Effect of the Sizing Removal Methods of Fiber Surface on the Mechanical Performance of Basalt Fiber-Reinforced Concrete

: In this study, comprehensive analyses were used to evaluate the physical and chemical properties of basalt fibers, employing a variety of instruments. Additionally, heat treatment and solvent treatment methods were used to eliminate the sizing present on fiber surfaces. The heat treatment process involved determining the optimal temperature and duration required to remove the sizing from the basalt fibers. The appearance, chemical composition, and crystal structure of the original fibers were examined, including those subjected to heat treatment and those treated with solvents. These treated fibers were then incorporated into concrete to create basalt fiber-reinforced concrete (BFRC) specimens for mechanical tests, which assessed their compressive, flexural, and splitting tensile strengths. The results revealed that heat treatment at 300 ◦ C for 180 min effectively removed the sizing on the basalt fibers, and the heat-treated basalt fibers exhibited uniform dispersion inside the BFRC specimens. In addition, solvent treatment primarily removed the soluble components of the sizing. The mechanical properties of specimens with sizing-removed basalt fibers were better than the specimens with original basalt fibers and the benchmark specimens. Crucially, the mechanical test results demonstrated that BFRC incorporating heat-treated basalt fibers exhibited a superior mechanical performance compared to BFRC incorporating original fibers or fibers subjected to the solvent treatment.


Introduction
Concrete is a composite material with low tensile strength and tensile strain limits.Adding fiber to concrete can enhance its flexural strength, tensile strength, impact resistance, toughness, and abrasion resistance [1,2].Fiber-reinforced concrete (FRC) has been widely used in infrastructure, road construction, bridges, and offshore platforms [3,4].In accordance with previous research, the addition of fibers to concrete necessitates careful consideration of various factors, including the fineness modulus (FM) of the aggregate, fiber type, fiber length, the amount of fiber added, the dispersion of fibers in concrete, and the bonding between the fibers and concrete [5][6][7][8][9].
Incorporating fibers into concrete can effectively suppress the formation and propagation of cracks, as it significantly mitigates concrete shrinkage [10,11].Fiber lengths suitable for concrete varies with aggregate size.Fibers of 6 mm enhance the compressive strength of FRC, but 24 mm fibers bolsters the flexural strength of FRC [12][13][14].Previous research indicates that adding fibers within the range of 0.75% to 1% (w/w) results in the most better than with 0.5% and 1.5%; thus, 1% chopped basalt fibers of 24 mm (Sheng Peng Applied Materials Co., Ltd., Yunlin County, Taiwan) was used in this study.The linear density and diameters of the basalt fibers used in this study were 300 tex and 11.7 ± 1.93 µm (N = 15), respectively.Figure 1 displays the appearance of the basalt fibers.
especially in regard to flexural strength, tensile strength, and impact resistance.Furthermore, the mechanical properties of BFRC with a ratio of 1% for fiber to cement was better than with 0.5% and 1.5%; thus, 1% chopped basalt fibers of 24 mm (Sheng Peng Applied Materials Co., Ltd., Yunlin County,Taiwan) was used in this study.The linear density and diameters of the basalt fibers used in this study were 300 tex and 11.7 ± 1.93 μm (N = 15), respectively.Figure 1

Surface Treatment of Basalt Fibers
The heat treatment method and the solvent treatment method were used to remove the sizing from the surfaces of basalt fibers, and the treated fibers were then added to the concrete for reinforcement.The physical-chemical properties of the original, heat-treated, and solvent-treated basalt fibers were observed and analyzed using thermogravimetric analysis (TGA), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometer (XRD); a filament tensile test was also performed.

Heat Treatment
In order to analyze the treatment temperature and time of the basalt fibers in the heat treatment method, a TGA (SDTA851e, Mettler Toledo, Greifensee, Switzerland) was performed.Nitrogen was used as a protection gas with a flow rate of 30 mL/min.The sample, weighing about 20 mg, was placed in an alumina ceramic crucible, and the reaction gas was zero-grade air with a flow rate of 30 mL/min.The target temperature for sizing removal was determined by the TGA, with a temperature range of 25 °C to 700 °C and a heating rate of 10 °C/min.The sample was gradually heated to the target temperature at a heating rate of 10 °C/min after being held at a 25 °C isothermal state.The temperature was maintained for a specific time before being gradually raised to 700 °C at a heating rate of 10 °C/min.The mass loss was then checked at different time points under the target temperature after determining said target temperature.

Solvent Treatment
The sizing on the surfaces of the basalt fibers was extracted using the solvent treatment method with isopropyl alcohol (IPA, 99.8%, Emperor Chemical Co., Ltd., Taipei City, Taiwan).The basalt fibers were first dried at 80 °C for 24 h and weighed using an electronic scale (TX223L, Shimadzu, Kyoto, Japan).Then, the basalt fibers were immersed in an IPA solution at 23 °C for 24 h and dried in an oven (KOE-270, Kotsao, New Taipei

Surface Treatment of Basalt Fibers
The heat treatment method and the solvent treatment method were used to remove the sizing from the surfaces of basalt fibers, and the treated fibers were then added to the concrete for reinforcement.The physical-chemical properties of the original, heat-treated, and solvent-treated basalt fibers were observed and analyzed using thermogravimetric analysis (TGA), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometer (XRD); a filament tensile test was also performed.

Heat Treatment
In order to analyze the treatment temperature and time of the basalt fibers in the heat treatment method, a TGA (SDTA851e, Mettler Toledo, Greifensee, Switzerland) was performed.Nitrogen was used as a protection gas with a flow rate of 30 mL/min.The sample, weighing about 20 mg, was placed in an alumina ceramic crucible, and the reaction gas was zero-grade air with a flow rate of 30 mL/min.The target temperature for sizing removal was determined by the TGA, with a temperature range of 25 • C to 700 • C and a heating rate of 10 • C/min.The sample was gradually heated to the target temperature at a heating rate of 10 • C/min after being held at a 25 • C isothermal state.The temperature was maintained for a specific time before being gradually raised to 700 • C at a heating rate of 10 • C/min.The mass loss was then checked at different time points under the target temperature after determining said target temperature.

Solvent Treatment
The sizing on the surfaces of the basalt fibers was extracted using the solvent treatment method with isopropyl alcohol (IPA, 99.8%, Emperor Chemical Co., Ltd., Taipei City, Taiwan).The basalt fibers were first dried at 80 • C for 24 h and weighed using an electronic scale (TX223L, Shimadzu, Kyoto, Japan).Then, the basalt fibers were immersed in an IPA solution at 23 • C for 24 h and dried in an oven (KOE-270, Kotsao, New Taipei City, Taiwan) at 80 • C for 24 h to remove the solvent.After that, the mass of the basalt fibers was measured to determine the amount of surface sizing removed.Furthermore, the remaining sizing on the treated fibers was analyzed by TGA, with a temperature range of 25 • C to 700 • C and a heating rate of 10 • C/min.

Sizing Composition Analysis of Basalt Fibers
In this study, the soluble components of the sizing on the surface of the original basalt fibers by IPA were analyzed using gas chromatography/mass spectrometry (GC/MS, 7890B/5977A, Agilent Technologies, Santa Clara, CA, USA).A capillary column (19091S-433, Agilent Technologies, Santa Clara, CA, USA) was utilized for the pending sample separations in GC, with a column length of 30 m, inner diameter of 0.25 mm, and a film thickness of 0.25 µm.Helium (He) gas was employed as the carrier gas, flowing at a rate of 1 mL/min.The ultrasonic extraction method was employed for sample preparation, wherein the basalt fibers were soaked in a toluene solution to extract the sizing, and then passed through a 13 mm PTFE syringe filter with a pore size of 0.22 µm to obtain the sample.The pending sample was injected into the GC/MS in a splitless mode with an injection volume of 1 µL and an inlet temperature of 270 • C to gasify the pending sample.The oven's initial temperature was set at 230 • C for 6 min, followed by heating to 280 • C at a heating rate of 50 • C/min and holding for 5 min.Subsequently, the oven's temperature was raised to 310 • C at the heating rate of 60 • C/min and held for 3 min.

Microscopic Observation and Analysis of Basalt Fibers
The morphologies of the original, the heat-treated, and the solvent-treated basalt fibers were examined using SEM (TM4000plus II, Hitachi High-Tech, Kyoto, Japan) with a test voltage of 15 kV.The chemical composition of the fibers was analyzed using EDS (MICS F+/X-STREAM-2, Oxford, UK).Additionally, the crystal structure of the basalt fibers was analyzed using an XRD (D2 Phaser, Bruker, Rheinstetten, Germany), with Cu Kα radiation as the X-ray source and a test voltage of 30 kV.The scan step size was set at 0.02 • , and the range of the 2θ was from 10 • to 80 • .

Tensile Force of Basalt Filament
The tensile force and displacement of the original, the heat-treated, and the solventtreated basalt fiber filaments were obtained by the tensile testing machine (QC-513M1, Cometech, Taichung City, Taiwan) at a tensile rate of 0.5 mm/min based on ASTM D3822-07 [43].The loadcell capacity of the tensile testing was 300 gf.In the single-filament tensile test of this study, the fiber testing length was 25 mm.Initially, a single fiber was extracted and glued on a standard paper frame.The paper frame was used to protect the fiber from direct gripping.Both sides of the frame were cut before the test, leaving only the filament to be pulled.The load was recorded with the grip displacement.The fiber was brittlely fractured at the peak load.At least 15 pieces were tested for each type of fiber.

BFRC Test Method
The BFRC specimens were created using 24 mm chopped basalt fibers, Type I Portland cement (Taiwan Cement Corp., Taipei City, Taiwan), aggregate, and water.The aggregate's fineness modulus (F.M.) was 6.31, and the test was conducted following the ASTM C33/C33M-18 standard [44].The weight ratio of cement to fine aggregate and coarse aggregate was set to 1:1.05:2.25, and the water-cement ratio was 0.6.The basalt fiber-tocement weight ratio was 10‰ for the BFRC specimens.Three different types of basalt fibers were used in this study, including original, heat-treated, and solvent-treated basalt fibers.According to the TGA analysis results, the heat-treated basalt fibers were obtained from the muffle furnace (PF-40, Prema, Taipei City, Taiwan), heated at 300 • C for three hours.In addition, the solvent-treated basalt fibers were immersed in IPA solution at 23 • C for 24 h and dried in the oven at 80 • C for 24 h in order to remove the solvent.Finally, those fibers were pneumatically dispersed by an air compressor under 90 psi for one minute.The production process of the BFRC specimen was in accordance with past research [18].The compressive, flexural, and splitting tensile tests were conducted following ASTM C39/C39M-21, ASTM C496/C496M-17, and ASTM C293/C293M-16, respectively [45][46][47].All specimens were tested after 28 days of curing.Table 1 explains the naming of all specimens for the above tests.For example, HBFRC-F referred to the specimen with heat-treated basalt fibers for the flexural test.

Physical-Chemical Property Results
The surface treatment results were divided into heat treatment and solvent treatment.The test results of the heat and solvent treatments for the removal of sizing from basalt fibers are described in the following subsections.The microscopic, chemical elements, and crystal structures of the original, heat-treated, and solvent-treated basalt fibers were as follows.

Surface Treatment Results
The weight loss ratio of the basalt fibers after heat treatment and solvent treatment were shown as follows.

Heat Treatment
The relationship between the mass ratio and temperature of basalt fibers was analyzed using TGA and presented in Figure 2. The temperature range was set from 25 • C to 700 • C, with a heating rate of 10 • C/min.As shown in Figure 2, the basalt fibers experienced less than 0.1% mass loss below 260 • C, while the primary mass loss occurred between 260 • C and 488 • C, resulting in a total mass loss of about 1%.This indicates that the sizing of the basalt fibers began to decompose at 260 • C, which coincides with the decomposing temperature of most commercial sizing of basalt fibers, around 300 • C [33,34].Thus, a target temperature of 300 °C was chosen for sizing removal, and three differ ent holding time durations (60, 180, and 300 min) were tested to determine the optima heat treatment process.The mass ratio-time relationship of the 20 mg basalt fiber unde different heating rates is presented in Figure 3.The mass loss for holding times of 60, 180  Thus, a target temperature of 300 • C was chosen for sizing removal, and three different holding time durations (60, 180, and 300 min) were tested to determine the optimal heat treatment process.The mass ratio-time relationship of the 20 mg basalt fiber under different heating rates is presented in Figure 3.The mass loss for holding times of 60, 180, and 300 min at 300 • C were 0.60%, 0.68%, and 0.72%, respectively.Thus, a target temperature of 300 °C was chosen for sizing removal, and three different holding time durations (60, 180, and 300 min) were tested to determine the optimal heat treatment process.The mass ratio-time relationship of the 20 mg basalt fiber under different heating rates is presented in Figure 3.The mass loss for holding times of 60, 180, and 300 min at 300 °C were 0.60%, 0.68%, and 0.72%, respectively.

Solvent Treatment
The original basalt fibers had a mass of 25.0 g, while the solvent-treated basalt fibers weighed 24.82 g, and the amount of sizing extracted from the surface of the basalt fibers was 0.7%.In order to determine the effectiveness of the solvent treatment method, the mass ratios of the 20 mg original and solvent-treated basalt fibers were tested using TGA, as shown in Figure 2. The maximum mass losses for the original and solvent-treated basalt fibers were 1% and 0.68%, respectively.Notably, the maximum mass loss for the solventtreated basalt fibers was lower than that of the original basalt fibers, indicating that the part of the sizing was successfully extracted using the solvent-treatment method.As suggested by other researchers, parts of the sizing still remained on the surfaces of the basalt fibers after solvent extraction [48].

Solvent Treatment
The original basalt fibers had a mass of 25.0 g, while the solvent-treated basalt fibers weighed 24.82 g, and the amount of sizing extracted from the surface of the basalt fibers was 0.7%.In order to determine the effectiveness of the solvent treatment method, the mass ratios of the 20 mg original and solvent-treated basalt fibers were tested using TGA, as shown in Figure 2. The maximum mass losses for the original and solvent-treated basalt fibers were 1% and 0.68%, respectively.Notably, the maximum mass loss for the solvent-treated basalt fibers was lower than that of the original basalt fibers, indicating that the part of the sizing was successfully extracted using the solvent-treatment method.As suggested by other researchers, parts of the sizing still remained on the surfaces of the basalt fibers after solvent extraction [48].In the chromatography at the 7th to 9th minutes, an unknown substance was observed to decompose at 280 • C, yielding two peaks via GC/MS.The mass-to-charge ratio and potential substance of these two unknown peaks are shown in Figures 5a and 5b, respectively.Based on the database from the NIST mass spectrometry data center, the major unknown substance in "peak A" was identified as "adipic acid, nonyl 2-octyl ester (C 23 H 44 O 4 )", with match factor (MF) and reverse match factor (RMF) scores of 750 and 802, respectively.This positive identification suggests it was used as an intermediate for organic synthesis, as well as in the production of lubricants, coatings, adhesives, sealants, elastomers, and other applications.

Sizing Composition Analysis
unknown substance in "peak A" was identified as "adipic acid, nonyl 2-octyl e (C23H44O4)", with match factor (MF) and reverse match factor (RMF) scores of 750 and respectively.This positive identification suggests it was used as an intermediate for ganic synthesis, as well as in the production of lubricants, coatings, adhesives, seala elastomers, and other applications.The unknown substance in "peak B" was identified as "1,2-cyclohexanedicarbox acid dinonyl ester (C26H48O4)", with match factor (MF) and reverse match factor (R decompose at 280 °C, yielding two peaks via GC/MS.The mass-to-charge ratio and potential substance of these two unknown peaks are shown in Figure 5a and Figure 5b, respectively.Based on the database from the NIST mass spectrometry data center, the major unknown substance in "peak A" was identified as "adipic acid, nonyl 2-octyl ester (C23H44O4)", with match factor (MF) and reverse match factor (RMF) scores of 750 and 802, respectively.This positive identification suggests it was used as an intermediate for organic synthesis, as well as in the production of lubricants, coatings, adhesives, sealants, elastomers, and other applications.The unknown substance in "peak B" was identified as "1,2-cyclohexanedicarboxylic acid dinonyl ester (C26H48O4)", with match factor (MF) and reverse match factor (RMF) The unknown substance in "peak B" was identified as "1,2-cyclohexanedicarboxylic acid dinonyl ester (C 26 H 48 O 4 )", with match factor (MF) and reverse match factor (RMF) scores of 832 and 856, respectively, which shows the positive identification.This substance, "1,2-cyclohexanedicarboxylic acid dinonyl ester", is commonly used to lubricate components in plastic production.
Like most of the sizing research on fiber surfaces, this result indirectly confirmed that parts of siloxane-based addition agents were imperfectly dissolved by the organic solvent [48][49][50].

Microscopic Observation and Analysis
The surface morphology, chemical elements, and crystal structures of the basalt fibers were observed and analyzed using SEM, EDS, and XRD. Figure 6a-e present the SEM observations of the original basalt fibers, those heat-treated for varying durations, and the solvent-treated basalt fibers' surface morphologies.It is evident that the original and heat-treated basalt fibers at 300 • C for 60 min, and solvent-treated basalt fibers' surfaces, had sizing or impurities.However, sizing was removed from the surfaces of the basalt fibers by heat treatment at 300 • C for 180 and 300 min.The above results indicate that a heat treatment at 300 • C for 180 min effectively removes the sizing from the surfaces of basalt fibers.Therefore, the optimal condition for the mechanical test of the BFRC specimens was a temperature of 300 • C for 180 min.The EDS results reveal that the chemical elements present in the basalt fibers are Si, Fe, Al, Ca, and Mg, as indicated in Table 2.The weight percentage concentration of Si decreased from 25.3% to 23.7% in the original, heat-treated, and solvent-treated basalt fibers.The Si content in the basalt fibers was reduced after heat treatment.Heating the basalt fibers for longer durations could result in the growth of iron, as evidenced by an increase in iron weight percentage concentration from 8.5% to 10.7% when the heating time was 300 min.

Microscopic Observation and Analysis
The surface morphology, chemical elements, and crystal structures of the basalt fibers were observed and analyzed using SEM, EDS, and XRD. Figure 6a-e present the SEM observations of the original basalt fibers, those heat-treated for varying durations, and the solvent-treated basalt fibers' surface morphologies.It is evident that the original and heattreated basalt fibers at 300 °C for 60 min, and solvent-treated basalt fibers' surfaces, had sizing or impurities.However, sizing was removed from the surfaces of the basalt fibers by heat treatment at 300 °C for 180 and 300 min.The above results indicate that a heat treatment at 300 °C for 180 min effectively removes the sizing from the surfaces of basalt fibers.Therefore, the optimal condition for the mechanical test of the BFRC specimens was a temperature of 300 °C for 180 min.The EDS results reveal that the chemical elements present in the basalt fibers are Si, Fe, Al, Ca, and Mg, as indicated in Table 2.The weight percentage concentration of Si decreased from 25.3% to 23.7% in the original, heat-treated, and solvent-treated basalt fibers.The Si content in the basalt fibers was reduced after heat treatment.Heating the basalt fibers for longer durations could result in the growth of iron, as evidenced by an increase in iron weight percentage concentration from 8.5% to 10.7% when the heating time was 300 min.The XRD patterns of the different fibers are shown in Figure 7.The original basalt fiber specimens were amorphous, with no crystals or diffraction peaks present in the diffraction spectrum.This observation agrees with the analysis by Chakartnarodom et al. [51].The XRD patterns of the heat-treated and solvent-treated basalt fibers showed a corresponding peak of 29.4 • (2θ), indicating the presence of a crystalline phase of calcite (CaCO 3 , JCPDF-47-1743).
2.0 1.9 2.0 2.2 2.0 The XRD patterns of the different fibers are shown in Figure 7.The original basalt fiber specimens were amorphous, with no crystals or diffraction peaks present in the diffraction spectrum.This observation agrees with the analysis by Chakartnarodom et al. [51].The XRD patterns of the heat-treated and solvent-treated basalt fibers showed a corresponding peak of 29.4° (2θ), indicating the presence of a crystalline phase of calcite (CaCO3, JCPDF-47-1743).

Peak Tensile Force of Basalt Filament
Table 3 shows the peak tensile force and its corresponding displacement for both the original and surface-treated basalt fiber filaments.The average peak tensile force and displacement of the original filament were 22.31 gf and 0.55 mm, respectively, with corresponding standard deviations (σ) of 0.74 gf and 0.01 mm, respectively.The average peak tensile force of the heat-treated filament at 300 °C for 60, 180, and 300 min, and the solventtreated filament were 19.00 gf, 21.76 gf, 14.60 gf, and 14.12 gf, respectively.Compared to

Peak Tensile Force of Basalt Filament
Table 3 shows the peak tensile force and its corresponding displacement for both the original and surface-treated basalt fiber filaments.The average peak tensile force and displacement of the original filament were 22.31 gf and 0.55 mm, respectively, with corresponding standard deviations (σ) of 0.74 gf and 0.01 mm, respectively.The average peak tensile force of the heat-treated filament at 300 • C for 60, 180, and 300 min, and the solvent-treated filament were 19.00 gf, 21.76 gf, 14.60 gf, and 14.12 gf, respectively.Compared to the original filament, the heat-treated filament at 300 • C for 60 min, 180 min, and 300 min, and the solvent-treated filament experienced a decrease in the average peak tensile force of about 15%, 2%, 35%, and 37%, respectively.These results suggest that the peak tensile force of basalt fibers is affected by the heat treatment time, with longer heating times resulting in lower tensile forces.Assuming a normal distribution for the single filament tensile forces under various conditions, with a significance level of 0.05, a sample size of 15 in a single group, and degrees of freedom amounting to 14, the analysis conducted using the F-test for variance yielded the results presented in Table 3.The p-values of the one-tailed F-test were 5.20 × 10 −8 , 3.59 × 10 −4 , 3.00 × 10 −6 , and 3.29 × 10 −6 , respectively, indicating unequal variances between the two populations.Consequently, this study proceeded with a t-test: two-sample assuming unequal variances and with a significance level of 0.05.The null hypothesis (H 0 ) assumed no significant difference in average value (µ 1 = µ 2 ), while the alternative hypothesis (H a ) suggested a significant difference in average value (µ 1 ̸ = µ 2 ).The p-values of the two-tailed t-test for "300 • C, 60 min BF", "300 • C, 300 min BF", and "solventtreated BF" were 7.07 × 10 −3 , 3.57 × 10 −8 , and 1.38 × 10 −8 , respectively.These results led to the rejection of the null hypothesis, signifying a significant difference in average values (µ 1 ̸ = µ 2 ).However, for "300 • C, 180 min BF," the p-value of the two-tailed t-test was 3.31 × 10 −1 , suggesting no significant difference in tensile strength compared to the original BF.

Mechanical Test of BFRC
The following section describes the materials, test methods, and mechanical test results of BFRC specimens.The specimens were made of both original and surface-treated basalt fibers, and the mechanical tests conducted were compressive, flexural, and splitting tensile tests.

Compressive Test Result
The BFRC specimens consisted of three different types of basalt fibers: original, heattreated, and solvent-treated.Table 4 shows the compressive strengths of both the benchmark and BFRC specimens after 28 days of curing.The average compressive strengths of specimens B-C, OBFRC-C, HBFRC-C, and SBFRC-C were 23.76 MPa, 24.27 MPa, 24.44 MPa, and 25.16 MPa, respectively.The compressive strengths of the BFRC specimens increased by 2.2% for OBFRC-C, 2.9% for SBFRC-C, and 5.9% for HBFRC-C compared to the benchmark specimen.The BFRC specimens with heat-treated basalt fibers had the highest average compressive strengths among all BFRC specimens.

Flexural Test Result
Table 5 shows the flexural strength of both the benchmark and BFRC specimens after 28 days of curing.The average flexural strengths of specimens B-F, OBFRC-F, HBFRC-F, and SBFRC-F were 4.18 MPa, 4.61 MPa, 5.56 MPa and, 4.82 MPa, respectively.Compared to the benchmark specimen, the average flexural strengths of the BFRC specimens were increased by 10.2% for OBFRC-F, 15.2% for SBFRC-F, and 32.9% for HBFRC-F.The BFRC specimens with heat-treated basalt fibers had the highest average flexural strength among all specimens.Chopped basalt fibers possess toughness, whereas concrete is an inherently brittle material.These results are consistent with previous research; when combined with concrete, these fibers contribute to an increased toughness, mitigating damage due to flexural forces and minimizing crack formation in order to improve the flexural strength of concrete [2,52].

Splitting Tensile Test Result
The splitting tensile strength of the benchmark and BFRC specimens, after 28 days of curing, are listed in Table 6.The average splitting tensile strengths for each specimen were 2.57 MPa in B-S, 2.62 MPa in OBFRC-S, 2.76 MPa in HBFRC-S, and 2.67 MPa in SBFRC-S.Compared to the benchmark specimen, the average splitting tensile strengths of OBFRC-S, HBFRC-S, and SBFRC-S were improved by 2.1%, 7.4%, and 4.0%, respectively.Notably, the BFRC specimen with heat-treated basalt fibers had the highest splitting tensile strength among all specimens.

Discussion
Based on the test results, it is evident that sizing-removed basalt fibers can improve the mechanical performance of BFRC specimens.Among the BFRC specimens, those with heat-treated basalt fibers exhibited higher mechanical strengths compared to those with the original and solvent-treated basalt fibers, as seen in Figure 6.This could be due to the absence of sizing on the surfaces of fibers, which strengthened the bonding force between the cement and the fibers.Figure 8 illustrates the different types of basalt fibers used in BFRC specimens after failure.The original fibers were difficult to disperse in concrete due to the sizing on their surfaces, while the solvent-treated basalt fibers had residual sizing, resulting in a slippage failure mode [53].Despite the heat-treated basalt filaments achieving lower tensile force, it could be uniformly dispersed in concrete, resulting in higher mechanical strengths in the BFRC specimens compared to those with the original and solvent-treated basalt fibers.

Conclusions
Based on the results of the physical-chemical properties analysis of basalt fiber and the mechanical tests of BFRC, the following conclusions can be drawn: 1. Based on a GC/MS analysis with the NIST database of extraction liquid of basalt fibers, the results showed that the two unknown substances might be "adipic acid, nonyl 2-octyl ester (C23H44O4)" and "1,2-cyclohexanedicarboxylic acid dinonyl ester (C26H48O4)", respectively.2. The original basalt fibers were amorphous, with no crystals or diffraction peaks in the diffraction spectrum.3. The SEM images indicate that the heat treatment method was more effective in re-

Conclusions
Based on the results of the physical-chemical properties analysis of basalt fiber and the mechanical tests of BFRC, the following conclusions can be drawn: 1.
Based on a GC/MS analysis with the NIST database of extraction liquid of basalt fibers, the results showed that the two unknown substances might be "adipic acid, nonyl 2-octyl ester (C 23 H 44 O 4 )" and "1,2-cyclohexanedicarboxylic acid dinonyl ester (C 26 H 48 O 4 )", respectively.2.
The original basalt fibers were amorphous, with no crystals or diffraction peaks in the diffraction spectrum.

3.
The SEM images indicate that the heat treatment method was more effective in removing sizing from the basalt fibers compared to the solvent treatment method.4.
Based on the TGA mass ratio-temperature and mass ratio-time relationships of the basalt fibers, the optimal conditions for removing sizing from the surfaces of the basalt fibers were a target temperature of 300 • C and a holding time of 180 min.

5.
The SEM image observation of the failed BFRC specimens indicated that the heattreated basalt fibers were more uniformly dispersed in the concrete than the specimens with original and solvent-treated basalt fibers.6.
The results indicated that the mechanical properties of specimens with sizing-removed basalt fibers (HBFRC and SBFRC) were better than the specimens with original basalt fibers (OBFRC) and the benchmark specimens (B). 7.
The compressive, flexural and split tensile test results show that BFRC specimens with heat-treated basalt fibers exhibited higher strengths compared to those with original and solvent-treated basalt fibers.

Figure 2 .
Figure 2. TGA mass ratio-temperature relationship of the basalt fibers.

Figure 2 .
Figure 2. TGA mass ratio-temperature relationship of the basalt fibers.

Figure 3 .
Figure 3. TGA mass ratio-time relationship of the basalt fiber.

Figure 3 .
Figure 3. TGA mass ratio-time relationship of the basalt fiber.

Figure 4
Figure 4 displays the chromatography of the extractive solution of the basalt fibers.In the chromatography at the 7th to 9th minutes, an unknown substance was observed to decompose at 280 • C, yielding two peaks via GC/MS.The mass-to-charge ratio and potential substance of these two unknown peaks are shown in Figures5a and 5b, respectively.Based on the database from the NIST mass spectrometry data center, the major unknown substance in "peak A" was identified as "adipic acid, nonyl 2-octyl ester (C 23 H 44 O 4 )", with match factor (MF) and reverse match factor (RMF) scores of 750 and 802, respectively.This positive identification suggests it was used as an intermediate for organic synthesis, as well as in the production of lubricants, coatings, adhesives, sealants, elastomers, and other applications.

Figure
Figure Chromatography of the sizing-extracted solution; (a) unknown peak A; and (b) unknown peak B.

Figure
Figure SEM images of the original and the surface-treated basalt fibers: (a) original; (b) heat treatment at 300 • C, 60 min; (c) heat treatment at 300 • C, 180 min; (d) heat treatment at 300 • C, 300 min; (e) solvent treatment.

Figure 7 .
Figure 7.The XRD analysis of the original and the surface-treated basalt fibers.

Figure 7 .
Figure 7.The XRD analysis of the original and the surface-treated basalt fibers.

Figure 8 .
Figure 8. SEM images of a BFRC specimen after failure: (a) BFRC specimen with original basalt fiber; (b) BFRC specimen with solvent-treated basalt fiber; (c) BFRC specimen with heat-treated basalt fiber.

Table 1 .
Material properties of carbon fiber.

Table 2 .
EDS results of the original, heat-treated and surface-treated basalt fibers.

Table 3 .
Peak tensile forces and displacements of the original and the surface-treated basalt fiber filaments.Force is the peak tensile force of BF filament; Displ. is the displacement of BF filament; Avg. is the average value; σ is the standard deviation.

Table 4 .
Compressive strength of the benchmark and the BFRC specimens.

Table 5 .
Flexural strength of the benchmark and the BFRC specimens.
Avg. is the average value; σ is the standard deviation.

Table 6 .
Splitting tensile strength of the benchmark and the BFRC specimens.
Avg. is the average value; σ is the standard deviation.Fibers 2024, 12, 10