ORIGINAL ARTICLE

Long term durability properties of class F fly ashgeopolymer concrete

David W. Law • Andi Arham Adam • Thomas K. Molyneaux •

Indubhushan Patnaikuni • Arie Wardhono

Received: 9 August 2013 / Accepted: 7 February 2014 / Published online: 19 February 2014

� RILEM 2014

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 the assessment of a number of key durability

parameters for geopolymer concrete made from fly ash

activated with sodium silicate and sodium hydroxide.

Properties investigated have included workability,

compressive strength, water sorptivity, carbonation,

chloride diffusion and rapid chloride permeability.

Microstructure studies have been conducted using

scanning electron microscopy and energy dispersive

X-ray spectroscopy. The results showed that both the

geopolymer concretes with activator modulus 1.00

and 1.25 gave durability parameters comparable to

Ordinary Portland and blended cement concretes of

similar strength, while the geopolymer concrete with

an activator modulus of 0.75 displayed lower durabil-

ity performance. However, there is a concern over the

long term performance of the geopolymer concretes

with activator modulus of 1.00 and 1.25 when

considering chloride induced corrosion of reinforcing

steel due to the initial pH and long term chloride

diffusion coefficient.

Keywords Geopolymers � Durability �Microstructure � Permeability � Carbonation �Chloride

1 Introduction

It is widely known that the production of Portland

cement consumes considerable energy and at the same

time contributes a large volume of CO2 to the

atmosphere. 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 [33].

Therefore the production of 1 ton of PC produces

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

School of Civil, Environmental and Chemical

Engineering, RMIT University, Melbourne, VIC 3000,

Australia

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

T. K. Molyneaux

e-mail: tom.molyneaux@rmit.edu.au

I. Patnaikuni

e-mail: indu.patnaikuni@rmit.edu.au

A. Wardhono

e-mail: arie.wardhono@rmit.edu.au

A. A. Adam

Department of Civil Engineering, Tadulako University,

Palu, Indonesia

e-mail: adam.arham@gmail.com

A. Wardhono

Department of Civil Engineering, State University of

Surabaya, Surabaya, Indonesia

Materials and Structures (2015) 48:721–731

DOI 10.1617/s11527-014-0268-9

approximately 1 ton of CO2 in the atmosphere.

Increased environmental concerns coupled with

increased development and use of concrete in con-

struction has prompted a search for more environ-

mentally friendly materials.

A possible alternative is the use of alkali-activated

binder using industrial by-products containing silicate

materials [25, 40]. One of the most common industrial

by-products used as binder materials is fly ash (FA).

FA has been widely used as a pozzolanic material to

enhance the physical, chemical and mechanical prop-

erties of cements and concretes. It has been estimated

that the energy requirement of geopolymers is 60 %

less than that of PC [34].

Previous research has shown that it is possible to

use 100 % FA as the binder by activating it with an

alkali component, such as; caustic alkalis, silicate

salts, and non silicate salts of weak acids [9, 48].

Activation of FA involves using a highly alkaline

solution. This reaction forms an inorganic binder

through a polymerization process [14, 19]. The term

‘‘Geopolymeric’’ is used to characterise this type of

reaction. The geopolymeric reaction differentiates

geopolymers from other types of alkali activated

materials (such as; alkali activated slag).

Research has shown that geopolymer concrete can

achieve comparable strengths to Ordinary Portland

(OP) and blended cement concretes [10, 26, 27, 30, 39,

41, 49]. Durability studies have investigated acid

attack [11, 46], sulphate attack [12] and fire resistance

[32], while Miranda has investigated the chloride

resistance of geopolymer mortar specimens [35]. The

research has shown that geopolymer concrete has the

potential to be a suitable alternative material to OP and

blended cement concretes. However, the long term

durability of the material has yet to be established,

particularly with reference to the protection of rein-

forcing steel.

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 and acidic gases such as CO2 that initiate

corrosion in the reinforcing steel. In order to be a

successful alternative to PC and blended cement

concretes, geopolymer concrete must show similar

durability properties and be able to resist attack from

these aggressive agents. This paper reports a set of

experiments analysing the strength gain and durability

properties of geopolymer concrete.

2 Materials and methodology

A range of geopolymer concrete and mortar specimens

were cast varying the mix composition and activator

modulus. Testing included compressive strength,

sorptivity, chloride diffusion, rapid chloride perme-

ability and carbonation. In addition, scanning electron

microscopy (SEM) and energy dispersive X-ray

analysis were also undertaken.

The FA used to manufacture the geopolymer

concrete was a low calcium FA (class F fly ash)

conforming to AS 3582.1-1998 [4], Table 1. The

fineness of the FA had 86.82 % passing a 45 lm sieve.

The SO3 is less than 1 % which should ensure high

volume stability which is desirable for good durability.

The alkaline activator used in this study was a sodium

silicate based solution containing sodium silicate

solution (wt. ratio: Na2O/SiO2 = 2) and 10 M sodium

hydroxide. The chemical composition of the sodium

silicate solution by mass was Na2O = 14.7 %,

SiO2 = 29.4 % and water = 55.9 %.

The blended sodium silicate and sodium hydroxide

solutions are characterized by the activator modulus

(Ms), which is defined as the mass ratio of SiO2 to

Na2O; and the dosage, corresponding to the percentage

Na2O. The adopted nomenclature for the specimens

was GX-Y, where X is dosage and Y is Ms, i.e. G7.5-

1.0 for a 7.5 % dosage and a 1.0 activator modulus.

Table 1 Chemical composition of class F fly ash

Component Percentage

SiO2 49.45

Al2O3 29.61

Fe2O3 10.72

CaO 3.47

MgO 1.3

K2O 0.54

Na2O 0.31

TiO2 1.76

P2O5 0.53

Mn2O3 0.17

SO3 0.27

LOI 1.45

722 Materials and Structures (2015) 48:721–731

Different dosage and activator modulus are

achieved by varying the ratio of sodium silicate and

sodium hydroxide. The sodium hydroxide solution

was prepared in a fume cupboard by dissolving

sodium hydroxide pellets in deionised water at least

1 day prior to mixing.

Both coarse and fine aggregate were prepared in

accordance with AS 1141.5-2000 [5]. The moisture

condition of the aggregate was in a saturated surface

dry 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 mix design was based on previous tests on

mortar specimens [3]. The total aggregate in the

concrete was kept to 64 % of the entire mixture by

volume for all mixes. The ratio of ingredients

(cementitious materials, chemical activator, aggre-

gate, and water) was calculated based on the absolute

volume method [37], 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 con-

stant. Table 2 summarizes the mix details for the

geopolymer concrete.

The concrete mix was designed to provide a 28 day

compressive 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 7.5 % Na2O dosage was selected. 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 FA,

the solid in the sodium silicate solution and the sodium

oxide pellets. Liquid sodium silicate and sodium

hydroxide were blended in different proportions,

providing an activator modulus (Ms) ranging from

0.75 to 1.25. In general class F fly ash based

geopolymer concretes exhibit longer setting times

and slower strength development at room temperature

[50], with curing generally being between 60 and

90 �C. This temperature can be reduced by using high

calcium (class C) FA [27] or by adding calcium

compound [49] as the higher calcium content can

produce C–S–H which can be cured at room temper-

ature. As these experiments used a 100 % class F fly

ash, a curing temperature of 90 �C was adopted [3].

The slump test was undertaken in accordance with

Australian Standard AS 1012.1-1993 [6]. Compres-

sive strength tests were performed with a loading rate

of 20 MPa/min according to AS 1012.9-1999. Sorp-

tivity tests were undertaken with 100 mm diameter

and 50 mm height specimens in accordance with

ASTM C1585-04 [7]. The sides of the specimens were

coated with epoxy to allow free water movement only

through the bottom face (unidirectional flow). The

results were plotted against the square root of the time

to obtain a slope of the best fit straight line. According

to Hall [28], the penetration of water under capillary

action (sorptivity, S) can be modeled by:

I ¼ Aþ St1=2 ð1Þ

where I is the cumulative absorbed volume after time

t per unit area of inflow surface, I = Dw/ar, Dw being

the increase in weight, a the cross-sectional area and

r the density of water.

Chloride diffusion tests were based on AASHTO

T259 [1] and rapid chloride permeability testing

(RCPT) was performed according to ASTM C1202-

07 [8] and AASHTO T277 [2]. Accelerated carbon-

ation tests were undertaken in a purpose built carbon-

ation chamber, Fig. 1 at a CO2 concentration of 5 %, a

temperature of 20 ± 1 �C and a relative humidity of

70 ± 1 % for 28 days. The CO2 concentration was

correlated relative to the O2 concentration in the

chamber. Testing for carbonation depth was under-

taken using Phenolphthalein indicator. Pore water

from mortar samples was obtained using a purpose

built pore press. The pH was measured electronically

using a pH electrode.

Table 2 Mix proportion of fly ash-based geopolymer concrete

Mix Fly ash (kg) Aggregate (kg) Activator (kg) Added water (kg) w/b

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

G7.5-0.75 1,050 1,728 763 1,528 198 209 88 0.34

G7.5-1.00 1,030 1,728 763 1,528 262 165 84 0.32

G7.5-1.25 1,016 1,728 763 1,528 324 123 79 0.32

Quantities for kg/m3

Materials and Structures (2015) 48:721–731 723

The microstructure was observed using SEM imag-

ing employing both secondary and backscatter electron

detectors. To prepare the samples for SEM analysis the

specimens were cut using a diamond saw to a size of

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

samples were subsequently gold coated for imaging.

Samples were mounted on the SEM sample stage with

conductive, double-sided carbon tape. A total of 3

samples were investigated for each mix.

3 Results and discussion

3.1 Material properties

The FA-based geopolymer displayed a very high

slump with all mixes giving a slump in excess of

200 mm. This is attributed to the spherical shape of FA

particles combined with the lubricating effect of

sodium silicate solution increasing the flowability.

The high slump across the mix types was not crucial

as the basic requirements were that the fresh concrete

did not segregate when vibrated and little bleeding

occurred, both of which requirements were fulfilled by

the geopolymer concrete. Following heat curing the

geopolymer mix produced a concrete of comparable

appearance to a standard OP concrete.

The strengths of the FA-based geopolymer concrete

are shown in Table 3. There was a significant increase

in strength between MS = 0.75 and 1.0. However,

there was only a small further increase to the

MS = 1.25 concrete. This is attributed to an increase

in the alkali modulus leading to an increase in soluble

silicates and consequently an increase in the reaction

rate. The proposed reaction mechanism for geopoly-

merisation is a multistage process [23] consisting of

five stages: (1) dissolution, (2) speciation equilibrium,

(3) gelation, (4) reorganization, and (5) polymeriza-

tion and hardening. The dissolution process starts with

an attack on the FA particles by alkaline solution [24].

As a result the reaction product is generated both

inside and outside the shell of the FA particle sphere

until the ash particle is completely or almost com-

pletely consumed. At the same time, precipitations of

reaction products occur as the alkaline solution

penetrates the larger sphere and fills up the interior

space with reaction product, forming a dense matrix.

Due to the precipitation of reaction products, some

portions of smaller particles are covered with the

products providing crust which prevents contact with

the alkaline solution resulting in unreacted FA parti-

cles. The increase in strength observed between

MS = 0.75 and MS = 1.0 is attributed to an increase

in the dissolution process, thus resulting in a higher

Fig. 1 Schematic of

accelerated carbonation

chamber

724 Materials and Structures (2015) 48:721–731

reaction rate and in fewer unreacted FA particles. The

relatively small variation between MS = 1.0 and 1.25,

suggests that little further dissolution of the FA arises

from the increase in activator modulus. This would

indicate that either all the FA has been dissolved once

a MS = 1.0 has been reached, or that the increase does

not result in any further dissolution of the protective

crust on the FA particles. Thus the formation of the

[Mz(AlO2)x(SiO2)y�nMOH�mH2O] gel, the dominant

step in formation of an amorphous structure of

geopolymers, which relies on the extent of dissolution

of alumino-silicate materials, reaches a limiting value

as the activator modulus increases above 1.0.

As would be expected with heat curing there was only

a small increase in strength from 28 to 90 days for all the

specimens, irrespective of activator modulus. Both the

MS = 1.0 and 1.25 concretes achieved a strength in

excess of 50 MPa at 28 days. The compressive

strengths observed are in agreement with those observed

by other authors using class F fly ash [10, 26, 41, 43].

3.2 Sorptivity

The sorptivity curve of the geopolymer concretes at 56

and 90 days are given in Fig. 2. The results show a

non-linearity in the initial stages, which is in agreement

with previous research [22], which found that the

sorptivity curve of geopolymer concrete was less linear

compared to that of OP and blended cement concretes.

During the setting period the geopolymer concrete

exhibited an increased degree of bleeding compared to

the OP concretes. This would be expected to result in

high quantities of cement paste at the surface. This high

concentration of cement paste is hypothesised as

leading to a rapid saturation of the outer paste layer.

Once this layer is saturated the area of absorption is

reduced due to the presence of aggregates. The level of

bleeding was higher in the specimens with an MS of

0.75, which corresponds to the highest value of

sorptivity, Table 4, and the least linear plot.

The increase in activator modulus from 1.0 to 1.25

showed little change in the sorptivity values. These

changes are consistent with those for the strength data,

which also showed an improvement in performance

from MS 0.75 to 1.0, but little further improvement

from MS 1.0 to 1.25. This improvement can be

accounted for by an increase in the SiO2 content and

increased dissolution of the FA, which results in a

higher rate of reaction, leading to a denser structure

with a reduction in the porosity of the geopolymer

concrete. The correlation coefficients, R, of all the

sorptivity data exceed 0.98. Overall the sorptivity

parameters of the geopolymer concretes are compara-

ble to those of OP and blended cement concretes of

similar strengths [22, 38].

3.3 Chloride diffusion

The results are presented in Table 5. The apparent

diffusion coefficient (Da) and surface concentration

(Cs) were calculated by plotting the chloride profiles

and determining the best fitted curve using Fick’s 2nd

Law of Diffusion [18].

Table 3 Compressive strength of geopolymer concrete

Mix Compressive strength (MPa)

7 days 28 days 90 days

G7.5-0.75 39.1 ± 3.5 44.4 ± 3.4 46.1 ± 2.1

G7.5-1.00 51.3 ± 5.2 53.3 ± 2.6 53.6 ± 5.5

G7.5-1.25 52.5 ± 4.6 56.9 ± 3.3 57.3 ± 2.0

0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20

t1/2 (min1/2)

i (m

m3 /m

m2 )

G7.5-0.75 (56 days)

G7.5-1 (56 days)

G7.5-1.25 (56 days)

G7.5-0.75 (90 days)

G7.5-1 (90 days)

G7.5-1.25 (90 days)

Fig. 2 Absorption (i) versus square root of time for geopolymer

concrete at 56 and 90 days

Table 4 Sorptivity correlation parameters

Mix Sorptivity parameters

56 days 90 days

Si (mm/min1/2) R Si (mm/min1/2) R

G7.5-0.75 0.101 0.996 0.101 0.997

G7.5-1 0.078 0.996 0.075 0.998

G7.5-1.25 0.071 0.997 0.066 0.998

Materials and Structures (2015) 48:721–731 725

The geopolymer specimens display a similar diffu-

sion coefficient for all activator moduli. The diffusion

coefficients of the FA geopolymer concretes are

comparable with those for OP and blended cement

concretes, indeed being in the lower range of values

reported in the literature [13, 17]. This data would

indicate a high level of resistance to chloride ingress

for geopolymer concretes. However, one factor that

should be taken into account when considering long

term performance is that OP and to a greater extent

blended cement concretes show a reduction in the

chloride diffusion coefficient with time. This is

represented as the maturity factor, m [13]. This

improved performance is attributed to on going

hydration of the concrete with time. For geopolymer

concrete, which is produced by heat curing, little if any

further reaction will take place, illustrated by a

minimal increase in compressive strength with time

when compared to OP and blended cement concretes

cured at ambient temperatures. Hence, it may be

expected that little improvement in the diffusion

coefficient will occur over time when compared with

OP and blended cement concretes. This would mean

that the chloride diffusion coefficient after 20 years

would be similar to that observed in these tests. Thus at

20 years the diffusion coefficients for geopolymer

concretes may be higher than those for OP and blended

cement concretes.

RCPT were undertaken on the geopolymer speci-

mens. However, rapid heating of the specimens was

observed, Table 6, with all specimens reaching 60 �C

before the conclusion of the test. The flow of electric

current through a conductor generates heat which will

in turn increase the mobility of the ions that carry the

current, which will itself raise the total current flow

producing more heat in a cyclic process. Given the

rapid rises in temperature observed it can no longer be

assumed that Ohm’s law (V = IR) applies, which is

the basic principle of the RCPT method. Hence the

RCPT is assessed as not giving data which can be used

to evaluate the performance of geopolymer concrete.

The RCPT is often used as a standard test to assess the

chloride resistance of concrete in severe exposure

conditions. The heating observed in these tests indi-

cates that the RCPT should not be applied as a method

for assessing the chloride resistance of geopolymer

concretes and an alternative method should be used to

assess their performance such as the Nord tests NT

BUILD 433.

3.4 Carbonation

The geopolymer concrete displayed no clear border

between the coloured and colourless area when

sprayed with phenolphthalein indicator. There was a

graduation in colour as the outer part was lighter in

colour compared to the inner part. In this case the

carbonation ‘front’ was not clear, as such it is not

possible to measure accurately the penetration depth

using a phenolphthalein indicator. Even after full

carbonation, some light pink colour was found in the

outer part of the specimens.

When compared to carbonated OP and blended

concrete the overall colour of non-carbonated geo-

polymer concrete was lighter than that observed in

uncontaminated OP or blended cement concretes. This

suggests that the pH of the pore solution of the

carbonated geopolymer concrete was lower and the

pH was not significantly affected by the CO2. The

colouring observed is in agreement with Davidovits

who stated that the pH of geopolymer concrete is in the

region 11.5–12.5 and that the carbonation products of

geopolymer concrete have a minimum pH of 10–10.5

which is much higher than the pH from the calcium

carbonate which can have a pH lower than 9 [20].

In order to asses the pH of the geopolymer material

mortar specimens using a dosage and Ms the same as

the concrete specimens, together with those with a

dosage of 15 % used in previous mortar tests were

investigated [3], Table 7. The pore water was obtained

Table 5 Apparent chloride diffusion coefficient and surface

chloride value

Mix Cs (%) Da 9 10-11 m2/s

G7.5-0.75 0.16 3.1

G7.5-1 0.17 3.1

G7.5-1.25 0.14 3.7

Table 6 Rapid chloride permeability test data

Mix Solution temperature �C Duration (Min)

Initial Final

G7.5-0.75 26.1 60 60

G7.5-1 25.1 60 200

G7.5-1.25 22.4 60 270

726 Materials and Structures (2015) 48:721–731

from the mortar specimens using a pore press and the

pH was tested at different time periods until full

carbonation of the specimens had been achieved after

28 days exposure. Spraying with phenolphthalein

gave similar colours to that obtained for the concrete

specimens. The results are given in Table 8.

The mortar specimens all had an initial pH of

between 11.75 and 12, with little variation between the

G7.5 or G15 mixes. However, higher initial pH values

have also been reported depending on the activator pH

and the raw materials used [36, 42, 45]. The pH

reduced slowly with time until full carbonation was

achieved after 28 days. At 3 days no discernable

difference in pH was detectable, by 7 days a reduction

in pH was clearly measured and by 28 days the G7.5

specimens had a pH in the range 10.4–10.9 and the

G15 in the range 11–11.25. These results show a

similar initial pH to that reported by Davidovits,

though the final pH is higher than that reported [20].

The variation in the initial and final pH recorded for

the mortars compared with that of OP and blended

cements is attributed to the difference in the materials

produced. The OP cements produce Ca(OH)2 and

C–S–H gel, while the geopolymers produced

[Mz(AlO2)x(SiO2)y�nMOH�mH2O] gel. The pH of

fresh, OP and blended concrete, is above 13 and that

of carbonated concrete below 9 [29], due to the

formation of calcium carbonate.

In the fresh OP cement concrete the Ca(OH)2 and

C–S–H gel provide buffering to maintain the pH above

13. In the geopolymer concrete the [Mz(AlO2)x

(SiO2)y�nMOH�mH2O] will not provide a similar

buffering. The pH from the mortars indicate that

following the geopolymeric reaction a pH in the pore

solution of around 12 for all geopolymer mortars is

achieved despite the variation in hydroxyl ion

concentrations in the initial mix. This would

indicate that there is no buffering by the [Mz(AlO2)x

(SiO2)y�nMOH�mH2O] comparable to that provided

by the C–S–H gel. Rather the pH of the geopolymers

produced is controlled by the sodium hydroxide from

the activator that is contained in the pore solution. A

number of factors are hypothesised as controlling this

pH, including the pH of the activator, the degree of

reaction that occurs and hence the residual activator

remaining and the chemical composition of the binder

material, which will determine the composition of the

geopolymeric material produced.

In the geopolymer concrete the carbonation is

hypothesised as the reaction of the sodium hydroxide

with CO2 forming sodium carbonate hydrates. The

result of this is only a minimal reduction of pH to a value

of approximately pH 11. The geopolymer mortars with

the lower dosage (7.5 %) having a slightly lower final

pH than those with the higher dosage (15 %).

These results are comparable with those reported

for alkali activated slag concretes which have not

shown any detrimental effects due to carbonation [21,

44] and that final a pH of between 10 and 12, could be

expected [15].

Given the results from the concrete and mortar

specimens, coupled with other reported results sug-

gests that a final pH in the region of pH 11 can be

expected for geoploymer concrete. This is a value that

could provide protection to reinforcing steel following

carbonation.

3.5 Microstructure

The microstructure of all three geopolymer concrete

mixes were similar, Fig. 3. All the geopolymer

Table 7 Mix proportion of fly ash-based geopolymer mortar

Mix Fly

ash

(kg)

Fine

sand

(kg)

Activator (kg) Added

water

(kg)Na2SiO3

(liquid)

NaOH

(15 M)

G7.5-0.75 0.523 1.440 0.128 0.101 0.082

G7.5-1.0 0.522 1.431 0.124 0.133 0.064

G7.5-1.25 0.521 1.438 0.167 0.046 0.108

G15-1 0.505 1.388 0.193 0.148 0.046

G15-1.25 0.500 1.376 0.255 0.117 0.033

G15-1.5 0.496 1.364 0.316 0.087 0.020

Quantities for kg/m3

Table 8 Carbonation data for geopolymer mortar specimens,

pH

Mix pH

0 days 3 days 7 days 28 days

G7.5-0.75 11.86 11.88 11.01 10.88

G7.5-1.0 11.94 11.91 11.35 10.46

G7.5-1.25 11.73 11.71 11.39 10.73

G15-1 11.96 11.97 11.50 11.05

G15-1.25 11.99 11.88 11.50 11.00

G15-1.5 11.97 11.98 11.77 11.23

Materials and Structures (2015) 48:721–731 727

specimens contained both unreacted FA and silica.

Some particles of FA were also found to have been

partially dissolved by alkali Fig. 4. The most distinc-

tive difference in the microstructure was a denser pore

structure as the modulus increased. Analysis of the

number of unreacted grains in each sample noted a

slight decrease in the number of unreacted FA parti-

cles, with an increase activator modulus, while the

number of partially reacted grains observed remained

similar as the activator modulus increased. This would

indicate that there is an additional dissolution of the FA

due to the increase in activator modulus but that the

increase does not result in any further dissolution of the

protective crust on the FA particles. The decrease in the

number of unreacted FA particles was more pro-

nounced between the G7.5-0.75 to G7.5-1 mixes, than

from the G7.5-1 to the G7.5-1.25 mixes.

According to Steveson and Sagoe-Crentsil [47],

unreacted components in FA-based geopolymer

binder make up a significant proportion of the total

volume of the binder. These components are compos-

ites, hence the strength of the unreacted particles, the

interface between them and geopolymer matrix is

expected to have a significant bearing on the overall

strength of the material. The reduction in the number

of unreacted particles of FA as the activator modulus

increases would explain the higher strength and lower

sorptivity observed.

3.6 Long term durability

The compressive strength of the geopolymer concrete

is in excess of the specified minimum strength in AS

3600 for the design of concrete structures in exposure

categories A1 and A2 and B1 and B2. This indicates

that from a structural perspective the use of geopoly-

mer concrete is a feasible alternative to OP and

blended concretes used at present.

Fig. 3 Microstructure of the three geopolymer concrete mixes

Fig. 4 Partially dissolved fly ash grain

728 Materials and Structures (2015) 48:721–731

However, the long term durability of reinforced

concrete is dependent upon the protection of reinforc-

ing steel in concrete which is provided by the passive

layer formed on the steel surface due to the high pH in

OP and blended cement concretes [16]. It is the

breaking down of this layer by carbonation or

chlorides that results in the corrosion of the reinforce-

ment. Thus the initial pH would be expected to have a

significant factor in the chloride induced corrosion of

reinforcing steel. The Cl/OH ratio is regarded as one of

the controlling factors in the initiation of corrosion

[31]. The pH produced following carbonation will also

be a significant factor in the long term durability of

geopolymer concrete.

The data obtained suggests that carbonation of

geopolymers may not be as potentially deleterious as

carbonation of OP and blended cement concretes as

the pH remains at a level that can provide protection to

the reinforcing steel. However, for chloride induced

attack the long term protection provided by geopoly-

mer concrete may be lower than for OP and blended

cement concretes. The lower initial pH may lead to a

lower concentration of chloride ions being required at

the rebar to initiate corrosion and, as discussed, the

long term chloride diffusion coefficient may be higher

than for OP and blended cement concretes. To

determine this effect more clearly longer term testing

of geopolymer samples is required to determine what

the maturity factor is for geopolymer concrete.

4 Conclusions

There is a significant increase in strength from the

MS = 0.75 to the MS = 1.0 and 1.25 concrete which

is attributed to an increase in the dissolution of the FA

grains and a resultant increase in the reaction rate.

Minimal variation in strength is observed between the

MS = 1.0 and 1.25 concretes and little increase in

strength is observed for any of the geopolymer

concretes with time, which is attributed to the heat

curing.

The geopolymer concretes display a non-linearity

in the sorptivity data in the initial stages compared to

OP and blended cement concretes. This non linearity

is hypothesized as being due to an increased bleeding

in geopolymer concretes giving a cement rich surface

layer allowing higher initial absorption. The sorptivity

values of the geopolymer concretes are comparable to

similar strength OP and blended cement concretes.

The MS = 0.75 gave the highest sorptivity value while

the MS = 1.0 and 1.25 gave similar values.

The apparent chloride diffusion values were similar

for all the geopolymer concretes. The values were

favorably comparable to similar OP and blended

cement concretes, though due to the heat curing there

may be no long term reduction in the diffusion

coefficient for geopolymer concretes, as compared to

OP and blended cement concretes. The RCPT results

in rapid heating in geopolymer concrete.

Carbonation testing indicated that the initial pH of

geopolymer concrete is less than that of OP and

blended cement concretes but is higher after carbon-

ation. The final pH after carbonation may be sufficient

to provide protection for reinforcing steel in carbon-

ated geopolymer concrete.

SEM analysis showed a denser pore structure with

increasing activator modulus, consistent with the

durability test results.

The durability test program indicates that both the

MS = 1.0 and 1.25 geopolymer concretes gave dura-

bility parameters comparable to OP and blended

cement concretes of similar strength. However, con-

cern exists over the long term performance when

considering chloride induced corrosion of reinforcing

steel due to the lower initial pH and potentially

constant chloride diffusion coefficient.

Due to the different composition of the geopolymer

concrete compared to OP and blended concretes the

direct application of the current standards and dura-

bility test methods is not considered appropriate and

before the use of geopolymer concrete is adopted the

development of a specific standard accounting for the

mix design and testing of the long term performance is

recommended.

Acknowledgments The authors wish to express their thanks

to PQ Australia for the supply of materials for the research

project, and Cement Australia Ltd for providing the FA. The

authors also wish to acknowledge the facilities, and the scientific

and technical assistance, of the Australian Microscopy &

Microanalysis Research Facility at the RMIT Microscopy &

Microanalysis Facility, at RMIT University.

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