Properties and microstructure of reactive powder concrete having a high content of phosphorous slag powder and silica fume

Construction and Building Materials 101 (2015) 482–487

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Properties and microstructure of reactive powder concrete having a high content of phosphorous slag powder and silica fume Peng Yanzhou a,b,c,⇑, Zhang Jun b,c, Liu Jiuyan b,c, Ke Jin b,c, Wang Fazhou a a

State Key Laboratory of Silicate Materials for Architecture, Wuhan University of Technology, Wuhan 430070, Hubei, China College of Civil Engineering & Architecture, China Three Gorges University, Yichang 443002, Hubei, China c Collaborative Innovation Center for Geo-Hazards and Eco-Environment in Three Gorges Area, Hubei Province, Yichang 443002, Hubei, China b

h i g h l i g h t s
Phosphorous slag powder was utilized to produce reactive powder concrete (RPC).
Mechanical strength, freeze–thaw and sulfate resistance of RPCs are investigated.
The content of phosphorous slag could be as high as 35% (by the weight of binder).
RPCs prepared herein have excellent mechanical and durability properties.
The diameter of the most probable pore in RPC samples is less than 10 nm.

a r t i c l e

i n f o

Article history: Received 17 April 2015 Received in revised form 16 August 2015 Accepted 14 October 2015

Keywords: Reactive powder concrete Phosphorous slag powder Strength Durability Microstructure Sequential hydration effect

a b s t r a c t Reactive powder concrete (RPC) specimens whose content of phosphorous slag powder (PS) and silica fume was about 50% (by the weight of binder) were produced after they had been cured in 95 °C steam for a given duration. The test results of strength (compressive and flexural), freeze–thaw and sulfate resistance verified the excellent mechanical and durability properties of RPC containing a high content of PS. The investigation of selected RPC compositions by Thermogravimetric Analysis, Mercury Intrusion Porosimetry and Scanning Electronic Microscope made it possible to better understand their mechanical and durability properties depending on their microstructure. Thermogravimetric Analysis and Scanning Electronic Microscope demonstrated the sequential hydration effect of cementitious composites during heat treatment. Mercury porosimetry results showed that RPC had very low porosity and the diameter of the most probable pore was less than 10 nm. These microstructural characteristics would enable RPC to have excellent mechanical and durability properties. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Reactive powder concrete (RPC) is developed through microstructural enhancement techniques and is characterized by super-high strength, extreme durability and superior toughness [1,2]. The mechanical properties that can be achieved include the compressive strength of the range between 200 MPa and 800 MPa, the flexural strength of the range between 30 MPa and 60 MPa, fracture energy of the range between 1200 J/m2 and 40,000 J/m2, Young’s modulus of the range between 50 GPa and 60 GPa, and ultimate tensile strain at the order of 1% [1–3]. RPC with superior performance has been applied extensively in ⇑ Corresponding author at: College of Civil Engineering & Architecture, China Three Gorges University, China. E-mail address: [email protected] (Y. Peng). http://dx.doi.org/10.1016/j.conbuildmat.2015.10.046 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

civil, petroleum, nuclear power, municipal, marine and military facilities, as well as in other projects [4,5]. However, cement dosage of conventional RPC is generally high and silica fume (SF) content is often over 25% (by the weight of cement), which not only increases the production costs, but also has negative effects on the hydration heat and may cause shrinkage problems. Replacing cement with mineral admixtures and decreasing SF content seemed to be a feasible solution to these problems [6–12]. Electric furnace phosphorus slag which is different from granulated blast furnace slag (GBFS) is a kind of industrial waste and mainly contains SiO2, CaO and Al2O3. Its total content of SiO2 and CaO is more than 85% (by weight) and the SiO2/CaO ratio ranges from 0.8 to 1.4. Ground granulated electric furnace phosphorous slag has a glassy microstructure which is similar to that of granulated blast furnace slag and the weight percentage of the glassy structure could be as high as 90% [13,14]. Therefore, ground

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granulated electric furnace phosphorous slag may be used as reactive composition of RPC. Previous findings demonstrated that reactive powder concrete containing high volume binary blends (SF + PS) had no significant mechanical performance loss and incorporation of phosphorous slag powder (PS) in RPC was feasible [15]. This paper aims to achieve the following objectives: to obtain RPCs with strength grade of C200 by utilizing phosphorous slag powder; to examine the freeze–thaw and sulfate resistance of RPCs containing high content of phosphorous slag powder; and to reveal the relationship between the performance and microstructure of RPCs containing phosphorous slag powder through Thermogravimetry, Mercury porosimetry and Scanning Electronic Microscope.

2. Experimental programs 2.1. Materials The RPCs considered here was prepared by the following ingredients. Cement: ordinary Portland cement P.O 52.5, which complies with Chinese Standard GB 1752007, from Huaxin (Yidu, Hubei Province, China) Cement Co. Ltd. Phosphorous slag (PS): granulated electric furnace phosphorous slag produced by Yichang Yatai Chemical Co., Ltd. (Hubei Province, China). Silica fume (SF): undensified silica fume with average size of 0.1 lm–0.2 lm provided by China Construction Ready Mixed Concrete Co. Ltd. Superplasticizer: a polycarboxylate based superplasticizer provided by Jiangxi Building Materials Scientific Research and Design Institute. Fine aggregate: quartz sand with size of 0.16 mm–0.63 mm was used as fine aggregate. Furthermore, brass-coated steel fiber with diameter of 0.2 mm was used to improve the ductility of concrete. The tensile strength and aspect ratio (length-to-diameter ratio) of steel fiber is 2800 MPa and 65 respectively. The pertinent chemical and physical properties of the cement, PS and SF used in this study are given in Table 1.

2.2. Mix proportions of RPC and samples preparation Table 2 shows the mix proportions of RPC produced in this study. For each mixture, all components (cement, PS, SF and quartz sand) were mixed, cast, and vibrated in a similar sequence as conventional concrete. Initially dry powders (cement, PS and SF) and quartz sand were mixed for about 3 min. The water and superplasticizer were then added and mixed for about 6 min. Subsequently, steel fibers was added (if necessary) and mixed for another 3 min. The entire mixing process took about 12 min before the concrete mixture was ready to cast. When RPC mixture was ready, it was poured into the required molds which had been sprayed with mold oil to reduce the friction at the interface between the molds and RPC mixture. The RPC mixture was compacted using a vibrating table and hand tamping. The cast molds were covered by plastic sheets before demolded to prevent moisture in the concrete from evaporation. These specimens were demolded at least 48 h after casting because of the high PS content which required longer setting time.

Table 1 Chemical composition and physical properties of cement, phosphorous slag powder and silica fume. Items

Cement

Phosphorous slag (PS)

Silica fume (SF)

21.05 5.11 2.90 61.46 1.34 3.64 0.18 2.36

38.84 3.46 1.40 46.09 1.83 1.34 2.45 0.24

96.90 0.08 0.03 0.12 0.08 0.50 0.02 1.95

Specific surface (m2/kg)

379 (Blaine)

423 (Blaine)

20,000 (Nitrogen Ab.)

Compressive strength (MPa)

36.7 53.5

/ /

/ /

Chemical composition (%)

SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 Loss in ignition

3 days 28 days

Table 2 Mix proportions of RPCs (by weight ratio). No.

PS30-0 PS30-1 PS35-0 PS35-1 RPC-0 a b

Binders PS

Cement

SF

0.30 0.30 0.35 0.35 0.35

0.55 0.55 0.50 0.50 0.50

0.15 0.15 0.15 0.15 0.15

W/Ba

Quartz sand

Superplasticizer

Steel fibresb (%)

0.16 0.16 0.16 0.16 0.16

1.0 1.0 1.0 1.0 0

0.02 0.02 0.02 0.02 0.02

0 1 0 1 0

Including water from superplasticizer. Volume percentage.

2.3. Test methods For each RPC mixture, i.e., mixture PS30-0, PS30-1, PS35-0 and PS35-1, prismatic specimens (40 mm  40 mm  160 mm, 100 mm  100 mm  400 mm) and cubic specimens (100 mm  100 mm  100 mm) were cast to determine the strength (compressive and flexural), freeze–thaw resistance and sulfate attack resistance respectively. After demolded, these specimens were moved in a ZKY400B Steam Curing Container for Concrete to be cured at 95 °C for 3 days. Then, they were placed in a water tank at 20 °C until the age of 7 days. The strength (compressive and flexural) was then tested according to Chinese Standard GB/T17671-1999 while sulfate attack resistance and freeze–thaw resistance were performed according to GB/T 50082-2009 (see Fig. 1). Mixture RPC-0 shown in Table 2 was selected for Thermogravimetric Analysis. Thermogravimetry was measured on a NETZSCH STA 449 C thermogravimetric analyzer under dry air atmosphere. A temperature range between 20 °C and 1000 °C with a 10 °C per minute heating rate was selected. Samples were prepared by taking small pieces from cubic specimens (40 mm  40 mm  40 mm) which had been cured at 95 °C for a given duration (2 days, 3 days and 4 days respectively) and then all specimens were cured in water at 20 °C till 7-day age. Finally, they were ground to a fine powder with the particle size about 10 lm. For Mercury Intrusion Porosimetry and Scanning Electronic Microscope analysis, cubic specimens (40 mm  40 mm  40 mm) were cast according to mixture PS35-0. After demolded, these specimens were initially exposed to steam curing at 95 °C for a given duration (2 days, 3 days and 4 days respectively) and then were cured in 20 °C water till 7-day age. Samples were splinters taken from these specimens and were oven-dried at 60 °C for 24 h. Porosimetric measurements were carried out on a POREMASTER 33G porosimeter, and 3 nm to 400 lm pore sizes were investigated by this technique. Microstructure of the RPC was investigated by using a ULTRA PLUS Scanning Electronic Microscope.

3. Results and discussions 3.1. Strength RPC specimens containing a high content of PS and SF were obtained according to Table 2. The compressive and flexural strength results of these specimens are summarized in Table 3. From Table 3, it is noted that the flexural and compressive strength of these RPCs are about 21 MPa and 150 MPa respectively. The addition of 1% (by volume) steel fiber gave 187 MPa or more compressive strength of RPC. The result implied that the hydration activity of phosphorous slag powder (PS) was intensified. On one hand, owing to the small particle of phosphorous slag, a lot of small phosphorous slag particles filled the interspaces among cement particles, which would increase the packing density of cementitious composites and then improved the strength of hardened paste. This is the so-called packing effect [16]. On the other, a small particle size meant many lattice distortions and chemical bonds breakages existing in the surface of phosphorous slag particles [17,18]. During the process of heat treatment (curing at 95 °C for 3 days), both the quantity and speed of ionic species, such as Ca2+, [SiO4]4 and [AlO4]5 diffusing from cementitious composites particles increased greatly. The hydration reaction of reactive mineral admixtures, such as SF and PS, was activated and occurs sequentially [7], which continuously consumed portlandite Ca (OH)2 produced by cement hydration. As a result, many Ca(OH)2 were transformed into another type of hydration product (Calcium

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Fig. 1. Test setup for measurement of strength (flexural and compressive).

Table 3 Strength of RPC specimens at 7-day age. Mixture No. Flexural strength (MPa) Compressive strength (MPa)

PS30-0 21.7 151.2

PS30-1 28.2 190.3

PS35-0 22.9 156.8

PS35-1 29.7 187.0

Silicate Hydrate, or C–S–H). These products filled the capillary pores existing in hardened paste [19]. Therefore, both the pore diameter and the total porosity decreased, and the microstructure became gradually compact (see Sections 3.3–3.5). These characteristics would definitely give the prominent mechanical properties of RPC specimens. Nevertheless, the cracking resistance, enhancing and toughening effect of micro-steel fibers also had a significant impact on improving the mechanical properties [20,21]. 3.2. Freeze–thaw resistance and sulfate resistance The results of freeze–thaw and sulfate resistance of RPC specimens are shown in Tables 4 and 5. The result shown in Table 4 demonstrates the excellent freeze– thaw resistance of RPCs containing high content of PS: after 250 freezing and thawing cycles, none of specimens has mass loss or dynamic elastic modulus decline. After 350 freezing and thawing cycles, the mass loss is less than 1.0% and the relative dynamic elastic modulus still exceeds 99%. Table 5 indicates the excellent sulfate resistance of RPCs. It is generally agreed that the durability, including freeze–thaw resistance and sulfate resistance of concrete mainly depends on the pore structure of hardened paste, especially the amount of capillary and their degree of connectivity. Thus, the results of freeze–thaw and sulfate resistance of RPC specimens imply that RPCs prepared in this study have a small capillary diameter and a low porosity, which is verified by Mercury porosimetry in the following experiment.

Table 5 Sulfate resistance of RPCs containing a high volume of phosphorous slag powder (5% Na2SO4). Mixture No.

PS30-1 PS35-1

Mass loss of RPC specimens after n drying and wetting cycles, Dmn (%)

Anti-erosion coefficient for compressive strength of RPC specimens after n drying and wetting cycles, Kf (%)

30

60

90

120

150

30

60

90

120

150

0 0

0 0

0 0

0 0

0 0

101.3 100.6

100.7 101.4

102.3 101.9

99.6 98.7

98.2 98.1

and TG-4d correspondingly) was analyzed by Thermogravimetry. Results are independent of measurement atmosphere (air or argon). The thermogravimetric curves of those samples are given in Fig. 2. Analysis on thermogravimetric results of cement sample has been discussed in details in Refs. [7,19,22]. We have considered in this paper that the weight loss between 20 °C and the temperature of the first Derivative thermogravimetric curve (DTG) peak is due to loss of water not chemically bound in hydrates. For temperatures higher than that of the first DTG peak, water losses are due to dehydration of C–S–H and dehydroxylation of portlandite Ca (OH)2. The DTG peak between 440 °C and 450 °C was identified as portlandite dehydroxylation (weight loss LCH). The DTG peak between 630 °C and 700 °C was attributed to decarbonation of calcite CaCO3 (weight loss LCalcite). For each sample, free water content (Wf) denotes the weight loss between 20 °C and the first DTG peak in the present paper. Free water content (Wf) and bound water content (Wb, constitutive of C–S–H hydrates) add up to total water content WT (see Eq. (1)). Moreover, the total water content (WT) is calculated as the total weight loss between 20 °C and 1000 °C (LT) minus the weight losses due to dehydroxylation of portlandite Ca(OH)2 (LCH) and calcite decarbonation (LCalcite) (Eq. (2)). Furthermore, the total portlandite content of each sample (PT) is evaluated as Eq. (3):

3.3. Thermogravimetry Mixture RPC-0 with varied heat-curing durations at 95 °C (duration 2, 3 and 4 days respectively, denoted as Sample TG-2d, TG-3d

Table 4 Freeze–thaw resistance of RPCs containing a high volume of phosphorous slag powder. Mixture No.

PS30-1 PS35-1

Mass loss of RPC specimens after n freezing and thawing cycles, DWn (%)

Relative dynamic elastic modulus of RPC specimens after n freezing and thawing cycles, P (%)

150

200

250

300

350

150

200

250

300

350

0 0

0 0

0 0

0.3 0.3

0.6 0.5

100 100

100 100

100 100

99.6 99.7

99.5 99.3

WT ¼ Wb þ Wf

ð1Þ

W T ¼ LT  LCH  LCalcite

ð2Þ

PT ¼ LCH  74=18 þ LCalcite  74=44

ð3Þ

The Thermogravimetric Analysis results of samples are displayed in Table 6. As given by Table 6, long heat treatment durations lead to a higher bound-water/total water percentage. When the heat-curing duration was 2 days (Sample TG-2d), the bound-water percentage was 46.4% while it reached 51.5% when the duration was 3 days. It is generally considered that the bound-water percentage reflects the amount of hydration product (C–S–H) to some degree. The increase of bound-water percentage

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(a) TG-2d, age=7 days 100

0.4

0.4 0.3

TG DSC 0.2 DTG/(%/min)

TG /%

0.0 80

-0.2

DTG

70

0.1 0.0 -0.1

DSC /(mW/mg)

0.2

90

-0.2 -0.3

-0.4

60

-0.4

3.4. Mercury Intrusion Porosimetry

-0.5

0

200

400

600

800

1000

Temperture /Celsius degrees

(b) TG-3d, age=7 days

0.4

0.4

0.2

0.2

TG /%

90 0.0

80

DTG

70

0

200

400

600

800

0.0

-0.2

-0.2

-0.4

-0.4

DSC /(mW/mg)

DSC

TG

DTG /(%/min)

100

In addition, the total portlandite content, PT, decreases with the heat treatment duration. For durations longer than 3 days, the Ca (OH)2 content is less than 10%. This is mainly attributed to the sequential hydration of cementitious composites containing SF and PS. The sequential hydration continuously consumed Ca (OH)2 produced by hydration of cement, and more hydration products (C–S–H) had been generated. Furthermore, unhydrated particles filled in paste evenly and enhanced the microstructure of paste. Thus the microstructure became increasingly compact (see Sections 3.4 and 3.5), which would endow the hardened paste with excellent mechanical properties.

1000

The Mercury porosimetry results of mixture PS35-0 cured at 95 °C for a given durations (2 days, 3 days and 4 days respectively, i.e., Sample T95-2d, T95-3d and T95-4d correspondingly) are displayed in Table 7. As shown in Table 7, the major portions of pores existed in those samples belong to the innocuous pores or the harmless pores [23], and the proportion of the harmful pores is only 3.8–9.6% (by the total volume of pore). In addition, Table 7 also testifies that longer curing duration leads to a lower porosity, a higher proportion of the innocuous pores and a smaller diameter of the most probable pore which declines from 5.1 to 3.6 nm. The reason was that during the process of heat treatment, the hydration reaction of cement, SF and PS was activated and took place sequentially. As a result, many Ca(OH)2 were transformed into C–S–H and these products filled some pores in hardened paste [7,19]. Thus, both the pore diameter and the total porosity decreased, and the microstructure of the paste became increasingly compact.

Temperture /Celsius degrees

3.5. Scanning Electronic Microscope (c) TG-4d, age=7 days

0.4

0.4 DSC

100

TG

0.0

80

0.0

-0.2

-0.2

-0.4

-0.4

DSC /(mW/mg)

90

0.2 DTG /(%/min)

TG /%

0.2

DTG

70 0

200

400

600

800

1000

Temperture /Celsius degrees Fig. 2. Thermogravimetric curves of Mixture RPC-0 with varied heat-curing durations.

demonstrated that more hydration product had been formed, which would improve microstructure and thus reinforce mechanical and durability properties of the hardened paste.

The microstructure of mixture PS35-0 is shown in SEM images (Fig. 3). It can be seen from Fig. 3 that the hardened paste contains many hydration products and a few unhydrated particles (Fig. 3a). These unhydrated particles are tightly wrapped by hydration products. In addition, some hydration products have been observed existing in pores of the hardened paste (Fig. 3b and d) and the microstructure of the paste is very compact. In fact, during the heat-curing process, the cementitious composites (C + SF + PS) hydrated step by step. The sequential hydration continuously consumed Ca(OH)2. So there was only a little amount of Ca(OH)2 left in paste (see Section 3.3 and Fig. 3b and c). Meanwhile, more hydration products (C–S–H) came into being. These products filled in pores of the paste (Fig. 3d and e). Thus, the pore diameter and the total porosity decreased (see Section 3.4) and the microstructure would be enhanced. It is generally agreed that the interfacial transition zone (ITZ) is one of the most important factors for performance of cementbased materials. In this study, the microstructure of aggregate– paste interfacial transition zone (i.e., quartz–paste ITZ) of RPC was investigated by SEM. The results of Sample PS35-0, whose age is 7 days and the duration of heat curing is 3 days, are shown in Fig. 4.

Table 6 Thermogravimetry results of Mixture RPC-0 with varied curing duration. Sample No.

Wf (%)

LCH (%)

LCalcite (%)

LT (%)

Wb (%)

Wb (%)

Percentage of Wb/total water (%)

Portlandite content, PT (%)

TG-2d TG-3d TG-4d

1.87 1.71 1.51

2.43 1.77 1.68

1.77 1.57 1.54

12.40 12.08 12.14

6.33 7.03 7.41

8.20 8.74 8.92

46.37 51.49 54.28

12.97 9.92 9.50

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Table 7 Mercury Intrusion Porosimetry results of mixture PS35-0 samples. Sample No.

Duration of heat curing (day)

Porosity (ml/ g)

Diameter of the most probable pore (nm)

Pore diameter distribution (%) 2–10 (nm)

10–20 (nm)

20–100 (nm)

100–200 (nm)

>200 (nm)

T95-2d T95-3d T95-4d

2 3 4

0.0180 0.0169 0.0165

5.1 3.9 3.6

63.78 68.42 73.26

15.34 16.14 16.32

11.28 9.45 6.64

7.49 4.23 2.76

2.11 1.76 1.02

(b) ×5 000

(c) ×10 000

(d) ×5 000

(e) ×10 000

(a) ×2 000

Fig. 3. SEM image of mixture PS35-0 (age = 7 days).

Hardened paste

Quartz

Quartz

(a) ×20 000

Hardened paste

(b) ×20 000

Fig. 4. Microstructure of the aggregate–paste interfacial zone in mixture PS35-0 (age = 7 days).

Fig. 4 demonstrates that the quartz–paste ITZ in mixture PS35-0 has a low porosity. There is no obvious crack existing in ITZ and the width of ITZ is quite small. The boundary of the quartz is blurred and the quartz–paste bond is more closely. These interfacial characteristics are also attributed to the sequential hydration effect and the dense packing effect of the cementitious composites, which would improve the properties of interfacial transition zone [19,24]. 4. Conclusions (a) Reactive powder concrete specimens containing high content of phosphorous slag powder and silica fume were produced after they had been cured in 95 °C steam for a given duration in this study. The compressive and flexural strength of specimens, whose content of PS was 35% (by weight of the

binder) and volume percentage of steel fiber was 1%, were 187 MPa and 29.7 MPa respectively. The results of freeze– thaw and sulfate resistance verified the excellent durability properties of RPCs. (b) The investigation of selected RPC compositions by Thermogravimetry, SEM and Mercury Intrusion Porosimetry made it possible to better understand their mechanical and durability properties depending on their microstructure. Thermogravimetric Analysis and SEM confirmed the sequential hydration effect of the compound cementitious composites during heat treatment, and a large number of hydration products came into being while only a little amount of Ca (OH)2 was left in the hardened paste (less than 10%). Moreover, a few unhydrated particles were observed in hardened paste by SEM and these unhydrated particles were tightly

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wrapped by hydration products. Porosimetric studies by mercury intrusion testified the very low porosity of RPC. The diameter of the most probable pore was less than 10 nm. These microstructure characteristics would endow RPC specimens with the excellent mechanical and durability properties.

Acknowledgments This research work was financially supported by the Open Fund of State Key Laboratory of Silicate Materials for Architecture in Wuhan University of Technology (Grant No. SYSJJ2014-05), the Science and Technology Supporting Project of Hubei Province (Grant No. 2014BCB035) and the Construction Science and Technology Project of the Department of Housing and Urban-Rural Development of Hubei Province. References [1] P. Richard, M. Cheyrezy, Composition of reactive powder concretes, Cem. Concr. Res. 25 (7) (1995) 1501–1511. [2] P. Richard, M. Cheyrezy, Reactive powder concretes with high ductility and 200–800 MPa compressive strength, ACI SP 144 (24) (1994) 507–518. [3] Halit Yazıcı, Mert Yücel Yardımcı, Serdar Aydın, Anıl S. Karabulut, Mechanical properties of reactive powder concrete containing mineral admixtures under different curing regimes, Constr. Build. Mater. 23 (2014) 1223–1231. [4] M. Cheyrezy, Structural application of RPC, Concrete 33 (1) (1999) 20–23. [5] Wenzhong Zheng, Baifu Luo, Ying Wang, Compressive and tensile properties of reactive powder concrete with steel fibres at elevated temperatures, Constr. Build. Mater. 41 (2013) 844–851. [6] Halit Yazıcı, Hüseyin Yigiter, Anıl S. Karabulut, Bülent Baradan, Utilization of fly ash and ground granulated blast furnace slag as an alternative silica source in reactive powder concrete, Fuel 87 (12) (2008) 2401–2407. [7] Yanzhou Peng, Shuguang Hu, Qingjun Ding, Preparation of reactive powder concrete using fly ash and steel slag powder, J. Wuhan Univ. Technol.-Mater. Sci. Ed. 25 (2) (2010) 349–354. [8] Hüseyin Yig˘iter, Serdar Aydın, Halit Yazıcı, Mert Yücel Yardımcı, Mechanical performance of low cement reactive powder concrete (LCRPC), Compos. Part B: Eng. 43 (8) (2012) 2907–2914.

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