SLAGS AND SLAG CEMENTS

Mark Alexander

Visiting Professor, IITM

30 January 2014

(University of Cape Town)

1

Introduction

Features of slags: o Non-metallic by-products of metal manufacturing –

therefore many types o Blasfurnace slag: non-metallic by-product of the iron

manufacturing process from a blast furnace (normally)

Consists of silicates and alumino-silicates of calcium and other metal alkalis

o BFS can be ‘tapped’ in 2 ways: As air-cooled slag – stable, crystalline, used for aggregates

etc. As quenched slag, which causes granulation and a glassy

structure (super-cooled liquid)

o The latter, when finely ground, becomes Ground Granulated Blastfurnace Slag (GGBS)

2

Pics of BF etc. IRON

BLASTFURNACE

3

Pics of BF etc.

IRON BLASTFURNACE

4

TAPPING MOLTEN SLAG

5

Pics of BFS etc.

BFS CLINKER

6

AIR-COOLED SLAG

7

GRANULATED SLAG

8

GRANULATED BFS HEAP

9

Other types of slag

• The ‘normal’ slag used in cement and concrete is from a blastfurnace – called BFS

• Other types of metallurgical slags exist – Steel slags

– Ferro-manganese slags

– Etc.

• Many are not suitable for cement-making due to unsuitable chemistry, mineralogy etc.

• Another iron-steel slag is available in SA, China:

COREX slag – from a direct reduction furnace

10

COREX Slag, Saldanha, Western Cape, SA

11

Benefits of slag use

• Economics – generally cheaper than PC

• Use of ‘waste’ material – avoidance of dumping and pollution

• Durability performance – usually much enhanced

o improved microstructure and chemical resistance

• Sustainability aspects – ‘triple bottom line’

o Economical

o Environmentally-friendly

o Societal benefits

12

Properties

• Specific gravity: ~ 2.9 • Bulk density: 1200-1300 kg/m3 • Blaine : typically > 350 kg/m2 (c.f. PC ~ 310 kg/m2 Blaine) • Primary constituents: silica (SiO2), alumina (Al2O3)

Typically replaces 20-50% of PC in concrete

Portland Blast Furnace Cement (PBFC): mixture of PC and GGBS (Inter-blended or inter-ground)

(but recall EN cement spec. lecture – CEM II or CEM III)

Slag is harder than clinker and so usually separate ground

13

Ground Granulated Blastfurnace Slag

(GGBS)

14

RECAP: Hydraulic Cements/Binders

“Cements or binders which, when mixed with water, set or harden in air or water by a process of hydration, forming compounds which are volumetrically stable, durable, and increase in strength with age”.

• Basic constituents are oxides of Ca, Si, Al, Fe

• Ca0/Si02 ratio ≈ 2,6 – 3,6, typically 2,8

• Implies excess of calcium in the system

15

Most common example: Portland Cement

Table : Composition of Portland cement clinker (From Fulton 9)

Oxide % by mass

CaO 63 – 69

SiO2 19 – 24

Al2O3 4 – 7

Fe2O3 1 - 6

MgO 0.5 – 3.6

Na2O + 0.658 K2O 0.2 – 0.8

Significant feature is presence of CaO – ‘quicklime’ Formation of hydrated calcium silicates

16

“Binders which, when mixed with water, will harden very

slowly (generally too slowly for engineering purposes), and therefore require an activator to accelerate the hydration.”

• Comprise same basic oxides as hydraulic binders, but in different proportions.

• Ca0/Si02 ratio ≈ 0,92 – 1,05, typically 1,02 – therefore a “deficiency” of calcium to form calcium silicates

Latent Hydraulic Binders

17

Most common example: ground granulated blast furnace slag or GGBS Also Corex slag (GGCS)

Table : Chemical composition of South African GGBS (From Fulton 9)

Oxide % by mass

SiO2 34 – 40

CaO 32 – 37

Al2O3 11 – 16

MgO 10 – 13

FeO 0.3 – 0.6

MnO 0.7 – 1.2

K2O 0.8 – 1.3

S 1.0 – 1.7

TiO2 0.7 – 1.4 18

“Materials which are siliceous or alumino-siliceous and in themselves possess little or no cementitious properties, but can react with lime in the presence of water to form stable hydrated cementitious compounds”.

Examples: Volcanic ashes and earths; calcined shales and clays; fly ash (FA); condensed silica fume (CSF).

In common use in concrete in SA: FA (CSF rarely used)

Pozzolanic Materials

19

For FA: Ca0/Si02 ratio ≈ 0,09 to 0,13, but can vary widely. For CSF: Ca0/Si02 ratio ≈ 0,01, very low Ca0 content. Table : Chemical composition of South African FA (ex Matla, Lethabo & Kendal) and CSF (From Fulton 9)

Oxide % by mass

FA CSF

SiO2 48 – 55 92 – 96

Al2O3 28 – 34 1.0 – 1.5

CaO 4 – 7 0.3 – 0.6

Fe2O3 2 – 4 1.0 – 1.6

MgO 1 – 2 0.6 - 0.8

Na2O + 0.658 K2O 1 - 2 0.8 – 1.3

20

Compositional ranges of cementitious materials

21

Slag cements

Slag can be incorporated into cement and concrete in different ways:

• As an additive during cement manufacture –

either interground or interblended

See next slide

22

Slag cements

EN 197 cement standard (cement composition):

• CEM I Portland cements

• CEM II Portland composite cements

• CEM III Blastfurnace cements

• CEM IV Pozzolanic cements

• CEM V Composite cement

23

• During concrete manufacture, by adding it at the

mixer or in the batching plant.

In this case, the slag is delivered to the plant in bulk

Slag cements

24

RMC PLANT WITH MULTIPLE

BINDER BINS

25

W

Recap: EN 197 Cement Specs.

26

EN 197 requirements for slag:

• ≥ 2/3 must consist of ‘glass’, i.e. amorphous material capable of chemical reaction

• ≥ 2/3 of total mass must consist of [CaO+MgO+Al2O3]

• Ratio: [CaO+MgO]/ SiO2 > 1.

27

Slag Hydraulicity Hydraulic activity influenced by:

• Chemical composition and thermal history of the slag

• Glass content and glass composition

• Particle fineness and PSD

• Alkali concentration (pH) of the reacting system

• Temperature during early phases of hydration process

Compounds that increase chemical reactivity:

CaO, MgO and Al2O3

SiO2 reduces its hydraulicity

28

• Hydraulicity characterised by

Slag Hydraulic Index (HI)

- first indicator to assess potential of a slag to form

cementitious hydration products in an alkaline medium

• HI = [CaO+MgO+Al2O3]/ SiO2

• Rather simplistic - hydration reactions are much more

complex than indicated by the formula

• Alternative is: slag activity index (SAI) (ASTM C 989).

SAI = [Strength of slag-blended cement mortar cubes

(50% replacement level)] /

[Compressive strength of plain cement reference

mortar cubes]

29

Slag hydration

• GGBS is a ‘latent hydraulic binder’. Therefore, requires an alkali activator to catalyse hydration

• Normally provided by primary hydration of PC – liberation of Ca(OH)2 [CH]

• Debate as to whether slag uses CH from the PC hydration, or relies on its own CaO content for hydration

• Nevertheless, slag reacts with CH and water to form C-S-H and calcium aluminates

• Slag hydration contributes to pore refinement, blocking of diffusing paths for, e.g. Cl-

30

Properties of slag concretes

• Compressive strength

• Heat of hydration

• Durability …

• ASR – later lecture

31

GGBS strength curves

32

GGCS strength curves

33

E.g. w/c required for a Grade 30 concrete as function of different binders

All Grade 30 concretes Δ ≈ 0.20

34

Heat of Hydration Hydration of Portland cements and cement extenders produces

substantial amounts of heat (‘heat of hydration’)

Graph below shows rate of heat evolution in early stages Stage 1: Early rapid heat evolution – mainly C3A Stage II: Induction or dormant period Stage III: Initial set 2-4h, followed by accel. period to max. heat rate (4-8h) – C3S hydration Stage IV: Reaction slows Stage V: steady state

0.1 1 10 100 35

Heat of Hydration (cont’d)

Total heats of hydration of PC and

other binders

• CEM I - Range for SA CEM I:

270 - 320 kJ/kg

• Proportion of the above for typical blended binders – – 50% GGBS: ≈ 60%

– 30% FA: ≈ 55%

– 5% CSF: ≈ 90%

Note: there is large variability in the above – need for specific testing in critical cases

36

Heat rate curves for typical SA cement blends in concrete (From Ballim et al)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 10 20 30 40 50 60 70 80 90 100

Heat

Rate

(W

/kg

)

t20 hours

100% CEM 1 42,5

50% GGBS

5% CSF

35% FA

37

Total heat curves for SA cement blends in concrete – low heat PC clinker (From Ballim et al)

0

50

100

150

200

250

0 50 100 150 200 250 300 350 400 450 500

To

tal H

eat

(kJ/k

g)

t20 hours

100% CEM 1 42,5

50% GGBS

5% CSF

35% FA

38

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100

Ma

turi

ty H

ea

t R

ate

(W

/kg

)

Arrhenius Maturity (t20 hours)

100 CEM I

5% CSF

10% CSF

15% CSF

Heat rate curves for GGBS, FA, CSF cement blends (From Ballim et al)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100

Ma

turi

ty H

ea

t R

ate

(W

/kg

)

Arrhenius Maturity (t20 hours)

100% CEM 1

20% GGBS

40% GGBS

60% GGBS

80% GGBS

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100

Ma

turi

ty H

ea

t R

ate

(W

/kg

)

Arrhenius Maturity (t20 hours)

100% CEM 1

20% FA

40% FA

60% FA

80% FA

GGBS

CSF

FA

39

Durability of slag concretes

Chloride binding • Influences chemical chloride binding capacity; partial

blocking of pores from formation of calcium chloro-aluminates (Friedel’s salt)

• Main hydration products in chloride binding: tricalcium-aluminate (C3A)

tetracalcium-aluminoferrite (C4AF)

Sulphate attack • Can resist sulphate attack; slag replacement needs to

be > 50%. However, poorly proportioned & poorly cured slag concretes may be more susceptible!

40

Durability of slag concretes – Potential durability as measured by durability indexes

10

.4 10

.6

10

.7

9.8

10

.7

10

.5

10

.5

9.8

10

.5

9.8

10

.6 1

0.8

10

.6

10

.7

9.9

11

.0

10

.5

10

.9

9.9

10

.6

10

.0

10

.6 1

0.8

10

.7

10

.8

9.9

11

.3

10

.6

10

.9

9.9

10

.6

10

.0

10

.9

9

9.5

10

10.5

11

11.5

12

CEM I [CT]: greywacke

CEM I [CT]: FA:

greywacke

CEM I [PE]: FA:

greywacke

CEM I [CT]: GGCS:

greywacke

CEM II A-S [DB]: Slagmore: greywacke

CEM I [CT]: quartzite

CEM I [PE]: FA:

quartzite

CEM I [CT]: GGCS:

quartzite

CEM I [CT]: tillite

CEM I [CT]: GGCS: tillite

CEM II A-S [DB]: Slagmore:

tillite

OP

I

Concrete mix constituents

28 days 91 days 182 days

OPI values of 0.55 w/b ratio concrete mixes at 28, 91 and 182 days

41

1.3

9

1.0

0

0.9

7

0.3

7

0.4

2

1.3

6

1.0

3

0.3

8

1.3

7

0.3

8

0.4

4

0.8

8

0.5

8

0.5

8

0.2

3

0.2

1

0.8

9

0.5

1

0.2

3

0.8

9

0.2

2

0.2

2

0.7

8

0.1

8

0.1

9

0.1

9

0.1

8

0.8

0

0.1

9

0.2

0

0.7

8

0.1

9

0.1

9

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

CEM I [CT]: greywacke

CEM I [CT]: FA:

greywacke

CEM I [PE]: FA:

greywacke

CEM I [CT]: GGCS:

greywacke

CEM II A-S [DB]: Slagmore: greywacke

CEM I [CT]: quartzite

CEM I [PE]: FA:

quartzite

CEM I [CT]: GGCS:

quartzite

CEM I [CT]: tillite

CEM I [CT]: GGCS: tillite

CEM II A-S [DB]: Slagmore:

tillite

Ch

lori

de

co

nd

uct

ivit

y (m

S/cm

)

Concrete mix constituents

28 days 91 days 182 days

Durability of slag concretes – Potential durability as measured by durability indexes

CCI values of 0.55 w/b ratio concrete mixes at 28, 91 and 182 days

42

Extender Effects Suitability for use in

Mass

concrete

Marine

exposure

ASR

GGBS

(SLAG)

Fresh concrete

May improve workability

Slightly retards setting

Hardened concrete

Slower strength development

Improved long term strength

Reduced permeability

Prevents or retards ASR

Binds chlorides and reduces

chloride ingress

Lower heat of hydration rate

High GGBS

contents

> 50%) help

reduce risk

of thermal

cracking.

GGBS

particularly

suited to

marine

conditions;

provides

substantial

resistance to

chloride

ingress and

controls reinf.

corrosion.

Requires >

40% GGBS

content to

control

potential ASR

for

susceptible

aggregate

types.

SUMMARY: Effects of slags on properties of concrete (Table 1.5, Fulton 9).

43

Table: Effects of fly ash on properties of concrete (from Table 1.5, Fulton 9).

Exten-

der

Effects Suitability for use in

Mass concrete Marine

exposure

ASR

FA

(FLY

ASH)

Fresh concrete

Improves workability and reduces

water content

Slightly retards setting

Hardened concrete

Slower strength development

Improved long term strength

Refines pore structure, reduces

permeability

Prevents or retards ASR Binds

chlorides and reduces chloride

ingress

Lower heat of hydration rate

FA content of ≥

30%

significantly

reduces risk of

thermal

cracking.

Fly Ash

content of

≥ 30%

enhances

resistance to

chloride

ingress and

reinf.

corrosion

due to

chlorides.

Requires

FA content

of > 20%

to control

potential

ASR for

suscep.

agg.

types.

44

Table: Effects of CSF on properties of concrete (from Table 1.5, Fulton 9).

Extender Effects Suitability for use in

Mass

concrete

Marine exposure ASR

CSF

(CON-

DENSED

SILICA

FUME)

Fresh concrete

Reduces workability

Increase cohesiveness

Significantly reduces

bleeding

Hardened concrete

Increases strength

Reduces permeability

Substantially refines pore

structure

Not

suitable

for use in

mass

concrete.

CSF significantly

reduces physical

permeability, but

does not bind

chlorides

effectively.

Nevertheless can

improve resistance

to chloride ingress.

Requires

CSF content

of > 15% to

control

potential

ASR for

susceptible

aggregate

types.

45

Practical use of slags in cement and concrete

• Recall: slag can be used as an already blended cement (PBFC), OR alternatively, GGBS can be added at the concrete mixer as a direct replacement of PC.

• Cements with high GGBS can be used as low heat cements.

• Higher replacement levels of slag - excessive delays in setting times, slower strength development, possible plastic shrinkage cracking

• Lower replacement levels - may not produce all the technical benefits, e.g. improved durability

46

CASE STUDY Otieno, M., Beushausen, H., Alexander, M., Effect of chemical composition of slag on

chloride penetration resistance of concrete, Cement & Concrete Composites (2013), doi: http://dx.doi.org/10.1016/ j.cemconcomp.2013.11.003

Properties of concretes made with 3 different metallurgical slags

• Ground granulated blastfurnace slag (GGBS)

• Ground granulated Corex slag (GGCS)

• Ground granulated FeMn arc-furnace slag (GGAS)

47

Property / composition PC GGCS GGBS GGAS

SiO2 20.39 33.67 37.80 33.74

Al2O3 3.82 15.23 12.74 6.16

Fe2O3 2.64 0.90 0.65 0.07

Mn2O3 0.63 0.07 1.01 6.93

TiO2 0.33 0.72 1.43 0.27

CaO 64.71 37.23 34.17 36.96

MgO 2.15 11.32 11.14 14.79

P2O5 0.03 0.01 0.01 0.01

K2O 0.44 0.60 0.80 0.03

Na2O 0.15 0.18 0.26 0.07

LOI 3.10 -1.95 -1.28 -0.52

Glass content, % na* 99 98 93

Blaine fineness, cm2/g 3550 4100 3850 4150 48

Setting times

2

3

4

5

6

7

8

0 10 20 30 40 50

Slag replacement level (%)

Sett

ing

tim

e (

ho

urs

)

GGCS initial

GGAS initial

GGBS initial

GGCS final

GGAS final

GGBS final

49

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100

Maturity t20 hours

He

at

Ra

te in

W/k

g o

f B

ind

er

Cem I GGCS GGBS GGAS

Heat rate

50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500 600

Maturity t20 hours

To

tal h

ea

t in

kJ

/kg

of

bin

de

r

Cem I GGCS

GGBS GGAS

Total Heat

51

56-day compressive strength

52

30

40

50

60

70

80

0 10 20 30 40 50 60

56

-day

co

mp

ress

ive

stre

ngth

(M

Pa)

Slag replacement level (%)

GGCS GGAS GGBS

0.40 w/b

0.60 w/b

w/b 0.60

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 20 40 60

Age (days)

f c,

sla

g c

on

cre

te /

fc,

PC

GGBS

GGCS

GGAS

Strength ratios (slag concrete/PC concrete)

53

20

40

60

80

100

120

140

0 5 10 15 20 25 30

Time (days)

SA

I (%

)

GGCS

GGBS

GGAS

Slag activity index (SAI)

54

28-day chloride conductivity results for PC and slag-blended concretes – 0.60 w/b ratio

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

PC/GGCS PC/GGBS PC/GGAS PC/GGCS PC/GGBS PC/GGAS PC/GGCS PC/GGBS PC/GGAS

PC 50/50 PC/slag 65/35 PC/slag 80/20 PC/slag

Ch

lorid

e c

on

du

cti

vit

y (

mS

/cm

)

0.60 w/b ratio

0.42

0.65

0.80

0.65 0.66

1.22

1.01 0.98

1.31

1.48

55

Effect of slag replacement level on concrete porosity (0.60 w/b ratio)

8

9

10

11

12

15 20 25 30 35 40 45 50 55

Por

osit

y (%

)

Slag replacement level (%)

0.60 w/b ratio 80/20 PC/GGCS

80/20 PC/GGBS

80/20 PC/GGAS

56

PhD study by Otieno – corrosion rate of cracked and uncracked RC beams

Effect of slags on concrete resistivity

Atlantic ocean

Indian ocean

(A)

N

Hout Bay

Kalk Bay

Location of field specimens

(tidal/splash zone)

Robben Island

Muizenberg

Buffels Bay

Simon's Town

Noordhek Beach

Table Bay

(B)

N

57

Lab and field measurements

58

Wenner 4-point resistivity measurements

Current flow lines

Equipotential lines

I

a a a

V

Concrete

Voltage (Inner probes)Current (Outer probes)

59

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Re

sis

tivit

y (

kΩ

-cm

)

Time (weeks after exposure)

Field specimens (Marine tidal zone, Cape Town)

100% PC, 0.40 w/b

70/30 PC/FA, 0.40 w/b

70/30 PC/FA, 0.55 w/b

50/50 PC/GGBS, 0.40 w/b

50/50 PC/GGBS, 0.55 w/b

Note: 2-week moving average trends

week 0 ≡ 80d after casting

Resistivity results from Otieno (PhD work) Field specimens (CT Harbour)

60

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Re

sis

tivit

y (

kΩ

-cm

)

Time (weeks after exposure)

Lab specimens (cyclic 3d wetting (5% NaCl soln) &

4d air-drying)

100% PC, 0.40 w/b

70/30 PC/FA, 0.40 w/b

70/30 PC/FA, 0.55 w/b

50/50 PC/GGBS, 0.40 w/b

50/50 PC/GGBS, 0.55 w/b

Note: 2-week moving average trends

week 0 ≡ 80d after casting

61

PC-0.40

Slag-0.55

PC-0.55

Slag-0.40

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50

Co

rro

sio

n r

ate

(µ

A/c

m2)

Resistivity (kΩ-cm)

Corrosion rates as a function of resistivity and crack width

0.7 mm crack

0.4 mm crack

Incipient- cracked

Uncracked

62

Thank You! Questions?

63

64