UDK 678.86
Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 47(4)535(2013)
MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF THE FLY-ASH-BASED GEOPOLYMER PASTE AND
MORTAR
MEHANSKE IN MIKROSTRUKTURNE LASTNOSTI GEOPOLIMERNIH VEZIV IN MALT IZ LETEČEGA PEPELA
Radomir Zejak1, Irena Nikolic2, Dragoljub Blecic2, Vuk Radmilovic3, Velimir Radmilovic3
iUniversity of Montenegro, Faculty of Civil Engineering, Džordža Vašingtona bb, 81000 Podgorica, Montenegro 2University of Montenegro, Faculty of Metallurgy and Technology, Džordža Vašingtona bb, 81000 Podgorica, Montenegro 3Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia
irena@ac.me
Prejem rokopisa — received: 2012-12-03; sprejem za objavo - accepted for publication: 2012-12-14
In this paper we have investigated the influence of synthesis parameters on the mechanical properties of the fly-ash-based geopolymer paste and mortars. Moreover, the influence of an addition of limestone sand on the microstructure of fly-ash-based geopolymers was investigated as well. An addition of limestone sand increased the compressive strength of the fly-ash-based geopolymer mortar, in comparison to the strength of the geopolymer paste, by changing the composition of the gel phase in the geopolymer structure and with the physical strengthening due to the presence of well connected grains of the sand. The results have shown that the compressive strength and Young's modulus of elasticity of the fly-ash-based geopolymer mortar are correlated. The values of these parameters increase as the alkali and silicate dosages increase. At one point, they reach the maximum values and then start to decrease. On the other hand, the parameters that considerably increase the compressive strength slightly decrease the flexural strength of the geopolymer mortar.
Keywords: geopolymerization, fly ash, mortar, flexural strength, compressive strength, Young's modulus
Članek obravnava raziskavo vplivov parametrov sinteze na mehanske lastnosti geopolimernih veziv in malt iz letečega pepela. Preiskovan je bil tudi vpliv dodatka peska iz apnenca na mikrostrukturo geopolimera iz letečega pepela. Dodatek peska iz apnenca je povečal tlačno trdnost geopolimerne malte iz letečega pepela v primerjavi s trdnostjo geopolimernega veziva s spreminjanjem sestave želirne faze v strukturi geopolimera in s fizikalnim utrjevanjem zaradi prisotnosti dobro povezanih zrn peska. Rezultati so pokazali pri geopolimeru iz letečega pepela povezavo med tlačno trdnostjo in Youngovim modulom elastičnosti. Vrednosti teh parametrov naraščajo z naraščanjem dodatkov silikatov in alkalij. V določeni točki dosežejo največjo vrednost in se potem začnejo zmanjševati. Po drugi strani pa parametri, ki občutno povečajo tlačno trdnost, rahlo zmanjšajo upogibno trdnost geopolimerne malte.
Ključne besede: geopolimerizacija, leteči pepel, malta, upogibna trdnost, tlačna trdnost, Youngov modul
1 INTRODUCTION
Geopolymerization is an innovative technology that can transform a variety of aluminosilicate materials with alkali activation into useful, environmentally friendly materials known as geopolymers (inorganic polymers). These materials may be successfully applied in civil engineering as a replacement for cement binders. Geopolymerization is also known as a waste-minimization technology because it may utilize waste material (coal fly ash and metallurgical slag) as raw material.
The mechanism of geopolymerisation may be summarized in a few steps:
(1)	Dissolution process, in which a breaking of Al-O-Si and Si-O-Si bonds in the source material and a liberation of Al3+ and Si4+ in an alkali silicate solution occur. Together with the dissolution, a hydrolysis of dissolved Al and Si occurs, leading to the formation of aluminate and silicate monomeric species;
(2)	Polycondensation of the Al and Si species in an amorphous three-dimensional aluminosilicate network (gel), in which the Si4+ and Al3+ cations are tetrahedrally
coordinated as [SiO4]4- and [AlO4]5- and linked by oxygen bridges.1 The negative charge of the AlO4- group is charge-balanced by alkali cations (Na+ and/or K+)2; (3) Hardening of aluminosilicate gel. The empirical formula of a geopolymer is: Mn [(-SiO2)z AlO2]n wH2O, where Mn is a cation, usually an alkali (Na+ or K+), n is a degree of polycondensation, w £ 3 and z is 1, 2 or 3.3
Geopolymers may be characterized with a variety of properties like a good thermal resistance4-6 and resistance to aggressive environment,7 8 but the basic engineering request is a high compressive strength. Depending on the processing conditions, geopolymers may show different values of compressive strength. Engineering demands for an improvement in the mechanical properties of geopolymers have resulted in considering possible geopolymer applications in the form of mortars or concrete.9-13
Fly ash, the by-product from coal-fired power stations, is mainly used for a geopolymer synthesis. Although fly ash is not considered to be a hazardous waste, the high quantity of ash produced all over the
world imposes the necessity of finding a solution for this environmental problem. In this sense, a utilization of fly ash through the geopolymerization process may be considered as a promising solution.
The goal of this study was to investigate the influence of the synthesis parameters, the alkali and silicate dosages, on the mechanical properties of the fly-ash-based geopolymer paste and mortar and to investigate the influence of a sand addition on the microstructural properties of the fly-ash-based geopolymers.
2 EXPERIMENT
The fly ash sourced for this investigation was supplied from the coal-fired power station Pljevlja in Montenegro. Its chemical composition is given in Table 1.
Table 1: Chemical composition of fly ash Tabela 1: Kemijska sestava letečega pepela
Compound	Content, w/%
SiO2	49.45
Fe2O3	5.23
Al2O3	21.77
TiO2	0.66
CaO	13.34
Na2O	0.46
ZnO	4.5-10-3
MgO	1.29
MnO	0.02
P2O5	0.24
K2O	1.4
LOI*	4.35
*Loss on ignition
The geopolymer paste was prepared by mixing fly ash with an alkali silicate solution, in a solid-to-liquid ratio of 1. An alkali silicate activator was prepared by mixing the NaOH and Na2SiO3 solutions at the mass ratios of 1, 1.5 and 2. The concentrations of the NaOH solution were (7, 10 and 13) M and a commercial sodium silicate solution (w(Na2O) = 8.5 %, w(SiO2) = 28.5 %, a density of 1.4 g/cm3) was used. The geopolymer paste was cast in a cylindrical plastic mould, sealed with lead and left to rest in an oven for 48 h at the temperature of 65 °C.
Geopolymer mortar was prepared by mixing the geopolymer paste with crushed limestone sand. The granulometry distribution obtained with the sieve method is given in Table 2. The prepared mixtures were cast into the standard 40 mm x 40 mm x 160 mm prism moulds covered with polyethylene film sheets and left in an oven for 48 h at 65 °C. After this time, the samples were allowed to cool down, then removed from the moulds and left to rest for additional 14 d at ambient temperature before any testing was performed. The samples were tested for the compressive and flexural strengths as well
as the modulus of elasticity. All the tests were performed using a minimum of three samples per mortar mix.
Table 2: Analytical sieving analysis of limestone sand Tabela 2: Analitična sejalna analiza peska iz apnenca
Sieve size opening mm	Cumulative mass fraction of a sample pass through each sieve %
0.063	3.45
0.09	7.2
0.125	10.24
0.25	15.88
0.5	28.41
1.0	54.47
2.0	74.78
4.0	97.90
8.0	100.00
The samples of the geopolymer paste were tested only for the compressive strength, while the samples of the geopolymer mortar were subjected to testing for all the mechanical properties (compressive strength, flexural strength and Young's modulus of elasticity).
The flexural strength was determined from the modulus of the rupture test, while the Young's modulus of elasticity was calculated from the linear stress/strain response prior to failure. The samples remaining from the flexural-strength tests were used for testing the compressive strength of the geopolymer mortar.
Microstructural investigations of the geopolymer mortar were carried out using the FEI 235DB focused-ion-beam system at the National Center for Electron Microscopy at Berkeley, equipped with an EDAX Genesis energy dispersive spectrometer (EDS). The SEM images were recorded with a secondary electron detector (SED). A cross-sectioning of the geopolymer paste (without a sand addition) was carried out using 1000 pA gallium ion beam and a microstructural characterization was performed using an electron beam at the 5 kV operating voltage. In order to minimize electron charging effects, a through-lens back-scattered electron detector (TLD-B) was used for image recording.
3 RESULTS AND DISCUSSION
3.1 Mechanical properties of fly-ash-based geopoly-
mers
Mechanical properties of fly-ash-based geopolymers are of primary importance regardless of possible applications in civil engineering. One of the basic conditions for a possible use of geopolymers in construction is a satisfactory strength, which can be significantly enhanced with an addition of sand. Two of the most important factors that greatly influence mechanical properties are alkali and silicate dosages. The influence of an alkali dosage on the mechanical properties of the geopolymer paste and mortar was evaluated through the
change in the NaOH concentration, while the influence of a silicate dosage was investigated through the change in the w(Na2SiO3)/w(NaOH) ratio.
The results have shown that the mechanical properties of the fly-ash-based geopolymer paste and mortar greatly depend on the reaction parameters of the geopolymerization process. Depending on the reaction parameters, the compressive strength of fly-ash-based geopolymers may be considerably increased with an addition of sand. The change in the compressive strength of the fly-ash-based geopolymer paste and mortar as a function of the reaction parameters is given in Figures 1 and 2. It is evident that the compressive strength of the geopolymer mortar is considerably higher than the com-pressive strength of the geopolymer paste. One of the reasons for that is the fact that well connected grains of sand physically strengthen the fly-ash-based geopoly-mers. The compressive strengths of both the geopolymer paste and mortar increase with the increase in the NaOH concentration (Figure 1) and the w(Na2SiO3)/w(NaOH) mass ratio (Figure 2). At one point, they reach the maximum values and then decrease. The maximum values of the compressive strength were obtained using the 10 M NaOH at the w(Na2SiO3)/w(NaOH) mass ratio of 1.5. Moreover, the compressive strength of the geopolymer mortar is higher than the strength of the geopolymer paste by 58.7 %.
The results of the investigation of geopolymer mortars have shown that the Young's modulus of elasticity is related to the compressive strength. The increase in the NaOH concentration from 7 M to 10 M leads to an increase in the Young's modulus. The maximum value of the Young's modulus of elasticity (9.1 GPa) was reached using 10 M NaOH. A further increase in the alkaline (NaOH) dosage to 13 M slightly reduced the modulus of elasticity of the geopolymer mortar (Figure 3). The same behaviour of the fly-ash-based geopolymer mortar was
25
■ft .
10
m
1	1,5	2
NajSiCVNaOH Geopolymer paste ■ Geopolymer mortar
Figure 2: Change in the compressive strength of geopolymer paste and mortar as a function of the w(Na2SiO3)/w(NaOH) mass ratio Slika 2: Spreminjanje tlačne trdnosti geopolimernega veziva in malte v odvisnosti od masnega razmerja w(Na2SiO3)/w(NaOH)
observed with the change in the silicate dosage (Figure 4). The fly-ash-based geopolymer mortar reached the maximum value of the Young's modulus at the w(Na2SiO3)/w(NaOH) ratio of 1.5. A further increase in the silicate dosage, i.e., a change in the w(Na2SiO3)/ w(NaOH) ratio, led to a reduction in the Young's modulus of elasticity. A similar observation of the influence of the silicate dosage on the Young's modulus of elasticity was observed by Duxon et. al.14
On the other hand, the flexural strength of the fly-ash-based geopolymer composites is inversely related to the compressive strength and Young's modulus of elasticity. The parameters that considerably increase the compressive strength and Young's modulus of elasticity slightly reduce the flexural strength of the fly-ash-based geopolymer composites. The minimum values of the flexural strength correspond to the maximum values of the compressive strength and Young's modulus of
Figure 1: Change in the compressive strength of geopolymer paste and mortar as a function of the NaOH concentration Slika 1: Spreminjanje tlačne trdnosti geopolimernega veziva in malte v odvisnosti od koncentracije NaOH
Figure 3: Change in the flexural strength and Young's modulus of elasticity of geopolymer mortar as a function of the NaOH concentration Slika 3: Spreminjanje upogibne trdnosti in Youngovega modula elastičnosti geopolimerne malte v odvisnosti od koncentracije NaOH
m
Figure 4: Change in the flexural strength and Young's modulus of elasticity of geopolymer mortar as a function of the w(Na2SiO3)/ w(NaOH) mass ratio
Slika 4: Spreminjanje upogibne trdnosti in Youngovega modula elastičnosti geopolimerne malte v odvisnosti od masnega razmerja w(Na2SiO3)/w(NaOH)
Figure 5: a) Microstructure of geopolymer mortar prepared using 10 M NaOH at the w(Na2SiO3)/w(NaOH) mass ratio of 1.5; b) geopolymer paste (as a part of geopolymer mortar); A - gel; B - unreacted fly ash particles
Slika 5: a) Mikrostruktura geopolimerne malte, pripravljene z 10 M NaOH, pri masnem razmerju w(Na2SiO3)/w(NaOH) 1,5; b) geopoli-merno vezivo (kot del geopolimerne malte); A- vezivo; B- nereagirani delci letečega pepela
elasticity that were reached using 10 M NaOH (Figure 3), at the w(Na2SiO3)/w(NaOH) ratio of 1.5 (Figure 4). The highest value of the flexural strength was obtained using 7 M NaOH at the w(Na2SiO3)/w(NaOH) mass ratio of 1.
3.2 SEM -EDS microanalysis
The microstructure of the geopolymer mortar is characterized by the presence of sand grains and geopolymer paste (Figure 5a). The geopolymer paste consists of a gel phase (A) and unreacted fly-ash particles (B), (Figure 5b). The strength of the geopolymer mortar primarily depends on the structure of the gel, while the presence of the well connected grains of sand additionally strengthen the structure of the mortar. Besides, the unreacted fly-ash particles play the role of microaggregates contributing to the overall strength of the geopolymer mortar. The results of an EDS analysis of the gel phase in the mortar (Figure 6a) have shown a quantity of Ca that is considerably higher than the Si, Al, and Na contents. On the other hand, the results of an EDS analysis of the gel phase of the geopolymer paste prepared without an addition of sand, but with the same reaction parameters, have shown the quantities of Si, Al, and Na that are considerably higher than the Ca content.
Figure 6: EDS of the: a) gel phase of fly-ash-based geopolymer mortar and b) paste (in the absence of sand)
Slika 6: EDS veziva: a) geopolimerne malte iz letečega pepela in b) veziva (brez peska)
In this case, Ca in the gel phase only originates from the fly ash (Figure 6b).
The previous research showed that the main gel forming elements are Si, Al and Na and that the structure of the geopolymer gel is determined by the w(Si)/w(Al) ratio.15 But, in the geopolymer mixture with a high Ca content, the influence of Ca must be taken into account because the properties of the gel also greatly depend on the Ca content.1617
Table 3: Content of elements (w/%) and their ratios in the gel phases of geopolymer mortar and paste
Tabela 3: Vsebnost elementov (w/%) in njihov delez v vezivu geo-polimerne malte in lepila
Ratio	Geopolymer mortar	Geopolymer paste
w(Na)/w(Si)	0.48	0.42
w(Na)/w(Al)	1.23	1.17
w(Si)/w(Al)	2.65	2.8
w(Ca)/w(Si)	3.04	0.17
Si	6.31	17.14
Al	2.38	6.11
Ca	19.17	2.85
There is no considerable difference between the w(Na)/w(Si), w(Na)/w(Al) and w(Si)/w(Al) ratios in the gel phase of the geopolymer paste and mortar (Table 3). But, in the case of w(Ca)/w(Si), the ratio is considerably higher in the gel phase of the geopolymer mortar than in the geopolymer paste, which indicates a strong influence of Ca on the structure of the geopolymer mortar. On the other hand, in the gel phase of the geopolymer mortar the observed quantities of Al and Si are considerably lower than their contents in the gel phase of the geopoly-mer paste without an addition of limestone sand (Table 3). It could be that the presence of Ca in some way depletes the Al and Si dissolution, but this assumption needs to be further studied.
As the limestone sand was used for the geopoly-mer-mortar synthesis, it is necessary to take into account the presence of Ca in the gel phase as a result of the limestone dissolution in a strongly alkaline medium. Besides, a certain amount of Ca in the gel originates from fly ash. The reaction mechanism of the geopolyme-risation varies significantly, depending on the presence of Ca in the geopolymer mixture. In the absence of Ca, the main product of the geopolymerisation process is an aluminosilicate inorganic polymer (the gel), while in the presence of Ca, additional Ca-bearing phases are formed.18 The reaction pathway during the geopoly-merization is strongly influenced by the Ca presence, because it strongly reacts with the soluble silicate and aluminate species provided during the dissolution step. This leads to the formation of a calcium silica hydrate and calcium aluminate hydrate gel phases.19 It is considered that these Ca phases additionally strengthen the geopolymer structure, but a clear understanding of how Ca influences the reaction mechanism of the geopolymerization has not yet been established. It is assumed that the calcium silica hydrate gel fills the voids
and pores within the geopolymer paste leading to an increase in the compressive strength.18 Moreover, it is also considered that Ca is capable of acting as a charge-balancing cation within the geopolymer structure participating, in this way, in the geopolymerization process.20
4	CONCLUSIONS
In this paper, the influence of the synthesis parameters and a limestone-sand addition on the mechanical and microstructural properties of the fly-ash-based geopolymer paste and mortar was investigated. The results have shown that the compressive strength of the geopolymer paste may be increased by adding limestone sand. Moreover, the compressive strength correlates with the Young's modulus of elasticity of the fly-ash-based geopolymer mortars. Both the compressive strength and Young's modulus of elasticity increase with an increase in the NaOH concentration and the w(Na2SiOs)/ w(NaOH) mass ratio, reaching their maximum values at one point and then decreasing. On the other hand, the parameters that increase the compressive strength slightly decrease the flexural strength of the fly-ash-based geopolymer mortar. The maximum value of the compressive strength corresponds to the minimum value of the flexural strength of the fly-ash-based geopolymer mortar, indicating that the fly-ash-based geopolymers may be considered to be brittle materials. An addition of limestone sand increases the compressive strength of the fly-ash-based geopolymers by changing the structure of the geopolymeric gel phase. The gel phase of the geopolymer mortar is characterized by a content of Ca that is considerably higher than in the geopolymer paste, resulting in a higher compressive strength of the geopolymer mortar. In addition, the physical presence of the well connected grains of sand also contributes to the strengthening of the geopolymer mortar.
Acknowledgements
The authors would like to acknowledge the financial support of the Montenegrin Ministry of Science in the framework of the project No.01-460. The FIB-SEM analysis was performed at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory, funded by the U.S. Department of Energy under the Contract DE-AC02-05CH11231.
5	REFERENCES
1 P. Duxson, A. Fernández-Jiménez, J. L. Provis, G. C. Lukey, A. Palomo, J. S. J. van Deventer, Geopolymer technology: the current state of the art, Journal of Materials Science, 42 (2007) 9, 2917-2933 2F. Pacheco-Torgal, J. Castro-Gomes, S. Jalali, Alkali-activated binders: A review: Part 1. Historical background, terminology, reaction mechanisms and hydration products, Construction and Building Materials, 22 (2008), 1305-1314
3 J. Davidovits, Geopolymers, inorganic polymeric new materials, Journal of Thermal Analysis, 37 (1991), 1633-1656
4D. L. Y. Kong, J. G. Sanjayan, Effect of elevated temperatures on geopolymer paste, mortar and concrete, Cement and Concrete Research, 40 (2010), 334-339
5D. L. Y. Kong, J. G. Sanjayan, K. Sagoe-Crentsil, Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures, Cement and Concrete Research, 37 (2007), 1583-1589
6	D. L.Y. Kong, J. G. Sanjayan, Damage behavior of geopolymer composites exposed to elevated temperatures, Cement and Concrete Composites, 30 (2008), 986-991
7	V. Sata, A. Sathonsaowaphak, P. Chindaprasirt, Resistance of lignite bottom ash geopolymer mortar to sulfate and sulfuric acid attack, Cement and Concrete Composites, 34 (2012), 700-708
8R. R. Lloyd, J. L. Provis, J. S. J. van Deventer, Acid resistance of inorganic polymer binders. 1. Corrosion rate, Materials and Structures, 45 (2012), 1-14
9	M. Steinerova, Mechanical properties of geopolymer mortars in relation to their porous structure, Ceramics -Silikaty, 55 (2011) 4, 362-372
10	S. Jumrat, B. Chatveera, P. Rattanadecho, Dielectric properties and temperature profile of fly ash-based geopolymer mortar, International communications in Heat and Mass Transfer, 38 (2011), 242-248
11	J. Temuujin, A. van Riessen, K. J. D. MacKenzie, Preparation and characterization of fly ash based geopolymer mortars, Construction and Building Materials, 24 (2010), 1906-1910
12	P. K. Sarker, R. Haque, K. V. Ramgolam, Fracture behaviour of heat cured fly ash based geopolymer concrete, Materials and Design, 44 (2013), 580-586
13	X. S. Shia, F. G. Collins, X. L. Zhaob, Q. Y. Wanga, Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete, Journal of Hazardous Materials, 237-238 (2012), 20-29
14	P. Duxson, J. L. Provis, G. C. Lukey, S. W. Mallicoat, W. M. Kriven, J. S. J. van Deventer, Understanding the relationship between geo-polymer composition, microstructure and mechanical properties, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 269 (2005), 47-58
15	H. Xu, J. S. J. van Deventer, Effect of Source Materials on Geopoly-merization, Industrial and Engineering Chemistry Research, 42 (2003), 1698-1706
16	J. Temuujin, A. van Riessen, R. Williams, Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes, Journal of Hazardous Materials, 167 (2009), 82-88
17	K. Dombrowski, A. Buchwald, M. Weil, The influence of calcium content on the structure and thermal performance of fly ash based geopolymers, Journal of Materials Science, 42 (2007), 3033-3043
18	D. Khale, R. Chaudhary, Mechanism of geopolymerization and factors influencing its development: a review, Journal of Materials Science, 42 (2007), 729-746
19	P. Duxson A. Fernández-Jiménez, J. L. Provis, G. C. Lukey, A. Palomo, J. S. J. van Deventer, Geopolymer technology: the current state of the art, Journal of Materials Science, 42 (2007), 2917-2933
20	L. Chao, S. Henghu, L. Longtu, A review: The comparison between alkali-activated slag (Si+Ca) and metakaolin (Si+Al) cements, Cement and Concrete Research, 40 (2010), 1341-1349