Activation of Ground Granulated Blast Furnace Slag … · Activation of Ground Granulated Blast...

Activation of Ground Granulated Blast Furnace Slag Cement by Calcined Alunite

Hyung-Seok Kim1, Joo-Won Park2, Yong-Jun An3, Jong-Soo Bae2 and Choon Han2;*

1Minerals & Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Daejeon, Korea2Department of Chemical Engineering, Kwangwoon University, Seoul, Korea3Department of Resource Recycling Engineering, University of Science and Technology, Daejeon, Korea

To enhance the early strength of grounded granulated blast furnace slag (GGBFS) blended cement, the activation characteristics ofGGBFS were examined by a potassium aluminum sulfate (PSA) clinker, consisting of KAl(SO4)2 and amorphous Al2O3 by calciningalunite [K2SO4�Al2(SO4)3�4Al(OH)3] at 650�C for 30min. The PSA clinker reacted with calcium hydroxide and gypsum to formettringite (3CaO�Al2O3�3CaSO4�32H2O, AFt) by following reaction: 2KAl(SO4)2 þ 2Al2O3 þ 13Ca(OH)2 þ 5(CaSO4�2H2O)þ 74H2O !3(3CaO�Al2O3�3CaSO4�32H2O)þ 2KOH. Mortar was prepared by mixing a blended cement of GGBFS and ordinary Portland cement (OPC)with PSA clinker as activator. The compressive strength of the GGBFS blended cement mortar was compared with that of OPC mortar. Whenthe PSA clinker and gypsum activator was added to the blended cement of GGBFS and OPC, the hydration products investigated by DTA and X-ray diffraction were mainly ettringite and calcium silicate hydrate(C-S-H) gel. The early and long-term strengths of the GGBFS blended cementwere higher than those of OPC. Therefore, PSA clinker as activator was shown to improve the early and long-term strengths of GGBFS blendedcement. [doi:10.2320/matertrans.M2010350]

(Received October 14, 2010; Accepted November 11, 2010; Published December 22, 2010)

Keywords: ground granulated blast furnace slag (GGBFS), activator, alunite, potassium aluminum sulfate, ettringite

1. Introduction

Grounded granulated blast furnace slag (GGBFS) is aglassy granular material formed when molten GGBFS israpidly cooled, usually by immersion in water, and thenground to improve its reactivity. The major components ofGGBFS are SiO2, CaO, MgO, and Al2O3, which are commoncomponents in commercial silicate glasses. It has been usedas a pozzolanic admixture in Portland cement paste.1–6)

GGBFS is effective in reducing the hydration heat ofcement and has a high resistance to freezing, thawing,chemicals and seawater.7,8) It is therefore, recommended forconcrete structures that require high durability.

GGBFS is also environmentally-friendly as it reduces theuse of ordinary Portland cement (OPC) clinkers in proportionto the amount of GGBFS that is substituted for OPC. It alsodecreases the amount of CO2(g) generated from the thermaldecomposition of limestone, a material used to produce OPCclinkers.9) However, the use of GGBFS as a cement additivehas its drawbacks, including delaying the setting time ofconcrete,10) which prolongs construction.

Although slag without an activator does react with water,the rate of hydration is very slow. Its hydraulic reactivitydepends on chemical composition, glass phase content,particle size distribution, and surface morphology.1–3)

The reduction of initial strength of GGBFS cement couldbe overcome if the fineness of GGBFS were increased topromote hydration speed. However, increasing the finenessof slag by pulverization could increase the manufacturingcost of GGBFS.

A coating of aluminosilicate forms on the surfaces of slaggrains within a few minutes of exposure to water, and thesecoatings were impermeable to water. Unless a chemicalactivator is present, further hydration is inhibited. In general,Portland cement, gypsum, and many alkalies have been

used as activators and the rate of hydration is faster at highalkali concentrations. The surface of slag is amorphous,and its dissolving behavior is very similar to that of silicateglasses.3)

GGBFS is a low performance cementitious material,which can achieve high compressive strength when analkaline activator is used. However, crucial investigationsabout the activation of GGBFS had been already made.11)

Alkaline activators are classified in six groups;12) M is analkaline:(a) Caustic alkalis, MOH(b) Non-silicate weak acid salts: M2CO3, M2SO3, M3PO4,

MF, etc.(c) Silicates, M2O�nSiO2.(d) Aluminates, M2O�nAl2O3.(e) Aluminosilicates, M2O�nAl2O3�(2-6)SiO2.(f) Non-silicate strong acid salts, M2SO4

13,14)

Of all these activators, NaOH, Na2CO3, Na2O�nSiO2 andNa2SO4 are the most widely available and economicalchemicals. Some potassium compounds have been used inlaboratory studies. However, their potential applicationswill be very limited due to their availability and costs.Conversely, the properties of sodium and potassium com-pounds are very similar.

This study compares the compressive strength and hydra-tion properties of GGBFS mixed cement with the calcinedalunite (KAl(SO4)2 and amorphous Al2O3) as the activator,which could be regarded as aluminates or non-silicate strongacid salts, with those of OPC.

2. Experimental Methods

2.1 Raw materialsOPC was obtained from a Korean cement company,

alunite from Gasa Island in Korea, and GGBFS from aKorean steel mill. GGBFS was in an amorphous state, itsbasicity [b ¼ ðCaOþMgOþ Al2O3Þ=SiO2] was 1.85 and its*Corresponding author, E-mail: [email protected]

Materials Transactions, Vol. 52, No. 2 (2011) pp. 210 to 218#2011 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

http://dx.doi.org/10.2320/matertrans.M2010350

fineness [blaine] was 4,060 cm2/g. The chemical composi-tions of OPC, GGBFS, gypsum and alunite are shown inTable 1.

When alunite is heated between 500 and 600�C, as shownin Fig. 1, it becomes PSA clinker, consisting of KAl(SO4)2and Al2O3 by the following dehydration reaction:15,16)

K2SO4�Al2(SO4)3�4Al(OH)3! 2KAl(SO4)2 þ 2Al2O3 þ 6H2O ð1Þ

The PSA clinker used as activator was prepared bysintering alunite at 550�C for 1 h. Figure 2 and Table 2 showthe results of XRD analysis and the chemical composition ofPSA clinker, respectively.

2.2 MethodThree activators were used: gypsum, PSA clinker, and a

mixture of the two with a mass ratio of 100 : 21 as shownin Table 3. In the OPC-GGBFS-activator ternary system,the activators were mixed at 0, 7.5 and 15.0%, as shown inFig. 3.

Mortars were prepared, stored and tested according to themethod described by KSL ISO 679. Blended cement, sandand water were mixed with a mass ratio of 1 : 3 : 0:5 asshown in Table 4. Prepared mortar was stored in 5 cm cubicmolds, in a humidity chamber. After 24 h, it was removed andcured in water at 20�C. The compressive strengths of themortars were measured using a universal testing machine(Heung Jin, Korea).

Hydration of the slag was performed by cement paste beingprepared with a 0.5 water/cement ratio and curing in water at23� 2�C. The hydration reaction of the cement paste wasinterrupted with acetone and hydrate analyzed with XRD(Philips, Netherlands) and TG-DTA (Shimadzu, Japan).

Table 1 Chemical composition of raw materials.

Raw

materialsIg. loss SiO2 Fe2O3 Al2O3 CaO MgO Na2O K2O SO3

OPC — 19.68 5.48 5.08 60.66 1.92 — — 1.45

GGBFS — 34.30 0.11 14.50 43.15 5.61 0.26 0.24 0.04

Gypsum — 2.96 0.38 0.41 29.38 0.40 — — 15.74

Alunite 37.99 6.67 1.16 33.86 0.10 0.01 0.72 9.33 35.46

60

70

80

90

100

12731073873673473

∆T

TGA

Wei

gh

t, m

ass

%

Temperature, T/K

0

DTA

Fig. 1 TG and DTA analyses of alunite.

Table 2 Chemical composition of calcined alunite.

Composition SiO2 Al2O3 Fe2O3 K2O Na2O TiO2 CaO MgO P2O3 SO3

mass% 7.60 38.60 1.32 10.64 0.82 0.10 0.12 0.10 0.27 40.42

10

P: KAl(SO4)2

Q: Quartz

QQ

Q

Q

PP

P

Inte

nsi

ty, a

rb. u

nit

Diffraction angle, 2θ /degree

P

6050403020

Fig. 2 XRD pattern of calcined alunite.

Table 3 The tested activators.

Activators Add amount (mass%)

Gypsum 0, 7.5, 15

PSA clinker 0, 7.5, 15

PSA + Gypsum 0, 7.5, 15

0.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

9

Slag

experimentalAc

tivat

or

OPC

1

10 2

11 3

12 4

1.00.90.80.70.60.50.40.30.20.1

5

6

7

8

Fig. 3 Ternary diagram of the admixture of raw materials.

Activation of GGBFS Cement by Calcined Alunite 211

3. Results and Discussion

3.1 Compressive strength propertyTable 5 shows the compressive strengths of the mortars at

each curing time according to the mixture of OPC andGGBFS. The compressive strengths of OPC after 3, 7, 28 and56 days were 18.3, 23.7, 32.4 and 36.8MPa, respectively.After 3 days, the compressive strength of the blended cementconsisting of OPC and GGBFS had decreased from 88% to68% of that of OPC with increasing GGBFS content from30 to 60%. However, increasing GGBFS content led to thecompressive strength of GGBFS blended cement surpassingthat of OPC after 28 days.

Table 5 shows the compressive strengths of the blendedcements being lower than those of OPC for up to 7 days.This is why gypsum is generally used as an activatorto enhance the compressive strength of GGBFS blendedcement.

Table 6 shows the compressive strengths of mortars atvarious curing times based on the proportions of the mixtureof OPC, GGBFS and gypsum.

When 7.5% gypsum was used as activator, the compres-sive strength of GGBFS blended cement after 3 days waslower than that of OPC when the proportion of GGBFS wasbelow 60% but surpassed that of OPC after 7 days. With 15%gypsum, the compressive strength of GGBFS blended cementwas always lower than that of OPC indicating that excessiveaddition of gypsum reduces the early and long-term strengthsof GGBFS blended cement.

Although the long-term strength of blended cementconsisting of OPC, BFS and gypsum improved withincreasing BFS content, its early strength was lower thanthat of OPC. Therefore, a PSA clinker of KAl(SO4)2 andamorphous Al2O3 was used as activator instead of gypsum.The results of compressive strength tests of GGBFS cementblended by using solely PSA clinker as activator are shown inTable 7. Table 8 shows results from the use of a mixedactivator of gypsum and PSA.

Blended cement with PSA clinker, GGBFS and OPCcontents of 7.5, 30 and 62.5%, respectively, had a highercompressive strength than OPC. With GGBFS contents of>40%, the compressive strength of the blended cement was

Table 4 The tested mortar (KSL ISO 679).

Materials Admixture ratio (mass ratio)

OPC + slag + activator 1

Sand 3

Water 0.5

Table 5 Effect of GGBFS on cement’s compressive strength.

MortarDosage (mass%) Compressive strength (MPa)

OPC GGBFS 3 days 7 days 28 days 56 days

S1 100 0 18.3 23.7 32.4 36.8

S2 70 30 16.0 18.4 36.3 44.0

S3 60 40 15.8 20.7 40.5 48.4

S4 50 50 12.7 21.6 43.2 48.8

S5 40 60 12.4 24.3 44.0 46.9

Table 6 Effect of gypsum on cement’s compressive strength.

MortarDosage (mass ratio) Compressive strength (MPa)

OPC GGBFS gypsum 3 days 7 days 28 days 56 days

�G1 0.625 0.3 0.075 12.3 21.1 35.1 37.1

G2 0.525 0.4 0.075 11.5 26.2 38.1 40.0

G3 0.425 0.5 0.075 11.2 28.6 38.5 43.5

G4 0.325 0.6 0.075 15.6 29.6 41.3 44.6

G5 0.55 0.3 0.15 9.3 12.0 17.5 19.3

G6 0.45 0.4 0.15 8.7 13.1 18.8 28.1

G7 0.35 0.5 0.15 8.5 14.1 21.1 37.3

G8 0.25 0.6 0.15 8.1 17.4 27.3 35.6

�G: Gypsum (CaSO4�2H2O)

Table 7 Effect of PSA on cement’s compressive strength.


OPC GGBFS Activator 3 days 7 days 28 days 56 days

�P1 0.625 0.3 0.075 19.5 35.4 44.6 46.5

P2 0.525 0.4 0.075 15.3 33.5 43.5 49.4

P3 0.425 0.5 0.075 15.6 32.7 40.1 45.3

P4 0.325 0.6 0.075 18.1 23.7 29.8 31.9

P5 0.55 0.3 0.15 3.5 4.6 5.1 7.9

P6 0.45 0.4 0.15 3.1 4.3 4.7 6.6

P7 0.35 0.5 0.15 — — — —

P8 0.25 0.6 0.15 — — — —

�P: PSA (potassium sulfoaluminate + amorphous Al2O3)

212 H.-S. Kim, J.-W. Park, Y.-J. An, J.-S. Bae and C. Han

initially lower than that of OPC but became higher after 7days. With 15.0% PSA clinker, mortars expanded excessive-ly with negligible development of compressive strength.However, both 7.5 and 15% of the mixed activator of PSAclinker and gypsum formed GGBFS blended cement withcompressive strengths higher than those of OPC at all curingtimes for GGBFS contents of <50%. Therefore, this mixedactivator was able to increase both the early and long-termstrengths of GGBFS blended cement better than PSA clinkeralone.

Tables 7 and 8 shows that the compressive strength ofGGBFS blended cement varies sensitively by activator and

the mixing ratio. Accordingly, the compressive strengths ofthe OPC-GGBFS-activator ternary system for various curingtimes were applied to the secondary regression equation ineq. (2), and compressive strength graphs drawn in MATLAB7.0.1 are displayed in Figs. 4, 5 and 6.

y ¼ b1x1 þ b2x2 þ b3x3 þ b12x1x2 þ b13x1x3 þ b23x2x3 ð2Þ

y compressive strength (MPa)x1 OPC ratiox2 GGBFS ratiox3 activator’s ratiobi constants.

Table 8 Effect of PSA and gypsum admixture on cement’s compressive strength.


OPC GGBFS Activator 3 days 7 days 28 days 56 days

�PG1 0.625 0.3 0.075 24.1 32.1 43.3 46.9

PG2 0.525 0.4 0.075 20.0 30.7 45.5 49.3

PG3 0.425 0.5 0.075 22.1 33.6 46.9 49.4

PG4 0.325 0.6 0.075 18.2 27.5 42.3 45.6

PG5 0.55 0.3 0.15 23.0 32.5 44.6 42.0

PG6 0.45 0.4 0.15 25.4 32.6 45.0 46.9

PG7 0.35 0.5 0.15 21.7 30.7 37.9 40.7

PG8 0.25 0.6 0.15 5.6 15.8 24.9 32.8

�PG: PSA + gypsum [1 : 1 (mass ratio)]

(a) 3-day strength, MPa

(c) 28-day strength, MPa (d) 56-day strength, MPa

(b) 7-day strength, MPa

Fig. 4 Composition for optimum compressive strengths of OPC-GGBFS-gypsum cements: strengths after (a) 3, (b) 7, (c) 28, and (d) 56

days.


When only gypsum was used as activator, there was muchvaration of compressive strength of GGBFS blended cementwith different mixtures of OPC, GGBFS and gypsum.Furthermore, good early compressive strengths of GGBFSblended cement were not achieved, with all the tested blendsbeing initially weaker than OPC. However, GGBFS blendedcement had improved long term strength when less than 5%gypsum was used.

When only 4–7% PSA clinker was used, the compressivestrengths of GGBFS blended cement after 3 and 7 days werehigher than those of OPC. However, its increase from 7.5to 15% reduced significantly the compressive strength ofGGBFS blended cement. However, after 28 days, cementblended with 30–60% GGBFS with activator content <7:5%was greater compressive strengths than OPC.

A mixed activator of PSA clinker and gypsum producedGGBFS blended cements of higher compressive strengthsthan OPC under the following conditions: after 3 days with30–50% GGBFS and >7:5% activator; 7 days with 40–55%GGBFS and 7.5–15% activator; after 28 days with 40–55%GGBFS and 3–10% activator.

3.2 Hydration propertyThe hydration characteristics of PSA clinker as activator

were tested by mixing with calcium hydroxide (Ca(OH)2)and gypsum (CaSO4�2H2O) at various molar ratios rangingfrom 1 : 13 : 1 to 1 : 13 : 5, and reaction with water for 28days. The XRD results of the hydration are shown in Fig. 7.

With increasing gypsum content, ettringite peaks in-creased. At the mole ratio of 1 : 13 : 5, only ettringite wasformed, indicating that PSA had undergone hydration by thefollowing reaction:

2KAl(SO4)2 þ 2Al2O3 þ 13Ca(OH)2

þ 5CaSO4�2H2Oþ 73H2O

! 3(3CaO�Al2O3�3CaSO4�32H2O)þ 2KOH ð3ÞEttringite has been detested as ‘a cement-bacillus’ which

causes expansive fracture of concrete. In 1936, Lossier17)

began a study to produce chemically prestressed concrete andthereafter Lafuma18) and Klein19) took over the study to buildup a foundation of the expansive cement.

ACI standards (proposal) in the USA20) include three typesof expansive cement as follows:(1) K-type: Portland cement mixed with anhydrous hauyne

(3CaO�3Al2O3�CaSO4), gypsum (CaSO4) and quicklime (CaO).

(2) M-type: Portland cement mixed with alumina cementand gypsum (CaSO4) at a reasonable ratio.

(3) S-type: normal Portland cement mixed with largeramount of tricalcium aluminate (C3A) and gypsum(CaSO4�2H2O)

In general, K-type calcium sulfoaluminate (3CaO�3Al2O3�CaSO4), reacts with water to form calcium monosulfatoalu-minate hydrate, 3CaO�Al2O3�CaSO4�12H2O (AFm), andhydrates of Al2O3, without forming ettringite.21) However,when at least 2mol of CaSO4 is mixed with 1mol of

(a) 3-day strength, MPa (b) 7-day strength, MPa

(c) 28-day strength, MPa (d) 56-day strength, MPa

Fig. 5 Composition for optimum compressive strengths of OPC-GGBFS-PSA cements: strengths after (a) 3, (b) 7, (c) 28, and (d) 56 days.


3CaO�3Al2O3�CaSO4, ettringite is produced by the followingreaction:22)

3CaO�3Al2O3�CaSO4 þ 2CaSO4�2H2Oþ 36H2O

! 3CaO�Al2O3�3CaSO4�32H2Oþ 2Al2O3 ð4ÞAlso, total conversion of 3CaO�3Al2O3�CaSO4 to ettringiterequires additional CaO and CaSO4 for the followingreaction:23)

3CaO�3Al2O3�CaSO4 þ 6Ca(OH)2 þ 8CaSO4 þ 74H2O

! 3(3CaO�Al2O3�3CaSO4�32H2O) ð5Þ

The microstructure of ettringite is strongly dependent onthe presence of lime.24) Ettringite formed in absence of limeby the reaction in eq. (4) has been reported nonexpansive andcan develop high early strengths of cement.25) However,when formed in the presence of lime by the reaction ineq. (5), ettringite is expansive which can be exploited inspecial applications such as shrinkage-resistant and selfstressing cement.22)

In order to examine the activation effects of gypsum andPSA, mixtures of GGBFS and with either gypsum or PSAwere prepared and subjected to the hydration reaction for 28days. The XRD results from the hydrates are shown in Figs. 8and 9.

Figure 8 shows that as gypsum reacts with GGBFS to formettringite, it strongly influences the activation of GGBFS.Due to this reason, gypsum is generally used as an activatorfor GGBFS blended cement. However, Tables 5 and 6 showthat its use reduced the early strength of GGBFS blendedcement below that of OPC.

Figure 9 shows that as the amount of PSA clinkerincreases from 5 to 20%, the gypsum diffraction peaksincreased, and also ettringite was formed. This indicates thatthe PSA clinker reacted with GGBFS to form gypsum, whichreacted with GGBFS to form ettringite.

Figure 10 displays the XRD results of the formed hydrateswhen GGBFS, calcium hydroxide and PSA clinker weremixed at a mass ratio of 10 : 4 : 2 and reacted with water for28 days.

(a) 3-day strength, MPa

(c) 28-day strength, MPa

(b) 7-day strength, MPa

(d) 56-day strength, MPa

Fig. 6 Composition for optimum compressive strengths of OPC-GGBFS-PSA cements: strengths after (a) 3, (b) 7, (c) 28, and (d) 56 days.

10

E

Calcined alunite:Ca(OH)2:Gypsum

(molar ratio)E

E

EE

EE

E

E

EE

E

E

E

E: EttringiteQ: Quartz


Inte

nsi

ty, a

rb.u

nit

Q

Q

Q

Q

Q1 : 13 : 5

1 : 13 : 4

1 : 13 : 3

1 : 13 : 2

1 : 13 : 1

E

80706050403020

Fig. 7 Hydration products of the PSA clinker by curing time.


After 3 days, diffraction peaks of ettringite, C4AH13 andunreacted calcium hydroxide were observable. As thehydration progressed, the diffraction peaks of calciumhydroxide reduced. After 28 days, calcium hydroxide wasundetectable but hydrates such as ettringite, C4AH13, andC-S-H were observed.

The exact reaction mechanism, which explains the settingand hardening of alkali-activated binders, is not yet quiteunderstood, although it is thought to be dependent on theprime material as well as on the alkaline activator.13)

The hydration products of alkali-activated slag cementshave been investigated and reported. It is generally agreedthat its main hydration product is C-S-H. There is no doubtthat the minor hydration products of alkali-activated slagcement will change with the nature of the slag andactivator.14) The chemical composition of the slag varieswith the type of iron being made and the type of ore beingused. There is no doubt that the chemical composition ofGGBFS has a significant effect on the hydration process,hydration product and properties of hardened alkali-activatedslag cements. In many cases, the MgO content of GGBFS islow and the slag can be described by the CaO-SiO2-Al2O3

system. The phase diagram of the CaO-SiO2-Al2O3-H2Osystem, indicates that five different products, such as C-S-H,Ca(OH)2, C4AH13, C2ASH8 and CS2H could appear in thissystem, while calcium hydroxide and gehlenite hydrate cannot co-exist at equilibrium. Also, ettringite is one of themain components of expansive, shrinkage-resistant, rapidhardening, high early strength and low energy cements.26–28)

Therefore, PSA clinker was shown to have a significant effecton the activation of GGBFS.

As we can see in Fig. 7, for the alumina and sulfatecomponents of PSA clinker to form ettringite and gypsum,additional calcium hydroxide and gypsum are required.Therefore, Figs. 11 and 12 shows XRD and DTA data ofhydrates formed at different curing times when 8% gypsum,PSA clinker or a mixture of both were blended with OPC andGGBFS, which had a mass ratio of 35 : 65.

When gypsum was used as activator (Fig. 11(a)) on day 1,initially the diffraction peaks of ettringite, Ca(OH)2 andgypsum were mainly observed. After 3 days, the diffractionpeaks of ettringite and Ca(OH)2 but not gypsum wereobserved. Since the crystallinities of the C-S-H hydrateswere very low, their diffraction peaks were difficult toobserve. The DTA results suggest that C-S-H was formedas the existence of an endothermic peak by the dehydrationof C-S-H appearing at around 100�C, although this peakof C-S-H overlapped with that of ettringite (approx. 90–110�C).

The endothermic peak of gypsum dehydration was initiallyobserved at around 135�C. However, after 3 days, it haddisappeared, indicating that all the gypsum had reacted withthe GGBFS blended cement to form ettringite. Therefore,when gypsum was used as activator, the main products ofthe hydration reaction were C-S-H, ettringite and Ca(OH)2,which likely develop the compressive strength of GGBFSblended cement.

When PSA clinker was used as activator, as shown inFig. 11(b), ettringite was immediately formed as hydrates.However, diffraction peaks of gypsum and Ca(OH)2, thatcould have been formed through the hydration of PSA andOPC, were not observed.

As shown in Fig. 12(b), endothermic peak due to thedehydration of C-S-H hydrates and ettringite were observedat around 100�C but those of Ca(OH)2 and gypsum was notobserved.

10


E E EEEE

GG

E EEEE GG

Inte

nsi

ty, a

rb. u

nit

70 : 30

80 : 20

85 : 15

90 : 10

95 : 5

E: EttringiteG: Gypsum

EG

Slag : Gypsum(wt. ratio)

80706050403020

Fig. 8 XRD patterns of hydrates formed at various mixing ratios of

GGBFS and gypsum.

10


EE

Q:QuartzE:Ettringite

G:Gypsum

G

GQG

G

Inte

nsi

ty, a

rb. u

nit

95 : 5

80 : 20

85 : 15

90 : 10

G

Slag : Calcined alunite (wt. ratio)

80706050403020

Fig. 9 XRD patterns of hydrates formed at various mixing ratios of

GGBFS and calcined alunite.

10


S

S

S

S

S: CSH

Q

Q

A

A

AL

L

A

A

E LL

28 days

Inte

nsi

ty, a

rb. u

nit

7 days

3 days

A: C4AH13

C: Calcite

A

C

Q: QuartzE

A

A

E

L: Ca(OH)2

E: Ettingite

EQE

E

E

80706050403020

Fig. 10 XRD patterns of hydrates formed in a system comprising GGBFS,

Ca(OH)2, and calcined alunite.


When the mixture of PSA clinker and gypsum was used asactivator, as shown in Fig. 11(c), ettringite was stablyproduced in the early stages of hydration. Its diffractionpeaks became smaller and those of monosulfate increasedafter 14 days. The presence of ettringite, monosulfate,C4AH13, and Ca(OH)2 were confirmed after 28 days.

Figure 12(c) shows endothermic peaks after 14 days ataround 180�C caused by the dehydration of the monosulfate.The very small endothermic peak due to the dehydration ofCa(OH)2 formed by the hydration of OPC emerged at atemperature of around 460�C. The formation of monosulfateshowed that a small amount of ettringite transformed tomonosulfate due to the lack of gypsum but did not stronglyinfluence the compressive strength of GGBFS blendedcement as shown in Table 8 and Fig. 6.

Therefore when the mixed PSA clinker and gypsumactivator was used, KAl(SO4)2 in PSA clinker reacted withCa(OH)2, a hydrate of OPC, to form CaSO4�2H2O. Thenewly formed gypsum reacted with C3A in OPC and theAl2O3 component of PSA clinker or GGBFS to form thehydration products of alkali-activated slag cements suchas ettringite, C-S-H, Ca(OH)2 and C4AH13. This series ofhydrations is thought to improve stably both the early andlong-term strengths of GGBFS-OPC blended cement.

4. Conclusions

The hydration properties and the compressive strength ofOPC and GGBFS mixed cement were investigated by usingcalcined alunite, consisting of KAl(SO4)2 and amorphousAl2O3, as activator. To transform PSA clinker into ettringite,additional gypsum and calcium hydroxide is required. PSAclinker appears to have undergone hydration by the followingreaction: 2KAl(SO4)2 þ 2A12O3 þ 13Ca(OH)2 þ 5CaSO4�2H2Oþ 73H2O! 3(3CaO�A12O3�3CaSO4�32H2O)þ 2KOH.When >15w% PSA clinker was mixed into blended cementconsisting of OPC and GGBFS, the compressive strength ofthe blended GGBFS cement decreased due to expansion bythe formation of an excessive amount of ettringite in the

10

PAB

AB

AA,B


Inte

nsi

ty, a

rb. u

nit

CEA

AB

PP

GG

G

EEE P

P

E

EEE

E: Ettringite, A: C3S

B: C2S, C:CSH, G: Gypsum

P: Ca(OH)2

28days

14days

7days

3days

1day

E

(a) Gypsum


A

A

EE

B

B

A

E

E

E

B

B

E A,BA A,B

2θ

Inte

nsi

ty, a

rb. u

nit P

EEEE

EEE

E: EttringiteA: C3S,B:C2S, C:CSHP: C4AH18

28days

14days

7days

3days

1day

E

(b) PSA clinker


E

B

B

A,B

B

B

E

E

E

A

BD

E

M

C M

PA,BA

A,B

Inte

nsi

ty, a

rb. u

nit

PP

EEE P

P

E

EE

E

E: Ettringite, M: MonosulfateA: C3S,B:C2S, C:CSH, D:C4AH13

G: Gypsum, P: Ca(OH)2

28days

14days

7days

3days

1day

E

(c) PSA clinker + gypsum

6050403020

10 6050403020

10 6050403020

Fig. 11 XRD patterns of hydrates formed in OPC-GGBFS-activator

systems.

28days

14days

7days

3days

1day

DTA

(uV

)

873773673573473

Temperature, T/K

373

(a) Gypsum

873773673573473

28days

14days

7days

3days

DTA

(uV

)

Temperature, T/K

1day

373

28days

14days

7days

3daysDTA

(uV

)

1day

873773673573473

Temperature, T/K

373

(b) PSA clinker (c) PSA clinker + gypsum

Fig. 12 DTA analysis of hydrates formed in OPC-GGBFS-activator

systems.


initial stage of the hydration. However, when a mixedactivator of equal masses of PSA clinker and gypsum wasused, the compressive strengths of the GGBFS blendedcement tested after 3 and 7 days were raised above that ofOPC in proportion to the amount of the activator and GGBFSused and the hydration products of alkali-activated slagcements such as ettringite, C-S-H, Ca(OH)2 and C4AH13

were formed. Therefore, PSA clinker containing KAl(SO4)2and amorphous Al2O3 can be used as an activator to improvethe early and long-term strengths of blended cementconsisting of OPC and GGBFS.

Acknowledgments

This research was conducted by Korea Evaluation Instituteof Industrial Technology, Rep. of Korea.

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