impact of sodium sulfate solution on mechanical properties and structure of fly ash based...

ORIGINAL ARTICLE

Impact of sodium sulfate solution on mechanical propertiesand structure of fly ash based geopolymers

Zvezdana Bascarevic • Miroslav Komljenovic •

Zoran Miladinovic • Violeta Nikolic •

Natasa Marjanovic • Rada Petrovic

Received: 23 August 2013 / Accepted: 29 April 2014

� RILEM 2014

Abstract In this paper, geopolymers based on two

different fly ash samples were exposed to sodium

sulfate (Na2SO4) solution (50 g/l) over a period of

365 days. It was found that sulfate solution attack

caused a small decrease in strength of geopolymer

mortars. Analysis of the Na2SO4 solutions by optical

emission spectroscopy indicated that exposing of the

geopolymer samples to the Na2SO4 solution had

caused leaching of one of the elements of the

aluminosilicate gel, silicon. Mineralogical analyses

of geopolymer samples did not show formation of any

new phases due to a reaction with sulfate ions.

Changes in aluminosilicate geopolymer gel due to

sulfate attack were investigated by electron micros-

copy and nuclear magnetic resonance. It was found

that treatment of geopolymer samples with the sulfate

solution caused breaking of –Si–O–Si– bonds in

aluminosilicate gel structure. Breaking of the –Si–

O–Si– bonds and leaching of Si were consequences of

the increase in the pH value of sulfate solution during

testing.

Keywords Geopolymer � Fly ash � Durability �Sulfate attack

1 Introduction

Alkali activated materials (AAM) are relatively new

group of binder materials. They are formed by the

reaction of different silicate materials with alkaline

solutions. Geopolymers are part of the group of AAM

generated by alkali activation of aluminosilicate

materials such as metakaolin or fly ash (FA). Geo-

polymers are characterized by highly cross-linked

three-dimensional aluminosilicate structure (geopoly-

mer gel), in which the negative charge of aluminum in

tetrahedral coordination is neutralized by alkali metal

ions [1, 2]. In order to widespread the uses of AAM as

alternative binder materials, these materials are

expected to meet all the requirements in terms of

quality and durability specified for Portland cement.

One of the durability requirements is resistance to

sulfate attack.

It is well known that sulfate ions from natural or

waste water may cause deterioration of building

elements based on hydrated Portland cement. During

the contact of hydrated Portland cement with sulfate

Z. Bascarevic (&) � M. Komljenovic �V. Nikolic � N. Marjanovic

Institute for Multidisciplinary Research, University of

Belgrade, Kneza Viseslava 1, 11030 Belgrade, Serbia

e-mail: [email protected]

Z. Miladinovic

Institute of General and Physical Chemistry, Studentski

trg 12-16, 11000 Belgrade, Serbia

R. Petrovic

Faculty of Technology and Metallurgy, University of

Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia

Materials and Structures

DOI 10.1617/s11527-014-0325-4

solution, diffusion of sulfate ions into the structure of

the hardened cement occurs, followed by a reaction

with some of the phases present in the structure. The

main products of sulfate attack on hydrated Portland

cement structure are gypsum, ettringite, thaumasite or

mixtures of these phases [3]. The formation of these

phases can cause stress, strength loss, expansion and

finally deterioration of the material. Sulfate attack on

hydrated Portland cement is usually a slow process.

Occurrence and the extent of deterioration of hardened

Portland cement structure in contact with sulfate

solution depend on many factors: type of Portland

cement, water/binder ratio, pH of the sulfate solution,

concentration of sulfate ions, cation accompanying the

sulfate ions, temperature etc. [4–6]. Although there are

numerous investigations on the effects of sulfate

solutions on hydrated Portland cement structure [4], at

the moment, there is no standard method in Europe for

testing the resistance of cementitious materials to

sulfate attack [7].

Currently, there is also an ongoing work on

providing recommendations regarding appropriate test

methodologies for testing durability of AAM [8]. One

of the areas which will be given special attention is

sulfate attack on AAM. Previous investigations on

sulfate attack on AAM were carried out mainly in

sodium sulfate solutions in concentrations of 44–50 g/l

[9–15]. Several studies used also magnesium sulfate

(MgSO4) solution [13–16]. Other experimental con-

ditions, in terms of type of AAM samples (paste,

mortar or concrete), sample size, sulfate solution/

AAM samples ratio, duration of testing, testing

conditions (agitation and/or renewal of the sulfate

solution, the pH value of the solution) varied from one

study to another.

Most of the previous studies concluded that AAM

are quite resistant to sulfate attack, especially to the

attack of Na2SO4 solution [9, 11–13, 15]. In particular,

the investigations of the action of Na2SO4 solution on

aluminosilicate AAM, geopolymers, have shown that

this solution has little impact on the mechanical

properties of the material [2, 9, 11, 13]. However, in

several cases, it was observed that exposure to Na2SO4

solution had caused decrease or fluctuations in the

strength of the geopolymer samples [9, 14]. These

changes in the mechanical properties of the geopoly-

mer samples have been attributed to diffusion of alkali

metal ions from the geopolymer structure into the

sulfate solution [9, 14].

It is often stated that geopolymers are generally more

resistant to chemical corrosion than Portland cement [2].

Mineral (phase) composition and microstructure of

geopolymers differ significantly from the structure of

hydrated Portland cement, as well as degradation

processes of the materials [2, 11]. The main product of

geopolymerization reaction is alkali aluminosilicate gel,

which is responsible for good mechanical properties and

high chemical durability of geopolymers. However,

geopolymer gel is amorphous and X-ray structural

analysis (XRD) provides only limited information about

geopolymers‘ structure. Valuable pieces of information

about the structure and composition of the aluminosil-

icate geopolymer gel were obtained by infrared spec-

trometry (IR) and electron microscopy [17–19]. An

instrumental technique that has, over the last decade,

contributed substantially to the understanding of the

structure of geopolymer gel is nuclear magnetic reso-

nance (NMR) [20–22].

In previous research works, effects of Na2SO4

solution on geopolymers‘ structure were studied by

electron microscopy and infrared spectroscopy [9, 11,

14]. Electron microscopy of alkali-activated FA, after

being exposed to Na2SO4 solution for 365 days,

showed crystals of Na2SO4 salts in the pores of the

material [11]. This was attributed to the migration of

ions from the solution [11]. An earlier study showed

that, after the exposure of alkali-activated metakaolin

to the effects of various aggressive solutions (deion-

ized water, sea water, sulfuric acid, Na2SO4 solution),

no significant changes in the structure could be

detected by the IR analysis of the material [9].

However, based on infrared analysis of FA based

geopolymers, Bakharev concluded that sulfate attack

had caused an increase of a chain length of alumino-

silicate gel [14]. The same study showed that, when

sodium silicate was used as an alkaline activator,

exposure of the geopolymer samples to Na2SO4

solution resulted in increase of the Si/Al atomic ratio

in the aluminosilicate gel [14]. Nevertheless, although

the sulfate solution was analyzed by optical emission

spectroscopy, which confirmed the diffusion of alkali

ions from the geopolymer structure into the sulfate

solution, aluminum ions were not detected (or were

not analyzed), which would corroborate the results of

the infrared analysis [14].

On the other hand, it is well known that the extent of

the structural changes caused by sulfate attack on

building materials depends significantly on the quality


of the material [3–5, 7]. Chemical durability of

building materials is strongly affected by their

permeability, i.e. porosity [3]. It is recommended that

testing of the resistance of hydrated Portland cement to

sulfate attack should be performed on mortars

prepared with high water/cement ratio [7]. One recent

study on the effects of sulfate solutions on AAM

structure also showed that decreasing the water/binder

ratio increases the resistance to sulfate attack [15].

In our study, effects of the sulfate solution on

mechanical properties and structure of geopolymers were

studied on the geopolymer samples based on two

different alkali activated FA samples. The selection of

the FA samples was based on our previous research work,

which showed that, due to different reactivity of various

FA samples, geopolymer mortars with different mechan-

ical properties were obtained by alkali activation of the

FA samples [23, 24]. Different FA samples required

different amounts of water in order to obtain equal

workability of mortars [23, 24]. In this work, selection of

the two different FA samples for the synthesis of the

geopolymer samples enabled us to examine the impact of

the sulfate solution on geopolymers of different quality,

i.e. different initial strength and porosity.

The geopolymer samples were exposed to the effects

of the Na2SO4 solution (50 g/l) over a period of 365 days.

Any changes in the geopolymer gel due to sulfate attack

were studied using 29Si MAS NMR. Changes in the

sulfate solution were monitored by measuring the pH

values and testing the composition of the solution using

optical emission spectrometry. Formation of any new

phases in the structure, due to a reaction with sulfate ions,

was investigated by XRD of the material. Furthermore,

changes in the geopolymers‘ structure were examined by

scanning electron microscopy (SEM).

The aim of this work was to investigate the effects

of Na2SO4 solution on mechanical properties and

structure of FA based geopolymers and to determine

whether the exposure of the geopolymer samples to

the sulfate solution results in changes in the alumino-

silicate geopolymer gel.

2 Materials and methods

2.1 Materials

In this study, two FA samples from Serbian thermal

power plants (TPP) were used: FA Svilajnac TPP

‘‘Morava’’, Svilajnac and FA Kolubara, TPP ‘‘Kolu-

bara’’, Veliki Crljani. In order to increase the amount

of the more reactive FA particles, fractions of the FA

samples smaller than 63 lm were used [23].

Alkaline activator was sodium silicate with silicate

modulus (SiO2/Na2O mass ratio) of 3.04 (‘‘Galenika-

Magmasil’’, Serbia, 8.72 % Na2O, 26.5 % SiO2,

64.78 % H2O). Silicate modulus of the activating

solution was adjusted by adding 10 M solution of

NaOH (‘‘ZorkaPharm’’, Serbia, p.a. 98 %).

Sulfate solution was prepared using Na2SO4 of

99 % purity (Superlab, Serbia).

2.2 Preparation of geopolymer samples

Sodium silicate solution with modulus of 1.5 was used

as an activating solution. The activator solution/FA

ratio was the same for all the geopolymer samples with

10 % Na2O content with respect to the FA mass [23].

Geopolymer mortars were prepared by adding the

silicate solution to water and then mixing the solution

with FA and sand. Different amounts of water were

required in order to obtain equal workability of the

mortars based on the two FA samples (mortar flow

measured on a flow table was 125 ± 5 mm), so the

water/FA ratio was 0.56 for the mortars based on FA

Svilajnac, and 0.76 for the mortars based on FA

Kolubara (water in water/FA ratio was calculated as

water from sodium silicate solution ? water added for

consistency). Sand/FA ratio was 3/1. The molds with

three mortar prisms (40 9 40 9 160 mm) were

placed in plastic bags to avoid moisture loss during

curing. Geopolymer mortars were cured at 95 �C for

24 h. The curing temperature was selected after an

extensive optimization of the geopolymer synthesis

conditions for the FA samples used in this study.

Previously, curing of the geopolymer samples based

on FA Kolubara and FA Svilajnac was performed at

room temperature for 1 day followed by curing at

55 �C for 6 days [23] and at 55, 80 and 95 �C for 24 h

(unpublished data). It was found that the highest

compressive strengths of the geopolymer mortars were

achieved after curing of the samples at 95 �C for 24 h.

Geopolymer pastes were prepared by mixing the FA

with the alkaline activator solution in the same

proportion as in the preparation of the geopolymer

mortars. Water/FA ratio was 0.49 for the pastes

prepared with FA Svilajnac and 0.69 for the pastes

based on FA Kolubara. Dimensions of the paste


samples were 25 9 25 9 30 mm. Curing conditions

were the same as for the geopolymer mortars.

Nine series of geopolymer mortar and paste sam-

ples were prepared based on both of the FA samples

(Table 1). The first samples were tested after curing at

95 �C for 24 h (starting samples). Eight series of

mortar and paste samples were prepared for testing

two groups of samples after four terms: 28, 90, 180 and

365 days. The first group was cured in the humid

chamber (temperature 20 ± 2 �C, humidity *98 %)

and will be hereinafter referred to as reference

samples. The second group of samples was exposed

to the Na2SO4 solution.

After the period of curing, (in the humid chamber or

in the Na2SO4 solution), in order to stop further

reaction, the paste samples were crushed and milled

(Netzsch Pulverisette) in isopropyl alcohol for 60 min.

After milling, the samples were washed with acetone,

dried at 50 �C for 60 min and packed in plastic bags.

Paste samples were prepared for scanning electron

microscopy (SEM) characterization by soaking the

fragments of the samples in isopropyl alcohol for 24 h

and drying at 50 �C for 60 min afterwards.

2.3 Experimental design

The Na2SO4 solution (50 g/l) was prepared by

dissolving of Na2SO4 in distilled water. Series of the

three geopolymer mortar prisms and the correspond-

ing paste sample were immersed in sealed plastic

containers with sulfate solution and kept at 20 ± 2 �C

for a period of up to 365 days. In order to ensure that

the entire sample surface was in contact with the

solution, geopolymer samples were placed on plastic

nettings (thickness 0.5 cm, hole size *3 cm). Solu-

tion/geopolymer mass ratio was 4/1. The sulfate

solutions were not renewed during the experiment,

thereby static testing conditions were established.

Instead, testing was performed for a longer period of

time (365 days), with high sulfate solution/geopoly-

mer samples ratio (4/1) and with high concentration of

the sulfate ions (50 g/l Na2SO4) in the solution.

2.4 Analytical methods

Chemical composition of the starting FA samples was

determined by energy dispersive X-ray fluorescence

spectrometer ED2000, Oxford instruments. The particle

size distribution (content of the particles smaller than

43 lm) was determined by sieving the FA samples

through the mesh. Compressive strength of the geo-

polymer mortars was tested according SRPS EN 196-1

[25] using Tony Technik (Toninorm) testing machine.

Nitrogen adsorption–desorption isotherms were

determined using a Micromeritics ASAP 2020 instru-

ment. Testing was performed on the powdered mortar

samples. The samples were crushed to pass through

100 lm sieve. Samples were degassed at 105 �C for

10 h under reduced pressure. The total pore volume

was given at p/p0 = 0.998. Specific surface areas of

the geopolymer samples were obtained by applying

the Brunauer–Emmett–Teller method.

The pH values of the Na2SO4 solutions before and after

the testing of the geopolymer samples were measured by

pH-meter (pH Testr30, Eutech Instruments).

The inductively coupled plasma optical emission

spectrometer (ICP-OES, SpectroGenesis EOP II,

Spectro Analytical Instruments GmbH, Kleve) was

used to determine the ion exchange between the

geopolymers and the Na2SO4 solutions. Prior to

analysis, Na2SO4 solutions were diluted with deion-

ized water in ratio 1:100. The concentration of each

element was calculated as the difference between the

concentration measured after curing the geopolymer

Table 1 Sample labels

Time (days) FA Svilajnac FA Kolubara

Humid chamber Sulfate solution Humid chamber Sulfate solution

0* G-Svil-0 G-Kol-0

28 G-Svil-Ref-28 G-Svil-S-28 G-Kol-Ref-28 G-Kol-S-28




* Samples cured for 24 h at 95 �C


samples in the Na2SO4 solution and the initial element

concentration in the same solution. Testing of the

sulfate solutions in which mortars based on FA

Kolubara were cured was performed in the same

terms as the compressive strength tests (28, 90, 180

and 365 days). The compositions of the Na2SO4

solutions in which FA Svilajnac geopolymers were

cured, except in these terms, were tested after 7 and

60 days, in order to further investigate the changes in

the initial period of curing.

XRD was performed by Philips PW 1710, with

CuKa X-rays operating at 30 mA and 40 kV, with

0.02� 2h steps, 0.5 s step-1 in 5–50� 2h range.

Morphological characterization of the geopolymer

samples was done by SEM (VEGA TS 5130 MM,

Tescan). Energy dispersive X-ray spectroscopy (EDS)

was performed by INCAPentaFET-x3 (OXFORD

Instruments). The samples were Au–Pd coated. Sam-

ples for EDS analysis were prepared by grinding and

polishing (MTI Corporation) of epoxy impregnated

samples. Grinding was done using SiC grinding papers

(300, 600, 1,200 and 2,000 grit, 3 min each) with

acetone as lubricant. Final polishing of the samples

was done using polishing cloths and diamond pastes

(0.5 lm and 0.25 lm, MTI Corp., 3 min each).29Si MAS NMR spectra were obtained at Larmor

frequency of 79.49 MHz using Bruker MSL 400

system, Apollo console upgraded (Tecmag). Single

pulse sequence was used for data acquisition with

pulse width of 4 ls, corresponding to p/2 flip angle.

Recycle time was typically 20 s, the number of scans

was 4,096 and the spectral width was 20,000 Hz for all

recorded spectra. Chemical shifts were externally

referenced to 2,2-dimethyl-2-silapentane-5-sulfonate

(DSS) standard. Gaussian peak deconvolution of the

obtained spectra was performed using DMFIT appli-

cation [26]. Prior to 29Si MAS NMR analysis, iron

content of the geopolymer samples was reduced by

exposing the samples to a strong magnetic field.

3 Results and discussion

3.1 Properties of the starting FA and geopolymer

samples

Chemical composition of the starting FA samples and

content of fine FA particles (smaller than 43 lm) in the

samples are given in Table 2. Both FA samples were

class F [27]. Although a fraction of less reactive FA

particles in both of the starting FA samples was reduced

by sieving [23], FA Svilajnac still contained a signif-

icantly higher amount of fine particles (Table 2). Due to

different properties of the starting FA samples, geo-

polymer mortars with different properties were obtained

by alkali activation of the FA samples (Table 3).

After 24 h of curing, geopolymer mortars based on

FA Svilajnac showed about two times higher com-

pressive strength compared to FA Kolubara geopoly-

mers. Also, porosity of these mortar samples was

significantly lower than porosity of mortars based on

FA Kolubara (Table 3). Different porosity of the

mortar samples is partially a consequence of different

fine particles content in the FA samples (Table 2). Due

to the different particle size distribution of the FA

samples, in the preparation of the mortar samples,

different quantities of water had to be added in order to

provide equal workability. Except for the higher

water/FA ratio, lower compressive strength of the

FA Kolubara geopolymer was also a result of mineral

composition of the FA samples. Previous mineralog-

ical characterization of FA Kolubara showed that this

FA sample had high content of quartz, a crystalline

phase which remains un-reacted during the alkali

activation reaction [24].

Table 2 Chemical composition (wt%) and particle size dis-

tribution of the starting FA samples

Chemical composition FA sample

Svilajnac Kolubara

LOI at 1,000 �C 1.51 2.08

SiO2 49.89 55.51

Al2O3 21.66 21.59

Fe2O3 6.87 6.30

CaO 9.71 8.05

MgO 2.88 2.87

SO3 1.12 0.76

MnO – 0.20

P2O5 – 0.05

Na2O 0.63 0.55

K2O 1.21 0.94

Total 95.48 98.90

SiO2 ? Al2O3 ? Fe2O3* 78.42 83.40

Particle content \43 lm 52.65 29.85

* According to [27]: SiO2 ? Al2O3 ? Fe2O3 C 70 % for class

F FA


3.2 Effects of the Na2SO4 solution on mechanical

properties of the geopolymers

Changes in the compressive strength of the geopoly-

mer mortars during the testing are shown on Fig. 1.

The results are presented as relative strength (relative

to the starting compressive strength after 24 h at

95 �C, Table 3).

Strength of both groups of the reference geopoly-

mer mortars (based on FA Svilajnac and on FA

Kolubara) increased with time. After 365 days of

testing, compressive strengths of both groups of

samples were approximately 15 % higher than the

starting strengths (Fig. 1).

Compressive strength of the FA Svilajnac geopoly-

mer mortars exposed to the effects of the sulfate

solution showed only slight changes during the first

180 days of testing. A more significant reduction in

strength of these mortar samples was noticed after

365 days of testing (strength loss of about 12 %,

Fig. 1a). On the other hand, the strength of the FA

Kolubara mortars decreased already after the first

28 days of exposure to the Na2SO4 solution (by about

10 % compared to the strength of the starting samples,

Fig. 1b). After an initial decrease in strength, the

subsequent investigation showed there was a notice-

able strength increase of the FA Kolubara geopolymer

mortars, i.e. a structural recovery occurred.

Differences in strength changes of the geopolymer

mortars based on FA Svilajnac and FA Kolubara can

be explained by different porosity of the starting

mortar samples (Table 3). Due to the higher porosity

of the FA Kolubara mortars, ingress of the sulfate

solution into the geopolymer structure occurred more

rapidly. Characterization of the geopolymers‘ struc-

ture after the exposure to the Na2SO4 solution should

provide an explanation of the observed dependence of

the mortars strengths on the time of exposure.

3.3 Analyses of the Na2SO4 solution

The pH values of the starting sulfate solutions used in

this study were about 6 (Table 4). During the testing of

the geopolymer mortars, in contact with the samples,

pH values of the sulfate solution reached the value of

about 12 already after 28 days of testing. The observed

increase in pH of the sulfate solution is a result of

leaching of pore solution alkalis [9, 14, 28].

Table 3 Properties of the starting geopolymer samples

Geopolymer mortar Compressive strength,

24 h at 95 �C (MPa)

Specific surface area (m2/g) Porosity (cm3/g) Average pore

diameter (nm)

FA Svilajnac 43.1 ± 0.75 13.40 0.055 14.18

FA Kolubara 21.4 ± 0.49 27.58 0.160 22.05

Fig. 1 Strength of the geopolymer mortar samples: a FA Svilajnac geopolymers, b FA Kolubara geopolymers


By the ICP-OES analysis of the sulfate solutions

high and variable concentrations of S and Na were

detected. Generally, concentrations of S in the sulfate

solutions after the testing of the geopolymer samples

were lower than the initial S concentrations in the

solutions, while an increase in the concentration of Na

was observed (data not shown here). These results

were consistent with the findings of the previous

studies [9, 11, 14] and they indicated that diffusion of

sulfate ions in the geopolymers‘ structure and diffu-

sion of alkali metal ions from the structure into the

sulfate solution had occurred.

In addition to high concentrations of S and Na, the

results of ICP-OES analysis showed that exposing of

the geopolymer samples to the Na2SO4 solution had

caused leaching of one of the elements of the

aluminosilicate structure, silicon (Fig. 2). Leaching

of Si is probably less associated with an impact of the

sulfate ions than with the high pH of the sulfate

solution (Table 4). It is well known that the main

degradation mechanism of aluminosilicate glasses and

zeolites at high pH is hydrolysis of the siloxane bonds

(–Si–O–Si–) caused by a nucleophilic attack by OH-

at Si sites [29, 30]. Due to this phenomenon, treatment

with alkaline solutions is used for desilication of the

aluminosilicate structure of the zeolites with high Si/

Al atomic ratio [31–33]. One recent study showed that

exposure of alkali activated FA to highly alkaline

solution (14 M NaOH) resulted in a significant

leaching of Si [34]. Previous study on the effects of

alkaline solutions on geopolymers‘ structure and

properties found that treatment of fly ash based

geopolymers with solutions of up to pH 14 resulted

in a very small changes in the geopolymers‘ structure,

while in a more alkaline solution (pH[14) both silicon

and aluminum were leached [35]. However, the

authors noted that the investigated geopolymer sam-

ples had low silicate content in the activating solutions

(SiO2/Na2O ratios were 0.0, 0.2 and 0.79) and that the

geopolymer samples synthesized at a higher silicate

contents (SiO2/Na2O = 2.0) showed quite limited

resistance to immersion in alkaline solutions [35]. In

our work, sodium silicate with SiO2/Na2O ratio of 1.5

was used as the activator solution. High silicate

content in the activator solution is probably the reason

why leaching of Si was observed already in the

solution of the pH value of about 12 (Table 4). The

second element of the aluminosilicate structure,

aluminum, was not detected. The maximum concen-

trations of Si in the sulfate solutions (Fig. 2) were

detected in the same terms as the largest observed

decline in strength of the geopolymer mortars (Fig. 1).

These results suggest that Si was probably leached

from the aluminosilicate geopolymer gel.

3.4 Effects of the Na2SO4 solution on structure

of the geopolymers

3.4.1 XRD analysis

XRD analysis of the geopolymer samples after the

exposure to the Na2SO4 solution did not indicate

formation of any new phases formed due to a reaction

with sulfate ions (Fig. 3). Both the reference geopoly-

mer samples and the samples treated with the sulfate

solution contained only the phases originating from

the starting FA samples [24]: quartz, feldspar, anhy-

drite, hematite and mullite. The presence of calcite,

formed as a result of carbonation during curing, was

also detected. The intensity of calcite peaks increased

with the time of curing in the humid chamber. On the

Table 4 pH values of the Na2SO4 solutions

Time (days) Geopolymer samples

FA Svilajnac FA Kolubara

0 5.90 5.64

28 11.90 11.90

90 12.13 12.11

180 12.06 11.86

365 11.67 11.68

Fig. 2 Concentration of Si leached into the sulfate solution


other hand, while it seems that there was a decrease in

calcite peak intensity in the XRD patterns of the

G-Svil-S samples, no significant changes were

observed in the XRD patterns of the G-Kol-S samples.

Based on these results, it is difficult to draw a

conclusion on the role of calcite in sodium sulfate

attack on the FA based geopolymers. Previous studies

have shown that carbonation of Portland cement

concrete [36] and granulated blast furnace slag

concrete [37] prior to sulfate attack can increase their

sulfate resistance.

3.4.2 SEM and EDS analyses

By the SEM analysis of the geopolymer samples

Na2SO4 crystals (thenardite) were observed on the

fracture surface of the FA Kolubara geopolymer

samples after the exposure to the sulfate solution for

365 days (Fig. 4). The presence of Na2SO4 crystals is

probably associated with the migration of the sulfate

ions in the geopolymer structure, as noticed in the

previous study [11]. Thenardite crystals were

observed not only in the pores of the geopolymer

samples, but on the fracture surface of the sample as

well. The presence of the Na2SO4 crystals on the

surface of the sample suggests that the salt has

precipitated during the preparation of the sample for

the SEM analysis, since no washing of the samples

after the exposure to the sulfate solution has been

done. After the treatment with the Na2SO4 solution the

geopolymer samples were immediately crushed,

soaked in isopropyl alcohol and dried (Section 2.2).

Detailed EDS analysis of the polished geopolymer

samples confirmed the leaching of Na and Si from the

structure. The Si/Al and Na/Al atomic ratios of the

geopolymer samples treated with the Na2SO4 solution

were somewhat lower than the main atomic ratios of

the reference samples (Table 5).

3.4.3 NMR analysis

29Si MAS NMR spectra of the geopolymer samples

are shown in Fig. 5. Broad 29Si MAS NMR resonances

are typical for geopolymers [20, 38] and they indicate

poorly ordered aluminosilicate structure. A 29Si MAS

NMR spectra of FA based geopolymers consist of the

overlapping resonances attributed to silicon sites in

different silicate and aluminosilicate components

present in the material: aluminosilicate geopolymer

gel and un-reacted glassy and crystalline phases (e.g.

quartz, mullite) originating from the starting FA [21,

22, 24].

It is well known that the chemical shift at which

maximum peak intensity of the 29Si MAS NMR

spectrum occurs depends on the Si/Al atomic ratio of

the material [20, 39]. The 29Si MAS NMR spectra of

the starting FA Svilajnac geopolymer sample and the

reference samples after 28 and 365 days of curing

Fig. 3 XRD analyses of the starting geopolymer samples and the geopolymer samples after 28 and 365 days in the humid chamber and

in the Na2SO4 solution: a FA Svilajnac geopolymers and b FA Kolubara geopolymers


were centered at approximately -93 ppm (Fig. 5a).

Shifting of the maximum peak intensities of the 29Si

MAS NMR spectra of the geopolymer samples treated

with the Na2SO4 solution to higher frequencies (less

negative chemical shifts) indicated a decrease in the

Si/Al atomic ratio of these samples, i.e. confirmed

Fig. 4 SEM/EDS analysis of the G-Kol-S-365 sample

Table 5 EDS analysis of the geopolymer samples

Time (days) FA Svilajnac geopolymer samples FA Kolubara geopolymer samples

Reference samples Samples treated with

Na2SO4

Reference samples Samples treated with

Na2SO4

Si/Al Na/Al Si/Al Na/Al Si/Al Na/Al Si/Al Na/Al

1 2.86 (0.058)* 0.98 (0.043) 2.86 (0.058) 0.98 (0.043) 2.97 (0.011) 1.03 (0.017) 2.97 (0.011) 1.03 (0.017)

28 2.95 (0.034) 0.92 (0.003) 2.92 (0.039) 0.81 (0.015) 3.00 (0.053) 1.17 (0.034) 2.87 (0.033) 0.95 (0.015)

365 2.86 (0.051) 0.65 (0.008) 2.80 (0.041) 0.70 (0.012) 2.99 (0.042) 0.95 (0.027) 2.87 (0.021) 0.96 (0.018)

* Standard deviation

Fig. 5 29Si MAS NMR analysis of the starting geopolymer samples and the geopolymer samples after 28 and 365 days in the humid

chamber and in the Na2SO4 solution: a FA Svilajnac geopolymers and b FA Kolubara geopolymers


leaching of Si (Fig. 2). Results of the 29Si MAS NMR

analysis of the FA Svilajnac geopolymer samples

showed good correlation with the analyses of the

sulfate solution. The results of the ICP-OES analysis

showed that concentration of Si in the sulfate solutions

in which FA Svilajnac geopolymer samples were

cured increased with time (Fig. 2). Shifting of the

maximum peak intensities of the 29Si MAS NMR

spectra of the FA Svilajnac geopolymer samples

treated with the Na2SO4 solution from -92 ppm after

28 days to -90 ppm after 365 days in the solution

(Fig. 5a) confirmed that the Si/Al atomic ratio of these

samples decreased with the time of the treatment.

The maximum peak intensity of the G-Kol-0

sample 29Si MAS NMR spectrum was centered at

approximately -94 ppm (Fig. 5b). After curing of the

geopolymer samples in the humid chamber for

365 days, the maximum peak intensity of the 29Si

MAS NMR spectrum was shifted to approximately

-93 ppm (Fig. 5b). This was probably due to the

continuation of the alkali activation reaction, as

further dissolution of FA led to minor changes in the

composition of the aluminosilicate gel. A decrease in

the Si/Al atomic ratio of the samples treated with the

Na2SO4 solution was confirmed by shifting of the

maximum peak intensity of 29Si MAS NMR spectra to

less negative chemical shift. The 29Si MAS NMR

spectra of the G-Kol-S-28 and G-Kol-S-365 samples

were centered at approximately -92 ppm (Fig. 5b).

These results are consistent with the results of ICP-

OES analysis which showed that the concentration of

Si leached form the FA Kolubara geopolymers into the

sulfate solution changed only slightly after the first

28 days of investigation (Fig. 2).

It has become a common practice to quantify broad29Si MAS NMR spectra of geopolymers by means of

Gaussian deconvolution [20–22, 24, 40–42]. Decon-

volution of 29Si NMR spectra assuming constant line

widths for the Q4(mAl) peaks is commonly used for

zeolites and crystalline aluminosilicates [39], but

proved to be useful also for analyzing 29Si MAS

NMR spectra of alkali activated fly ash [21, 22, 24, 40–

42]. It is generally accepted that 29Si MAS NMR

spectra of geopolymers consist of all five Q4(mAl)

silicon species, with Q4(4Al), Q4(3Al), Q4(2Al),

Q4(1Al) and Q4(0Al) resonating at approximately

-84, -89, -93, -99 and -108 ppm, respectively

[20–22, 24, 38, 40–42]. The frequency of the individual

Q4(mAl) peaks varies depending on the Si/Al atomic

ratio of the material [39]. The peaks width is also

affected by changes in the Si/Al atomic ratio [20]: as the

Si/Al atomic ratio decreases, the peak width also

decreases. The 29Si MAS NMR spectra in this work

were deconvoluted using Gaussian line shapes, the same

number of peaks and constant line widths (Figs. 6, 7;

Tables 6, 7). The peak positions for each of the five

possible Q4(mAl) silicon species, ranging from 4 to 0 Al

neighbors, were adopted from the previous 29Si MAS

NMR studies of crystalline aluminosilicates [39] and

geopolymers [20–22, 24, 40–42]. The chemical shifts

and the peak widths were confined to those reported

previously [20–22, 40–42]. In addition to the Q4(mAl)

silicon peaks, in all the 29Si MAS NMR spectra in this

study a small resonances at approximately -78, -107.5

and -115 ppm were observed (Figs. 6, 7). A sharp

resonance at -107.5 ppm, indicating an ordered struc-

ture, can be attributed to quartz originating from the

starting FA [24, 39]. The small peak at -78 ppm is

usually ascribed to less condensed silicon species (Q1

and Q2) present in the material [21, 22, 39, 40]. The peak

at -115 pm is more difficult to identify and it can be

attributed to different silica polymorphs [41, 42] or to

Q4(0Al) units in the products of alkali activation

reaction [22].

Previous analysis of the 29Si MAS NMR spectra of

the reference FA Svilajnac geopolymer samples

showed that increase in compressive strength of the

reference geopolymer mortars was associated with a

small increase of peak areas of aluminum rich

components in the 29Si MAS NMR spectra,

Q4(mAl), m = 2, 3, 4 [24]. The results of the earlier29Si MAS NMR analyses of FA based geopolymer

samples showed that evolution of signals intensity of

individual Q4(mAl) components with reaction time

depends on the properties of the starting FA sample

[40], as well as on the composition of the activator

solution [21]. In our study, the activator solution with

high silicate content was used. It appears that contin-

uing of the alkali activation reaction of the investi-

gated FA Svilajnac geopolymer samples in the 29Si

MAS NMR spectra was manifested by an increase in

the fraction of aluminum rich components, Q4(mAl),

m = 2, 3, 4 (Table 6).

Deconvolution results of the 29Si MAS NMR

spectra of the FA Svilajnac geopolymer samples

treated with the Na2SO4 solution indicated a signifi-

cant decrease of peak areas of silicon rich components

in the 29Si MAS NMR spectra, Q4(mAl), m = 0, 1


(Table 6). Decrease in the fraction of the Q4(1Al) and

Q4(0Al) components was also observed in the 29Si

MAS NMR spectra of zeolites upon desilication

treatment [33]. The reduction of the signals intensity

of the silicon rich components in the 29Si MAS NMR

spectrum was attributed to the preferential leaching of

silicon which is not surrounded by aluminum in the

structure [32, 33]. Besides the observed decrease in

peak areas of the Q4(mAl), m = 0, 1 components, in29Si MAS NMR spectra of the FA Svilajnac geopoly-

mer samples treated with the Na2SO4 solution, an

increase in the fraction of less condensed silicon

species was observed (Q1 and Q2, Table 6). It seems

that breaking of –Si–O–Si– bonds and leaching of Si

had caused an increase in the fraction of silicon atoms

that are not involved in three-dimensional alumino-

silicate gel network, i.e. in Q4(mAl) structural units.

The observed decline in the strength of the

geopolymer samples in sulfate solution (Fig. 1)

resulted from the disruption in continuity of alumino-

silicate structure and the formation of structural

defects due to the leaching of Si.

Deconvolution of the 29Si MAS NMR spectra of the

reference FA Kolubara geopolymer samples showed a

more significant increase of peak areas of aluminum

rich components, Q4(mAl), m = 2, 3, 4 (Table 7),

compared to the deconvolution results of the FA

Svilajnac geopolymer 29Si MAS NMR spectra

(Table 6). Analysis of the deconvolution results of

the 29Si MAS NMR spectra of the FA Kolubara

geopolymer samples treated with the Na2SO4 solution

indicated a decrease in peak areas of the Q4(0Al)

components, as well as an increase in the fraction of

less condensed silicon species, Q1 and Q2, (Table 7).

The observed decrease in strength of the FA

Kolubara geopolymer mortars after 28 days of

Fig. 6 29Si MAS NMR

deconvoluted spectra of FA

Svilajnac geopolymers: a G-

Svil-0, b G-Svil-Ref-28,

c G-Svil-S-28, d G-Svil-

Ref-365, e G-Svil-S-365


exposure to the Na2SO4 solution was followed by a

structural recovery during further investigation

(Fig. 1). The increase in strength of the FA Kolubara

geopolymer samples is probably a result of continuing

of the alkali activation reaction in the sulfate solution.

Conditions to which the geopolymer samples were

subjected throughout the testing of the sulfate attack,

i.e. high concentration of Na in the solution and high

pH of the solution (Table 4) were favorable for

continuing the alkali activation reaction. The increase

in strength of the FA Kolubara geopolymer mortars in

the sulfate solution is consistent with the work of

Ismail et al., who investigated effects of sulfate

solutions on the structure of alkali activated fly ash/

slag mixture and suggested that geopolymer binder

continues to develop in the Na2SO4 solution [15]. A

similar phenomenon was observed during the expo-

sure of alkali activated slag to the effects of sulfate

solution, whereby an increase in strength of the

material in the Na2SO4 solution was attributed to the

continuing hydration [43]. However, no significant

differences could be observed by comparing the

deconvolution results of the 29Si MAS NMR spectra

of the G-Kol-S-28 and G-Kol-S-365 samples

(Table 7). As noted previously, 29Si MAS NMR

spectra of FA based geopolymers consist of the

overlapping resonances attributed to silicon sites in

aluminosilicate geopolymer gel and un-reacted FA.

Structural changes that occurred over extended expo-

sure of the FA Kolubara geopolymer samples to the

sulfate solution did not result in significant changes in29Si MAS NMR spectra of the geopolymer samples

(Figs. 5b and 7).

Nevertheless, the 29Si MAS NMR analysis of the

geopolymer samples provided useful information

regarding the degradation mechanism of geopolymer

gel in the sulfate solution. The 29Si MAS NMR

analysis of the geopolymer samples showed that,

Fig. 7 29Si MAS NMR

deconvoluted spectra of FA

Kolubara geopolymers: a G-

Kol-0, b G-Kol-Ref-28, c G-

Kol-S-28, d G-Kol-Ref-365,

e G-Kol-S-365


under the applied testing conditions, treatment of the

geopolymer samples with the Na2SO4 solution caused

breaking of –Si–O–Si– bonds and leaching of Si,

whereby breaking of –Si–O–Si– bonds occurred

preferentially in silicon rich components of the

aluminosilicate gel (Q4(0Al) and Q4(1Al) components

of the 29Si MAS NMR spectra, Tables 6 and 7).

However, the observed degradation of the geopoly-

meric material was not a result of reaction with sulfate

ions, but rather a consequence of the high pH values of

Table 6 29Si MAS NMR spectral deconvolution of the FA Svilajnac geopolymer samples (Fig. 6)

Geopolymer sample Q1, Q2* Q4(4Al) Q4(3Al) Q4(2Al) Q4(1Al) Q4(0Al) quartz** Q4**

G-Svil-0 d (ppm) -77.8 -84.0 -88.9 -94.2 -99.8 -106.9 -107.6 -115.2

Width (ppm) 7.0 7.0 7.0 7.0 7.0 7.0 1.2 7.0

Area (%) 2.9 9.0 22.4 27.4 21.6 13.0 1.2 2.7

G-Svil-Ref-28 d (ppm) -77.8 -84.1 -88.8 -94.1 -99.9 -107.1 -107.8 -114.0

Width (ppm) 7.0 7.0 7.0 7.0 7.0 7.0 1.2 7.0

Area (%) 2.4 9.2 22.3 27.9 22.0 12.4 1.2 2.8

G-Svil-S-28 d (ppm) -75.5 -82.5 -88.2 -93.7 -99.3 -105.6 -107.5 -110.0

Width (ppm) 6.8 6.8 6.8 6.8 6.8 6.8 1.0 6.8

Area (%) 3.5 11.4 25.6 27.8 18.7 9.7 1.2 2.1

G-Svil-Ref-365 d (ppm) -77.2 -84.2 -89.3 -94.8 -100.7 -107.9 -107.7 -116.1

Width (ppm) 7.0 7.0 7.0 7.0 7.0 7.0 1.2 7.0

Area (%) 2.8 9.2 22.3 28.6 20.5 11.8 1.0 3.8

G-Svil-S-365 d (ppm) -74.7 -82.3 -87.7 -93.1 -98.4 -104.8 -107.7 -109.0

Width (ppm) 6.6 6.6 6.6 6.6 6.6 6.6 1.3 6.6

Area (%) 4.5 12.5 27.1 28.2 17.8 6.6 1.2 2.1

* Less condensed silicon species, Q1 and Q2 [21, 22, 39, 40]

** Quartz (-107.5 ppm) and other Q4 silicon units [22, 39, 41, 42]

Table 7 29Si MAS NMR spectral deconvolution of the FA Kolubara geopolymer samples (Fig. 7)

Geopolymer sample Q1,2* Q4(4Al) Q4(3Al) Q4(2Al) Q4(1Al) Q4(0Al) quartz** Q4**

G-Kol-0 d (ppm) -78.7 -84.0 -89.4 -95.0 -100.7 -107.3 -107.7 -114.0

Width (ppm) 7.0 7.0 7.0 7.0 7.0 7.0 1.5 7.0

Area (%) 1.8 9.1 23.1 28.6 18.7 12.0 1.5 5.2

G-Kol-Ref-28 d (ppm) -76.0 -83.3 -89.2 -94.6 -100.4 -107.4 -107.5 -114.6

Width (ppm) 7.0 7.0 7.0 7.0 7.0 7.0 1.3 7.0

Area (%) 2.3 10.5 23.3 27.4 18.7 12.2 1.9 3.8

G-Kol-S-28 d (ppm) -75.5 -83.3 -89.1 -94.5 -100.2 -107.2 -107.4 -115.1

Width (ppm) 6.8 6.8 6.8 6.8 6.8 6.8 1.0 6.8

Area (%) 3.2 12.2 24.8 27.7 18.2 10.6 1.4 2.0

G-Kol-Ref-365 d (ppm) -74.3 -82.4 -88.4 -93.7 -99.6 -106.9 -107.7 -114.5

Width (ppm) 6.9 6.9 6.9 6.9 6.9 6.9 1.4 6.9

Area (%) 2.4 12.3 23.7 28.2 19.7 10.1 1.6 2.0

G-Kol-S-365 d (ppm) -75.1 -82.9 -88.9 -94.3 -99.9 -107.2 -107.7 -115.5

Width (ppm) 6.8 6.8 6.8 6.8 6.8 6.8 1.6 6.8

Area (%) 3.0 11.8 25.9 27.7 18.1 10.1 1.4 2.0

* Less condensed silicon species, Q1 and Q2 [21, 22, 39, 40]

** Quartz (-107.5 ppm) and other Q4 silicon units [21, 39, 41, 42]


the sulfate solution (about 12, Table 4). High pH

values of the sulfate solution caused breaking of –Si–

O–Si– bonds and formation of structural defects,

which led to the observed strength decrease of the

geopolymer samples (Fig. 1). The obtained results

emphasize the importance of controlling pH value and

composition of the sulfate solution when testing

sulfate attack on geopolymers.

4 Conclusions

In this paper, the impact of the Na2SO4 solution (50 g/l)

on mechanical properties and structure of geopolymer

samples based on two different FA samples was

investigated over a period of 365 days.

It was found that the treatment with the sulfate

solution had caused a small decrease in strength of the

geopolymer samples (about 10 %). Decline in strength

of the less porous geopolymer samples was observed

only after 365 days of investigation. The strength of

the more porous geopolymer samples decreased after

the first 28 days of testing. The subsequent exposure to

the sulfate solution caused an increase in strength of

the more porous geopolymer samples, probably due to

continuing of the alkali activation reaction in the

sulfate solution.

Analysis of the sulfate solution showed that, in

addition to diffusion of sulfate ions in the geopolymer

structure and leaching of Na from the structure,

treatment of the geopolymer samples with the Na2SO4

solution caused leaching of Si. During the entire

testing period, the pH value of the sulfate solution was

about 12.

No new phases were detected due to a reaction of

the geopolymeric material with sulfate ions. Occur-

rence of Na2SO4 crystals on the fracture surface of the

geopolymer samples was attributed to migration of

sulfate ions in geopolymer structure.29Si MAS NMR analysis of the geopolymer sam-

ples showed that, in the applied testing conditions,

treatment of the geopolymer samples with Na2SO4

solution had caused breaking of –Si–O–Si– bonds and

decrease in Si/Al atomic ratio in the samples. Decon-

volution of the 29Si MAS NMR spectra indicated that

breaking of –Si–O–Si– bonds occurred preferentially

in silicon rich components of the structure, showed by

a decrease of Q4(mAl), m = 0, 1 components of the29Si MAS NMR spectra. It was concluded that the

leaching of Si was a consequence of the increase in the

pH value of sulfate solution during the sulfate attack

investigation.

Acknowledgments This work was carried out within the

Project TR34026 funded by the Ministry of Education, Science

and Technological Development, Republic of Serbia. Authors

are grateful to Dr. Aleksandra Rosic (Faculty of Mining and

Geology, Belgrade University) for XRD analyses, Ljiljana

Milicic (Institute for testing materials Serbia) for XRF analyses,

MSc Ivona Jankovic-Castvan (Faculty of Technology and

Metallurgy, Belgrade University) for porosity measurements

and Prof. Miroslav Nikolic (Plant and Soil Laboratory, Institute

for Multidisciplinary Research) for ICP-OES analysis.

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