Effect of Alkali Metals on the Preferential Geopolymerization of

However, it should be noted that the distinctive structures for geopolymeric materials and ...... Barrer, R. M. Hydrothermal Chemistry of Zeolites; Ac...
0 downloads 0 Views 96KB Size
Download PDF
Ind. Eng. Chem. Res. 2001, 40, 3749-3756

3749

Effect of Alkali Metals on the Preferential Geopolymerization of Stilbite/Kaolinite Mixtures H. Xu, J. S. J. van Deventer,* and G. C. Lukey Department of Chemical Engineering, The University of Melbourne, Victoria, Australia 3010

Stilbite is used as the main aluminosilicate oxide source to determine the effect of various factors on the extent of geopolymerization of stilbite/kaolinite mixtures. Increasing the M2O/H2O ratio (where M ) Na and/or K) results in an increase in the dissolution of aluminum and silicate species from stilbite and kaolinite, which therefore leads to an improvement in the compressive strength of the geopolymer. The SiO2/M2O ratio affects significantly the degree to which polymerization between Al(OH)4- and Si complexes can occur. The results also show that Na+ compared with K+ enhances the dissolution of aluminum and silicate species. However, K+ improves the compressive strength of the geopolymer compared with Na+. It has been established for the first time that kaolinite and stilbite react differently with different alkali metals. The successful geopolymerization of stilbite/kaolinite mixtures can occur only by homogeneously dispersing condensed Na2SiO3 in a MOH solution prior to adding solids. Introduction A geopolymer is a three-dimensional aluminosilicate mineral polymer that contains a variety of amorphous to semicrystalline phases. A geopolymeric product is formed by chemical dissolution and subsequent recondensation of various aluminosilicate oxides (Al3+ in IVVI-fold coordination) and silicates in a MOH solution (where M ) Na and/or K). A geopolymeric reaction is best described as1

The term “geopolymer” was proposed and first used by Joseph Davidovits in 1978,1 and over the past 20 years, Davidovits has conducted extensive research on the geopolymerization process. In particular, Davidovits has improved the chemical and mechanical characteristics of geopolymeric products and has identified a variety of industrial applications for these products.2,3 Davidovits primarily used kaolinite and calcined kaolinite (metakaolin) as the source of aluminosilicate oxides. Helferich and Shook4 and Neuschaeffer et al.5 published separate patents on the synthesis of amorphous aluminosilicate polymers. The aluminosilicate polymers described in these patents are similar in chemical and mechanical properties as well as in preparation procedures to the geopolymer materials proposed by Davidovits.2,3 In 1980, Mahler6 patented his work on the production of poly(aluminosilicates) using an aqueous alkali-metal aluminate and silicic acid as reactants, instead of solid aluminosilicate oxides. By using calcined kaolinite with the addition of sand as a reinforcing component, Palomo et al.7 produced a geopolymer having a compressive strength of 84.3 MPa after a setting time of only 24 h. During the late 1990s, Van Jaarsveld et al.8,9 and Van Jaarsveld and Van Deventer10 conducted research on geopolymers derived from * To whom correspondence should be addressed. Tel: +613-8344-6620. Fax: +61-3-8344-4153. E-mail: jannie@ unimelb.edu.au.

waste materials such as fly ash. The focus of this work was on the industrial use of fly ash based geopolymeric materials in a wide range of applications, including the immobilization of toxic metals. Many aluminosilicate oxide sources including kaolinite, calcined kaolinite, blast furnace slag, industrial construction wastes, and also different types of fly ash have been investigated previously for their ability to undergo geopolymerization. However, natural aluminosilicate minerals, which are the largest source of aluminosilicate oxides in the world, were not investigated until 1998. Xu and Van Deventer11 studied the ability of 16 naturally occurring aluminosilicate minerals to undergo geopolymerization. Each of these minerals has a different crystal structure, chemical composition, density, hardness, and degree of paragenesis. It was found by Xu and Van Deventer11 that various natural aluminosilicate minerals behaved significantly differently because of their individual properties. In particular, it was observed that stilbite, a natural zeolite mineral, possessed a high reactivity in geopolymeric reactions. Zeolites are the largest group of minerals among the natural silicates.12 According to Davidovits,1 geopolymerization can be viewed as the analogue of zeolite synthesis, and therefore the geopolymerization reaction would generally involve similar chemical and physical conditions. However, it should be noted that the distinctive structures for geopolymeric materials and zeolites are amorphous and crystalline, respectively. The mechanisms involved in the synthesis of zeolites are well understood and, therefore, may be used to aid in the understanding of the phenomena observed in geopolymerization. The present work investigates the geopolymerization of stilbite/kaolinite mixtures for the purpose of determining the factors that dominate the geopolymeric reaction and thereby affect the compressive strength of geopolymers. In particular, the effect of different alkali metals on the reactivity and therefore the preferential geopolymerization of different minerals is studied. The related chemical mechanisms involved in the geopolymerization process are also investigated. It is important to note that any combination of minerals could have

10.1021/ie010042b CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001

3750

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001

been studied, but stilbite and kaolinite were chosen because of their ability to undergo geopolymerization readily. The results obtained in the present work will have a significant impact on the current understanding of other geopolymeric systems containing different reactive components. Experimental Methods Materials. Stilbite obtained from Geological Specimen Supplies, Australia, was ground to -80 µm. Kaolinite, grade HR1/F, was purchased from Commercial Minerals, Australia, and had a particle size of 50% less than 0.5 µm and 1% greater than 38 µm. The compositions of stilbite and kaolinite were determined by X-ray fluorescence (XRF) analysis, and the results are presented in Table 1. A sodium silicate solution (Vitrosol N40) was supplied by PQ Australia, and it had a composition of 8.9 wt % Na2O and 28.7 wt % SiO2. Distilled water and analytical-grade NaOH and KOH were used throughout all experiments. Geopolymerization. A total of 15 g of stilbite was dry mixed with 7.5 g of kaolinite for 10 min before the addition of Na2SiO3 and a MOH solution (where M ) Na and/or K). The subsequent mixture was mixed by hand for a further 5 min. The resulting slurry was transferred to a steel mould (20 × 20 × 20 mm) and placed in an oven at 35 °C for 24 h. The sample was then removed from the mould and kept in an oven for another 2 days at 35 °C. The compressive strength of each sample was tested using a Tinus Tolsen compressive strength testing machine. Three samples of each geopolymer mixture were tested, with average values listed in Table 2. Samples 4-6 and 12-14 were prepared in the same manner as those described above, except with an additional 3 days of hardening at 35 °C in the oven. Samples 7, 8, 15, and 16 were prepared by mixing 70 g of stilbite and 35 g of kaolinite for 10 min by hand followed by 10 min using a Fritsch vibratory shaker. MOH and Na2SiO3 solutions were then added to the dry mixture. The resulting paste was shaken for another 10 min before the paste was poured into a steel cylinder mould (27 × 54 mm) and set under a specified pressure for 30 min at ambient temperature. The demoulded sample was placed in an oven for further setting and hardening at 35 °C for 3 days. It was observed that, after the pressured setting, the height of samples 7, 8, 15, and 16 was less than 54 mm. A maximum decrease in height of 15 mm was observed for sample 16. Consequently, the height/diameter ratio for these four samples lay between 1.4 and 1.9, instead of the standardized 2.0. This means that the compressive strength values reported in Table 2 for these four samples are slightly higher than the true values. For the M2O/H2O and SiO2/ M2O molar ratios specified in the present investigation, M2O is contributed by both the MOH solution and the Na2SiO3 solution, with SiO2 and H2O being counted only from the Na2SiO3 and MOH solutions, respectively. Small samples have been studied because homogeneously mixed natural stilbite was available only in limited supply. These small samples are below the minimum required in standard testing specifications, so the obtained compressive strength values (MPa) should not be interpreted in absolute but rather in relative terms. Kaolinite is an inexpensive aluminosilicate material, and it has been used in most previous studies on

Table 1. Composition of Kaolinite and Stilbite As Determined by XRF Analysis (mass %) element as oxide

kaolinite HR1/F

stilbite

SiO2 Al2O3 MgO Fe2O3 CaO K2O Na2O TiO2 loss on ignition

54.5 29.4 0.2 1.4 0.2 0.2 0.2 2.8 11

59.2 14.8 0.07 0.23 7.65 0.03 0.18 0.03 15.87

geopolymerization1,8-11 as a secondary source of soluble Si and Al in addition to waste or natural aluminosilicates. In the present study, kaolinite is added during geopolymerization so that the desired initial gel composition can be obtained. This ensures that comparisons between the geopolymerization of stilbite and the previously published results are possible. Leaching Tests. A specified mass of stilbite or kaolinite ((0.01 g) was mixed with 10 mL of a MOH solution at room temperature for 18 h using a magnetic stirrer. After centrifuging, the clear liquid part of the solution was diluted to 0.2 M alkaline concentration and neutralized by condensed HCl. A Perkin-Elmer Optima 3000 ICP-OES was used to analyze for concentrations of silicon and aluminum leached in the solution. Scandium was used as an internal standard. The leaching results of stilbite and kaolinite for three alkaline concentrations 2, 5, and 10 M (these correspond to the M2O/H2O ratios of 0.018, 0.045, and 0.09, respectively) are presented in Table 3 and Figures 1-4. X-ray Diffraction and MAS NMR Measurement. X-ray powder diffraction (XRD) patterns [from 5 to 70° (2θ)] were recorded on a Philips PW 1800 diffractometer using Cu KR as a radiation source and a scanning rate of 2°/min. 29Si and 27Al MAS NMR spectra were obtained on a Varian 300/solid-state spectrometer equipped with a Doty MAS probe. Results and Discussion Effect of the M2O/H2O Ratio (Where M ) Na and/ or K) on Compressive Strength. Although not clearly stated in the published literature,1-3,5-11 the geopolymerization process involves four main steps: (a) the dissolution of solid aluminosilicate oxides in a MOH solution; (b) the diffusion of dissolved Al and Si complexes from particle surfaces to the interparticle space; (c) the formation of a gel phase resulting from the polymerization between an added silicate solution and Al and Si complexes; and (d) hardening of the gel phase by the exclusion of spare water to form a monolithic geopolymeric product. Because of the difficulties that exist in practical experimental techniques, it is difficult to distinguish between the dissolved Al and Si complexes, the gel phase, and the spare water. As a consequence, a quantitative research investigation on the geopolymerization process can only be conducted on the dissolution step. The effect of increasing the M2O/ H2O ratio on both the leaching of Al and Si species and also the extent of geopolymerization has been investigated in the current work in order to elucidate the relationship between the dissolution of stilbite and kaolinite and the final compressive strength of the geopolymer. The three M2O/H2O ratios that have been investigated are 0.018, 0.054, and 0.09.

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3751 Table 2. Compressive Strength of Geopolymers Formed by Stilbite sample no.

Na2O/H2O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0.035 0.062 0.107 0.035 0.062 0.107 0.107 0.107 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.107 0.107 0.017 0.017 0.107 0.107 0.017 0.017

K2O/H2O

0.018 0.045 0.09 0.018 0.045 0.09 0.09 0.09 0.09 0.09 0.09 0.09

M2O/H2O (M ) Na and/or K)

SiO2/M2O (M ) Na and/or K)

setting pressure (MPa)

mixing procedurea

compressive strength (MPa)

0.035 0.062 0.107 0.035 0.062 0.107 0.107 0.107 0.035 0.062 0.107 0.035 0.062 0.107 0.107 0.107 0.107 0.107 0.107 0.107 0.107 0.107 0.107 0.107

0.74 0.34 0.18 0.74 0.34 0.18 0.18 0.18 0.74 0.34 0.18 0.74 0.34 0.18 0.18 0.18 0.26 0.00 0.26 0.00 0.18 0.18 0.18 0.18

0 0 0 0 0 0 0.22 0.89 0 0 0 0 0 0 0.22 0.89 0 0 0 0 0 0 0 0

a a a a a a a a a a a a a a a a a a a a b c b c

10.4 7.8 8.5 10.1b 11.5b 12.7b 15.1 17.6 19.2 16.3 18.9 18.9b 20.3b 23.8b 33.3 59.7 15.5 3.2 25.1 3.5 4.6 3.4 5.1 4.3

a a represents the mixing of MOH and Na SiO solutions before the addition of solid sample. b represents the mixing of a MOH solution 2 3 with a solid sample first, followed by the addition of a Na2SiO3 solution. c represents the mixing of a solid sample with a Na2SiO3 solution, b followed by the addition of a MOH solution. Treated by 5 days of hardening in an oven at 35 °C.

Table 3. Aluminum and Silicon Leached from Kaolinite or Stilbite in 10 M MOH (10 mL) Solutions, Calculated as [Al] ppm, [Si] ppm, and [Al2O3] and [SiO2] Molar Concentrations, Respectively kaolinite (g)

stilbite (g)

4.5 4.5 5 5

Na2O/H2O

K2O/H2O

0.09 0.09 0.09 0.09 6 6 7 7

0.09 0.09 0.09 0.09

The concentration of leached Al and Si from stilbite and kaolinite using either NaOH or KOH solutions is shown in Figures 1-4. It is important to note that the solid (g)/liquid (mL) ratio (S/L) varies from 0.1 to 0.5 (Figures 1 and 2) and from 0.1 to 0.7 (Figures 3 and 4), respectively. For both kaolinite and stilbite, the concentration of leached Al and Si increases with increasing M2O/H2O ratio. It is also shown that a NaOH solution has a greater propensity to leach more Al and Si than a KOH solution. Despite kaolinite having a smaller particle size, it has also been found for each M2O/H2O ratio (0.018, 0.045, or 0.09) that the dissolution of Al and Si from kaolinite is significantly lower than from stilbite in both NaOH and KOH solutions. This implies that, during geopolymerization, stilbite will contribute more dissolved Al and Si complexes than kaolinite into the gel phase. The dissolution procedures for kaolinite and stilbite in an alkaline solution can be described as: dissolution

kaolinite(s) {\ } Al + Si complexes precipitation dissolution

} Al + Si complexes stilbite(s) {\ precipitation

(2) (3)

For the case when 1 g of stilbite or kaolinite has been added to 10 mL of a MOH solution, only the partial dissolution of the mineral occurs after 18 h of mixing. As the S/L ratio is increased from 0.1 to 0.3 and from 0.1 to 0.4 for kaolinite and stilbite, respectively, the

[Al] (ppm)

[Si] (ppm)

[Al2O3] (M)

[SiO2] (M)

925 984 945 995 3300 1180 3510 1200

1725 1673 1750 1709 11250 4890 12320 4900

1.92 × 10-2 1.71 × 10-2 1.75 × 10-2 1.84 × 10-2 6.11 × 10-2 2.19 × 10-2 6.5 × 10-2 2.22 × 10-2

6.16 × 10-2 5.85 × 10-2 6.25 × 10-2 6.1 × 10-2 4.02 × 10-1 1.75 × 10-1 4.4 × 10-1 1.75 × 10-1

surface area available for dissolution is also increased. This results in the linear increase in concentration of leached Al and Si in solution. For S/L ratios greater than 0.35 for kaolinite and 0.45 for stilbite, the total amount of Al and Si leached approaches a constant value. It is proposed that this observed decrease in the extent of leaching of Al and Si from each mineral for higher S/L ratios is due to the high concentrations of Al and Si complexes in the solution. These complexes hinder the further dissolution of Al and Si from the solid phase to the liquid phase. At S/L ratios of 0.5 and 0.7 for kaolinite and stilbite, respectively, the dissolution equilibrium represented by eqs 2 and 3 is attained. However, at this equilibrium only the partial dissolution of Al and Si from the mineral has occurred. It has also been found that when the S/L ratio for kaolinite and stilbite changes from 0.045 to 0.5 and from 0.6 to 0.7, respectively, the concentration of Al and Si complexes in a MOH solution (after 18 h mixing) remains almost constant (Table 3). These results suggest that, for geopolymerization systems having S/L ratios for kaolinite and stilbite far greater than those used in the present study, the corresponding concentration of leached Al and Si complexes can be assumed to be the same as the values obtained at S/L ratios of 0.5 and 0.7, respectively. It is often assumed that an increase in the dissolution of the aluminosilicate oxide source corresponds to an increase in the extent of geopolymerization and thereby results in a higher compressive strength of the geopoly-

3752

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001

Figure 1. Leached [Si] (ppm) of kaolinite in 2, 5, and 10 M MOH (M ) Na and /or K) solutions.

Figure 3. Leached [Si] (ppm) of stilbite in 2, 5, and 10 M MOH (M ) Na and/or K) solutions.

Figure 2. Leached [Al] (ppm) of kaolinite in 2, 5, and 10 M MOH (M ) Na and/or K) solutions.

mer product. In view of the above discussion, the greater M2O/H2O ratio and Na+ cation (instead of K+) should favor the geopolymerization of the stilbite and kaolinite system. However, the results presented in Table 2 show that the compressive strengths of samples 1-3 and 9-11 decrease with an increase in the M2O/H2O ratio from 0.035 to 0.062 and then increase when the M2O/ H2O ratio is varied from 0.062 to 0.107. It is also noted that the Na+ alkali-metal cation, compared with K+, shows a significant negative influence on the compressive strength of each sample. From Table 2 it is shown that for samples placed in the oven for an additional 2 days at a temperature of 35 °C (samples 4-6 and 1214), the compressive strength increased progressively as the M2O/H2O ratio increased. This suggests that the differences in the compressive strength caused by different hardening times in the oven may be due to the viscosity. Samples with a higher M2O/H2O ratio have a higher viscosity possibly because of the attractive force that exists between the alkali-metal cations and water molecules. Consequently, samples with a higher M2O/ H2O ratio need a longer hardening time because more energy is required to overcome the attractive forces between the alkali-metal cations and water molecules. It was observed in the present work that 2 days of hardening in the oven at 35 °C was satisfactory for a M2O/H2O ratio of 0.035 but 5 days was better for the 0.062 and 0.107 cases. Effect of the SiO2/M2O Ratio (Where M ) Na and/ or K) on Compressive Strength. It is shown in Table 2 that the compressive strengths of samples 3, 11, and 17-20 increase with an increase of the SiO2/M2O ratio.

Figure 4. Leached [Al] (ppm) of stilbite in 2, 5, and 10 M MOH (M ) Na and/or K) solutions.

It should also be noted that samples 18 and 20 (which were prepared without the addition of a Na2SiO3 solution) exhibit significantly lower compressive strengths compared to the other samples tested. The lower compressive strengths of samples 18 and 20 were expected because Davidovits1 has reported previously that the addition of silicates or a silicate solution is one of the essential conditions for the geopolymerization process. It is proposed that the geopolymeric gel phase formed by the dissolution of only kaolinite and stilbite is too weak to bind the remaining undissolved Al-Si particles, thereby resulting in a poorly formed geopolymer. It is also shown in Table 2 that increasing the SiO2/ M2O ratio from 0.18 to 0.26 results in an increase in the compressive strengths of the geopolymers from 8.5 to 15.5 MPa and from 18.9 to 25.1 MPa for the NaOH and KOH cases, respectively (samples 3, 7, 11, and 19). SiO2/M2O ratios of greater than 0.26 were also tested, but no significant increase in the compressive strength was observed for either NaOH or KOH. It is worth noting that by addition of a Na2SiO3 solution the compressive strengths of the resultant geopolymers increased substantially. This implies that some kind of reaction mechanism for geopolymerization must involve the Na2SiO3 species. The leaching results presented in Table 3 show that the maximum condensed gel phase formed by the dissolution of kaolinite and stilbite in a 10 M NaOH solution (Na2O/H2O ) 0.09) is composed of [(6.25 × 10-2)kaolinite + (4.4 × 10-1)stilbite] M SiO2 and

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3753

Figure 5. XRD patterns of stilbite, kaolinite, and geopolymers formed by stilbite and kaolinite mixtures at a kaolinite/stilbite mass ratio of 6.5, a Na2O/H2O ratio of 0.107, and SiO2/Na2O ratios of 0.0, 0.18, and 0.26, respectively. Table 4. 27Al and Kaolinite sample stilbite kaolinitec geopolymer 1f geopolymer 2f

29Si

MAS NMR Chemical Shifts (ppm) of Stilbite, Kaolinite, and Geopolymers Formed by Stilbite and Si(4Al)a

Si(3Al)a -91.5 -92.4 -90.03

Si(2Al)a

Si(1Al)a

Si(0Al)a

Al(4Si)a

-98b

-101.5b

-108b

64.5e

-98.8

-104.7 -101.1

-114.2 -106.6

63.5

a

Al(3Si)a

Ald

70.1

0.0 1.7 9.9

b

Si(nAl) and Al(nSi) designate the SiO4 and AlO4 tetrahedra connected through shared oxygen atoms. Data from Lippmaa et al.15 Data from Madani et al.16 d Al associated with octahedral Al. e Sodalite data from Lippmaa et al.15 f Geopolymer 1: geopolymer formed by kaolinite/stilbite mass ratio ) 6.5, Na2O/H2O ) 0.107, SiO2/Na2O ) 0.0. Geopolymer 2: geopolymer formed by kaolinite/stilbite mass ratio ) 6.5, Na2O/H2O ) 0.107, SiO2/Na2O ) 0.26.

c

[(1.92 × 10-2)kaolinite + (6.5 × 10-2)stilbite] M Al2O3. This composition is significantly lower than the minimum gel polymerization concentrations quoted by Barrer13 and therefore could be a reason for the lower compressive strengths of samples 18 and 20. It is believed that the compressive strengths of geopolymers are derived from a combination of factors, including (a) gel-phase strength, (b) the ratio of the gel phase/undissolved Al-Si particles, (c) the distribution of the undissolved Al-Si particle sizes, and (d) the surface reaction between the gel phase and the undissolved Al-Si particles. When a Na2SiO3 solution is added into a geopolymer mixture, an improvement in at least one, if not all, of the above listed factors is expected. To gain a qualitative insight into the gel-phase behavior of samples produced with and without the addition of the Na2SiO3 solution, XRD and MAS NMR techniques have been applied. XRD experiments were conducted on geopolymer samples having a Na2O/H2O ratio of 0.107 and a kaolinite/stilbite mass ratio varying from 9.0 to 0.1. The XRD patterns for stilbite, kaolinite, and geopolymers formed using a kaolinite(g)/stilbite(g) ratio equal to 6.5, and different SiO2/Na2O ratios are presented in Figure 5. It is important to note that both stilbite and kaolinite contained a minor amount of quartz as an impurity (d spacing at 4.267 and 3.33). It is observed from patterns “c” and “d” that stilbite and kaolinite show lower intensities of their characteristic peaks when the SiO2/ Na2O ratio is increased from 0.0 to 0.18. This means that the crystallized structure of stilbite and kaolinite decreases as the SiO2/Na2O ratio is increased, with the subsequent formation of an amorphous gel phase which is not detected by XRD. It is also noted that in XRD

pattern “e” the characteristic peak “A” for stilbite is not present. This suggests that nearly all crystallized stilbite, or at least certain of its crystal faces, is leached into the gel phase when the SiO2/Na2O ratio is greater than or equal to 0.26. Therefore, it can be concluded that increasing the SiO2/M2O ratio in geopolymerization (by the addition of a Na2SiO3 solution) results in a higher extent of geopolymerization because greater amounts of kaolinite and stilbite are dissolved into the gel phase. The 29Si and 27Al chemical shifts observed from MAS NMR analyses on aluminosilicate minerals,14 including stilbite15 and kaolinite,16 as well as geopolymers formed by stilbite/kaolinite mixtures are presented in Table 4. It is important to mention that kaolinite and stilbite have different 29Si NMR spectra, and so the interpretation of 29Si NMR spectra of geopolymers from kaolinite and stilbite is possible. Geopolymer 1 listed in Table 4 was prepared using a mass ratio of kaolinite/stilbite equal to 6.5 and SiO2/Na2O and Na2O/H2O ratios of 0.0 and 0.107, respectively. The 27Al chemical shift of 63.5 ppm observed for geopolymer 1 can be attributed to the tetrahedral framework structure of stilbite. This is because a study by Lippmaa et al.14 has reported similar chemical shift values for a variety of aluminosilicate minerals, in particular for sodalite, which possesses a structure similar to that of stilbite. According to Davidovits,1 the 27Al chemical shift at 1.7 ppm can be attributed to the octahedral layer structure of kaolinite. Characteristic 29Si chemical shifts for both stilbite and kaolinite were detected in geopolymer 1, which means that the majority of stilbite and kaolinite still exists as their original crystallized structures. When the SiO2/ Na2O ratio is increased to 0.26 (geopolymer 2), three

3754

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001

main differences for the 29Si and 27Al chemical shifts appear in the MAS NMR spectra. The first two differences are the disappearance of the characteristic 29Si and 27Al chemical shifts at -98 and 63.5 ppm, respectively, for geopolymer 2. As discussed previously, the 27Al chemical shift at 63.5 ppm can be attributed to the tetrahedral framework structure of aluminosilicate minerals, including stilbite. Therefore, the absence of this peak in the MAS NMR spectrum for geopolymer 2 indicates that an increase in the SiO2/Na2O ratio results in stilbite being totally transferred into the geopolymeric gel phase. The third difference in the MAS NMR spectrum for geopolymer 2 is that the 27Al chemical shifts are observed at 70.1 and 9.9 ppm. This can be attributed to tetrahedrally coordinated Al in the gel phase and also the presence of undissolved kaolinite particles. Although, it should be noted that a variation from 0.0 to 9.9 is observed for the chemical shift of Al. The reasons as to why or how the SiO2/M2O ratio can have such a significant influence on geopolymeric reactions still remain unclear. At this stage, the other possible effects of the SiO2/M2O ratio on the size distribution of undissolved Al-Si particles and the surface reactions between the gel phase and undissolved Al-Si particles in geopolymerization are not well understood because of the limitation of currently available experimental techniques. In general, it is well accepted that, in a highly alkaline solution (pH ≈ 14), Al and Si complexes exist as Al(OH)4-, (OH)2SiO22-, (OH)SiO33-, and other silicate oligomer anions.13,17-22 Without the addition of a Na2SiO3 solution, the Al and Si complexes leached from stilbite and kaolinite in the present investigation are Al(OH)4-, (OH)2SiO22-, (OH)SiO33-, and the silicate dimer.20,24 The concentrations of these complexes are lower than that required for polymerization to occur between Al(OH)4- and silicate complexes.13 Barrer,13 Andersson et al.,18 and McCormick et al.20 have discovered previously using 29Si NMR that increasing the SiO2/M2O ratio promotes the formation of larger silicate oligomers. Furthermore, these studies have shown that the reaction rate between silicate oligomers and Al(OH)4increases substantially at high pH conditions, which has been attributed to the Al(OH)4- complex having a greater ability to polymerize with larger silicate oligomers.20 It is possible that this could be the reason in geopolymerization that addition of silicate solids or a condensed silicate solution is required. It has been proposed by Andersson et al.18 that Al-Si polymerization occurs predominantly between Al(OH)4- and larger silicate oligomers, because it was found that, after such a polymerization, the amount of smaller silicate species (e.g., monomer and dimer species) remained constant while the amount of larger silicate species decreased. This implies that the aluminosilicate polymeric reaction takes place predominantly between Al(OH)4- and larger silicate oligomers. It is therefore proposed in the present work that by the addition of silicate solids or a Na2SiO3 solution a simultaneous polymerization between Al(OH)4and larger silicate oligomers occurs. This polymerization decreases the concentration of Al(OH)4- complexes in solution, thereby promoting the further dissolution of Al-Si particles from stilbite or kaolinite. This suggests that the optimum SiO2/M2O ratio in geopolymerization is a function of all of the factors that affect the solubility of the Al-Si oxide sources used, such as the crystal

structure, the reaction time, and also the particle size distribution. Effect of Alkali-Metal Cations (Na+ and K+) on Compressive Strength. It has been shown previously that metal cations affect each stage of the geopolymerization process, including the dissolution of Al-Si oxide sources, polymerization of dissolved Al(OH)4- and silicate complexes, and also setting and hardening.1,11,25 In particular, Van Jaarsveld and Van Deventer25 have proposed that the alkali-metal cation affects the dissolution, gel hardening, and eventual crystal formation processes by ordering H2O and dissolved Al and Si complexes in solution as well as playing a structuredirecting role in polymerization. The structure-directing role of metal cations has also been proposed by Gabelica et al.26 during the synthesis of ZSM-5 zeolite. It should be noted, however, that geopolymerization occurs in highly alkaline conditions, while a combination of acidic and basic conditions is required for the synthesis of zeolites. Another significant difference between geopolymerization and zeolite synthesis is that the former occurs in paste form having a S (g)/L (mL) ratio of between 2.8 and 4.0,8-11 while the latter occurs in solutions having a S/L ratio lower than 0.1. Despite these differences, the knowledge gained in the synthesis of zeolites can be used to understand further the dissolution and early stages of geopolymerization. Previous research on the dissolution and gelling stages of zeolite synthesis and also on highly alkaline silicate solutions has observed an ion pair effect.18,19,23 In highly alkaline solution, silicate monomer anions are believed to pair with metal cations to form dimers, trimers, and other larger oligomers. As expected, metal cations with a smaller size and a higher positive charge are found to favor ion pairing with smaller silicate oligomers, such as monomers and dimers.14-23 This implies that Na+ will have a greater ability to stabilize silicate monomers, while the larger K+ cation will stabilize silicate oligomers.17 For the same alkaline concentration, previous research11,25 has shown that more Al and Si can be leached from Al-Si oxide sources using a NaOH solution rather than a KOH solution. This phenomenon is also observed in the present work (Table 3). However, it is shown in Table 2 that the compressive strength of the geopolymers obtained showed significantly higher values when stilbite or kaolinite was leached with a KOH solution. It is expected that in a dilute aluminosilicate solution, derived from the dissolution of Al-Si oxide sources, only silicate monomers and possibly dimers exist. These silicate species would be better stabilized by Na+ rather than K+, and therefore a greater amount of Al and Si can be leached using a NaOH solution. Upon the addition of silicate solids1 or a silicate solution (e.g., Na2SiO3 or K2SiO3)11,25 into the geopolymer mix, the concentration of larger silicate oligomer anions is increased. These larger silicate oligomer anions are ionpaired by K+ cations in preference to Na+ cations, thereby reducing the negative charge and also stabilizing the larger silicate oligomer anions. Therefore, a KOH solution not only contains a greater amount of the larger silicate oligomers for any given time but also contains a higher ratio of larger silicate oligomers/ smaller oligomers than does a NaOH solution. As a result, the polymerization between Al(OH)4- and large silicate oligomers as well as between larger polymerized

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3755

Figure 6. XRD patterns of geopolymers formed by stilbite and kaolinite at a M2O/H2O ratio of 0.107, a SiO2/M2O ratio of 0.26, a kaolinite/ stilbite mass ratio of 6.5, and a stilbite/kaolinite mass ratio of 6.5, in NaOH and KOH solutions.

particles is expected to occur more rapidly in a KOH solution. This directly results in a faster setting time for the geopolymer.25 Although the K+ cation can accelerate the polymerization between Al(OH)4- and larger silicate oligomers, the ability of K+ to increase the extent of polymerization, to improve the gel-phase composition, or to change the distribution between gel phase and undissolved Al-Si particles has not been established. This is important because it is believed that these factors affect the final compressive strength of geopolymers. Because of experimental restrictions, a direct investigation on the extent of geopolymerization, the composition of the gel phase, and the distribution between the gel phase and undissolved Al-Si particles cannot be conducted. However, through the application of XRD techniques, some important qualitative results can be obtained. Figure 6 shows XRD patterns of geopolymers formed by stilbite and kaolinite using NaOH and KOH solutions and also for kaolinite/stilbite mass ratios of 6.5/1 and 1/6.5, respectively. It is shown by Figure 6 that the characteristic peak for stilbite (labeled “A”) is not present in the XRD pattern labeled a, but this peak is detectable in the pattern labeled b. The difference between these XRD patterns is the type of metal cation used during the dissolution of stilbite. This result suggests that, for a constant kaolinite/stilbite mass ratio of 6.5/1, the Na+ cation promotes the formation of an amorphous gel phase with a corresponding decrease in the crystallinity of stilbite (as noted by the absence of the characteristic peak for stilbite in pattern a). In contrast, it is shown by the lower intensity of the characteristic peak for kaolinite (labeled B) in pattern c compared with d that the K+ cation promotes the dissolution of kaolinite into the gel phase. This suggests that, for a constant kaolinite/stilbite mass ratio of 1/6.5, the Si/Al molar ratio in the K+ gel phase is lower than that in the Na+ gel phase because kaolinite has a lower SiO2/Al2O3 molar ratio than stilbite [i.e., (SiO2/Al2O3)kaolinite ) 3.15 and (SiO2/Al2O3)stilbite ) 6.81]. Although only qualitatively described, the results obtained using XRD have determined the effect of metal cations on the gel-phase composition. This is an important result because the gel-phase composition directly

influences the final compressive strength of the geopolymer. As stated earlier, kaolinite has a much smaller particle size (50% < 0.5 µm and 1% > 38 µm) than stilbite (10% < 1.6 µm and 25% > 38 µm), and so kaolinite possesses a much larger surface area. As a consequence, in a mixture containing a fixed mass of kaolinite and stilbite, a reduction in the total surface area of the particles will occur for the same amount of kaolinite being transferred into the gel phase as in the case of stilbite. Therefore, the type of metal cation (either Na+ or K+) not only affects the gel-phase composition but also changes the ratio of gel/(undissolved Al-Si surface area), with K+ favoring a higher gel/(undissolved Al-Si surface area) ratio than the Na+ cation. Effect of Setting Pressure and Mixing Procedure on Compressive Strength. Table 2 shows that the setting pressure significantly increases the compressive strengths of geopolymers in both NaOH and KOH cases. It was observed during the pressured setting of samples 7, 8, 15, and 16 that solution was squeezed out from the joints in the sample mould. Consequently, it is believed that pressured setting can exclude the spare solution contained in the gel phase resulting in an increase in the gel density. In addition, pressured setting will also cause the gel to evenly coat undissolved Al-Si particles, thereby narrowing the interparticle space which is a result of the evaporation of water. It is proposed that these phenomena result in an increase in the compressive strength of the geopolymer. The effect of different mixing procedures on the final compressive strengths of geopolymers is shown by samples 3, 6, and 21-24 in Table 2. Samples 21 and 23 were prepared by first mixing a MOH solution with a Al-Si solid, followed by the addition of a Na2SiO3 solution. In contrast, samples 22 and 24 were prepared by mixing the Al-Si solid with a Na2SiO3 solution followed by a MOH solution. Table 2 shows that both mixing procedures, b (samples 21 and 23) and c (samples 22 and 24), result in a geopolymer of lower compressive strength than the geopolymer formed using mixing procedure a. Mixing procedure a involved the mixing of MOH and Na2SiO3 solutions, followed by the gradual addition of a Al-Si solid oxide source. These results

3756

Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001

suggest that MOH and Na2SiO3 solutions play a joint role, rather than separate roles, in geopolymerization. This observation is consistent with earlier results found in the current work. It is possible that because of the high viscosity of a Na2SiO3 solution, whether mixing solid Al-Si oxide first with a MOH solution (procedure b) or first with a Na2SiO3 solution (procedure c), neither mixing procedure is able to produce a paste which is as homogeneously mixed as that of procedure a. Therefore, only a certain part of solid Al-Si particles take part in geopolymerization, which results in a large sacrifice in compressive strength. Conclusions A series of geopolymerization experiments have been conducted on stilbite/kaolinite mixtures to determine the effect of different factors on the final compressive strengths of geopolymers. The results of this work show that the M2O/H2O ratio affects the solubility of stilbite and kaolinite as well as the final compressive strength of the geopolymer. In particular, it has been found that increasing the M2O/H2O ratio results in an increase in the dissolution of Al and Si species and also an increase in the final compressive strength. The SiO2/M2O ratio plays an essential role in the polymerization between Al(OH)4- and silicate species. A high SiO2/M2O ratio results in the formation of predominantly large silicate oligomers in solution. These large silicate oligomers initiate the polymerization with Al(OH)4- species, resulting in a decrease in the concentration of both Al and Si species in solution. This means that additional dissolution of Al and Si species from the Al-Si solid occurs because of the shift in the dissolution equilibrium. An optimum SiO2/M2O ratio of 0.26 was determined in this study for a M2O/H2O ratio of 0.107. At these optimum conditions, the compressive strength of the resultant geopolymer was 15.5 and 25.1 MPa for NaOH and KOH, respectively. The setting pressure has been shown to improve the compressive strengths of geopolymers formed using NaOH or KOH solutions. This improvement has been attributed to a reduction in the internal spare space and more effective coating of the gel phase on undissolved Al-Si particles. It has also been found that a substantial improvement in the compressive strength is achieved when MOH and sodium silicate solutions are mixed prior to the addition of the Al-Si solid. The other mixing procedures investigated were not able to produce a homogeneously mixed paste, resulting in a geopolymer of low compressive strength. It has also been established that the metal cations, Na+ and K+, have multiple effects on the dissolution and polymerization steps, with K+ resulting in a better compressive strength. It has also been established that K+ increases the dissolution of kaolinite, while Na+ increases the leaching of stilbite. The results of the present investigation show that stilbite, which is a naturally occurring aluminosilicate mineral, can be used as a source of Al and Si species for geopolymeric reactions. Literature Cited (1) Davidovits, J. Geopolymers: Inorganic Polymeric New Materials. J. Mater. Educ. 1994, 16 (2 & 3), 91. (2) Davidovits, J.; Davidovits, M.; Davidovits, N. Process for Obtaining a Geopolymeric Alumino-Silicate and Products thus Obtained. U.S. Patent 5342595, 1994. (3) Davidovits, J. Mineral Polymers and Methods of Making Them. U.S. Patent 4349386, 1982.

(4) Helferich, R. L.; Shook, W. B. Aluminosilicate Hydrogel Bonded Aggregates Articles. U.S. Patent 4432798, 1984. (5) Neuschaeffer, K. H.; Splelau, P.; Zoche, G.; Engels, H. W. Aqueous Curable Molding Compositions Based on Inorganic Ingredients and Process for the Production of Milder Parts. U.S. Patent 45339, 1985. (6) Mahler, W. Process for Preparing Amorphous Particulate Poly(aluminosilicate). U.S. Patent 4213950, 1980. (7) Palomo, A.; Maclas, A.; Blanco, M. T.; Puertas, F. Physical, Chemical and Mechanical Characterisation of Geopolymers. Proc. 9th Int. Congr. Chem. Cem. 1992, 505. (8) Van Jaarsveld, J. G. S.; Van Deventer, J. S. J.; Lorenzen, L. The Potential Use of Geopolymeric Materials to Immobilise Toxic Metals: Part I. Theory and Applications. Miner. Eng. 1997, 10, 659. (9) Van Jaarsveld, J. G. S.; Van Deventer, J. S. J.; Lorenzen, L. Factors Affecting the Immobilisation of Metals in Geopolymerised Fly Ash. Met. Mater. Trans. B 1998, 29, 283. (10) Van Jaarsveld, J. G. S.; Van Deventer, J. S. J. The Potential Use of Geopolymeric Materials to Immobilise Toxic Metals: Part II. Material and Leaching Characteristics. Miner. Eng. 1999, 1, 75. (11) Xu, H.; Van Deventer, J. S. J. The Geopolymerisation of Aluminosilicate Minerals. Int. J. Miner. Process 2000, 59, 247. (12) Tsitsishvili, G. V.; Andronikashvili, T. G.; Kirov, G. N.; Fillzova, L. D. Natural Zeolites; Ellis Horwood Ltd.: London, 1992. (13) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982; p 123. (14) Lippmaa, E.; Samoson, A.; Ma¨gi, M. High-Resolution 27Al NMR of Aluminosilicates. J. Am. Chem. Soc. 1986, 108, 1730. (15) Lippmaa, E.; Ma¨gi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G. Investigation of the Structure of Zeolite by Solid-State High-resolution 29Si NMR Spectroscopy. J. Am. Chem. Soc. 1981, 103, 4992. (16) Madani, A.; Aznar, A.; Sanz, J.; Serratosa, J. M. 29Si and 27Al NMR Study of Zeolite Formation from Alkali-Leached Kaolinites. Influence of Thermal Preactivation. J. Phys. Chem. 1990, 94, 760. (17) Harris, R. K.; Knight, C. T. G.; Hull, W. E. NMR Studies of the Chemical Structure of Silicates in Solution. In Soluble Silicates; ACS Symposium Series 194; American Chemical Society: Washington, DC, 1982; p 79. (18) Andersson, K. R.; Dent Glasser, L. S.; Smith, D. N. Polymerization and Colloid Formation in Silicate Solutions. In Soluble Silicates; ACS Symposium Series 194; American Chemical Society: Washington, DC, 1982; p 115. (19) Dent Glasser, L. S.; Lachowski, E. E. Silicate Species in Solution. Part 2. The Structure of Polymeric Species. J. Chem. Soc., Dalton. Trans. 1980, 399. (20) McCormick, A. V.; Bell, A. T. The Solution Chemistry of Zeolite Precursors. Catal. Rev. Sci. Eng. 1989, 31 (1 & 2), 97. (21) McCormick, A. V.; Bell, A. T.; Radke, C. J. Evidence From Alkali-metal NMR Spectroscopy for Ion Pairing in Alkaline Silicate Solutions. J. Phys. Chem. 1989, 93, 1733. (22) McCormick, A. V.; Bell, A. T.; Radke, C. J. Multinuclear NMR Investigation of the Formation of Aluminosilicate Anions. J. Phys. Chem. 1989, 93, 1741. (23) Swaddle, T. W.; Salerno, J.; Tregloan, P. A. Aqueous Aluminates, Silicates, and Aluminosilicates. J. Chem. Soc. Rev. 1994, 319. (24) Gasteiger, H. A.; Frederick, W. J.; Streisel, R. C. Solubility of Aluminosilicates in Alkaline Solutions and a Thermodynamic Equilibrium Model. Ind. Eng. Chem. Res. 1992, 31, 1183. (25) Van Jaarsveld, J. G. S.; Van Deventer, J. S. J. Effect of the Alkali Metal Activator on the Properties of Fly Ash-Based Geopolymers. Ind. Eng. Chem. Res. 1999, 38, 3932. (26) Gabelica, Z.; Blom, N.; Derouane, E. G. Synthesis and Characterisation of ZSM-5 Type Zeolites III: A Critical Evaluation of the Role of Alkali and Ammonium Cations. Appl. Catal. 1983, 5, 227.

Received for review January 10, 2001 Revised manuscript received June 3, 2001 Accepted June 6, 2001 IE010042B