Waste Management 29 (2009) 539–543

Contents lists available at ScienceDirect

Waste Management

journal homepage: www.elsevier.com/ locate /wasman

Comparative study on the characteristics of fly ash and bottom ash geopolymers

Prinya Chindaprasirt a, Chai Jaturapitakkul b, Wichian Chalee c, Ubolluk Rattanasak d,*

a Depart ment of Civil Engi neer ing, Fac ulty of Engi neer ing, Khon Kaen Uni ver sity, Khon Kaen 40002, Thai landb Depart ment of Civil Engi neer ing, Fac ulty of Engi neer ing, King Mong kut’s Uni ver sity of Tech nol ogy Thon buri, Bang kok 10140, Thai landc Depart ment of Civil Engi neer ing, Fac ulty of Engi neer ing, Bu ra pha Uni ver sity, Chon buri 20131, Thai landd Depart ment of Chem is try, Fac ulty of Sci ence, Bu ra pha Uni ver sity, Chon buri 20131, Thai land

a r t i c l e i n f o

Article history:

Accepted 11 June 2008

Available online 19 August 2008

0956-053X/$ - see front matter © 2008 Else vier L

doi:10.1016/j.wasman.2008.06.023

* Cor re spond ing author. Tel.: +66 38103066; fa

E-mail address: ub olluk@buu.ac.th (U. Rattana

a b s t r a c t

This research was con ducted to com pare geo poly mers made from fly ash and ground bot tom ash. Sodium

hydrox ide (NaOH) and sodium sil i cate (Na2SiO3) solu tions were used as acti va tors. A mass ratio of 1.5

Na2SiO3/NaOH and three con cen tra tions of NaOH (5, 10, and 15 M) were used; the geo poly mers were

cured at 65 °C for 48 h. A Fou rier trans form infra red spec trom e ter (FT-IR), dif fer en tial scan ning cal o rim-

e ter (DSC), and scan ning elec tron micro scope (SEM) were used on the geo poly mer pastes. Geo poly mer

mor tars were also prepared in order to inves ti gate com pres sive strength. The results show that both fly

ash and bot tom ash can be uti lized as source mate ri als for the pro duc tion of geo poly mers. The prop er ties

of the geo poly mers are depen dent on source mate ri als and the NaOH con cen tra tion. Fly ash is more reac-

tive and pro duces a higher degree of geo po ly mer iza tion in com par i son with bot tom ash. The mod er ate

NaOH con cen tra tion of 10 M is found to be suit able and gives fly ash and bot tom ash geo poly mer mor tars

with com pres sive strengths of 35 and 18 MPa.

© 2008 Else vier Ltd. All rights reserved.

1. Intro duc tion

Every year, there is an increase in con sump tion of fos sil fuels

to pro duce energy. Coal is mainly used to gen er ate the steam for

indus trial oper a tion and elec tric ity gen er a tion. Con se quently, coal

ash is obtained as a waste prod uct that needs to be dis posed of

in an envi ron men tally friendly way. How ever, because of the large

quan tity pro duced, most types of ash, includ ing fly and bot tom

ash, are dis posed of in land fills. This dis posal pro cess leads to envi-

ron men tal and other prob lems.

In Thai land, the annual output of fly ash and bot tom ash from

large and small power plants totals about 4.0 mil lion tons. Approx i-

mately 1.8 mil lion tons of fly ash is used as a poz zo la nic mate rial in

the con crete indus try. The partial replace ment of Port land cement

with fly ash reduces the heat of hydra tion and improves the work-

abil ity and dura bil ity of the con crete. How ever, the level of fly ash

replace ment is nor mally restricted to less than 40% of Port land

cement (Me hta, 1998).

Bot tom ash, in con trast, does not pos sess the same enhanced

work abil ity. How ever, the chem i cal con stit u ents of these two mate-

ri als are very sim i lar, with the main dif fer ence being par ti cle shape

and size. Bot tom ash is larger in size and very irreg u lar, con tain-

ing pores and cav i ties. Ground to a proper fine ness, bot tom ash

td. All rights reserved.

x: +66 38393494.

sak).

can be used as a poz zo lan that pro duces rel a tively high strength

con crete (Ja turapitakkul and Cheera rot, 2003). Since fly ash is not

yet com pletely used, a very large amount of bot tom ash is still dis-

carded.

Man u fac tur ing of Port land cement is an energy inten sive pro-

cess that releases a very large amount of green house gas (Ma hol-

tra, 2002). There fore, the use of po zzo lans to replace part of Port-

land cement is receiv ing a lot of atten tion. Other efforts have also

been made to develop alter na tive cemen ti tious mate ri als. One

prom is ing find ing is the use of an alu mi no-sil i cate mate rial called

“geo poly mer”. Since fly ash con tains a large amount of sil ica and

alu mina, it is a suit able source mate rial for mak ing geo poly mers

(Lee and Van De vent er, 2002). The fly ash geo poly mer is prepared

by incor po rat ing high alka line solu tion and sodium sil i cate and is

acti vated with tem per a ture cur ing. The poly con den sa tion reac tion

pro vides an alu mi no-sil i cate cemen ti tious com pound. Apart from

fly ash, other mate ri als, such as cal cined ka ol i nite and burnt clay,

can also be used as source mate ri als. Bot tom ash, which has sim i-

lar chem i cal ingre di ents as fly ash, should also be used to pro duce

geo poly mers. The bot tom ash, how ever, may need some grind ing

to improve its reac tiv ity.

The uti li za tion of coal ash in the pro duc tion of geo poly mers

offers an alter na tive cemen ti tious mate rial. There fore, this research

focuses on the phys i cal and chem i cal struc tures of coal ash geo-

poly mer. Original fly ash and ground bot tom ash were used to pre-

pare geo poly mers by mix ing with sodium hydrox ide and sodium

540 P. Chinda pra sirt et al. / Waste Management 29 (2009) 539–543

sil i cate solu tions. Mix pro por tions var ied in order to ana lyze their

effects on strength and other mate rial prop er ties.

2. Mate ri als and meth ods

2.1. Mate ri als

Fly ash and bot tom ash from Mae Moh power plant in the north

of Thai land were used in this research. Bot tom ash was ground

to a sim i lar par ti cle size as fly ash. Table 1 shows the sig nifi cant

chem i cal com po si tion of coal ash using X-ray fluo res cence (XRF).

XRD pat terns of fly ash and bot tom ash dis played in Fig. 1 show that

fly ash con tained a higher con tent of amor phous phase par ti cles

com pared to the bot tom ash. The crys tal line phases are pre dom i-

nantly quartz and mull ite. Sodium hydrox ide solu tion (NaOH) at 5,

10, and 15 M con cen tra tion and sodium sil i cate solu tion (Na2SiO3)

with Na2O 9% and SiO2 30% by weight were used. The vis cos i ties

of 5, 10, and 15 M NaOH solu tions were 3.9, 9.3, and 14.3 cps (cen-

ti pois es), respec tively. The sodium sil i cate solu tion’s vis cos ity was

higher at 60.6 cps.

2.2. Mix ing pro ce dure for the geo poly mer paste

Coal ash was mixed with NaOH solu tion for 10 min to allow the

leach ing of ions. Sodium sil i cate solu tion was then added to the

mix ture and mixed until uniform, usu ally about 60 s. The mass

ratio of Na2SiO3/NaOH of 1.5 was used. The mix pro por tions are

tab u lated in Table 2. Com po si tions of geo poly mers in mole ratios

are shown in Table 3. After mix ing, paste spec i mens were molded

into 25 mm diam e ter £ 25 mm height plas tic con tain ers. They

were then cured at 65 °C for 48 h, based on pre vi ous work done

on fly ash (Chinda pra sirt et al., 2007). FT-IR, Dif fer en tial Scan ning

Cal o rim e try (DSC), XRD and EDX anal y ses were per formed on the

hard ened sam ple.

2.3. Mix ing pro ce dure and tests of the geo poly mer mortar

When mak ing the mortar, sand was added to the paste mix-

ture at a sand-to-coal ash ratio of 2:1 (by weight) and mixed for

another two 2 min. The mix ture was then cast into 50 mm3 molds

in accor dance with ASTM C109 and cov ered with cling film to

avoid mois ture evap o ra tion dur ing heat cur ing. The mix ture was

sub se quently cured in an oven at 65 °C for 48 h to com plete the

geo po ly mer iza tion. After that, the spec i mens were cooled to room

tem per a ture and tested for strength. The results are reported as an

aver age of three sam ples.

3. Results and dis cus sion

3.1. IR spec tra

FT-IR was used to study the geo po ly mer iza tion of the paste.

The dis tinct band near 460 cm¡1 can be ascribed to the O–Si–O

bend ing mode (Barb osa et al., 2000; Gün zler and Grem lich, 2002).

The Si–O–Si stretch ing vibra tion was detected at the wave num-

ber range of 1200–950 cm¡1. The Si–O–Si stretch ing vibra tion was

more prom i nent than the O–Si–O bend ing mode. It is, there fore,

log i cal to use the Si–O–Si vibra tion to indi cate the degree of geo-

po ly mer iza tion.

The results of the IR spec tra are shown in Fig. 2a and b. The sig-

nifi cant broad bands are located at approx i mately 3450 cm¡1 and

1650–1600 cm¡1 for O–H stretch ing and O–H bend ing, respec tively.

Si–O–Si position is shifted to the right position or lower fre quency

com pared with the original ash, imply ing a chem i cal change in the

matrix. The band at 1460 cm¡1 rep re sents the sodium car bon ate

result ing from the car bon ation (Barb osa et al., 2000).

The peak areas and peak heights are fre quently used in quan-

ti ta tive assess ment of the reac tion. The ratios of peak area of

Si–O–Si stretch ing vibra tion (AS) are tab u lated in Table 3. For the

fly ash sys tem, the AS is rel a tively low. The AS of the geo poly mer

paste increases with an increase in the con cen tra tion of sodium

hydrox ide (up to 10 M). At a higher con cen tra tion of 15 M NaOH,

the AS of paste is still high (Table 4). The AS ratios of the 5, 10,

and 15 M NaOH paste to that of the fly ash are 1.43, 4.53, and

3.61, respec tively. This sug gests that a rel a tively high degree of

geo po ly mer iza tion is obtained with the use of 10 M NaOH. At a low

con cen tra tion, the geo po ly mer iza tion is low, due to the low con-

Table 1

Chem i cal com po si tion and phys i cal prop er ties of coal ash

Com po si tion (%) Fly ash (FA) Ground bot tom ash (BT)

SiO2 38.7 38.8

Al2O3 20.8 21.3

FeO3 15.3 12.1

CaO 16.6 16.5

Na2O 1.3 1.0

TiO2 0.5 0.8

MgO 1.3 1.7

K2O 2.1 2.5

SO3 2.6 2.4

LOI 0.8 2.9

Retained on sieve no.

325 (% by weight)

32 29

0 10 20 30 40 50

2Theta(deg)

Inte

nsit

y bottom ash

fly ash

Q

Q

QQM M

M

Q

QM

M M QM

Fig. 1. XRD pat terns of fly ash and bot tom ash. Q = quartz, M = mull ite.

Table 3

Com po si tion of geo poly mers prepared from fly ash and bot tom ash

Sam ple NaOH (M) Mole ratio

Na2O/SiO2 SiO2/Al2O3 H2O/Na2O Na2O/Al2O3

FA5 5 0.15 4.14 19.95 0.64

FA10 10 0.21 4.14 13.33 0.89

FA15 15 0.26 4.14 10.40 1.08

BT5 5 0.15 4.05 19.95 0.62

BT10 10 0.21 4.05 13.33 0.87

BT15 15 0.26 4.05 10.40 1.05

Table 2

Mix pro por tion of coal ash geo poly mer paste

Mate ri als Mix pro por tion (% by weight)

Fly ash or bot tom ash 60

NaOH (5, 10, and 15 M) 16

Na2SiO3 24

P. Chinda pra sirt et al. / Waste Management 29 (2009) 539–543 541

cen tra tion of base and, hence, less leach ing of sil ica and alu mina

from the source mate rial. At the high con cen tra tion of 15 M NaOH,

although the con cen tra tion of base is high, the matrix becomes

very stiff, as the vis cos ity of the 15 M NaOH is 14.3 cps in com par i-

son to the 9.3 cps of the 10 M NaOH. The high vis cos ity hin ders the

leach ing of the sil ica and alu mina, result ing in a lesser degree of

geo po ly mer iza tion as com pared to that of the 10 M NaOH paste.

The peak height also gives sim i lar indi ca tions of the degree of geo-

po ly mer iza tion. For the bot tom ash sys tem, the results are sim i lar

to those of the fly ash sys tem. The 10 M NaOH paste also gives a

high degree of poly mer i za tion. The bot tom ash pastes behave

in a sim i lar man ner to the fly ash pastes as they con tain sim i lar

amounts of sil ica and alu mina and come from the same source.

3.2. Dif fer en tial Scan ning Cal o rim e try (DSC)

Dif fer en tial scan ning cal o rim e try was used to mea sure a num-

ber of char ac ter is tic prop er ties of the geo poly mer pastes. Using

this tech nique, it is pos si ble to observe exo ther mic and endo ther-

mic events, as well as glass tran si tion tem per a tures (Tg). The range

of inves ti ga tion is between ¡30 and 100 °C. The results of the DSC

ther mo grams (exo ther mal up) of the fly ash, ground bot tom ash,

and geo poly mer mixed with 10 M NaOH are shown in Fig. 3. The

DSC ther mo grams of fly ash and bot tom ash are rel a tively straight,

indi cat ing no sign of reac tion. The ther mo grams of coal ash geo-

poly mers show sev eral peaks, indi cat ing some degree of geo po ly-

mer iza tion. A sig nifi cant peak rep re sent ing the melt ing point of

water is observed in all geo poly mer sam ples at 0 °C (Phair et al.,

2003). The exo ther mal peaks found in the fly ash geo poly mer at

the tem per a ture of approx i mately 20 °C reflect the glass tran si tion

tem per a ture (Tg). The bot tom ash geo poly mer reveals sev eral small

peaks in the tem per a ture range of 10–40 °C. The peaks at this range

of tem per a ture can be attrib uted to the crys tal li za tion tem per a ture

(Tc), at which an amor phous solid could become less vis cous and

obtain enough free dom of motion to spon ta ne ously arrange into

a crys tal line form. This tran si tion from amor phous solid to crys tal-

line solid is an exo ther mic pro cess, and it results in a peak in the

DSC sig nal (Dean, 1995).

Within the tem per a ture range of 30–100 °C, the slope of the

DSC ther mo gram for the fly ash geo poly mer is rel a tively flat as

com pared to that of the bot tom ash geo poly mer. This indi cates a

dif fer ence in the degree of geo po ly mer iza tion of the two geo poly-

mers. For the bot tom ash geo poly mer, although the DSC ther mo-

gram at this range con tains sev eral dis tinct peaks, the slope of the

ther mo gram is about the same as that of the base mate rial (i.e.,

bot tom ash). This sug gests a low degree of geo po ly mer iza tion of

the bot tom ash geo poly mer. The slope of the fly ash geo poly mer,

how ever, is sig nifi cantly dif fer ent from that of fly ash, indi cat ing a

higher degree of geo po ly mer iza tion.

3.3. Micro struc ture

The typ i cal SEM-EDX of hard ened fly ash geo poly mer is shown

in Fig. 4. The paste shows unre acted and/or par tially reacted grains

40080012001600200024002800320036004000

Wavenumber (cm-1)

FA

5M-FA

10M-FA

15M-FA

O-H

O-H

Si-O

40080012001600200024002800320036004000

Wavenumber (cm-1)

BT

5M-BT

10M-BT

15M-BT

O-H

O-H

Si-O

(a) fly ash geopolymer (b) bottom ash geopolymer

Fig. 2. FT-IR spec tra of fly ash and bot tom ash geo poly mers.

Table 4

Inverted peak area and AS ratio from IR spec tra of pastes at Si–O–Si stretch ing vibra-

tion

Sam ple NaOH (M) Loca tion of Si–O–Si

(cm¡1)

AS ratio Peak height

ratio

Fly ash par ti cles – 1016 1 1

1 5 1016 1.43 1.74

2 10 1001 4.53 4.14

3 15 1008 3.61 3.93

Bot tom ash par ti cles – 1023 1 1

4 5 1023 3.23 2.75

5 10 1016 4.56 3.82

6 15 1020 3.11 2.38

-40 -20 20 40 60 80 100Temperature (ºC)

Hea

t F

low

(m

W)

10M-BT

BT

10M-FA

FA

0

Fig. 3. DSC ther mo gram of geo poly mer pastes using 10 M NaOH.

542 P. Chinda pra sirt et al. / Waste Management 29 (2009) 539–543

of fly ash and a con tin u ous mass of alu mi no-sil i cate. A large pro por-

tion of fly ash still does not com pletely react, espe cially the large

par ti cles. Although the matrix is con tin u ous and rel a tively dense,

voids and cracks are eas ily observed. This would limit the bind-

ing capac ity and strength of the geo poly mer. For the bot tom ash

geo poly mer, as shown in Fig. 5, the paste also shows a con tin u ous

mass of alu mi no-sil i cate with unre acted and/or par tially reacted

grains of irreg u lar coal ash par ti cles. The irreg u lar par ti cles are

porous and would thus exert a neg a tive influ ence on the strength

of the geo poly mer.

The results of the EDX anal y ses of the fly ash and bot tom ash geo-

poly mers are also shown in Figs. 4 and 5. The major ele ments are Si

and Al, with some Na and Ca also pres ent. The pres ence of Ca is from

the source mate ri als as the fly ash and bot tom ash both con tain large

amounts of CaO. The ratios of Si/Al for the fly ash and bot tom ash geo-

poly mers are sig nifi cantly dif fer ent. The ratio of Si/Al for the fly ash

geo poly mer is 3.0, and the same ratio for the bot tom ash geo poly mer

is much higher at 6.0. This indi cates that the leach ing of alu mina in

the fly ash geo poly mer matrix is bet ter than that in the bot tom ash

matrix. The higher ratio of Si/Al results in geo poly mers with lower

strength and higher elas tic ity (Fletcher et al., 2005).

3.4. Com pres sive strength

Fig. 6 shows the com pres sive strength of geo poly mer mor tars

when heat cured at 65 °C for 48 h. The fly ash geo poly mer mortar

gives a higher com pres sive strength in com par i son to the bot tom

ash geo poly mer. The results also indi cate that the use of a low NaOH

con cen tra tion of 5 M gives geo poly mer mor tars with rel a tively low

strength. For the fly ash geo poly mer, the use of 5 M NaOH gives a

mortar with a mod er ate strength of 24 MPa. Higher strengths of 35

and 33 MPa are obtained with the use of 10 M and 15 M NaOH. The

com pres sive strength results are con sis tent with the results of the

FT-IR, ther mo graph, and SEM.

For the bot tom ash, the com pres sive strengths of mor tars are

lower than those of the fly ash geo poly mer. The strengths of 5,

10, and 15 M geo poly mer mor tars are 10, 14, and 18 MPa, respec-

tively. The degree of poly mer i za tion of the bot tom ash geo poly-

mer is lower than that of the fly ash poly mer, as sug gested by the

ther mo graph. The dis so lu tion of bot tom ash in the NaOH solu tion

would be lower than that of the fly ash. Bot tom ash thus requires

a higher con cen tra tion of NaOH for the dis solv ing of alu mina and

sil ica and for geo po ly mer iza tion. The fact that a large num ber of

Fig. 4. SEM-EDX anal y sis of fly ash geo poly mer.

Fig. 5. SEM-EDX anal y sis of bot tom ash geo poly mer.

P. Chinda pra sirt et al. / Waste Management 29 (2009) 539–543 543

the bot tom ash par ti cles are porous also con trib utes to the lower

mortar strength.

The anal y sis of geo poly mer paste char ac ter is tics and micro-

struc ture, using FT-IR, DSC ther mo graphs, and SEM, rein forces the

impor tance of a phys i cal prop erty, i.e., the com pres sive strength of

geo poly mer mor tars.

4. Con clu sions

Fly ash and ground bot tom ash are suit able source mate ri als

for pro duc ing geo poly mers. The results of the FT-IR, DSC ther mo-

gram, SEM, and XRD anal y ses indi cate that fly ash is more reac-

tive than bot tom ash and gives a higher degree of geo po ly mer iza-

tion. The com pres sive strength of the fly ash geo poly mer mortar

is rea son ably high at 35 MPa, and it is sig nifi cantly higher than

the 18 MPa of the bot tom ash geo poly mer mortar. The strength

of a geo poly mer is also depen dent on NaOH con cen tra tion. The

opti mum NaOH con cen tra tion of 10 M is suit able for both ash

mate ri als.

Acknowl edg ments

The authors grate fully acknowl edge the finan cial sup port from

the Thailand Research Fund (TRF) under TRF New Researcher

Scholar Con tact No. MRG4980145, TRF Senior Research Scholar Con-

tact no. RTA5080020, and the Com mis sion on Higher Edu ca tion,

Min is try of Edu ca tion, Thai land. Appre ci a tion is also extended to

PERCH-CIC Pro gram.

Ref er ences

Barb osa, V., Mac Ken zie, K., Thau mat urgo, C., 2000. Syn the sis and char ac ter-isa tion of mate ri als based on inor ganic poly mers of alu mina and sil ica: sodium po lysi a late poly mer. Inter na tional Jour nal of Inor ganic Mate ri als 2, 309–317.

Chinda pra sirt, P., Chare erat, T., Si ri vi van anon, V., 2007. Work abil ity and strength of coarse high cal cium fly ash geo poly mer. Cement and Con crete Com pos ites 29, 224–229.

Dean, J.A., 1995. The Ana lyt i cal Chem is try Hand book. McGraw Hill, Inc., New York, USA.

Fletcher, R.A., Mac ken zie, K.J.D., Nich ol son, C.L., Shi mad a, S., 2005. The com po si tion rang of alu mi no sil i cate geo poly mers. Jour nal of the Euro pean Ceramic Soci ety 25, 1471–1477.

Gün zler, H., Grem lich, H., 2002. IR Spec tros copy: An Intro duc tion. Wiley-VCH Ver-lag GmbH, Ger many.

Ja turapitakkul, C., Cheera rot, R., 2003. Devel op ment of bot tom ash as poz zo la nic mate rial. Jour nal of Mate ri als in Civil Engi neer ing 15, 48–53.

Lee, W.K.W., Van De vent er, J.S.J., 2002. Struc ture re or ga ni sa tion of class F fly ash in alka line sil i cate solu tions. Col loids and Sur faces A: Phys ico chem ist ry Engi neer-ing Aspects 211, 49–66.

Ma hol tra, V.M., 2002. Intro duc tion: Sus tain able devel op ment and con crete tech nol-ogy. ACI Con crete Inter na tional 24, 22–23.

Me hta, P.K., 1998. Role of poz zo la nic and cemen ti tious by-prod ucts in sus tain able devel op ment of the con crete indus try. In: Sixth CAN MET/ACI/JCI Con fer ence: Fly Ash, Sil ica Fume, Slag and Nat u ral Po zzo lans in Con crete, Bang kok, Thai-land.

Phair, J.W., Smith, J.D., Van De vent er, J.S.J., 2003. Char ac ter is tics of alu mi no sil i cate hydro gels related to com mer cial ‘Geo poly mer’. Mate ri als Let ters 57, 4356–4367.

0

5

10

15

20

25

30

35

40

5M NaOH

NaOH Concentration (M)

Com

pres

sive

str

engt

h (M

Pa)

FA geopolymer

BT geopolymer

15M NaOH10M NaOH

Fig. 6. Com pres sive strength of geo poly mer mor tars.