coatings Investigation of Mechanical Properties for Basalt Fiber/Epoxy Resin Composites Modiﬁed with La

: As an efﬁcient reinforcing material in resin matric composites, the application of basalt ﬁbers (BFs) in composites is limited by the poor interfacial adhesion between BFs and the resin matrix. In this study, to obtain the basalt ﬁbers/epoxy resin composites with enhanced mechanical properties, the modiﬁcation solution containing different concentrations of Lanthanum ions (La 3+ ) was synthesized to modify the BFs surfaces to enhance the poor interfacial adhesion between BFs and the matrix. The morphology, the chemical structure and the chemical composition of the modiﬁed BFs surface were observed and detected by scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, respectively. The results show that, after BFs were soaked in the modiﬁcation solution, the more active groups (C=O, –OH, C–O, etc.) were introduced to the BFs surfaces and effectively enhanced the bond strength between BFs and the resin matrix. The obtained mechanical performances of prepared basalt ﬁbers/epoxy resin composites showed that the tensile strength, bending strength and interlaminar shear strength (ILSS) were improved with the modiﬁed BFs, and reached to 458.7, 556.7 and 16.77 Mpa with the 0.5 wt.% La. Finally, the enhancement mechanism of the modiﬁcation solution containing La element is analyzed.


Introduction
In the past years, many studies have been conducted on the applicability of basalt fibers (BFs). As a reinforcing material for composites, BFs (mainly composed by SiO 2 ) can be prepared from basalt rocks using conventional equipment with the advantage on the low-cost process of melting and drawing [1]. Compared with other fiber materials, BFs have the excellent properties such as high tensile strength, high E-modulus, high abrasion strength, high temperature resistance, high resistance to aggressive media, excellent thermal and sound insulation, good chemical stability [2]. Moreover, the mechanical properties of BFs can be kept without significant decrease in the operating temperature range of 200-600 • C [3]. In addition, the environmentally friendly preparation and recycling process of BFs significantly improves its demand for the modern market [1,[4][5][6]. Thus, the BFs might be a promising reinforced material as the candidate of glass and carbon fibers in composite materials, and have been attractive in the application of friction materials, corrosion resistance materials, heat shields, thermal insulting barriers and hot fluid transportation pipes in many fields [7].
Especially in the field of flywheel rotors' structural materials, the fiber-reinforced resin matrix composite flywheel was generally to replace the traditional metal flywheel due to The used BFs cloth (223 g/km, unidirectional cloth) with an average fiber of diameter (5.9 µm) was manufactured by Shanxi Basalt Fiber Technology Co., Ltd., Taiyuan, China. The elastic modulus and the tensile strength of the BFs cloth were about 7.5 and 1835 MPa, respectively. The epoxy resin (LY564) and the curing agent (22964, aliphatic polyamine curing agent) were from Huntsman Co., Ltd. (The Woodlands, TX, USA). The following chemical reagents were used: acetone and nitric acid (analytical reagent, provided by Tianjin Tianda Chemical Reagent Factory, Tianjin, China), absolute ethyl alcohol (analytical reagent, produced by Tianjin Tianli Chemical Reagent Co., Ltd., Tianjin, China), citric acid and urea (analytical reagent, provided by Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China), lanthanum chloride (99.99%, manufactured by Jining Zhongkai New Type Material Co., Ltd., Jining, China).

Treatment of Fibers
The BFs cloth was cut into 260 × 35 mm 2 . After immersed in acetone for 48 h at room temperature to remove the protective coating on the surface of BFs, the fibers were dried in a drying oven at 100 • C for 1 h. The dried fiber cloth was soaked in the concentrated nitric acid with constant temperature water bath at 60 • C for 2 h to improve the roughness and the -OH numbers of the fibers' surfaces [29]. Washing the fibers 3-4 times with distilled water to remove acid fluid from their surfaces, the PH value on the surface of the fibers was adjusted to 7. Finally, the fibers were dried in a drying oven at 100 • C for 1 h and conserved in the dryer.
According to a certain weight ratio, the composition of the BFs modification solution were uniformly mixed at room temperature, as shown in Table 1. Additionally, five kinds of rare earth modification solution were prepared with different La 3+ concentrations of 0.1, 0.3, 0.5, 0.7, and 0.9 wt.% After immersed in the rare earth modification solution for 2 h, the treated BFs were dried in the drying oven at 100 • C for 1 h.

Fabrication and Measurements of Basalt Fiber/Epoxy Resin Composites
The prepared BFs was immersed in epoxy resin/curing agent mixture at volume ratio 4:1. With the hand lay-up method, the BF/ERCs were paved as three layers in the mold, and fabricated after curing in a drying oven at 120 • C for 15 min and 140 • C for 2 h, respectively.
The change of BFs' chemical structure was determined in the transmission mode using Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer 100, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) using a PHI 5700 ESCA System equipped with an Al-Ka X-ray source (NewYork, NY, USA), analyzed the chemical composition of the BFs surfaces. The surface morphologies of BFs and the fracture surface morphologies of the composites were examined using scanning electron microscopy (SEM, JSM-6480, Tokyo, Japan).
The tensile test and the bending test were carried out to determine the tensile strength and the bending strength of BF/ERCs, respectively. The ILSS of BF/ERCs was measured by the three-point short-beam bending test. Due to the details of the mechanical tests shown in Table 2 below, the specimens were cut according to the following dimensional requirement.

FT-IR Spectra of BFs
The chemical structures of untreated and treated BFs were obtained with FT-IR spectroscopy, as shown in Figure 1. The characteristic peaks of -OH at 3450 cm −1 and C=O at 1630 cm −1 were observed in the untreated BFs. In addition, the peaks of -CH 2 appeared at 2921 and 2844 cm −1 , respectively. As the main composition of BFs, the peak of Si-O from SiO 2 of BFs appeared at 860 cm −1 .
the La 3+ concentration of 0.9 wt.%, the peak disappearances of C-O at 1260 c other weakened characteristic peaks were observed, implying that superabun groups introduced by the excess La 3+ were aggregated on the BFs to be sup and further destroyed formed macromolecular membrane on the BFs' surfac the loss of active groups.   For BFs treated with the 0.1 wt.% La 3+ rare earth modification solution, the new absorption peaks of C=O at 1660 cm −1 and -OH at 1384 cm −1 appeared in the range of 4000-1330 cm −1 , and the peaks of -CH 2 at 2917 and 2845 cm −1 from the citric acid were broader than those of the untreated BFs, indicating that the La 3+ was successfully absorbed on the BFs' surface with the introduced -CH 2 -from the acyl group and citric acid, which also brought C=O. The absorption band at the peak of 1046 cm −1 referred to the formation of along with the peak of shifting from 860 to 863cm −1 , stating that the Si-OH from the hydrolysis of SiO 2 react with itself or the introduced -COOH under dehydration condensation reaction to produce Si-O-Si and more Si-O. With La 3+ concentration increasing to 0.3 wt.% in the rare earth modification solution, the new peak of C-O from the citric acid along with the introduced La 3+ at 1116 cm −1 appeared in the FT-IR spectrum. Moreover, the peaks of -CH 2 at 2922 and 2859 cm −1 exhibited obviously. It indicated that the La 3+ was absorbed to the surface with -COOH and -CH 2 from citric acid. When La 3+ concentration was up to 0.5 wt.%, the vibration frequency of C-O increased from 1116 to 1260 cm −1 , and that of C=O stretched from 1668 to 1716 cm −1 , accompanied by the urea in the modification solution. Due to the strong affinity of La 3+ to nonmetal element, the La 3+ could combine with urea and citric acid under the coordination reaction to introduce the C=O and C-O from the modification solution, resulting that more active oxygen-contained functional groups were introduced onto the fiber surfaces with the increasing La 3+ concentration.
With the La 3+ concentration above 0.5 wt.%, the peak of C=O began to disappear. At the La 3+ concentration of 0.9 wt.%, the peak disappearances of C-O at 1260 cm −1 and the other weakened characteristic peaks were observed, implying that superabundant active groups introduced by the excess La 3+ were aggregated on the BFs to be supersaturated and further destroyed formed macromolecular membrane on the BFs' surface to trigger the loss of active groups.  Figure 2 and Table 3, the XPS survey spectra and the detected atomic concentration of the elements C, O, N and Si on the BFs surfaces were respectively obtained to analyze the chemical composition of the BFs surface. On the surfaces of BFs treated with the rare earth modification solution, the concentration of element C decreased and then increased with the increase of La 3+ concentration, but the changes of the elements O and N were contrary. The proportions of O/C and N/C on the BFs surfaces had the same variation trend with the concentration of element C. When La 3+ concentration was 0.5 wt.%, the proportions of O/C and N/C reached maximum values, confirming that the BFs surfaces contained the largest number of active oxygen-containing groups.

XPS Results of BFs
As shown in Figure 2 and Table 3, the XPS survey spectra and the detected atomic concentration of the elements C, O, N and Si on the BFs surfaces were respectively obtained to analyze the chemical composition of the BFs surface. On the surfaces of BFs treated with the rare earth modification solution, the concentration of element C decreased and then increased with the increase of La 3+ concentration, but the changes of the elements O and N were contrary. The proportions of O/C and N/C on the BFs surfaces had the same variation trend with the concentration of element C. When La 3+ concentration was 0.5 wt.%, the proportions of O/C and N/C reached maximum values, confirming that the BFs surfaces contained the largest number of active oxygen-containing groups.

XPS Results of La on BFs Surfaces
The fitted curves of La3d spectra on BFs surface treated with different modification solution containing La 3+ are presented in Figure 3. The spectra peaks of La3d in the treated BFs with the different La 3+ modification solution concentrations (0.1, 0.3, 0.5, 0.7, and 0.9 wt.%) were corresponding to the bonding energies of 851.02, 851.77, 851.9, 852, and 852.1 ev, respectively. All the bonding energies were lower than that (853.0 ev) of the La corresponding to La3d in LaCl 3 , indicating that the coordination chemical reactions occurred between the La and the O, C, N element of the treated fibers' surfaces to the generate lanthanum coordination compounds on fibers' surfaces.

XPS Characterization of Element C on BFs Surfaces
The fitted curves of C 1s spectra and the detected functional groups contents on the BFs surfaces are presented in Figure 4 and Table 4, respectively. Three small peaks of C-C, C-O-and C=O appeared under the C 1s peak of the untreated BFs surfaces in Figure 4a, indicating that C atoms were combined with O atoms. The percentage of C atoms in the corresponding chemical state was represented with the area ratio of every small peak to the C 1s peak. Due to the low content of element N on BFs surfaces, the C-N bond could be ignored.
As seen in Figure 4b, with the peak appearance of -COOH in the fitted curves of C 1s spectra of the BFs, new functional groups were generated on the surface of BFs treated with the rare earth modification solution, besides the original ones. According to the low-to-high order of binding energy, these functional groups can be arranged as alcoholic hydroxyl (C-OH) or ether bond (C-O-C), carbonyl (C=O) and carboxyl (COOH). The -C-C content on the BFs surfaces firstly decreased and then increased with La 3+ concentration increasing, but the changes of the carbon-oxygen bond were contrary. When La 3+ concentration was 0.5 wt.%, with the sum of C-OH/-C-O-C-, C=O and -COOH reaching to a maximum value (72.87%), the number of oxygen-containing groups on the BFs surfaces were the largest. As La 3+ concentration exceeded 0.5 wt.%, the content of carbon-oxygen bond decreased. That was attributed to the fact that the abundant oxygen-containing groups introduced by excess La were aggregated on the fibers' surfaces and could not be stably absorbed to the fibers' surfaces with the weak Van der Waals force between La 3+ . Moreover, the aggregated supersaturated active oxygen-containing groups could also trigger the break of the origin formed macromolecular membrane on the fibers' surfaces, which led to the loss of active groups.
BFs with the different La 3+ modification solution concentrations (0.1, 0.3, 0.5, 0.7, and 0.9 wt.%) were corresponding to the bonding energies of 851.02, 851.77, 851.9, 852, and 852.1 ev, respectively. All the bonding energies were lower than that (853.0 ev) of the La corresponding to La3d in LaCl3, indicating that the coordination chemical reactions oc curred between the La and the O, C, N element of the treated fibers' surfaces to the gen erate lanthanum coordination compounds on fibers' surfaces.

XPS Characterization of Element C on BFs Surfaces
The fitted curves of C 1s spectra and the detected functional groups contents on the BFs surfaces are presented in Figure 4 and Table 4, respectively. Three small peaks of C-C, C-O-and C=O appeared under the C 1s peak of the untreated BFs surfaces in Figure  4a, indicating that C atoms were combined with O atoms. The percentage of C atoms in the corresponding chemical state was represented with the area ratio of every small peak to the C 1s peak. Due to the low content of element N on BFs surfaces, the C-N bond could  bond decreased. That was attributed to the fact that the abundant oxygen-containing groups introduced by excess La were aggregated on the fibers' surfaces and could not be stably absorbed to the fibers' surfaces with the weak Van der Waals force between La 3+ Moreover, the aggregated supersaturated active oxygen-containing groups could also trigger the break of the origin formed macromolecular membrane on the fibers' surfaces which led to the loss of active groups.

XPS Characterization of Si Element on BFs Surfaces
The functional group contents on the BFs surfaces are presented in Table 5, according to the fitted curves of Si 2p spectra in Figure 5.  increased with La 3+ concentration increasing, but the content changes of Si-O-Si and Si-OH were contrary. When La 3+ concentration was 0.5 wt.%, the SiO2 content reached a min imum value (36.12%) corresponding to the highest contents of Si-O-Si and Si-OH. With the La 3+ concentration beyond 0.5 wt.%, the ether bond was generated with the reaction of Si-OH and active groups in the rare earth modification solution. Accordingly, the hy drolysis of SiO2 was compressed and the content of SiO2 increased.   It can be seen that the SiO 2 content on the BFs surfaces firstly decreased and then increased with La 3+ concentration increasing, but the content changes of Si-O-Si and Si-OH were contrary. When La 3+ concentration was 0.5 wt.%, the SiO 2 content reached a minimum value (36.12%) corresponding to the highest contents of Si-O-Si and Si-OH. With the La 3+ concentration beyond 0.5 wt.%, the ether bond was generated with the reaction of Si-OH and active groups in the rare earth modification solution. Accordingly, the hydrolysis of SiO 2 was compressed and the content of SiO 2 increased. Figure 6 reveals the changes of BFs surface morphology after modification. The surfaces of untreated BFs were smooth without obvious grooves or salients in Figure 6a. Some particles formed through the process of the active groups drawn onto the fiber surfaces by the rare earth element, were attached to the surfaces of BFs treated with the rare earth modification solution containing La 3+ concentration 0.1 wt.% in Figure 6b. Figure 6c,d shows that a trend of increasing particles was accompanied by an increasing concentration of La 3+ . When La 3+ concentration was up to 0.5 wt.%, a great number of small particles uniformly covered the fiber surfaces to form the fibers' coatings.  Figure 6 reveals the changes of BFs surface morphology after modification. The surfaces of untreated BFs were smooth without obvious grooves or salients in Figure 6a. Some particles formed through the process of the active groups drawn onto the fiber surfaces by the rare earth element, were attached to the surfaces of BFs treated with the rare earth modification solution containing La 3+ concentration 0.1 wt.% in Figure 6b. Figure  6c,d shows that a trend of increasing particles was accompanied by an increasing concentration of La 3+ . When La 3+ concentration was up to 0.5 wt.%, a great number of small particles uniformly covered the fiber surfaces to form the fibers' coatings.

Basalt Fiber Surface Morphology
As shown in Figure 6e,f, the particles decreased on the fiber surfaces, as La 3+ concentration increased to 0.7 wt.%. When La 3+ concentration was up to 0.9 wt.%, the excess rare earth elements were attached onto the fiber surfaces with the saturation of active groups, resulting that only a few particles of inhomogeneous size were on the fibers. Moreover, these active groups were easy to bulge and even be broken under the function of tension. As shown in Figure 6e,f, the particles decreased on the fiber surfaces, as La 3+ concentration increased to 0.7 wt.%. When La 3+ concentration was up to 0.9 wt.%, the excess rare earth elements were attached onto the fiber surfaces with the saturation of active groups, resulting that only a few particles of inhomogeneous size were on the fibers. Moreover, these active groups were easy to bulge and even be broken under the function of tension.

Mechanical Properties of Basalt Fibers/Epoxy Resin Composites
Tensile, bending and ILSS standard tests were performed to analyze mechanical behavior of BF/ERCs. According to the obtained experimental data shown in Figures 7-9, the mechanical properties, including the tensile strength, the bending strength and ILSS of BF/ERCs, were improved by BFs treated with La 3+ concentration increasing from 0.1 to 0.5 wt.%. The active functional groups attached to the fibers' surfaces, such as C=O and O-H, improved the fiber surface roughness and activity, and thus enhanced the adhesion between fibers and the composite matrix. The maximum values of the tensile strength, the bending strength and ILSS were up to 458.7, 556.7 and 16.77 Mpa, and increased by 56.22%, 103.32% and 88% compared with those of the unmodified ones, respectively.

Mechanical Properties of Basalt Fibers/Epoxy Resin Composites
Tensile, bending and ILSS standard tests were performed to analyze me havior of BF/ERCs. According to the obtained experimental data shown in the mechanical properties, including the tensile strength, the bending streng of BF/ERCs, were improved by BFs treated with La 3+ concentration increasing 0.5 wt.%. The active functional groups attached to the fibers' surfaces, such O-H, improved the fiber surface roughness and activity, and thus enhanced t between fibers and the composite matrix. The maximum values of the tensile s bending strength and ILSS were up to 458.7, 556.7 and 16.77 Mpa, and i 56.22%, 103.32% and 88% compared with those of the unmodified ones, respe When La 3+ concentration exceeded 0.5 wt.%, the mechanical strength of decreased as La 3+ concentration increased. This may be ascribed to the fact th active groups introduced by La 3+ were aggregated to be supersaturated, and t break of saturated active groups happened, leading to the loss of active group the bits of grains on the interface of composite formed by the introduction o La 3+ to the fibers' surface also reduced the interface bonding strength.   Tensile, bending and ILSS standard tests were performed to analyze m havior of BF/ERCs. According to the obtained experimental data shown in the mechanical properties, including the tensile strength, the bending stren of BF/ERCs, were improved by BFs treated with La 3+ concentration increasin 0.5 wt.%. The active functional groups attached to the fibers' surfaces, such O-H, improved the fiber surface roughness and activity, and thus enhanced between fibers and the composite matrix. The maximum values of the tensile bending strength and ILSS were up to 458.7, 556.7 and 16.77 Mpa, and 56.22%, 103.32% and 88% compared with those of the unmodified ones, resp When La 3+ concentration exceeded 0.5 wt.%, the mechanical strength o decreased as La 3+ concentration increased. This may be ascribed to the fact th active groups introduced by La 3+ were aggregated to be supersaturated, and break of saturated active groups happened, leading to the loss of active group the bits of grains on the interface of composite formed by the introduction La 3+ to the fibers' surface also reduced the interface bonding strength.

Fracture Surfaces Morphology of Basalt Fibers/Epoxy Resin Composites
The fracture surfaces of untreated and treated BF/ERCs in the tensile te served by SEM. In Figure 10a, smooth fibers and grooves can be seen on the fr faces of untreated composites. Caused by the worse interfacial adhesion betw and the resin matrix, the fibers can be easily separated or pulled out from the re exhibiting the low tensile strength of composites.
There were a few resins on the fiber surfaces of composites treated with th modification solution containing La 3+ concentration 0.1 wt.%, as shown in Figu ins increased and grooves decreased on the fracture surfaces with the increase centration, as seen in Figure 10c,d. These residual resins on the fibers' surfac tributed in the improved adhesion between fibers and the resin matrix, when stretched from the resin matrix under the stress. When La 3+ concentration wa wt.%, the gaps between fibers were filled with the resins in Figure 10d. BFs an matrix adhered so tightly that external load was delivered to the fibers from and the fibers mainly tolerated the breaking stress. It exhibited the obviously mechanical properties of BF/ERCs in the better reinforcing effect.
When La 3+ concentration exceeded 0.5 wt.%, the decrease in resins on th faces with increasing La 3+ concentration led to the reappeared grooves on the re when the smooth fibers pulled out, as seen in Figure 10 e,f. It demonstrated th facial strength between the fibers and the resin matrix reduced, and the mecha erties of composites correspondingly decreased. When La 3+ concentration exceeded 0.5 wt.%, the mechanical strength of composites decreased as La 3+ concentration increased. This may be ascribed to the fact that the excess active groups introduced by La 3+ were aggregated to be supersaturated, and the triggered break of saturated active groups happened, leading to the loss of active groups. Moreover, the bits of grains on the interface of composite formed by the introduction of the excess La 3+ to the fibers' surface also reduced the interface bonding strength.

Fracture Surfaces Morphology of Basalt Fibers/Epoxy Resin Composites
The fracture surfaces of untreated and treated BF/ERCs in the tensile test was observed by SEM. In Figure 10a, smooth fibers and grooves can be seen on the fracture surfaces of untreated composites. Caused by the worse interfacial adhesion between fibers and the resin matrix, the fibers can be easily separated or pulled out from the resin matrix, exhibiting the low tensile strength of composites.
There were a few resins on the fiber surfaces of composites treated with the rare earth modification solution containing La 3+ concentration 0.1 wt.%, as shown in Figure 10b. Resins increased and grooves decreased on the fracture surfaces with the increase of La 3+ concentration, as seen in Figure 10c,d. These residual resins on the fibers' surfaces were attributed in the improved adhesion between fibers and the resin matrix, when fibers were stretched from the resin matrix under the stress. When La 3+ concentration was up to 0.5 wt.%, the gaps between fibers were filled with the resins in Figure 10d. BFs and the resin matrix adhered so tightly that external load was delivered to the fibers from the matrix, and the fibers mainly tolerated the breaking stress. It exhibited the obviously improved mechanical properties of BF/ERCs in the better reinforcing effect.
When La 3+ concentration exceeded 0.5 wt.%, the decrease in resins on the fiber surfaces with increasing La 3+ concentration led to the reappeared grooves on the resin matrix, when the smooth fibers pulled out, as seen in Figure 10e,f. It demonstrated that the interfacial strength between the fibers and the resin matrix reduced, and the mechanical properties of composites correspondingly decreased. and the fibers mainly tolerated the breaking stress. It exhibited the obviously improved mechanical properties of BF/ERCs in the better reinforcing effect.
When La 3+ concentration exceeded 0.5 wt.%, the decrease in resins on the fiber surfaces with increasing La 3+ concentration led to the reappeared grooves on the resin matrix, when the smooth fibers pulled out, as seen in Figure 10 e,f. It demonstrated that the interfacial strength between the fibers and the resin matrix reduced, and the mechanical properties of composites correspondingly decreased. The monomolecular layer theory can be supplemented to analyze the effect of element La concentration on the tensile performances of BF/ERCs with the model shown in Figure 11. At low concentrations, few La 3+ were absorbed onto the BFs surfaces. As shown in Figure 11a, the arrangement of La atoms on BFs surfaces was discontinuous with a lot of voids, resulting in the failure of BF/ERCs occurring firstly on the location without La atoms under external load. Thus, the discontinuous interface between BFs and the epoxy resin is insufficient to improve the mechanical performance of BF/ERCs obviously. When La element increased to the best concentration, 0.5 wt.%, the uniform and compact monomolecular layer on the BFs surfaces exhibits high adhesive strength between BFs and the matrix, as shown in Figure 11b. The mechanical performances of BF/ERCs reached to the highest value, including the tensile strength, the bending strength, and ILSS.
When La 3+ concentration exceeded 0.5 wt.%, excessive La atoms were assembled on the BFs surfaces to form multimolecular layer, as shown in Figure 12. Under external load, failure of BF/ERCs firstly occurred on the interlayer of La atoms interlinked by weak Van der Waals force, resulting in the decreased performance of BF/ERCs. The monomolecular layer theory can be supplemented to analyze the effect of element La concentration on the tensile performances of BF/ERCs with the model shown in Figure 11. At low concentrations, few La 3+ were absorbed onto the BFs surfaces. As shown in Figure 11a, the arrangement of La atoms on BFs surfaces was discontinuous with a lot of voids, resulting in the failure of BF/ERCs occurring firstly on the location without La atoms under external load. Thus, the discontinuous interface between BFs and the epoxy resin is insufficient to improve the mechanical performance of BF/ERCs obviously. When La element increased to the best concentration, 0.5 wt.%, the uniform and compact monomolecular layer on the BFs surfaces exhibits high adhesive strength between BFs and the matrix, as shown in Figure 11b. The mechanical performances of BF/ERCs reached to the highest value, including the tensile strength, the bending strength, and ILSS.
When La 3+ concentration exceeded 0.5 wt.%, excessive La atoms were assembled on the BFs surfaces to form multimolecular layer, as shown in Figure 12. Under external load, failure of BF/ERCs firstly occurred on the interlayer of La atoms interlinked by weak Van der Waals force, resulting in the decreased performance of BF/ERCs. matrix, as shown in Figure 11b. The mechanical performances of BF/ERCs reach highest value, including the tensile strength, the bending strength, and ILSS.
When La 3+ concentration exceeded 0.5 wt.%, excessive La atoms were assem the BFs surfaces to form multimolecular layer, as shown in Figure 12. Under exter failure of BF/ERCs firstly occurred on the interlayer of La atoms interlinked by w der Waals force, resulting in the decreased performance of BF/ERCs.

Analysis about Modification Mechanism of La 3+
In Figure 1 and Figures 2-5, the analysis based results of BFs shows that, compared to untreated BFs, groups on the fiber surfaces increased, once BFs is tre solution. That was ascribed to the mechanism of La as follows.
The rare earth elements with 4f electronic shel polarized, they have strong affinity to the nonmeta ments in alcohol solution with the typical nonmetal e 28]. Consequently, with the results in Section 3.2.2.,

Analysis about Modification Mechanism of La 3+
In Figures 1-5, the analysis based on the FT-IR spectra and the XPS results of BFs shows that, compared to untreated BFs, active oxygen-containing functional groups on the fiber surfaces increased, once BFs is treated with the rare earth modification solution. That was ascribed to the mechanism of La element in the modification solution as follows.
The rare earth elements with 4f electronic shell have good chemical activity. Once polarized, they have strong affinity to the nonmetal elements and turn into active elements in alcohol solution with the typical nonmetal elements such as C, H, O and N [26][27][28]. Consequently, with the results in Section 3.2.2., the coordination bonds between La ions and nonmetal elements are formed and La elements are attached onto the fiber surfaces, as shown in Figure 12.
The coordination number of La element changes from 3 to 12, while the majority level remains 8. In this study, the rare earth modification solution was the alcohol solution with a variety of organic compounds, such as urea and citric acid, as shown in Figure 13a. As the active chemical cores, La ions attached on the BFs surfaces were combined with a lot of active organic groups in the solution to generate the multicomponent complex [26][27][28], shown in Figure 13b. With the active organic groups absorbed onto the fiber surfaces to form the macromolecular membrane, the chemical activity on the fiber surfaces could be improved obviously, as shown in Figure 13c. As seen in Figure 6, the La ions can also be embedded in the defect points on the BFs surfaces to generate more active chemical cores, and hence the chemical activity of the fiber surfaces can be further improved. a variety of organic compounds, such as urea and citric acid, as shown in the active chemical cores, La ions attached on the BFs surfaces were comb of active organic groups in the solution to generate the multicomponent co shown in Figure13b. With the active organic groups absorbed onto the fi form the macromolecular membrane, the chemical activity on the fiber sur improved obviously, as shown in Figure 13c. As seen in Figure 6, the La io embedded in the defect points on the BFs surfaces to generate more active c and hence the chemical activity of the fiber surfaces can be further improve oatings 2021, 11, 666 However, the abundant active groups introduced by the excess La 3+ w on the fibers' surfaces to be supersaturated, resulting in the break of the m membrane and loss of active groups, as shown in Figure 4e,f. Meanwhile, w La 3+ , the generated Si-OH between SiO2 of the fibers' surface and the mod tion under hydrolytic action restrain the further hydrolytic action of SiO2, decrease of Si-OH and Si-O-Si in Figure 4e,f, which further reduced the s of fibers. Thus, the La 3+ concentration of 0.5% was the most suitable. However, the abundant active groups introduced by the excess La 3+ were aggerated on the fibers' surfaces to be supersaturated, resulting in the break of the macromolecular membrane and loss of active groups, as shown in Figure 4e,f. Meanwhile, with the excess La 3+ , the generated Si-OH between SiO 2 of the fibers' surface and the modification solution under hydrolytic action restrain the further hydrolytic action of SiO 2 , leading to the decrease of Si-OH and Si-O-Si in Figure 4e,f, which further reduced the surface activity of fibers. Thus, the La 3+ concentration of 0.5% was the most suitable.

Analysis of Enhancement Mechanism of BFs/ECR's Mechanical Property Modified with La 3+
In the process of the resin matrix synthesis, the following crosslinking reaction of epoxy resin with ali-phatic polyamine curing agent took place and generated two oxhydryl groups, as shown in Figure 14.
However, the abundant active groups introduced by the excess La 3+ were aggerated on the fibers' surfaces to be supersaturated, resulting in the break of the macromolecular membrane and loss of active groups, as shown in Figure 4e,f. Meanwhile, with the excess La 3+ , the generated Si-OH between SiO2 of the fibers' surface and the modification solution under hydrolytic action restrain the further hydrolytic action of SiO2, leading to the decrease of Si-OH and Si-O-Si in Figure 4e,f, which further reduced the surface activity of fibers. Thus, the La 3+ concentration of 0.5% was the most suitable.

Analysis of Enhancement Mechanism of BFs/ECR's Mechanical Property Modified with La 3+
In the process of the resin matrix synthesis, the following crosslinking reaction of epoxy resin with ali-phatic polyamine curing agent took place and generated two oxhydryl groups, as shown in Figure 14. Based on the FT-IR spectra and the XPS results of BFs, the active functional groups increased on the surface of BFs treated with the rare earth modification solution. With the oxhydryl generated in the above crosslinking reaction, the active functional groups took part in the dehydration condensation reaction, when BFs were in sufficient contact with epoxy resin. The bonding of BFs and epoxy resin was based on the chemical bonds, such as the ester group and the ether bond, so that the bonding force improved significantly, as shown in Figure 15a. Thus, when La 3+ concentration was 0.5 wt.%, the absorbed active groups reached to the maximumand the bonding force was correspondingly highest, as well as the tensile strength, the bending strength, and ILSS.
However, parts of La ions, which were absorbed on the modified BFs surfaces, did not take part in the coordination bond with active groups. When the modified BFs were put into the epoxy resin, these La ions can be combined with oxhydryl generated in the above crosslinking reaction, because of the typical chemical activities and the variable coordination number of La 3+ . As shown in Figure 15b, under the effect of the formed coordination bond, the adhesion strength between BFs and epoxy matrix was further improved, as well as the mechanical performances of BF/ERCs. Based on the FT-IR spectra and the XPS results of BFs, the active functional groups increased on the surface of BFs treated with the rare earth modification solution. With the oxhydryl generated in the above crosslinking reaction, the active functional groups took part in the dehydration condensation reaction, when BFs were in sufficient contact with epoxy resin. The bonding of BFs and epoxy resin was based on the chemical bonds, such as the ester group and the ether bond, so that the bonding force improved significantly, as shown in Figure 15a. Thus, when La 3+ concentration was 0.5 wt.%, the absorbed active groups reached to the maximumand the bonding force was correspondingly highest, as well as the tensile strength, the bending strength, and ILSS.

Conclusions
In this research, the modification of basalt fibers with the modificat different La 3+ concentrations was carried out. Moreover, the effect of mo different La 3+ concentrations and the mechanical properties for basalt fib composites modified with La 3+ were evaluated.
The study about the chemical composition, the functional groups and ogy of untreated and treated BFs' surface with the rare earth modification several analytical techniques (FTIR, XPS, and SEM), confirmed that La elem earth modification solution could link active oxygen-containing functiona However, parts of La ions, which were absorbed on the modified BFs surfaces, did not take part in the coordination bond with active groups. When the modified BFs were put into the epoxy resin, these La ions can be combined with oxhydryl generated in the above crosslinking reaction, because of the typical chemical activities and the variable coordination number of La 3+ . As shown in Figure 15b, under the effect of the formed coordination bond, the adhesion strength between BFs and epoxy matrix was further improved, as well as the mechanical performances of BF/ERCs.

Conclusions
In this research, the modification of basalt fibers with the modification solution of different La 3+ concentrations was carried out. Moreover, the effect of modification with different La 3+ concentrations and the mechanical properties for basalt fiber/epoxy resin composites modified with La 3+ were evaluated.
The study about the chemical composition, the functional groups and the morphology of untreated and treated BFs' surface with the rare earth modification solution using several analytical techniques (FTIR, XPS, and SEM), confirmed that La element in the rare earth modification solution could link active oxygen-containing functional groups to the BFs surfaces to improve the roughness and the activity of the fiber surfaces. When the La 3+ concentration in the rare earth modification solution was 0.5 wt.%, the activity of BFs' surface reached the strongest with the most introduced active groups. It is also determined that the mechanical performances of BF/ERCs, including the tensile strength, the bending strength and ILSS, could be significantly enhanced by the modified BFs due to the formed chemical reaction between resin matrix and the more active groups introduced by La 3+ . With the La 3+ concentration in the rare earth modification solution reaching 0.5 wt.%, the bonding between the resin matrix and BFs is demonstrated to be the best and the tensile strength, the bending strength and ILSS were up to 458.7, 556.7 and 16.77 Mpa, and improved by 56.22%, 103.32% and 88% compared with those of the unmodified ones, respectively.
In addition, the modification mechanism of La 3+ and enhancement mechanism of BFs/ECR's mechanical property modified with La 3+ were stated with the strong affinity of La to the nonmetal elements, indicating that the modification of BFs with La 3+ was effective and the mechanical property of the prepared BFs/ECR composites was excellent.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available as the data also form part of an ongoing study.

Conflicts of Interest:
The authors declare no conflict of interest.