- Email: [email protected]
Influence of coexistent salt on hydrothermal synthesis of smectite and film-formability of the smectite
Applied Clay Science 46 (2009) 209–215
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Influence of coexistent salt on hydrothermal synthesis of smectite and film-formability of the smectite Hyun-Jeong Nam a, Takeo Ebina a,⁎, Ryo Ishii a, Hiroshi Yokota b, Fujio Mizukami a a b
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, 983-8551, Japan New Applied Materials R&D Center, Hitachi Chemical Co., Ltd., Tarasaki 1380-1, Hitachinaka, Ibaraki, 312-0003, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 31 July 2009 Accepted 6 August 2009 Available online 12 August 2009 Keywords: Hydrothermal synthesis Stevensite Salt amount Film formability Coating film Self-standing film
a b s t r a c t To evaluate the influence of coexistent salt in the Mg–Si precursor for the hydrothermal synthesis of smectite, stevensite was synthesized hydrothermally from Mg–Si precursors with different salt concentrations. Prepared homogeneous Mg–Si precursors were washed with distilled water before hydrothermal treatment. The salt concentration of the Mg–Si precursors was differentiated according to the washing cycle number. The salt concentration decreased concomitantly with increased washing cycles. Prepared smectite samples were characterized using ICP, XRD, TG-DTA, TEM, EDX, and viscosity measurement. Trioctahedral smectite peaks were observed in the XRD charts of all samples; NaNO3 peaks were observed in the sample with excess salt. Excessive salt interrupted smectite synthesis, whereas salt of twice of ideal composition accelerated the smectite synthesis. Results also show that salt concentration affects smectite crystallinity. To investigate film formability of the derived smectite samples, coating films on glass substrates and binder-free self-standing films were prepared from aqueous dispersions of the derived smectite samples. The film formability improved with decreasing salt concentration of smectite. Smectite with low salt concentration forms a highly transparent film. Film formation processes were discussed through observation of film preparation using clay dispersions with different salt concentrations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Smectites have been used widely in industrial fields (ceramic manufacturing, polluted environment purification, etc.) because they have specific characteristics such as high surface area, cation exchange capacities, and gelation properties (Thomas et al., 2000; Harvey and Lagaly, 2006). Recently, smectite has been applied to clay films and nanocomposite films. Ebina and Mizukami (2007) developed a flexible transparent clay film that has excellent properties: oxygen gas barrier properties over 400 times better than those of a typical plastic film; greater than 90% total-visible-light-transmittance; heat durability up to 350 °C; and mechanical strength sufficient that it can be handled. The improved properties derive from a high clay loading of greater than 80 wt.% in the film. Therefore, this film has potential for use in solar batteries, or as a gas-barrier film, a heat-resistant film, a substrate for display products, etc. Expanding the application fields described above, the film formability of smectite becomes an important property in addition to conventional requirements such as gelation, porosity, specific surface area, and cation exchange capacity (CEC). To form a transparent clay film, removal of impurities that cause coloration and low whiteness is necessary. For that reason, natural clay
⁎ Corresponding author. E-mail address: [email protected] (T. Ebina). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.08.005
is unsuitable. The flexible transparent clay film produced by Ebina and Mizukami (2007) used synthetic smectite. In general, synthetic smectite has lower film formability than some natural smectite samples (Nam et al., 2009). We have attempted to form clay films using commercially available synthetic and natural smectite samples with no organic binder because the binder depresses the films' heat durability and gas barrier property. Results show that the commercially available synthetic smectites—saponite, stevensite, and hectorite—formed clay films of poor quality with many cracks and distortions, whereas natural smectites—montmorillonite, beidellite, and hectorite—formed flat uniform clay films. The unusually small particles of the synthetic smectite provide one of the reasons for their low film formability. Therefore, we improved the film formability by particle enlargement of the synthetic smectite samples using a hydrothermal treatment (Nam et al., 2009). Another reason is considered to be the excess amount of salt in the precipitates. In most synthetic smectite samples, salt is retained in the sample by insufficient washing or is added intentionally for gelation. Neumann, who is famous for work with synthetic hectorite, reported no serious viscosity problem in smectite synthesis with much excess salt in precipitation (Neumann, 1976). Neumann also reported that excess salt improves the rheological and optical properties of synthetic smectite, so that excess salt is favorable for the successful synthesis of smectite (Neumann and Sansom, 1973). Probably for the same reason, Torri and Iwasaki (1986) and Buck et al. (1991) did not report details of the washing process for salt removal.
210
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
The effect of excess salt has not been evaluated when the synthetic smectite is used for conventional purposes. No reports have described the importance of the washing process, the level of sufficient purification, or the influence of salt. However, salt is considered to affect the clay/nanocomposite film formability. From our experience, commercially available synthetic smectite tends to crack or distort during drying, perhaps because of the concentration of salt and the resultant film contraction. Salt removal from synthetic smectite is possible before hydrothermal treatment by washing, whereas purification of commercially available synthetic smectite is difficult because of its high gelation. Therefore, we synthesized stevensite, a trioctahedral smectite, using different salt concentrations by changing repeated washing cycles. To investigate the influence of the salt concentration on properties of the smectite, crystallization, and thermal properties of the samples, XRD and DTA analyses were conducted. Additionally, to evaluate film formability, coating films on glass substrate and self-standing films were prepared from the smectite aqueous dispersions with no additive used as a binder.
2. Experimental 2.1. Synthesis of smectite Trioctahedral smectite was synthesized from homogeneous Mg–Si gel under hydrothermal conditions. The synthetic process is depicted in Fig. 1. The Si and Mg form tetrahedral and octahedral sheets, respectively; the elemental ratio was fixed at 4:2.85, corresponding to the ideal chemical composition of stevensite, Na0.33Mg2.85Si4O10 (OH)2·nH2O. The silica source was a sodium silicate aqueous solution (water glass; Koso Kagaku Co. Ltd.). The magnesium source was Mg (NO3)2·6H2O (analytical reagent grade; Wako Pure Chemical Industries, Ltd.). In addition, HNO3 (analytical reagent grade, purity 70%; Wako Pure Chemical Industries, Ltd.) was used for acidification in the gel preparation, and NaOH (Wako) was used as the sodium source and for alkalinization in the gel preparation. First, sodium silicate (200 g) was dissolved in distilled water (1000 ml). Then HNO3 (70 ml) was poured to the sodium silicate solution; a solution of Mg(NO3)2·6H2O (182 g) and distilled water (182 g) was then added to the acidic silicate solution. A precipitate was obtained from the solution by adding NaOH aqueous solution (5 N). This precipitate was kept at room temperature for one night. The final pH value was around 10. The precipitate was divided equally into six vessels. Next, the precipitate was separated by centrifugation. To remove excess salt from the precipitate, repeated washing was performed as follows. First, 250 ml of distilled water was poured in to
each vessel. The precipitate was dispersed using a homogenizer. The dispersions were centrifuged and the supernatant was discarded. This washing process was conducted repetitively a maximum of six times. Finally, six washed precipitates were prepared with different washing cycle numbers. The washed precipitates were treated hydrothermally to synthesize smectite. Each sample with a different washing cycle was poured into a 120 ml Teflon container. Then they were placed and sealed in stainless steel autoclaves and maintained at 200 °C for 48 h under autogeneous pressure. After hydrothermal treatment, the treated products were dried in an oven at 60 °C. The dried products were ground using an automatic agate mortar and a pestle. 2.2. Characterization of the product Chemical compositions of the supernatant liquid and interlayer cation of the products were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES, SPS 1500R; Seiko Instruments Inc.). Additionally, the chemical compositions of the products were analyzed using energy dispersive X-ray spectroscopy (EDX, S-800; Hitachi Ltd.). The crystallinity and the crystallographic composition of the products were determined using an X-ray diffractometer with CuKα radiation (XRD, M21X; Mac Science Co., Ltd.). The XRD data were obtained using oriented samples prepared on glass substrates in the 2θ range 2–50° at a scanning speed of 2° min− 1. To confirm the formation of smectite, X-ray data were also obtained from powder samples in the 2θ range of 2–70°. Thermogravimetric analyses of the samples were obtained using differential thermal analysis equipment (TG-DTA, Thermo plus TG 8120; Rigaku Corp.). The TG-DTA operated at the heating rate of 10 °C/min at 30–1000 °C in a dry-air-flow atmosphere. Approximately 20 mg of finely ground sample was heated in an open platinum crucible. The respective microstructures of the samples were observed using transmission electron microscopy (TEM, operating voltage 200 kV, JEM-2000EXII; JEOL). Smectite samples were prepared from aqueous suspensions. Trace amounts of suspension were deposited on Cu mesh grids that had been coated with a thin carbon film; then they were dried. Viscosity of the aqueous dispersion (3 wt.%) was determined using a Brookfield rotating-type viscometer (TV-22, model B; Toki Sangyo Co. Ltd.). Measurements were performed at room temperature using M20, M21, and M22 spindles at 10, 20, and 30 rpm, respectively, immediately after shaking dispersions. Viscosities of aqueous dispersions (2 wt.%) of some commercially available smectite were also measured for comparison. 2.3. Preparation of clay film Coating films of the clay samples were prepared by depositing the aqueous suspensions (1 wt.%) with no binder on borosilicate glass substrates and drying them at room temperature. Self-standing films were prepared using the following casting method. The aqueous suspensions (1 wt.%, 40 ml) were placed on polypropylene trays (internal size 55 × 85 × 14 mm, average surface RMS roughness is 2.1 μm); then the suspensions were dried at room temperature. The dried films were peeled from the tray. 2.4. Characterization of clay film
Fig. 1. Schematic diagram showing the synthetic process.
The total light transmission and haze of the coating films were measured using a turbidimeter (NDH5000; Nippon Denshoku Industries Co. Ltd.). Film formability of the self-standing was evaluated by visual observation. The film thickness was measured using a digital micrometer (293-666 MDQ 30M; Mitutoyo Corp.).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
211
stevensite, the Na value was calculated as an atomic proportion relative to Si=4. The Na values of samples 1, 2, 3, 4, 5, and 6 were, respectively, 5.15, 1.35, 0.68, 0.56, 0.36, and 0.38. Sample 1 has an extremely great amount of sodium; it is considered that much of the amount of sodium is near the surface region of the synthesized particles. The sodium ratio decreased with the washing cycle number; it approached the ideal composition ratio in samples 5 and 6. These results suggest that five repeated washings are sufficient to remove excess sodium and to obtain a precipitate of ideal stevensite composition. The sodium ratio does not decrease much beyond the ideal ratio in spite of numerous repeated washings.
3.3. Crystallization of smectite
Fig. 2. Sodium concentration in the supernatant separated by centrifugation.
3. Results and discussion 3.1. Removal of excess sodium ion in the precipitate by repeated washing The synthesis of stevensite can be represented by the following. ðSiO2 Þ3 ðNa2 OÞþ afMgðNO3 Þ2 gþ bHNO3 þ cðNaOHÞða; b; c : mole numberÞ þ
−
→ðSiO2 Þ3 ðMgOÞa ·mH2 O þ ð2 þ cÞNa þ ð2a þ bÞðNO3 Þ
ð1Þ
By this synthesis, (SiO2)3(MgO)a·mH2O is a hydrated gel in precipitate. Other ions aside from the hydrated gel were removed during the washing process. To elucidate the effect of the washing, Na+ concentration in supernatants was analyzed using ICP-AES. Fig. 2 shows the Na+ concentration in the supernatant of each sample. The sodium concentrations decreased concomitantly with the number of washing cycles: over 95% of sodium had been removed after the third washing. The concentrations of sodium in the fourth, fifth, and sixth supernatants are similarly low, which suggests that more than four washings are not effective for salt removal. 3.2. Sodium concentration in the product Sodium concentration in the product was measured using EDX. From the ideal chemical composition (Na0.33Mg2.85Si4O10(OH)2·nH2O) of
Fig. 3(a) shows the powder XRD pattern of sample 1. The assigned NaNO3 peaks are indicated. Sample 1 showed many sharp peaks for NaNO3. This result is expected from equation 1; the XRD pattern shows that excess Na+ and NO− 3 remained as NaNO3 crystals. Fig. 3(b) presents an expanded chart of Fig. 3(a); it is consistent with a typical smectite pattern. This result shows that smectite was synthesized, even though the yield is low, if the reactant precursor contained an excess amount of Na+. From samples 1–4, the intensity of the smectite peaks increased concomitantly with increasing washing cycle number. In contrast, those of the NaNO3 peaks decreased concomitantly with increasing washing cycles; the peaks had disappeared in samples 3–6. Fig. 4 shows XRD patterns of sample 4 for the powder state and the film coating on a glass substrate. The XRD peaks of the powder state presented a typical smectite pattern. A d(060) peak is apparent at 0.153 nm, indicating trioctahedral smectite. The XRD pattern suggests synthesis of trioctahedral smectite. The XRD pattern of the film shows strong (001) reflections at around 2θ = 6°, suggesting crystal stacking along with a highly ordered c-axis. The self-assembled lamination is also a characteristic of smectite (Faust and Murata, 1953). The intensity for (001) reflection peaks increased concomitantly with increasing washing cycles; it was the highest in sample 4. Then it decreased in samples 5 and 6 (Fig. 5). This result might be affected by coexistent Na+. Otsuka et al. (1972, 1979) and Sakamoto et al. (1981) reported that stevensite formation is accelerated by sodium. They investigated hydrothermal synthesis of stevensite from natural minerals and reported that sodium improves the crystallinity of stevensite and broadens stable temperatures of stevensite. It is natural to conclude that some excess sodium accelerates the crystallization of stevensite in sample 4, although too much excess sodium interrupts it.
Fig. 3. Powder XRD patterns of once washed smectite (sample 1; Na = 5.15, Si = 4) (a) and the expanded chart (b).
212
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
Fig. 4. XRD patterns of 4 times washed smectite (sample 4; Na = 0.56, Si = 4) for the powder state (a) and the film coated on a glass substrate (b).*: A unit is nm.
Fig. 6 portrays a TEM image of sample 5. The image shows a folded thin clay particle of ca. 2 μm in planar extension. The TEM image also suggests that smectite synthesis has occurred. 3.4. Thermal property of synthesized smectite The DTA curves of these samples are presented in Fig. 7. Samples 4 and 6 show a sharp exothermic peak at 750–800 °C corresponding to a phase transition from smectite to enstatite (Takahashi et al., 1997). This peak shifted to a higher temperature with increased washing. In contrast, samples 1 and 2 showed DTA peaks that differed from those of samples 4 and 6. Samples 1 and 2 showed an endothermic peak for NaNO3 decomposition at ca. 306 °C. This result is consistent with the results of chemical and XRD analyses. 3.5. Separation of smectite from the product We tried to separate the smectite and NaNO3 after synthesis. After dispersing sample 1 using distilled water, it was centrifuged; then the
Fig. 5. XRD patterns of the smectite films coated on glass substrates prepared through 4 times washing (sample 4; Na = 0.56, Si = 4), 5 times washing (sample 5; Na = 0.36, Si = 4), and 6 times washing (sample 6; Na = 0.38, Si = 4).
supernatant and the sediment were dried separately on glass substrates. These were dried separately on glass substrates. Fig. 8 depicts their XRD patterns. The sedimentation showed only the smectite pattern, although the supernatant showed the NaNO3 pattern. It shows that separation of smectite from salt is possible after synthesis. However, the washing process is better if carried out before hydrothermal synthesis for two reasons. First, a low crystallinity of smectite is expected if the hydrothermal synthesis is conducted without washing, as shown in the XRD patterns (Fig. 3) and DTA curves (Fig. 7). Second, complete separation is difficult.
3.6. Viscosity of aqueous dispersions Fig. 9 shows the viscosity of 3 wt.% aqueous dispersion for samples 4, 5, and 6. Samples 5 and 6 showed a similar low viscosity although sample 4 had a high viscosity. This is because of the higher salt content of sample 4. The salt in the dispersion affects the electro-diffuse layer and increases the viscosity (Suzuki et al., 2005). Additionally, the viscosity of a 2 wt.% aqueous dispersion of commercially available synthetic smectite samples was measured. The viscosity values of two synthetic
Fig. 6. TEM image of 5 times washed smectite (sample 5; Na = 0.36, Si = 4).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
213
Fig. 9. Viscosity of 3 wt.% aqueous dispersions of 4, 5, and 6 times washed smectites (sample 4, 5, and 6) at 10, 20, and 30 rpm of the spindle rotation. Fig. 7. DTA peaks of samples with different washing cycle number. Sample 1; once washed smectite (Na= 5.15, Si= 4): sample 2; twice washed smectite (Na= 1.35, Si= 4): sample 4; 4 times washed smectite (Na = 0.56, Si= 4): sample 6; 6 times washed smectite (Na = 0.38, Si= 4).
hectorite samples (Thixopy; Kyowa Chemical Industry Co., Ltd. Japan, SWN; Co-op Chemical Co., Ltd. Japan) are 1000 and 504 mPa s, respectively, at 30 rpm of spindle rotation. Even with the high concentration of the dispersion of our smectite, it is shown that the viscosity of samples 5 and 6 decreased drastically through purification. 3.7. Film formability of the synthetic smectite and the property of each film 3.7.1. Coating film The aqueous dispersions of samples were deposited on glass substrates and dried at room temperature. Dispersions of samples 1 and 2 showed little sediment, but the others were transparent gels. After drying, white powder remained on the glass substrate for the dispersion of sample 1. Sample 2 yielded a transparent film with a small amount of white powder dispersed in the film. The other dispersions formed uniform transparent films on the glass substrates.
The total light transmittance and haze of these films were measured after subtracting those of the glass substrate as the baseline (Table 1). All films except of samples 1 and 2 showed total light transmittance greater than 99%. The haze of these films decreased concomitantly with increasing washing cycle number. 3.7.2. Self-standing film The smectite samples produced were used in an attempt to make self-standing films with no binder. Dispersions were poured in to polypropylene trays and dried at room temperature. Fig. 10 presents results for samples 1, 2, 4, and 6. Sample 1 did not produce a film but instead produced a powdery precipitate because it contained too much NaNO3. Sample 2 became transparent in spite of coexisting excess NaNO3, but it shrank during drying and became a distorted film. Sample 4 produced a transparent film with curly edges. Sample 6 produced a flat transparent film of ca. 110 μm thickness to have the same size of the tray, with no shrinkage. Curling and distortion of the films were remarkable when there was much more excess salt ion in the sample. This result suggests that coexistent salt strongly affects the film formation. The film formability improves with increasing purity of smectite. Apparently, clay particles were aggregated in the concentrated salt solution. Sample 4 contains about two times the sodium used for the ideal composition. Excess salt is considered to induce aggregation of clay sheets even though it was not detected in the XRD pattern. To elucidate reasons for film distortion in dispersions with excess salt ions, the concentration of salt in the top and bottom surfaces of the films were measured by EDX. The concentrations of salt on both surface sides were not different; they were also the same on interior and exterior surfaces of the curled film, as in sample 2, so it appears that the salt concentration is uniform throughout the film. Solubility of NaNO3 is 92 g/100 g in distilled water at 25 °C (NaNO3, ICSC#0180). The salt in the smectite sample was dissolved completely in the 1 wt.% dispersion used for the self-standing film preparation. It is assumed that the salts contract the film through evaporation; then the smectite sheets are
Table 1 Total light transmittance and haze of films coated on glass substrates.
Fig. 8. XRD patterns of the product, the separated supernatant, and the sedimentation by centrifugation.
Sample number
1
2
3
4
5
6
Washing cycle number Total light transmittance (%) Haze (%)
1 89.6 74.0
2 98.8 19.2
3 99.6 4.5
4 99.3 2.4
5 100.0 1.4
6 99.9 1.3
Glass substrate 92.1 0.3
214
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
Fig. 10. Film preparation results using once washed smectite (sample 1; Na = 5.15, Si = 4) (a), twice washed smectite (sample 2; Na = 1.35, Si = 4) (b), 4 times washed smectite (sample 4; Na = 0.56, Si = 4) (c), and 6 times washed smectite (sample 6; Na = 0.38, Si = 4) (d).
drawn near. The film is shrunk because evaporation occurs more rapidly from the top than the bottom surface. The dispersions with low salt concentrations form a flat film by a transition from the card-house structure to the self-assembled laminated structure through evaporation. The film formation process is presented in Scheme 1. Both the smectite dispersions with high and low salt concentration are changed to the clay films through a card-house structure process and a laminated structure process by evaporation. Therefore, the dispersion with high salt concentration becomes a deformed film due to shrinkage of whole film caused by a decrease in the volume of salts, while that with low or
no salt concentration becomes a flat film through a uniform evaporation in whole film. The results presented have confirmed that purity (salt concentration) is important when synthesizing smectite for producing transparent clay films. 4. Conclusion To synthesize smectite with high film formability, stevensite samples with different salt concentration were synthesized using the hydrothermal method. The smectite samples prepared were characterized in terms of salt contents, crystalloscopic properties, thermal properties, and viscosities of dispersion. Coating films on glass substrates and self-standing films were prepared from aqueous dispersions of the smectite samples with no binder. Transparent self-standing films were formed from well-washed samples that have a nearly ideal composition. Moderate amounts of excess salt accelerated the crystallization of smectite, whereas too much prevented it. With increasing purity of smectite, the transparency of the coating films increased. Moreover, self-standing film formability improved with increasing purity (low salt concentration) of smectite. These results suggest that purity and crystallinity are important for the use of synthetic smectite as a film material. References
Scheme 1. Smectite film formation process by evaporation using aqueous dispersion with different salt concentration.
Buck, M.J., Crofts, R.D., Purchase, L.J., 1991. Synthesis of smectite clay minerals, European patent 614445B1. Ebina, T., Mizukami, F., 2007. Flexible transparent clay films with heat-resistant and high gas-barrier properties. Adv. Mater. 19, 2450–2453. Faust, G.T., Murata, K.J., 1953. Stevensite, redefined as a member of the montmorillonite group. Am. Mineral. 38, 973–987. Harvey, C.C., Lagaly, G., 2006. Developments in clay science. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Elsevier Ltd., Amsterdam, pp. 506–540. Nam, H.-J., Ishii, R., Ebina, T., Mizukami, F., 2009. Flexible transparent self-standing binderless clay film prepared by hydrothermally treated synthetic clay. Mater. Lett. 63, 54–57. Neumann, B.S., 1976. Production of Magnesium Silicates, United States patent 4049780. Neumann, B.S., Sansom, K.G., 1973. Synthesis of hydrous magnesium silicates, United States patent 3954943. Otsuka, R., Imai, N., Sakamoto, T., 1972. Stability of stevensite under hydrothermal conditions. Ann. Sch. Sci. Eng., Waseda Univ. 36, 37–56. Otsuka, R., Sakamoto, T., Suzuki, S., Shinoda, S., Koshimizu, H., 1979. Hydrothermal treatment of pectolite — experimental studies of the genesis of stevensite and “hydrated talc” (3rd report). Koubutugaku-Zasshi 14, 170–186 (in Japanese).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215 Sakamoto, T., Koshimizu, H., Otsuka, R., 1981. Hydrothermal treatment of wollastonite — experimental studies of the genesis of stevensite and “hydrated talc” (4th report). Koubutugaku-Zasshi 15, 55–65 (in Japanese). Suzuki, S., Prayongphan, S., Ichikawa, Y., Chae, B.-G., 2005. In situ observation of the swelling of bentonite aggregates in NaCl solution. Appl. Clay Sci. 29, 89–98. Takahashi, N., Tanaka, M., Satoh, T., Endo, T., Shimada, M., 1997. Study of synthetic clay minerals. Part IV: synthesis of microcrystalline stevensite from hydromagnesite and sodium silicate. Microporous Mater. 9, 35–42.
215
Thomas, P.J., Baker, J.C., Zelazny, L.W., 2000. An expansive soil index for predicting shrinkswell potential. Soil Sci. Soc. Am. J. 64, 268–274. Torri, K., Iwasaki, T., 1986. Synthesized swelling silicate and its production, Japanese Patent 62292616.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Influence of coexistent salt on hydrothermal synthesis of smectite and film-formability of the smectite Hyun-Jeong Nam a, Takeo Ebina a,⁎, Ryo Ishii a, Hiroshi Yokota b, Fujio Mizukami a a b
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, 983-8551, Japan New Applied Materials R&D Center, Hitachi Chemical Co., Ltd., Tarasaki 1380-1, Hitachinaka, Ibaraki, 312-0003, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 31 July 2009 Accepted 6 August 2009 Available online 12 August 2009 Keywords: Hydrothermal synthesis Stevensite Salt amount Film formability Coating film Self-standing film
a b s t r a c t To evaluate the influence of coexistent salt in the Mg–Si precursor for the hydrothermal synthesis of smectite, stevensite was synthesized hydrothermally from Mg–Si precursors with different salt concentrations. Prepared homogeneous Mg–Si precursors were washed with distilled water before hydrothermal treatment. The salt concentration of the Mg–Si precursors was differentiated according to the washing cycle number. The salt concentration decreased concomitantly with increased washing cycles. Prepared smectite samples were characterized using ICP, XRD, TG-DTA, TEM, EDX, and viscosity measurement. Trioctahedral smectite peaks were observed in the XRD charts of all samples; NaNO3 peaks were observed in the sample with excess salt. Excessive salt interrupted smectite synthesis, whereas salt of twice of ideal composition accelerated the smectite synthesis. Results also show that salt concentration affects smectite crystallinity. To investigate film formability of the derived smectite samples, coating films on glass substrates and binder-free self-standing films were prepared from aqueous dispersions of the derived smectite samples. The film formability improved with decreasing salt concentration of smectite. Smectite with low salt concentration forms a highly transparent film. Film formation processes were discussed through observation of film preparation using clay dispersions with different salt concentrations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Smectites have been used widely in industrial fields (ceramic manufacturing, polluted environment purification, etc.) because they have specific characteristics such as high surface area, cation exchange capacities, and gelation properties (Thomas et al., 2000; Harvey and Lagaly, 2006). Recently, smectite has been applied to clay films and nanocomposite films. Ebina and Mizukami (2007) developed a flexible transparent clay film that has excellent properties: oxygen gas barrier properties over 400 times better than those of a typical plastic film; greater than 90% total-visible-light-transmittance; heat durability up to 350 °C; and mechanical strength sufficient that it can be handled. The improved properties derive from a high clay loading of greater than 80 wt.% in the film. Therefore, this film has potential for use in solar batteries, or as a gas-barrier film, a heat-resistant film, a substrate for display products, etc. Expanding the application fields described above, the film formability of smectite becomes an important property in addition to conventional requirements such as gelation, porosity, specific surface area, and cation exchange capacity (CEC). To form a transparent clay film, removal of impurities that cause coloration and low whiteness is necessary. For that reason, natural clay
⁎ Corresponding author. E-mail address: [email protected] (T. Ebina). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.08.005
is unsuitable. The flexible transparent clay film produced by Ebina and Mizukami (2007) used synthetic smectite. In general, synthetic smectite has lower film formability than some natural smectite samples (Nam et al., 2009). We have attempted to form clay films using commercially available synthetic and natural smectite samples with no organic binder because the binder depresses the films' heat durability and gas barrier property. Results show that the commercially available synthetic smectites—saponite, stevensite, and hectorite—formed clay films of poor quality with many cracks and distortions, whereas natural smectites—montmorillonite, beidellite, and hectorite—formed flat uniform clay films. The unusually small particles of the synthetic smectite provide one of the reasons for their low film formability. Therefore, we improved the film formability by particle enlargement of the synthetic smectite samples using a hydrothermal treatment (Nam et al., 2009). Another reason is considered to be the excess amount of salt in the precipitates. In most synthetic smectite samples, salt is retained in the sample by insufficient washing or is added intentionally for gelation. Neumann, who is famous for work with synthetic hectorite, reported no serious viscosity problem in smectite synthesis with much excess salt in precipitation (Neumann, 1976). Neumann also reported that excess salt improves the rheological and optical properties of synthetic smectite, so that excess salt is favorable for the successful synthesis of smectite (Neumann and Sansom, 1973). Probably for the same reason, Torri and Iwasaki (1986) and Buck et al. (1991) did not report details of the washing process for salt removal.
210
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
The effect of excess salt has not been evaluated when the synthetic smectite is used for conventional purposes. No reports have described the importance of the washing process, the level of sufficient purification, or the influence of salt. However, salt is considered to affect the clay/nanocomposite film formability. From our experience, commercially available synthetic smectite tends to crack or distort during drying, perhaps because of the concentration of salt and the resultant film contraction. Salt removal from synthetic smectite is possible before hydrothermal treatment by washing, whereas purification of commercially available synthetic smectite is difficult because of its high gelation. Therefore, we synthesized stevensite, a trioctahedral smectite, using different salt concentrations by changing repeated washing cycles. To investigate the influence of the salt concentration on properties of the smectite, crystallization, and thermal properties of the samples, XRD and DTA analyses were conducted. Additionally, to evaluate film formability, coating films on glass substrate and self-standing films were prepared from the smectite aqueous dispersions with no additive used as a binder.
2. Experimental 2.1. Synthesis of smectite Trioctahedral smectite was synthesized from homogeneous Mg–Si gel under hydrothermal conditions. The synthetic process is depicted in Fig. 1. The Si and Mg form tetrahedral and octahedral sheets, respectively; the elemental ratio was fixed at 4:2.85, corresponding to the ideal chemical composition of stevensite, Na0.33Mg2.85Si4O10 (OH)2·nH2O. The silica source was a sodium silicate aqueous solution (water glass; Koso Kagaku Co. Ltd.). The magnesium source was Mg (NO3)2·6H2O (analytical reagent grade; Wako Pure Chemical Industries, Ltd.). In addition, HNO3 (analytical reagent grade, purity 70%; Wako Pure Chemical Industries, Ltd.) was used for acidification in the gel preparation, and NaOH (Wako) was used as the sodium source and for alkalinization in the gel preparation. First, sodium silicate (200 g) was dissolved in distilled water (1000 ml). Then HNO3 (70 ml) was poured to the sodium silicate solution; a solution of Mg(NO3)2·6H2O (182 g) and distilled water (182 g) was then added to the acidic silicate solution. A precipitate was obtained from the solution by adding NaOH aqueous solution (5 N). This precipitate was kept at room temperature for one night. The final pH value was around 10. The precipitate was divided equally into six vessels. Next, the precipitate was separated by centrifugation. To remove excess salt from the precipitate, repeated washing was performed as follows. First, 250 ml of distilled water was poured in to
each vessel. The precipitate was dispersed using a homogenizer. The dispersions were centrifuged and the supernatant was discarded. This washing process was conducted repetitively a maximum of six times. Finally, six washed precipitates were prepared with different washing cycle numbers. The washed precipitates were treated hydrothermally to synthesize smectite. Each sample with a different washing cycle was poured into a 120 ml Teflon container. Then they were placed and sealed in stainless steel autoclaves and maintained at 200 °C for 48 h under autogeneous pressure. After hydrothermal treatment, the treated products were dried in an oven at 60 °C. The dried products were ground using an automatic agate mortar and a pestle. 2.2. Characterization of the product Chemical compositions of the supernatant liquid and interlayer cation of the products were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES, SPS 1500R; Seiko Instruments Inc.). Additionally, the chemical compositions of the products were analyzed using energy dispersive X-ray spectroscopy (EDX, S-800; Hitachi Ltd.). The crystallinity and the crystallographic composition of the products were determined using an X-ray diffractometer with CuKα radiation (XRD, M21X; Mac Science Co., Ltd.). The XRD data were obtained using oriented samples prepared on glass substrates in the 2θ range 2–50° at a scanning speed of 2° min− 1. To confirm the formation of smectite, X-ray data were also obtained from powder samples in the 2θ range of 2–70°. Thermogravimetric analyses of the samples were obtained using differential thermal analysis equipment (TG-DTA, Thermo plus TG 8120; Rigaku Corp.). The TG-DTA operated at the heating rate of 10 °C/min at 30–1000 °C in a dry-air-flow atmosphere. Approximately 20 mg of finely ground sample was heated in an open platinum crucible. The respective microstructures of the samples were observed using transmission electron microscopy (TEM, operating voltage 200 kV, JEM-2000EXII; JEOL). Smectite samples were prepared from aqueous suspensions. Trace amounts of suspension were deposited on Cu mesh grids that had been coated with a thin carbon film; then they were dried. Viscosity of the aqueous dispersion (3 wt.%) was determined using a Brookfield rotating-type viscometer (TV-22, model B; Toki Sangyo Co. Ltd.). Measurements were performed at room temperature using M20, M21, and M22 spindles at 10, 20, and 30 rpm, respectively, immediately after shaking dispersions. Viscosities of aqueous dispersions (2 wt.%) of some commercially available smectite were also measured for comparison. 2.3. Preparation of clay film Coating films of the clay samples were prepared by depositing the aqueous suspensions (1 wt.%) with no binder on borosilicate glass substrates and drying them at room temperature. Self-standing films were prepared using the following casting method. The aqueous suspensions (1 wt.%, 40 ml) were placed on polypropylene trays (internal size 55 × 85 × 14 mm, average surface RMS roughness is 2.1 μm); then the suspensions were dried at room temperature. The dried films were peeled from the tray. 2.4. Characterization of clay film
Fig. 1. Schematic diagram showing the synthetic process.
The total light transmission and haze of the coating films were measured using a turbidimeter (NDH5000; Nippon Denshoku Industries Co. Ltd.). Film formability of the self-standing was evaluated by visual observation. The film thickness was measured using a digital micrometer (293-666 MDQ 30M; Mitutoyo Corp.).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
211
stevensite, the Na value was calculated as an atomic proportion relative to Si=4. The Na values of samples 1, 2, 3, 4, 5, and 6 were, respectively, 5.15, 1.35, 0.68, 0.56, 0.36, and 0.38. Sample 1 has an extremely great amount of sodium; it is considered that much of the amount of sodium is near the surface region of the synthesized particles. The sodium ratio decreased with the washing cycle number; it approached the ideal composition ratio in samples 5 and 6. These results suggest that five repeated washings are sufficient to remove excess sodium and to obtain a precipitate of ideal stevensite composition. The sodium ratio does not decrease much beyond the ideal ratio in spite of numerous repeated washings.
3.3. Crystallization of smectite
Fig. 2. Sodium concentration in the supernatant separated by centrifugation.
3. Results and discussion 3.1. Removal of excess sodium ion in the precipitate by repeated washing The synthesis of stevensite can be represented by the following. ðSiO2 Þ3 ðNa2 OÞþ afMgðNO3 Þ2 gþ bHNO3 þ cðNaOHÞða; b; c : mole numberÞ þ
−
→ðSiO2 Þ3 ðMgOÞa ·mH2 O þ ð2 þ cÞNa þ ð2a þ bÞðNO3 Þ
ð1Þ
By this synthesis, (SiO2)3(MgO)a·mH2O is a hydrated gel in precipitate. Other ions aside from the hydrated gel were removed during the washing process. To elucidate the effect of the washing, Na+ concentration in supernatants was analyzed using ICP-AES. Fig. 2 shows the Na+ concentration in the supernatant of each sample. The sodium concentrations decreased concomitantly with the number of washing cycles: over 95% of sodium had been removed after the third washing. The concentrations of sodium in the fourth, fifth, and sixth supernatants are similarly low, which suggests that more than four washings are not effective for salt removal. 3.2. Sodium concentration in the product Sodium concentration in the product was measured using EDX. From the ideal chemical composition (Na0.33Mg2.85Si4O10(OH)2·nH2O) of
Fig. 3(a) shows the powder XRD pattern of sample 1. The assigned NaNO3 peaks are indicated. Sample 1 showed many sharp peaks for NaNO3. This result is expected from equation 1; the XRD pattern shows that excess Na+ and NO− 3 remained as NaNO3 crystals. Fig. 3(b) presents an expanded chart of Fig. 3(a); it is consistent with a typical smectite pattern. This result shows that smectite was synthesized, even though the yield is low, if the reactant precursor contained an excess amount of Na+. From samples 1–4, the intensity of the smectite peaks increased concomitantly with increasing washing cycle number. In contrast, those of the NaNO3 peaks decreased concomitantly with increasing washing cycles; the peaks had disappeared in samples 3–6. Fig. 4 shows XRD patterns of sample 4 for the powder state and the film coating on a glass substrate. The XRD peaks of the powder state presented a typical smectite pattern. A d(060) peak is apparent at 0.153 nm, indicating trioctahedral smectite. The XRD pattern suggests synthesis of trioctahedral smectite. The XRD pattern of the film shows strong (001) reflections at around 2θ = 6°, suggesting crystal stacking along with a highly ordered c-axis. The self-assembled lamination is also a characteristic of smectite (Faust and Murata, 1953). The intensity for (001) reflection peaks increased concomitantly with increasing washing cycles; it was the highest in sample 4. Then it decreased in samples 5 and 6 (Fig. 5). This result might be affected by coexistent Na+. Otsuka et al. (1972, 1979) and Sakamoto et al. (1981) reported that stevensite formation is accelerated by sodium. They investigated hydrothermal synthesis of stevensite from natural minerals and reported that sodium improves the crystallinity of stevensite and broadens stable temperatures of stevensite. It is natural to conclude that some excess sodium accelerates the crystallization of stevensite in sample 4, although too much excess sodium interrupts it.
Fig. 3. Powder XRD patterns of once washed smectite (sample 1; Na = 5.15, Si = 4) (a) and the expanded chart (b).
212
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
Fig. 4. XRD patterns of 4 times washed smectite (sample 4; Na = 0.56, Si = 4) for the powder state (a) and the film coated on a glass substrate (b).*: A unit is nm.
Fig. 6 portrays a TEM image of sample 5. The image shows a folded thin clay particle of ca. 2 μm in planar extension. The TEM image also suggests that smectite synthesis has occurred. 3.4. Thermal property of synthesized smectite The DTA curves of these samples are presented in Fig. 7. Samples 4 and 6 show a sharp exothermic peak at 750–800 °C corresponding to a phase transition from smectite to enstatite (Takahashi et al., 1997). This peak shifted to a higher temperature with increased washing. In contrast, samples 1 and 2 showed DTA peaks that differed from those of samples 4 and 6. Samples 1 and 2 showed an endothermic peak for NaNO3 decomposition at ca. 306 °C. This result is consistent with the results of chemical and XRD analyses. 3.5. Separation of smectite from the product We tried to separate the smectite and NaNO3 after synthesis. After dispersing sample 1 using distilled water, it was centrifuged; then the
Fig. 5. XRD patterns of the smectite films coated on glass substrates prepared through 4 times washing (sample 4; Na = 0.56, Si = 4), 5 times washing (sample 5; Na = 0.36, Si = 4), and 6 times washing (sample 6; Na = 0.38, Si = 4).
supernatant and the sediment were dried separately on glass substrates. These were dried separately on glass substrates. Fig. 8 depicts their XRD patterns. The sedimentation showed only the smectite pattern, although the supernatant showed the NaNO3 pattern. It shows that separation of smectite from salt is possible after synthesis. However, the washing process is better if carried out before hydrothermal synthesis for two reasons. First, a low crystallinity of smectite is expected if the hydrothermal synthesis is conducted without washing, as shown in the XRD patterns (Fig. 3) and DTA curves (Fig. 7). Second, complete separation is difficult.
3.6. Viscosity of aqueous dispersions Fig. 9 shows the viscosity of 3 wt.% aqueous dispersion for samples 4, 5, and 6. Samples 5 and 6 showed a similar low viscosity although sample 4 had a high viscosity. This is because of the higher salt content of sample 4. The salt in the dispersion affects the electro-diffuse layer and increases the viscosity (Suzuki et al., 2005). Additionally, the viscosity of a 2 wt.% aqueous dispersion of commercially available synthetic smectite samples was measured. The viscosity values of two synthetic
Fig. 6. TEM image of 5 times washed smectite (sample 5; Na = 0.36, Si = 4).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
213
Fig. 9. Viscosity of 3 wt.% aqueous dispersions of 4, 5, and 6 times washed smectites (sample 4, 5, and 6) at 10, 20, and 30 rpm of the spindle rotation. Fig. 7. DTA peaks of samples with different washing cycle number. Sample 1; once washed smectite (Na= 5.15, Si= 4): sample 2; twice washed smectite (Na= 1.35, Si= 4): sample 4; 4 times washed smectite (Na = 0.56, Si= 4): sample 6; 6 times washed smectite (Na = 0.38, Si= 4).
hectorite samples (Thixopy; Kyowa Chemical Industry Co., Ltd. Japan, SWN; Co-op Chemical Co., Ltd. Japan) are 1000 and 504 mPa s, respectively, at 30 rpm of spindle rotation. Even with the high concentration of the dispersion of our smectite, it is shown that the viscosity of samples 5 and 6 decreased drastically through purification. 3.7. Film formability of the synthetic smectite and the property of each film 3.7.1. Coating film The aqueous dispersions of samples were deposited on glass substrates and dried at room temperature. Dispersions of samples 1 and 2 showed little sediment, but the others were transparent gels. After drying, white powder remained on the glass substrate for the dispersion of sample 1. Sample 2 yielded a transparent film with a small amount of white powder dispersed in the film. The other dispersions formed uniform transparent films on the glass substrates.
The total light transmittance and haze of these films were measured after subtracting those of the glass substrate as the baseline (Table 1). All films except of samples 1 and 2 showed total light transmittance greater than 99%. The haze of these films decreased concomitantly with increasing washing cycle number. 3.7.2. Self-standing film The smectite samples produced were used in an attempt to make self-standing films with no binder. Dispersions were poured in to polypropylene trays and dried at room temperature. Fig. 10 presents results for samples 1, 2, 4, and 6. Sample 1 did not produce a film but instead produced a powdery precipitate because it contained too much NaNO3. Sample 2 became transparent in spite of coexisting excess NaNO3, but it shrank during drying and became a distorted film. Sample 4 produced a transparent film with curly edges. Sample 6 produced a flat transparent film of ca. 110 μm thickness to have the same size of the tray, with no shrinkage. Curling and distortion of the films were remarkable when there was much more excess salt ion in the sample. This result suggests that coexistent salt strongly affects the film formation. The film formability improves with increasing purity of smectite. Apparently, clay particles were aggregated in the concentrated salt solution. Sample 4 contains about two times the sodium used for the ideal composition. Excess salt is considered to induce aggregation of clay sheets even though it was not detected in the XRD pattern. To elucidate reasons for film distortion in dispersions with excess salt ions, the concentration of salt in the top and bottom surfaces of the films were measured by EDX. The concentrations of salt on both surface sides were not different; they were also the same on interior and exterior surfaces of the curled film, as in sample 2, so it appears that the salt concentration is uniform throughout the film. Solubility of NaNO3 is 92 g/100 g in distilled water at 25 °C (NaNO3, ICSC#0180). The salt in the smectite sample was dissolved completely in the 1 wt.% dispersion used for the self-standing film preparation. It is assumed that the salts contract the film through evaporation; then the smectite sheets are
Table 1 Total light transmittance and haze of films coated on glass substrates.
Fig. 8. XRD patterns of the product, the separated supernatant, and the sedimentation by centrifugation.
Sample number
1
2
3
4
5
6
Washing cycle number Total light transmittance (%) Haze (%)
1 89.6 74.0
2 98.8 19.2
3 99.6 4.5
4 99.3 2.4
5 100.0 1.4
6 99.9 1.3
Glass substrate 92.1 0.3
214
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215
Fig. 10. Film preparation results using once washed smectite (sample 1; Na = 5.15, Si = 4) (a), twice washed smectite (sample 2; Na = 1.35, Si = 4) (b), 4 times washed smectite (sample 4; Na = 0.56, Si = 4) (c), and 6 times washed smectite (sample 6; Na = 0.38, Si = 4) (d).
drawn near. The film is shrunk because evaporation occurs more rapidly from the top than the bottom surface. The dispersions with low salt concentrations form a flat film by a transition from the card-house structure to the self-assembled laminated structure through evaporation. The film formation process is presented in Scheme 1. Both the smectite dispersions with high and low salt concentration are changed to the clay films through a card-house structure process and a laminated structure process by evaporation. Therefore, the dispersion with high salt concentration becomes a deformed film due to shrinkage of whole film caused by a decrease in the volume of salts, while that with low or
no salt concentration becomes a flat film through a uniform evaporation in whole film. The results presented have confirmed that purity (salt concentration) is important when synthesizing smectite for producing transparent clay films. 4. Conclusion To synthesize smectite with high film formability, stevensite samples with different salt concentration were synthesized using the hydrothermal method. The smectite samples prepared were characterized in terms of salt contents, crystalloscopic properties, thermal properties, and viscosities of dispersion. Coating films on glass substrates and self-standing films were prepared from aqueous dispersions of the smectite samples with no binder. Transparent self-standing films were formed from well-washed samples that have a nearly ideal composition. Moderate amounts of excess salt accelerated the crystallization of smectite, whereas too much prevented it. With increasing purity of smectite, the transparency of the coating films increased. Moreover, self-standing film formability improved with increasing purity (low salt concentration) of smectite. These results suggest that purity and crystallinity are important for the use of synthetic smectite as a film material. References
Scheme 1. Smectite film formation process by evaporation using aqueous dispersion with different salt concentration.
Buck, M.J., Crofts, R.D., Purchase, L.J., 1991. Synthesis of smectite clay minerals, European patent 614445B1. Ebina, T., Mizukami, F., 2007. Flexible transparent clay films with heat-resistant and high gas-barrier properties. Adv. Mater. 19, 2450–2453. Faust, G.T., Murata, K.J., 1953. Stevensite, redefined as a member of the montmorillonite group. Am. Mineral. 38, 973–987. Harvey, C.C., Lagaly, G., 2006. Developments in clay science. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Elsevier Ltd., Amsterdam, pp. 506–540. Nam, H.-J., Ishii, R., Ebina, T., Mizukami, F., 2009. Flexible transparent self-standing binderless clay film prepared by hydrothermally treated synthetic clay. Mater. Lett. 63, 54–57. Neumann, B.S., 1976. Production of Magnesium Silicates, United States patent 4049780. Neumann, B.S., Sansom, K.G., 1973. Synthesis of hydrous magnesium silicates, United States patent 3954943. Otsuka, R., Imai, N., Sakamoto, T., 1972. Stability of stevensite under hydrothermal conditions. Ann. Sch. Sci. Eng., Waseda Univ. 36, 37–56. Otsuka, R., Sakamoto, T., Suzuki, S., Shinoda, S., Koshimizu, H., 1979. Hydrothermal treatment of pectolite — experimental studies of the genesis of stevensite and “hydrated talc” (3rd report). Koubutugaku-Zasshi 14, 170–186 (in Japanese).
H.-J. Nam et al. / Applied Clay Science 46 (2009) 209–215 Sakamoto, T., Koshimizu, H., Otsuka, R., 1981. Hydrothermal treatment of wollastonite — experimental studies of the genesis of stevensite and “hydrated talc” (4th report). Koubutugaku-Zasshi 15, 55–65 (in Japanese). Suzuki, S., Prayongphan, S., Ichikawa, Y., Chae, B.-G., 2005. In situ observation of the swelling of bentonite aggregates in NaCl solution. Appl. Clay Sci. 29, 89–98. Takahashi, N., Tanaka, M., Satoh, T., Endo, T., Shimada, M., 1997. Study of synthetic clay minerals. Part IV: synthesis of microcrystalline stevensite from hydromagnesite and sodium silicate. Microporous Mater. 9, 35–42.
215
Thomas, P.J., Baker, J.C., Zelazny, L.W., 2000. An expansive soil index for predicting shrinkswell potential. Soil Sci. Soc. Am. J. 64, 268–274. Torri, K., Iwasaki, T., 1986. Synthesized swelling silicate and its production, Japanese Patent 62292616.