Hydrothermal Synthesis of Humidity-Regulating Material from

Mar 12, 2013 - the distribution of the Chinese loess is shown in Figure 1. Loess .... The dried raw material (80 °C, 24 h) was first ... was calcined...
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Hydrothermal Synthesis of Humidity-Regulating Material from Calcined Loess Yi Zhang,† Zhenzi Jing,*,† Xinwei Fan,† Junjie Fan,† Lei Lu,† and Emile Hideki Ishida‡ †

School of Materials Science and Engineering, Tongji University, 4800 Cao’an Road, Shanghai 201804, China Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan

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ABSTRACT: To simulate the thermal properties of cave dwellings, which are warm in winter and cool in summer, a mesoporous material with a good humidity-regulating performance was synthesized hydrothermally from loess. Through the calcination of loess, which both provides active calcium through the decomposition of calcite within loess and improves the reactivity by dehydroxylation, a tough mesoporous material could be synthesized without any additives, and tobermorite formation was found to exert a positive effect on its strength and porosity. Mesopores seemed to exert a positive influence on the humidity-regulating performance of the material from loess. Although calcination destroyed the original porosity of the loess, the porosity and thus the humidity-regulating performance could be recovered greatly through hydrothermal treatment. As such, using only calcined loess, a tough mesoporous material exhibiting good humidity-regulating performance was synthesized that could be used as a “cave-dwelling” building material in cities to save energy.

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INTRODUCTION Loess is widely distributed in Middle Asia, Russia, North America, and the Middle East. In China, the loess plateau covers approximately 640000 km2 of landscape, and a map1 of the distribution of the Chinese loess is shown in Figure 1. Loess

the original mesopores within loess, which provides excellent thermal insulation and humidity self-regulating properties. However, to date, loess cannot be directly used as a material in cities because of some issues such as strength, durability, and workability. On the other hand, common brick, made mainly from loess, is still widely used as a primary building material in China and some developing countries not only because it has good mechanical properties but also because loess is very abundant and inexpensive. However, common brick has poor humidityregulating properties because of its high firing temperature during production. Ishida3 reported that the temperature of manufacture of earthen products must be lower than 500 °C to sustain the inherent properties and performance of the original materials. Therefore, to produce a porous material from loess whose original mesopores remain after manufacture, a lowtemperature synthesis technology is needed. A low-temperature (≤200 °C) hydrothermal method has been developed to synthesize mesoporous earthen materials4,5 and other materials.6,7 Previous work found that a mesoporous material from diatomaceous earth could be hydrothermally synthesized with slaked lime addition,7,8 although the addition of too much slaked lime could destroy the original microstructure of material, that is, the porous structure of sepiolite.9 The hardening mechanism for the hydrothermal synthesis of earthen materials showed that tobermorite exerts the most important effects as a strength-producing constituent of the synthesized materials.9,10 The molar ratio of CaO to SiO2 (C/ S) is regarded as the most important factor. To favor the formation of tobermorite, the C/S ratio of the starting materials should be about equal to the stoichiometric C/S ratio of

Figure 1. Map of the distribution of Chinese loess.1

is an aeolian sediment formed by the accumulation of windblown silt,2 20% or less clay, and the balance equal parts sand and silt that are loosely cemented by calcium carbonate so that loess is usually homogeneous and highly porous. Cave dwellings, which commonly serve as houses in the loess plateau, have been in use for centuries. Cave dwellings are wellknown to have excellent properties in terms of warming in winter and cooling in summer with no need for the use of air conditioners throughout the year. Such properties are due to © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4779

November 27, 2012 March 7, 2013 March 12, 2013 March 12, 2013 dx.doi.org/10.1021/ie303232g | Ind. Eng. Chem. Res. 2013, 52, 4779−4786

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tobermorite (0.83). According to the composition of loess, the SiO2 content is higher than the CaO content (C/S = 0.41), and lime or calcic materials should, therefore, be added for tobermorite formation. The introduction of lime for the hydrothermal synthesis of loess not only will raise the synthesis costs but also could possibly damage the porous structure. Therefore, the hydrothermal synthesis of loess without lime (slaked lime) addition and with enhanced reactivity of loess is very important. Calcination is often used to improve the reactivity of clayey materials by dehydroxylation, for example, to transform kaolinite into metakaolinite, which is an amorphous aluminum silicate with a disordered structure.11,12 As mentioned in the preceding paragraph, to obtain a tough and porous material from loess, hydrothermal technology might be suitable for synthesizing loess while retaining its inherent porosity. The chemical and mineralogical analysis of loess shows that the SiO2 content is much higher than the CaO content and the content of calcite is also high, which suggests that, for the hydrothermal synthesis of loess, slaked lime should be added in abundance to form tobermorite. However, the introduction of excess slaked lime might destroy the porosity of loess. Calcination of loess might have the capability of both offering active calcium to form tobermorite through the decomposition of CaCO3 (calcite) into CaO (lime) and improving its reactivity by dehydroxylation. To the best of our knowledge, however, the hydrothermal synthesis of humidity-regulating material with calcined loess has not been reported in the literature. The objective of the work reported herein was thus to (1) improve the reaction activity of loess and produce lime from the decomposition of the calcite within loess by means of calcination; (2) hydrothermally synthesize a tough and porous material with the calcined loess without any additives (i.e., use of only the decomposed lime); and (3) investigate the hardening mechanisms, porosity evolutions, and humidity-regulating properties of raw loess, calcined loess, and hydrothermally synthesized calcined loess samples. The results are expected to provide practical information on the manufacturing of tough and mesoporous materials from loess that could be used as “cave-dwelling” building materials in cities to both enhance comfort and save energy.

Figure 2. Hydrothermal apparatus used for curing samples.

universal testing machine (XO-106A, Xie Qiang Instrument Technology), and all flexural strength values reported herein are averages of three measurements obtained from three samples. The crushed samples were then investigated using several analytical technologies: for crystalline phases, X-ray diffraction (XRD, D/max2550VB3+/PC); for crystal structures, Fourier transform infrared (FTIR) spectroscopy (EQUINOXSS); for fracture surface morphology, environmental scanning electron microscopy (ESEM, Quanta 200 FEG); for macropore size distribution, mercury intrusion porosimetry (MIP, model Poremaster 33P, Quantachrome); for specific surface area, total pore volume, and pore size distribution, nitrogen gas sorption analysis at −196 °C using an automatic adsorption apparatus (Autosorb-1 Quantachrome Instrument, Boynton Beach, FL); and for the water-vapor adsorption/desorption isotherms, water-vapor sorption analysis (Hydrosorb HS-12 Quantachrome Instruments). The humidity-regulating abilities of the samples were investigated according to Japanese Industrial Standards (JIS A 1470-1, 2:2002). The apparatus for measuring the humidity-regulating ability of materials is shown in Figure 3. The surfaces of the samples, except their top surface, was covered with aluminum tape so that adsorbed and desorbed water vapor could go through only the top surface. The samples were placed in a closed box with a relative humidity of 75% for 24 h, and then the relative humidity was

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EXPERIMENTAL SECTION Loess, obtained from Qian County, near Xi’an, China, was used as a raw material in this study. Its chemical composition, determined by X-ray fluorescence spectroscopy (XRF, SRS3400), was as follows: SiO2, 44.0%; CaO, 16.7%; Al2O3, 10.4%; Fe2O3, 3.65%; MgO, 2.12%; Na2O, 1.40%; K2O, 1.93%; and TiO2, 0.54%. The dried raw material (80 °C, 24 h) was first ground in a ball mill to pass 150 μm, and then the ground loess was calcined in a refractory electronic furnace at 800 °C for 2 h at a heating rate 10 °C/min. The raw loess (RL) and calcined loess (CL) were separately mixed with 10 mass % distilled water, and then the mixtures were compacted into a mold with a compaction pressure of 30 MPa. The demolded samples (40 mm × 15 mm × 8 mm) were hydrothermally treated under saturated steam pressure from room temperature to 200 °C for up to 48 h; the hydrothermal apparatus for curing the samples is shown in Figure 2. Afterward, all of the samples were airdried at 80 °C for 24 h and subsequently stored in desiccators until further study. The dried samples were used to measure the three-point flexural strength at a loading rate of 0.5 mm/min with a

Figure 3. Apparatus for measuring humidity-regulating properties of materials. 4780

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changed to 33% for measurement for another 24 h. The relative humidities of 33% and 75% were controlled using MgCl2·6H2O saturated solution and NaCl saturated solution, respectively. During this process, the amounts of water vapor adsorbed/ desorbed by the samples were measured every 10 min.

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RESULTS AND DISCUSSION Material Characterization. The thermogravimetry (TG)/ differential thermal analysis (DTA) curves of RL are shown in Figure 4. The weight loss (4.52%) occurring in the temperature

Figure 6. Environment scanning electron microscopy (ESEM) images of (a) RL and (b) CL.

morphologies of both RL and CL. The agglomerated and bonded appearance of CL might lead to a small shift of the particle size distribution toward larger sizes. The chemical compositions of RL and CL presented in Table 1 show that the main components of both were SiO2, CaO, and Table 1. Chemical Compositions of RL and CL (mass %)

Figure 4. Thermogravimetry (TG)/differential thermal analysis (DTA) curves of RL.

component

RL

CL

SiO2 CaO Al2O3 Fe2O3 MgO Na2O K2O TiO2 LOI

44.0 16.7 10.4 3.65 2.12 1.40 1.93 0.54 19.14

49.5 19.9 11.3 4.14 2.34 1.50 2.17 0.59 10.8

Al2O3, which means that they belong to the SiO2−CaO− Al2O3−H2O hydrothermal system. The mineralogical composition of loess identified from XRD patterns, as shown in Figure 7, mainly corresponded to quartz, calcite, chlorite, kaolinite,

range of 430−480 °C is due to the removal of structure water from the mineral and the dehydroxylation of clay in the loess,13,14 where SiO2 and Al2O3 might transform into an unstable amorphous or metastable structure, which would improve the reaction activity of the loess.15,16 The largest endothermic peak with an obvious weight loss (15.28%) at temperatures between 600 and 780 °C results from the decomposition of the calcite (CaCO3) in RL. The mixing of alkali carbonates or alkali halides into calcium carbonate could exert a catalytic effect on the calcination temperature, which could reduce the calcination temperature significantly.17−19 Therefore, some trace of minerals within loess seemed to function as “catalysts” to lower the theoretical decomposition temperature of CaCO3 (853 °C).20 Therefore, a loess calcination temperature of 800 °C was used in this study. The particle size distributions of RL and CL are shown in Figure 5, in which particle size of CL seemed to be larger than that of RL. The SEM images in Figure 6 reveal the particle

Figure 7. X-ray diffraction (XRD) patterns of RL and CL.

illite, feldspar, gehlenite, and muscovite. After calcination, however, the phases of calcite, chlorite, and muscovite tended to almost disappear, and at the same time, a phase of lime formed as a result of the decomposition of calcite.21 New phases corresponding to lime and portlandite became distinct, which should be reaction products of lime decomposed from calcite and of portlandite produced from damped lime. In addition, a broad and ridged diffraction peak tended to appear in the range between 28° and 32° for CL, and according to previous research,22,23 this material should be amorphous silicate or aluminosilicate due to calcination. The FTIR spectra of RL and CL are shown in Figure 8. According to the reported XRD results (Figure 7), namely, that calcite peaks could be observed, the 1436, 875, and 713 cm−1 peaks of RL, attributed mainly to the vibration modes of CO32− of calcite, disappeared or weakened significantly after

Figure 5. Particle size distributions of (a) RL and (b) CL. 4781

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that the reaction activity of loess increased significantly after calcination. As shown in Figure 7, lime (CaO) was decomposed from the calcite (CaCO3) in loess after calcination, and part of the lime was transformed into portlandite when contacted with moisture in air. The free Ca2+ derived from lime and portlandite was much more active than that from calcite in pure water. More Ca2+ derived during the hydrothermal process could result in the formation of more calcium silicate hydrate (C−S− H) materials, such as C−S−H gel and tobermorite, thereby enhancing the strength. On the other hand, the layer and framework silicate structure of loess would be broken to transform into unstable intermediates after calcination, which improved the activity of the loess. The hydrothermal reaction between silica and lime is mainly controlled by the solubility of silica, and the solubility of silica in pure water is much higher at 200 °C than at 100 °C,26 which also provides more silica formed to react with lime. More C−S−H gel or tobermorite formation resulted in the achievement of a higher strength at 200 °C. The effects of hydrothermal time on flexural strength of the HCL and HRL samples cured at 200 °C without any additives are shown in Figure 10. The flexural strength of the HRL

Figure 8. Fourier transform infrared (FTIR) spectra of RL and CL.

calcination, suggesting that the calcite in RL decomposed after calcination. A very sharp spike at 3643 cm−1corresponding to the stretching vibration of O−H was clearly observed in CL because of portlandite formation. Generally, the spectral features of silicates in loess appear as a complex group of bands in the range of 1200−900 cm−1, which is attributed to the asymmetrical stretching vibration of SiO4 tetrahedra.24,25 To facilitate the description of the silicate structure, the Q0−Q4 notation is commonly used to indicate the connectivity of silica, where Q represents the tetrahedron and the superscript represents the number of other Q units to which it is bonded. Bands corresponding to Q2 sites appear in the range of 1100− 900 cm−1, and Q1 sites appear near 800 cm−1, which means that the stretching bands of higher-connectivity SiO4 tetrahedra tend to appear at relatively higher wavenumber. The broad band centered at 1032 cm−1 in RL obviously moved to lower wavenumber (1009 cm−1) in CL. This suggests that the connectivity of the SiO4 tetrahedra decreased after calcination (i.e., the stability tended to be poor), which might produce higher amounts of metastable matter and amorphous silicate or aluminosilicate minerals. Strength Development and Hardening Mechanism. The effects of hydrothermal temperature on the flexural strength of the samples solidified with RL and CL cured for 12 h are shown in Figure 9. The flexural strength of hydrothermally synthesized calcined loess (HCL) samples increased slowly until 100 °C, after which it rose very rapidly and reached around 23 MPa at 200 °C. In contrast, the strength of the hydrothermally synthesized raw loess (HRL) samples exhibited little change as the temperature increased, indicating

Figure 10. Effects of hydrothermal time on the flexural strength of HRL and HCL samples cured at 200 °C.

sample was only 5 MPa even after 48 h, whereas the flexural strength of the HCL sample reached 23 MPa for only 6 h, which is nearly 4.5 times higher. After 6 h, the strength of HCL tended to drop, and after 12 h, it exhibited a constant value. XRD patterns of HCL samples for different hydrothermal times at 200 °C are given in Figure 11 to reveal the reason for the rapid increase in the strength of HCL. Quartz, muscovite, lime, and portlandite were confirmed in the green sample (0 h). With increasing time, the peaks of quartz decreased, and those of lime and portlandite disappeared after 3 h. At the same time, a new phase corresponding to 1.1-nm tobermorite [Ca5(OH)2Si6O16·4H2O] became distinct, and its peak intensity seemed to reach a maximum at 6 h and then decreased slightly. These results suggest that the decomposed lime from calcite or portlandite reacted mainly with quartz to form tobermorite. The tobermorite, shown in ESEM images (Figure 14d), forming around particles and filling the spaces between these particles, and thus leading to a dense matrix, played an important role in the strength enhancement.27 The high strength (∼23 MPa) achieved in a short time (6 h) indicates that calcination did improve the reactive activity of loess significantly. However, a longer curing time (12 h) caused strength deterioration. A longer curing time could result in tobermorite growth (Figure 14d), which might lead to a loose matrix, which might deteriorate the strength.7

Figure 9. Effects of hydrothermal temperature on the flexural strength of HRL and HCL samples cured for 12 h. 4782

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Figure 13. Evolutions of the pore distributions of HCL samples cured at 200 °C for different times.

Figure 11. XRD patterns of HCL samples cured for different times at 200 °C.

FTIR spectra of HCL samples cured for different times at 200 °C are shown in Figure 12. A very sharp spike at 3643

Figure 12. FTIR spectra of HCL samples cured for different times at 200 °C.

Figure 14. ESEM images of HCL samples cured at 200 °C for (a) 0, (b) 3, (c) 6, and (d) 12 h.

cm−1 due to the stretching vibration of O−H appeared at 0 h, and with increasing curing time, the band tended to disappear after 1 h. According to the XRD analysis (Figure 11), the peak should correspond to portlandite, which was derived from loess after calcination and was reacted completely at 3 h. A broad shoulder band appeared at about 903 cm−1 and decreased gradually with time, and a new band at 976 cm−1 due to Si−O stretching vibrations28 corresponding to C−S−H or tobermorite was observed from 3 h and became sharper afterward, which indicates a transformation from lime (portlandite) to tobermorite, which is in good agreement with the results obtained from the XRD analysis (Figure 11). A detailed investigation of the microstructure evolution for HCL samples with increasing curing time was also conducted by measuring the change in porosity (Figure 13). Before hydrothermal treatment (0 h), the pore size distribution had a high and sharp distribution between 0.03 and 2.0 μm with one high-frequency peak at about 1.0 μm. According to the ESEM image of the sample without curing (0 h) shown in Figure 14a, the peak corresponds mainly to the spaces between agglomerated or bonded particles of CL. At 3 h, the pore size distribution (main peak) tended to shift toward smaller

pores, and two new peaks formed at 0.02 and 0.04 μm. The peaks correspond to the spatial dimensions between the particles within the sample at the time of their formation. The shift and formation of the peaks suggest that the formed crystals filled the spaces between particles. According to the XRD results (Figure 11), the peaks formed mainly correspond to tobermorite formation. The platelike morphology of tobermorite crystals, as shown in the ESEM image (Figure 14b), filled the spaces between particles to reduce the spatial dimension, leading to a shift toward smaller pore size (peak at 0.4 μm), and the intercrystalline pores of tobermorite might cause the new peaks at 0.02 and 0.04 μm. With increasing curing time, more tobermorite formed at 6 h (Figures 11, 12, and 14c), causing a higher peak to form between 0.01 and 0.05 μm and a further shift of the main peak toward 0.2 μm (more crystals filled in the spaces). The smaller pore size distribution resulted in a denser matrix and thus enhanced the strength of the samples. The hydrothermal process can be considered as a dissolution/precipitation process, and a long curing time might result in crystal growth. The ESEM image of the samples cured for 12 h (Figure 14d) showed that the grown tobermorite 4783

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resulted in a shift of the pore size distribution toward a larger pore size (loose matrix), which might cause a deterioration in strength. Porosity and Humidity-Regulating Properties. Samples of RL, CL (without hydrothermal treatment), and HCL (hydrothermally synthesized calcined loess) were used to further investigate the microstructure and humidity-regulating properties. Table 2 reports the specific surface areas and total pore volumes of the samples. Sample CL had the smallest specific Table 2. Specific Surface Areas and Total Pore Volumes of Samples RL, CL, and HCL sample

specific surface area (m2/g)

total pore volume (cm3/g)

RL CL HCL

8.431 3.894 13.824

0.06185 0.03689 0.08326

Figure 16. Water-vapor isotherms at 25 °C for samples RL (squares), CL (triangles), and HCL (circles). Solid and open symbols represent adsorption and desorption isotherms, respectively.

sample RL, for which the amount of adsorbed/desorbed water changed from 15 to 35 mg/g as the relative water-vapor pressure changed from 0.3 to 0.8, which might reflect good humidity-regulating performance for ordinary cave-type dwellings. Sample HCL also exhibited good humidity-regulating performance, meaning that the adsorption/desorption properties of CL were renewed upon hydrothermal treatment. It should be noted that the isotherm of sample HCL contains a type IV hysteresis loop in the range of 0.3−0.8 relative watervapor pressure. Usually, according to theory, a type IV hysteresis loop is caused by cylindrical pores with open ends and “ink-bottle”-type holes; however, in this study, it is more likely to be caused by an s series of cross pore space such as in the clearance between tabular/rodlike particles and intergranular pores, as shown in Figure 14c. Figure 17 presents changes in the moisture contents of the samples with measurement time for 48 h at 25 °C and relative

surface and total pore volume, whereas sample HCL had the largest, suggesting that the porosity was deteriorated markedly after calcination but could be improved by hydrothermal treatment. To further investigate the microstructure of the samples, the mesopore size distribution, as shown in Figure 15, was

Figure 15. Mesopore size distribution curves calculated by the Barrett−Joyner−Halenda equation using N2 gas desorption isotherms for samples RL (squares), CL (triangles), and HCL (circles).

determined by the Barrett−Joyner−Halenda method from the nitrogen gas isotherms. Samples RL and HCL exhibited pore sizes greater than 8 nm, and RL seemed to have more mesopores, whereas for sample CL, the mesopore size was greater than 20 nm. This indicates that the mesopores of loess were destroyed partly after calcination; however, the mesopores could be re-formed through new crystal formation with hydrothermal treatment. As shown in the ESEM image in Figure 14c, the intertwining structure between the new phase and the particles of the starting materials due to tobermorite formation led to mesopore formation. As such, calcined loess can be used as a starting material for the hydrothermal synthesis of humidity-regulating building materials without any additives. The water-vapor adsorption/desorption isotherms of the samples at 25 °C are shown in Figure 16. As expected, the water-vapor adsorption/desorption properties of sample CL were very poor and had few changes between 0.3 and 0.8 relative water-vapor pressure (30−80% relative humidity). The adsorption/desorption performance seemed to be best for

Figure 17. Weight changes of samples RL (squares), CL (triangles), HCL (circles), and common brick (pentagons) during the measurement period at 33−75% relative humidity at 25 °C for 48 h.

humidities of 33% and 75%. For sample RL, the moisture content increased quickly with time when the relative humidity was 75%; however, it decreased when the relative humidity was changed to 33%. The increase and then decrease in moisture content between 75% and 33% relative humidities demonstrates the good humidity-regulating performance (i.e., respiration characteristics) of the RL material. The humidityregulation properties of this material mean that it will regulate the indoor humidity by adsorbing the indoor moisture when the relative humidity is high and desorbing the adsorbed 4784

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moisture when the relative humidity becomes low. By contrast, sample CL exhibited only a slow increase in moisture weight for 48 h, regardless of whether it was in 75% or in 33% relative humidity, which suggests that the material lost its respiration characteristics upon calcination. Sample HCL, however, exhibited a trend in moisture content changes similar to that of sample RL, suggesting that the humidity-regulating properties were restored by hydrothermal treatment. Based on the Arai modification of the Kelvin equation of capillary condensation29 and a consideration of the capacity and speed of adsorption/desorption of moisture, this mesoporous material can be considered to be appropriate for good regulating performance in the range of 75−33% relative humidity. Although sample HCL had the highest specific surface area and total pore volume, as shown in Table 2, it seemed to have fewer mesopores than sample RL (Figure 15), which might have caused the small difference in changes in moisture content between samples HCL and RL shown in Figure 17. RL exhibited outstanding humidity-regulating performance, but its strength was very low (Figures 9 and 10), similar to that of the material (loess) used in cave-type dwellings, which is not suitable for use in city buildings. However, a material with high strength with good porosity can be prepared from calcined loess (CL) by a hydrothermal treatment. Use of calcined loess as the starting material not only provides good strength but also saves on the cost of hydrothermal synthesis greatly because it does not require any additives. To further compare the adsorption/desorption of moisture, the data for a sample of common brick are also included in Figure 17. The humidityregulating performance of common brick, similarly to that sample of CL, is also very poor. Generally, common brick is produced by firing at 800−900 °C with loess as the main material, and these high temperatures undoubtedly destroy the mesoporous structure of loess, making common brick lose the humidity-regulating properties of the original raw material.

synthesize humidity-regulating building materials without any additives.

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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 Shanghai Science and Technology Committee Program (China) (No. 09JC1413900) and the National Natural Science Foundation of China (Nos. 51072138, 51272180).

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REFERENCES

(1) Jin, C.; Liu, Q. Remagnetization mechanism and a new age model for L9 in Chinese loess. Phys. Earth Planet. Inter. 2011, 187 (3−4), 261−275. (2) Donahue, M. S. Soils: An Introduction to Soils and Plant Growth, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1977. (3) 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. (4) Maeda, H.; Ishida, E. H. Hydrothermal preparation of diatomaceous earth combined with calcium silicate hydrate gels. J. Hazard. Mater. 2011, 185 (2−3), 858−861. (5) Maeda, H.; Ishida, E. H. Water vapor adsorption and desorption of mesoporous materials derived from metakaolinite by hydrothermal treatment. Ceram. Int. 2009, 35 (3), 987−990. (6) Shan, C.; Jing, Z.; Pu, L.; Pan, X. Solidification of MSWI Ash at Low Temperature of 100 °C. Ind. Eng. Chem. Res. 2012, 51 (28), 9540−9545. (7) Jing, Z.; Maeda, H.; Ioku, K.; Ishida, E. H. Hydrothermal synthesis of mesoporous materials from diatomaceous earth. AIChE J. 2007, 53 (8), 2114−2122. (8) Maeda, H.; Kato, S.; Ishida, E. H. Preparation of Hydrothermally Solidified Mesoporous Materials from Diatomaceous Earth for Moisture Control Application. Int. J. Appl. Ceram. Technol. 2009, 6 (3), 431−436. (9) 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 (0), 30−36. (10) Maeda, H.; Kato, S.; Ishida, E. H. Preparation of Hydrothermally Solidified Mesoporous Materials from Diatomaceous Earth for Moisture Control Application. Int. J. Appl. Ceram. Technol. 2009, 6 (3), 431−436. (11) Fabbri, B.; Gualtieri, S.; Leonardi, C. Modifications induced by the thermal treatment of kaolin and determination of reactivity of metakaolin. Appl. Clay Sci., published online Oct 15, 2012, 10.1016/ j.clay.2012.09.019. (12) Elimbi, A.; Tchakoute, H. K.; Njopwouo, D. Effects of calcination temperature of kaolinite clays on the properties of geopolymer cements. Constr. Build. Mater. 2011, 25 (6), 2805−2812. (13) Buchwald, A.; Hohmann, M.; Posern, K.; Brendler, E. The suitability of thermally activated illite/smectite clay as raw material for geopolymer binders. Appl. Clay Sci. 2009, 46 (3), 300−304. (14) Kakali, G.; Perraki, T.; Tsivilis, S.; Badogiannis, E. Thermal treatment of kaolin: The effect of mineralogy on the pozzolanic activity. Appl. Clay Sci. 2001, 20 (1−2), 73−80. (15) Berna, F.; Behar, A.; Shahack-Gross, R.; Berg, J.; Boaretto, E.; Gilboa, A.; Sharon, I.; Shalev, S.; Shilstein, S.; Yahalom-Mack, N.; Zorn, J. R.; Weiner, S. Sediments exposed to high temperatures: Reconstructing pyrotechnological processes in Late Bronze and Iron Age Strata at Tel Dor (Israel). J. Archaeolog. Sci. 2007, 34 (3), 358− 373.

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CONCLUSIONS Hydrothermal synthesis of mesoporous materials from calcined loess without any additives and subsequent investigation of their porosity and humidity-regulation properties have been carried out. The experimental results can be summarized as follows: (1) Calcination of loess can both improve its activity and provide lime to form tobermorite during hydrothermal processing. The reaction activity of loess can be improved obviously by dehydroxylation, and active calcium can be obtained by the decomposition of calcite through calcination. As a result, loess can be synthesized hydrothermally without addition of lime, thereby reducing costs. (2) During the hydrothermal synthesis of materials from calcined loess, the formed tobermorite exerts a positive effect on the strength development and pore size distribution. However, excessive curing time (>12 h) seems to cause crystal overgrowth, resulting in a decrease in the strength. (3) Mesopores seem to have a positive influence on humidity-regulating performance. The mesopores of loess are partially destroyed after calcination; however, they can be reformed by new crystal formation during hydrothermal treatment. The sample synthesized with calcined loess had a good humidity-regulating performance in the range of 75−33% relative humidity. As such, loess can be used as a starting material for calcination followed by hydrothermal treatment to 4785

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dx.doi.org/10.1021/ie303232g | Ind. Eng. Chem. Res. 2013, 52, 4779−4786