower than that of R-ECC.

Conclusions
In this paper, a series of studies on the physical and chemical characteristics of Chinese PVA fibers produced in Shanghai, Guangdong and Jiangsu were conducted and compared with fibers imported from Japan.Moreover, the mechanical properties of fiber-reinforced cement matrix composites based on different PVA fibers were studied.The specific conclusions are as follows.
(1) The strength of R-PVA was more than 1600 MPa, while others were about 9% lower.The strength retention rate of alkali treated J-PVA was 92%, while others were higher than 95% indicating good alkaline tolerance.High degree of molecular chain dehydration, oxidation and thermal contraction with obvious yellowing and fiber constriction occurred in alkali treated C-PVA and J-PVA.The alkali treatment was less variable in R-PVA, Y-PVA and S-PVA.
(2) The oil content of PVA fiber significantly affected the hydrophilic and hydrophobicity of the fiber surface, meanwhile, it was one of the influencing factors in alkaline resistance.J-PVA was almost oil-coated and hydrophilic.Significantly swelling after soaking the alkali solution showed relatively low alkaline resistance.R-PVA and C-PVA were both hydrophilic due to the low oil content.They had diverse degrees of swelling, but their alkaline resistance was still excellent.Higher oil content made Y-PVA and S-PVA hydrophobicity, among which S-PVA almost no swelling and had excellent alkaline resistance.
(3) The strength and interfacial characteristics of PVA fibers were the key influencing factors to the bending and tensile properties of PVA-ECC, while hardly affected the compressive strength.
Insufficient intrinsic strength of fibers significantly reduced bending resistance and tensile strength.Excessive oil agent treated PVA fibers performed lower interfacial interaction with the cement matrix, meanwhile original PVA fiber with high hydrophilicity was not conducive to the dispersion and bridging function.The bending resistance strength and tensile ductility of J-ECC were the lowest due to the poor dispersion of fiber and insufficient slippage.The lowest fiber intensity of C-PVA led to relatively low mechanical properties of C-ECC.Highest oil content made the Y-PVA fibers highly dispersed and realized the maximum slip pull-out.The elongation rate of Y-ECC was the highest, but the strain softening phenomenon appeared in the later stage.The fiber fracture state of S-ECC was diverse, similar to that of R-ECC, and the mechanical performance of S-ECC was optimal in all the Chinese PVA-ECC.(4) In order to prepare low-cost PVA-ECC, PVA fibers with high tensile strength should be selected.Flexible interface modification strategies were worth considering, such as functional gradient oil covering, to ensure the fibers to achieve the sliding pull out bridge connection, and also meet the high slip strengthening effect.
Figure1.Photographs of PVA fibers before and after alkali treatment Further conducting quantitative analysis, Carl Zeiss Microscopy GmbH optical microscope was used to observe various fibers.Figure2showed the optical microscope photographs of J-PVA fibers, which had the most significant changes before and after alkali treatment.It was clearly visible that the fibers bent from a straight state and entangled with each other.The diameter of fiber could be measured more accurately by microscope.For each group of fiber samples, 100 samples were randomly selected to calculate their diameter distribution, as shown in Figure3.The diameters of R-PVA, C-PVA and J-PVA single filament were concentrated between 30 and 35 μm, while those of Y-PVA and S-PVA were slightly larger between 40 and 45 μm.The fiber diameter size was ranked: C-PVA ≈ J-PVA <R-PVA <Y-PVA <S-PVA.After alkali treatment, most of the fibers swelling increased the diameter, among which the swelling of J-PVA was the most obvious, and the diameter increased from 31-33 μm to 44-47 μm, which was consistent with the morphological changes of J-PVA fibers shown in Figure1(d) and (i).The diameter of R-PVA, C-PVA and Y-PVA fibers increased by swelling, while the S-PVA

Figure 2 .
Figure 2. Optical microscope photographs of J-PVA fibers before and after alkali treatment

Figure 3 .
Figure 3. Statistics of diameter distribution of PVA fibers before and after alkali treatment

Figure 4 .
Figure 4. Contact angle of the PVA fibers

Figure 5 .
Figure 5. Oil content of each PVA fiberThe hydrophobicity of PVA fibers before and after alkali treatment was analyzed by contact angle measurement.The contact angle was measured after compressing the fibers tightly into a sheet, in which R-PVA, C-PVA and J-PVA all showed obvious hydrophilicity before and after the alkali treatment.The droplets were directly infiltrated into the internal pores of the fiber sheet.The contact angle of Y-PVA was up to 140.2 and close to the superhydrophobic properties, as shown in Figure4 (a).After alkali treatment, its hydrophobicity decreased and the contact angle decreased to 105.1 (Figure4(b)).S-PVA was changed from hydrophobic to hydrophilic after alkali treatment, as shown in Figure4(c) and (d).The oil content of fibers was plotted in Figure5, and the oil content was ranked: Y-PVA> S-PVA> R-PVA> C-PVA> -PVA> J-PVA.Y-PVA and S-PVA had high oil content,were 1.87% and 1.55%, respectively.Oil content of R-PVA and C-PVA were 0.37% and 0.21%, respectively.There was almost nothing on the surface of J-PVA.PVA fibers contained a lot of hydroxyl groups on the surface, so they usually showed hydrophobicity, while Y-PVA and S-PVA were hydrophobic due to the high amount of oil coated on the fiber surface, but their hydrophobicity decreases after the action of alkali solution, and S-PVA even changed into hydrophilic characteristics.The tensile strength of each fiber was tested by the single fiber force instrument, and the results were shown in Table2.The tensile strength of R-PVA imported from Japan was above 1600 MPa, while the four kinds of Chinese fibers did not exceed 1500 MPa, with the intensity range ranging from 1300 to 1400 MPa.The tensile strength of the fiber was ranked: R-PVA> S-PVA> J-PVA> Y-PVA> C-PVA.After alkali treatment, ηf of each fiber was higher than 95%, showing good alkaline resistance, except for J-PVA.The tensile strength of J-PVA decreased from 1438.6 MPa to 1323.5 MPa, mainly because of the lack of oil film protection on the fiber surface.The intensity attenuation was obvious under the action of high-temperature alkali solution.The increase in diameter and morphological structure changes of J-PVA were also the most significant of all the fibers.Although the surface hydrophobicity of S-PVA changed significantly after alkali treatment, the retention rate of tensile strength was still maintained at a high level, mainly due to the protection of the high oil content, which was also verified by almost no change in the fiber diameter.Interestingly, R-PVA and Y-PVA, the former had low oil content swelling much, but the retention rate of tensile strength was 96.9%, the latter had the highest oil content among the fibers, but the fiber still expanded, and the retention rate of tensile strength was less than the former.In a summary, the oil content is only one of the influencing factors for the alkaline resistance of PVA fiber.The type of surface coating oil and the differences in performance of fibers may affect the alkaline resistance.

Table 1 .
The mix proportion of ECC(kg/m 3 )

Table 2 .
Tensile strength of PVA fibers before and after alkali treatment

Table 3 .
Compressive strength and bending strength of different PVA-ECC