Article pubs.acs.org/IECR
Relationship between Porous and Mechanical Properties of Hydrothermally Synthesized Porous Materials from Diatomaceous Earth Yani Jing,† Zhenzi Jing,*,‡ and Emile Hideki Ishida§ †
Department of Electronic Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, China School of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China § Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan ‡
ABSTRACT: Hydrothermal solidification/synthesis of diatomaceous earth based material has been carried out so as to investigate the relationship between porous and mechanical properties. Compaction pressure could make contact of particles closer, which accelerated the hydrothermal reaction and thus improved the strength of the solidified specimen; however, it could shrink the pore volume yet seemed to exert a small influence on the surface area. Curing time and temperature affected the porous and mechanical properties of synthesized specimens significantly, and the toughest specimen seemed to possess the largest surface area and optimum pore size distribution. As such, a porous and tough humidity regulating material seemed to be manufactured easily by the hydrothermal solidification/synthesis technology.
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INTRODUCTION Humankind has utilized loess cleverly as a construction material in many ways, such as in its natural state loess has been in use as cave-type dwellings for centuries, and sun-dried loess bricks as construction materials are still used widely in a loess plateau area (China) to date. Cave-type dwellings, as well as the sundried loess brick buildings, are well-known to have excellent properties in terms of warming in winter and cooling in summer. Such properties are due to the loess containing incipient micropores that can be effective in imparting humidity self-regulating performance and thermal insulation. However, the use of loess as building materials is impossible in current construction practice due to problems regarding strength, durability, and workability. Solidified ceramics such as bricks, blocks, and tiles made their appearance early in history so as to solve these problems. However, ceramics are produced through high-temperature firing, which can possibly damage the inherent properties (porous microstructure) and performance (humidity regulating) of loess. To sustain the inherent properties and performance, the temperature of the manufacture of earthen products must be lower than 500 °C, and in the case of loess/ organic material composites, an even lower temperature is desirable.1 Therefore, a porous humidity regulating material seems not suitable to be manufactured by high-temperature firing. A humidity regulating material was produced with allophone by low-temperature firing (about 900 °C),1 and although some micro-/mesopores of allophone were damaged during firing, the strength of the products at least could meet construction requirements. A new low-temperature (≤200 °C) hydrothermal solidification/synthesis technology is developed to obtain a material with properties and performance between the sun-dried brick (room temperature) and ceramics (≥1000 °C), which might be suitable to manufacture tough materials that sustain the © 2013 American Chemical Society
inherent porous property of the raw materials. This technology has been used to solidify clay minerals and wastes.2−6 Diatomaceous earth (DE) is a naturally occurring clay material, comprised of very porous fossilized remains of planktons or green algae. Each granule of diatomite contains millions of microscopic, hollow, perforated cylindrical shells, resulting in an inert, lightweight, high porosity and thermally resistant, and powerful absorbent material. DE, therefore, has extensively been applied in many processes, such as building materials,7,8 porous materials,9,10 adsorbent materials,11,12 and thermal energy storage materials.13,14 Because of its light, porous, and thermal resistance properties, DE is considered as a perfect material for producing a regulating humidity material. As mentioned above, the new low-temperature (≤200 °C) hydrothermal solidification/synthesis technology, which can obtain a material with properties and performance between the sun-dried brick (low temperature favors retention of its porous microstructure) and ceramics (high-temperature firing causes its porosity to be destroyed), might be suitable to manufacture tough materials in which the inherent porous property of the raw materials is sustained. However, the evolutions of the strength and porosity, especially the relationship between strength and porosity, during hydrothermal synthesis have rarely been investigated. To manufacture a tough and porous humidity regulating material from clay materials, the present work is aimed at exploring the evolution of porosity and the strength of specimens from DE during hydrothermal synthesis and at investigating how to obtain a tough product but maintain its inherent porous microstructure which can provide good Received: Revised: Accepted: Published: 17865
June 28, 2013 November 8, 2013 November 14, 2013 November 14, 2013 dx.doi.org/10.1021/ie4020205 | Ind. Eng. Chem. Res. 2013, 52, 17865−17870
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humidity regulating performance. These results are expected to provide practical information on the hydrothermal manufacture of humidity regulating materials from clay minerals such as DE, loess, and sepiolite.
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EXPERIMENTAL SECTION Materials. Commercially available DE powder precalcined at 1200 °C (Brunauer−Emmett−Teller (BET) surface area: 36.3 m2/g), obtained from Showa Chemical Industry, Tokyo, Japan, was used in this study due to the fact that the raw DE frequently contains an important rate of impurities that can restrain its efficiency in many applications15 and also calcination of diatomite can improve the strength of products.7 Hydrate lime (reagent grade) from Wako Pure Chemical Industries, Osaka, Japan, was used as an additive. The particle size distributions of DE were determined by laser diffraction technology (X100, Microtrac). The chemical composition of DE, as measured by X-ray fluorescence (XRF; RIX3100, Rigaku), and the DE and specimens’ mineralogical compositions and crystalline morphologies, as measured by X-ray diffraction (XRD; MiniFlex, Rigaku) and scanning electron microscope (SEM; S-4100, Hitachi), are shown in Table 1 and Figures 1 and 2, respectively. Table 1. Composition of DE Used (mass %) SiO2 CaO Al2O3 TiO2 Fe2O3 K2O
86.4 0.9 9.2 0.3 2.5 0.7
Figure 2. SEM micrographs of (a) DE and specimens cured at 200 °C for (b) 3 h and (c) 12 h.
SiO2 content is higher than the CaO content, and then DE mixed with hydrate lime at C/S 0.9 was used as the starting material. A 15 g amount of starting material was first mixed with 4.5 mL of distilled water (30 mass %) in a mortar manually, and then the mixture was compacted in a column-shaped mold (30 mm diameter × 120 mm height) at 10−30 MPa. The demolded specimens were subsequently autoclaved under saturated steam pressure (0.2−1.55 MPa) at 120−200 °C, for up to 24 h. The Teflon (PTFE) lined stainless steel hydrothermal apparatus, as shown in our previous work,16 was used for curing the demolded specimens. After autoclaving, all synthesized specimens were dried at 80 °C for 24 h. Characterization. The dried specimens (30 mm diameter × 20 mm height) were used to measure the tensile strength using the Brazilian testing method.17 The Brazilian tests were
Figure 1. XRD patterns of DE and specimens cured at 200 °C for different curing times.
Hydrothermal Processing. Our previous work showed that, for hydrothermal solidification/synthesis, tobermorite (or CSH gel) formation was favored to enhance the strength of the solidified specimens. The molar ratio of CaO/SiO2 (C/S) was regarded as the most important factor for synthesis of tobermorite (Ca5(OH)2Si6O16·4H2O). In favor of the formation of tobermorite, the C/S of the starting materials should fall near the stoichiometric C/S of tobermorite (0.83). According to the chemical composition of DE (Table 1), the 17866
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performed in a universal testing machine (M1185, Instron) at a crosshead speed of 0.2 mm/min. All tensile strength measurements presented here were the average of three measurements on three different specimens. Then, the crushed specimens were investigated by several techniques: for crystalline phase analysis, XRD; for phase qualitative identification, energy dispersive X-ray spectroscopy (EDS; Genesis 7000, EDAX/ TSL); and for total pore volume, total surface area, and pore size distribution, mercury intrusion porosimetry (MIP; Poremaster 33P, Quantachrome).
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RESULTS AND DISCUSSION Properties of DE. The mineralogical composition of asreceived DE shown in Figure 1 corresponds mainly to cristobalite (SiO2), with a small proportion of quartz (SiO2), hematite (Fe2O3), and kyanite (Al2SiO5). The SEM micrograph (Figure 2a) shows that DE has cellular structure. The particle size distribution of DE is shown in Figure 3: its particle size ranges from 6 to 200 μm, and the average particle size (D50) is 47 μm.
Figure 4. Effects of compaction on the tensile strength, the total pore volume, and the total surface area of the specimens cured at 200 °C for 12 h with different compaction pressures. Solid circle marks are the measured strength values at each condition, respectively, and the line between the marks is the average tensile strengths based on three measured specimens.
Figure 3. Particle size distribution of the DE used.
Effect of Compaction Pressure. In hydrothermal solidification/synthesis processing, the starting material was first compacted and then cured in an autoclave for hardening. Therefore, the compaction pressure might affect the porous and mechanical properties of the solidified specimens. Figure 4 shows the evolutions of the tensile strength, pore volume, and surface area of the specimens synthesized at 200 °C for 12 h with different compaction pressures. The tensile strength, as expected, increased with increasing compaction pressure until 20 MPa and then remained constant afterward. The pore volume and surface area, however, decreased with increasing compaction pressure. Compaction pressure could influence the degree of the contact between particles within the green specimen, and closer contact of particles with larger compaction pressure might accelerate the hydrothermal reaction and thus improve the strength of the solidified specimen. However, a larger compaction pressure could shrink the pore volume but seemed to exert a small influence on the surface area, as shown in Figure 4. The evolution of the pore size distribution of the above specimens with different compaction pressures was also investigated. As shown in Figure 5, at 10 MPa, a broad pore size distribution ranging from 0.006 to 1.0 μm in diameter with two peaks at 0.015 and 0.3
Figure 5. Evolution of pore size distribution of specimens cured at 200 °C for 12 h with different compaction pressures.
μm, respectively, was observed. The peaks should correspond to both the structural pores of DE and the voids between particles within the solidified specimen. The peak of 0.3 μm shifted clearly toward a small pore size (peaks at 0.1, 0.05, and 0.025 μm) with larger compaction pressure (15, 20, and 30 MPa, respectively), showing that the matrix became denser, and the denser matrix thus resulted in an enhancement in strength. The shift of the pore size distribution also caused a decrease in the total pore volume shown in Figure 4. Effect of Curing Time. To elucidate the evolutions of porous and mechanical properties during the hydrothermal process, the effects of curing time on the strength, surface area, 17867
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and pore volume of specimens solidified at 200 °C were investigated (Figure 6). The tensile strength increased rapidly
3 h, however, the peak tended to shift toward a small pore size (at peak of 0.06 μm) and the pore distribution narrowed to ∼0.01−0.2 μm. The peak shift reflects the reaction progress within the matrix, which resulted in a change in pore dimension. The SEM micrograph of the specimen cured at 3 h (Figure 2b) indicated that the presence of a newly formed spongy crystal was filled in the voids, which reduced the pore dimension (Figure 7) and in turn enhanced the strength (Figure 6). The spongy crystal should be calcium silicate hydrate (CSH) gel, which is the precursor of tobermorite. The peak of the pore size distribution shifted further toward smaller pore size until 18 h, and then drifted back to large pore size slightly. The finest pore distribution was obtained at 18 h, at which its pore size distribution ranged wholly within the mesoporous area (2−50 nm). The shift of the pore size distribution toward small pore suggests that more crystals formed in the interspaces between particles. The SEM micrograph at 12 h revealed the platy crystal formation (Figure 2c), whose newly formed intercrystalline pores were filled with numerous individual fine crystal particles. The phase evolution with curing time is shown in Figure 1. The main phases corresponding to portlandite, cristobalite, and quartz for the specimen without curing (0 h) were confirmed. From curing times of 3 and 12 h, a trace of the phase corresponding to CSH and a new phase of 1.1 nm tobermorite tended to form, respectively, and the peak intensity of tobermorite also increased for the longer curing time. According to EDS analysis (Figure 8) and their morphologies, the platy crystal should be tobermorite and the fine particles CSH. The interlocked structure of tobermorite formed within the space between DE
Figure 6. Effects of curing time on the tensile strength, total pore volume, and total surface area of specimens cured at 200 °C. Solid circle marks are measured strength values at each condition, respectively, and the line between the marks is the average tensile strengths based on three measured specimens.
up to 18 h and declined afterward. The evolution of the surface area is very similar to that of the strength, while the pore volume seems to be indeclinable with increasing curing time. A detailed investigation of the evolution of the porosity with increasing curing time for the preceding specimens was also conducted by measuring the change in pore size distribution (Figure 7). Before hydrothermal treatment (0 h), the pores had a broad distribution between 0.02 and 3.0 μm with a peak at ∼1.0 μm. The pore dimension should correspond to the voids between particles within the green specimen. At curing time of
Figure 8. EDS spectra of specimens cured at 200 °C for 12 h: (a) platy crystals and (b) fine particles.
Figure 7. Evolution of the pore size distribution of specimens cured at 200 °C for different curing times. 17868
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particles filling with finer CSH particles has made the matrix denser, thus enhancing the strength. For a humidity regulating material, calculation from Kelvin’s equation of capillary condensation, corrected for the preexistence of a certain thickness of the adsorbed layer prior to capillary condensation,18 yielded pore radii values of 3.2 nm for 40% relative humidity and 7.4 nm for 70% humidity, which shows that high-humidity regulating performance can be expected from materials synthesized with their pore radii being controlled to be within this range, in which a person feels comfortable. However, according to the response of the humidity regulating speed, the pore size (diameter) usually fills about the mesoporous (2−50 nm) area. It should be noted that the maximum strength and surface area and the optimum pore size distribution (mesopores) occurred at the same curing time of 18 h, which suggests that for the hydrothermal solidification/synthesis, the specimen which has the highest strength will possess the best porous property (highest surface area and optimum pore size distribution). Such a relationship between the porous and mechanical properties during the hydrothermal process depends on the hydrothermally hardening mechanism, in which the hydrothermal reaction only occurs at the surface of the particles within the matrix due to mild reaction conditions and the formed crystals (e.g., tobermorite or/and CSH) on the surface of the particle (i.e., in the interspaces between particles) could reduce void dimension (pore size), enhance surface area (massive intercrystalline pores formation), and densify the matrix of the specimen (massive crystals filling in the interspaces within matrix) thus resulting in an increase in strength. Effect of Curing Temperature. The curing temperature will also affect the porous and mechanical properties of specimens during the hydrothermal process. As shown in Figure 9, both tensile strength and surface area increased with increasing curing temperature, while pore volume remained constant. It is noted that the increase in strength matches very well with that in surface area; e.g., both the tensile strength and surface area increase form 0 (room temperature) to 120 °C quickly, then give the almost constant value during 120−150 °C, and afterward increase quickly again until 200 °C. The microstructure evolution with increasing curing temperature for the preceding specimens is shown in Figure 10. Without curing (0 °C), the pores had a broad distribution between 0.02 and 3.0 μm with a peak at ∼1.0 μm. At 120 °C, the pore size distribution shifted toward small pore size and at the same time a new pore size distribution between 0.01 and 0.2 μm formed. The pore size distribution at 150 °C seemed to be similar to that at 120 °C, which might result in the almost constant value in the strength and total surface area shown in Figure 9. At 200 °C, the pore size distribution varied obviously, and more small pores ranging from 0.006 to 0.03 μm formed clearly. Those pores could be observed in Figure 2c, which should be formed by the intercrystalline pores of tobermorite and interspaces between those fine CSH particles. Tobermorite and particle formation, undoubtedly, exert a positive influence on the enhancements in the strength and total surface area, which also obtained the same conclusion that the more the strength increased, the better the porous property achieved (higher total surface area and smaller pore size distribution (mesopores)), and vice versa.
Figure 9. Effects of curing temperature on the tensile strength, total pore volume, and total surface area of specimens cured for 12 h. Solid circle marks are measured strength values at each condition, respectively, and the line between the marks is the average tensile strengths based on three measured specimens.
Figure 10. Evolution of the pore size distribution of specimens cured 12 h for different curing temperatures.
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CONCLUSIONS The relationship between porous and mechanical properties of hydrothermally synthesized materials has been investigated so as to understand how to manufacture readily humidity regulating materials by hydrothermal solidification/synthesis. Compaction pressure could influence the porous and mechanical properties of the hydrothermally synthesized specimens positively; however, a large compaction pressure could shrink the pore volume within the matrix but seemed to exert a small influence on the surface area. Curing time and temperature exerted a significant effect on the porous and mechanical properties of synthesized specimens, and strength developments seemed to match well with the surface area evolutions. The toughest specimen achieved at 18 h had the largest surface area and optimum pore size distribution 17869
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(15) Bamda, B.; Kessaissia, Z.; Donnet, J. B.; Wang, T. K. Analytical study of the variation of physico-chemical and structural properties of a Kieselguhr during its decarbonation. Analusis 1998, 26, 164−169. (16) Jing, Z.; Ran, X.; Jin, F.; Ishida, E. H. Hydrothermal solidification of municipal solid waste incineration bottom ash with slag addition. Waste Manage. 2010, 30, 1521−1527. (17) International Society for Rock Mechanics Commission on Standardization of Laboratory and Field Tests.. Suggested methods for determining tensile strength of rock materials. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 1978, 15, 99−103. (18) Arai, C.; Mizutani, T.; Murase, Y.; Hanakawa, T.; Sano, Y. Measurement of pore distribution by water vapour adsorption. J. Soc. Powder Technol., Jpn. 1983, 20, 115−121.
(mesopores). As such, the hydrothermal solidification/synthesis technology seemed to have the capability of manufacturing a tough material that synchronously possesses good porosity (surface area and pore size distribution) easily.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86 21 6958 0264. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work reported herein was supported by the National Natural Science Foundation of China (Grant Nos. 51072138, 51272180).
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REFERENCES
(1) Ishida, E. H. Soil-ceramics (earth), self-adjustment of humidity and temperature. In Encyclopedia of Smart Materials; Schwartz, M, Ed.; Wiley: New York, 2002; pp 1015−1029. (2) Zhou, L.; Jing, Z.; Zhang, Y.; Wu, K.; Ishida, E. H. Stability, hardening and porosity evolution during hydrothermal solidification of sepiolite clay. Appl. Clay Sci. 2012, 69, 30−36. (3) Zhang, Y.; Jing, Z.; Fan, X.; Fan, J.; Lu, L.; Ishida, E. H. Hydrothermal synthesis of humidity-regulating material from calcined loess. Ind. Eng. Chem. Res. 2013, 52, 4779−4786. (4) Jing, Z.; Fan, X.; Zhou, L.; Fan, J.; Zhang, Y.; Pan, X.; Ishida, E. H. Hydrothermal solidification behavior of municipal solid waste incineration bottom ash without any additives. Waste Manage. 2013, 33, 1182−1189. (5) Shan, C.; Jing, Z.; Pu, L.; Pan, X. Solidification of MSWI ash at low temperature of 100 °C. Ind. Eng. Chem. Res. 2012, 51, 9540−9545. (6) Rios, C. A.; Williams, C. D.; Fullen, M. A. Hydrothermal synthesis of hydrogarnet and tobermorite at 175 °C from kaolinite and metakaolinite in the CaO−Al2O3−SiO2−H2O system: A comparative study. Appl. Clay Sci. 2009, 43, 228−237. (7) Posi, P.; Lertnimoolchai, S.; Sata, V.; Chindaprasirt, P. Pressed lightweight concrete containing calcined diatomite aggregate. Constr Build. Mater. 2013, 47, 896−901. (8) Ergun, A. Effects of the usage of diatomite and waste marble powder as partial replacement of cement on the mechanical properties of concrete. Constr. Build. Mater. 2011, 25, 806−812. (9) Liu, D.; Yuan, P.; Tan, D.; Fan, M.; Yuan, A.; Zhu, J.; He, H. Effects of inherent/enhanced solid acidity and morphology of diatomite templates on the synthesis and porous of hierarchically porous carbon. Langmuir 2010, 24, 18624−18627. (10) Liu, D.; Yuan, P.; Tan, D.; Liu, H.; Wang, T.; Fan, M.; Zhu, J.; He, D. Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products. J. Colloid Interface Sci. 2012, 388, 176−184. (11) Aivalioti, M.; Papoulias, P.; Kousaiti, A.; Gidarakos, E. Adsorption of BTEX, MTBE and TAME on natural and modified diatomite. J. Hazard. Mater. 2012, 207−208, 117−127. (12) Yuan, P.; Liu, D.; Tan, D.; Liu, K.; Yu, H.; Zhong, Y.; Yuan, A.; Yu, W.; He, H. Surface silylation of mesoporous/macroporous diatomite (diatomaceous earth) and its function in Cu(II) adsorption: The effects of heating pretreatment. Microporous Mesoporous Mater. 2013, 170, 9−19. (13) Karaman, S.; Karaipekli, A.; Sar, A.; Bicer, A. Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage. Sol. Energy Mat. Sol. Cells 2011, 95, 1647−1653. (14) Sun, Z.; Zhang, Y.; Zheng, S.; Park, Y.; Frost, R. L. Preparation and thermal energy storage properties of paraffin/calcined diatomite composites as form-stable phase change materials. Thermochim. Acta 2013, 558, 16−21. 17870
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