Construction and Building Materials 76 (2015) 264–272

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Performance and microstructural examination on composition ofhardened paste with no-cement SFC binder

http://dx.doi.org/10.1016/j.conbuildmat.2014.11.0320950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: tpchang@mail.ntust.edu.tw (T.-P. Chang).

Chun-Tao Chen a, Hoang-Anh Nguyen a,b, Ta-Peng Chang a,⇑, Tzong-Ruey Yang a, Tien-Dung Nguyen a

a Department of Civil and Construction Engineering, National Taiwan University of Science and Technology (NTUST) (Taiwan Tech), Taipei 106, Taiwanb Department of Civil Engineering, College of Engineering Technology, Cantho University, Cantho City, Vietnam

h i g h l i g h t s

� A new no-cement SFC binder with slag, Type F fly ash and CFBC fly ash is proposed.� The hardened paste of SFC binder has a compressive strength of 68.4 MPa at 28 days.� The main hydration products are ettringite and calcium aluminum silicate hydrate.

a r t i c l e i n f o

Article history:Received 9 May 2014Received in revised form 6 October 2014Accepted 12 November 2014

Keywords:CFBC fly ashFly ashSlagCompressive strengthNo-cement

a b s t r a c t

This research aims to present the engineering performance and microstructural examination on compo-sitions of hardened paste with a no-cement SFC binder, which is fabricated with mixture of ground gran-ulated blast furnace slag (GGBFS) (S), Type F fly ash (FA) (F) and circulating fluidized bed combustion(CFBC) fly ash (C). Specimens with four FA/GGBFS weight ratios (0, 1/9, 3/7 and 1) are prepared to esti-mate the effects of FA on the engineering performance. CFBC fly ash, around 10–25 wt.% of the mixture, isused as the main activator. The results show that the optimum mixtures are obtained by 15–20 wt.% CFBCfly ash and 10–30 wt.% FA. They have higher compressive strengths than those without FA. The earlyhydration of CFBC fly ash and GGBFS provides sufficient alkalis and releases heat, both play a crucial rolein the pozzolanic reaction of FA particles. The main hydration products of the hardened paste are ettring-ite (AFt) and calcium aluminum silicate hydrate (C–A–S–H) gel, as examined by scanning electronmicroscopy (SEM) and X-ray diffraction (XRD).

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, the demand of green power generation strongly moti-vates the innovative clean combustion technique, such as the cir-culating fluidized bed combustion (CFBC) that significantlyreduces the SO2 and NOx emission when comparing with the con-ventional coal combustion [1,2]. However, this technique releasesa lot of fly and bottom ashes. In general, the ashes released fromthe CFBC method have physicochemical properties different fromthose of conventional coal fly ash due to different raw fuel compo-sitions and sulfur absorbent additives (particularly limestone). Thepresence of free lime (f-CaO) and SO3 in CFBC ashes limits theapplication of the ashes in construction because of the expansion[3]. In contrary, the ground granulated blast furnace slag (GGBFS)from by-products of the iron and steel industry and Type F flyash (FA) from the coal combustion have been considered as

important recycling industrial solid wastes. Those solid wasteshave been extensively introduced into many cementing systemsas a green cement used in concrete for buildings, roadways andbridges by partially or entirely replacing ordinary Portland cement(OPC) [4–12] to reduce the amounts of carbon dioxide emissioninto the atmosphere. On the other hand, the no-cement binderwith the binary or ternary solid waste without adding any strongalkalis (such as NaOH, KOH, etc.) are promising in futureresearches due to its competitive cost [13–16]. Most of these no-cement binders investigated are based on the sulfate-activatedpozzolanic reactions [4,8–10,12]. The combinations of the pozzo-lans, such as GGBFS and FA, and the other by-product materialscontaining rich sulfur, such as CFBC fly ash and flue gas desulfur-ization (FGD) gypsum, were adopted to produce the binding sys-tems with major compositions of AFt crystals and C–S–H/C–A–S–H gels [13]. To make the mixture powders more active, the prefer-able main powders are the plain GGBFS with reactive glassyphases, and the fly ash, treated by reducing the particle sizes[13,14,17]. However, the high cost and the energy consumption

0 10 20 30 40 50 60 702θ (degrees)

Type F fly ash

CFBC fly ash

A

P

Q

Q

Q: QuartzM: MulliteA: AnhydriteP: PortlanditeL: Lime

MM M M

AA P A

P

M

MQ M

A AP P A

GGBFS

LL

L

Fig. 1. XRD patterns of three blended powders.

C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272 265

during the treatment process limit the applications [18]. Moreover,the workability is poor due to the irregular shapes or the increasedspecific area of the raw materials. Therefore, only the combinationof GGBFS and regular shaped FA is commonly used to produceclinkerless cements [15,16]. During the production, the utilizationof FA significantly reduced the cost of cement by decreasing theaddition of GGBFS [18]. However, a likely negative effect by exces-sive FA in the GGFBS-based cementing binders was found [15]. Thecompressive strengths of the alkaline sulfate activated GGBFS weredecreased by the FA additive, particularly for those replaced by 10%or more. Such result of strength decrease could be explained by thelack of sufficient hydration products with the increase of FA addi-tion [15].

It has been always a goal for many researchers to keep pursuingbetter no-cement cementitious binders that meet the require-ments for low cost, low energy consumption, reduced environmen-tal pollution and acceptable engineering and durable properties.The aim of this study is to propose an innovative no-cement SFCbinder, simply fabricated by the ternary mixture of three industrialsolid wastes of GGBFS (S), FA (F) and CFBC fly ash (C), which hascomparable and satisfactory binding properties with those of nor-mal Portland cement. The CFBC fly ash acts as the main activator totrigger the binding mechanism, as investigated by Salain et al. [14]and Dung et al. [17]. Through the engineering tests, microstruc-tural examination and optimization analysis for the mix design,it is shown that the no-cement SFC binder has prospective applica-tions. The mixtures of GGBFS and FA not only significantly reducethe cost but also improve the engineering property of the SFC bin-der such that the no-cement SFC binder can apparently alleviatethe aforementioned detrimental problems and provide plenty ofpossibilities for practical construction application in buildings,bridges and roads.

2. Experimental program

2.1. Materials and mix proportions

Type F fly ash, GGBFS and CFBC fly ash were used in this investigation. Thechemical compositions and XRD patterns of these materials are given in Table 1and Fig. 1, respectively. Table 1 shows that the CFBC fly ash mostly includes CaOand SO3 while Type F fly ash mostly includes SiO2 and Al2O3. From Fig. 1, Type Ffly ash contains stable crystals of mullite and quartz, but, the CFBC fly ash containsportlandite (Ca(OH)2), transformed from the reaction between f-CaO and vaporizedwater in the air and anhydrite (CaSO4). The GGBFS mostly includes CaO, SiO2 andAl2O3 under amorphous phases, as shown in XRD patterns (Table 1 and Fig. 1).The amorphous phases make the GGBFS more active than the FA. Fig. 2 showsthe SEM images of three industrial waste particles, where the FA particles aremostly spherical, and on the other hand, most of the particles of GGBFS and CFBCfly ash are irregular. Fig. 3 shows the grain size distributions of three industrialwaste particles, where the GGBFS particle is significantly smaller than that of theFA. Thus, the hydration degree of GGBFS as reacting with CFBC fly ash would behigher and faster than that of FA with CFBC fly ash. In this investigation, the rawCFBC fly ash without further physicochemical treatment is directly used to lowerthe cost of the no-cement binder. As shown in Fig. 3, the particle size of CFBC flyash is larger than that of GGBFS. In this study, the water-to-binder ratio (W/B)

Table 1Physical properties and chemical compositions of three blended powders.

GGBFS Type F fly ash CFBC fly ash

Specific gravity 2.9 2.08 2.7Blaine fineness (cm2/g) 6000 – 3000SiO2 (wt.%) 34.9 61.12 5.22Al2O3 (wt.%) 13.53 24.31 2.21Fe2O3 (wt.%) 0.52 4.26 0.581CaO (wt.%) 41.47 4.65 56.8MgO (wt.%) 7.18 1.15 2.06SO3 (wt.%) 1.74 – 32.4K2O (wt.%) – 0.33 –TiO2 (wt.%) – 1.48 –L.O.I (wt.%) 4.72 2.7 –

was fixed at 0.35. Four weight ratios of the Type F fly ash (F) to GGBFS (S)(F/S = 0, 1/9, 3/7 and 1) and four weight ratios of CFBC fly ash (C) to the mixtureof GGBFS and FA (10, 15, 20 and 25 wt.%) were used to create the ternary no-cementSFC binder in this study. The mix proportions of SFC binder pastes are described inTable 2. In the table, the name of the mix numbers denotes the following meaning:the characters F and C refer to the Type F fly ash (FA) and CFBC fly ash, respectively.The numbers immediately following the characters represent the weight percent-age of the materials used in the SFC cement. For example, the mix numberF10C15 means WFA/(WFA + WGGBFS) = 10 wt.% and WCFBC/(WFA + WGGBFS) = 15 wt.%.All the mixtures are based on a fixed W/B ratio of 0.35, i.e., W/B = WW/(WCFBC + WFA + WGGBFS) = 35 wt.%.

2.2. Specimen preparation and test methods

The cubic specimens with dimensions of 50 � 50 � 50 mm were casted for thecompressive strength test. The specimens were demoulded 24 h after casting andcured at temperature of 27 ± 2 �C and relative humidity (RH) of 65% until the testingtime. The compressive strength test was conducted at ages of 3, 7, 14 and 28 days,respectively, in accordance to ASTM C109. The pieces of the broken samples afterthe compressive strength test were soaked in alcohol to stop hydration and thenexamined by a scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM/EDS) and X-ray diffraction (XRD). The fresh pastes were con-ducted by Vicat needles in accordance to ASTM C191. The hydration heat of thecement pastes was carried out by the semi-adiabatic calorimetry during the hydra-tion in accordance to RILEM119-TCE [19].

3. Results and discussions

3.1. Microstructural examinations on hydration products of SFC binder

The microstructures of hydration products of the hardened no-cement SFC binder pastes were examined by SEM/EDS (Figs. 4–6)and XRD (Fig. 7). The SEM micrographs in Figs. 4–6 show thatthe needle-like crystals of AFt (ettringite) and amorphous gelsare the major hydration products. The AFt crystals were formedat age of 3 days (Figs. 4(a), 5(a), (b) and 6(a)). The amount of AFtincreased with the increase of FA (Figs. 4(a), 5(a) and 6(a)) due tothe increase of active alumina (Al2O3) contributed by FA. The AFtcrystals were connected by the amorphous gels to form the skele-ton and contributed to the early-age engineering properties of theSFC binder pastes. At the later ages of 7 and 28 days, further hydra-tion produced more amorphous gels and resulted in condensedmicrostructure leading to the increased compressive strength(Figs. 4(b), 4(c), 5(c), (d), 6(b) and 6(c)). The characteristics of the

(a) GGBFS (b) FA

(c) CFBC fly ash

Fig. 2. SEM images of industrial waste particles of (a) GGBFS, (b) FA and (c) CFBC fly ash.

0.01Particle Size (μm)

00.5

11.5

22.5

33.5

44.5

55.5

66.5

Volu

me

(%)

GGBFSType F fly ashCFBC fly ash

0.1 1 10 100 1000

Fig. 3. Particle size distributions of three industrial waste particles.

Table 2Mix proportions for no-cement SFC binder pastes (unit: g).

Mixtures GGBFS FA CFBC Water

F10C10 900 100 100 385.0F30C10 700 300 100 385.0F50C10 500 500 100 385.0F00C15 1000 – 150 402.5F10C15 900 100 150 402.5F30C15 700 300 150 402.5F50C15 500 500 150 402.5F00C20 1000 – 200 420.0F10C20 900 100 200 420.0F30C20 700 300 200 420.0F50C20 500 500 200 420.0F00C25 1000 – 250 437.5F10C25 900 100 250 437.5F30C25 700 300 250 437.5F50C25 500 500 250 437.5

266 C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272

amorphous gels were examined by EDS analysis after the SEMexamination, as illustrated in Table 3. These results show that,the amorphous gels contained calcium, silicon and, especially therich aluminum, which likely relate to the calcium aluminum sili-cate gels (C–A–S–H) [20–22]. In this study, the C–A–S–H gels withthe ratios of Ca:Si and Si:Al were in the ranges of 1.5–1.7 and 2.6–2.8, respectively. When comparing with the composition of theneat OPC, as investigated by Richardson [23], the Ca:Si ratio inthe SFC binder was smaller.

The XRD patterns of three SFC binder pastes in Fig. 7 show thecrystalline hydration products. The humps at approximate 30� in2h revealed the amorphous C–A–S–H gels. The obvious peakswhich indicated the rod-like AFt formation were obtained at allages (see Fig. 7). As the curing age increased, the changing humpsand peaks in the patterns illustrated more amorphous gels. Byusing the peaks as the reference, the increase in FA additive ledto the increased AFt formation because of the increased active alu-mina in FA (Figs. 7(a)–(c)). The function of FA acted not only as thefiller but also as the pozzolan. The dissolved FA in the SFC binderscould result from the hydration of GGBFS [15]. However, the peaksof mullite and quartz were obviously detected in Fig. 7(b) and (c),showing the dissolution of the FA in the SFC binders was not com-plete. The result could be explained by the stabilized FA in theweak alkaline solution due to the CFBC fly ash dissolution.

3.2. Setting time and hydration heat

The setting time and the hydration heat evolution of the SFCbinder pastes are shown in Table 4 and Fig. 8, respectively. InTable 4, the long setting time for these binder pastes were due tothe slow dissolution of the SFC binder in water, particularly theslow dissolution of CFBC fly ash [24,25]. With the hydration ofthe SFC binder, the dissolution of the CFBC fly ash resulted in the

(a) 3 days (b) 7 days

(c) 28 days

Spectrum 1

Spectrum 2

Spectrum 3Spectrum 4

Fig. 4. SEM images of no-cement SFC binder pastes (F10C15) made by 15 wt.% of CFBC fly ash and FA/GGBFS = 1/9 at ages of (a) 3 days, (b) 7 days and (c) 28 days (spectrumanalyses are given in Table 3).

(a) 3 days (b) 3 days with magnification of 4000X

(c) 7 days (d) 28 days

Fig. 5. SEM images of no-cement SFC binder pastes (F30C15) made by 15 wt.% of CFBC fly ash and FA/GGBFS = 3/7 at ages of (a) 3 days, (b) 3 days with magnification of4000�, (c) 7 days and (d) 28 days.

C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272 267

(a) 3 days (b) 7 days

(c) 28 days

Fig. 6. SEM images of no-cement SFC binder pastes (F50C15) made by 15 wt.% of CFBC fly ash and FA/GGBFS = 1 at ages of (a) 3 days, (b) 7 days and (c) 28 days.

268 C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272

alkaline sulfate solution through the following two chemical reac-tions [24,25]:

CaOþH2O! Ca OHð Þ2 ð1Þ

CaSO4 þ 2H2O! CaSO4 � 2H2O ð2Þ

At early ages, the first chemical reaction could be preferred butretarded by the outer coating CaSO4 layers formed during theburning process [24,25]. The lack of hydroxyl (OH�) and/or sulfate(SO4

2�) ions delayed the AFt formation, thereby inducing the longsetting time of the SFC binder paste. In the SFC binder paste, theamount of FA and CFBC fly ash significantly affected the settingtime of pastes. Normally, the increases of FA and CFBC fly ashextended the initial and final setting time (Table 4). The increasedstable FA led to the decreased active GGBFS in the mixtures. There-fore, the hydration degree of the SFC binder paste at early ages waslow. In addition, the increased CFBC fly ash induced a high concen-tration of sulfate ion (SO4

2�), which delayed the dissolution ofCaSO4 layers so that the reaction between water and f-CaO trappedby outer coating CaSO4 layers was delayed.

The hydration heat evolution of the chosen pastes not only clar-ified the slow setting of the SFC binders but also essentiallydescribed their hydration mechanisms. In Fig. 8, the initial temper-ature increases of the mixtures during first 2 h were related to thehydration of CFBC fly ash in water, especially the dissolution ofanhydrite and/or the reaction between f-CaO and water asdescribed by Eqs. (1) and (2). The temperature decreases wereassociated with the increased hydroxyl (OH�) and sulfate (SO4

2�)ions. During this stage, the SFC binders did not set due to absenceof dissolved alumina and/or silica. Fig. 8 shows that the dissolvingstage took place at least 10 h. At the end of the dissolving stage, thehigh concentration of OH� ion increased the alkalinity of themixtures and accelerated the hydration of GGBFS. The dissolutionof GGBFS provided enough dissolved silica and alumina toreact with the calcium oxide and sulfate ion so that the AFt and

C–A–S–H/C–S–H gels were precipitated, influencing the settingand engineering properties of the SFC binders. The precipitationof the AFt and the gels could be described by the followingchemical reactions [26]:

2½AlðOHÞ4�� þ 3SO2þ

4� þ 16Ca2þ þ 4OH� þ 26H2O

! C3A � 3CaSO4 � 32H2O ðor ettringiteÞ ð3Þ

Y½SiOðOHÞ3�� þ XCa2þ þ ðZ� X� YÞH2Oþ ð2X� YÞOH�

! CX � SY �HZ ðor C� S�HÞ ð4Þ

However, during the precipitation of the gels as described by Eq.(4), the silica could be substituted by the reactive alumina to formcalcium aluminum silicate hydrate (C–A–S–H) [20–22], especiallyafter the age of 7 days [27]. During the early hydration of the SFCbinders, the FA additive delayed the setting, leading to the lowengineering properties of these binders. Fig. 8(a) shows that theincreased FA resulted in the longer dissolving period, therebyextending the setting time of the SFC binders. However, theincreased FA helped to disperse the particles in the SFC bindersand accelerated the hydration of the CFBC fly ash. As seen inFig. 8(a), the increased FA induced temperature increases duringthe first 2 h. Moreover, Fig. 8(a) shows that after 40 h, the FAs inthe mixes with the FA/GGBFS ratios from 3/7 to 1 started to dis-solve due to the high hydration heat. The dissolution of the FA pro-vided more active silica and alumina, leading to the morehydration products as suggested by Eqs. (3) and (4). Fig. 8(b) clar-ifies the effect of CFBC fly ash in the SFC binders without FA addi-tive (FA/GGBFS = 0). Although acting as a crucial role in triggeringthe pozzolanic reactions, the increased CFBC fly ash significantlyprolonged the dissolving period and setting time of the SFC bind-ers. In Fig. 8(b), in the range of 10–25 wt.%, more CFBC fly ashinduced longer dissolution time for CFBC fly ash. As aforemen-tioned, the delayed time was due to the slow reaction betweenf-CaO and water.

5

2θ (degrees)

5

2θ (degrees)

5

10 15 20 25 30 35

10 15 20 25 30 35

10 15 20 25 30 35

2θ (degrees)

E

E

Q

QME E

E: EttringiteM: MulliteQ: Quartz

(a)

(b)

(c)

MM

E

E

QQME EM

M

EE

Q

Q

E EM M

Q

Q

3 days

7 days

28 days

3 days

7 days

28 days

3 days

7 days

28 days

Fig. 7. XRD patterns of no-cement SFC binder pastes with 15 wt.% CFBC fly ash and(a) FA/GGBFS = 1/9, (b) FA/GGBFS = 3/7 and (c) FA/GGBFS = 1.

Table 3EDS analyses of the points in Fig. 4(c) (unit: atom%).

Elements Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4

C – – 13.56 13.68O 25.92 36.36 39.57 33.36Mg 4.97 4.31 4.22 3.62Al 8.56 7.13 5.20 6.07Si 23.41 19.68 13.66 16.99S – – 0.74 –Ca 37.13 32.52 23.05 26.28

Table 4Setting times of the no-cement SFC binder pastes.

FA/GGBFS CFBC fly ash Initial setting Final setting(wt.%) (min.) (min.)

Effect of FA amountF00C15 0 15 830 1035F10C15 1/9 15 860 1065F30C15 3/7 15 1135 1305F50C15 1 15 1215 1360Effect of CFBC fly ash amountF10C10 1/9 10 855 1060F10C15 1/9 15 860 1065F10C20 1/9 20 920 1080

(a) 15 wt.% CFBC fly ash

(b) FA/GGBFS = 0

Time (hours)

15

20

25

30

35

Tem

pera

ture

(0 C

)

F00C15F10C15F30C15F50C15

0 10 20 30 40 50 60 70 80

0 10 20 30 40 50 60 70 80Time (hours)

15

20

25

30

35F00C10F00C20F00C25

Tem

pera

ture

(C

)0

Fig. 8. Heat evolution of SFC pastes with (a) 15 wt.% CFBC fly ash and (b) FA/GGBFS = 0.

C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272 269

3.3. Compressive strength

3.3.1. Effect of CFBC fly ashThe effect of CFBC fly ash on the compressive strengths of the

SFC binder pastes is shown in Fig. 9. With the fixed FA, the mix-tures with 15–20 wt.% CFBC fly ash had the highest compressivestrengths regardless of curing ages, and thus 15–20 wt.% CFBC flyash could be regarded as the optimum range. The mixtures with

(a) FA/GGBFS = 0 (b) FA/GGBFS = 1/9

(c) FA/GGBFS = 3/7 (d) FA/GGBFS = 1

0 10 20 30Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F00C25F00C20F00C15

0 10 20 30Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F10C25F10C20F10C15F10C10

0 10 20 30

Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F30C25F30C20F30C15F30C10

0 10 20 30

Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F50C25F50C20F50C15F50C10

Fig. 9. Effect of CFBC fly ash on compressive strength of hardened no-cement SFC binder pastes with (a) FA/GGBFS = 0, (b) FA/GGBFS = 1/9, (c) FA/GGBFS = 3/7 and (d) FA/GGBFS = 1.

(a) 15 wt.% CFBC fly ash (b) 20 wt.% CFBC fly ash

0 6 12 18 24 30

Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F50C15F30C15F10C15F00C15

0 5 10 15 20 25 30

Age (days)

0

10

20

30

40

50

60

70

80

Com

pres

sive

Stre

ngth

(MPa

)

F50C20F30C20F10C20F00C20

Fig. 10. Effect of FA on compressive strength of hardened no-cement SFC binder pastes with (a) 15 wt.% CFBC fly ash and (b) 20 wt.% CFBC fly ash.

270 C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272

lower or higher amount of CFBC fly ash clearly showed the lowercompressive strengths at both early and longer ages, except forthe F00C25 mix, which had higher compressive strengths thanF00C15 mix at all ages (see Fig. 9(a)), and the F50C10 mix, whicharchived the highest compressive strength at 3 days (seeFig. 9(d)). The 15–20 wt.% CFBC fly ash in the SFC binders balancedthe chemical compositions of the mixtures and optimized theratios of AFt to C–S–H/C–A–S–H gels. Therefore, the condensedstructure of the pastes could be obtained. However, either higher

or lower amount of CFBC fly ash could break the balanced chemicalcompositions of the SFC binders. The lower amount of CFBC fly ashresulted in the alkaline concentration insufficient for the dissolu-tion of GGBFS and FA. The increased CFBC fly ash was associatedwith a reduced GGBFS and FA, both of which could also result ina less reactive silica and alumina to form the C–S–H/C–A–S–H gels.Moreover, the remained sulfate could cause the compressivestrength degradation of the SFC binder at later because of the pre-cipitation of secondary gypsum or delayed AFt.

C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272 271

3.3.2. Effect of Type F fly ash (FA)The effect of FA addition on the compressive strengths of the

SFC binder pastes is shown in Fig. 10. When the optimum CFBCfly ash was used, the compressive strengths of SFC binder pastesat very early age of 3 days increased with the increased FA from0 to 10 wt.%. Fig. 10(a) and Fig. 10(b) show that the compressivestrengths of SFC binder pastes with 10 wt.% FA (mixes F10C15and F10C20) were higher than those of the reference SFC pasteswithout FA (mixes F00C15 and F00C20), respectively. The reasoncould be the filling role of the FA particles in the SFC binder pastesat early ages. But the increased FA from 10 to 50 wt.% induced thedecreased compressive strengths of the SFC pastes at 3 days. After7 days, the increased FA up to 30 wt. % did not cause a negativeeffect on the compressive strengths of SFC pastes. In Fig. 10(a), as15 wt.% CFBC fly ash was used, the mix with 30 wt.% FA (mixF30C15) had the compressive strengths comparable with those ofthe mix with 10 wt.% FA (mix F10C15) at 7, 14 and 28 days. More-over, when 20 wt.% CFBC fly ash was used, the mix with 30 wt.% FA(mix F30C20) had the higher compressive strengths than the mixwith 10 wt.% FA (mix F10C20) at 14 and 28 days. The increasedcompressive strengths of the SFC binder pastes with FA couldresult from the pozzolanic reaction of FA, followed by the hydra-tion of CFBC fly ash and GGBFS. In the study on using desulphuri-zation gypsum to activate the mixture of GGBFS and Class C fly ash,the GGBFS replaced by fly ash additive (even 5 wt.%) was reportedto have the negative effects on the compressive strengths ofcements at all ages [15]. However, In this study, the GGBFSreplaced by 10–30 wt.% FA could be used to further improve thecompressive strengths of the SFC pastes with 15–20 wt.% CFBCfly ash. However, the 50 wt.% FA broke the optimum ratio of FA/GGBFS in the SFC binders, so that the compressive strengths of bin-der pastes were significantly reduced regardless of the ages.

3.3.3. Analysis for optimal mixtureTo prolong the sustainability and durability of SFC binder, the

AFt formation and free water in the specimens should be mini-mized. The AFt formation in the expansion-controlled SFC binderis associated with the optimized CFBC fly ash. The optimal CFBCfly ash depends on the active alumina in the mixtures of FA andGGBFS. In fact, the higher amount of CFBC fly ash results in the dis-continuous structure of the SFC binder due to the secondary gyp-sum and delayed AFt formation. In addition, the maximum FAadditive can help the SFC binders minimize the water demand byoptimizing particle distribution. Therefore, the engineering proper-ties and durability of the SFC binders can be improved. To reach thesustainable definition abovementioned, the SFC binder should beprepared by 15 wt.% CFBC fly ash to trigger the hydration. TheSFC binder with 15 wt.% CFBC fly ash also meets the threshold levelof SO3 in the expansion-controlled binder, as investigated by Shenget al. [28]. Moreover, the GGBFS can be replaced by 10–30 wt.% FAin the mixtures to meet different requirements. The SFC binderwith 10 wt.% FA can have high early compressive strengths whilethe SFC binder with 30 wt.% FA can have good workability andlong-term engineering properties. In this study, although FA addi-tion, as the replacement for GGBFS by 50 wt.%, induced negativeeffects on the compressive strengths of the SFC binder pastes, itis still possible used in the concretes/mortars with requirementsfor low to medium strengths, low cost and high flowability.

4. Conclusions

The no-cement SFC binder proposed in this study has paved apromising way for manufacturing alternative hydraulic cementwith the low energy consumption. Results in this study show thatthe compressive strengths of SFC binder pastes could reach 47.7

and 68.4 MPa at 3 and 28 days, respectively. Therefore, such SFCbinder has high potential for most practical applications in infra-structure construction. During the manufacture of SFC binder, theratios of CFBC fly ash to the mixtures of GGBFS and FA and theFA to the GGBFS are the two most important parameters affectingthe performances of the specimens. The optimum ratios to inducehigh compressive strength are related to the ratios of ettringite(AFt) formation to C–A–S–H gels. Results show that 15–20 wt.%CFBC fly ash and 10–30 wt.% FA, as replacement of GGBFS, werethe optimal values for the no-cement SFC binder to obtain the bestcompressive strengths.

Acknowledgement

The authors would like to acknowledge the financial aid fromNational Taiwan University of Science and Technology (NTUST)(Taiwan Tech) for this investigation.

References

[1] Anthony EJ. Fluidized bed combustion of alternative solid fuels; status,successes and problems of the technology. Prog Energy Combust Sci 1995;21(3):239–68.

[2] Anthony EJ, Granatstein DL. Sulfation phenomena in fluidized bed combustionsystems. Prog Energy Combust Sci 2001;27(2):215–36.

[3] Sheng G, Li Q, Zhai J. Investigation on the hydration of CFBC fly ash. Fuel 2012;98:61–6.

[4] Bijen J, Niël E. Supersulphated cement from blastfurnace slag and chemicalgypsum available in the Netherlands and neighbouring countries. Cem ConcrRes 1981;11(3):307–22.

[5] Chi M, Huang R. Binding mechanism and properties of alkali-activated fly ash/slag mortars. Constr Build Mater 2013;40:291–8.

[6] Dongxu L, Xuequan W, Jinlin S, Yujiang W. The influence of compoundadmixtures on the properties of high-content slag cement. Cem Concr Res2000;30(1):45–50.

[7] Duran Atis� C. Strength properties of high-volume fly ash roller compacted andworkable concrete, and influence of curing condition. Cement Concr Res2005;35(6):1112–21.

[8] Dutta DK, Borthakur PC. Activation of low lime high alumina granulated blastfurnace slag by anhydrite. Cem Concr Res 1990;20(5):711–22.

[9] Gruskovnjak A, Lothenbach B, Winnefeld F, Figi R, Ko SC, Adler M, et al.Hydration mechanisms of super sulphated slag cement. Cem Concr Res 2008;38(7):983–92.

[10] Midgley HG, Pettifer K. The micro structure of hydrated super sulphatedcement. Cem Concr Res 1971;1(1):101–4.

[11] Nath SK, Kumar S. Influence of iron making slags on strength andmicrostructure of fly ash geopolymer. Constr Build Mater 2013;38:924–30.

[12] Singh M, Garg M. Calcium sulfate hemihydrate activated low heat sulfateresistant cement. Constr Build Mater 2002;16(3):181–6.

[13] Rust D, Rathbone R, Mahboub KC, Robl T. Formulating low-energy cementproducts. J Mater Civ Eng 2012;24(9):1125–31.

[14] Salain IMAK, Clastres P, Bursi JM, Pellissier C. Circulating fluidized bedcombustion ashes as an activator of ground vitrified blast furnace slag, vol.202. Special Publication; 2001.

[15] Zhao F-Q, Ni W, Wang H-J, Liu H-J. Activated fly ash/slag blended cement.Resour Conserv Recycl 2007;52(2):303–13.

[16] Zhong S, Ni K, Li J. Properties of mortars made by uncalcined FGD gypsum-flyash-ground granulated blast furnace slag composite binder. Waste Manage2012;32(7):1468–72.

[17] Dung NT, Chang T-P, Chen C-T. Engineering and sulfate resistance properties ofslag-CFBC fly ash paste and mortar. Constr Build Mater 2014;63:40–8.

[18] Li D, Shen J, Chen Y, Cheng L, Wu X. Study of properties on fly ash–slagcomplex cement. Cem Concr Res 2000;30(9):1381–7.

[19] RILEM119-TCE. TCE1: Adiabatic and semi-adiabatic calorimetry to determinethe temperature increase in concrete due to hydration heat of the cement. MatStruct. 1997; 30(8): p. 451–64.

[20] Andersen MD, Jakobsen HJ, Skibsted J. Characterization of white Portlandcement hydration and the C–S–H structure in the presence of sodiumaluminate by 27Al and 29Si MAS NMR spectroscopy. Cem Concr Res2004;34(5):857–68.

[21] Pardal X, Pochard I, Nonat A. Experimental study of Si–Al substitution incalcium–silicate–hydrate (C–S–H) prepared under equilibrium conditions.Cem Concr Res 2009;39(8):637–43.

[22] Thomas JJ, Rothstein D, Jennings HM, Christensen BJ. Effect of hydrationtemperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solidphases in Portland cement pastes. Cem Concr Res 2003;33(12):2037–47.

[23] Richardson IG. The nature of C–S–H in hardened cements. Cem Concr Res1999;29(8):1131–47.

272 C.-T. Chen et al. / Construction and Building Materials 76 (2015) 264–272

[24] Anthony EJ, Iribarne AP, Iribarne JV. Study of hydration during curing ofresidues from coal combustion with limestone addition. J Energy Res Technol1997;119(2):89–95.

[25] Anthony EJ, Jia L, Wu Y. CFBC ash hydration studies. Fuel 2005;84(11):1393–7.[26] Shi C, Day RL. Pozzolanic reaction in the presence of chemical activators: Part II

— reaction products and mechanism. Cem Concr Res 2000;30(4):607–13.

[27] Yang K-H, Cho A-R, Song J-K, Nam S-H. Hydration products and strengthdevelopment of calcium hydroxide-based alkali-activated slag mortars. ConstrBuild Mater 2012;29:410–9.

[28] Sheng G, Zhai J, Li Q, Li F. Utilization of fly ash coming from a CFBC boiler co-firing coal and petroleum coke in Portland cement. Fuel 2007;86(16):2625–31.