Experimental study on the mechanical properties of fiber-reinforced alkali-activated slag concrete

ABSTRACT

The use of alkali-activated slag concrete (AASC), which is made from industrial by-product slag, strongly supports sustainable development and provides an environmentally friendly option for construction materials. Fundamental mechanical performance tests were conducted to investigate the impact of replacement ratios of slag and the ratios of basalt fiber (BF) to polypropylene alcohol fiber (PVA) on the physical and mechanical properties of fiber-reinforced AASC. Subsequently, microstructural tests were performed to analyze its microscopic morphological features. The results indicate that the slump value of fiber-reinforced AASC increases with the slag replacement ratio, while it decreases initially and then increases as the fiber replacement ratio increases. With increasing slag replacement ratio and curing ages, the specimen exhibits enhanced compressive strength, input and elastic energy density, whereas the dissipated energy density first increases and then decreases. As the BF content reduces, compressive and splitting tensile strength demonstrate an initial increase followed by a decrease. The microstructural analysis reveals that with increase in slag replacement ratios, the overall compactness of specimen improves. When the total fiber content is 0.2%, the compactness of specimen with equal ratios of BF and PVA is optimal.

Keywords:
Concrete; Slag; Fiber; Mechanics Properties; Microstructure Analysis

1. INTRODUCTION

Global warming is one of the most critical global environmental issues currently, and energy conservation and emission reduction have become a global consensus. Reducing carbon emissions in the cement industry plays a significant role in controlling global warming. YOSRA et al. [¹] analyzed the effects of substituting fly ash with varying proportions of limestone, marble, and basalt stone waste to promote the utilization of quarry waste. Similarly, slag, which is a by-product of the metallurgical industry, can effectively lower carbon emissions by partially replacing cement as a cementitious material. Additionally, slag contributes to the utilization of solid waste. Numerous studies have demonstrated that slag as a replacement for a cementitious material exhibits excellent mechanical properties and durability [², ³, ⁴, ⁵, ⁶, ⁷]. Moreover, slag as a replacement for a cementitious material demonstrates advanced early-age strength compared to ordinary cement [⁸, ⁹, ¹⁰]. HAN et al. [¹¹] investigated the mechanical characteristics of microwave-cured concrete with high slag content, revealing that microwave treatment effectively accelerated the early hydration process in alkali-activated slag systems and significantly enhances early strength. Additionally, the type and concentration of alkali activators have a significant influence on the mechanical performance of slag concrete. BAI et al. [¹²] conducted a systematic investigation on the influence of varying alkali concentrations on the macro-mechanical properties and meso-damage mechanisms of AASC. The research demonstrated that increased alkali concentration causes enhanced peak stress and elastic modulus in specimen, coupled with progressive densification of the microstructure. FANG et al. [¹³] discussed the impact of 2–6% alkali contents on the engineering properties of AASC. The results indicated that AASC with low alkali content exhibits flexural strength and hardened strength similar to those of cement concrete. In contrast, high alkali contents enhance compressive strength and simultaneously reduce chloride ion resistance. ESCALANTE-GARCIA et al. [¹⁴] prepared AASC using 4%, 6%, and 8% alkali concentrations. The results demonstrated that, regardless of slag content, the compressive strength increases with higher alkali concentration. However, excessively high alkali concentrations can cause a decrease in the compressive strength of the concrete. JI et al. [¹⁵] researched that when the Na₂O content exceeds 12%, excessive gel products form on the surfaces of bonded particles, hindering the polymerization process. Additionally, the presence of microcracks caused an increased proportion of large pores, which compromised compressive strength. Incorporating fiber reinforcement into concrete significantly enhances its toughness and splitting tensile strength [¹⁶, ¹⁷, ¹⁸], and reducing microcracks improves material continuity. PAN et al. [¹⁹] researched the effects of incorporating steel fibers with varying lengths and volume fractions on the physical properties, mechanical performance, and microstructure of AASC. Results demonstrated that adding steel fibers with a volume fraction of 1.5% significantly reduced the slump of the concrete. As the volume fraction of steel fibers increased, the compressive strength, split tensile strength, and sulfate resistance of the concrete correspondingly improved. Microstructural analysis revealed that the steel fibers formed a tightly bonded interface with the matrix, reduced porosity, effectively inhibited crack propagation. LI et al. [²⁰] explored the effects of fiber type and fiber content on the axial compressive mechanical properties of fiber-reinforced alkali-activated slag concrete. The results indicated that appropriate amounts of steel fibers, BF, and polypropylene fibers could effectively enhance the axial compressive strength, peak compressive strain, and elastic modulus of the AASC. LIU et al. [²¹] studied the tensile behavior of steel-PVA hybrid fiber-reinforced concrete containing fly ash and slag powder. The results indicated that the tensile strength of the hybrid fiber-reinforced concrete increased with the addition of steel fibers but decreased with the addition of PVA. PVA, characterized by high elastic modulus and tensile strength, has been widely used in cement-based reinforcement materials [²², ²³]. GÜLTEKİN et al. [²⁴] investigated the impact of curing methods and PVA inclusion on some engineering properties of fly ash-based geopolymer mortars. The findings indicated that sealing the specimens during curing increased the compressive strength, and these increases were 18% for the reference mortar and 18% and 12% for mortars produced with 6 mm and 12 mm PVA, respectively. BF is environmentally friendly, low-carbon, cost-effective, and can effectively enhance the tensile strength and flexural strength of concrete [²⁵]. ALI et al. [²⁶] investigated the flexural performance of metakaolin-based geopolymers by varying the substitution ratios of garnet for metakaolin and adjusting the amount of BF. The findings showed that when 10% of metakaolin was replaced with garnet and 1% BF was added, the flexural strength increased by 28.17% at 7 d. PEHLIVAN et al. [²⁷] conducted an investigation on the mechanical properties of cement mortar, which incorporating three different contents of BF, various amounts of nano-silica and two levels of silica fume. The findings demonstrated that adding BF significantly enhanced the flexural strength and toughness of the mortar. Additionally, when silica fume was present, the introduction of nano-silica led to a 23% increase in the flexural strength of the mortar. WANG et al. [²⁸] investigated the wear resistance of steel-polyvinyl alcohol hybrid fibers in geopolymer concrete through underwater abrasion tests. The results showed that when used the 0.6% PVA and the 1.5% steel fiber, the geopolymer concrete exhibited optimal wear resistance. LU et al. [²⁹] incorporated PVA into EPP concrete to develop a novel filling material for tunnel buffer layers and analyzed its failure mode through dynamic compression tests. The results indicated that the inclusion of PVA transformed the failure mode of the specimen from brittle to plastic failure. HUANG et al. [³⁰] investigated the pore structure of basalt-polyvinyl alcohol hybrid fiber-reinforced concrete under the coupled effects of sulfate erosion and freeze-thaw cycles. Microscopic experiments revealed that incorporating BF and PVA effectively improved the pore structure of the concrete. Based on the above research, this study examines the impact of incorporating various ratios of slag as a substitute for cement in cementitious materials, along with different ratios of BF to PVA, on the performance and microstructure of concrete.

2. MATERIALS AND METHODS

2.1. Mixture design and preparing specimen

The original materials for making concrete specimen are S95 grade slag, and its parameters are shown in Table 1; NaOH solids are more than 96% pure; the parameters of BF and PVA are shown in Table 2, and the apparent characteristics are shown in Figure 1; water; sand; stones; grade 42.5 ordinary cement; naphthalene highly concentrations powder superplasticizer, water reduction ratios of 18~28%.

Table 1
Chemical content of slag.

CATEGORY	DENSITY	SPECIFIC SURFACE AREA	28D ACTIVITY INDEX	FLUIDITY RATIOS	MOISTURE CONTENT
Slag	3100 kg∙m−3	429 m2∙kg−1	99%	98%	0.45%

Table 2
Fiber technical indexes.

CATEGORY	LENGTH	DIAMETER	DENSITY	TENSILE STRENGTH	ELASTIC MODULUS
BF	12 mm	17 μm	2620 kg∙m−3	1550 MPa	35.8 GPa
PVA	12 mm	15.3 μm	1290 kg∙m−3	1830 MPa	40 GPa

Figure 1
PVA (a) and BF fibers (b).

Prepared cube test specimen with a side length of 100 mm according to the mix proportion in Table 3, curing the specimens under standard conditions for 28 d, then conduct mechanical performance testing.

Table 3
Mix proportion (kg∙m⁻³).

CATEGORY	STONES	SAND	CEMENT	SLAG	NAOH	WATER	BF	PVA	SUPERPLASTICIZER
C1	1085	725	420	0	15.34	176.4	0	0	3.36
C2	1085	725	315	105	15.34	176.4	0	0	3.36
C3	1085	725	210	210	15.34	176.4	0	0	3.36
C4	1085	725	105	315	15.34	176.4	0	0	3.36
C5	1085	725	0	420	15.34	176.4	0	0	3.36
X0	1085	725	210	210	15.34	176.4	0	0	3.36
X1	1085	725	210	210	15.34	176.4	2.58	0	3.36
X2	1085	725	210	210	15.34	176.4	1.935	1.325	3.36
X3	1085	725	210	210	15.34	176.4	1.29	2.56	3.36
X4	1085	725	210	210	15.34	176.4	0.645	3.975	3.36
X5	1085	725	210	210	15.34	176.4	0	5.3	3.36

In Table 3, C1, C2, C3, C4, and C5 represent alkali-activated slag concrete specimen prepared by replacing cement with slag at mass percentages of 0%, 25%, 50%, 75%, and 100%, respectively. X0 denotes AASC with 50% slag content and without fiber reinforcement. X1, X2, X3, X4, and X5 correspond to AASC specimen containing BF and PVA with a total fiber content of 0.2%, where the replacement ratios of PVA to BF are 0%, 25%, 50%, 75%, and 100%, respectively.

2.2. Experimental equipment and methods

The main equipment used in the test includes a truncated cone-shaped slump cone to test the slump of specimen. The uniaxial compression test utilized a WAW-1000 compression machine featuring a load capacity of 1000 kN and a loading speed of 2 mm/min. The microstructural analysis employed a FlexSEM1000 scanning electron microscope SEM and an X-ray diffraction XRD.

3. RESULTS AND DISCUSSION

3.1. Slump measurement and analysis

Concrete must meet workability requirements in actual construction, with fluidity being a key indicator of workability. Poor fluidity in concrete can lead to pump pipe clogs during pumping. Slump is a crucial indicator for evaluating the flowability of concrete [³¹]. The main test procedure involves evenly filling the mixed concrete into the slump cone in three layers, leveling it off, and then vertically lifting the cone at a steady rate. The slump value represents the vertical difference measured between the original height of the slump cone and the elevation of the settled concrete surface.

Figure 2 is the slump curve of AASC under different slag replacement ratios, where “S” in the fitting curve represents the slump value, and “M_C” denotes the slag replacement ratios. It can be observed that the slump of specimen increases with the increases in slag replacement ratios. The primary reason is that the surface of ground slag is generally smoother than cement, which reduces interparticle frictional resistance and thereby enhances the fluidity of the paste. Additionally, the particle size distribution of slag may complement cement, filling the gaps between cement particles and reducing internal friction, thereby increasing the slump.

Figure 2
Slump value of specimen with various slag replacement ratios.

Figure 3 illustrates the slump curve of AASC under C2 mix ratio with different fiber mix ratios. Similarly, in the fitting curve, “S” represents the slump value, while “M” denotes the fiber mix ratios. Initially, the slump of the specimen decreases as the replacement ratios of PVA increases, but it subsequently rises. Due to PVA finer diameter and higher specific surface area compared to BF, which gives PVA a greater capacity for water absorption. As the proportion of PVA decreases, the total water absorption of the fibers increases, resulting a reduction in free water content, consequently, a decrease in the fluidity. In contrast, as the ratio of BF decreases, the resistance from their rough surface lessens, thereby enhancing the fluidity.

Figure 3
Slump value of specimen with different fibers mix ratios.

3.2. Study on static mechanical properties of specimen at different curing ages

3.2.1. Stress-strain curves of specimen at various curing ages

The varying replacement ratios of slag to cement significantly affect the early strength of concrete [³²]. Researching the early strength of AASC has a significantly implications for practical engineering applications. Figure 4 shows the stress-strain curves of AASC with different slag replacement ratios at various curing ages. As shown in Figure 4, the stress-strain curves of concrete with different slag replacement ratios can be generally divided into four stages: compaction, elastic, yielding, and failure. Stage I: Compaction stage, during this stage, internal cracks and pores in the concrete are continuously compressed, causing cracks closure and the gradual densification of the concrete. Stage II: Elastic stage, during which the specimen exhibits a rapid increase in stress, with the stress-strain curve approximating a straight line. Stage III: Yielding stage, the slope of the stress-strain curve gradually decreases, reaching the peak stress. Stage IV: Failure stage, the stress-strain curve exhibits a rapid decline with negative slope values, ultimately leading to specimen failure.

Figure 4
Stress-strain curves of specimen with different slag replacement ratios at various curing ages. (a) C1, (b) C2, (c) C3, (d) C4, and (e) C5.

3.2.2. Compressive strength of specimen at various curing ages

Figure 5 illustrates the compressive strength of specimen at various curing ages. As the slag content increases, the activation of slag under alkaline conditions generates more cementitious materials. This process enhances the density and strength of the specimen. For instance, at a curing age of 7 d, the strengths of specimen C1 through C5 are measured at 18.9 MPa, 23.2 MPa, 25.8 MPa, 27.0 MPa, and 31.2 MPa, respectively. This indicates that increasing the slag content effectively improves the internal structure of the specimen, thereby enhancing their compressive strength. At the same slag content, prolonged curing age can promote the development of gel materials and hydration products that fill pores and microcracks, which improves the strength of AASC. For example, the C3 specimen achieves compressive strengths of 19.1 MPa, 22.3 MPa, 25.8 MPa, 26.4 MPa, and 34.8 MPa at curing ages of 1 d, 3 d, 7 d, 14 d, and 28 d, respectively. Specifically, the strength of specimen C3 at 1, 3, 7, and 14 d reach 54.89%, 64.08%, 74.14%, and 75.86% of 28 d strength, respectively.

Figure 5
Compressive strength of specimen at various curing ages.

3.2.3. Energy characteristics of specimen at various curing ages

The loading and failure process of specimen involves energy storage, consumption, and release. The stored energy primarily originates from the work performed by the testing machine, with the majority being retained within the specimen as releasable elastic energy. The remaining energy is consumed through specimen deformation, crack propagation, and internal damage [³³]. The energy relationship can be expressed as follows:

(1) $$U=U_d+U_e$$

In the formula: U represents the input energy; U_d denotes the dissipated energy; U_e indicates the elastic energy.

Figure 6 illustrates the energy distribution diagram during the compression test, showing the relationship between dissipated energy and elastic energy. As shown in Figure 6, the total input energy corresponds to the area enclosed by the curve and the horizontal coordinate axis OAC. The elastic energy is represented by the area enclosed by triangle ABC, while dissipated energy corresponds to the blank area in the diagram.

Figure 6
Energy profile.

The calculation formulas for input energy; elastic energy; dissipated energy are as follows:

(2) $$U={\displaystyle {\int }_0^{{\epsilon }_i}\sigma d\epsilon }$$

(3) $$U_e=\frac{{\sigma }_i^2}{2E}$$

(4) $$U_d=U-U_e={\displaystyle {\int }_0^{{\epsilon }_i}\sigma d\epsilon -}\frac{{\sigma }_i^2}{2E}$$

In the formula: ε_i represents the peak strain; σ_i denotes the peak stress; E indicates the elastic modulus.

Figure 7 shows the input energy density curves of the C2 specimen at various curing ages. As illustrates in Figure 7, the input energy of specimen generally increases with extended curing ages. The main reason is the progressive hydration reaction generates increasing hydration products over time, which alter the internal pore structure of concrete. During the early curing age, the concrete exhibits larger and more interconnected pores. As curing progresses, these pores become gradually filled and refined by hydration products, resulting in reduced porosity and smaller pore dimensions. Such microstructural improvements concrete to transfer stress more effectively under loading, reducing energy dissipated and increasing the input energy of the specimen.

Figure 7
Input energy density of specimen at various curing ages.

Figure 8 presents elastic and dissipated energy density curves of the C2 specimen at various curing ages. From Figure 8, the elastic energy density of specimen increases with extended curing ages, while dissipated energy density decreases early and subsequently rises. The enhanced elastic energy density is attributed to the completion of hydration reactions at prolonged curing ages, where gels further fill pores form a dense three-dimensional network structure capable of storing more elastic strain energy. Due to pore filling and the formation of gel material, the dissipated energy density reduces with curing age from 1 d to 3 d, which decreases the plastic deformation capacity while increases the elastic deformation. As curing continues, the improved overall compactness of the specimen requires major energy dissipated to induce failure, resulting in the observed increase in dissipated energy density.

Figure 8
Elastic and dissipated energy density of specimen at various curing ages.

3.3. Study on mechanical properties of specimen with various slag replacement ratios

3.3.1. Stress-strain curves of specimen with different slag replacement ratios

Figure 9 shows the static compressive stress-strain curve of specimen with different slag replacement ratios. From Figure 9, as the slag replacement ratios increase, the compressive strength progressively increases, indicating the incorporation of slag contributes to enhancing the compressive properties. Compared to the 0% slag replacement ratio, the post-peak descent segments of other curves become slightly more gradual, which is primarily attributed to the increase slag replacement ratios delaying fracture propagation and enhancing ductility.

Figure 9
Stress-strain curve of specimen with different slag replacement ratios.

3.3.2. Compressive strength of specimen with different slag replacement ratios

Figure 10 presents the peak compressive strength of specimen under different slag replacement ratios. It can be observed that the compressive strength of specimen increases with the rise in slag contents. Compared to C1, the compressive strength of specimen C2, C3, C4, and C5 increased by 1.27%, 5.45%, 18.12%, and 25.09%, respectively. Which indicated that high slag replacement ratio can effectively enhance the compressive strength of AASC.

Figure 10
Compressive strength of specimen with different slag replacement ratios.

3.3.3. Energy characteristics of specimen different slag replacement ratios

Figure 11 indicates the input energy of specimen with different slag replacement ratios. From Figure 11, as the slag content increases, the input energy of the specimen also increases. This is attributed to the enhance hydration reaction generating more C-S-H gel with higher slag contents, which optimizes the pore structure. Consequently, the specimen can absorb more energy under loading, causing an increase in input energy density.

Figure 11
Input energy density of specimen with different slag replacement ratios.

Figure 12 displays elastic and dissipated energy density curves of specimen with different slag replacement ratios. It can be observed that as the slag replacement ratio increases, the elastic energy density of the specimen generally increases, while the dissipated energy density initially increases and then decreases. High slag replacement ratio can optimize the pore structure of the specimen and enhance the density of stored elastic energy. Furthermore, high slag replacement ratios can reduce the number of internal defects and microcracks, thereby decrease the energy dissipated through crack propagation.

Figure 12
Elastic and dissipated energy density of specimen with different slag replacement ratios.

3.4. Study on the mechanical properties of specimen with different fiber mixing ratios

3.4.1. Stress-strain curves of specimen with different fiber mixing ratios

Figure 13 represents the static stress-strain curves of specimen C3 with different fiber mixing ratios. As illustrates in Figure 13, the incorporation of fibers results in an extension of the stress-strain curve compaction stage, an increased slope in the elastic stage and an appropriate enhancement in peak stress. Mainly because the fibers improve the specimen’s continuity during the early loading stage, leading to a shorter compaction phase. Additionally, the fibers restrain the propagation of initial stage shrinkage microcracks. The interfacial bonding between fibers and the cementitious matrix enhances the structural integrity, allowing for more efficient stress transfer during the elastic stage. This mechanism reduces local stress concentrations, ultimately increasing the slope of the elastic stage.

Figure 13
Stress-strain curve of specimen with different fiber mix ratios.

3.4.2. Compressive strength of specimen with different fiber mixing ratios

Figure 14 illustrates the compressive strength of specimen with different fiber mix ratios. As illustrates in Figure 14, the compressive strength of the specimen firstly increases and then decreases with the increasing proportion of PVA. The compressive strength of specimen with fiber mixing ratios X0, X1, X2, X3, X4, and X5 was 34.8, 35.9, 36.2, 38.7, 37.0, and 35.1 MPa, respectively. Compared to X0, the compressive strength of X1, X2, X3, X4, and X5 increased by 3.16%, 4.02%, 11.21%, 6.32%, and 0.86%, respectively. The incorporation of fibers enhances the specimen strength, but the increase was not substantial. Among these, the highest strength improvement occurred when 50% of BF was replaced with PVA. The reason is that the high modulus of BF effectively transfers stress and restrains the development of microcracks.

Figure 14
Compressive strength of specimen with different fiber mix ratios.

3.4.3. Splitting tensile strength of specimen with different fiber mixing ratios

The splitting tensile strength serves as an indirect method for evaluating the tensile performance of concrete. By compressing specimen to induce splitting failure along the loading direction, this method calculates the concrete equivalent tensile strength, serving as a critical indicator for assessing cracks resistance and toughness of concrete. Additionally, the incorporation of fibers can significantly enhance the splitting tensile strength of concrete [³⁴, ³⁵, ³⁶]. The calculation formula is as follows:

(5) $${\sigma }_t=\frac{2P}{\pi A}=0.637\frac{P}{A}$$

Where: σ_t represents the splitting tensile strength, P is the failure load, and A denotes the area of the split surface.

Figure 15 illustrates the trend of tensile strength variations in specimen with different fiber mix ratios. The data shows that incorporating fibers enhance the tensile strength of specimen. As the ratio of BF decreases, the tensile strength initially increases and then decreases. The splitting tensile strength for specimen with fiber mix ratios X0, X1, X2, X3, X4, and X5 were 1.57, 1.95, 2.17, 2.53, 2.05, and 1.60 MPa, respectively. Compared to X0, the splitting tensile strength of specimens X1, X2, X3, X4, and X5 increased by 24.20%, 38.22%, 61.15%, 30.57%, and 1.91%, respectively.

Figure 15
Splitting tensile strength of specimen with different fiber mix ratios.

The primary reason that the splitting tensile strength reaches its maximum value with the X3 mix ratio is that, during the initial loading stage, the concrete experiences low stress, which is mainly supported by the matrix. As loading continues, small cracks begin to develop. The presence of BF, which has high strength and high modulus, helps prevent the propagation of these cracks through bonding and mechanical interlocking. Additionally, the PVA, owing to their high toughness, can absorb some of the stress, thereby enhancing the overall strength of the concrete. However, since the strength of the PVA is not as high as that of the BF, a higher content of PVA has a minimal effect on the splitting tensile strength of the concrete.

4. MICROSTRUCTURE EXPERIMENT AND ANALYSIS

Microstructure test utilizes X-ray diffraction XRD and scanning electron microscopy SEM to analyze the effects of factors such as slag replacement ratios and fibers on the mechanical properties of specimen from a microscopic perspective.

4.1. XRD test and analysis of specimen with different slag replacement ratios

Figure 16 illustrates the XRD patterns of AASC with different slag replacement ratios. As shown in Figure 16, with an increase in slag content, the C-S-H gel content increases significantly. This is because, in the alkaline solution, as the calcium-rich phases in the slag gradually dissolve, the silicon-rich phases in the slag also begin to dissolve, causing a reduction in the polymerization degree of the slag. With the continuous dissolution and depolymerization of slag, the CaCO₃ content decreases while hydrated C-S-H gel is generated. As the hydration reaction progresses, an abundant C-S-H gel forms at the surface defects and internal flaws of the slag particles, ultimately creating a tightly integrated structure between the slag and the C-S-H gel. The internal pores of the sample become filled with hydration products, causing increased compactness and strength of the specimens.

Figure 16
XRD patterns of specimen with different slag replacement ratios. (a) C1, (b) C2, (c) C3, (d) C4, and (e) C5.

4.2. SEM image analysis of specimen with different slag replacement ratios

Figure 17 shows SEM images of specimen with different slag replacement ratios. From Figure 17, both the quantity and length of internal cracks within the specimen decrease as the slag replacement ratio increases. When the slag replacement ratio reaches 100%, the SEM images reveal an almost crack-free surface and a denser microstructure. This occurs because the higher slag content promotes the formation of additional hydration products, which effectively fill internal pores and coat unreacted slag particles, thereby obstructing the paths of crack propagation [³⁷, ³⁸].

Figure 17
Microstructure of specimen with different slag replacement ratios. (a) C1, (b) C2, (c) C3, (d) C4, and (e) C5.

4.3. SEM image analysis of specimen with different fiber ratios

Figure 18 presents microstructural images of specimen with varying ratios of BF and PVA. From Figure 18, the specimen without fibers displays distinct cracks. When the ratio of BF increased, it creates a skeletal framework within the specimen. Additionally, a small amount of PVA fills the gap, forming a complex three-dimensional network [³⁹], which enhance the overall performance of specimens, with the X3 specimen demonstrating optimal results. However, when the PVA ratio exceeded 50%, it began to agglomerate excessively. This aggregation phenomenon limits the effective utilization of its high-toughness properties, leading to localized stress concentrations that ultimately degraded the overall performance of specimen.

Figure 18
Microstructure of specimen with different fiber mix ratios. (a) X0, (b) X1, (c) X2, (d) X3, (e) X4, and (f) X5.

5. CONCLUSIONS

Through experimental research involving concrete specimen with different slag replacement ratios, different proportions of BF and PVA, along with slump tests, compression tests, and microstructural analysis, main conclusions are summarized as follows:

Increasing the slag replacement ratio can effectively improve the concrete slump. However, as the substitution ratio of PVA to BF increases, the concrete slump shows a trend of first increasing and then decreasing. The slump reached the maximum when the BF content was 0.2% and the PVA content was 0.

The compressive strength of AASC increases with both curing ages and slag replacement ratios. Taking 60% slag replacement of cement as an example, the compressive strength at 7 d reaches 74.14% of the 28 d strength. Incorporating fibers has minimal impact on the compressive strength but significantly affects the splitting tensile strength. When the substitution ratios of PVA to BF reach 50%, the splitting tensile strength of specimen increases by 61.15% compared with X3, indicating the optimal mix ratio.

Microstructural analysis reveals that both the length of surface cracks and the quantity of pores in the specimen increase with higher slag replacement ratio. However, as the substitution ratios of PVA to BF increase significantly, PVA agglomeration occurs, causing stress concentration and performance reduction. Therefore, in practical engineering applications, fiber dispersants may be incorporated to prevent PVA agglomeration.

However, several limitations of current study must be acknowledged. First, the study was limited to early-age strength, without investigation of long-term durability such as wet-dry cycles, freeze-thaw cycles, sulfate attack, and high temperatures. Second, fiber agglomeration occurs in concrete specimens, especially in the X4 and X5 mix ratios, and the effect of agglomeration on the long-term durability of concrete should be considered.

6. ACKNOWLEDGMENTS

This study was financed by Anhui Provincial Natural Science Foundation (No. 2108085ME156 and No. 1808085QE148), China Postdoctoral Science Foundation (No. 2018M642504).

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Publication Dates

• Publication in this collection
  20 Oct 2025
• Date of issue
  2025

History

• Received
  10 July 2025
• Accepted
  21 Aug 2025