Arab J Sci Eng DOI 10.1007/s13369-014-1093-8
RESEARCH ARTICLE - CHEMICAL ENGINEERING
Effect of Sodium Silicate to Sodium Hydroxide Ratios on Strength and Microstructure of Fly Ash Geopolymer Binder M. S. Morsy · S. H. Alsayed · Y. Al-Salloum · T. Almusallam
Received: 9 April 2013 / Accepted: 31 May 2013 © King Fahd University of Petroleum and Minerals 2014
Abstract Geopolymerization can transform a wide range of waste aluminosilicate materials into building and mining materials with excellent chemical and physical properties. The present experimental study investigates the effect of sodium silicate/sodium hydroxide ratios on the feasibility of geopolymer synthesis at 80 ◦ C using fly ash. The sodium silicate/sodium hydroxide (S/N) ratios 0.5, 1.0, 1.5, 2.0 and 2.5 were studied. The result showed that the compressive and flexural strength increases as the curing age increases. Also, the compressive strength increases as the sodium silicate/sodium hydroxide ratio increases from 0.5 to 1.0 and then decreases. Morphology studies, conducted by SEM analysis of the geopolymer samples, indicated that geopolymers gel had the fly ash particles and pores embedded in a continuous matrix. At S/N = 1 a homogeneous and less porous microstructure was observed. Keywords Geopolymer · Microstructure · Compressive strength · Flexural strength
M. S. Morsy (B) · S. H. Alsayed · Y. Al-Salloum · T. Almusallam Department of Civil Engineering, Specialty Units for Safety and Preservation of Structures, King Saud University, Riyadh, Saudi Arabia e-mail:
[email protected] S. H. Alsayes e-mail:
[email protected] Y. Al-Salloum e-mail:
[email protected] T. Almusallam e-mail:
[email protected]
1 Introduction Ordinary Portland cement (OPC) is the most commonly used binder in the production of concrete. The production of OPC goes through energy-intensive processes, which release a large amount of greenhouse gases to the atmosphere [1]. Geopolymer concrete is emerging as a potential alternative to OPC-based cement concrete. The relative merits of geopolymer concrete are: (i) low heating temperature and thus low energy consumption; (ii) low CO2 emission making it a green material; (iii) rapid gain of strength; (iv) low permeability making it more durable; and (v) superior resistance to fire and acid attacks [2–9]. Geopolymers are thus considered as new generation materials for coatings and adhesives, a new binder for fiber composites and new cement for concrete. Moreover their superior properties, make the geopolymer a potential candidate for several industrial applications [6,9].
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Geopolymers provide the possibilities to prepare inorganic bonds using waste materials such as slag, silica fume, fly ash, kaolinite substances, etc. Geopolymers are inorganic polymeric materials with a chemical composition similar to zeolites but containing an amorphous structure and possessing ceramic-like structure and properties. The amorphous to semi-crystalline three-dimensional structure of silicate network consists of SiO4 and AlO4 tetrahedral, which is linked alternately by sharing all the oxygen to create polymeric Si– O–Al bonds [7]. The geopolymers contain negatively charged tetrahedral aluminum sites in the network which are charge-balanced by alkali metal cations such as sodium and/or potassium [6]. However, sodalite and hydroxysodalite, which are included in zeolite group, have been detected as reaction products in some of the metakaolinite’-fly ash geopolymer systems [10,11]. In the pioneering work on geopolymers, Davidovits [2,3, 12] used metakaolinite which was activated by alkali hydroxide and/or alkali silicate. Further studies [6,13–15] investigated the use of other aluminosilicate materials such as fly ash, furnace slag, kaoline, silica fume, and some natural minerals in the production of geopolymers with great success. Xu et al. [6] investigated geopolymerization of sixteen natural aluminosilicate minerals with the addition of kaolinite. The study demonstrated that a wide range of natural aluminosilicate minerals provide potential sources for synthesis of geopolymers. In the area of nanopores the geopolymer porosity is very similar without any regard to preparing conditions [16]. The scope of this work is to provide experimental data on the effect of sodium silicate/sodium hydroxide ratios on mechanical properties, microstructure and phase composition of low-calcium fly ash geopolymer.
2 Experimental 2.1 Materials Class F fly ash (FA) of Blaine surface area ≈ 3,800 and average diameter ≤ 10 µm was used in this investigation [6]. Figure 1 shows an SEM micrograph of raw fly ash particles. The mineral and chemical compositions of fly ash used in the investigation are shown in Fig. 2 and Table 1 respectively. It is mainly glassy with some crystalline inclusions of mullite, hematite and quartz. The oxide composition of fly ash is summarized in Table 1. The chemical analysis of the fly ash used as the starting material in this work showed it to be a high-silica ash with the mole ratio of SiO2 : Al2 O3 = 2.796. Commercial local red sand was used as fine aggregate in the mortar. cm2 /g
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Fig. 1 SEM micrograph of fly ash
Fig. 2 XRD of fly ash Table 1 Chemical composition of fly ash by XRF, mass % Oxide
FA
Mob/100g of fly ash
Composition CaO
4.29
0.0051
SiO2
52.87
0.8799
Al2 O3
33.08
0.3146
Fe2 O3
3.58
0.0224
MgO
0.9
0.0223
SO3
0.38
0.0447
Na2 O
0.3
0.0048
K2 O
0.79
0.0083
TiO2
1.89
0.0111
P2 O5
0.55
0.0038
MnO
0.05
0.0007
Total
98.68
–
1.32
–
Ignition loss
Sodium silicate (Na2 SiO3 ) mixed with sodium hydroxide (NaOH) as an alkaline activator has been used in this study. NaOH in pellet form with 97 % purity, and Na2 SiO3 which consists of Na2 O; 7.9 %, SiO2; 26.0 % and H2 O; 66.1 %,
Arab J Sci Eng Table 2 Alkaline activator, wt%
Mixes
Na2 SiO3 /NaOH
Ml
0.5
M2
1.0
M3
1.5
M4
2.0
M5
2.5
with weight ratio SiO2 /Na2 O; 3.2 and specific gravity 1.35 (at 20 ◦ C) were used in this study. 2.2 Mortar Preparation and Identification In the preparation of NaOH solution, NaOH pellets were dissolved in distilled water in a volumetric flask. The concentration of NaOH used in this investigation was 10 M. Alkaline activator with the combination of NaOH and Na2 SiO3 was prepared just before mixing with fly ash. The addition of sodium silicate was to enhance the process of geopolymerization [6]. The mass ratios of Na2 SiO3 /NaOH was 0.5, 1.0, 1.5, 2.0 and 2.5. The mass ratio of fly ash/alkaline activator was 2.5 [17]. The fly ash and alkaline activator were mixed together in the mixer until homogeneous paste was obtained. This mixing process can be handled within 5 min for each mixture with different Na2 SiO3 /NaOH. The fly ash geopolymer mortar was prepared using fly ash to sand ratio of 0.5. The total mass of each material used was kept constant for geopolymer mortar for all the five different types of samples with different Na2 SiO3 /NaOH ratio as shown in Table 2. The geopolymer mortars were molded into 50 mm cubes for compressive strength and 40 mm × 40 mm × 160 mm prisms for flexural strength tests. The geopolymer mortar samples were cured at 80 ◦ C for 24 h in electric furnace and then cured at room temperature until testing. The geopolymer mortar samples were tested at 3, 7, 28 and 60 days. The compressive and flexural strength tests were performed on cured specimens. At the dates of testing, three mortar cubes and prisms were tested for compressive and flexural strength determinations and the average value were recorded at each age. The crushed samples resulting from compressive strength tests were ground for X-ray diffraction studies (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM). The crystalline phases present in the geopolymer mortar were identified using the X-ray diffraction technique. Nickel-filtered Cu–Kα radiation at 40 KV and 20 mA was used throughout in a Philips PW 1390 diffractometer. Scanning speed of 2◦ /min. was used. The scanning electron microscope JWEL JSM 6360A was used for the identification of the changes occurred in the microstructure of the formed and/or the decomposed phases.
The analysis of infrared transmission spectra was done for the investigated geopolymer mortar by Jasco-6100 Fourier transform infrared spectrometer. The spectra were obtained in the absorption mode up to 4,000 cm−1 . 3 Results and Discussion The compressive strengths of low calcium fly ash geopolymer (FAG) mortars at the ages of 3, 7, 28 and 60 days are shown in Fig. 3. In general, the compressive strength of the fly ash geopolymer mortars increases with the increase in curing age. The 3 days compressive strengths of M1, M2, M3, M4 and M5 fly ash geopolymer mortars are 34.7, 61.6, 40.4, 40.5 and 22.3 MPa, respectively. It is observed from Fig. 3 that the compressive strength is a maximum when the ratio of Na2 SiO3 /NaOH (S/N) is 1.0 (i.e. M2). As the ratio of Na2 SiO3 /NaOH increases from 1 to 2.5 the compressive strength decreases. Whereas when the ratio increases from 0.5 to 1.0 there is significant increase in strength. The gain in M2 compressive strength at 3 days is 1.8 fold than M1 and also the gain in mixes M3 and M4 compressive strengths is 1.2 fold than mix M1. On the other hand the decline in compressive strength of M5 is 0.64 than M1. Basically, the initial curing at an elevated temperature (80 ◦ C) activates the geopolymerization processes. Moreover, at equal mass ratio of Na2 SiO3 /NaOH; the dissolution of silica and alumina was high leading to improvement in compressive strength of the fly ash geopolymer. Furthermore, the strength development of M2 is slow with curing age; but it is fast for M1, M3, M4 and M5. The slow development in strength is attributed to most of silica and alumina in fly ash are dissolved in the first contact with alkaline activator (S/N = 1) leading to accelerating geopolymerization process. This phenomena consume the silica and alumina in raw fly ash material. When a solid silicate is used, silicate species must first be dissolved from the solid source by reaction with alkali and water, lead-
Fig. 3 Compressive strength of low calcium fly ash geopolymer mortar versus curing age
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Fig. 4 Flexural strength of low calcium fly ash geopolymer mortar versus Na2 SiO3 /NaOH ratios at different curing age
ing to a more gradual release of silica to the geopolymer gel. This can potentially give greater control over the characteristics of the gel by controlling the rate of silicate release, which can be manipulated by altering the silicate solubility. Basically, the increase in the Na2 SiO3 /NaOH ratio was result in the increase of sodium content in the mixture. Sodium is important for the formation of geopolymers as it acts as charge balancing ions. However, the compressive strength decreases as more silicate is added into the system since excess sodium silicate hinders water evaporation and structure formation [16]. Figure 4 shows the flexural strength of low calcium fly ash geopolymer mortars at different ages versus curing age. The low calcium fly ash geopolymer mortars cured at 80 ◦ C for 24 h followed by curing at room temperatures till the testing ages. It is clear that, the flexural strength increases as the curing age increases, up to 60 days. Moreover, the flexural strength of fly ash geopolymer decreases as S/N ratios increases from 1.0 up to 2.5. Furthermore, the fly ash geopolymer formed at S/N = 1.0 (i.e M2) results in higher values of flexural strength at all curing ages. As the S/N ratios increases in the alkaline activation solution; the sodium and silica concentration increases. The presence of sodium during the geopolymer formation acts as charge balancing ions. The decrease of flexural strength as the S/N ratio increases is due to a more silicate in the system since excess sodium silicate hinders water evaporation and structure formation. It is observed from the XRD pattern of FA that the main crystalline phases are quartz (SiO2 ) and mullite (Al6 Si2 O13 ). The FA also consists of an X-ray amorphous aluminosilicate material which is, demonstrated by the broad hump for the range of 2 between 20 ◦ to 40 ◦ approximately. Figure 5 shows the XRD diffractograms obtained for M1, M2 and M5 geopolymer mortar. It can be seen from Fig. 6 that no significant change was observed in the intensity for
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Fig. 5 XRD pattern of low calcium fly ash geopolymer Mortar. Q quartz, M mullite, H hematite
Fig. 6 FT-IR spectra of low calcium fly ash geopolymer mortar
the quartz or mullite peaks. After geopolymerization process, as illustrated in Fig. 5, the resulting geopolymer has amorphous phases. The XRD pattern of the geopolymer is differ-
Arab J Sci Eng Fig. 7 SEM micrograph of low calcium fly ash geopolymer mortar: a M1, b, c M2 and d M5
ent from that of the original fly ash. As S/N ratio increases from 0.5 to 1.0, the sharp peaks, which are characteristics for
hematite, mullite and other identified minerals have disappeared. The ill-crystal and amorphous phases of the formed
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geopolymer gel are shown in Fig. 6a, b. This indicates the dissolution of fly ash glass phases and the formation of amorphous structure in the geopolymer. The sharp peak of quartz observed in M1 and M2 XRD micrograms is due to sand in mortar sample. Moreover, at S/N ratios 2.5 (i.e. M5); the crystalline peaks in microgram is due to low solubility of fly ash minerals at this concentration. Most of the crystalline phase of raw fly ash disappeared and the amorphous phase, which is characteristic of a geopolymer, is apparent in M1 and M2. This indicates that the crystalline aluminum silicate has been geopolymerized to amorphous geopolymer during synthesis. Figure 6 shows the IR spectra of fly ash and geopolymer products synthesized using sodium silicate solution and NaOH. Transformation taken place during the synthesis is indicated by the different transmittance frequencies of fly ash and the synthesized geopolymers [18]. The network of silico-aluminate-based geopolymers consists of SiO4 and AlO4 tetrahedral linked alternately by sharing all the oxygen [19]. The IR spectrum of fly ash shows main transmission bands at 1,103, 1,456, 2,360, and 3,748 cm−1 . The broad component at 1,004 cm−1 is due to the Si–O–Si and Al–O–Si asymmetric stretching vibration and it becomes sharper and shifts towards lower frequencies of 995, 979 and 975 cm−1 as S/N ratios increases from 0.5 to 2.5 in formed FA geopolymer. The peak around 1,113 cm−1 is attributed to Si–O vibration in SiO4 molecules, which vanished after geopolymerization reaction. Transmittance at 995 and 790 cm−1 are assigned as Al–OH and Al–O, respectively. A shift of the asymmetric bending of the bonds O–Si–O and O–Al–O to lower frequencies can be observed, which is in accordance to previous research [20]. The main band analyzed in IR spectrum of geopolymer is in the region of 900–1,300 cm−1 . The peak at 1,103 cm−1 (most intense for FA) is indicative of silicate stretching and it shifts after reaction. Following the hydrolysation of the raw materials, a structural reorganization occurs in which aluminium ions are incorporated into the SiO4 tetrahedra, forming the Si–O–Al network. The aluminium acts as a perturbation of silicate stretching vibrations as metal cations in other silicates do, such as sodium. The extent of the peak shift has been correlated with the amount of aluminium incorporated into the silicate structure, when the alkali content is kept constant. Furthermore, the addition of alkali, forming non-bridging oxygen of the form Si–O–Na, causes a lowering of the molecular vibration force constant and therefore shifting of the peak associated with the asymmetric stretching of Si–O–Si and Si–O–Al bond to lower wave numbers. The fracture surface morphology of the low calcium fly ash geopolymer mortar is shown in Fig. 7. The sample was examined using secondary beam and EDX modes of the SEM. The micrograph shows that the sample was not homo-
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geneous. Fly ash particles and pores embedded in a continuous matrix were identified. In addition, the elements namely sodium (Na), silicon (Si), aluminum (Al) and magnesium (Mg) shown on the map were not visible at places where especially large fly ash particles were located. For the geopolymer matrix, Na, Si and Al elements were distributed in a similar manner. The products were, therefore, the result of bonding formations between Na, Si and Al atoms. The relationship between the Si/Al ratio for geopolymer mortar M1, M2 and M5 (S/N = 0.5, 1 and 2.5) is shown in Fig. 7. It is clear that, the Si/Al ratio decreases as the S/N ratio increases. This indicates that leaching of Si and Al was dependent on S/N ratio. At low S/N ratio, leaching of Si was higher than of Al, and subsequent reactions resulted in a geopolymer mortar with an average Si/Al ratio of 1.54. At higher ratio (S/N = 2.5), rates of Al leaching were improved, resulting in geopolymer mortar with lower Si/Al ratios [21]. 4 Conclusions Based on the obtained data, the following conclusion can be summarized. • •
• • • •
The compressive strength of low calcium fly ash geopolymer cured at 80 ◦ C increases as the curing age increases up to 60 days. The S/N ratio of alkali activator was highly affecting the strength of low calcium fly ash geopolymer cured at 80 ◦ C. The highest strength was achieved using S/N = 1.0. The increase of S/N ratio beyond 1.0 decreases the compressive strength of fly ash geopolymer at all curing ages. SEM, EDX, XRD and FTIR studies showed that S/Nactivated fly ash geopolymerization occurred at 80 ◦ . The morphology of fly ash geopolymer gel contain fly ash particles and pores embedded in a continuous matrix. Fly ash geopolymer gel formed using alkaline activator (S/N = 1) had a homogeneous and less porous microstructures.
Acknowledgments The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the research group project No. RGP-VPP-310.Thanks are also extended to the MMB Chair for Research and Studies in Strengthening and Rehabilitation of Structures, at the Department of Civil Engineering, King Saud University for providing some technical support.
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