ORIGINAL ARTICLE

Durability assessment of alkali activated slag (AAS) concrete

David W. Law • Andi Arham Adam •

Thomas K. Molyneaux • Indubhushan Patnaikuni

Received: 18 August 2011 / Accepted: 28 January 2012 / Published online: 10 February 2012

� RILEM 2012

Abstract The environmental impact from the pro-

duction of cement has prompted research into the

development of concretes using 100% replacement

materials activated by alkali solutions. This paper

reports research into the durability of AAS concrete.

The durability properties of AAS have been studied

for a range of sodium oxide dosages and activator

modulus. Properties investigated have included mea-

surements of workability, compressive strength, water

sorptivity, depth of carbonation and rapid chloride

permeability. Microstructure studies have been con-

ducted using scanning electron microscopy and energy

dispersive X-ray spectroscopy. It was concluded that

an activator modulus of between 1.0 and 1.25 was

identified as providing the optimum performance for a

sodium oxide dosage of 5% and that AAS concretes

can exhibit comparable strength to concrete currently

produced using Portland cement (PC) and blended

cements. However, with regards to the durability

properties such as water sorptivity, chloride and

carbonation resistance; the AAS concretes exhibited

lower durability properties than PC and blended

concretes. This, in part, can be attributed to surface

microcracking in the AAS concretes.

Keywords Alkali activated slag � Durability �Permeability � Chloride diffusion � Microstructure

1 Introduction

The calcination of CaCO3 to produce 1 ton of ordinary

Portland cement (PC) releases 0.53 tons of CO2 into

the atmosphere, and if the energy source used in the

production of PC is carbon fuel then another 0.45 tons

of CO2 are produced [1]. Therefore the production of

1 ton of PC produces approximately 1 ton of CO2 in

the atmosphere. On the other hand, it has been reported

that the production of Ground granulated blast furnace

slag (GGBS) can release up to 80% less greenhouse

gas emission [2].

GGBS is a latent hydraulic material which can react

directly with water, but requires an alkali activator. In

concrete, this is provided by the Ca(OH)2 released

from the hydration of PC. Thus, when used as a

replacement material GGBS will not start to react until

some hydration has taken place [3]. However, it was

suggested as early as 1940 that it may be possible to

activate GGBS with alkali solution [4]. Further

research has shown that it is possible to use 100%

slag as the binder in mortar by activating with an alkali

component, such as; caustic alkalis, silicate salts, and

non silicate salts of weak acids [5–8]. Given the drive

D. W. Law (&) � T. K. Molyneaux � I. Patnaikuni

School of Civil Environmental and Chemical

Engineering, RMIT University, Melbourne 3001,

Australia

e-mail: david.law@rmit.edu.au

A. A. Adam

Department of Civil Engineering, Tadulako University,

Palu, Indonesia

Materials and Structures (2012) 45:1425–1437

DOI 10.1617/s11527-012-9842-1

to provide more ecologically acceptable and sustain-

able concrete the use of 100% slag as the binder has

the potential to be a major benefit to the construction

industry [9], though due to the energy consumed by the

chemical activators it has been suggested this can have

a detrimental effect on the over all environmental life

cycle costs [10, 11].

In a normal exposure environment with proper

design and production, concrete made with PC can be

a durable material. However, it has been long recog-

nized that traditional concrete can suffer from deteri-

oration due to the attack from aggressive agents such

as chloride ions and acidic gases such as CO2 that

initiate corrosion in the reinforcing steel. In order to be

a successful alternative to PC and blended concretes,

AAS concrete must show similar durability properties,

to be able to resist attack from these aggressive agents.

This paper reports a set of experiments analysing

the strength gain and durability properties of AAS

concrete.

2 Experimental

A range of AAS concrete specimens were cast with

varying the mix composition, based on previous

research on mortar specimens [12]. Testing included

compressive strength, sorptivity, rapid chloride per-

meability and carbonation. Comparative tests were

also undertaken with an OP cement concrete and a

range of OPC–GGBS blended concretes. In addition,

Scanning electron microscopy (SEM) and Energy

dispersive X-rays (EDS) analysis of the AAS concrete

were also undertaken.

2.1 GGBS

The most common cementitious material for AAS

binder is GGBS [13, 14]. Other types of slag [15, 16]

can also be activated however, their hydraulic activity

is not as high nor their activation as effective as

GGBS.

The hydraulic activity of slag can be measured in

various ways; one method is by calculating the

basicity coefficient. The basicity of a slag or glassy

material is the ratio between the total content of basic

constituents and the total content of acidic constit-

uents [17]

Kb ¼CaOþMgOþ Fe2O3þK2Oþ Na2O

SiO2 + Al2O3

ð1Þ

As the Fe2O3, K2O, and Na2O are minor compo-

nents (less than 1%) in the GGBS, some authors [18,

19] prefer to calculate basicity of slag using;

Kb ¼CaOþMgO

SiO2 + Al2O3

ð2Þ

Based on the basicity coefficient, as given in Eqs. 1

or 2, the slag can be classified into three groups: acid

(Kb \ 1), neutral (Kb = 1), and basic (Kb [ 1),

neutral and alkaline slags are preferred as starting

materials for AAS binder.

To be suitable as a binder material the GGBS

requires a chemical composition which fulfils the

criteria of a CaO/SiO2 ratio between 0.5 and 2.0 and an

Al2O3/SiO2 ratio between 0.1 and 0.6 [20]. The

chemical composition of the GGBS used is given in

Table 1.

The degree of hydration of AAS is influenced by the

hydration modulus (HM), where;

HM ¼ CaOþMgOþ Al2O3

SiO2

ð3Þ

To ensure good hydration properties, the HM

should exceed 1.4 [21]. As can be seen from Table 1,

the GGBS has a CaO/SiO2 ratio of 1.2, a Al2O3/SiO2

ratio of 0.4, and a HM of 1.83. Therefore it satisfies the

criteria required of an AAS binder. The basicity

coefficient (Kb) of the GGBS, calculated using Eqs. 1

and 2 gives values of 1.03 and 1.02, respectively,

Table 1 Chemical com-

position of GGBS (mass%),

as supplied by manufacturer

Blaine fineness, 460 m2/kg;

Particle size, 1–10 lm;

Relative density, 2.9; Bulk

density, 1,200 kg/m3

Component Percentage

SiO2 33.45

Al2O3 13.46

Fe2O3 0.31

CaO 41.74

MgO 5.99

K2O 0.29

Na2O 0.16

TiO2 0.84

P2O5 0.12

Mn2O3 0.40

SO3 2.74

LOI NA

1426 Materials and Structures (2012) 45:1425–1437

therefore the GGBS is a basic slag (Kb [ 1). The

properties of this slag conformed to AS 3582.2-2001.

2.2 Activator

Activation of the GGBS is achieved by using a low to

mild alkali of a material containing primarily silicate

and calcium which will produce calcium silicate

hydrate gel (C–S–H), similar to that formed in PCs,

but with a lower Ca/Si ratio. A number of alkali

activators have been used, including sodium hydrox-

ide, sodium silicate, sodium carbonate and potassium

hydroxide [19, 22, 23]. The majority of previous

research has found that activation with sodium silicate

or sodium silicate blended with NaOH gives the

highest strength. Therefore, blended sodium silicate

and sodium hydroxide activator was chosen for this

study.

The blended sodium silicate and sodium hydroxide

solutions are characterized by the activator modulus

(Ms), where

Ms ¼SiO2

Na2Oð4Þ

Different Ms values are achieved by varying the

ratio of sodium silicate and sodium hydroxide. The

sodium hydroxide solution (NaOH) was prepared by

dissolving sodium hydroxide pellets in deionised

water at least 1 day prior to mixing. A 5% Na2O

dosage was selected as other researchers had identified

that higher concentrations may result in efflorescence,

brittleness, possibly as a result of an excess of the

alkali in the mix, and high economic costs [24].

2.3 Aggregates

Both coarse and fine aggregate were prepared in

accordance with AS 1141.5-2000 and AS 1141.6.1-

2000. The moisture condition of the aggregate was in a

saturated surface dry (SSD) condition. The fine

aggregate was river sand in uncrushed form. The

coarse aggregate was crushed basalt aggregate with a

specific gravity of 2.99. The grading of the aggregates

is given in Table 2.

2.4 Mix design

The mix design was based on previous tests on mortar

specimens [12]. The total aggregate in the concrete

was kept to 64% of the entire mixture by volume for all

mixes. The proportioning of ingredients (cementitious

materials, chemical activator, aggregate, and water)

was calculated based on the absolute volume method

[3], as a result, the total weight of binder and water was

varied to keep the volume of material and water/binder

or water/solid ratio the same. Table 3 summarizes the

mix details for the AAS concrete.

The mix was designed to provide a 28 day compres-

sive strength of 40 MPa. This target strength was chosen

to replicate the strength for standard site concrete as

specified in AS 3600. To achieve this target strength a

5% Na2O dosage was selected for the AAS. The water in

the mix was taken as the sum of water contained in the

sodium silicate, sodium hydroxide and added water. The

solid is taken as the sum of GGBS, the solid in the

Na2SiO3 solution and the NaOH pellets. Liquid sodium

silicate and sodium hydroxide were blended in different

proportions, providing an activator modulus (Ms) in

solution ranging from 0.75 to 1.25.

For the control and the OPC–GGBS blended

concretes a water/binder ratio of 0.52 was used.

Table 4 shows the mix proportion of the blended, and

control concretes. The levels of GGBS replacement

were 30% for S30, 50% for S50, and 70% for S70 (of

the total binder). The mix design for the control (CTL)

concrete was a standard concrete mix based on the

British Method published by the Department of the

Environment (UK) [25].

The mixing procedure for control and blended

concrete was carried out in accordance with the

Australian standard AS 1012.2-1994. The mixing

procedure for the AAS concrete, incorporating the

mixing of the activator, is described in Fig. 1. The spec-

imens were demoulded after 24 h followed by water

Table 2 Mix proportion of AAS concrete

Sieve size Aggregates

7 mm 10 mm

19.00 mm 100.00 100.00

9.50 mm 99.99 74.86

4.75 mm 20.10 9.32

2.36 mm 3.66 3.68

1.18 mm 2.05 2.08

600 lm 1.52 1.47

300 lm 1.08 1.01

150 lm 0.62 0.55

Materials and Structures (2012) 45:1425–1437 1427

curing at 20�C for 6 days and then stored in an

environmental control room (50%RH, 20�C) prior to

testing.

2.5 Test methodology

The slump test was undertaken in accordance with

Australian Standard AS 2701.5. Compressive strength

tests were performed with a loading rate of 20 MPa/min

according to AS 1012.9-1999. Sorptivity tests were

undertaken with 100 mm diameter and 50 mm height

specimens in accordance with ASTM C1585-04, all

surfaces except the exposed surface being sealed with

epoxy. Rapid chloride permeability testing (RCPT)

was performed according to ASTM C1202-07 and

AASHTO T277 and accelerated carbonation tests were

undertaken in a purpose built carbonation chamber,

Fig. 2. The environment was set to a temperature of

20 ± 1�C, relative humidity of 70 ± 1%, and a CO2

concentration of 20 ± 1%. All tests are presented with

the mean data and the standard deviation.

3 Results and discussion

3.1 Workability and strength of AAS concrete

The workability, in terms of the slump test, is

presented in Table 5. The results showed that for

Table 3 Mix proportion of AAS concrete (kg/m3)

Mix GGBS (kg) Aggregate (kg) Activator (kg) Added water (kg)

sand 7 mm 10 mm Na2SiO3 (liquid) NaOH (10 M)

AAS5-0.75 419 784 346 693 53 56 137

AAS5-1.00 415 784 346 693 71 46 136

AAS5-1.25 412 784 346 693 87 33 135

Table 4 Mix proportion

blended OPC–GGBS and

control concretes (kg/m3)

Mix OPC (kg) GGBS (kg) Aggregate (kg) Added water (kg)

Sand 7 mm 10 mm

CTL 428 – 784 346 693 222

S30 296 127 784 346 693 220

S50 210 210 784 346 693 219

S70 125 293 784 346 693 217

Load coarse then fine

aggregates in to the mixer

Add a small portion of the

liquid and mix for 30 sec.

Add the binder and mix for

2.5 minutes.

Add the remaining liquid and

mix for 4 minutes.

Stop and measure slump.

Cast samples

Total time:

0 min

0.5 min

3 min

7 min

Fig. 1 Mixing procedure for casting AAS concrete

Fig. 2 Schematic of the accelerated carbonation chamber

1428 Materials and Structures (2012) 45:1425–1437

AAS concrete, the workability reduced as the activator

modulus increased. This is attributed to the increased

amount of sodium silicate at higher activator modulus,

which made the mix very sticky. However, up to

Ms = 1.25 the mix was still workable. While the

slump was high for the Ms = 0.75 the fresh concrete

did not segregate when vibrated and while no exces-

sive bleeding was observed in any specimens higher

rates of bleeding did occur in the AAS concrete.

Surface micro-cracking was observed on the AAS

specimens, consistent with observations by other

authors [26–28].

3.2 Durability analysis of AAS concrete

The compressive strength results are presented in

Table 5. The strength of the Ms = 0.75 AAS was lower

than that of the Ms = 1 and the Ms = 1.25 specimens,

both of which achieved a strength in excess of 40 Mpa at

28 days and 45 MPa at 90 days. This is above the

specified minimum strength in AS 3600 for the design

of concrete structures. Thus, both the AAS5-1.00 and

AAS5-1.25, demonstrated strengths that would be

acceptable in A1, A2, B1 and B2 exposure conditions,

Table 6. It should be noted that the acceptable strengths

for these exposure categories are based on OP and

blended cement concretes, for which the durability

properties are well established. That the AAS concretes

achieve this strength does not guarantee that their

durability will be comparable as durability does not

necessarily correlate to strength. Durability being

dependent such factors as resistance to chloride ingress

and carbonation, both of which can initiate corrosion.

Previous experiments with AAS mortars found that

the increasing alkali content up to a certain point

increased the strength [12]. This was attributed to an

increase in the alkalinity of the mix which assists in the

dissolution of the slag and the adsorption of ions in

solution on the surface of the slag. Thus increasing the

activator modulus results in an increase in soluble

silicates and consequently an increase in the reaction

rate (a higher concentration of reactants induces a

higher reaction rate). The results indicate that little

further increase in strength is achieved once an

Ms = 1 is reached and that the optimum value for

the Ms lies between Ms = 1 and Ms = 1.25. That a

maximum strength is observed suggests that a limiting

factor has been reached. Possible explanations are that

this is due to an excess of slag in mixes below this ratio

and beyond this point no further dissolution is

achieved, that the slag has been completely consumed,

or that the available slag has reacted to form a

protective crust which prevents further reaction. SEM

analysis of the concrete was undertaken to identify the

composition of the concrete matrix for each mix to

identify the reaction mechanisms taking place (Sect.

3.3).

The compressive strength results are comparable

with those reported by other authors, Collins and

Sanjayan [27] report values between 30 and 55 MPa

for AAS concrete made with sodium silicate

and lime slurry, Bakharev et al. report values of

40–60 Mpa, with an Ms of 0.75 and 5.4% Na2O [29],

while Hakkinen reports a compressive strength

between 49 and 64 MPa with NaOH activator at

5% weight of binder [30]. All the AAS specimens

show an increase in strength with time between 7

and 28 days and the Ms = 0.75 and Ms = 1.00 a

further increase to 90 days, while the Ms = 1.25

remains constant, These results are again in agree-

ment with those reported by other authors and is

indicative of on going hydration with strength being

influenced by the curing regime, chemical composi-

tion of the slag and activator used.

Table 5 Slump and

compressive strength

of blended GGBS–OPC,

and AAS concrete

Mix Slump Compressive strength (MPa)

(mm) 7 days 28 days 90 days

CTL 80 38.0 ± 1.0 51.8 ± 2.1 57.0 ± 4.1

S30 75 31.7 ± 2.5 46.5 ± 5.9 49.5 ± 1.0

S50 65 27.8 ± 0.5 46.9 ± 1.3 53.2 ± 5.3

S70 55 24.7 ± 1.8 35.6 ± 3.8 42.9 ± 1.0

AAS5-0.75 150 25.0 ± 0.1 32.9 ± 2.1 36.6 ± 0.3

AAS5-1.00 95 35.4 ± 4.3 44.3 ± 2.1 45.3 ± 4.0

AAS5-1.25 70 37.0 ± 2.6 43.5 ± 4.0 43.5 ± 2.2

Materials and Structures (2012) 45:1425–1437 1429

The sorptivity test is based on Darcy’s law of

unsaturated flow, where the cumulative water absorp-

tion (per unit area of the inflow surface), i increases as

the square root of the elapsed time t [31]:

i ¼ St1=2 ð5ÞWith the data points are fitted to the equation:

i ¼ Aþ St1=2 ð6ÞThe sorptivity curve of the AAS concretes, Fig. 3,

especially with low Ms were found to be less linear

compared to that of blended concretes, which was in

agreement with DeSouza [32]. These results would

indicate that during the first few minutes, depending

on the concrete characteristics, saturation of the paste

surface layer occurs; however after this initial period

of time, the area of absorption is smaller due to the

presence of aggregates. This leads to an initial non-

linear curve within the sorptivity curve with a much

higher rate of absorption. During the setting period the

AAS concrete exhibited a higher extent of bleeding

compared to the blended concrete, resulting in higher

amounts of cement paste at the surface, this phenom-

ena was more pronounced at low Ms. However the

correlation coefficients, R, in all the sorptivity data

exceed 0.98, Table 7. It was also found that the

relationship between 6 h absorption and sorptivity was

linear which would indicate that capillary absorption

can be represented by the sorptivity.

There was a large reduction in sorptivity in the AAS

concrete as the activator modulus was increased from

0.75 to 1.00. However, only a small increase in

sorptivity was observed in AAS concrete as the

modulus was increased from 1.00 to 1.25. This would

again indicate that an optimum mix is achieved

between Ms = 1 and Ms = 1.25. The results would

suggest that increasing the activator modulus reduces

the capillary absorption of the AAS concretes.

Table 6 Summary of exposure categories as defined in AS 3600

Surface and exposure environment Exposure classification

Surface members in contact with ground

(a) Members protected by a damp proof membrane A1

(b) Residential footings in non-aggressive-soils A1

(c) Other members in non-aggressive-soils A2

Surface members in interior environments

(a) Fully enclosed within a building except for a brief period of weather exposure during construction

i. Residential A1

ii. Non-residential A1

(b) In industrial buildings, the me,ber being subjected to repeated wetting and drying B1

Surfaces of members in above-ground exterior environments in areas that are;

(a) Inland ([50 km from coastline)

i. Non-industrial and arid climate zone A1

ii. Non-industrial and temperate climate zone A2

iii. Non-industrial and tropical climate zone B1

iv. Industrial and any climate zone B1

(b) Near coastal (1 to 50 km from coastline), any climate zone B1

(c) Coastal, any climate zone B2

Surface of members in freshwater B1

Surfaces of maritime structures permanently submerged B2

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.0 5.0 10.0 15.0 20.0

t1/2 (min1/2)

i (m

m3 /m

m2 )

CTLS30S50S70AAS5-0.75AAS5-1AAS5-1.25

Fig. 3 Rate of absorption versus square root of time

1430 Materials and Structures (2012) 45:1425–1437

A decrease in the capillary absorption is indicative of a

reduction in the porosity and a denser material. This

would correlate with the strength results, as a reduc-

tion in porosity and an increase in density would be

expected to give an increase in strength.

While the range of compressive strengths are of a

similar magnitude, Table 5, the sorptivity values for

the AAS concretes are all higher than the OPC and

blended concretes, Table 7. For the OPC control

concrete this ranges from 15% for the Ms = 1.00–70%

for the Ms = 0.75, at 56 days while by 90 days the

Ms = 1.00 remains at 15% and the Ms = 0.75 has

fallen to 60%. For the blended concretes the increase

in sorptivity values is even higher, with over a 200%

increase compared to the S70 mix. These observations

are consistent with those observed by Collins and

Sanjayan [29], The Sorptivity values obtained being in

the range 0.2–0.3 for the AAS concretes. A reduction

in sorptivity with time is observed as the activator

modulus decreases.

The Ms = 1.25 specimen showing no change in

sorptivity between 56 and 90 days while a reduction of

5% is observed for the Ms = 1.00 and 10% for the

Ms = 0.75 specimens, respectively. A decrease in

sorptivity with time would be indicative of an on going

hydration reaction. This is in agreement with the

compressive strength results which show an increase

in strength for the Ms = 0.75 specimen between 28

and 90 days of 3.7 MPa, an increase of 1.0 MPa for

the Ms = 1.00 specimen but no increase in strength for

the Ms = 1.125 specimen.

The sorptivity value can be taken as an indication of

the durability of the material. The lower the porosity of

the material generally the more resistant the material is

to the ingress of aggressive ions such as chlorides.

Previous studies have show that that AAS concrete has

a much finer pore size distribution and lower porosity

than OPC [33] which would suggest that the higher

sorptivity can be attributed to surface microcracking

observed.Thus the results would suggest that the

durability performance of the AAS concrete may be

inferior to that of GGBS blended concrete.

A summary of the results for the accelerated carbon-

ation (20 ± 1% CO2) at 20 ± 1�C and 75 ± 2.5% RH

are shown in Fig. 4. As found by previous authors the

depth of carbonation was higher when the level of GGBS

replacement was increased [34, 35].

The carbonation of AAS specimens was higher than

that of blended OPC–GGBS, and CTL specimens,

with the Ms = 0.75 AAS showing the highest value,

while similar depths of carbonation were found in

Ms = 1 and Ms = 1.25. This finding is in agreement

with other authors [29, 36, 37]. The depth of

Table 7 Sorptivity parameters of concrete

Mix Sorptivity parameters

Si (mm/min1/2) A (mm3/mm2) R i (mm3/mm2)

56 days 90 days 56 days 90 days 56 days 90 days 56 days 90 days

CTL 0.167 0.158 -0.071 -0.149 0.999 0.999 3.16 2.90

S30 0.135 0.133 -0.105 -0.079 0.999 0.998 2.51 2.50

S50 0.116 0.114 0.007 0.003 0.999 0.998 2.23 2.17

S70 0.088 0.099 0.110 0.171 1.000 1.000 1.78 2.04

AAS5-0.75 0.281 0.252 -0.567 -0.427 0.988 0.989 5.07 4.69

AAS5-1.00 0.192 0.181 -0.324 -0.263 0.988 0.990 3.53 3.40

AAS5-1.25 0.204 0.205 -0.265 -0.116 0.997 0.997 3.73 3.87

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Time (weeks)

Dep

th o

f C

arb

on

atio

n (

mm

)

CTLS30S50S70AAS5-0.75AAS5-1AAS5-1.25

Fig. 4 Depth of carbonation plotted against exposure period

(exposed to 20% CO2 at 20�C and 75% RH)

Materials and Structures (2012) 45:1425–1437 1431

carbonation (X) versus t1/2 data were fitted by linear

functions. It was found that the relationship gave a

good correlation coefficient (R), Table 8.

The carbonation rate of AAS concrete was approx-

imately three times that of the control concrete. The

highest rate was found in Ms = 0.75 and constant at

Ms = 1 and Ms = 1.25. The results are consistent with

the sorptivity results for AAS concrete, and suggest

that the matrix of Ms = 0.75 is more porous than that

of Ms = 1 and Ms = 1.25 thereby increasing the rate

of penetration of CO2 and capillary absorption of

water. These results are in agreement with Byfors et al.

[38]. The increased rate of carbonation in the AAS

concrete can be attributed to two factors. In OPC

concrete there is a pH buffer due to the presence of

CaO/Ca(OH)2 which maintains the pH between 12.5

and 13.5. However, in the AAS concrete there is no

reserve of CaO within the binder Thus, as in OPC–

GGBS blended concrete, there is less Ca(OH)2

available per unit area to react with the available

CO2, which may result in a faster movement of the

carbonation front. Secondly, the greater extent of

surface microcracking in AAS concrete is expected to

increase the speed of penetration of CO2. Bakharev

et al. [29], observed a reduction in pH at the surface

and crystallisation of calcite as well as decalcified

C–S–H when AAS concrete was exposed to an

atmosphere rich in CO2. The decalcification of C–S–H

increases the porosity as the main solid part of

hydration, C–S–H, will be decomposed into CaCO3

and silica gel. Due to the low Ca/Si ratio in C–S–H of

AAS concrete, the decalcification of C–S–H will have

a faster reaction compared to the carbonation of

Ca(OH)2, as in the PC concrete.

Current measurements taken during the RCPT of

blended and OPC–GGBS and AAS, concrete are

presented in Fig. 5. The initial currents of the

OPC–GGBS blended concretes were considerably

lower than for the control PC concrete and decreased

as the level of replacement increased. The plots for the

AAS concretes show a decrease in initial current with

an increase in the activator modulus.

The conductivity and charge passed of concrete

obtained from the RCPT, as presented in Fig. 6, show

that the addition of GGBS in blended concrete reduced

the conductivity and charge passed proportional to the

level of replacement. The lower the charge passed

the better the resistance to chloride diffusion. Thus the

results would indicate that the higher the volume of

GGBS in the blended cements the better the chloride

resistance, which is in agreement with previous

research of Shi et al. [39].

For the AAS concretes the results show that

increasing the activator modulus, reduces the conduc-

tivity—indicated by the increased charge passed. A

reduction in charge passed can be accounted for as a

result of reduced porosity of the concrete, again in

agreement with the sorptivity data and strength data.

A higher activator modulus also means a reduction

in NaOH in the activator this could also suggest a

Table 8 Regression parameters for carbonation

Mix C A R2

CTL 3.5 0.6 0.985

S30 4.4 0.4 0.994

S50 5.1 0.3 0.991

S70 7.1 0.0 0.998

AAS5-0.75 13.6 -0.6 0.997

AAS5-1.00 12.5 0.4 0.999

AAS5-1.25 12.3 -0.2 0.997

0

50

100

150

200

250

300

350

400

0 60 120 180 240 300 360

Time (minutes)

Cu

rren

t (m

A)

AAS5-0.75AAS5-1.00AAS5-1.25CTLS30S50S70

Fig. 5 Current versus time for RCPT

0

1000

2000

3000

4000

5000

6000

7000

0.0000 0.0050 0.0100 0.0150 0.0200 0.0250

Conductivity (s/m)

Ch

arg

e P

asse

d (

Co

ulo

mb

s)

6 h, Blended6 h, ASS

Fig. 6 Correlation between conductivity and charge passed for

blended and AAS concrete

1432 Materials and Structures (2012) 45:1425–1437

reduction in ionic concentration in the pore fluid of the

AAS concrete. A reduction in the ionic concentration

would provide fewer ions to conduct the charge, and

hence would be expected to lead to a lower charge

passed. This is dependant on the concentration of

NaOH remaining in the pore solution, rather than

being absorbed into the C–S–H gel to form N–A–S–H

gel [40].

It should, however, be noted that the RCPT test is a

measure of charge passed. The presence of supple-

mentary cementitious materials has been known to

affect the RCPT results [41]. The GGBS can change

the pore solution properties making it less conductive,

as a result less current will flow through the specimens

during the test. Therefore, for concrete containing

GGBS, the RCPT results will underestimate the actual

chloride permeability which is a physical flow of fluids

through the specimens. For two concretes with iden-

tical pore structure, the one with GGBS will have the

lower RCPT, although the permeability will be the

same.

A comparison of values between the different types

of material should not be directly made without

consideration of the chemistry of the material, the

composition of the free ions carrying the charge and

the porosity of the concrete matrix. Where similar

materials are compared, i.e., the OPC–GGBS blended

concretes, or the variation in Ms for the AAS, a direct

comparison between the specimens can be made.

However, no direct comparison between the AAS

concretes and the PC and blended concretes should be

made based on the relative RCPT values.

3.3 SEM and EDS analysis of AAS concrete

SEM and EDS analysis was undertaken on AAS

concrete samples following curing. The SEM imaging

was conducted using secondary as well as backscatter

electron detectors. The microscope was coupled with

an energy dispersive X-ray spectrometer (EDS) for

elemental analysis. Sample preparation for SEM

investigation was as follows: the AAS concrete

samples were cut using a diamond saw to a size of

2–4 mm in height and 5–10 mm in diameter. The

samples were left to dry before they were gold coated

for imaging. The samples for EDS work were left

uncoated. Samples were mounted on the SEM sample

stage with conductive, double-sided carbon tape. For

EDS analysis charging was eliminated by adjusting

the voltage and current. A total of three samples were

investigated for each AAS mix. In addition to general

spectra, five spot analyses were undertaken per

sample.

Collins and Sanjayan have shown that AAS con-

crete display high shrinkage strains [42] and that

improved curing can reduce microcracking [43]. The

visual inspection identified uniformly distributed

microcracking on the surface of all the AAS samples,

even though 6 day water curing was employed. The

observed microcracking is attributed to the drying out

that occurs following water curing. Similar results

have been reported by Malolepszy and Deja [44].

It was noted, however, that the number of cracks,

per sample, was reduced as the activator modulus

increased.

The C–S–H in the AAS matrix was seen to have a

foil-like morphology (Figs. 7, 8, 9) which had an

average ratio of 0.75–1 (Ca/Si). The main elements,

Na, Mg, Si, Al, S, and Ca of the matrix were

determined by EDS spectrum analysis. Analysis of

the SEM images noted that microcracks also occurred

within the AAS concrete matrix. These microcracks

were observed at partially dissolved slag grains, where

cracks had formed on the surface of the grains, Fig. 7.

These microcracks are of the order 20–50 lm and are

attributed to stresses built up as the reaction proceeds.

As the reaction occurs the microstructure becomes

more dense and thus the partially dissolved slag grains

Fig. 7 SEM image of partially dissolved slag grain shows

cracks formed on the surface of slag grain, AAS5-1

Materials and Structures (2012) 45:1425–1437 1433

become more confined, resulting in the formation of

microcracks.

The microstructure of the AAS5-0.75 and AAS5-

1.0 specimens were similar in nature, with a number of

partially dissolved slag grains, Figs. 8 and 9. This is

attributed to the amount of soluble silica available in

the mix to react with the slag grains. Analysis of the

number of unreacted slag grain in each sample

analysed noted a reduction in the number of unreacted

slag grains in the AAS5-1 concrete compared to the

AAS5-0.75 concrete.

The matrix of AAS5-1.25, Fig. 10, was different

from AAS5-0.75 and AAS5.1 as the microstructure

was more dense with fewer unreacted slag grains and

also a small quantity of unreacted silica, the EDS

analysis showing that these components consisted

solely of Si and O, Fig. 11. The number of micro-

cracks had also reduced but some wider cracks were

seen to form on the border of the C–S–H and unreacted

silica.

The reduction in unreacted grains between AAS5-

0.75 and AAS5-1.0 and the denser microstructure

observed correlates well with the durability and

strength data that all show a significant increase in

performance from activator modulus of 0.75 to 1.0.

This appears to be due to the increased reaction of the

Fig. 8 SEM image of AAS5-0.75

Fig. 9 SEM image of AAS5-1.00

Fig. 10 SEM image of AAS5-1.25

Fig. 11 EDS spectra, AAS5-1.25, unreacted silica

1434 Materials and Structures (2012) 45:1425–1437

available slag brought about by the increase in activa-

tor. While there is a further decrease in the number of

unreacted slag grains from AAS5-1.0 to AAS5-1.25,

unreacted silica is also observed. This would indicate

that during the reaction stage the available slag

concentration is reduced to a point where the remaining

slag grains do not come into contact with the silicates

before the reaction phase is complete.

The SEM/EDS results correlate well with the

durability tests which show a significant improvement

between AAS5-0.75 and AAS5-1.0,specimens but

little difference between the AAS5-1.0 and AAS5-

1.25 specimens. This is due to an increase in the

activator leading to a denser and less porous material.

The results indicate that an optimum performance is

achieved between Ms = 1 and Ms = 1.25, which is

attributed to the increased reaction of the slag grains

being offset by an excess of activator resulting in less

but larger microcracks.

The microcracking observed may well contribute to

a reduced durability performance in the AAS con-

cretes compared to the OP and OPC–GGBS blended

concretes. This is due to the microcracks allowing

ingress to water and carbon dioxide in the surface layer

of the concrete, reducing the carbonation resistance

and leading to higher water permeability. The result

emphasises the need for good curing if AAS concrete

is to provide comparable durability performance to OP

and blended concretes used at present.

4 Conclusion

• Increasing the activator modulus (Ms) up to 1.00

increases the strength of AAS concrete. However,

further increases of Ms have minimal impact, with

a small decrease observed.

• There was a large reduction in sorptivity for AAS

concrete as the activator modulus increased from

0.75 to 1.00. However, an increase in sorptivity

was observed in AAS concrete as modulus

increased from 1.00 to 1.25.

• The AAS concretes displayed a higher water

sorptivity than the OP and OPC–GGBS blended

concretes.

• The carbonation of alkali activated slag was higher

than that of blended OPC–GGBS, and OP concrete

with the Ms = 0.75 showing the highest depth,

while a similar depth of carbonation was found in

Ms = 1 and Ms = 1.25.

• In the RCPT, increasing the activator modulus

reduced the conductivity and charge passed of the

AAS concrete. Both conductivity and charge

passed for the AAS were comparable to the control

and blended concrete.

• The majority of the slag grains in the matrix of the

AAS have been dissolved in all mixes. However,

a number of unreacted slag grains were observed

in all mixes, with the number decreasing as the

activator modulus increased. In the AAS5-1.25

unreacted silica was observed, which was not

observed in the AAS5-0.75 and the AAS5-1.0.

• Microcracks were observed in the AAS specimens.

The number of microcracks reduced as the mod-

ulus increased. At Ms = 1.25, wider microcracks

formed in the interface between C–S–H and

unreacted silica.

• Overall the data indicates that an optimum mix

design exists between an activator modulus of 1.0

and 1.25. A similar performance for strength,

sorptivity and carbonation is observed for both

these mixes, which are both substantially better

than for the 0.75 modulus specimens. This is

attributed to an increase in the activator leading to

a denser and less porous material. However,

between Ms = 1 and Ms = 1.25, the increased

reaction of the slag grains is offset by an excess of

activator resulting in less but larger microcracks.

• The presence of the microcracks may well con-

tribute to the reduced durability performance

observed in the AAS concretes compared to the

OP and OPC–GGBS blended concretes.

References

1. Lawrence CD (2003) The ‘ents. Lea’s chemistry of cement

and concrete, 4th edn. Butterworth-Heinemann, Oxford,

pp 421–470

2. Roy DM, Idorn GM (1982) Hydration, structure, and

properties of blast-furnace slag cements, mortars, and con-

crete. J Am Concr Inst 79(6):444–457

3. Neville AM (1996) Properties of concrete, 4th edn edn.

Wiley, New York

4. Purdon AO (1940) The action of alkalis on blast-furnace

slag. J Soc Chem Ind 59:191–202

5. Talling B, Brandstetr J (1989) Present state and future of

alkali activated slag concretes, fly ash, silica fume, slag, and

Materials and Structures (2012) 45:1425–1437 1435

natural pozzolans in concrete, vol 2. In: Proceedings of the

third international conference, Trondheim, vol 114. Amer-

ican Concrete Institute SP, Norway, pp 1519–1546

6. Deja J, Malolepsy J (1989) Resistance to chlorides.

In: Proceedings of the third international conference,

vol 2. American Concrete Institute SP, Trondheim,

pp 1547–1564

7. Bakharev T, Patnaikuni I (1997) Microstructure and dura-

bility of alkali activated cementitious pastes. In: Proceed-

ings of the fifth international conference on structural

failure, durability and retrofitting. Singapore Concrete

Institute, Singapore

8. Brough AR, Atkinson A (2002) Sodium silicate-based,

alkali-activated slag mortars: part I. Strength, hydration and

microstructure. Cem Concr Res 32(6):865–879

9. Davidovits J (2002) Environmental drivers. In: Proceedings

geopolymer. Melbourne, ISBN 0-9750242-0-5

10. Habert G, d’Espinose de Lacaillerie JB, Roussel N (2011)

An environmental evaluation of geopolymer based concrete

production; reviewing current research trends. J Clean Prod

19:1229–1238

11. McGuire E, Provis JL, Duxon P, Crarford R (2011) Geo-

polymer concrete is there an alternative and viable tech-

nology in the concrete sector which reduces carbon

emissions? In: Proceedings concrete 11; building a sus-

tainable future. Perth

12. Adam AA, Molyneaux TCK, Patnaikuni I, Law DW (2007)

Strength of mortar containing activated slag. The fourth

international structural engineering and construction con-

ference (ISEC-4). Innovations in structural engineering and

construction. Taylor & Francis, Melbourne

13. Gjorv OE (1989) Alkali activation of a Norwegian granu-

lated blast furnace slag. Paper third international conference

on fly ash, silica fume, slag, and natural pozzolans in con-

crete. Trondheim

14. Wang SD, Scrivener KL (1995) Hydration products of alkali

activated slag cement. Cem Concr Res 25(3):561–571

15. Shi C, Li Y (1989) Investigation on some factors affecting

the characteristics of alkali-phosphorus slag cement. Cem

Concr Res 19(4):527–533

16. Pan Z, Cheng L, Lu Y, Yang N (2002) Hydration products

of alkali-activated slag-red mud cementitious material. Cem

Concr Res 32(3):357–362

17. McGannon H (1971) The making shaping and treating of

steel [S.l.]. United States Steel, Pittsburgh

18. Wang SD, Scrivener KL, Pratt PL (1994) Factors affecting

the strength of alkali-activated slag. Cem Concr Res

24(6):1033–1043

19. Li Y, Sun Y (2000) Preliminary study on combined-alkali-

slag paste materials. Cem Concr Res 30(6):963–966

20. Talling B, Brandstetr J (1989) Present state and future of

alkali-activated slag concretes. In: Proceedings third inter-

national conference on fly ash, silica fume, slag, and natural

pozzolans in concrete. Trondheim

21. Chang JJ (2003) A study on the setting characteristics of

sodium silicate-activated slag pastes. Cem Concr Res

33(7):1005–1011

22. Collins F, Sanjayan JG (1998) Early age strength and

workability of slag pastes activated by NaOH and Na2CO3.

Cem Concr Res 28(5):655–664

23. Song S, Sohn D, Jennings HM, Mason TO (2000) Hydration

of alkali-activated ground granulated blast furnace slag.

J Mater Sci 35:249–257

24. Fernandez-Jimenez A, Palomo JG, Puertas F (1999) Alkali-

activated slag mortars: mechanical strength behaviour. Cem

Concr Res 29:1313–1321

25. Teychenne DC, Franklin RE, Erntroy HC (1988) Design of

normal concrete mixes. Department of the Environment,

Watford

26. Escalante-Garcia JI, Espinoza-Perez LJ, Gorokhovsky A,

Gomez-Zamorano LY (2009) Coarse blast furnace slag as a

cementitious material, comparative study as a partial

replacement of portland cement and as an alkali activated

cement. Constr Build Mater 23(7):2511–2517

27. Collins FJ, Sanjayan JG (2001) Microcracking and strength

development of alkali activated slag concrete. Cem Concr

Compos 23:345–352

28. Hakkinen T (1993) The influence of slag concrete on the

microstructure, permeability and mechanical properties of

concrete, part 1 microstructural studies and basic mechan-

ical properties. Cem Concr Res 23(2):407–421

29. Bakharev T, Sanjayan JG, Cheng YB (2001) Resistance of

alkali-activated slag concrete to carbonation. Cem Concr

Res 31:9:1277–1283

30. Hakkinen T (1993) The influence of slag concrete on the

microstructure, permeability and mechanical properties of

concrete: part 2 technical properties and theoretical exam-

inations. Cem Concr Res 23(3):518–530

31. Hall C (1989) Water sorptivity of mortars and concretes: a

review. Mag Concr Res 41(147):51–61

32. DeSouza SJ (1996) Test methods for the evaluation of the

durability of covercrete. M.A.Sc. Thesis, University of

Toronto, Toronto

33. Collins F, Sanjayan JG (2000) Effect of pore size distribu-

tion on drying shrinking of alkali-activated slag concrete.

Cem Concr Res 30(9):1401–1406

34. Jones MR, Dhir RK, Magee BJ (1997) Concrete containing

ternary blended binders: resistance to chloride ingress and

carbonation. Cem Concr Res 27(6):825–831

35. Papadakis VG (2000) Effect of supplementary cementing

materials on concrete resistance against carbonation and

chloride ingress. Cem Concr Res 30(2):291–299

36. Al-Otaibi S (2008) Durability of concrete incorporating

GGBS activated by water-glass. Constr Build Mater 22(10):

2059–2067

37. Bertolini L, Elsener B, Pedeferri P, Polder R (2004) Cor-

rosion of steel in concrete. Wiley/Co. KGaA, Weinheim

38. Byfors K, Klingstedt G, Lehtonen V, Pyy H, Romben L

(1989). Durability of concrete made with alkali activated

slag. In: Proceedings third international conference on fly

ash, silica fume, slag, and natural pozzolans in concrete.

Trondheim

39. Shi C, Stegemann JA, Caldwell RJ (1998) Effect of sup-

plementary cementing materials on the specific conductivity

of pore solution and its implications on the rapid chloride

permeability test (AASHTO T277 and ASTM C1202)

results. ACI Mater J 95(4):389–394

40. Provis JL, Van Deventer SJ (2007) Direct measurement of

the kinetics of geopolymerisation by in situ energy disper-

sive X-ray diffractometry. Mater Sci 42:2974–2981

1436 Materials and Structures (2012) 45:1425–1437

41. Krizan D, Zivanovic B (2002) Effects of dosage and mod-

ulus of water glass on early hydration of alkali-slag cements.

Cem Concr Res 32(8):1181–1188

42. Collins F, Sanjayan JG (2000) Cracking tendency of alkali-

activated slag concrete subjected to restrained shrinkage.

Cem Concr Res 30(5):791–798

43. Kutti T, Berntsson L, Chandra S (1992) Shrinkage of

cements with high content blast furnace slag. Proceedings of

Fly ash, silica fume, slag and natural pozzolans in concrete.

Istanbul, pp 615–625 (supplementary papers)

44. Malolepsszy J, Deja J (1988) The influence of curing con-

ditions on the mechanical properties of alkali activated slag

binders. Silicon Ind 11–12:179–186

Materials and Structures (2012) 45:1425–1437 1437