ORIGINAL ARTICLE

Cementitious properties and microstructureof an innovative slag eco-binder

Nguyen Tien Dung . Ta-Peng Chang . Chun-Tao Chen .

Tzong-Ruey Yang

Received: 1 October 2013 / Accepted: 11 May 2015

� RILEM 2015

Abstract Circulating fluidized bed combustion fly

ash was used to activate the hydration of ground

granulated blast furnace slag to produce non-cement

SCA eco-binder without Portland cement (OPC). The

engineering properties of SCA paste and mortar with

air or water curing were evaluated. The microstructure

and hydration products of the SCA binder were

investigated by scanning electron microscope, and

X-ray diffraction. The hydration products of SCA

were ettringite (AFt), calcium silicate hydrate (C–S–

H) and calcium aluminate silicate hydrate (C–A–S–H)

so that the SCA paste had proper setting times, dense

microstructure and high strength. The compressive

strengths of SCA pastes and mortars reached up to

70 MPa at 28 days and even higher at longer ages. The

early expansion of SCA paste and mortar due to the

AFt formation compensated for their drying shrinkage

to lead to a very low ultimate shrinkage. The SCA

binder is a promising alternative to OPC.

Keywords Eco-binder � Slag � CFBC fly ash �Compressive strength � Drying shrinkage

1 Introduction

Since the development of ordinary Portland cement

(OPC) has been over 175 years, it has become the

dominant binder used in concrete for construction. The

annual global cement production has reached 2.8 bil-

lion tonnes nowadays, and is expected to increase to

4 billion tonnes. [23]. However, the OPC production

has a large environmental impact [3]. It contributes

approximately 5–8 %of global CO2 emissions [20, 30,

31, 35] and is always among the largest emissions

sources [4]. In addition, the process always involves

the destruction of natural quarries and energy con-

sumption to extract rawmaterials. It is reported that the

manufacturing of OPC consumes 2780–3050 TWh of

energy annually, approximately 2–3 % of global

primary energy use [18]. Hence, there is a great need

for alternative green binders in construction to reduce

greenhouse gas emissions and save energy and natural

resources. For this purpose, blended cements, in which

a portion of OPC is replaced by industrial waste

materials with pozzolanic properties, such as fly ash

from coal combustion or slag from iron production, are

used as binders for concrete. The pozzolanic properties

of fly ash, slag and metakaolin are due to the presence

of large quantities of reactive SiO2 and Al2O3. They

react with the Ca(OH)2, liberated during the hydration

of OPC to form calcium silicate hydrates (C–S–H) and

calcium aluminate silicate hydrates (C–A–S–H) phas-

es, and give strength for blended cement materials [13,

32]. However, because the OPC is typically replaced

N. T. Dung (&) � T.-P. Chang � C.-T. Chen � T.-R. YangDepartment of Civil and Construction Engineering,

National Taiwan University of Science and Technology,

(Taiwan Tech), Taipei 106, Taiwan

e-mail: tiendung568@gmail.com

Materials and Structures

DOI 10.1617/s11527-015-0630-6

by about 10 to 50 % in blended cement [18, 36], it just

reducedCO2 emission by 13–22 % [30].Moreover, the

replacement of fly ash/slag causes lower strength at

early ages [16]. On the other hand, although alkaline-

activated binder satisfies mechanical properties [5], it

is limited by some inherent problems such as rapid and

unsteady setting, cracking due to high shrinkage, and

huge energy consumption due to specific curing

requirement [21]. Therefore, there is a great need for

alternative eco-binders which only utilize common

industrial wastes. The environmental benefit for such

eco-binder is twofold; an alleviation of the environ-

mental impacts during the manufacturing process of

OPC, and a reduction of the cost for buildings and

landfill sites. Among various industrial wastes, the

circulating fluidized bed combustion (CFBC) fly ash is

a by-product fly ash resulting from CFBC technology

in thermal power plants. It becomes abundant world-

wide due to great advantages of applying CFBC

technology. However, CA particles are mainly coarse,

non-spherical, irregular [11] and have higher loss on

ignition, which may lead to high water demand [14]. In

addition, CA pastes exhibit a considerable amount of

expansion due to molar volumetric change. The

presence of calcium sulfate within Portland cement

or fly ash concrete can also result in additional

expansion due to formation of the ettringite (AFt) [9].

Furthermore, CA does not meet the specified require-

ments of ASTM 618 to qualify as Class C or Class F fly

ashes [10]. Therefore, to date, it is mostly disposed for

landfills [26] and might induce environmental prob-

lems, such as the contamination of groundwater and

surface waters, and the decreases of available landfill

sites [22, 33].

For these above-mentioned reasons, this research

developed an innovative eco-binder, named as SCA

binder, which only comprised the ground granulated

blast furnace (GGBF) slag (S), a by-product during the

iron production, and the CFBC fly ash (CA), a by-

product during the coal combustion, without adding

any OPC. The engineering properties of paste and

mortar made by such SCA binder with different

combination ratios of slag and CA with air or water

curing were examined for 91 days. In addition, to

evaluate the hydration products and reactivity

mechanism of SCA binder, X-ray diffraction (XRD)

and Energy Dispersive X-ray (EDX) were carried out

on SCA pastes. Moreover, the microstructure of SCA

paste and mortar was examined by Scanning Electron

Microscopy (SEM) micrographs. The experimental

results from this study show that the SCA binder is a

promising alternative to OPC with adequate engineer-

ing properties such as proper setting times, sufficient

high strength and less drying shrinkage, and suitable

for construction applications.

2 Research significance

This study involves an innovative cementitious SCA

binder simply made of two industrial wastes, ground

granulated blast furnace slag (S) and circulating

fluidized bed combustion fly ash (CA) without OPC

tomanufacture various mixtures of pastes andmortars.

Experimental results clearly show that the SCA binder

is appropriate for practical construction with adequate

strengths and resistance to shrinkage. Therefore, it is

likely to replace some of the OPC. In addition, this

study examines the microstructure of the SCA binder

and proposes mechanisms associated with the proper-

ties investigated. The outcome of this study are useful

for further researches on this promising SCA eco-

binder in the future. Furthermore, the use of S and CA

by-products to produce S eco-binder plays a pivotal

role to reduce environmental impacts, save natural

resources and create green materials.

3 Materials and testing methods

3.1 Materials

The fineness and chemical compositions of GGBF slag

and CA used in this experiment are shown in Table 1.

The XRD patterns in Fig. 1 indicate that the GGBF

slag is amorphous without clear peaks. Major miner-

alogical compositions of CA are anhydrite (CaSO4),

portlandite (Ca(OH)2), lime (CaO) and quartz (SiO2).

The presence of Ca(OH)2 is attributed to the reaction

between the free CaO in CA and moisture in air during

the storage. The alkalinity can break down glassy

structure of slag and react with the dissolved oxides of

slag and form hydration products to give strength.

The solution extracted from a pure CA paste with

water/CA of 0.40 was diluted by 1:100 using distilled

water and then measured for pH. The pH value of the

extracted solution is calculated as high as 12.5.

Materials and Structures

The engineering properties of the fine aggregates

used for the mortar tests are presented in Table 2. A

polycarboxylate superplasticizer (SP) was used to

obtain proper workability for SCA pastes and mortars.

3.2 Testing program

3.2.1 Specimen preparations

Based on the preliminary studies, a total of six mix

proportions of SCA pastes including three different

CA/binder ratios and two different W/B ratios were

tested in addition to the pure slag and pure CA pastes.

The mix proportions are shown in Table 3.

Furthermore, a total of 18 mix proportions of SCA

mortars including two different W/B ratios and three

different CA/binder ratios were selected. These SCA

mortars used 50–60 % fine aggregates by volume and

their mix proportions are shown in Table 4.

The specimens of SCA pastes andmortars were cast

in cubic molds of 50 9 50 9 50 mm for the com-

pressive strength test. They were also cast in prismatic

molds of 25 9 25 9 285 mm for expansion/shrink-

age test. Those specimens were de-molded after 24 h

of casting and then were equally divided into two

groups. The first group was cured in water with

temperature of 25 ± 2 �C, whereas the other group

was cured in air with relative humidity of 65 ± 5 %

RH and temperature of 25 ± 2 �C.

3.2.2 Test methods

(a) Setting time test

Vicat needle tests in according to ASTM C 191

[1] were conducted to determine the setting

time. In this study, penetration tests were

performed at regular time intervals of 10 min

until the final setting.

(b) Compressive strength test

The compressive strength of the SCA pastes and

mortars were tested in according to ASTM C

109 [6] at the ages of 7, 28, 56 and 91 days.

(c) Expansion/Shrinkage test

The expansion/shrinkage of the SCA pastes and

mortars were tested in accordance with ASTM

C 1148 [2] for 91 days.

(d) SEM/EDX observations

The sample for SEM observations was dried at

Table 1 Physical and chemical compositions of slag and CA

GGFB slag CA

Physical properties

Specific gravity 2.83 2.47

Blaine fineness (cm2/g) 6000 3000

Chemical compositions (%)

Loss of ignition 4.72 –

SiO2 34.9 5.22

Al2O3 13.53 2.21

CaO 41.47 56.80

MgO 7.18 2.06

SO3 1.74 32.40

S 0.65 –

Fe2O3 0.52 0.581

Fig. 1 XRD patterns of GGBF slag and CA

Table 2 Properties of fine aggregate

Fine aggregate

Specific gravity 2.65

Fineness modulus 2.64

Water absorption (%) 1.65

Materials and Structures

60 �C for 24 h. Afterwards, they were mounted

into aluminum stubs by double-sided adhesive

carbon disks and coated with gold before the

SEM observations. The instrument used for

SEM was a JSM-6390LV microscope equipped

with an OXFORD INCAX-7060 Energy Dis-

persive X-ray (EDX) spectrometer.

(e) XRD analyses

The SCA paste was ground until all passed

through the 75 lm sieve and then was dried at

60 �C for 24 h. The dried sample was prepared

for XRD analysis by a D2-phaser X-ray diffrac-

tometer with Cu Ka radiation. Step scan was

performed in the range of 5�–55� (2h) with a

stepping interval of 0.02� and a duration of

0.1 s.

4 Results and discussion

4.1 SCA pastes

4.1.1 Setting time tests of SCA pastes

The setting times of the pure slag, pure CA and SCA

pastes were presented in Table 3. The pure slag paste

is hard to set due to its glassy structure. However, the

SCA pastes utilizing 75–85 % slag and 25–15 % CA

can set easily. This is attributed to the portlandite

released from CA-water system breaks down and

dissolves glassy structure of slag [12] and then reacts

with the dissolved alumina and silica of slag to form

hydration products. As a result, the SCA pastes can set

and give strength. In general, the initial and final

setting times of the SCA paste are around 6–7 and

11.5–13.5 h, respectively. The long setting times of

SCA paste is attributed to slow hydration rate of CA

particles. The surface of CA particles is often covered

by CaSO4 layers, preventing the transport of H2O

molecules [25, 33] to result in the slow hydration rate

of CA. Therefore, the chemical reactions between the

hydrated CA products and the hydrated slag products

are delayed and the setting times of the SCA paste are

longer than those of the OPC paste.

These longer setting times of SCA paste might be

related to the use of SP, as mentioned by Singh et al.

[27]. For this reason, the initial setting of SCA pastes

with W/B of 0.35 is longer than that with W/B of 0.40

because the pastes with W/B of 0.35 used higher SP

amount than those with W/B of 0.40 (Table 3).

4.1.2 Compressive strength of SCA pastes

The compressive strengths of SCA pastes with W/B of

0.35 and 0.40 are shown in Figs. 2 and 3. The results

indicate that the pure slag or pure CA pastes give

Table 3 Mixture proportions and setting times of pastes

Mixture Binders W/

B

CA (kg/

m3)

Slag (kg/

m3)

Water (kg/

m3)

SP (kg/

m3)

Setting times (h)

Initial

setting

Final

setting

P15 W/B0.35 15 %CA ? 85 %

slag

0.35 209 1183 485 2.51 6.2 11.6

P20 W/B0.35 20 %CA ? 80 %

slag

0.35 277 1110 483 2.50 6.3 11.6

P25 W/B0.35 25 % CA ? 75 %

slag

0.35 346 1037 481 2.49 6.9 12.6

P0 W/B0.40 100 % slag 0.40 0.00 1314 524 1.05 34.3 54.3

P15 W/B0.40 15 % CA ? 85 %

slag

0.40 195 1106 519 1.04 6.3 12.6

P20 W/B0.40 20 % CA ? 80 %

slag

0.40 259 1037 517 1.04 6.0 13.8

P25 W/B0.40 25 % CA ? 75 %

slag

0.40 323 969 516 1.03 5.9 13.6

P100 W/

B0.40

100 % CA 0.40 1230 0.00 490 1.72 – –

Materials and Structures

compressive strengths less than 7 MPa (Fig. 3a).

However, the SCA pastes with both slag and CA

produce high compressive strengths. In particular, the

compressive strength of the SCA pastes ranges from

about 40 to 70 MPa at 28 days and even higher at

longer ages. It is evident that the CA plays an

important role as an activator. The CA as small as

15 % by mass can produce alkaline environment to

break down glassy structure of slag. As a consequence,

the portlandite of CA-water system can react with the

dissolved alumina and silica of slag to generate new

hydration products which produce high strengths of

SCA pastes. In general, the compressive strength of

SCA paste increases with age. The rate of strength

gain is high at early ages and gradually decreases at

later ages.

The compressive strength of the SCA paste de-

creases as the W/B increases. In particular, as the W/B

increases from 0.35 to 0.40, the compressive strength

of the SCA paste decreases by about 10 to 20 MPa.

This could be related to the fact that higher W/B leads

to more free water and more porous microstructure in

the SCA paste so that the compressive strength of SCA

paste with high W/B is less than that with low W/B.

The curing condition also affects the compressive

strength of the SCA paste. The water curing seems not

good for the compressive strength of SCA paste at

early ages, especially for those with 25 % CA. The

results demonstrate that, in most cases, the early

compressive strengths of specimens cured in water are

lower than those cured in air. This could be attributed

to abundant AFt formation during the CA hydration in

those specimens with water curing at early ages [25].

As a result, the AFt expansion occurs considerably at

early ages. By the time, the strength of specimens is

small so that micro-cracks occur and the compressive

strength is reduced. However, these micro-cracks may

allow water to come in contact easily with the CA

particles. Therefore, the hydration products are easily

produced in water than in air. Accordingly, at later

ages, these hydration products fill the cracks and

increase the strength. The AFt expansion becomes

harmless at this time because the specimens have

gained sufficient strength. Therefore, although the

compressive strengths of the SCA pastes cured in

water are lower than those cured in air at early ages,

they reach the same or even higher strengths than those

cured in air in the long term.

Table 4 Mix proportions of SCA mortars

Mixture W/B CA/

(CA ? slag) (%)

Volume

of fine

aggregate

(%)

CA (kg/m3) Slag (kg/m3) Fine aggregate

(kg/m3)

SP (kg/m3) Water

(kg/m3)

W/B0.35CA15S50 0.35 15 50 99 562 1325 0.20 231

W/B0.35CA15S55 0.35 15 55 87 502 1458 0.59 206

W/B0.35CA15S60 0.35 15 60 78 442 1590 1.04 181

W/B0.35CA20S50 0.35 20 50 132 527 1325 0.20 230

W/B0.35CA20S55 0.35 20 55 118 471 1458 0.88 205

W/B0.35CA20S60 0.35 20 60 104 415 1590 1.30 180

W/B0.35CA25S50 0.35 25 50 164 492 1325 0.20 229

W/B0.35CA25S55 0.35 25 55 147 440 1458 0.59 205

W/B0.35CA25S60 0.35 25 60 129 387 1590 1.03 180

W/B0.40CA15S50 0.40 15 50 93 525 1325 0.19 247

W/B0.40CA15S55 0.40 15 55 83 469 1458 0.55 220

W/B0.40CA15S60 0.40 15 60 73 413 1590 0.97 193

W/B0.40CA20S50 0.40 20 50 123 492 1325 0.18 246

W/B0.40CA20S55 0.40 20 55 110 440 1458 0.82 219

W/B0.40CA20S60 0.40 20 60 97 388 1590 1.21 193

W/B0.40CA25S50 0.40 25 50 153 460 1325 0.18 245

W/B0.40CA25S55 0.40 25 55 137 411 1458 0.82 218

W/B0.40CA25S60 0.40 25 60 121 362 1590 0.97 192

Materials and Structures

4.1.3 Expansion/shrinkage of SCA pastes

The expansion/shrinkage behavior of the SCA

paste is illustrated in Fig. 4. The results demon-

strate that the CA content and curing condition

highly affect the expansion/shrinkage behavior of

the SCA paste.

With water curing, the SCA paste strongly expands

at early ages and then gradually become stable at

longer ages. Figure 4 clearly shows that the amount of

CA greatly affects the expansion of SCA paste. In

particular, higher CA amount results in higher expan-

sion, especially when CA is larger than 20 %. During

the first 4 days, the SCA pastes with 15 % CA

(P15 W/B0.35 and P15 W/B0.40) and 20 % CA

(P20 W/B0.35 and P20 W/B0.40) expand to about

100 9 10-6 m/m and 200 9 10-6 m/m, respectively

and become steady afterwards. On the other hand,

during the first 42 days, the SCA pastes with 25 % CA

(P25 W/B0.35 and P25 W/B0.40) quickly expand to

around 400 9 10-6 m/m and then gradually stable.

The drastic expansion of SCA paste with high CA

content is attributed to larger AFt expansion [34] due

to the hydration of CA containing high content of

CaSO4 and CaO (Fig. 1). The water curing is also

beneficial to the expansion of AFt because CA can

absorb water easily [19, 25]. As a result, the pastes

with 25 % CA cured in water give large expansion.

The W/B slightly affects the expansion of SCA

paste cured in water. Higher W/B results in higher

expansion, especially for specimens with 25 % CA

(P25 W/B0.40). This phenomenon may be related to

(a) Air curing (b) Water curing

Fig. 2 Compressive

strength of SCA pastes with

W/B of 0.35

(a) Air curing (b) Water curing

Fig. 3 Compressive

strength of SCA pastes with

W/B of 0.4

Materials and Structures

the fact that higher W/B specimens have porous

microstructures, resulting in opening spaces for the

hydration of CA and the expansion due to the AFt

formation.

By contrast, with air curing, the SCA paste expands

during the first few days and then shrinks afterwards. It

can be seen that the early expansion of the SCA paste

compensates for its later drying shrinkage. The highest

expansion is nearly 100 9 10-6 m/m for specimens

with both W/B ratios. The SCA paste shrinks soon

after expansion. The drying shrinkage increases with

time and the rate of drying shrinkage decreases rapidly

with time. The drying shrinkage seems to be constant

after 21 days of exposure. The test data indicate that

higher amount of CA results in lower drying shrinkage

with the exception of the SCA paste with 20 %CA and

W/B of 0.35 (P20 W/B0.35), which has lower

shrinkage than that of the SCA paste with 25 % CA

and W/B of 0.35 (P25 W/B0.35). This might be

attributed to the fact that higher expansion resulted

from higher CA amount (as explained above) com-

pensates for drying shrinkage more effectively. After

91 days of exposure, the largest shrinkage (about

-200 9 10-6 m/m) and the smallest shrinkage (about

-100 9 10-6 m/m) were observed in the SCA paste

with 15 % CA and W/B of 0.40 (P15 W/B0.40) and

that with 20 % CA and W/B of 0.40 (P20 W/B0.35),

respectively.

The SCA paste cured in air simultaneously expands

and shrinks so that the expansion is small and only

occurs in a few days at the beginning. Therefore, there

is no clear influence of the W/B on the expansion/

shrinkage behavior of SCA paste cured in air.

4.1.4 SEM observations of SCA pastes

The SEM images in Fig. 5 show that the hydration

products of the pure slag paste do not have good

bonding capability due to many interior pores

(Fig. 5a). The pure CA paste also has similar poor

microstructure (Fig. 5b). This is the reason why the

pure slag and pure CA paste only produce very small

compressive strengths at all ages. However, the SCA

pastes produce more hydration products being bonded

together to form a continuous solid phase, acting like

cement (Fig. 5c, d, e). This might be the main reason

why the SCA paste has satisfactory cementitious

property.

The microstructure of SCA paste with 20 % CA

and W/B of 0.40 (P20 W/B0.40) (Fig. 5d) exhibits

the most compact matrix among the SCA pastes

with W/B of 0.40 cured in air. It is consistent with

the fact that the highest compressive strength at

91 days was observed in the P20 W/B0.40 paste. In

contrast, the lowest compressive strength of the

SCA paste with 25 % CA and W/B of 0.40

(P25 W/B0.40) paste could be attributed to its

porous microstructure (Fig. 5e).

4.1.5 Hydration products of SCA pastes

The hydration products of SCA binder were investi-

gated by SEM/EDX and XRD, and the results are

shown in Figs. 5 and 6, respectively. The SEM results

show that the SCA paste generates several hydrated

phases bonded together and thus the SCA paste has

(a) Air curing

Time of exposure (days)

-250

-200

-150

-100

-50

0

50

100

Len

gth

chan

ge (

×10

- 6m

/m)

P15W/B0.35

P20W/B0.35

P25W/B0.35

P15W/B0.40

P20W/B0.40

P25W/B0.40

(b) Water curing

0 20 40 60 80 100

0 20 40 60 80 100Time of exposure (days)

0

100

200

300

400

500

600

700

Len

gth

chan

ge (

×10

-6m

/m)

P15W/B0.35

P20W/B0.35

P25W/B0.35

P15W/B0.40

P20W/B0.40

P25W/B0.40

Fig. 4 Length change of SCA pastes

Materials and Structures

Fig. 5 SEM micrographs of pastes cured in air at 91 days

Materials and Structures

self-cementitious properties. In Fig. 6, the intensities

of diffraction peaks for all hydration products increase

with age, explaining that the increase of hydration

products leads to the increase in strength and expan-

sion with time. However, the increasing rate is fast at

the first 7 days and then slow afterwards, explaining

why its increase in strength and expansion is fast at

early ages and gradually slows down at longer ages. In

addition, the XRD results indicate that the diffraction

peaks of hydration products are more intensified in

water than in air. This ought to result in higher

compressive strength of the SCA pastes cured in water

than those cured in air. However, at early ages, the

larger AFt expansion of specimens cured in water

causes lower compressive strength. Therefore, the

SCA paste cured in water has lower strength at early

ages but obtains equivalent or higher strength at later

ages than the SCA paste cured in air, as mentioned in

Sect. 4.1.2.

The hydration products of CA contain gypsum

(CaSO4. 2H2O) and portlandite (Ca(OH)2). The gyp-

sum is attributed to the hydration of the anhydrite

(CaSO4) in CA, whereas the portlandite is attributed to

the hydration of free-CaO [15]. The solution from the

CA-water system is a strong alkaline solution with pH

of 12.5 (as reported in Sect. 3.1) so that it is able to

dissolve the active Al2O3 and SiO2 of slag [7, 12, 24].

Thus, in the SCA paste, the Ca(OH)2 of CA-water

system could react with the active Al2O3 dissolved

from slag. As a consequence, the AFt observed in

SEM images (Fig. 5) and in XRD patterns (Fig. 6) is

formed due to the reaction, as shown in Eq. (1) [25]:

3Ca(OH)2 þ 3CaSO4 � 2H2Oþ Al2O3 þ 23H2O

¼ Ca6Al2ðSO4Þ3ðOH)12 � 26H2O(AFt) ð1Þ

The AFt is very important for the hardening and the

compressive strength development of the SCA paste at

early stage [8, 25]. The XRD results indicate that the

diffraction peaks of AFt are higher for specimens

cured in water than in air, causing higher expansion, as

seen in Fig. 4a and b.

Moreover, at high pH, the Ca(OH)2 from CA-water

system could react with the active SiO2 dissolved from

(a) Air curing (b) Water curing

Fig. 6 XRD patterns of

SCA paste with 20 % CA

and W/B of 0.35 (P20 W/

B0.35)

Materials and Structures

slag to generate calcium silicate hydrate (C–S–H), as

shown in Eq. (2) [25]:

xCa(OH)2 þ SiO2 þ ðy� xÞH2O ¼ CxSHyðC�S�HÞð2Þ

The C–S–H is very important for the later strength

development of the SCA paste. However, C–S–H is

difficult to be found in XRD patterns because of its

amorphous structures [14, 25]. Moreover, calcium

alumina silicate hydrates (C–A–S–H) is produced

from the crystallization process of the dissolved

alumina and the silica in slag. The XRD results

indicate the existence of C–A–S–H (CA2S6H8) in the

SCA paste.

Several locations of the SCA pastes were investi-

gated for their composition by EDX. The EDX results

shown in Table 5 identify the atomic ratio of Si/Ca

ranging from 0.48 to 0.66 and the Al/Ca ranging from

0.17 to 0.41. The locations 1, 2, 4 and 5 with the Si/Ca

from 0.48 to 0.66 and the Al/Ca from 0.17 to 0.26

contain C–S–H phases [17, 28, 29], whereas the

location 3 with the Si/Ca of 0.54 and Al/Ca of 0.41

contains C–A–S–H phase.

4.2 SCA mortars

4.2.1 Compressive strength of SCA mortars

The compressive strengths of SCA mortars with W/B

of 0.35 and 0.40 are presented in Figs. 7 and 8,

respectively. The results indicate that it is feasible to

produce the SCA mortar with a compressive strength

ranging from 45 to 70 MPa at 28 days. The compres-

sive strength of the SCA mortar decreases as the W/B

increases. In most cases, as the W/B increases from

0.35 to 0.40, the compressive strength of SCA mortar

decreases by about 10 MPa.

Similar to the SCA paste, the compressive strength

of SCA mortar increases with age. The rate of strength

gain is high at early ages and gradually decreases at

later ages. However, unlike the SCA paste, the water

curing does not adversely affect the compressive

strength of SCA mortar at early ages. This might be

due to the fact that the fine aggregates can resist the

large deformation caused by AFt. In most cases, the

compressive strength of specimens cured in water is

similar to those cured in air.

The results in Figs. 7 and 8 suggest that, to obtain

high compressive strength, the fine aggregate of the

SCAmortar with W/B of 0.35 should not exceed 55 %

by volume. In contrast, with W/B of 0.40, the fine

aggregates should not be less than 55 % by volume.

It is worth of noting that, in most cases, the

compressive strength of SCA mortar is equivalent

or even higher than that of the SCA paste with the

same W/B. It is evident that the bonding capability

between fine aggregates and the SCA paste is quite

adequate.

4.2.2 Expansion/Shrinkage of SCA mortars

The expansion/shrinkage behavior of SCAmortars are

shown in Fig. 9. The results indicate that the CA

content and curing condition significantly influence

the expansion/shrinkage behavior of the SCA mortar.

Similar to the SCA paste, the SCA mortar cured in

water expands with time, and the expansion rate

decreases rapidly with time, especially after 7 days

(Fig. 9b). It is consistent with the fact that the AFt

formation slows down after 7 days (Fig. 6). For SCA

mortar, the rate of expansion is similar, but not as

rapid as that of the SCA paste. The fine aggregate

resists the expansion of the SCA paste so that the

expansion of SCA mortar is less than that of the SCA

Table 5 Proportions of

components analyzed by

EDX

Location Elements Si/Ca Al/Ca

Ca (%) Si (%) Al (%) O (%)

1 (Fig. 5c) 33.06 19.12 7.20 40.62 0.57 0.22

2 (Fig. 5c) 31.12 19.37 7.64 41.87 0.62 0.25

3 (Fig. 5d) 27.16 14.65 11.26 46.93 0.54 0.41

4 (Fig. 5d) 25.45 16.83 6.55 51.17 0.66 0.26

5 (Fig. 5e) 36.09 17.32 6.04 40.56 0.48 0.17

Materials and Structures

paste when compared at the sameW/B. The amount of

expansion is constant after 21 days. With the same

W/B and volume of fine aggregate, the SCA mortars

with 25 % CA give expansion about 80 9 10-6 m/m

higher than that of those with 20 % CA. The

expansion is related to the AFt formation caused by

the presence of CaSO4 in CA (as explained in Sect.

4.1.5) so that higher CA amount results in higher

expansion of the SCA paste and mortar.

Because the expansion of SCA mortars with

20 % CA is low (less than 100 9 10-6 m/m), the

effect of the W/B and the volume of fine aggregate

(a) Air curing

(b) Water curing

Fig. 7 Compressive strength of SCA mortars with W/B of 0.35

Materials and Structures

on the expansion of SCA mortars with 20 % CA is

not clear. However, for the SCA mortars with 25 %

CA, higher volume of fine aggregate shows higher

restraining effect on the expansion of SCA mortar.

Thus, after 91 days of expansion, the highest

expansion (about 150 9 10-6 m/m) and the lowest

expansion (about 60 9 10-6 m/m) of SCA mortars

were observed in W/B0.40CA25S50 (W/B of 0.40,

25 % CA and 50 % fine aggregates by volume) and

W/B0.40CA20S60 (W/B of 0.40, 20 % CA and

60 % fine aggregates by volume), respectively, as

seen in Fig. 9b.

With air curing, the SCA mortar simultaneously

expands and shrinks so that their expansion is not

(a) Air curing

(b) Water curing

Fig. 8 Compressive strength of SCA mortars with W/B of 0.4

Materials and Structures

clear. The results show that the SCA mortars with

25 % CA only expand on the first day and then shrink

afterwards, while the SCA mortars with 20 % CA

shrink right after it was exposed to air (Fig. 9a). The

early expansion of SCA mortars with 25 % CA

compensates for their later drying shrinkage so that

their drying shrinkage is less than that of those mortars

with 20 % CA.

Similar to the SCA paste, the shrinkage rate of SCA

mortar is high at early ages and gradually decreases at

longer ages. The drying shrinkage of the SCA mortar

is roughly constant after 28 days of exposure. Because

the drying shrinkage of SCA mortar is minor, there is

no clear influence of the W/B and the amount of fine

aggregate on the drying shrinkage behavior of SCA

mortar cured in air. However, it can be seen that the

(a) Air curing

(b) Water curing

0 20 40 60 80 100Time of exposure (days)

-125

-100

-75

-50

-25

0

25

Leng

thch

ange

(×10

- 6m

/m)

0 20 40 60 80 100Time of exposure (days)

0

25

50

75

100

125

150

175

Leng

thch

ange

(×10

-6m

/m)

W/B0.40CA25S50W/B0.40CA25S55W/B0.40CA25S60

W/B0.40CA20S50W/B0.40CA20S55W/B0.40CA20S60

W/B0.35CA20S50W/B0.35CA20S55W/B0.35CA20S60

Fig. 9 Length change of

SCA mortars

Materials and Structures

fine aggregate restrains the drying shrinkage of SCA-

water system so that the drying shrinkage of SCA

mortars was less than that of the SCA pastes.

4.2.3 SEM observations of SCA mortars

Figure 10 shows the microstructural characteristics of

the SCA mortars with 55 % fine aggregate (by

volume) using W/B of 0.40. A plenty of massive

continuous hydration products of the SCA mortars

were observed in SEM images, especially in those of

the SCA mortars with 20 and 25 % CA. Such

hydration products explain why the SCA mortars with

55 % fine aggregate can produce a compressive

strength as high as 50–70 MPa at 91 days. In addition,

the densest microstructures of the SCA mortars with

20 and 25 % CA explain why their compressive

strengths are higher than those with 15 % CA.

5 Conclusions

An innovative slag eco-binder (SCA) without OPC

was created in this study by using CFBC fly ash (CA)

to activate the hydration of GGBF slag (S). The

engineering properties and microstructure of the SCA

paste and mortar were investigated. Through the

experimental results, the following conclusions can be

drawn:

1. The Ca(OH)2 from CA-water system can dissolve

slag and then react with the active SiO2 and the

active Al2O3 of slag to form AFt, C–S–H and C–

A–S–H, all of which enable the SCA paste to set

properly and form dense microstructure.

2. A plenty of massive continuous hydration prod-

ucts with highly dense microstructures give

compressive strength of the SCA paste and mortar

as high as 40–70 MPa at 28 days, which is

suitable for the construction in practice. The

compressive strengths increase with time but the

increasing rate also decreases with time. Such

results are consistent with the formation of

hydration products, which are produced fast at

early ages and gradually slow at later ages.

3. In general, lower W/B results in higher compres-

sive strength of SCA paste and mortar. The SCA

pastes cured in water without fine aggregate suffer

drastic expansion of AFt so that they have lower

early compressive strength than those cured in air.

But, their strengths in the long terms are

equivalent to or even higher than those cured in

air due to the mass hydration products.

Fig. 10 SEM micrographs of SCA mortars cured in air at

91 days

Materials and Structures

4. With water curing, the SCA paste and mortar

expand fast at early ages and then the expansion

rate rapidly decreases. Higher CA results in higher

expansion. The fine aggregate significantly re-

duces expansion of the SCA paste. With air

curing, however, the SCA paste and mortar

expand during the first few days and shrink later.

The early expansion compensates for its drying

shrinkage, resulting in low drying shrinkage.

Acknowledgments The authors gratefully acknowledge the

financial support of this work by National Taiwan University of

Science and Technology (Taiwan Tech) through the

scholarships.

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