DURABILITY STUDY OF LOW CALCIUM FLY ASH GEOPOLYMER CONCRETE

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ACF International Conference-ACF/VCA 2008

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D.21

DURABILITY STUDY OF LOW CALCIUM FLY ASH

GEOPOLYMER CONCRETE

R.Sathia- PhD Scholar, K. Ganesh Babu- Professor, Manu Santhanam- Assistant Professor

Indian Institute of Technology Madras, Chennai, India

ABSTRACT: The objective of the present work is to find the durability of the fly ash based

Geopolymer prepared with sodium silicate and sodium hydroxide as activators. The concretes

were prepared with varying fly ash content of 350, 450 & 550 Kg/m3

and activator solution to

fly ash ratio of 0.4 and 0.5. Compressive strength in the range of 10-60 MPa was obtained.

The performance of these concretes in aggressive environments was also studied, using tests

on absorption, acid resistance and potential. Results indicated that the water absorption

decreased with an increase in the strength of the concrete and the fly ash content. All

geopolymer concretes showed excellent resistance to acid attack (3% H2SO4) compared to the

normal concrete.

KEYWORDS: Geopolymer, alkali activated fly ash, permeability, mechanical properties

1. INTRODUCTION

Geopolymer [1] is a new material which is being used for construction all over the world. As

a new material for construction not much of information is available on the durability of

geopolymer concrete. The durability of concrete is an important requirement for the

performance of the structure in aggressive environments throughout its design life period.

The durability of concrete primarily depends upon its permeability characteristics.

Impermeable concretes can resist the ingress of aggressive ions into the concrete and there by

reduce the damages occurring due to the deterioration of concrete and the corrosion of steel in

concrete. However, there appears to be no comprehensive information of the permeability

characteristics and deterioration characteristics of geopolymer concretes Moreover, for such a

comprehensive understanding it is also essential that these concretes should be well designed

at any particular strength.

2. MATERIALS AND EXPERIMENTS

2.1. Material characterization

The physical properties and chemical composition as determined by XRF is shown in Table1.

For the preparation of geopolymer concrete locally available river sand with a specific gravity

of 2.62 was used as fine aggregate.

Fly ash itself is a slowly reactive material and has a very strong silica alumina glassy chain. In

order to enhance the reaction process the strong chain has to be broken; hence, the alkali

activators are used to enhance the reaction process. The OH¯ concentration of the activators is

an important constituent which is primarily necessary for the disintegration of the strong silica

– alumina glassy chain. Higher the OH¯ concentration of the activators, more rapid is the

disintegration of the chain, resulting in the production of large number of active groups [4].

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The choice of the activators mainly depends upon the reactivity and the cost of the activators

[8]. Literature indicates that sodium silicate solution in combination with sodium hydroxide

and potassium hydroxide is an effective activator. Compared to potassium hydroxide, sodium

hydroxide is cheaper and equally reactive. The chemical composition of activators used in

this study is shown in Table 2. Sodium hydroxide solution of 16M was prepared by mixing

the pellets with distilled water. The percentage of solids and liquids in the NaOH solution is

tabulated in Table 2. In order to improve the workability, a high range water reducing

admixture (SNP based) was used.

Table1. Chemical composition of fly ash

Oxides SiO2 Al2O3 Fe2O3 CaO Na2O MgO K2O SO3 LOI

% by

mass

61.16 30.08 4.62 1.75 0.76 0.18 0.36 0.19 0.60

Table 2. Composition of Activator solutions

SiO2 Na2O SiO2/ Na2O Specific gravity

Sodium

silicate 28% 8% 3.5 1.48 g/cm

3

Sodium

Hydroxide

Molarity Solids % Liquids % Specific gravity

16M 44.4 55.6 1.34 g/cm3

2.2. Manufacture of geopolymer concrete 2.2.1 Mix proportioning

All geopolymer concrete were made with low calcium fly ash, with SiO2 to Al2O3

ratio of 2.5. All the concretes were designed similar to normal concrete, such that the density

was approximately equal to 2400 kg/m3

[2]. The total aggregate of the geopolymer concrete

varied depending upon the amount of fly ash and activator solution used. The total aggregate

content normally occupies about 60 – 80 % of the mass of the concrete. The sodium silicate to

sodium hydroxide ratio was fixed as 1.5 and the concentration of NaOH was taken as 16M.

Extra water was added to certain mixes depending upon the workability of the mix required.

High range water reducing naphthalene based superplasticizer at 2.5 % by mass of fly ash was

added in order to improve the workability. The mixture proportions are shown in Table 3.

2.2.2 Mixing, curing and casting of geopolymer concrete

The activator solutions were prepared one day before use. Fly ash and the aggregates were

dry mixed in an 80 liter capacity pan mixer for 4 minutes. This was followed by the addition

of the activator solutions, extra water and superplasticizer followed by a final mixing of

another 5 min. Right after mixing, the slump of the fresh geopolymer concrete was

determined in accordance with ASTM C173. After the determination of slump, the fresh

concrete was placed in the mould. The specimens were compacted with three – layer placing

and tamping using a rod. This was followed by an additional vibration of 10seconds using a

vibrating table. The specimens were then wrapped with thin vinyl sheet to avoid loss of water

due to evaporation. All the concrete specimens were then transferred to an oven set at a

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temperature of 85°C, and stored for 24hours.After curing; the specimens were allowed to cool

in air, demoulded and kept in open until the day of testing.

Table 3. Details of concrete mixes

Note : NC = normal concrete , C = Cement and GC = Geopolymer concrete

2.3 Mechanical properties 2.3.1 Compressive strength

Compressive strength tests were performed at the age of 3, 7 and 28 days in accordance with

IS: 516-1959. The reported strengths in Table 3 are the average of three tests. It can be seen

from the results that the compressive strength increases with an increase in the fly ash content

and the amount of the activator solution. Increase in the activator solution showed increase in

the strength, this is due to the increase in the Na2O content of the solution, which is mainly

required for geopolymerization. It is noted that the in case of geopolymer concrete, 90% of

the final compressive strength is reached within 7 days and there is not much variation in

compressive strength after 7 days.

2.4 Effect of water content

Similar to Portland cement concrete the water content in the mix plays an important role in

the strength achievement of geopolymer concrete. The reaction occurring in the case of

geopolymer concrete is different from that of Portland cement concrete [8]. In case of

geopolymer concrete, water is required to improve workability, but is expelled during curing

at elevated temperature, increasing the porosity of concrete. It can be inferred from the results

that as the H2O to Na2O ratio of the mix increases, the compressive strength decreases. This is

due to increase in the porosity of the concrete due to the evaporation of water during curing at

elevated temperature. It was found that the H2O to Na2O ratio of 10-14 proposed by the

author [9] could be used only for the concrete designed with a fly ash content of 408 Kg/m3

and these ratios were changing, on variation of fly ash content.

3. DURABILITY OF GEOPOLYMER CONCRETE

The durability of concrete has been evaluated in this study through parameters related to the

permeability, chemical attack and corrosion potential of steel reinforcement. The permeability

has been assessed through measurements of water absorption. The chemical attack has been

Concrete fly ash

Kg/m3 Total solution Extra water w/s Na2O:SiO2 H2O:Na2O

Compressive

strength ( Mpa)

NC20 C=240 150 0.77 25

GC1 550 275 7.5 0.26 0.14 11.61 24

GC2 450 180 26.5 0.26 0.12 13.44 27

GC3 350 140 11.2 0.24 0.12 12.30 24

NC40 C=356 185 0.52 47

GC4 550 220 16.5 0.24 0.12 12.22 44

GC5 450 180 17.3 0.24 0.12 12.57 47

GC6 350 175 11.2 0.28 0.14 12.24 41

NC60 C=428 185 0.38 62

GC7 550 220 7.5 0.22 0.12 11.52 60

GC8 450 225 0 0.25 0.14 11.14 65

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studied through the acid attack; the corrosion potential of reinforcement has been studied

using potentials.

3.1 Absorption studies This study was done to know the relative porosity or permeability characteristics of the

concretes, and was carried out according to ASTM C 642-82 at 28 days. The specimens used

for this test were 100 mm cubes. The percentage absorption was calculated using the equation

(2)

Absorption (%) = (w2 – w1)/ w1 x100 (2)

Where w1 = weight of specimen after complete drying at 105°C

w2 = final weight of surface dry sample after immersion in water at least 24 hours

The results of this study for all the concretes are presented in Fig.1. Fig 2 presents a typical

variation of absorption with time for the GC2 (20 MPa) concrete. The initial absorption

values (at 30 min) for all the concretes were compared with recommendations given by

Concrete Society (CEB) [10], and this comparison is presented in Table 4. From these results,

it can be seen that absorption values of the geopolymer concretes at all strength levels were

lower than the limit of 3% specified for good concretes. The final absorption results of these

mixes shows that the geopolymer concretes were having lower absorption rate compared to

normal concretes, and also decreasing with increasing strength.

Table 4 Assessment criteria for absorption (CEB 1989)

Absorption (%)

at 30 minutes Absorption rating Concrete quality

< 3.0 low good

3.0 to 5.0 average average

> 5.0 high poor

0

1

2

3

4

5

Ab

so

rpti

on

(%

)

20MPa 60MPa

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

Time (hrs)

Ab

so

rpti

on

(%

)

G2

Fig.1 Variation in water absorption Fig.2 Variation of absorption with

time (20 MPa concrete)

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3.2 Acid attack studies To perform the acid attack studies, in the present investigation immersion techniques was

adopted. After 28 of casting days, 100mm cube specimens were immersed in 3% H2SO4

solution. The solution was kept at room temperature and the solution was stirred regularly, at

least twice a day to maintain uniformity. The solution was replaced at regular intervals to

maintain concentration of solution throughout the test period. The evaluations were conducted

after 5,15,30,60 and 90 days from the date of immersion. After removing the specimens from

the solution, the surfaces were cleaned with a soft nylon wire brush under the running tap

water to remove weak products and loose material from the surface. Then the specimens were

allowed to surface dry and all the measurements were taken. From the initial measurement

and measurements at particular intervals, the loss/ gain of the weight were studied. All the

geopolymer concrete were showing percentage of mass loss less than 0.5 % (see Table 5).

The percentage of mass loss for geopolymer concrete was only a fraction compared to the

normal concrete of equal strength grades. Hence geopolymer concrete showed an excellent

resistance to acid attack.

Table 5 Change in mass of concrete exposed to sulphuric acid solution (3%)

3.3 Open circuit potential (OCP) measurements

Corrosion potential is a technique used to detect the state of reinforcement without disturbing

the structure [10]. This is important because the intensity of corrosion of steel in concrete is

generally known only after the concrete has cracked or disrupted. At this stage, the

maintenance or rehabilitation of structures becomes very expensive. Therefore, it is essential

to know the state of reinforcement whether it is in active or passive condition well before the

spalling or cracking of concrete occurs. The open circuit potential of steel in different

concretes was measured by using a saturated calomel (i.e., mercury in saturated mercuric

chloride) electrode (SCE). The procedure given in ASTM C 876 was followed. The potentials

obtained are presented in Fig 3, where, we can see that the geopolymer concretes exhibited

low potentials compared to the other concretes. The potentials vary from -35 mV to -300 mV,

which shows probable corrosion indications. Generally to after 3 years only we will get stable

potentials, so, the half-cell potentials “may or may not be indicators of corrosion current”

(ASTM C 876) and, therefore, cannot be taken as absolute indicators of corrosion reactions.

Concrete Weight loss % (90 days)

NC20 25

G2 0.5

NC40 18

G8 0.47

NC60 22

G9 0.4

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0

100

200

300

400

500

90 d

ay p

ote

nti

al (S

CE

, -m

V)

Fig.3 Potentials of embedded steel

4. CONCLUSIONS

The salient conclusions of this study can be listed as following

1. Compressive strength increases with increase in the fly ash content and increase in

the activator solution. This is due to the increase in the sodium oxide content which

is mainly required for the geopolymerization reaction.

2. Similar to normal concrete the split tensile strength of geopolymer concrete were

varying from 7to 11% of compressive strength which is in good agreement with the

earlier information [9]

3. The absorption characteristics, which indirectly reflects the permeability, show that

the initial 30 minutes absorption values for all the concretes was lower than the

limits specified for “good concrete” by Concrete society

4. The deterioration of geopolymer concrete assessed in 3% H2SO4 solution shows that

there is no significant variation in weight loss with increasing fly ash content. All the

concrete showed weight loss of less that 0.5 % at 90 days.

5. The potentials taken with SCE were reducing with time to indicate a lower

probability of corrosion. At 90 days, the potentials of steel in geopolymer concretes

were tending to lower a probability of corrosion and were also decreasing with

increasing strength

It can thus be concluded that geopolymer concrete possesses excellent mechanical

properties and durability for aggressive environments.

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REFERENCES

1. Davidovits, J., (1988), Soft mineralogy and Geopolymers. Paper presented at the

Geopolymer ‘88’, First European conference on soft Mineralogy, Compiegne,

france.(1)

2. Rangan, B.V., and Hardjto, D. (2005), Development and properties of low calcium

fly ash based geopolymer concrete. Research report GC-1, Faculty of Engineering,

Curtin University of Technology, Perth, Australia.

3. Bakharev, T. (2005), Geopolymeric materials prepared using class Fly ash and

elevated temperature curing, Cement and Concrete Research, Vol.35, 1224-1232

4. Rangan, B.V., and Wallah, S.E. (2006), Low- calcium fly ash based geopolymer

concrete: long term properties. Research report GC-2, Faculty of Engineering,

Curtin University of Technology, Perth, Australia

5. Fernandez – Jimenez, A., and Palomo, A. (2006), Engineering properties of alkali

activated fly ash concrete. ACI material journal, Vol.106, 106-112.

6. Swanepoel, J.C., and Strydom, C.A, (2002), Utilization of fly ash in a geopolymeric

material, Applied Geochemistry, Vol.17, 1143-1148

7. Jo, B.–W., Park, S.-K and Park, M.-S. (2007). Strength and hardening

characteristics of activated fly ash mortars, Magazine of concrete research, Vol.39,

121-129.

8. Dinakar , P., and Ganesh Babu, K.(2004) High performance self compacting

concrete- A critical assessment, Proceedings of the International conference of fibre

composities, High performance concretes and smart materials, Vol II, 747-756,

Chennai, India.

9. Neville A. M. (1995) Properties of concrete, 4th Ed. New York: John Wiley & Sons.

10. CEB – FIP (1989) Diagnosis and assessment of concrete structures – state of

artreport,CEB Bulletin, 83-85.

10. Broomfield J. P. (1997) Corrosion of steel in concrete: understanding, investigation

and repair, E & FN SPON, London

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DURABILITY STUDY OF LOW CALCIUM FLY ASH GEOPOLYMER CONCRETE