polymers

Article
Mechanical Properties and Microstructure of
Polypropylene–Glass-Fiber-Reinforced Desert Sand Concrete
Lina Hou 1, * , Baojun Wen 2 , Wei Huang 3 , Xue Zhang 1 and Xinyu Zhang 1

1 College of Architecture Engineering, Xi’an Technological University, Xi’an 710021, China

2 Xi’an Survey, Design and Research Institute Co., Ltd. of CREC, China Railway Liu Yuan, Xi’an 710021, China
3 College of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
* Correspondence: houlina-163@163.com

Abstract: In order to improve the performance of desert sand concrete, polypropylene fiber (PF)
and glass fiber (GF) were used to prepare desert sand concrete (DSC) with different fiber and
volume content, and the basic mechanical properties, such as cube compressive, tensile and flexural
strengths, were tested and studied. Based on the mercury injection method (MIP) and scanning
electron microscopy (SEM), the evolution of pore structure and interface structure was analyzed.
The mechanism of fiber toughening was revealed at the microscopic level. The results show that
the slump of DSC decreases with the increase in fiber content. The slump of glass-fiber-reinforced
DSC (GFRDSC) is smaller than that of polypropylene-fiber-reinforced DSC (PFRDSC). The strength
enhancement of DSC by fibers is in the order of flexural strength > split tensile strength > compressive
strength. The flexural strength of hybrid-fiber-reinforced DSC (HyFRDSC) (0.1% PF + 0.1% GF) is
increased by 40.7%. Meanwhile, fibers can improve the toughness of DSC. The MIP results show
that the porosity of HyFRDSC decreased by 50.01%, and the addition of fiber can effectively refine
the large pore size. The SEM results show that the incorporation of PF and GF causes the formation
of a uniform and dense structure between the fibers, cement and aggregate. The two can give full
play to the crack-resisting and toughening effect in different loading stages, thus improving the
macromechanical properties of DSC.

Citation: Hou, L.; Wen, B.; Keywords: desert sand concrete; hybrid fiber; mechanical properties; pore structure; microstructure
Huang, W.; Zhang, X.; Zhang, X.
Mechanical Properties and
Microstructure of Polypropylene–
Glass-Fiber-Reinforced Desert Sand 1. Introduction
Concrete. Polymers 2023, 15, 4675.
Accelerated global infrastructure construction unprecedentedly increases the demand
https://doi.org/10.3390/polym
for concrete and sand. However, meeting engineering and construction needs is challenging
15244675
given the limited natural sand resources [1]. The long-distance transport of natural sands
Academic Editor: Chenggao Li raises construction costs and greenhouse gas emissions, leading to serious environmental
Received: 7 November 2023
problems and river sand resource consumption [2–4]. For this reason, alternatives to sand
Revised: 24 November 2023
resources are urgently required [5,6]. Desert sand, a weathering product of rock, has a small
Accepted: 3 December 2023 fineness modulus [7] and huge deposits in vast desert areas worldwide. Partially replacing
Published: 11 December 2023 natural sand with desert sand has far-reaching significance in reducing construction costs,
protecting the ecological environment and satisfying engineering application standards
and conditions.
Desert sand can refine microstructure and fill pores, improving cement slurry den-
Copyright: © 2023 by the authors. sity [8–10]. Therefore, the mechanical properties of desert sand concrete (DSC) are very
Licensee MDPI, Basel, Switzerland. close to those of ordinary concrete, and the desert sand can be used as a fine aggregate in
This article is an open access article
civil engineering concrete to meet the general engineering requirements. However, due
distributed under the terms and
to its low fineness modulus, high water requirement and easy cracking, shrinkage and
conditions of the Creative Commons
bleeding, the application of desert sand resources is limited [11–14].
Attribution (CC BY) license (https://
Fiber-reinforced concrete technology has been widely used, and the bonding effect
creativecommons.org/licenses/by/
between fiber and the concrete matrix can enhance the tensile properties of concrete and
4.0/).

Polymers 2023, 15, 4675. https://doi.org/10.3390/polym15244675 https://www.mdpi.com/journal/polymers

Polymers 2023, 15, 4675 2 of 20

reduce the occurrence of crack generation and expansion in the matrix, improving the
mechanical properties of concrete. Fiber-reinforced concrete can be widely used in construc-
tion engineering, water conservancy engineering, road and bridge engineering and tunnel
and military engineering [15,16]. Hybrid-fiber-reinforced concrete is a high-performance
composite construction material formed by mixing various fibers into concrete according
to a certain ratio. Conventional fiber combinations include macrofibers (steel fiber, glass
fiber (GF), carbon fiber, aramid fiber (AF), etc.) and microfibers (polyvinyl alcohol fiber
(PVAF), polypropylene fiber (PF), polyester fiber, polyacrylonitrile fiber, etc.) [17].
Studies have shown that hybrid-fiber-reinforced concrete has better mechanical prop-
erties than single-mixed-fiber-reinforced concrete. The influences on the mechanical prop-
erties of mixed-fiber-reinforced concrete come mainly from the type of fiber and the mixing
ratio. For example, the compressive strength, splitting tensile strength and bending strength
of concrete mixed with steel fibers and PFs were superior to those of concrete only mixed
with steel fibers; the maximum increase in compressive strength could reach 50.1% [18,19],
and the optimum fiber content was 1.5% steel fibers and 1.5% PFs [19]. Blending basalt
fibers, PVAFs and steel fibers could achieve the optimal synergistic performance, and
the compressive and flexural strength of concrete was significantly increased [20]. The
favorable blending effect of steel and carbon fibers largely also improved the compressive
strength and toughness of concrete [21]. Steel fibers when blended with polyolefin fibers,
PVAFs, GFs, polyester fibers and basalt fibers, respectively, increased the tensile strength of
UHPC more than single steel fibers [22]. The best mechanical properties of concrete were ob-
tained with 0.8% basalt fibers and 0.25% steel fibers [23]. The flexural and tensile strength of
PVA–steel-fiber-reinforced concrete was largely increased with the elevating mix proportion
of steel fibers [24]. The compressive strength and elastic modulus of PVAF/PPF-reinforced
concrete dropped with the rising volume ratio of PFs [25]. The mixture of 0.15% basalt fiber
and 0.033% PFs had the best synergistic effect [26]. In addition, the shape of steel fibers
dramatically enhanced the flexural strength of steel-fiber–PF-reinforced concrete [27]. The
length-to-diameter ratios of PFs and AFs greatly influenced the early mechanical properties
of concrete [28].
PFs and GFs have shown a favorable performance in improving the properties of
concrete and are extensively applied in engineering projects such as road pavements,
bridges and harbors [29]. PFs are favored in concrete reinforcement due to their low cost,
easy processing, high strength, low density and good corrosion resistance. Adding PF to
concrete can considerably improve the economic benefits and sustainability [30,31]. PF
greatly enhances the mechanical properties and crack resistance of concrete [32] in the case
of 0.5% fiber content. The splitting strength and flexural strength of concrete were increased
by 12.01% and 17.15%, respectively [33]. Additionally, 0.1–0.3% PF can reduce the plastic
shrinkage of concrete by 12–25%, and the tension–compression ratio can be increased by
46% [34–36].
Alkali-resistant GF is a new fiber material characterized by high ductility, strong
temperature resistance, noncombustibility, favorable corrosion resistance, large tensile
strength and good thermal, acoustic and electrical insulation. GF can cross over and inhibit
the development of macroscopic cracks [37], increasing the compressive strength, flexural
strength and tensile strength of concrete by 15%, 44.6% [38] and 10–17%; in addition, the
deformability and energy absorption capacity were also intensified [39]. In view of the ex-
cellent hybrid effect of different fibers among blended groups, the combination of GF with a
higher modulus of elasticity and PF with a lower modulus of elasticity can achieve comple-
mentary advantages to improve the cracking resistance and reinforcement properties of the
materials. It was reported that when PF and GF dosages were less than 0.45%, the concrete
properties were highly improved; at 1.5% hybrid-fiber content, the 28-day compressive and
flexural strengths were increased by 9.35% and 20.36%, respectively [40,41].
Fiber-reinforced concrete technology has been applied in DSC. Some researchers
experimentally found that mixing steel fibers could substantially enhance the compres-
sive strength and splitting tensile strength of DSC, and the enhancement of the latter
 0.45%, the concrete properties were highly improved; at 1.5% hybrid-fiber content, the 28-
day compressive and flexural strengths were increased by 9.35% and 20.36%, respectively
[40,41].
Fiber-reinforced concrete technology has been applied in DSC. Some researchers ex-
perimentally found that mixing steel fibers could substantially enhance the compressive
Polymers 2023, 15, 4675
strength and splitting tensile strength of DSC, and the enhancement of the latter strength 3 of 20

was relatively obvious [42–44]. Steel fibers account for the most commonly used option in
the current literature, yet they have the disadvantages of self-weighting and easy corro-
sion [45].was
strength Additionally, existing[42–44].
relatively obvious research mainly
Steel fibers focuses
account onfor single-fiber-doped
the most commonlyDSC; used in-
vestigation into HyRDSC is still a research gap; mixed PF and GF reinforced DSCand
option in the current literature, yet they have the disadvantages of self-weighting espe-
easy corrosion
cially have not [45].beenAdditionally,
reported. existing research mainly focuses on single-fiber-doped
DSC; investigation into HyRDSC is still a research gap; mixed PF and GF reinforced DSC
In this paper, to further enhance DSC performance, HyRDSC with various fibers and
especially have not been reported.
volume admixtures was prepared using PFs and GFs, and its basic mechanical properties
In this paper, to further enhance DSC performance, HyRDSC with various fibers and
(compressive, flexural and splitting tensile strength) were experimentally investigated. In
volume admixtures was prepared using PFs and GFs, and its basic mechanical properties
addition,
(compressive,the toughening mechanism
flexural and splitting of these
tensile fibers
strength) werewas further explored
experimentally at the micro-
investigated. In
scopic
addition,level
thebased on mercury
toughening mechanisminjection capillary
of these pressure
fibers was further(MIP) testsatand
explored scanning elec-
the microscopic
tron
levelmicroscopy (SEM).injection
based on mercury The current findings
capillary are significant
pressure (MIP) testsforandenvironmental protection,
scanning electron mi-
shortage alleviation of sand and gravel resources, achievement of
croscopy (SEM). The current findings are significant for environmental protection, shortagesustainable develop-
ment strategy
alleviation and and
of sand carbon emission
gravel reduction.
resources, achievement of sustainable development strategy
and carbon emission reduction.
2. Materials and Methods
2. Materials and Methods
2.1. Raw Materials
2.1. Raw Materials
Grade PO·42.5 cement produced by Conch Cement Co., Ltd. (Xianyang, China) was
Grade PO·42.5 cement produced by Conch Cement Co., Ltd. (Xianyang, China) was
used. The cement parameters are listed in Table 1, conforming to the provisions of GB/T
used. The cement parameters are listed in Table 1, conforming to the provisions of GB/T
175-2020 [46]. The gravel coarse aggregates in this study have a maximum particle size of
175-2020 [46]. The gravel coarse aggregates in this study have a maximum particle size
20 mm.
of 20 mm. TheThesand forfor
sand fine aggregate
fine aggregate was
was collected
collectedfromfromthetherivers
riversaround
aroundXi’an,
Xi’an,and
andthe
desert sandsand
the desert waswasobtained
obtainedfromfrom
the Mu Us Desert
the Mu Us Desertin Yulin City;City;
in Yulin bothboth
are shown in Figure
are shown in
1. The physical parameters and grading curve of the fine aggregates
Figure 1. The physical parameters and grading curve of the fine aggregates are presented are presented in Table
2inand Figure
Table 2 and1 Figure
(GBT 14684-2011) [47]. Grade
1 (GBT 14684-2011) [47].I Grade
fly ashIfrom Henan
fly ash from Borun
HenanCo., Ltd.
Borun (Anyang,
Co., Ltd.
China)
(Anyang, was selected,
China) and the corresponding
was selected, parameters
and the corresponding are given
parameters are in Table
given in 3. PFs3.and
Table PFsGFs
and theirand
and GFs performance parameters
their performance are shown
parameters are shownin Figure
in Figure2 and
2 andTable
Table4,4,respectively.
respectively. A
naphthalene-type
A naphthalene-typesuperplasticizer
superplasticizer withwith aa water-reducing
water-reducingrate rateofof 16%
16% waswas employed.
employed. TapTap
water
water inin Xi’an
Xi’an was
was utilized
utilizedfor forconcrete
concretemixing.
mixing.

(a) (b) (c)

Figure 1. Fine
Figure 1. Fine aggregates
aggregatesand
andsieving
sievingcurve:
curve:(a)(a)river
river sand;
sand; (b)(b) desert
desert sand;
sand; (c) (c) grading
grading curve.
curve.

Table 1. Cement parameters and chemical compositions.

Ignition Specific Flexural Compressive

Initial Setting Final Setting
Loss/% Surface Area Duration/min Duration/min Stability Strength Strength Cl− /% SO3 /% MgO/%
m2 /kg /MPa /MPa
3.49 349 187 246 Qualified 5.8 30.7 0.032 2.41 3.32
Note: The flexural and compressive strengths are both 3D strengths.
ers 2023, Polymers
15, x FOR PEER REVIEW
2023, 15, 4675 4 of 20
4 of 20

(a) (b)
Figure 2.Figure
Fibers: (a) PFs;
2. Fibers: (a) (b)
PFs;GFs.
(b) GFs.

Table 1.Table
Cement parameters and chemical compositions.
2. Physical properties of fine aggregates.

Fine Aggregate Bulk Density Apparent Density Fineness Modulus Mud Content Superplasticizer
Specific Flexural Compressive
tion Unit Initial Setting
kg/m3 Final Setting
kg/m3 % %
Surface Stability Strength Strength Cl−/% SO
0.8 3/% MgO/%
s/% River sandDuration/min
1558 Duration/min
2578 2.3 2.5
Area Desert
m2/kgsand 1542 2602 /MPa
1.1 /MPa
1.2 2.1

49 349 187 246 Qualified 5.8 30.7 0.032 2.41 3.32

Table 3. Parameters and chemical composition of fly ash.
Note: The flexural and compressive strengths are both 3D strengths.
Ignition Moisture Density Bulk Density Free
AI2 O3 /% AIO2 /% Cl− /% SO3 /% CaO/% Fe/%
Table 2.g/cm
Physical properties of fine aggregates.
Loss/% Content/% 3 g/cm3 CaO/%
2.8 0.85 2.55 1.12 24.2 45.1 0.015 2.1 5.6 0.85/ 0.85
ine Aggregate Bulk Density Apparent Density Fineness Modulus Mud Content Superplasticizer
Unit kg/m3 Table 4. Performance
kg/m3 parameters of PFs and GFs. % %
River sand 1558 2578 2.3
Tensile Elastic2.5 0.8
Elongation after
Fiber Type Density/g·cm−3 Diameter/µm Length/mm
Desert sand 1542 2602 1.1
Strength/MPa 1.2
Modulus/GPa Fracture/% 2.1
PFs 0.9 31.86 12 567 5.2 39
GFs
Table 3. Parameters and chemical composition of fly ash.
2.4 18 2500 70 3.6

nition Moisture Density Bulk Density

2.2. Mix Proportions and Specimen Preparation
AI2O3/% AIO2/% Cl−/% SO3/% CaO/% Fe/% Free CaO/%
ss/% Content/% g/cm 3 g/cm 3
The concrete strength grade is C40. Based on extensive data [34,38,40,41] and repeated
2.8 0.85 2.55 tests,1.12
the desert sand24.2 45.1
replacement 0.015
ratio was 2.1and its benchmark
set at 30%, 5.6 0.85/ 0.85
is shown in
Table 5 according to JGJ 55-2011 [48]. The selected mix ratio and the effects of fiber type
and content on the compressive, flexural and splitting tensile strengths of DSC were
Table 4.studied.
Performance
Twelveparameters of PFs andwere
groups of specimens GFs.designed, as shown in Table 6. According to
GB/T50008-2019 [49], the specimens for testing compressive and tensile strengths were
mm3 ,Length/mm Tensile Elongation
mm. Threeafter
er Type Density/g·cm−3 Diameter/μm
100 and those for flexural Elastic100
strength measured Modulus/GPa
mm × 100 mm × 400
test blocks without visibleStrength/MPa
defects were selected for each group. Fracture/%
PFs 0.9 31.86 A 12
dry-mixing method was 567
applied to concrete to 5.2
ensure fiber dispersion. Firstly, 39
coarse
aggregates, fine aggregates, cement and fly ash were added successively. The fibers were
GFs 2.4 18 2500 70
added after dry-mixing for 30 s, followed by the addition of water and a water-reducing
3.6
agent after mixing for another 90 s. Secondly, the mixture was stirred for 1 min. Finally, the
2.2. Mixconcrete
Proportions and Specimen
was loaded Preparation
into a mold, and the blocks were demolded after 24 h. The tests were
carried out after standard curing for 28 d.
The concrete strength grade is C40. Based on extensive data [34,38,40,41] and re-
peated Table
tests,5.the desert
Concrete sand mix
reference replacement
ratio/(kg·m−ratio
3 ). was set at 30%, and its benchmark is shown
in Table 5 according to JGJ 55-2011 [48]. The selected mix ratio and the effects of fiber type
Desert Sand Water Cement Fly Ash Sand Stone Water-Reducing Agent
and content on the compressive, flexural and splitting tensile strengths of DSC were stud-
ied. Twelve225 groups of 181
specimens
362
were40
designed,
524
as 1333
shown in Table 3.2
6. According to
GB/T50008-2019 [49], the specimens for testing compressive and tensile strengths were 100
mm3, and those for flexural strength measured 100 mm × 100 mm × 400 mm. Three test
blocks without visible defects were selected for each group.
A dry-mixing method was applied to concrete to ensure fiber dispersion. Firstly,
Polymers 2023, 15, x FOR PEER REVIEW
P0.1 0.9 5 of 20
P0.15 1.35
G0.1 0
Polymers 2023, 15, 4675 Table 5. Concrete reference mix ratio/(kg·m ). −3 5 of 20
G0.2 0
Water-Reducing
Desert Sand Water G0.3 Cement
Table 6. Test groups/(kg·m−3 ).
Fly Ash Sand 0
Stone
Agent
225 PG0.05
181 + 0.1
Specimen No.
362 40
PF/(kg·m−3 )
524 0.45
1333
GF/(kg·m−3 )
3.2

PG0.1 + 0.05
Table 6. Test groups/(kg·m
M0 ).−3
0 0.9 0
PG0.05
P0.1 No. + 0.15
P0.05
Specimen
0.45
PF/(kg·m
0.9
−3) 0.45GF/(kg·m
0
0
−3)

PG0.1 + 0.1
M0
P0.15 1.350 0.9 2.4 00
P0.05
G0.1 0.45
0 0
PG0.15
G0.2
P0.1 + 0.05 0
0.9 1.35 4.80
G0.3 0 7.2
Note: P, G and
P0.15
PG0.05 + 0.1 PG indicate PF, 0.45
GF
1.35and PF-GF hybrid 2.4
fibers,
0 respectively
G0.1 0.90 1.22.4
represent the fiber content. For example, PG0.05 + 0.1 refers
3.64.8 to a mix of 0.
PG0.1 + 0.05
G0.2
PG0.05 + 0.15 0.450
the reference
G0.3 control group is M0.0
PG0.1 + 0.1 0.9 2.4
7.2
PG0.15 + 0.05 1.35 1.2
PG0.05
Note: P, G and + 0.1 PF, GF and PF-GF hybrid fibers,
PG indicate 0.45respectively. The subsequent numbers
2.4 represent the
PG0.1 + 0.05 0.9 1.2
2.3. Test Methods
fiber content.
group is M0.
For example, PG0.05 + 0.1 refers to a mix of 0.05% PFs and 0.1% GFs, and the reference control
PG0.05 + 0.15 0.45 3.6
2.3. TestThe effects of PFs and GFs
PG0.1 + 0.1 0.9 on the mechanical 2.4properties and
Methods
PG0.15 + 0.05 1.35 1.2
were investigated.
The effects of PFs and GFs The
on theexperimental
mechanical properties methods are
and microstructure depicted
of DSC
Note: P, G and PG indicate PF, GF and PF-GF hybrid fibers, respectively. The subsequent numbers
in
slump,themechanical,
represent
mechanical,
pore PG0.05
fiber content. For example, structure and
+ 0.1 refers
pore structure and microstructural tests.
microstructural
were investigated. The experimental methods are depicted in Figures 3–5, including slump,
to a mix tests.
of 0.05% PFs and 0.1% GFs, and
the reference control group is M0.

2.3. Test Methods

The effects of PFs and GFs on the mechanical properties and microstructure of DSC
were investigated. The experimental methods are depicted in Figures 3–5, including
slump, mechanical, pore structure and microstructural tests.

Figure 3.
3. Slump
Slumptest.
test.
Figure 3. Slump test.
Figure

Compression Flexural Tensile

Compression Flexural

Figure 4.
Figure 4. Mechanical
Mechanicaltest.
test.

Figure 4. Mechanical test.

Polymers 2023, 15, x2023,
Polymers FOR15,PEER
4675 REVIEW 6 of
6 of 20 20

MIP

SEM

FigureFigure
5. Pore
5. structure andand
Pore structure microstructure tests.
microstructure tests.

The workability
The workabilityof fresh DSC
of fresh DSC was
was assessed
assessedthrough
through slump conetests
slump cone testsaccording
according to to
GB/T 50080-2016 [50]. As illustrated in Figure 3, the slump is measured
GB/T 50080-2016 [50]. As illustrated in Figure 3, the slump is measured in mm and is as in mm and is as
precise as 1 mm.
precise as 1 mm.
The loading scheme and system for the macromechanical property tests were designed
The loading
referring scheme
to GB/T and system
50081-2019 [49]. Allfor the
tests macromechanical
were property
conducted on a TSY-2000 tests were de-
electrohydraulic
signed referring
pressure to GB/T
testing 50081-2019
machine purchased [49].
from AllZhejiang
tests were Ludaconducted
Mechanicalon a TSY-2000
Instrument Co.,electro-
Ltd.
hydraulic pressure testing machine purchased from Zhejiang Luda
(Shaoxing, China), with a maximum test force of 2000 kN (Figure 4). This machine was Mechanical Instru-
ment loaded
Co., LTD. (Shaoxing,the
by controlling China), withloading
force. The a maximum
rates fortest
theforce of 2000 kN (Figure
cube compressive strength4). This
tests
machinewerewas5–8 loaded
kN/s and by0.5–0.8
controlling
kN/s for the force. and
bending Thesplitting
loadingtensile
rates strength
for the cube
tests. compressive
The failure
strength
loadstests were
of the 5–8 kN/s
specimens wereand 0.5–0.8and
recorded, kN/s for bending
the mean and splitting
of three specimens was tensile strength
determined as
the test value. Nonstandard specimens of 100 mm × 100 mm × 100
tests. The failure loads of the specimens were recorded, and the mean of three specimens mm were selected for
cube compressive and splitting tensile strength tests and 100 mm × 100 mm × 400 mm for
was determined as the test value. Nonstandard specimens of 100 mm × 100 mm × 100 mm
flexural strength tests. The strength values are calculated as follows [49]:
were selected for cube compressive and splitting tensile strength tests and 100 mm × 100
mm × 400 mm for flexural strength tests. The strength F
Compressive strength f c =values
0.95 × are calculated as follows(1) [49]:
A
𝐹𝐹
Compressive strength 𝑓𝑓𝑐𝑐 = 0.95 ×2F (1)
Splitting tensile strength f sp = 0.85 × 𝐴𝐴 (2)
πA
2𝐹𝐹
SplittingFlexural
tensileStrength
strengthf f𝑓𝑓𝑠𝑠𝑠𝑠 Fl
= = 0.85 × (3)(2)
𝜋𝜋𝜋𝜋
bh2
where 0.95 and 0.85 are the size conversion coefficients when 𝐹𝐹𝐹𝐹 converting nonstandard test
Flexural Strength 𝑓𝑓𝑓𝑓 = 2 and splitting tensile strengths(3)
pieces into standard ones; f c and f sp present the compressive
𝑏𝑏ℎ

whereof0.95
the specimens; F is the ultimate load on the loading curve; and b, h and A denote the
and 0.85 are the size conversion coefficients when converting nonstandard test
width, height and area of the specimens, respectively.
pieces into standard ones; 𝑓𝑓𝑐𝑐 and 𝑓𝑓𝑠𝑠𝑠𝑠 present the compressive and splitting tensile
The porosity and micropore size distribution of DSC samples were analyzed using
strengths of theAfter
MIP tests. specimens;
standardFcuring
is theforultimate load onwith
28 d, a sample the aloading curve; and
10 mm diameter wasb,selected
h and A
denote thethe
from width, height
specimen and
core andarea of the
soaked specimens,
in isopropyl respectively.
anhydrous ethyl alcohol for 24 h to stop
The porosity and micropore size distribution of DSC samples
hydration. Before the tests, the samples were dried to a constant weight were analyzed
in an ovenusing
at
MIP tests.
60 C After
◦ for 48 standard curing
h [51]. Then, for were
the tests 28 d,performed
a sample onwith a 10 mm diameter
a Poremaster33 was
automatic selected
mercury
from injection
the specimen core
aperture and soaked
analyzer. in isopropyl
The aperture rangedanhydrous
from 0.003 to ethyl
1000alcohol
µm, andfor
the24 h to stop
pressure
hydration. Before the tests, the samples were dried3 to a constant weight in an oven at 60
was 30,000 Pa (Figure 5).
Before the tests, the cube specimen (100 mm ) was broken to obtain a sample with
°C for 48 h [51]. Then, the tests were performed on a Poremaster33 automatic mercury
a diameter of about 10 mm, and the obtained sample was washed and dried naturally.
injection aperture analyzer. The aperture ranged from 0.003 to 1000 µm, and the pressure
Then, it was ground and pasted on a conductive film and gold-sprayed in Cressington
was 30,000 Pa (Figure
108 coater 5). scientific instruments Ltd., Watford, UK) at 40 s injection time and
(Cressington
Before the tests, the
30 mA current to ensure cube specimen
electrical (100 mm3)SEM
conductivity. wasimages
brokenatto obtain magnifications
different a sample with a
diameter
wereof about 10
obtained mm,
using and the obtained
a JSM-7610F sample
mode field wasscanning
emission washedelectron
and dried naturally.
microscope Then,
(Japan
it wasElectronics
ground and pasted
Co., on a conductive
Ltd., Tokyo, film5).and gold-sprayed in Cressington 108 coater
Japan) (Figure
(Cressington scientific instruments Ltd., Watford, UK) at 40 s injection time and 30 mA
current to ensure electrical conductivity. SEM images at different magnifications were ob-
tained using a JSM-7610F mode field emission scanning electron microscope (Japan Elec-
tronics Co., Ltd, Tokyo, Japan) (Figure 5).
 Polymers 2023, 15, x FOR PEER REVIEW 7 of 20
Polymers 2023, 15, 4675 7 of 20

3. Results and Discussion

3. Results and Discussion
3.1.Workability
3.1. Workability
Slumpisisan
Slump animportant
importantindicator
indicatorof ofconcrete
concreteworkability
workability[52].[52].The
Theslump
slumptest testresults
results
of DSC are summarized in Figure 4, where the slump decreases
of DSC are summarized in Figure 4, where the slump decreases with the increase in fiber with the increase in fiber
content. Consistent results were reported in the previous literature.
content. Consistent results were reported in the previous literature. For example, SINGH M For example, SINGH
Mal.
et et [53]
al. [53]
foundfound
thatthat excessive
excessive fiberfiber incorporation
incorporation mitigated
mitigated the workability
the workability of theofconcrete
the con-
mix and affected fiber dispersion. It can be seen from Figure 6 that the slump ofofDSC
crete mix and affected fiber dispersion. It can be seen from Figure 6 that the slump DSC
decreases by 45.6%, 28.6% and 35.4% at the maximum GF,
decreases by 45.6%, 28.6% and 35.4% at the maximum GF, PF and hybrid-fiber contents PF and hybrid-fiber contents
of0.3%,
of 0.3%,0.15%
0.15%andand0.2%,
0.2%,respectively,
respectively,indicating
indicatingthat thatthe
thefiber
fibercontent
contentsignificantly
significantlyaffects
affects
the fluidity of DSC. The main reason is that the cement mortar
the fluidity of DSC. The main reason is that the cement mortar wraps around the fiber wraps around the fiber
surface,and
surface, andaahigher
higherfiberfibercontent
contentrequires
requiresmoremore mortar
mortar ononthethe surface,
surface, leaving
leaving less
less mor-
mortar
tarencapsulate
to to encapsulate the aggregate.
the aggregate. Consequently,
Consequently, the physical
the physical friction
friction between
between the aggre-
the aggregate
gatethe
and and the mortar
mortar is enhanced,
is enhanced, and the andDSCthefluidity
DSC fluidity is reduced,
is reduced, i.e., thei.e., the slump.
slump. A com-
A comparison
parison between
between Figure 6a,b Figure 6a,b suggests
suggests that theofslump
that the slump GFRDSC of GFRDSC
is slightlyis slightly
smaller smaller
than that than
of
that of PFRDSC under the same fiber content. As shown
PFRDSC under the same fiber content. As shown in Figure 6c, the slump of HyFRDSC in Figure 6c, the slump of
HyFRDSCwith
decreases decreases with the GF
the upgrading upgrading
content. GF Withcontent. Withcontent
0.2% fiber 0.2% fiberandcontent and the
the PF-to-GF PF-
ratio
to-GF
of ratio
1:3, the of 1:3,ofthe
slump theslump of theDSC
reinforced reinforced
is 35.4% DSC is 35.4%
lower than lower
that ofthan that of theDSC.
the reference reference
The
DSC.reasons
main The main arereasons are as(1)
as follows: follows:
With a(1) Withwater
higher a higher water absorption
absorption rate thanratePF, than PF, GF
GF surface
surface adsorbs
adsorbs less freeless
water,freeleading
water, leading
to lower toDSC
lowerfluidity.
DSC fluidity. (2) Under
(2) Under the samethe same
weight, weight,
GFs
GFs awith
with a smaller
smaller diameterdiameter thanhave
than PFs PFs ahave a greater
greater specificspecific
surface surface area, which
area, which requiresrequires
more
more cement
cement slurry to slurry
wraptoaround
wrap around the surfaces
the surfaces and the
and reduces reduces the freeslurry,
free cement cement slurry, in-
intensifying
tensifying
DSC slumpDSC slump[54].
reduction reduction [54].

Slump Slump
147 150 147
150 142

120 123
105
Slump/mm
Slump/mm

100
100 100
80

50 50

0 0
0.0 0.1 0.2 0.3 0.00 0.05 0.10 0.15
Volume content of GF/% Volume content of PF/%
(a) (b)
Slump
150 147

104 108
Slump/mm

99 100
100 95

50

0
0.1 .1+0.05 5+0.15 .1+0.1 0.05
0 0.05+ 0 0.0 0 0.15+
Volume content of PF-GF/%
(c)
Figure6.6.Slump
Figure Slumpof
ofFRDSC
FRDSCmixtures:
mixtures:(a)
(a)GFRDSC;
GFRDSC;(b)
(b)PFRDSC;
PFRDSC;(c)
(c)HyFRDSC.
HyFRDSC.
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3.2.
3.2. Failure
3.2.Failure Process
FailureProcess
Process
The
The failure
Thefailure morphologies
failuremorphologies
morphologiesof of each
ofeach specimen are
eachspecimen
specimen are shown
are shown in
shown in Figures
in Figures7–9.
Figures 7–9.In
7–9. InFigures
In Figures7a,
Figures 7a,
7a,
8a
8a and
and 9a,
9a, initial
initial cracks
cracks immediately
immediately occur
occur at
at the
the corner
corner of
of the
the reference
reference
8a and 9a, initial cracks immediately occur at the corner of the reference DSC specimen DSC
DSC specimen
specimen
under stresses
understresses
under and
stressesand gradually
andgradually penetrate
graduallypenetrate through the
penetrate through
through the specimen;
the specimen; these
specimen; thesecracks
these crackscontinue
cracks continuetoto
continue to
expand
expand in rangerange and
and width,
width, resulting
resulting in in large
large spalling
spalling areas
areas aroundaround
expand in range and width, resulting in large spalling areas around the specimen. The the specimen.
the specimen. The DSCThe
DSC
DSC specimens
specimens
specimens
underunder tensile
tensile andbending
and bending
under tensile and bending
loads loads suddenly
suddenly
loads suddenly
break break
into two into
break into twowhen
parts
two partsthe
parts when the
loads
when the
loads
reach reach
the the bearing
bearing capacity,
capacity, and and
obviousobvious
brittle brittle
failure failure characteristics
characteristics are
loads reach the bearing capacity, and obvious brittle failure characteristics are observed. are
observed.observed.

(a) (b) (c) (d)

(a) (b) (c) (d)
Figure
Figure7.7.7.Compressive
Compressive failure morphologies. (a) Reference DSC; (b) PFRDSC; (c) GFRDSC; (d)
Figure Compressivefailure morphologies.
failure (a) (a)
morphologies. Reference DSC;
Reference (b) (b)
DSC; PFRDSC; (c) (c)
PFRDSC; GFRDSC; (d)
GFRDSC;
HyFRDSC.
HyFRDSC.
(d) HyFRDSC.

(a) (b)
(a) (b)

(c) (d)
(c) (d)
Figure 8. Flexural failure morphologies. (a) Reference DSC; (b) PFRDSC; (c) GFRDSC; (d)
Figure
Figure8.8. Flexural
HyFRDSC. Flexuralfailure
failure morphologies.
morphologies. (a) Reference
(a) Reference DSC;
DSC; (b) (b) PFRDSC;
PFRDSC; (c) (d)
(c) GFRDSC; GFRDSC; (d)
HyFRDSC.
HyFRDSC.
Polymers 2023,15,
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x FOR PEER REVIEW 9 9ofof20
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(a) (b) (c) (d)

Figure 9.
Figure 9. Splitting
Splittingtensile
tensilefailure
failuremorphologies.
morphologies.(a) (a)
Reference DSC;
Reference (b) (b)
DSC; PFRDSC; (c) GFRDSC;
PFRDSC; (d)
(c) GFRDSC;
HyFRDSC.
(d) HyFRDSC.

After adding
After adding fibers,
fibers, the
the damage
damage characteristics
characteristics of of the
the specimen
specimen changed
changed greatly.
greatly. AsAs
shown in Figure 7b–d, the original cracks mostly appear at the corners
shown in Figure 7b–d, the original cracks mostly appear at the corners under compression under compression
loading and
loading and expand
expand longitudinally,
longitudinally, and and the
the “sizzle”
“sizzle” sound
sound of of fiber
fiberfracture
fracturecan canbebeheard.
heard.
The crack range and width of the specimen are smaller than those
The crack range and width of the specimen are smaller than those of the reference DSC, of the reference DSC,
and
and only a few fragments are spalling, indicating good integrity.
only a few fragments are spalling, indicating good integrity. When the ultimate bearing When the ultimate bear-
ing capacity
capacity is reached,
is reached, thethe specimen
specimen does
does notbreak
not breaksuddenly
suddenlyand and shows
shows certain
certain ductility
ductility
instead. The
instead. The loading
loading speed
speed ofof the
the press
press machine
machine slowly
slowly drops
drops to to zero.
zero.
Figure 8b–d
Figure 8b–d demonstrate
demonstrate that that microcracks
microcracks appear
appear on on the
the surface
surface of of DSC
DSC specimens
specimens
considering the flexural strength, accompanied by fiber fracture sounds weaker thanthose
considering the flexural strength, accompanied by fiber fracture sounds weaker than those
upon breakage.
upon breakage. When Whenthe thespecimen
specimen sustains
sustainsdamage
damage under
under continuous
continuous loading, crack
loading, ex-
crack
pansion and penetration occur, and the specimen breaks into two
expansion and penetration occur, and the specimen breaks into two parts with a large parts with a large split-
ting sound.
splitting sound.Additionally, thethe
Additionally, loading
loading speed
speeddecreases
decreases totozero,
zero,andandthethespecimen
specimenshows
shows
obvious plastic
obvious plastic failure
failure characteristics.
characteristics.
It can
It can be be seen
seen from
from Figure
Figure 9b–d9b–d that
that the
the splitting
splitting tensile
tensile damage
damage morphologies
morphologies of of
mono- and
mono- andhybrid-FRDSC
hybrid-FRDSC specimens
specimens are are consistent.
consistent. With With the increasing
the increasing load, mi-
load, microcracks
crocracks
emerge in emerge
the middle in the
of middle of the specimen
the specimen with the
with the sound soundpulling
of fibers of fibersout pulling out and
and breaking.
breaking. Upon the ultimate bearing capacity, these small cracks
Upon the ultimate bearing capacity, these small cracks evolve into multiple macrocracks evolve into multiple
macrocracks
and and failthe
fail to penetrate tospecimen,
penetrate resulting
the specimen, resulting
in relatively in relatively
complete damage complete
withoutdamage
obvious
without obvious
fragmentation. fragmentation.
Similarly, the loadingSimilarly,
speedthe loading
slowly speed
drops slowly
to zero, anddrops to zero, and
the specimen the
exhibits
specimen
plastic exhibits
failure plastic failure characteristics.
characteristics.

3.3. Strength
3.3. Strength Analysis
Analysis
3.3.1. Compressive Strength
3.3.1. Compressive Strength
Figure
Figure 88 shows
shows that
that compared
compared with the reference
with the DSC, the
reference DSC, the compressive
compressive strength
strength ofof
FRDSC decreases first and increases before decreasing again with the
FRDSC decreases first and increases before decreasing again with the increasing fiber con-increasing fiber con-
tent.
tent. DSC
DSC specimens
specimens with 0.05% PFs
with 0.05% PFs oror 0.1%
0.1% GFs
GFs exhibit
exhibitreduced
reducedcompressive
compressivestrength
strengthby by
1.4% and 3.1%, respectively. In contrast, those with more PFs (0.1% and
1.4% and 3.1%, respectively. In contrast, those with more PFs (0.1% and 0.15%) or GFs 0.15%) or GFs (0.2%
and
(0.2%0.3%)
and display an increased
0.3%) display compressive
an increased strength
compressive (9.6% (9.6%
strength and 6.2%
andvs.
6.2%6.7%
vs. and
6.7%0.5%).
and
According to the above analysis, an appropriate fiber content has the
0.5%). According to the above analysis, an appropriate fiber content has the optimal en- optimal enhancing
effect
hancingon effect
DSC. on Excessively low or high
DSC. Excessively low orfiber contents
high adversely
fiber contents affect the
adversely compressive
affect the com-
strength of DSC; the former is insufficient for the fiber to fully play
pressive strength of DSC; the former is insufficient for the fiber to fully play a leading a leading role inrole
the
matrix crack expansion, and the latter leads to poor fiber dispersion
in the matrix crack expansion, and the latter leads to poor fiber dispersion and agglomer-and agglomeration.
The weak
ation. Thelayers
weak in the interfacial
layers transition
in the interfacial zone ofzone
transition DSCof are increased,
DSC and theand
are increased, compressive
the com-
strength decreases. LIN et al. [25] reported that that the compressive
pressive strength decreases. LIN et al. [25] reported that that the compressive strength strength of someof
PP-PVA-reinforced concrete was lower than those of PVA-fiber-reinforced
some PP-PVA-reinforced concrete was lower than those of PVA-fiber-reinforced concrete concrete alone
due to excessive fiber admixture.
alone due to excessive fiber admixture.
Figure 10a,b suggest that PFs have a significantly better effect on DSC compressive
Figure 10a,b suggest that PFs have a significantly better effect on DSC compressive
strength than GFs. As shown in Figure 8c, the compressive strength of HyFRDSC shows an
strength than GFs. As shown in Figure 8c, the compressive strength of HyFRDSC shows
increasing trend overall. With a hybrid-fiber content of 0.15% and a 2:1 PF-to-GF ratio, the
an increasing trend overall. With a hybrid-fiber content of 0.15% and a 2:1 PF-to-GF ratio,
compressive strength of DSC maximizes and surpasses that of the reference one by 9.1%.
the compressive strength of DSC maximizes and surpasses that of the reference one by
With a hybrid-fiber content of 0.2% and a PF-to-GF ratio of 1:1, the compressive strength of
9.1%. With a hybrid-fiber content of 0.2% and a PF-to-GF ratio of 1:1, the compressive
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strength of DSC is increased by 7.2%. Moreover, the positive effect of the hybrid fiber is
fullyDSC
achieved, andby
is increased the7.2%.
crack development
Moreover, is restricted.
the positive effect of the hybrid fiber is fully achieved,
and the crack development is restricted.

48 50 15
Compressive strength 10 Compressive strength
Rate of strength improvement Rate of strength improvement

Rate of strength improvement/%

Rate of strength improvement/%

Compressive strength/MPa

Compressive strength/MPa
45 44.7 45.9 10
45 44.5
5
41.9 42.1
42 41.9 5
41.3
40.6

0 40
39 0

36 -5 35 -5
0.0 0.1 0.2 0.3 0.00 0.05 0.10 0.15
Volume content of GF/% Volume content of PF/%

(a) (b)
50 15
Compressive strength
Rate of strength improvement

Rate of strength improvement/%

Compressive strength/MPa

45.7 10
44.9
45
43.33
42.3
41.9 5
41

40
0

35 -5
0.1 .1+0.05 5+0.15 .1+0.1 0.05
0 0.05+ 0 0.0 0 0.15+
Volume content of PF-GF/%

(c)
Figure 10. 10.
Figure Compressive strength.
Compressive strength.(a)
(a)GFRDSC; (b)PFRDSC;
GFRDSC; (b) PFRDSC;(c)(c) HyFRDSC.
HyFRDSC.

3.3.2. Flexural
3.3.2. Strength
Flexural Strength
As shown in Figure 11, the flexural strength of the reference DSC is 3.39 MPa, and the
As shown in Figure 11, the flexural strength of the reference DSC is 3.39 MPa, and
fibers considerably improve the flexural strength, which increases first and then decreases
the with
fiberstheconsiderably improve the flexural strength, which increases first and then de-
elevating fiber content. For 0.1% PFs or 0.2% GFs, the DSC flexural strength
creases
increases by 23.9% or 17.4%,fiber
with the elevating content.The
respectively. For 0.1%is that
reason PFs the
or tightly
0.2% bonded
GFs, the DSC
fiber andflexural
the
strength increases
cement-based by 23.9%
composite or 17.4%,
material bear respectively. The generated
part of the stresses reason is by
that
thethe
DSCtightly bonded
shrinkage
fiberand
and the cement-based
deformation composite
during loading material
with the bear part
appropriately of the fiber
increasing stresses generated
content, by the
inhibiting
DSCtheshrinkage and of
development deformation
small cracksduring loading with
and improving the appropriately
the flexural strength. Theincreasing
enhancingfiber
content, inhibiting the development of small cracks and improving the flexural
effect of PFs on DSC flexural strength is slightly greater than that of GFs, as strength.
illustrated
The enhancing effect of PFs on DSC flexural strength is slightly greater than that
in Figure 11a,b. Figure 11c showcases that the hybrid fibers noticeably enhance of GFs,
flexural
strength. With a hybrid-fiber content of 0.2% and a PF-to-GF ratio of 1:1, the DSC flexural
as illustrated in Figure 11a,b. Figure 11c showcases that the hybrid fibers noticeably en-
strength is increased by 40.7%, achieving an optimal enhancing effect. Furthermore, the
hance flexural strength. With a hybrid-fiber content of 0.2% and a PF-to-GF ratio of 1:1,
effect of the hybrid fibers on the DSC flexural strength is better than that of monofibers.
the DSC flexural strength is increased by 40.7%, achieving an optimal enhancing effect.
Furthermore, the effect of the hybrid fibers on the DSC flexural strength is better than that
of monofibers.
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5
Rupture strength 30
Rate of strength improvement

Rate of strength improvement/%

4.2 25

Rupture strength/MPa
4 3.94
20
3.61
3.39
15

3
10

2 0
0.00 0.05 0.10 0.15
Volume content of PF/%

(a) (b)

(c)
Figure 11. Flexural
Figure strength.
11. Flexural (a)(a)GFRDSC;
strength. GFRDSC;(b)
(b) PFRDSC; (c)HyFRDSC.
PFRDSC; (c) HyFRDSC.

3.3.3.
3.3.3. Splitting
Splitting Tensile
Tensile Strength
Strength
Figure 12 shows that adding fibers dramatically improves the DSC splitting tensile
Figure 12 shows that adding fibers dramatically improves the DSC splitting tensile
strength. As seen in Figure 12a,b, the DSC splitting tensile strength increases with the
strength. As of
addition seen
GFsin orFigure 12a,b, athe
PFs, showing DSC
first splitting
increasing andtensile strength increases
then decreasing trend. Thewith the ad-
splitting
dition of GFs
tensile or PFs,
strength showingare
increments a the
firstlargest
increasing
in DSCand then decreasing
specimens with 0.1% trend. The splitting
PFs or 0.2% GFs,
tensile strength increments are the largest in DSC specimens with
reaching 17.65% and 10.70%, respectively. This is attributed to the fact that the 0.1% PFs or evenly
0.2% GFs,
reaching 17.65%
dispersed andin10.70%,
fibers respectively.
the matrix This isrole
play a bridging attributed to the fact
in transferring thatrelieving
loads, the evenlythe dis-
persed fibers in the matrix play a bridging role in transferring loads, relieving the concen-
concentrated stress at the crack edge and maintaining the uniform and continuous matrix
trated stress
stress. at the crackthe
Consequently, edgeDSCand maintaining
splitting the uniform
tensile strength and continuous
is improved. matrix stress.
With the increase in
fiber content, the weak transition zone at the DSC interface expands,
Consequently, the DSC splitting tensile strength is improved. With the increase in fiber resulting in stress
concentration at the crack and splitting tensile strength deterioration. Research [40] found
content, the weak transition zone at the DSC interface expands, resulting in stress concen-
that the splitting tensile strength of fiber-reinforced concrete containing 1.35% PF was
tration
72.8%at lower
the crack
than and splitting
that of tensile which
plain concrete, strength
wasdeterioration.
mainly due to the Research
high-fiber[40] found that
admixture.
the splitting
Figure 12c shows that the hybrid fibers significantly enhance the DSC splitting was
tensile strength of fiber-reinforced concrete containing 1.35% PF 72.8%
tensile
lower than that
strength. of presence
In the plain concrete,
of 0.15%which was mainly
hybrid-fiber contentdue and to2:1the high-fiber
PF-to-GF ratio,admixture.
the splitting Fig-
ure tensile
12c shows
strengththat the hybrid
increases fibers
by 17.11%, significantly
exhibiting an optimal enhance the effect.
enhancing DSC splitting tensile
strength. In the presence of 0.15% hybrid-fiber content and 2:1 PF-to-GF ratio, the splitting
tensile strength increases by 17.11%, exhibiting an optimal enhancing effect.
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5 5
Splitting tensile strength Splitting tensile strength 15
Rate of strength improvement Rate of strength improvement

Rate of strength improvement/%

Rate of strength improvement/%

Splitting tensile strength/MPa

Splitting tensile strength/MPa

15 4.4

4.01 10
4.14 4 3.94
10 3.74
4
3.74 3.75 3.78

5
5
3

3 0 0
0.0 0.1 0.2 0.3 0.00 0.05 0.10 0.15
Volume content of GF/% Volume content of PF/%

(a) (b)

(c)
Figure 12. Splitting
Figure tensile
12. Splitting strength.
tensile strength.(a)
(a)GFRDSC; (b) PFRDSC;
GFRDSC; (b) PFRDSC;(c)(c)HyFRDSC.
HyFRDSC.

In conclusion, thethe
In conclusion, hybrid
hybridfibers
fiberswith
with different contentsand
different contents and mixing
mixing ratios
ratios significantly
significantly
improve the DSC splitting tensile strength and outperform the monofibers. Forthing,
improve the DSC splitting tensile strength and outperform the monofibers. For one one thing,
a
higher PF content has a “bleed air” effect, which increases the small and stable bubbles in the
a higher PF content has a “bleed air” effect, which increases the small and stable bubbles
specimen and inhibits the development of early primary cracks. For another, the distributed
in the specimen and inhibits the development of early primary cracks. For another, the
GF bears the tensile stress of the matrix, hindering the development of macrocracks in the
distributed GFThis
later stage. bears the tensilecan
combination stress of theimprove
effectively matrix,the
hindering the development
DSC splitting of mac-
tensile strength.
rocracks in the later stage. This combination can effectively improve the DSC splitting
tensile strength.
3.3.4. Flexural/Compressive and Tensile/Compressive Strength Ratios
• Flexural/compressive strength ratio.
3.3.4. Flexural/Compressive andstrength
The flexural/compressive Tensile/Compressive Strength
ratio roughly measures theRatios
concrete toughness [55],
• Flexural/compressive
and its variation trend with strength ratio. is depicted in Figure 13. The bending pressure of
fiber contents
each fiber-reinforced group is significantly higher than that of the reference DSC. In contrast,
The flexural/compressive strength ratio roughly measures the concrete toughness
a higher fiber content exhibits a stronger compressive strength and flexural strength of
[55],DSC,
and and
its variation trend with fiber contents is depicted in Figure 13. The bending pres-
the effect on the flexural strength is significantly greater than that of compressive
surestrength.
of each Akca
fiber-reinforced
et al. [34] alsogroup
found is significantly
that the effect of higher than
PF on the that strength
flexural of the reference
of concreteDSC.
In contrast, a higher fiber content exhibits
was greater than on the compressive strength. a stronger compressive strength and flexural
strength of DSC, and the effect on the flexural strength is significantly greater than that of
compressive strength. Akca et al. [34] also found that the effect of PF on the flexural
strength of concrete was greater than on the compressive strength.
Polymers 2023, 15, x FOR PEER REVIEW 13 of 20

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(a) (b)

(c)
Figure 13.13.
Figure Flexural/compressive strength
Flexural/compressive strengthratio.
ratio.(a)
(a)GFRDSC;
GFRDSC;(b)
(b)PFRDSC;
PFRDSC; (c)
(c) HyFRDSC.
HyFRDSC.

It is
It indicated
is indicated thatthat
the flexural/compressive
the flexural/compressive strength ratio is
strength improved
ratio in a first
is improved in increas-
a first
ingincreasing
and then and decreasing trend with
then decreasing thewith
trend increasing fiber content.
the increasing Figure 13a,b
fiber content. Figureshow
13a,bthat
showthe
flexural/compressive strength ratio in 0.2% GF or 0.1% PF content maximizes at 9.88%,
that the flexural/compressive strength ratio in 0.2% GF or 0.1% PF content maximizes
13.58% higher
at 9.88%, than higher
13.58% the reference
than theDSC. FigureDSC.
reference 11c demonstrates that with the
Figure 11c demonstrates hybrid-fiber
that with the
hybrid-fiber content of 0.2% and the GF-to-PF ratios of 1:3 and 1:1, the flexural/compressive
content of 0.2% and the GF-to-PF ratios of 1:3 and 1:1, the flexural/compressive strength ratio
strength ratio maximizes, reaching an increase of 30.86% in comparison to the reference
maximizes, reaching an increase of 30.86% in comparison to the reference DSC. In sum, add-
DSC. In sum, adding fibers significantly improves the DSC flexural strength, and the
ing fibers significantly improves the DSC flexural strength, and the compressive strength
compressive strength improvement remains relatively stable, suggesting an improved
improvement remains relatively stable, suggesting an improved flexural/compressive
flexural/compressive strength ratio and toughness.
strength ratio and toughness.
• Tensile/compressive strength ratio.
• Tensile/compressive strength ratio.
The tensile/compressive strength ratio assesses the brittleness of concrete. A smaller
The tensile/compressive strength ratio assesses the brittleness of concrete. A smaller
tensile/compressive strength ratio means greater brittleness and lower toughness [56].
tensile/compressive strength ratio means greater brittleness and lower toughness [56]. Fig-
Figure 14 reveals the significant increase in FRDSC tensile/compressive strength ratios.
ure 14 reveals the significant increase in FRDSC tensile/compressive strength ratios. As
As shown in Figure 14a,b, the growth rates of tensile/compressive strength ratios are the
shown
highestin inFigure 14a,b,with
specimens the0.2%
growth
GFs rates
or 0.1%of PFs,
tensile/compressive
reaching 9.88% and strength
13.58%,ratios are the
respectively,
highest in specimens with 0.2% GFs or 0.1% PFs, reaching 9.88% and 13.58%,
suggesting that PFs have a more significant effect on DSC splitting tensile strength than GFs. respectively,
suggesting
As shownthat PFs have
in Figure a more
12c, the significant effect
tensile/compressive on DSC
strength splitting
ratio growthtensile strength
rates in than
each group
GFs. As shown in Figure 12c, the tensile/compressive strength ratio
with GFs or PFs is above 10% except for the 0.15% GF and 0.05% PF groups, and the highestgrowth rates in each
group with GFs
is 30.86%. or PFs ishybrid
In contrast, above fibers
10% except
deliverfora the 0.15%
better GFonand
effect the0.05% PF groups, and
tensile/compressive
thestrength
highestratiois 30.86%. In contrast,The
than monofibers. hybrid
GF-PF fibers
hybriddeliver a betterimproves
combination effect on the
thecompressive
tensile/com-
pressive strength ratio than monofibers. The GF-PF hybrid combination improves the
Polymers 2023, 15, x FOR PEER REVIEW 14 of 20
Polymers 2023, 15, 4675 14 of 20

compressive and splitting tensile strength of the DSC matrix and effectively overcomes
theand
shortcomings of high
splitting tensile brittleness
strength andmatrix
of the DSC low toughness.
and effectively overcomes the shortcomings
of high brittleness and low toughness.

(a) (b)

(c)
Figure 14.14.
Figure Tensile/compressive strength
Tensile/compressive strengthratio.
ratio.(a)
(a)GFRDSC;
GFRDSC;(b)
(b)PFRDSC;
PFRDSC; (c)
(c) HyFRDSC.
HyFRDSC.

The Thevariation trends

variation trendsofof
the flexural/compressive
the flexural/compressive and tensile/compressive
and tensile/compressive strength ra-
strength
tios are insufficient to determine whether a higher fiber content leads to more DSC defects,
ratios are insufficient to determine whether a higher fiber content leads to more DSC defects,
which
which reduce
reducethe
thecompressive
compressive strength andaffect
strength and affectthe
theflexural
flexural strength
strength andand splitting
splitting ten-
tensile
sile strength. However, the flexural/compressive and tensile/compressive strength ratios
strength. However, the flexural/compressive and tensile/compressive strength ratios are
arestill
stillhigher
higherthan those
than of the
those reference
of the DSC.
reference Mixing
DSC. a proper
Mixing quantity
a proper of PF of
quantity and/or GF into
PF and/or GF
DSC improves the concrete toughness.
into DSC improves the concrete toughness.
3.4. Pore Structure
3.4. Pore Structure
Specimen groups P0.1, G0.2, PG0.1 + 0.5 and M0 (control group) were selected for
Specimen
mercury groups
injection P0.1, pressure
capillary G0.2, PG0.1
tests,+ and
0.5 the
andeffects
M0 (control
of pore group) wereand
distribution selected for
porosity
mercury injection capillary pressure tests, and the effects of pore distribution
on the mechanical properties of specimens under the optimal fiber content were studied and porosity
onthrough
the mechanical properties
microstructure of specimens under the optimal fiber content were studied
analysis.
through Asmicrostructure
shown in Figure analysis.
15, the porosity and average pore size are significantly reduced
As shown
compared in Figure
to the 15,DSC.
reference the porosity and the
For instance, average pore
porosity of size
PFDSCareand
significantly reduced
GFDSC decreases
compared
by 10.7%to the5.93%,
and reference DSC. ForThe
respectively. instance,
porosity theofporosity
HyFRDSC of PFDSC and GFDSCby
drops significantly decreases
50.01%.
by 10.7% and 5.93%, respectively. The porosity of HyFRDSC drops significantly by
The average pore sizes of PFDSC and GFDSC are 31.04 nm and 35.74 nm, respectively,
50.01%.
reducingTheby average pore18.68%
29.4% and sizes of PFDSC and
compared withGFDSC are 31.04
the reference DSC.nmTheand 35.74 pore
average nm, size
respec-
of
tively, reducing
HyFRDSC by 29.4% most
is decreased and 18.68% compared
markedly by 33.61%. withItthe referencethat
is indicated DSC. The average
hybrid pore
fibers have
size
theofbest
HyFRDSC is decreased
effect mainly most
due to the markedly
addition by 33.61%.GFs
of hydrophilic It isincreasing
indicated the
thatwater–binder
hybrid fibers
have the best effect mainly due to the addition of hydrophilic GFs increasing the water–
Polymers 2023, 15, x FOR PEER REVIEW 15 of 20
Polymers2023,
Polymers 15,4675
2023,15, x FOR PEER REVIEW 15
15 of 20
of 20

binder ratio. In so doing, the binding strength of GFs and cement substrate is strong, sig-
binder
ratio. Inratio.
nificantly In sothe
so doing,
reducing doing, the binding
binding
the porosity.
strengthstrength
PFs have good of
of GFs andGFs andsubstrate
cement
dispersion cement
and smallsubstrate
is strong,issignificantly
fiber spacing,strong, sig-
allow-
nificantly
reducing
ing increasedreducing
the porosity.the porosity.
PFs
DSC density
have PFs haveporosity.
good good dispersion
dispersion
and reduced
and small and small
fiber
A reasonable
spacing,fiber spacing,
allowing
hybrid-fiber allow-
increased
proportion
ing
DSC
can increased
density and
give DSC
full play density
reduced
to the and reduced
porosity.
positive porosity.
A reasonable
hybridrefine
effect, A reasonable
hybrid-fiber hybrid-fiber
proportion can giveproportion
full play
to the
can positive
give full hybrid
play effect,
to the effectively
positive theeffectively
hybrid effect, effectively
refineimprove
pore structure,
refine
the porethe
structure, im-
density and
the pore structure, im-
prove
reduce the density
the density and
porosityand reduce
andreduce the
average porosity and average pore size.
prove the thepore size. and average pore size.
porosity

Figure 15. Porosity and average pore diameter of FRDSC.

Figure15.
Figure 15. Porosity
Porosityand
andaverage
average pore
porediameter
diameterof
ofFRDSC.
FRDSC.
Figure 16 shows the pore size distribution and peak pore size (all below 50 nm) of
Figure16
Figure
the specimens. 16shows
shows
It can the
betheseen
poresize
pore size
from distribution
distribution
Figure and
and
16 that peak
peak
the pore
pores pore
sizesize
with (all
pore(all
belowbelow 50greater
50 nm)
structure nm)
of theof
the specimens.
specimens. It canIt can
be be
seen seen
from from
FigureFigure
16 16
that that
the the
pores pores
with
than 200 nm account for 34.36% of the total pore volume. Compared with the reference with
pore pore structure
structure greatergreater
than
thannm
200
DSC, 200
the nm account
account
pore volume for
for 34.36%
ratio 34.36%
of the of
in the thepore
total
small totalvolume.
pore pore volume.
segment belowCompared
Compared 20 nmwithand with
the thenm
reference
20–50 reference
isDSC,
sig-
DSC,
the the
pore pore
volume volume
ratio in ratio
the in
smallthe small
pore pore
segment segment
below 20below
nm 20
and
nificantly increased in the DSC mixed with fiber, while the pore volume ratio in the harm- nm
20–50 andnm 20–50
is nm is sig-
significantly
nificantly
increased increased
in the DSC in the
mixed DSCwith mixed
fiber,with
whilefiber,
the while
pore the pore
volume
ful pore segment is correspondingly decreased in the large pore segment. Hybrid fibers volume
ratio in ratio
the in the
harmful harm-
pore
ful increase
can pore is
segment segment
the volumeis correspondingly
correspondingly ratiodecreased decreased
in
of pores below the large
20 nm in the
pore
to large and
segment.
22.46% pore segment.
Hybrid
reduce the Hybrid
fibers fibers
can increase
volume ratio
can
the increase
volume the
ratio volume
of pores ratio
belowof pores
20 nm below 20
to 22.46% nm to
and 22.46%
reduceand
of pores above 200 nm to 22.77%. In this sense, fiber reinforcement can improve the inter-
thereduce
volume the volume
ratio ratio
of pores
of pores
above 200above
nm to200 nm
22.77%. to 22.77%.
In In
this sense, this sense,
fiber fiber reinforcement
reinforcement can improve
nal pores of DSC, effectively limit the formation of harmful pores and reduce the conver- can improve
the internal the inter-
pores
nalDSC,
of
sion pores of DSC,
effectively
of harmful poreseffectively
limit the limit theless
formation
to harmless and formation
harmfulofones
of harmful harmful
pores and pores
[39,40]. Theand
reduce reduce the conver-
the conversion
pore size distribution
of
ofsion
harmfulof harmful
HyFRDSC
pores poresthan
to
is better to harmless
harmless and and less harmful onesDSC,
less harmful
that of monofiber-reinforced
ones [39,40].
[39,40]. The The porethat
pore
indicating
size sizea distribution
distribution
reasonable
of
of HyFRDSC
HyFRDSC
hybrid-fiber is better
is better
content
than
canthan that
that
fully
ofof monofiber-reinforced
monofiber-reinforced
reduce the large pores in the DSC,
DSC, indicating
indicating
DSC matrix and thata
that areasonable
reasonable
improve the
hybrid-fiber
hybrid-fiber content
content
internal compactness. can
can fully
fully reduce
reduce the
the large
large pores
pores in
in the
the DSC
DSC matrix
matrix and
and improve
improve the
the
internal compactness.
internal compactness.

Figure
Figure 16.
16. Pore
Pore size
size distribution
distribution and
and peak
peak pore
pore diameter.
diameter.
Figure 16. Pore size distribution and peak pore diameter.
Polymers 2023,
Polymers 15, 4675
2023, 15, x FOR PEER REVIEW 1616of
of 20
20

3.5.
3.5. Micromorphological
Micromorphological Analysis
Analysis
3.5.1.
3.5.1. Cement
Cement Slurry
Slurry Microstructure
Microstructure
The
The cement
cement slurry
slurry microstructure is presented
microstructure is presented in in Figure
Figure 15.
15. The
The microstructure
microstructure of of the
the
reference DSC is
reference DSC is significantly
significantly different
different from
from that
that of
of the
the fiber-reinforced DSC. As
fiber-reinforced DSC. As shown
shown inin
Figure 17a, the internal structure of the reference DSC under magnification of 300 times is
Figure 17a, the internal structure of the reference DSC under magnification of 300 times is
loose, and the bond between cement and fine fine aggregate is not tight enough, with obvious
pores and microcracks. Figure 17b shows the micromorphology of the hydration products
in the reference
in the reference DSC.
DSC. Calcium
Calcium silicate
silicate hydrate
hydrate (C-S-H),
(C-S-H), Ca(OH)
Ca(OH)22 crystal
crystal (CH)
(CH) andand aft-
aft-
ettringite
ettringite (AFt)
(AFt) generated
generated during hydration show
during hydration flocculent and
show flocculent and honeycomb
honeycomb structures
structures
under 4000××magnification.
under 4000 magnification.AAlarge largenumber
numberof of C-S-H
C-S-H gels
gels bind
bind and
and cling, covering and
cling, covering and
wrapping crystals (i.e., CH). In addition, the independent aggregate, cement and
wrapping crystals (i.e., CH). In addition, the independent aggregate, cement other
and other
hydration
hydration products
products in in the
the matrix
matrix are tightly bonded
are tightly bonded andand form
form aa spatial
spatial grid
grid structure
structure to
to
maintain the DSC strength. However, the original dense stress structures of some
maintain the DSC strength. However, the original dense stress structures of some thin
thin
interfacial DSC
interfacial DSC cause
cause macromechanical
macromechanical properties
properties to to deteriorate
deteriorate [51–53].
[51–53].

Hole AFt

Hole

Aggregate CH

10 μm 10 μm

(a) (b)

AFt Cracks

Hole
CH

CH AFt
Hole Hole

Cement matrix
10 μm 10 μm 100 μm

(c) (d) (e)

Figure 17. SEM
Figure 17. SEM images
images of
of FRDSC:
FRDSC: (a)
(a) reference
reference DSC;
DSC; (b)
(b) hydration
hydration product
product of
of the
the reference
reference DSC;
DSC;
(c) GFRDSC; (d) PFRDSC; (e) HyFRDSC.
(c) GFRDSC; (d) PFRDSC; (e) HyFRDSC.

According to Figure
According to 17c–e, the
Figure 17c–e, the fibers
fibers improve
improve the
the overall
overall microstructure
microstructure density
density and
and
uniformity
uniformity of the cement. The FRDSC surface is smoother and denser than that
of the cement. The FRDSC surface is smoother and denser than that of the
of the
reference DSC. Due to the good hydration reaction of the gelled material and the
reference DSC. Due to the good hydration reaction of the gelled material and the fillingfilling
and plugging of
and plugging of various
various hydrates,
hydrates, the
the sizes
sizes of
of pores
pores and
and microcracks
microcracks in
in the matrix are
the matrix are
obviously reduced, consistent with the phenomenon found in the literature. This is similar
Polymers 2023, 15, x FOR PEER REVIEW 17 of 20
Polymers 2023, 15, 4675 17 of 20

to the phenomenon found in the microscopic study of steel-fiber–PF-reinforced concrete

obviously reduced, consistent with the phenomenon found in the literature. This is similar
intothe
theliterature
phenomenon [19].found in the microscopic study of steel-fiber–PF-reinforced concrete in
the literature [19]. GF specimens contain substantial C-S-H gels, of which the joints are
Both PF and
joinedBothandPF setand
together. In this case,
GF specimens hydration
contain products
substantial C-S-H and
gels,solute
of whichparticles are tightly
the joints are
packed withset
joined and C-S-H gels to
together. Inform a dense
this case, and continuous
hydration products phase. As illustrated
and solute particles are in Figure
tightly 15,
the HyFRDSC
packed microstructure
with C-S-H gels to formisa the densest
dense and mostphase.
and continuous uniformAs of all specimens.
illustrated in Figure The
17e,C-S-
Hthegels are alsomicrostructure
HyFRDSC well developed, and
is the the solidified
densest and most gels exhibit
uniform of allaspecimens.
closed bonding surface
The C-S-H
and
gelsembed
are alsowith
welleach other, granting
developed, the cement
and the solidified gelsslurry strong
exhibit bonding
a closed bonding properties.
surface and
embed with each other, granting the cement slurry strong bonding properties.
3.5.2. Fiber Microstructure and Toughening Mechanism
3.5.2. Fiber Microstructure and Toughening Mechanism
Figure 16 shows the micromorphology of fibers. In Figure 18a, the uniformly and
Figure 16 shows the micromorphology of fibers. In Figure 18a, the uniformly and
randomly distributed PF filaments adhere to the DSC matrix through the “bridging” ef-
randomly distributed PF filaments adhere to the DSC matrix through the “bridging” effect,
fect, which improves the structural integrity and strength. In addition, PFs break due to
which improves the structural integrity and strength. In addition, PFs break due to yielding
yielding
with the with the increasing
increasing stress. in
stress. As shown AsFigure
shown 18b,inGFs
Figure 16, aGFs
exhibit exhibit
fracture a fracture
failure mode with failure
mode
obvious pull-out traces, which are induced by the phenomenon that the internal stress inter-
with obvious pull-out traces, which are induced by the phenomenon that the in
nal
GFsstress
exposedin GFs exposed
to early to early
loading loading
is greater than is
thegreater thanforce
interfacial the between
interfacialtheforce
fiber between
and DSC the
fiber and DSC
and smaller and
than smaller
the thanload.
fiber yield the fiber
Whenyield load.increases,
the load When the theload increases,
internal stress isthe internal
greater
stress is greater
than the than
yield load ofthe yield
GFs, loadtooffracture
leading GFs, leading to fracture
and failure [39]. and failure [39].

Polypropylene Pullout Trace

Fiber

Glass Fiber Fracture

80 μm 80 μm

(a) (b)
Figure
Figure18.
18. Fiber
Fiber micromorphology. (a)PFs;
micromorphology. (a) PFs;(b)
(b)GFs.
GFs.

Under
Under thethe loading conditions,internal
loading conditions, internalstresses
stressesinindifferent
different directions
directions appear
appear in the
in the
DCS matrix. The reference DSC matrix bears the external load alone, and the cracks de-
DCS matrix. The reference DSC matrix bears the external load alone, and the cracks develop
velop
from from the inside
the inside to the to the surface
surface with
with the the progressive
progressive loading,loading,
breaking breaking the specimen.
the specimen. The
resistance stress of FRDSC mainly includes the interfacial adhesion
The resistance stress of FRDSC mainly includes the interfacial adhesion between thebetween the matrix andma-
the fiber, the tensile strength of the fibers, the friction force between the aggregates and
trix and the fiber, the tensile strength of the fibers, the friction force between the aggregates
the bearing capacity of the matrix. With the increasing internal stress, independent and
and the bearing capacity of the matrix. With the increasing internal stress, independent
incomplete microcracks appear in the FRDSC matrix. Since the elastic modulus of the fiber
and incomplete microcracks appear in the FRDSC matrix. Since the elastic modulus of the
is greater than that of the DSC matrix, internal stress tends to appear in the microcracks,
fiber
which is develop
greater rapidly.
than that of the
Figure 18aDSC
showsmatrix,
that when internal
thesestress tends expand
microcracks to appear in fiber,
to the the mi-
crocracks, which develop rapidly. Figure 18a shows that when these microcracks
the fiber exerts its “bridging” effect. If the internal stress is below the PF yield load, it expand
toabsorbs
the fiber,
andthe fiber exerts
transmits its “bridging”
the remaining effect.
stress and If theotherwise.
fractures internal stress is point,
At this belowthetheenergy
PF yield
load, it absorbs and transmits the remaining stress and fractures otherwise.
released from the PF broken end is dispersed in a “ring” manner to the matrix and the At this point,
the energy released from the PF broken end is dispersed in a “ring” manner
surrounding fiber. The random distribution of PFs in the matrix can effectively prevent the to the matrix
and the surrounding fiber. The random distribution of PFs in the matrix can effectively
prevent the generation and development of microcracks, thus improving the bearing
Polymers 2023, 15, 4675 18 of 20

generation and development of microcracks, thus improving the bearing capacity. When
the load increases further, macrocracks appear on the surface. GFs can cross the cracks and
restrain crack development until breakage due to high elastic modulus and tensile strength,
effectively hindering the formation and development of macrocracks [51,55]. In this sense,
the positive effect of hybrid fibers can be fully utilized under appropriate fiber contents
and mixing ratios, greatly improving the flexural strength of DSC.

4. Conclusions
(1) The slump of FRDSC with different fibers decreases with the increasing fiber content.
The slump of HyRDSC decreases with the increase in GF volume. When the mixture
amount is 0.2% and the mixture ratio of PF to GF is 1:3, the slump of HyFRDSC
decreases by 35.4%.
(2) FRDSC shows obvious plastic characteristics upon compressive, flexural and splitting
tensile fractures. The strength improvements from fiber reinforcement rank as flexural
strength > splitting tensile strength > cube compressive strength. The optimum PF and
GF contents are 0.1% and 0.2%, respectively. The effect of PFs on the DSC compressive
and tensile strengths is better than that of GFs. In the case of the hybrid fiber of 0.1%
PF + 0.1% GF, the DSC flexural strength is increased by 40.7%. For 0.1% PF + 0.05%
GF content, the compressive strength and splitting tensile strength are enhanced by
9.1% and 17.11%, respectively.
(3) Compared with the reference DSC, the flexural/compressive strength ratios of GFRDSC,
PFRDSC and HyFRDSC are increased by 9.88%, 13.58% and 30.86%, respectively; their
tensile/compressive strength ratios are increased by 4.5%, 5% and 18%, respectively.
Fibers enhance DSC toughness, and the improvement effect of the hybrid fiber is
relatively significant.
(4) The effect of hybrid fibers on the internal pores of DSC is more significant than that
of single fibers. The porosity and average pore size of HyDSC decrease by 50.01%
and 33.61%, respectively. In addition, the pore volume ratio below 20 nm increases to
22.46% and that above 200 nm decreases to 22.77%.
(5) HyFRDSC has the most dense and homogeneous microstructure among the mixtures.
PFs alleviate the stresses in a “bridging” manner until yielding, and the damage form
is dominated by fracture. For GFs, at the initial stress stage, the internal stress is
greater than the interface force between the fiber and DSC and less than the yield load
of the fiber, so the fiber has obvious drawing marks. When the internal stress is greater
than the yield load of the fiber, the fiber will also eventually have fracture damage.

Author Contributions: L.H.: writing—review and editing, writing—original draft, resources. W.H.:
methodology, validation. B.W.: writing—review and editing, supervision. X.Z. (Xue Zhang): data
curation, formal analysis, investigation. X.Z. (Xinyu Zhang): writing—review and editing. All
authors have read and agreed to the published version of the manuscript.
Funding: The authors wish to acknowledge the financial support from the National Natural Science
Foundation of China (51978566), Key R & D Projects of Shaanxi Province—Key Industry Innovation
Project (2020ZDLNY06-04, 2021ZDLSF05-11) and the Natural Science Foundation of Shaanxi Province
(2021JM-435).
Institutional Review Board Statement: Not applicable.
Data Availability Statement: Data are contained within the article.
Conflicts of Interest: Author Baojun Wen was employed by the company China Railway Liuyuan
Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any
commercial or financial relationships that could be construed as a potential conflict of interest.
Polymers 2023, 15, 4675 19 of 20

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