The Effect of Sodium Silicate and Sodium Hydroxide on the Strength of
Aggregates Made from Coal Fly Ash using the Geopolymerisation Method
Hamzah Fansuria,b1, Didik Prasetyokoa, Zezhi Zhangb and Dong-ke Zhangb aDepartment of Chemistry, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS
Sukolilo, Surabaya 60111 – Indonesia bCentre for Petroleum, Fuels and Energy, University of Western Australia, 35 Stirling
Highway, Crawley WA 6009, Australia
Abstract
Geopolymerisation of coal fly ash to produce synthetic aggregates as a potential means of utilising coal combustion by-product has been investigated. It has been revealed that the geopolymerisation strongly depends on the physico-chemical properties of the fly ash, the availability of soluble silicates and aluminates, and the concentration of added sodium hydroxide. The presence of sodium hydroxide increases the amount of soluble silicates and aluminates in the mixture through fly ash solubilisation. Solubility tests on various fly ash samples have shown that the solubility increases as the concentration of sodium hydroxide in the fly ash increases, which also increases the strength of the resulting geopolymer aggregates. The compression strength of the geopolymer aggregates also increases to a maximum before decreasing again as the amount of sodium silicate is increased.
1 Corresponding author: phone: 06287861228242 or 062315992090; fax:
062315928314; Email address: [email protected]
1 Keywords: fly ash, geopolymerisation, silicates, aluminates
1. Introduction
Utilisation of coal combustion product (CCP) or coal ash from coal-fired power stations is of great interest. Each year a huge amount of coal ash is produced.
According to the Ash Development Association of Australia (ADAA), in 2005, 13 Mt
(million tonnes) of coal ash was produced from coal-fired utilities in Australia and
New Zealand [1] and this figure is increasing year by year. The estimated worldwide annual coal ash production is around 600 million tonnes, with fly ash constituting about 500 million tonnes [2]
One of the desirable coal ash utilisation approaches is to convert fly ash into aggregates as replacements for natural aggregates. The reasons favouring this approach are: (1) fly ash constitutes up to 90% of the total ash produced, (2) the natural aggregate resource is depleting, and (3) demand for aggregates is large and increasing continuously [3]. Aggregate may account for 7080% by mass of concrete.
Aggregates can also be used as soil conditioners, water savers, and soil and sand stabilisers. Therefore, the successful and economical manufacture of aggregates from fly ash will not only reduce the impact of fly ash disposal on the environment but also provide a great benefit to the economy.
There are five common ways of converting fly ash into aggregate, namely high- temperature sintering, adhesive material binding, cementitious reaction, high-pressure
2 mechanical compression, and geopolymerisation [4]. Geopolymers are rapid-setting binders and there are claims [57] that geopolymer binders have the potential to replace ordinary Portland cement in construction materials as low-CO2 cements for a sustainable future.
Fly ash geopolymerisation has been reported widely in the literature since 1990. In
1998, van Jaarsveld et al. [8] reported the utilisation of fly ash geopolymer to immobilize heavy metals. Since then, van Deventer’s group has published many reports on ash geopolymerisation [911].
To make geopolymer from fly ash, scientists normally refer to the preparation of geopolymer from soluble reactants such as soluble silicate and aluminates, as well as from kaolinite. Factors that control geopolymerisation from these soluble reactants have been reasonably understood, some of which are related to the metal oxide ratios of the soluble reactants such as SiO2/M2O, SiO2/Al2O3, M2O/H2O, and M2O/Al2O3, where M is either sodium or potassium. The best geopolymer can be made if these ratios fall within the limits defined by Eq. 1 [12, 13]:
0.2 < M2O/SiO2 < 0.48
3.3 < SiO2/Al2O3 < 4.5 Eq. 1
10 < H2O/M2O < 25
When the formula is applied to a broad range of fly ash geopolymerisations, it gives products with variable strengths and no clear relationship has yet been found between the compositions and the properties of the geopolymer products. Some researchers
3 [14, 15] have since discovered that these molar oxide ratios are just an indication of the approximate composition and are not very critical, particularly when dealing with
Si-Al minerals from waste materials such as fly ash. This is the case because, although these molar ratios are based on chemical analyses, it is highly unlikely that all of the silica and alumina will actually take part in the synthesis reaction. An extensive geopolymerisation study over a wide range of fly ash compositions, using the oxide molar ratios in Eq. 1, revealed that the fly ash geopolymerisation depends more strongly on the internal properties of the fly ash itself [16] rather than the oxide molar ratio.
In a recent publication, Chindaprasirt et al. [17] related the workability and strength of geopolymer from a mixture of sand and high-calcium (class C) fly ash to the sodium silicate and sodium hydroxide concentrations in the geopolymer gel. They found that the workability of the gel and the geopolymer strength were dependent on the amounts of sodium silicate and sodium hydroxide present in the mixture.
The relationship between the effects of sodium silicate and sodium hydroxide and the strength of fly ash geopolymer can be rationalised in terms of the mechanism of geopolymer formation proposed by Provis and Deventer [18], based on the aluminosilicate weathering model of Faimon [19]. The geopolymerisation is initiated by the dissolution of silicate and aluminate monomers from the source materials
(here, fly ash), which occurs at high alkaline concentration [20]. These soluble monomers then polymerise into the long-chain geopolymer.
4 The dissolution of silicates and aluminates from fly ash is not as simple as the dissolution of these materials from kaolinite as in the Provis and Deventer model [18] since kaolinite is very close to a single phase and is less heterogeneous than fly ash.
As a result, the silicate and aluminate composition in a geopolymer gel from fly ash cannot be determined merely from the chemical composition of the fly ash and other reactants. It is therefore not possible to use oxide molar ratios as in Eq. 1 as guidance for making a fly ash geopolymer because the ratio depends strongly on the typical physico-chemical properties of the fly ash.
In order to understand the relationship between the physico-chemical properties and fly ash geopolymerisation, researchers at the Centre for Fuels and Energy have investigated geopolymerisation for a wide range of type F fly ashes under various reaction conditions [16]. In this paper, we report the roles played by sodium silicate and sodium hydroxide solution in fly ash geopolymerisation in relation to the physico- chemical properties of the ash.
2. Experimental
2.1. Materials.
Fly ash samples were collected from several power stations in Australia. The chemical composition of the ash is given in Table 1.
Table 1. Chemical composition (in wt%) of fly ashes by the XRF method
Components FA-A FA-B FA-C FA-D FA-E
SiO2 69.60 67.20 66.10 55.70 52.30
Al2O3 24.40 26.40 30.70 26.60 32.40
5 Fe2O3 1.80 1.40 0.54 10.80 11.00 CaO 0.29 0.35 0.06 1.10 1.00 MgO 0.30 0.30 0.13 0.65 0.80
Na2O 0.22 0.29 <0.05 0.23 0.10
K2O 2.50 2.90 0.28 0.47 0.22
TiO2 1.00 1.10 2.10 1.60 2.10
Mn2O3 0.03 0.02 <0.02 0.03 0.20
SO3 <0.02 0.03 <0.02 0.03 <0.02
P2O5 0.09 0.08 0.07 1.40 0.07 BaO 0.05 0.06 0.04 0.32 0.04 SrO 0.03 0.03 <0.02 0.33 <0.02 ZnO <0.02 <0.02 0.08 0.05 <0.02
V2O5 0.02 0.03 0.05 0.03 0.02
XRD analyses, together with the use of the Rietveld refinement method, were used to quantify the mineral phases in the fly ashes. The ashes consisted mainly of an amorphous phase along with quartz and mullite. Some of them also contained hervynite and mahgemite. The phase composition determined by quantitative XRD analysis is shown in Table 2.
Table 2. Phase compositions (in wt %) of fly ashes
Mineral/Phase FA-A FA-B FA-C FA-D FA-E Quartz 6.4 13.8 6.8 9.9 5.7 Mullite 10.1 21 23.3 25.9 25.3 Hercynite - 0.6 0.2 0.1 0.3 Mahgemite - - - 3.9 4.1 Amorphous 83.5 64.6 69.7 60.2 64.6 Total 100 100 100 100 100
Sodium silicate solution was obtained from BDH Chemicals. The SiO2/Na2O ratio,
1 SiO2 concentration, and density were 3.5, 6.03 M, and 1.35 ± 0.07 g mL , respectively. Laboratory grade NaOH and distilled water were used in all geopolymerisation experiments.
6 2.2. Silicon and aluminium leaching tests
Silicon and aluminium leaching tests were carried out using a solid-to-liquid ratio of
1:5. As the alkaline solution, various concentrations of NaOH were used, specifically
0.1, 0.5, 1, 2, and 4 mol L1. To leach the silicon and aluminium, the fly ash was stirred in the alkaline solution for 24 hours at room temperature. The leached silicon and aluminium were analysed by means of atomic absorption spectrometry (AAS).
2.3. Geopolymer preparation and compressive strength tests
All geopolymers were prepared on the basis of 300 g samples of fly ash. The mixtures contained fly ash, sodium silicate solution, sodium hydroxide (NaOH), and distilled water. A certain amount of water was added to each mixture to produce a workable geopolymer gel. In the mixtures, both fly ash and sodium hydroxide are regarded as solids, while additional water and sodium silicate solution are regarded as liquids.
The amount of sodium silicate added during geopolymer preparation was varied from
45 to 120 g. The amounts of sodium hydroxide and water added were also varied, as indicated in Table 3. In the table, the SiO2/Na2O ratio indicates the ratio in the liquid phase, i.e. in the solution containing sodium silicate, sodium hydroxide, and water.
The concentrations of silicate and alkali metal ions in the fly ash were considered constant. Therefore, their influence on the total SiO2/Na2O ratio was assumed to be the same in each experiment.
7 Table 3. An example of variation in the amounts of sodium hydroxide and water added to the geopolymer matrix in preparations with 45 g of added sodium silicate
NaOH H2O Code SiO2/ Na2O mol g g A-1 3.36 0.00 0.00 0.00 A-2 3 0.01 0.55 0.08 A-3 2.5 0.04 1.59 0.24 A-4 2 0.08 3.15 0.47 A-5 1.5 0.14 5.75 0.86 A-6 1 0.27 10.95 1.64 B-1 3.36 0.00 0.00 30.75 B-2 3 0.01 0.55 30.75 B-3 2.5 0.04 1.59 30.75 B-4 2 0.08 3.15 30.75 B-5 1.5 0.14 5.75 30.75 B-6 1 0.27 10.95 30.75 C-1 3.36 0.00 0.00 69.75 C-2 3 0.01 0.55 69.75 C-3 2.5 0.04 1.59 69.75 C-4 2 0.08 3.15 69.75 C-5 1.5 0.14 5.75 69.75 C-6 1 0.27 10.95 69.75 D-1 3.36 0.00 0.00 69.75 D-2 3 0.01 0.55 69.75 D-3 2.5 0.04 1.59 69.75 D-4 2 0.08 3.15 69.75 D-5 1.5 0.14 5.75 69.75 D-6 1 0.27 10.95 69.75 E-1 3.36 0.00 0.00 39.75 E-2 3 0.01 0.55 39.75 E-3 2.5 0.04 1.59 39.75 E-4 2 0.08 3.15 39.75 E-5 1.5 0.14 5.75 39.75 E-6 1 0.27 10.95 39.75
The combined materials were mixed in a mechanical mixer for 10 minutes to form a geopolymer gel. The thus formed gel was then moulded in a cylindrical plastic mould with 2:1 height to diameter ratio and left in an oven at 60 C for 4 days to cure the geopolymer into aggregate pellets. Following the curing process, the formed aggregates were left at room temperature and humidity for a further 24 days for aging prior to carrying out the compressive strength tests. The total time for curing and aging thus amounted to 28 days.
8 The strengths of the geopolymer aggregates were tested using an INSTRON 1196 universal testing machine operating at a speed of 2 mm min1. The strength of the aggregate pellets is reported in terms of the maximum load needed to break the aggregate.
3. Results and Discussion
3.1. Variation in the liquid-to-solid (L/S) ratio and strength of FA (fly ash)- geopolymers.
Each fly ash used in this research was unique, especially in its ability to adsorb water and interact with liquid reactant, which affected the workability of the resulting geopolymer gel (here it is discussed as liquid-to-solid (L/S) ratio). Fly ashes have very narrow ranges of L/S ratio. Fly ash A, for example, can only be used to prepare a geopolymer at an L/S ratio of 0.15, not more or less. In contrast, fly ash B to E adsorb much more water than fly ash and, therefore, more water need to be added. Hence, the
L/S ratios were much higher than 0.15. In attempting to prepare geopolymers outside of this L/S range, their workability was either very poor or the paste became too thin.
In such cases, the geopolymer aggregates were ultimately very weak. The suitable L/S ratio ranges of fly ashes are presented in Table 4.
Table 4. Liquid-to-solid (L/S) ratio ranges of fly ash for preparing geopolymer gel
Fly ash A B C D E L/S (mass ratio) ranges 0.15 0.250.27 0.370.47 0.370.47 0.270.32
9 Geopolymers with lower L/S ratio are normally stronger than those prepared with higher L/S [2123], as is also the case upon the hydration of ordinary Portland cement
(OPC). Analogous results were also seen with fly ash geopolymer, as indicated in
Figure 1. The strength of the geopolymer aggregates increases with decreasing L/S and aggregates prepared from fly ash A with the lowest L/S ratio were much stronger than the others. Water provides a medium for the soluble silicates and aluminates to polymerise. The polymerisation is actually a condensation reaction that produces water molecules. An excessive amount of water will reduce the condensation rate as it modifies the equilibrium state of the reaction. Furthermore, excess water causes segregation in the geopolymer mixture.
At this stage, it was not clear as to why fly ash A geopolymer was stronger than the other products. The strengths of geopolymers FA-B and –E, which have the same L/S ratio, were found to be similar and therefore it was concluded that the strength is controlled by the L/S ratio. For geopolymers FA-C and –D, however, which also have the same L/S ratio, the strengths were found to be different. Geopolymer FA-C was apparently stronger than geopolymer FA-D. This indicates that there must be other factors besides the L/S ratio that affect the strength, which may also affect the strengths of other FA geopolymers.
Chindaprasirt et al. [17] reported that the sodium hydroxide concentration also affects the strength of the resulting geopolymer. The same trend can also be seen in Figure 1, which shows an increase in geopolymer strength as a result of an increase in sodium hydroxide concentration. It can be concluded that the increase in the FA geopolymer
10 strength depends on the availability of soluble silicates and aluminates, which are more soluble in solutions with higher alkaline concentrations.
Further evidence of the effect of increasing sodium hydroxide concentration on the geopolymer strength is provided by the SEM images in Figure 2. With no additional
NaOH, the fly ash particles are bonded by a matrix of geopolymer binder and they show no sign of dissolution. As the concentration of NaOH is increased, the dissolution becomes more evident. At the highest added NaOH concentration, most of the fly ash particles no longer exist as individual particles, with only a few of them remaining. Instead, they become a blend of soluble ash particles. Figure 2b clearly shows the effect of solubilisation of the ash particles at the highest added sodium hydroxide concentration.
3.2. The solubility of silicates and aluminates from fly ashes in alkaline solution
The solubilities of the silicates and aluminates from all of the fly ashes used in this research are shown in Figure 3. It can be seen that the solubility of the silicates and aluminates increases with increasing sodium hydroxide concentration, resulting in stronger fly ash geopolymer. For example, if the curve in Figure 1 were extrapolated to an SiO2/Na2O ratio of 0.5, the compressive strength of the FA-A geopolymer would reach up to 70 MPa. Geopolymers made from fly ashes B, C, and E would have strengths around 20 MPa, and that made from fly ash D would have a strength of about 6 MPa. It is, however, impossible to obtain such materials since sodium hydroxide or silicate would start to precipitate at the required composition under the conditions used in the experiments.
11 3.3. The effect of sodium silicate and sodium hydroxide solution on geopolymer strength
Figure 4 shows the effect of increasing added sodium silicate concentration on the fly ash geopolymer. Not all of the fly ashes have been included in the figure since only three of them, namely fly ashes C, D, and E can be treated over a wide range of S/L ratios. The figure shows an increase in the compressive strength of the fly ash geopolymer as more sodium silicate was incorporated into the geopolymer preparation. The increase in compressive strength of the FA-C geopolymer was less pronounced than the increases seen for FA-D and –E. Fly ash E proved to be the most sensitive raw material to the increase in sodium silicate concentration and the FA-E geopolymer was the strongest one.
An interesting result was shown by fly ash C geopolymer upon increasing the amount of added sodium silicate solution. The strength of the geopolymer was expected to increase with increasing sodium silicate concentration, as shown by the dashed purple line in Figure 4a. However, it can be seen in the figure that the strength diminished when the highest sodium silicate concentration was added to make the FA-C geopolymer. The figure seemingly shows a saturation trend with respect to the sodium silicate concentration, which shows a limitation when making FA-C geopolymer aggregate.
The saturation process may be explained schematically, as illustrated in Figure 5.
When less sodium silicate is available in the mixture, empty spaces between the
12 particles in the geopolymer matrix will result (a). These particles are fly ash particles that were not soluble during the geopolymerisation process. At optimum sodium silicate addition, these gaps are filled (b). The geopolymer has its ideal composition when the geopolymer gel acts as a binder of the insoluble ash particles and fills the gaps between them. The strength of the geopolymer stems mainly from the strength of the particles. When more sodium silicate is available, most of the space is occupied by the geopolymer binder and the particles merely act as fillers (c). The geopolymer matrix becomes the main contributor to the strength of the aggregate.
Under the conditions depicted in Figure 5c, the strength of the geopolymer aggregate depends on the strength of geopolymer matrix. The matrix consists mainly of solid sodium silicate, which has a strength of 9.8 MPa [24]. When the geopolymer is in a state as depicted in Figure 5b, bonded fly ash particles contribute to the strength of the aggregate. The particles consist mainly of mullite and/or quartz, which are stronger than sodium silicate (1310 and 1110 MPa, respectively). On the other hand, a lack of particle binding, as shown in Figure 5a, results in weakness of the aggregate. Thus, it can be seen why geopolymer aggregate is weak at low added sodium silicate concentration, reaches a maximum, and then becomes weaker when there is too much silicate.
4. Conclusions
The geopolymerisation strongly depends on the physico-chemical properties of the fly ash, the availability of soluble silicates and aluminates, and the concentration of added
13 sodium hydroxide, which is able to increase the amounts of soluble silicates and aluminates in the mixture through fly ash solubilisation.
The amount of water required for fly ash geopolymerisation strongly depends on the nature of the fly ash. There are wide variations in the amounts of water required for the various fly ash sources. However, for each fly ash, the less water added, the stronger the geopolymer product. This is consistent with the effect of water on ordinary Portland cements.
The amounts of sodium silicate and sodium hydroxide that can be added are limited by the intrinsic properties of the fly ash and the workability of the geopolymer mixture. The strength of the geopolymer increases with increasing amounts of sodium hydroxide added. This increase is related to the dissolution of silicates and aluminates from the fly ash. Solubility tests on various fly ashes have shown that the solubility of silicates and aluminates from fly ashes increases with increasing sodium hydroxide concentration and at the same time the strength of the resulting geopolymer also increases.
The strength of the geopolymer also increases with increasing amount of sodium silicate solution added, up to a certain limit. After this point, further addition of sodium silicate solution reduces the strength of the geopolymer.
5. Acknowledgement
14 This research has been funded by the CRC on Coal in Sustainable Development
(CCSD) and partly funded by the Indonesian State Ministry of Research and
Technology under the Insentif Riset Terapan program on fly ash leaching tests. The writing of this manuscript was funded by the Indonesian Ministry of Education under the 2009 PAR-B project. H. Fansuri wishes to thank Abdul Hakim, who carried out most of the leaching tests in the Inorganic Laboratory, Department of Chemistry,
Institut of Teknologi Sepuluh Nopember (ITS) Surabaya.
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