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#### Journal of the Korea Concrete Institute

##### ISO Journal TitleJ Korea Concr Inst.
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1. 공주대학교 건축공학과 박사과정 (Graduate Student, Department of Architectural Engineering, Kongju National University, Cheonan 31080, Rep. of Korea)
2. 공주대학교 건축공학과 교수 (Professor, Department of Architectural Engineering, Kongju National University, Cheonan 31080, Rep. of Korea)

전기로 환원 슬래그, 강섬유, C12A7, 복합섬유 보강 시멘트 복합체, 섬유분산
ladle furnace slag, steel fiber, C12A7, hybrid fiber reinforced cementitious composite, fiber distribution

## 1. Introduction

The sustainability in the cement industry influences on global sustainability directly. Sabnis (2015) reported three criteria that must be satisfied for an increase in the global sustainability in the concrete industry: (1) the consumption of less concrete; (2) the consumption of less cement in concrete mixtures; and (3) the consumption of less clinker in cement. Decreasing the amount of clinker employed could help improve sustainability in the cement industry (Li and Kim 2019). Supplementary cementitious materials (SCMs) are critical to blend cement production. However, SCMs cannot process hydration reactions independently without an alkaline activator. More importantly, mass application of SCMs will degrade the engineering properties, particularly early strength. Therefore, a new hydraulic binding material is required in construction industry.

In recent years, a rapid cooling method for the recycling of ladle furnace slag using high-pressure air was successfully industrialized (Choi et al. 2016; Fuente-Alonso et al. 2017). The slag, which is obtained from the ladle furnace and then rapidly cooled down using high-pressure air is referred to as rapid cooling ladle furnace slag (RC-LFS). Previous studies (Choi et al. 2016; Fuente-Alonso et al. 2017), indicate that the RC-LFS can be hydrated rapidly and continuously without an alkaline activator.

##### (1)
$C_{12}A_{7}+51H_{2}0\to 6C_{2}AH_{8}+AH\to 4C_{3}AH_{6}+6AH+18H_{2}O$

##### (2)
$C_{12}A_{7}+12Ca SO_{4}+137H_{2}0\to 4C_{3}A.3Ca SO_{4}.32H_{2}O+6AH$

As report by Juenger et al. (2011), the hydration of C12A7 occurs in phases and prevents the continuous hydration of C12A7. By adding gypsum to the matrix, ettringite is formed. Thus, strength can be developed early on and continuously. Additionally, both pulverized RC-LFS and gypsum can be obtained from the recycling process, this blend exhibits significant eco-friendly properties. Therefore, using the blending of pulverized RC-LFS and gypsum is a new approach to attain sustainability in the cement industry and benefit the construction field (Choi and Kim 2019).

However, the low tensile strength of hydraulic cementitious materials, a common limitation, requires improvement. has to be improved. Moreover, the hydration of C12A7 is associated with the generation of significant amount of energy. Hence, micro- cracks may occur during the hardening process. To compensate the drawbacks, studies were conducted on the improvement of engineering properties by introducing hybrid fiber reinforced system to this matrix. For experimental study, both single-fiber reinforcing and hybrid-fiber reinforcing methods were employed in this study. As mentioned above, the hydration of C12A7 is always a associated with a huge amount of energy emission. For controlling the hydration speed and heat, pulverized ground granulated blast furnace slag (GGBFS), gypsum, and chemical retarder (citric acid, anhydrous) were employed. The basic information about raw materials are provided in the below section.

Fiber is the most common material for cementitious material reinforcement. There are various types of fibers that can be mixed with cementitious material. Depending on the different morphology properties, fiber can be divided into straight fiber and deformed fiber. Nowadays, the majority of the fibers used for concrete reinforcement are deformed fibers. The performance and classification of the fibers were reported in several papers, technical documents, and guidelines (ACI Committee 506 1998; EN 14889-1 2006; Shah et al. 2010; Soltanzadeh et al. 2015; ASTM A820 2016). Although steel fiber is the most commonly used fiber material in concrete, the hybrid fiber reinforced method is gaining attention as a new approach for concrete reinforcement (ACI Committee 544 1993; Yazici et al. 2007; Orbe et al. 2012). Steel fiber hybrid with other types of fibers can provide a more efficient reinforcement for cementitious composite (Zollo 1997; Yao et al. 2003; Hsie et al. 2008; Blunt and Ostertag 2009; Fuente-Alonso et al. 2017; Nguyen et al. 2017). The principal benefit of introducing discontinuous fibers into cementitious materials is that the embedded fibers can contribute to cracks control, improving performance. As it is known that the embedded fibers increases the critical cracking strength of concrete, and enhances the post-cracking behavior due to the improved stress transfer provided by the bridging effect (Blunt and Ostertag 2009; Nguyen et al. 2017), the bond properties between the fibers and matrix are critical to the stress transfer (Mudadu et al. 2018). The reinforcement effect is mainly dependent on the following four factors: ① the strength of matrix, ② volume factor of fibers, ③ distribution and orientation of embedded fibers, and ④ strength and toughness of fibers. These factors define the properties of the binding matrix and fibers. In this study, hooked-end steel fibers and polypropylene fibers (PPFs) were introduced to the matrix formed by RC-LFS, gypsum, and GGBFS.

The properties of fiber reinforced concrete based on ordinary Portland cement have been evaluated by many researchers (Lee et al. 2016). However there is limited research on fiber reinforced RC-LFS based hydraulic composite. Because the high early strength can be achieved by RC-LFS based composite, there is huge probability that this type of material can be used as a special cement for tunnel projects, underground engineering, military engineering, and shooting concrete. Thus, the studies on fiber reinforced RC-LFS composite are necessary.

This study evaluates the physical performances, i.e., the mechanical properties and embedded fiber condition of hybrid fiber reinforced RC-LFS based composite.

## 2. Experimental Approach

### 2.1 Sample preparation

2.1.1 Materials

The binder materials and fibers used in this study were sourced from the South Korean market. The binder material as the blending of pulverized RC-LFS, α semi-hydrate gypsum, and GGBFS. Fiber materials include polypropylene fiber (PP) and hooked-end- glued steel fiber will be co-mixed with a binder, such as ordinary purified river sand and water. The chemical composition of the powder, fineness of the binder, and surface morphology properties of the fiber were analyzed using x-ray diffraction (XRD), a fineness tester, and scanning electron microscopy (SEM) respectively.

Table 1. Basic physical and chemical properties of LFS powder, gypsum, and GGBFS

 Binder RC-LFS Gypsum GGBFS Density (g/cm3) 2.97 2.72 2.84 Fineness (g/cm3) 6,300 1,100 4,200 SiO2 10.9 2.6 30.3 CaO 44.5 40 44.6 Al2O3 26.6 0.9 13.8 Fe2O3 4.3 0.4 0.5 MgO 6.6 0.3 4.5 SO3 - 55.8 4.4

Table 2. Basic physical and chemical properties of LFS powder, gypsum, and GGBFS

 Type of fiber PP fiber Steel fiber Section Single round Glued round Length (mm) 12 30 Tensile strength (MPa) 500 1,250 Density (g/cm3) 0.91 7.86 Aspect ratio 300 30 Acid/Alkali resistance Excellent Well

Fig. 1. Physical condition of steel fiber and PP fibers

Fig. 2. Surface condition of reinforced fiber tested by SEM: steel fiber (left side) and PP fiber (right side)

The properties of materials as listed in Table 1 and 2 and shown in Fig. 1 and 2. Ordinary river sand (SSD) was used as the aggregate in this study.

2.1.2 Mix proportion

The mix proportioning is summarized in Table 3. To control the hydration of C12A7 and also to achieve continuous development of strength, gypsum and retarder (citric acid, anhydrous) were introduced to the matrix.

2.1.3 Mixing procedure and casting procedure

The mixing process presented in Fig. 3 was employed. First, sand and binder material were carefully introduced into the mixer bowl for 30 seconds of dry mixing. Second, 90 % of the mixing water was poured into the bowl for 30 seconds of mixing. Third, PP fibers and steel fibers were added to the bowl for 30 seconds of mixing. Finally, the rest of mixing water was added in the bowl and mixed at a high speed for 90s. Superplasticizer and retarder were pre-dissolved in the mixing water. The casting method employed is presented in Fig. 4. Introduced mortar in the mould did not exceed a half volume of the mould. The mould was vibrated for 10 second to flatten the surface. After that, the procedure described above is repeated.

Table 3. Mixing proportions of mortar samples ($kg$/$m^{3}$)*

 ID GS0P0 GS25P75 GS50P50 GS75P25 GS100P0 Component RC-LFS 381.9 381.9 381.9 381.9 381.9 Gypsum 127.3 127.3 127.3 127.3 127.3 GGBFS 165.9 165.9 165.9 165.9 165.9 Sand 1,275.8 1,275.8 1,275.8 1,275.8 1,275.8 Water 270 270 270 270 270 Steel fiber 0 0.25 0.5 0.75 1 PP fiber 0 0.75 0.5 0.25 0 SP 1 2.5 2.5 1.25 1.25 Retarder 0.017 0.017 0.017 0.017 0.017

*Steel fiber, PP fiber, SP, Retarder mentioned above are calculated by weight % of total binder

Fig. 3. Mixing method

Fig. 4. Casting procedure

### 2.2 Experimental plan

Table 4 summarizes the experimental plan. The total dosage of steel fiber and PP fiber was maintained at 1 % to the cement weight. Five different samples with different embedded fibers were prepared. To evaluate the physical and mechanical properties of RC-LFS mortar with embedded steel fibers and PP fibers, the flexural strength, compressive strength, deformation of samples under the flexural load, and fiber conditions were investigated. In this study, two different types of samples were prepared. The compressive and flexural strengths were measured by using samples with dimensions of 4 cm×4 cm×16 cm. The deformation of samples under the flexural load and fiber distribution properties were measured by using samples with dimensions of 10 cm×10 cm×40 cm.

Table 4. Experimental plan

 ID GS0P0 GS25P75 GS50P50 GS75P25 GS75P25 Steel fiber (binder weight %) 0 0.25 0.5 0.75 1 PP fiber (binder weight %) 0 0.75 0.5 0.25 0 w/c 40 % Curing method Water curing Test items - Flow - Air content - Unit weight - Compressive strength (4h,1.7.28d) - Flexural strength (4h,1.7.28d) - Deformation of samples under the flexural load (35d) - Fiber distribution - Fiber orientation

### 2.3 Test items and method

2.3.1 Air content and flow

Air content and flow of all mixtures were evaluated by ASTM C185 (2015), ASTM C1437-15 (2015), and Orbe (2012) reported that additional fibers in the matrix trend to decrease the flowability. To maintain appropriate flowability, polycarboxylic acid based superplasticizer was involved. Because it is an active surface agent, the air content of the mixtures was influenced. Thus we evaluated air content and flow in the fresh state.

2.3.2 Compressive strength and flexural strength

The experiments of compressive and flexural strength were conducted in accordance with the ASTM C348-18 (2018) and ASTM C349-18 (2018), and the load indicator was as shown in Fig. 5. The flexural strength was first measured using the load indicator, and the compressive strengths of the same samples were measured after the flexural strength test. To ensure the reliability of the results, a minimum of three samples were used for each test.

2.3.3 Deformation of samples under the flexural load

Samples with dimensions of 10 cm×10 cm×40 cm were used to evaluate the deformation of samples under the flexural load and embedded fiber conditions (fiber distribution and orientation). The deformation of samples under the flexural stress was measured using linear variable differential transformer (LVDT) (Kazmi et al. 2019), as shown in Fig. 6.

2.3.4 Fiber distribution and orientation test

The steel fiber has the most significant influence on the mechanical properties of mixtures. In addition, the distribution and orientation of the embedded fibers have a direct influence on the final mechanical performance. Hence, it is necessary to evaluate the properties of the embedded fibers (Lee et al. 2002; Kang 2017; Song et al. 2018).

Fig. 5. Load indicator and illustration of compressive strength, flexural strength test

Fig. 6. (a) Data logger used during the experiment, and (b) LVD (35 days)

Fig. 7. Sampling method for image analysis of fiber distribution. sample size: (10 cm×10 cm×40 cm)

An image analysis (IA) and scanning electron microscope (SEM) were employed to evaluate the conditions of the steel and PP fibers in the hardened composite. Depending on the general size of fibers, steel fiber is considered to be macro fiber which could be identified by the naked eye or an ordinary microscope. Thus it is possible to evaluate the morphological properties of steel fiber using the IA software. Unlike steel fiber, PP fiber is considered to be micro fiber. Thus the evaluation of PP fiber will be processed using an SEM.

The sampling method is presented in Fig. 7. Four slices of concrete were sampled to illustrate the steel fiber conditions. For each slice, the negative and positive surfaces were dyed black and then polished to ensure the exposure of the steel fiber sections. For data collection, both surfaces of each slice were labelled as “-” and “+.” Thereafter, the counting process was initiated. The morphological properties of each steel fiber were then recorded, and used for further analysis.

The orientation of fiber considerably influences the mechanical performance deeply. As observed by several researchers (Lee et al. 2002; Song et al. 2018), the pullout strength of inclined fibers tends to be higher than that of aligned fibers. The optimum inclination is in the range 0°~20°. However, for an inclination greater than 30°, the occurrence of fiber rupture and matrix spalling is highly probable owing to the bending effect and concentration of the frictional stress at the exit point of the fiber. The orientation properties of the embedded fiber are dependent on several factors such as the matrix properties, mixing procedures, fiber properties, and casting procedures. Traditionally, the condition of fiber orientation is illustrated to be random. However, for deeper understanding and illustration, a more specified description is necessary. A calculated model as shown in Fig. 8 was adapted in this study.

Fig. 8. Fiber orientation calculated model

On the polished surface of the concrete samples, the mor-phological configuration of the steel fibers was measured. The absolute value of $\sin(\theta)$ can be provided as the parameter of the steel fiber orientation. Each steel fiber within the composite had an absolute value of $\sin(\theta)$. According to their orientation properties, the value was closer to one when the fiber was vertically oriented to the section, and approached zero when the fiber was parallel to the section, as shown in Equation (3). Based on the method presented in Fig. 8, a calculation method for an average value of absolute value of $\sin(\theta)$ was implemented using $d/l_{1}$.

##### (3)
$$|\sin \theta|=\left\{\begin{array}{l}{1\left(l_{1}=d\right)} \\ {\frac{d}{l_{1}}\left\{l_{1} | d<l_{1}<L\right\}} \\ {0\left(l_{1}=L\right)}\end{array}\right\}$$

$\theta$: the angle between the steel fiber and assumed fraction surface

$l_{1}$: the longer diameter of the steel fiber section

$d$: the diameter of the steel fiber

$L$: the length of steel fiber

## 3. Results and Discussion

### 3.1 Properties of fresh concrete

The properties of fresh concrete include the flow, air content, and unit weight, as listed in Table 5. From the experimental results, the PP fibers tended to cause a decrease in the flow and increase in the air content during the mixing. This can be attributed to the higher aspect ratio and large surface areas of PP fibers.

The superplasticizer promoted the increase in flowability. However, the superplasticizer increased the air content of the composite during the mixing. Because it is a type of surface active agent. Therefore, antifoamer is necessary for fiber reinforced composite.

However, to obtain the direct effect of superplasticizer in the matrix, antifoamer is not introduced to any matrix in this study. Based on the experimental results, the amount of superplasticizer used should be carefully controlled, and strong mixing should be avoided. The PP fibers exhibited the lowest density of the all the components. The addition of PP fibers into the cementitious composite led to a decrease in the unit weight; whereas the increase in the unit weight of the component due to the addition of steel fibers was not significant. Among all the mixtures, only the GS100P0 mixture exhibited a higher unit weight than the control mixture (GS0P0).

### 3.2 Hardened concrete properties

3.2.1 Flexural and compressive strength

Table 5. Fresh properties of hybrid fiber reinforced RC-LFS based composite

 ID Flow (mm) Air content (%) Unit weight ($kg$/$m^{3}$) GS0P0 200 2.6 2,258 GS25P75 190 6.4 1,966 GS50P50 183 5.6 2,161 GS75P25 190 2.7 2,244 GS100P0 190 1.6 2,317

The flexural and compressive strengths are the most critical parameters with respect to the evaluation of the mechanical properties. Using samples with dimensions of 4 cm×4 cm×16 cm, the values of the compressive and flexural strengths were measured in accordance with the ASTM C348 (2018) and C349 (2018). The experimental results are presented in Fig. 9 and Fig. 10. The results indicate that a high early strength was achieved. Compared with the control mixture, the fiber reinforced mixtures exhibited a higher flexural strength at the same curing age. The average flexural strength of GS100P0 (the sample with 1 % steel fiber), obtained over 28days, was 12.1 MPa, which was a 53.2 % improvement when compared with the control mixture. Moreover, GS75P25, which had 0.75 % of steel fibers combined with 0.25 % of PP fibers, exhibited a 49.4 % improvement. The flexural strength increased in accordance with an increase in the amount of steel fibers. There are two reasons for this. First, the steel fibers have a higher tensile strength, particularly with the hooked- shaped ends. Hence, the steel fibers can provide a better connection with the matrix and bridge the cracked region after the generation of the first crack. Second, the mixture between the PP fibers and superplasticizer led to a general increase in the air content. Third, for the same volume, the PP fibers exhibited a higher density than the steel fibers, which increased the inner flow of the total matrix.

Fig. 9. Flexural strength

Fig. 10. Compressive strength

The same trends were observed in the compressive strength data. The GS25P75 mixture, which was 0.25 % of steel fiber with 0.75 % PP fiber, exhibited the minimum compressive strength value, which was 49.52 % of that of the control mixture. The composite between the steel and PP fibers improved the compressive strength to a lesser extent. The strength depression of GS25P75 may be caused by the air entrapped by PP fiber during the mixing process. Because GS25P75 exhibited the highest air content among all mixtures (see Table 5). Therefore, it is necessary to introduce antifoamer to prevent unintentional air in fiber reinforced composite. On the other hand, the GS100P0 mixture, which was 1.0 % of steel fiber without PP fiber, resulted in a compressive strength that was 11.43 % higher than that of the control mixture.

3.2.2 Deformation of samples under the flexural load

Generally, the embedded fibers do not have an influence on the cracking strength; however, they have an influence on the post-cracking behavior owing to the remarkable residual post- cracking strength resulting from the enhanced material properties. Hence, in addition to the compressive and flexural strength tests, the deformation of samples under the flexural stress was evaluated. The data recorded by LVDT are presented in Fig. 11. The control mixture (GS0P0) exhibited high brittleness. The other mixtures with additional fibers exhibited improved deformation properties after reaching the heaviest load. The GS75P25 mixture exhibited the most superior properties. The GS25P75 mixture, which contained the minimum volume of steel fibers, exhibited different trends with other reinforced mixtures. After the heaviest load was reached, there was a significant decrease in the stress, followed by a slight increase. This can be attributed to the decrease in the steel fiber volume after the crack generation thus, the sample trends to collapse. The bumps of the curves can be considered as the partial removal of the steel fibers from the matrix. The LVDT results were in good agreement with the abovementioned trends.

Fig. 11. Deformation of samples under the flexural load

3.2.3 Distribution of embedded fibers

The properties of the embedded fibers were tested using SEM, as presented in Fig. 12. In particular, Fig. 12 (a)~(c) present the PP fiber properties in the mixture. Fig. 12 (d) presents the steel fiber condition in the mixture. The SEM images shows that the gaps between the PP fibers and the matrix are presented in the Fig. 12. During the loading process, the cracks were generated from the surface between the PP fibers and matrix. With the crack propagation, the fibers were eventually pulled out of the matrix. There were hollows where the PP fibers were pulled out, as shown in Fig. 12 (c). The micro cracks around the steel fiber were as presented in Fig. 12 (d).

The fiber orientation and distribution are the main influencing factors of the engineering performance (Lee et al. 2002; Song et al. 2018); hence, they should be investigated. In fiber-reinforced cementitious composites, the fibers are typically randomly distributed and oriented, depending on the fiber conditions of the casting method and matrix properties. Form the reinforced mechanization, the fibers should bridge the crack sections to delay and retard crack generation and propagation.

The orientations of the steel fibers were determined based on their sizes and cross-sectional shapes. The sampling method employed is presented in Fig. 5. The cross-section after dying and polishing is presented in Fig. 13.

Fig. 12. (a)~(c) Embedded PP fiber and (d) steel fiber

Fig. 13. Cross-sections after dying and polishing

Fig. 14. Fiber distribution condition

The black-colored part indicates the hardened matrix, and the white dots indicate the section surfaces of the steel fibers. After the application of the IA software, the steel fibers exhibited black-colored dots, as presented in Fig. 14.

By observing the morphological properties of the steel fiber sections, several typical conditions were observed, as provided in Table 6. The conditions can be classified as “positive” and “negative,” depending on the cross-sectional shapes of the steel fibers. As presented in Fig. 14, several cross-sections exhibited ellipse-like shapes, which indicated the presence of the out-of- plane angle ($\theta$). The angle ($\theta$) can be calculated using the value of sine ($\theta$) as has been illustrated in Fig. 5.

Table 6. Typical conditions of steel fibers

The images obtained from the application of the computer software are presented in Fig. 14. As can be seen, the number of steel fibers on the friction surface increasing with the amount of steel fibers. Moreover, with an increase in the fiber amount, the orientations of the fibers exhibited differences.

The GS75P25 mixture exhibited a good distribution and orientation. Another critical result is the variation in the distribution in accordance with an increase in the amount of steel fibers. For the GS100P0 mixture, several fibers were found to be clustered. This case was observed more frequently when the fiber amount was increased.

This indicates that 80 % of the steel fibers were oriented nearly vertical to the fraction surface and vertical to the load direction. Furthermore, several fibers were nearly parallel to the friction surface. Those paralleled fibers contribute nothing to flexural strength improvement. While in this experiment, only 0.3 % to 0.8 % of fibers were parallel to the fraction surface, which is very low for the composite. The average value of the angle was approximately 60°. The experimental and analysis results are summarized in Table 7.

Another phenomenon that can be observed form Fig. 14 is that RC-LFS based matrix can hold steel fibers well. Steel fiber has the highest density among all others materials. During the casting process, vibration is common procedure for fiber reinforced composite. However, if the vibration is too strong or the matrix has low viscosity, high density fibers sediment toward the bottom of a sample. However, as shown in Fig. 14, there is no significant difference in the amount of fiberbetween the bottom parts and the top parts. It is because RC-LFS composite is a rapid setting and hardening material. After the final set was achieved, the embedded fibers cannot be moved.

Table 7. Analysis results of $bold d/l_{1}$ values

 Mix d/l1 Aggregate Average $d/l_{1}$ Present of total num. (%) Counting numbers Average $\theta$ GS25P75 0.8-1 0.8< $d/l_{1}$ ≤1.0 0.849 80.8 413 58.1 0.6-0.8 0.6< $d/l_{1}$ ≤0.8 14.3 73 0.4-0.6 0.4< $d/l_{1}$ ≤0.6 3.5 18 0.2-0.4 0.2< $d/l_{1}$ ≤0.4 1.2 6 0-0.2 0≤ $d/l_{1}$ ≤0.2 0.2 1 GS50P50 0.8-1 0.8< $d/l_{1}$ ≤1.0 0.867 82.8 961 60.1 0.6-0.8 0.6< $d/l_{1}$ ≤0.8 10.4 121 0.4-0.6 0.4< $d/l_{1}$ ≤0.6 4.7 54 0.2-0.4 0.2< $d/l_{1}$ ≤0.4 1.8 21 0-0.2 0≤ $d/l_{1}$ ≤0.2 0.3 4 GS75P25 0.8-1 0.8< $d/l_{1}$ ≤1.0 0.862 78.1 1,317 59.5 0.6-0.8 0.6< $d/l_{1}$ ≤0.8 13.8 232 0.4-0.6 0.4< $d/l_{1}$ ≤0.6 4.7 79 0.2-0.4 0.2< $d/l_{1}$ ≤0.4 3 50 0-0.2 0≤ $d/l_{1}$ ≤0.2 0.5 8 GS100P0 0.8-1 0.8< $d/l_{1}$ ≤1.0 0.852 81.9 1,556 58.4 0.6-0.8 0.6< $d/l_{1}$ ≤0.8 11.2 213 0.4-0.6 0.4< $d/l_{1}$ ≤0.6 4.5 86 0.2-0.4 0.2< $d/l_{1}$ ≤0.4 1.6 31 0-0.2 0≤ $d/l_{1}$ ≤0.2 0.8 15

## 4. Conclusion

This paper presents the experimental results of the mechanical properties, in addition to the fiber distribution and orientation of the hybrid fiber reinforced pulverized RC-LFS based composite. In this study we focus on the fresh properties, strength performance and fiber distribution properties. The following conclusions were drawn from the discussion above:

1) The matrix formed by pulverized RC-LFS with GGBFS and gypsum can provide excellent early strength.

2) The 30-mm hook-end glued steel fibers effectively increased the flexural and compressive strengths. The maximum effect on the flexural strength, which was an 89 % increase, was observed with 1 % of the steel fibers of the mixture with GGBFS with a 1-day strength.

3) A negative effect is observed when the volume of PP fibers is greater than 50 % of the total fiber volume. In this experiment, the mixtures with 0.75 % PP fibers and 0.25 % steel fiber were found to have negative effects on the compressive and flexural strengths. The main reason of low compressive strength is the air entrapped by PP fiber (high aspect ratio) during mixing process. Therefore, it is necessary to introduce antifoamer to prevent unintentional air in fiber reinforced composite.

4) The angle of the steel fibers within the composite were calculated using a given mixing procedure. The average angle of the steel fibers was 61.5°. Although the dosages of steel fiber were different, the average values showed a slight difference when the matrix and mixing process are the same.

## Acknowledgements

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20182010202100), and is also supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program (10077570) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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