The Influence of Alkali Activator Concentration to Mechanical Properties of Geopolymer Concrete with Trass as a Filler

Puput Risdanareni1,a, Januarti Jaya Ekaputri2,b , and Triwulan2,c

1 Civil Engineering Departement ,State University of Malang, Jalan Semarang No 5 Malang 65145, East Java, Indonesia

2 Civil Engineering Departement, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo Surabaya 60111, East Java, Indonesia

Email : a pu2t_risdanareni@yahoo.com, b januarti_je@yahoo.com, c triwulan_marwan@yahoo.com

Keyword: geopolymer, fly ash, trass, NaOH concentration, ratio Na2SiO3 /NaOH, compressive strength

Abstract. This paper describes one of the varying factors influencing the mechanical properties of

geopolymer concrete. Fly ash and volcanic material called trass was used as raw materials, while

sodium Hydroxide (NaOH) and Sodium silicate (Na2SiO3) was applied as an alkali activator.

Mechanical properties were assessed by compressive test, the concentration of NaOH used in this

study was eight and ten Molars, and ratio of Na2SiO3 to NaOH by mass was 0.5, 1, 1.5, 2, and 2.5.

Test results indicated that the strength of geopolymer concrete and binder were hardly influenced by

concentration of NaOH in solution and the activator ratio. Binder with activator ratio of 2.5 has the

highest compressive strength both in 8M and 10M NaOH system. However, in concrete, there are a

few difference, concretes made with 8M NaOH and activators ratio of 2 have the highest

compressive strength. This result might due to the low workability of fresh geopolymer concrete.

On the other hand, binder and concrete made with 10M NaOH, showed the highest compressive

strength when they were made with the activator ratio of 2,5.

Introduction

Recently, the demand of concrete in the world was increased tremendously. This condition also

makes consumption of cement as a concrete material based increase. Meanwhile in cement

production process release carbon dioxide which caused greenhouse effect [1,4]. Due to this

condition, many efforts to find alternative material to substitute cement have been published. One of

the new alternative material which can be used to replace cement is geopolymer materials [1,4].

Geopolymer material is an inorganic material which contains silica (SiO2) and alumina (Al2O3)

as raw materials, and alkali activator. In general, geopolymer concrete can be produced with fly ash

as a based material [3]. However, different content of alumina and silica can give different

performance of geopolymer concrete [9]. In order to study the influence of alumina and silica

content in geopolymer properties, natural pozzolan called trass was used as an alternative material

as fly ash substitute.

Trass , is a volcanic eruption material which can be used as raw material in geopolymer

concrete because it is rich in silica and alumina. Other research stated that vulcanic ash could be

use as an eco-friendly cement replacement material [8]. In this paper, the addition of trass into

concrete mixture is expected to increase the mechanical property of fly ash based geopolymer

concrete.

Fly ash is one of the residues produced in combustion that comes out with the outlet gases in

the electric power plant station. The chemical properties of the fly ash are largely influenced by the

chemical content of the coal burned [12]. Due to it is rich contain of silica and alumina, fly ash

could be used as raw material for geopolymer concrete [3].

In polymerization reaction,a reactant from alkali group is required to release unessential

ions.[2] This research used Natrium Hydroxide or NaOH which has strong alkali characteristic as

alkali reactant and Sodium Silica or Na2SiO3 as a catalyst to accelerate polimerization reaction.

Materials Science Forum Vol. 803 (2015) pp 125-134© (2015) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.803.125

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 36.81.110.125-18/08/14,11:13:58)

Previous research [5] stated that NaOH as activator play important role in geopolymer properties. In

order to study the influence of Sodium hidroxyde (NaOH) and sodium silicate (Na2SiO3) as alkali

activator in polymerization reaction, ratio of Na2SiO3 to NaOH by mass was used.

Previous research [13] about geopolymer binders stated that water content played an important

role in geopolymer concrete properties. Other research [10] stated that the compressive strength of

the geopolymer concrete was increased with the increasing concentration of NaOH. However, the

inclusion of water beyond certain limit resulted in bleeding and segregation of fresh concrete and

decreased the compressive strength of the concrete [11]. In order to study the influence of water

content in geopolymer properties, a variety of activator concentration in solution eight and ten

Molars have been made.

Materials

Fly Ash and Trass

This research used fly ash type F (ASTM C61eight-03) from Jawa Power Paiton. Table 1 showed

the different chemical composition between fly ash and trass. The major constituents are SiO2 with

52.24% and 87.5% for fly ash and trass, respectively. The content of Al2O3 for both materials is

38.58 % and 0.2%. This shows that the content of silica in trass is higher than fly ash which is

suitable to use as a raw material for geopolymer. However, the content of alumina for trass is lower

than fly ash.

Table 1 Chemical Composition of Fly Ash and Trass by XRF

Particle Fly Ash

% mass

Trass

% mass

SiO2 52.24 87.5

Al2O3 38.58 0.2

Fe2O3 2.94 1.9

CaO 0.69 1.7

Na2O 0.52 0.8

K2O 0.44 0.41

TiO2 2.42 0.9

MgO 0.49 0.5

P2O5 0.13 -

SO3 1.21 6.2

SO2 -

LOI 1.39

Trass that obtained from PT Semen Gresik which origin from Pasuruan was used. The

chemical composition of trass can be seen in table 1. XRD test result indicated that trass has

amorphous characteristic due to its high silica content. The XRD pattern of trass can be seen in

Figure 1.

126 Geopolymer and Green Technology Materials

Fig. 1: X-Ray Diffraction Pattern of trass

Alkali Activator

A clear gel of Na2SiO3 and the flake shape of NaOH were selected as alkali activator. The mass

ratio of Na2SiO3 to NaOH was in range of 0.5 to 2.5. And then, NaOH flakes were diluted by

distilled water to obtain eight and ten Molars NaOH solutions. The alkali solution was prepared a

days before it is used.

Aggregate

Fine and coarse aggregate were collected from Lumajang with density of 2670 kg/m3 and 2700

kg/m3 respectively. The Coarse aggregate has maximum size of 20 mm.

Specimens and Mix Proportion

The specimen of geopolymer concrete is a cylinder with 100 mm diameter and 200 mm height.

Whereas, specimen of geopolymer binders is a cylinder with 20 mm diameter and 40 mm height.

There was ten composition of geopolymer binders and concrete. Labeling of geopolymer binders

and concrete can be seen in table 2. Mix proportion of geopolymer binders and concrete can be seen

in figure 2 and 3.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00

SiO2tri

NaAlSiOSiO2mono SiO2orto

CaSiO4

Materials Science Forum Vol. 803 127

Fig. 2: Geopolymer Binder Mix Proportion

Table 2 Composition and labeling of geopolymer binder and concrete

Binder’s

label

Concentration

of NaOH

Na2SiO3/

NaOH A 8M 0.5

B 8M 1.0

C 8M 1.5

D 8M 2

E 8M 2.5

F 10M 0.5

G 10M 1.0

H 10M 1.5

I 10M 2

J 10M 2.5

Geopolymer Binder 2x4 cm2

74 % Fly ash 26 % Activator Trass 50% from fly ash

total mass Water 12 % from fly

ash mass ash

NaOH 8M NaOH 10M =

NaOH

SiONa 32 1.0

=

NaOH

SiONa 32 1.5

=

NaOH

SiONa 32 2.5

=

NaOH

SiONa 32 2.0

=

NaOH

SiONa 32 0.5 =

NaOH

SiONa 32 0.5

=

NaOH

SiONa 32 2.5

=

NaOH

SiONa 32 2.0

=

NaOH

SiONa 32 1.5

=

NaOH

SiONa 32 1.0

128 Geopolymer and Green Technology Materials

Fig. 3 : Geopolymer Concrete Mix Proportion

Laboratorium Test

All of testing was held at The Concrete and Building Material Laboratory of ITS, Surabaya.

The testing result was the average evaluation of 3 specimens test. Concrete compressive strength

was tested using compressive strength ASTM C 39-03 testing code and tested at age 3, 7, 14, 21

and 28 days.

Fig. 4: Compressive strength test of Geopolymer Concrete

Curing

Immediately after casting, specimens were covered with plastic cover and left in room

temperature for 4 days to reduce evaporation. The cover then removed and the specimens were left

in room temperature.

NaOH 8M NaOH 10M

Trass 50 % from fly ash mass

Geopolymer concrete 100 x 200 mm2

75 % Aggregate 25 % (Fly ash + activator)

coarse : fine aggregate = 2 : 1 26 % activator 74 % fly ash

Water 12 %

from fly ash

mass

=

NaOH

SiONa 32 0.5

=

NaOH

SiONa 32 2.5

=

NaOH

SiONa 32 2.0

=

NaOH

SiONa 32 1.5

=

NaOH

SiONa 32 1.0

=

NaOH

SiONa 32 0.5

=

NaOH

SiONa 32 2.5

=

NaOH

SiONa 32 2.0

=

NaOH

SiONa 32 1.5

=

NaOH

SiONa 32 1.0

Materials Science Forum Vol. 803 129

Fig. 5: Curing at room temperature of geopolymer concrete

Result and Discusion

Effect of NaOH concentration on geopolymer binder compressive strength. In Figure 6 and 7

showed an increasing trend of geopolymer binder compressive strength to age. Figure 6 showed

binder geopolymer compressive strength of 8M NaOH system .Binders with 8M NaOH system

have almost the same compressive strength increasing trend. These specimens have a significant

increasing compressive strength during age 0 to 3 days. Figure 7 showed binder geopolymer

compressive strength of 10M NaOH system. The compressive strength trend is almost the same

between binder 8M and 10M NaOH system. However, at binder made with 10M NaOH system,

Speciment F and G already have a constant compressive strength in age of 28 days. The conclusion

from this compressive strength data is geopolymer binder did not have their 100 % strength at age

28 days. According to this data, increasing compressive strengh of geopolymer binder is still

possible after age of 28 days. This condition was a little bit different with OPC concrete, which has

constant compressive strength after age of 28 days. This result was apropriate with previous

research which was conducted by Ekaputri [5] that volcanic material was slow reacted in low

concentrate of alkali solution.

Fig. 6: The effect of age to compressive strength of geopolymer binder with NaOH concentration

of eight Molars

0.05.0

10.015.020.025.030.035.040.045.0

0 7 14 21 28 35

Co

mp

ress

ive

stre

ng

ht

(MP

a )

age (days)

A

B

C

130 Geopolymer and Green Technology Materials

Fig. 7: The effect of age to compressive strength of geopolymer binder with NaOH concentration of ten

Molars

Fig. 8: The effect of ratio Na2SiO3/NaOH on compressive strength of geopolymer binder

Effect of ratio Na2SiO3/NaOH on geopolymer binder compressive strength. This research used

5 vary ratio of Na2SiO3 to NaOH by mass 0.5,1,1.5,2 and 2.5. In Figure 8, shows that activator ratio

and concentration in a solution of NaOH play an important role in compressive strength of

geopolymer binder. Test result showed that binder made with an activator ratio of 2.5 , has the

highest compressive of strength both in 8M and 10M NaOH concentration. Experimental results

shown that, the compressive strength increase almost linearity with the concentration of Na2SiO3

and NaOH. This result has a good agreement with the research which was conducted by

Madheswaran [10] that the compressive strength of the geopolymer concrete was increased with the

increasing concentration of NaOH.

Effect of NaOH concentration on geopolymer concrete compressive strength Figure 9 and 10

showed an increasing trend of geopolymer concrete compressive strength to age. In general,

increasing trend of geopolymer concrete with the concentration of NaOH eight and ten Molars was

almost the same. Geopolymer concrete made with activator ratio of 2 and 2.5, have 50% of their

compressive strength in range age of 3-7 days. On the other hand, geopolymer concrete made with

activator ratio of 0.5, 1 and 1.5 have a slow increasing compressive strength. As expected, the

amount of Na2SiO3 in mixture proportion influenced geopolymer concrete compressive strength. As

catalyst, Na2SiO3 enabled to accelerate polymerization reaction at geopolymer concrete. Within the

plenty amount of Na2SiO3 in geopolymer concrete, the bonding formation of polymer will be fast.

0.05.0

10.015.020.025.030.035.040.045.050.0

0 7 14 21 28 35Co

mp

ressiv

e s

tren

gh

t(

MP

a )

Age (days)

F

GHIJ

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

0 0.5 1 1.5 2 2.5 3

Co

mp

ress

ive

stre

ng

ht

( M

Pa

)

Na2SiO3/NaOH

Binder 8 Molar

Binder 10 Molar

Materials Science Forum Vol. 803 131

As a result, an increase amount of Na2SiO3 causing geopolymer concrete has a high compressive

strength at early age.

Fig. 9: The effect of age to compressive strength of geopolymer concrete with NaOH concentration

of eight Molars

Fig 10: The effect of age to compressive strength of geopolymer concrete with NaOH concentration

of ten Molars

Fig. 11: The effect of activator mass ratio and concentration of NaOH to compressive strength of

geopolymer concrete

0

5

10

15

20

25

30

35

40

0 7 14 21 28 35 42 49 56 63Com

pre

ssiv

e st

ren

gh

t (

MP

a )

age (days)

A

B

C

D

E

05

1015202530354045

0 7 14 21 28 35 42 49 56 63Com

pre

ssiv

e st

ren

gh

t (

MP

a

)

age( days )

FGHIJ

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3

Co

mp

ressiv

e s

tren

gh

t(

MP

a )

Na2SiO3/ NaOH

8M concrete

10M concrete

132 Geopolymer and Green Technology Materials

Effect of ratio Na2SiO3/NaOH on geopolymer concrete compressive strength. Figure 11 show

the correlation between activator ratio with compressive strength of geopolymer concrete. At

Geopolymer concrete with 8M NaOH system, highest compressive strength owned by geopolymer

concrete made with activator ratio Na2SiO3/NaOH of 2. Whereas, at geopolymer concrete with 10M

NaOH concentration, the highest compressive strength is owned by geopolymer concrete made with

activator ratio Na2SiO3/NaOH of 2.5. The decreased compressive strength of geopolymer concrete

with 8M NaOH system might due to the low workability of geopolymer concrete. An experimental

result obtained that geopolymer concrete with a high activator ratio has a low slump test value that

cause difficulties when mixing and casting. This result confirm the previous finding by Ekaputri [5]

that solution which is too concentrated due to the concentration of NaOH or content of Na2SiO3

could decrease the compressive strength due to difficulties when mixing and casting.

Effect of trass addition on compressive strength. Figure 12 show the effect of trass addition on

compressive strength of geopolymer concrete. The compressive strength data of fly ash based

geopolymer concrete was obtained from previous research which was conducted by Ekaputri [14].

As seen at figure 12, the addition of trass into the mixture proportion is proven to reduce the

strength of geopolymer concrete. The highest compressive strength of geopolymer concrete made

with 100% fly ash was 48.59 MPa. With the addition of trass, geopolymer concrete strength was

decrease into 30.97 MPa. According to these data, it can be conclude that the addition of trass into

mixture proportion could reduce almost 36% of geopolymer concrete strength. This condition might

due to the silica and alumina composition of trass. As listed at table 1, trass is richer in silica but

less in alumina, compared to fly ash. Since there is not enough alumina substance, some silica in fly

ash and trass will remain unreacted with NaOH. As the result, it would cause the decrease strength

of geopolymer concrete. These results appropriate with previous research conducted by Faten [15]

that the compressive strength and the workability of the formed samples are influenced by the

proportions and properties of the components of the reaction mixture.

Fig. 12: Effect of trass addition on compressive strength of geopolymer concrete

Conclusion

1. Concentration of alkali activator NaOH influenced mechanical properties of geopolymer binder

and concrete. The higher of NaOH concentration, the higher mechanical strength of geopolymer

concrete and binder produced. Optimum Concentration in solutions of Sodium hydroxide as an

alkali activator will be further investigated.

2. Activator mass ratio Na2SiO3/NaOH more influenced compared to concentration of NaOH in

mechanical properties of geopolymer concrete. The increment of an activator ratio

Na2SiO3/NaOH in certain limit can increase compressive strength of geopolimer concrete. The

100 % fly ash

8M

100 % fly ash

10M,

fly ash +

trass 8M

fly ash +

trass 10M

0

10

20

30

40

50

60

Co

mp

ress

ive

Str

en

gth

(M

Pa

)

Materials Science Forum Vol. 803 133

optimum activator mass ratio of 2 in NaOH concentration of eight Molars and 2.5 in NaOH

concentration of ten Molars contribute maximum compressive strength of geopolymer concrete.

3. The low workability of fresh geopolymer concrete was predicted caused decreasing

compressive strength of geopolymer concrete with NaOH concentration eight Molars.

4. Geopolymer concrete which has a concentration of NaOH ten Molars with activator mass ratio

of Na2SiO3/NaOH 2.5 can be used for structural concrete because it has quite high compressive

strength, about 30,97 MPa.

5. The addition of trass into mixture proportion could reduce almost 36% of geopolymer concrete

strength. This might due to proportion of alumina and silica on trass and fly ash.

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