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Effect of vacuum dehydration on gel structure and properties of metakaolin-based geopolymers
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Effect of vacuum dehydration on gel structure and properties of metakaolin-based geopolymers Wei Li, Patrick N. Lemougna, Kaituo Wang, Yan He, Zhangfa Tong, Xuemin Cui
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School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Geopolymer Metakaolin Water Vacuum dehydration Gel
In aluminosilicate mineral materials, water plays a significant role during the process of geopolymer reaction and is related closely to the development of the strength and structure. In this work, a geopolymer is synthesized via the reaction of metakaolin with alkaline silicate solutions, including water glass and a NaOH solution. This study investigated the effect of water on the gel structure and properties of metakaolin-based geopolymers. Geopolymer samples were found to fast release water under vacuum and allowed the development of low-defect porous materials. After 3 days of curing in suitable conditions, the samples were treated under vacuum, during which the compressive strength did not decrease. Under vacuum at 120 °C, the residual water of the sample produced using water glass was 8.31% and that of the sample produced using NaOH solution was only 1.08%. The results of XRD and FTIR demonstrated that the sample produced using NaOH solution consisted primarily of NaA zeolite. SEM analysis indicated that the pores in sample growed with increasing H2O/Na2O molar ratio. Additionally, vacuum dehydration can suppress the formation of micro cracks and reduce the shrinkage of the samples.
1. Introduction Geopolymers, first introduced by Davidovits [1], are aluminosilicate mineral polymer materials synthesized by geosynthesis or geochemistry. Geopolymers are based on amorphous silicate and aluminosilicate mineral materials in alkali activated aqueous solutions, in which silica oxygen tetrahedral and aluminum oxygen tetrahedral units connect in different ways. Geopolymers can be classified as a family of mineral binders closely related to artificial zeolites [2,3]. Metakaolinbased geopolymers is a type of high-performance binder material that exhibit high strength, long-term durability, and the property of being environmentally friendly [2,4]. The development of geopolymer technology has been driven by the application rather than the technology, meaning that a full understanding of the reaction processes and reaction mechanisms of geopolymers could enable the development of additional applications. In the past decades, researchers have discussed and studied geopolymers extensively; most research has been focused on geopolymerization mechanisms [5] and kinetics [6], microstructure [7,8], mechanical properties [9,10] and different alkali activators [11,12]. Although water is a key factor of the reaction process of geopolymers, few studies have examined its influence on the formation process of geopolymers and their microstructure.
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In the formation process of geopolymers, water, as a reaction medium, contributes to the dissolution of aluminosilicate precursors and promotes various ions transfer and polycondensation of Al and Si monomeric and oligomeric species. At the completion of geopolymerization, water still exists in the pores and the structure of the threedimensional network [13]. In a study on the solidification of geopolymer mortar, Perera et al. [14] established that water exists in geopolymers with different forms during the curing process. Approximately 60% of water is called free water, predominantly inhabiting the geopolymer pores, i.e., the interstitial spaces between the gel particles. There is also the interstitial water, accounting for 35%, which is associated with the active cation, such as Si and Al. The remaining water, which represents 5% of the total, is in the form of hydroxyl groups in geopolymers and exhibits a relatively low specific gravity. These hydroxyl groups provide the chemical bonding and remain in the geopolymers within small frameworks of pores. The additional weight loss of water does not affect the Si-Al framework of geopolymers but affects their porous structure, which can provide a useful way to obtain porous geopolymer materials by dehydration [15,16]. On the other hand, by increasing the amount of water, geopolymer can be porous, which is further beneficial to the removal of water during drying [17]. In previous work, although the
Corresponding author. E-mail address: [email protected] (X. Cui).
http://dx.doi.org/10.1016/j.ceramint.2017.07.190 Received 4 July 2017; Received in revised form 24 July 2017; Accepted 26 July 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.07.190
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influence of water content on the structural properties of geopolymers and the relevant research on dehydration process was addressed to some extent, the context was a relatively high-temperature environment [14,18–20]. Although between 600 and 800 °C，geopolymers perform better than Portland cement, such as the compressive strength of these materials rises. But at around 600–800 °C, there is cracks on the surface bending strength is substantially impacted, the properties of geopolymer materials are seriously affected [21–23]. Especially for metakaolin-based geopolymers，900 °C is an important transition temperature, where the geopolymer partially melts and coagulates locally [24]. Considering the water lost at high temperatures, the weight loss at lower temperatures can ensure that the gel structure of the geopolymer synthesis mixture is not destroyed [25]. It is well known that the silica-alumina gel in the geopolymers can crystallize at high temperature. The crystallization phases, consisting of different structures, are based on leucite and contain a small amount of mullite and silica glass. The experiments of this study were conducted in a lowtemperature vacuum environment. This method, based on the alkali aluminosilicate gel, ensures the integrity of the alkali aluminosilicate gel structure and takes advantage of the vacuum environment to increase the amount of water removal. The effects of water loss are further analyzed in the context of the geopolymerization process and the properties of the resulting metakaolin-based geopolymers.
Table 1 Composition of geopolymer samples. Activator
Na2O/Al2O3
H2O/Na2O
Sample label
Water glass
0.57 0.67 0.77 0.67
11
NaSi-0.57-11 NaSi-0.67-11 NaSi-0.77-11 NaSi-0.67-11 NaSi-0.67-13 NaSi-0.67-15 Na-0.75-10.62 Na-0.85-10.62 Na-0.95-10.62 Na-0.85-10.62 Na-0.85-12.62 Na-0.85-15.62
NaOH solution
0.75 0.85 0.95 0.85
11 13 15 10.62
10.62 12.62 15.62
of H2O/Na2O. The compositions of the geopolymer samples are given in Table 1. In this report, we focus primarily on the influence of water content on the properties of metakaolin-based geopolymers. 2.3. Methods X-ray powder diffraction data were recorded using a Rigaku MiniFlex 600 instrument with Ni-filtered and Cu (Kα) radiation operating at (40 kV, 15 mA) with a dwell time of 3 s, a 2θ range of 5–70°, and a scanning rate of 10°/min. The compressive strength of the samples was examined using a DNS100 compression-testing machine (Changchun, China). Six parallel test samples from each mixture were placed in the DNS100 compression-testing machine, and the average compressive strength (in MPa) was recorded. The standard deviation of strength was less than 5%. BET was measured by a BET surface area (Gemini VII, Micromeritics Instrument Corporation, USA). Geopolymer samples were weighed between 0.1 and 0.15 g and subjected to isothermal nitrogen adsorption measurements. The pore size is in the range of 1.70–300.00 nm. The whole vacuum dehydration process was completed in a vacuum device at constant temperature. The samples were placed in the vacuum device where the temperature was set to 120 °C and the vacuum degree was set to 0.1 kPa. After recording the initial mass, the samples were weighed periodically to observe the change of mass. The whole process lasted for 24 h. What the geopolymers lost was the evaporated water. FT-IR of the geopolymers was performed using a Nicolet iS50 infrared spectrometer (Thermo Scientific Company, America). The samples were mixed with KBr for studying the FT-IR spectra over the range from 400 to 4000 cm−1 with a resolution ratio of 4 cm−1. Scanning electron microscopy (SEM) was performed on the sample surfaces using an S-3400N device (Hitachi Limited Company, Japan). The samples were cleaned with absolute ethyl alcohol, underwent ultrasonic treatment for 5 min, and were polished with SiC paper and then coated with gold as a conductive coating.
2. Experimental procedure 2.1. Raw materials In this experiment, metakaolin was used as the main source material for geopolymer samples. The primary raw material was kaolin, which was produced by Chinese Beihai Yankuang Kaolin Co. Ltd. The kaolin was placed in a muffle furnace, and the heating temperature was increased from room temperature to 800 °C at a rate of 5 °C/min. Then raw material was held at 800 °C for 2 h and then allowed to cool to room temperature. The metakaolin was prepared from kaolin by a synthesis process. The chemical composition of the metakaolin was SiO2, 52.89%; Al2O3, 43.50%; K2O, 1.80%; Fe2O3, 1.38%; and MgO, 0.44%, which was determined by X-ray fluorescence (XRF). All experiments were performed using the same reagents and starting materials. The alkaline activator was water glass (solid content of 37.63%), which was produced in Nanning, Guangxi. Chemical grade NaOH was mixed into water glass to prepare modified water glass (with modulus 1.3), which was used as the activating solution. This solution remained stationary for 24 h at room temperature prior to use. The other alkaline activator was a NaOH solution. The sodium hydroxide was produced by the Xilong Chemical Company. All of the raw materials described above were from the same batches of reagents. 2.2. Geopolymer synthesis A mixture of metakaolin, water glass (with modulus 1.3) and deionized water was used as the alkali aluminosilicate solution, and it was mixed on magnetic stirrers for 7 min in a beaker to obtain a homogenous mixture. Then, the solution was poured into cubic molds (20 mm × 20 mm × 20 mm). The molds were sealed in the curing box and cured at 25 °C for 24 h, and then the sample was removed from the molds. Metakaolin was also mixed with the prepared 11 M NaOH solution in the proportion (n(Na2O/Al2O3) = 0.85). As specified earlier [17], the mixed solution was then poured into a sealed mold, and the sample was cured at 60 °C for 24 h. Then the solidified sample, activated by NaOH solution, was removed from the sealed mold, it was maintained at 25 °C before any test was conducted. In this experiment, the ratio of Na2O/Al2O3 was determined by the Na2O content in the samples. According to the ratio of Na2O/Al2O3, metakaolin-based geopolymers were prepared by adjusting the ratio
3. Results and discussion 3.1. Mechanical properties of geopolymers The compressive strength of alkali-activated geopolymers prepared using water glass is represented in Fig. 1(a). The compressive strength reaches its maximum level when n(H2O/Na2O) = 11 and n(Na2O/ Al2O3) = 0.67, corresponding to NaSi-0.67-11-GP. The strength of the samples aged for 3 days and 7 days is 41.26 MPa and 51.71 MPa, respectively. The origin of this high strength is the alkali aluminosilicate gel that fills metakaolin-based geopolymers, increasing the surface density and compressive strength. Fig. 1(b) shows the compressive 2
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Fig. 1. (a) Compressive strength of NaSi-11-GP cured for 3 days and 7 days with different ratios of Na2O/Al2O3; (b) compressive strength of Na-10.62-GP cured for 3 days and 7 days with different ratios of Na2O/Al2O3; (c) changes in the compressive strength of NaSi-0.67-GP (with different ratios of H2O/Na2O) under different conditions; (d) changes in the compressive strength of Na-0.85-GP (with different ratios of Na2O/Al2O3) under different conditions.
Fig. 2. SEM of NaSi-0.67-GP with different H2O/Na2O molar ratios during curing at room temperature (a. NaSi-0.67-11-GP; b. NaSi-0.67-13-GP; c. NaSi-0.67-15-GP); SEM of NaSi0.67-GP with different H2O/Na2O molar ratios during curing at 120 °C under vacuum (d. NaSi-0.67-11GP; e. NaSi-0.67-13-GP; f. NaSi-0.67-15-GP).
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Fig. 3. SEM of Na-0.85-GP with different H2O/Na2O molar ratios during curing at room temperature (a. Na-0.85-10.62-GP; b. Na-0.85-12.62-GP; c. Na-0.85-15.62-GP); SEM of Na0.85-GP with different H2O/Na2O molar ratios during curing at 120 °C under vacuum (d. Na-0.85-10.62-GP; e. Na-0.85-12.62-GP; f. Na-0.85-15.62-GP).
tion of the monomers into polymeric structures and the different forms of water present at the cured geopolymer stage.
strength of alkali-activated geopolymers prepared using NaOH. The compressive strength reaches its maximum level when n(H2O/Na2O) = 10.62, and n(Na2O/Al2O3) = 0.85, corresponding to Na-0.85-10.62GP. The strength of the samples aged for 3 days and 7 days is 16.16 MPa and 19.33 MPa, respectively, which is lower than that of NaSi-0.67-GP mentioned above. From these results and the SEM results (Figs. 2 and 3), Na-0.85-GP exhibits high porosity and low density, making the compressive strength weaker. NaSi-0.67-GP and Na-0.85-GP were chosen to conduct vacuum dehydration experiments with different H2O/Na2O molar ratios of geopolymers. Fig. 1(c) and (d) indicate that the geopolymers lose a substantial amount of water at 120 °C under vacuum, although the compressive strength does not decrease. After 24 h vacuum treatment at 120 °C, the compressive strength of all the samples increased. Duxson et al. [26] confirmed that lost water is recovered at the end of the geopolymer reaction. That is to say, water acts as the reaction medium in the mechanism of reaction, which does not participate in the chemical reaction directly. Appropriate heating can accelerate the polymerization process and help the slurry solidify in a short time and is conducive to form a geopolymer gel phase. Furthermore, a vacuum treatment at 120 °C can promote the evaporation of water and form a network structure with a relatively high strength. In this experiment, most of the recycled water belongs to free water. This type of free water is physically bonded water that exists in the pores and crystalline grains during geopolymerization, and its short-term loss does not influence the compressive strength. Fig. 1(c) and (d) show the different amounts of water in the NaSi0.67-GP and Na-0.85-GP samples. For further analysis, we can identify the effect of water content on geopolymer concrete compressive strength through horizontal comparative research. From the test results, it is clear that an increase in the H2O/Na2O molar ratio causes the corresponding compressive strength to decline. The total water calculated for geopolymer is increasing; thus, as discussed above, a large quantity of free water is located in the gel particles and pores of the geopolymer, which increases the porosity of the geopolymer samples. As we increase the curing time, free water is released from the polymeric gel material, leaving a large amount of pores and resulting in a decreased density in the geopolymer samples. It can be concluded from the compressive strength results that water has an important influence on the mechanical properties of geopolymers in every phase, which is primarily coupled to the dissolution of Si and Al atoms from the geopolymer precursor, polycondensa-
3.2. SEM and BET of metakaolin-based geopolymers before and after vacuum dehydration In Fig. 2, under the function of water glass, metakaolin-based geopolymers form an amorphous substance with a lamellar structure. Unreacted metakaolin particles exist in the samples, which consists primarily of geopolymeric presoma with a loose structure. The gel phase and shrinking percentage are strongly coupled. Under the same experimental conditions, the generated gel phase shrinks during the curing process because of the high concentration of basic ions. Thus, cracks appear in both Fig. 2(a) and (b). With increases in water content, the alkalinity and concentration of silicate ions decrease, leading to decreased gelation and reduced shrinkage. Fig. 2(c) reveals the reduction of cracks. The micrograph indicates that the micro cracks of the samples disappear under the treatment of vacuum dehydration at 120 °C, associated with densification of the gel structure. These results can be correlated to those under the treatment of vacuum dehydration at 120 °C in Fig. 1(c). As expected, the elevated temperature accelerates the formation of a hard structure, particularly in the early stages of the geopolymerization reaction [27]. In Fig. 3, no crack appears in the samples of metakaolin-based geopolymer under the treatment of NaOH solution, and there is a relatively clear porosity structure. The more water that was added, the larger the pores. Metakaolin transformed its solid phase into the liquid phase of alkali-activated material in NaOH solution, and then alkaliactivated material transformed into sodium aluminosilicate gel. However, the amount of gel of Na-GP was lower than that of the sample prepared using water glass. Part of the gel turned into zeolite crystal, and Na-GP failed to form a hard structure. No cracks can be detected in Fig. 3. During the process of crystallization, although NaGP formed NaA zeolite and part of the water transformed into crystal water, the porous regions of the gel allowed water to move easily between the macro pores. Two types of alkali-activated materials were characterized through N2 adsorption experiments, and the results of BET measurements were shown in Table 2. The surface area of NaSi-0.67-15-GP increase from 25.786 to 29.2621 cm2/g after the treatment of vacuum dehydration at 120 °C. The additional weight loss of water affects their porous structure, which the original position occupied by the water will be 4
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was added into the slurry, suitable slurry viscosity and surface tension contributed to the formation of pores. With increasing water content, the quantity of the pores of the geopolymers increased, and evenly distributed pores became connected, which allows the water to discharge easily under vacuum [15,25]. The water loss rate is rapid in the first eight hours, representing primarily the free water on the surface of the geopolymers. Over time, the intergranular water and bound water combined with cations are lost, but some water remains in the pore that is difficult to discharge.
Table 2 BET results of geopolymer samples with different treatment. Geopolymer samples
NaSi-0.6715-GP
NaSi-0.67-15GP 120 °C vacuum treatment
Na-0.8515.62-GP
Na-0.8515.62-GP 120 °C vacuum treatment
Total BET surface area m2/g
25.7865
29.2621
3.7308
4.0300
free after they remove from geopolymer. This finding can provide a useful way to obtain porous geopolymer materials by dehydration. The same trend result was shown in Table 2 that the BET of Na-0.85-15.62GP increase from 3.7308 to 4.0300 cm2/g. The surface area of Na-0.8515.62-GP dramatically lower than NaSi-0.67-15-GP, likely due to samples were transformed into a NaA zeolite (Fig. 5(a)) which result in loose structure. In the measured range, the proportion of pores less than 200 nm is reduced, resulting in a decrease in BET data.
3.4. XRD and FT-IR of metakaolin-based geopolymers prepared under different conditions From Fig. 5(b), the results of X-ray diffractograms show that geopolymers activated by the NaOH solution changed into NaA zeolite (Na2Al2Si1.85O7.7·5.1H2O) crystals with many sharp peaks. In the course of the reaction, its gel phase forms NaA zeolite after rearrangement. The amorphous hump in the figure disappears, which contrasts with the results of XRD in Fig. 5(a). By analyzing the samples of geopolymers before and after vacuum dehydration, the XRD spectrum does not present significant changes, which indicates that water does not participate in the geopolymer reaction and that vacuum dehydration does not affect the results. In the presence of water glass, the XRD spectrum shows that there are no nanometer-sized zeolitic crystal nucleii at room temperature or at 120 °C under vacuum, which contrasts with the experimental results shown in Fig. 5(b). Through analysis of these diffractograms in Fig. 5(a), no sharp peaks to which NaA zeolite crystals would correspond can be located in the figure, which indicates that the NaSi-0.85-15.62-GP samples do not transform into the zeolite phase. Note that unreacted metakaolin is located at 2θmax = 22° [28]. The reason for the observation of unreacted metakaolin is that geopolycondensation and gelation processes are actually faster in geopolymers with water glass as an alkali activator. At this point, there is no apparent limit in the reaction process; thus, undissolved metakaolin is constrained within the gel in the early stages, which hinders the reaction. A broad peak that represents the amorphous Si structure exists at 2θ = 18–25°, also known as the "amorphous peak". This feature indicates that the gel is primarily amorphous and that the products are aluminosilicate materials with a threedimensional network structure. In the X-ray pattern for NaSi-0.8515.62-GP, the peak intensity appears at 2θ = 26–33°, which indicates that the amorphous structure of metakaolin is involved in the reaction and forms a gel phase after the reaction. The FT-IR results of the alkali-activated cementitious materials are shown in Fig. 6. The FT-IR date collected for two different treatments
3.3. Water loss rate of geopolymers under vacuum In this experiment, NaOH solution and water glass were chosen as different alkali activators to form metakaolin-based geopolymers, referred to as Na-0.85-GP and NaSi-0.67-GP, respectively, and samples were produced in different H2O/Na2O molar ratio conditions. The aim was to investigate the water loss of metakaolin-based geopolymers in a vacuum environment. Perera et al. [14] have shown that free water that primarily inhabits geopolymer pores is lost when the temperature is increased from room temperature to 150 °C. Once the temperature reaches 300 °C, the interstitial water that adheres to the Si-Al framework bands of the geopolymer structure is almost entirely lost. As the geopolymers react longer under vacuum at 120 °C, the water content gradually decreases until the amount of remaining water becomes stable. For the Na-0.85-GP sample with a H2O/Na2O molar ratio of 11, the residual water content of Na-0.85-11-GP at 120 °C under vacuum for 24 h was 24.20%. When H2O/Na2O molar ratio for Na-GP was increased to 15, referred to as Na-0.85-15-GP, the residual water content was 8.31% after aging. Comparing Na-0.85-GP using NaOH as an alkali activator with NaSi-0.67-GP, 18.85% of the residual water decreased to 1.08% when the H2O/Na2O molar ratio of 10.62 for Na0.85-GP was increased to 15.62. This finding is consistent with the SEM experimental results discussed earlier. As shown in Figs. 4 and 5, the residual water of metakaolin-based geopolymers at 120 °C under vacuum decreases with increasing H2O/Na2O molar ratio. After water
Fig. 4. (a) Changes in water content of NaSi-0.67-GP with different H2O/Na2O molar ratios under the treatment of vacuum dehydration at 120 °C for 24 h; (b) changes in water content of Na-0.85-GP with different H2O/Na2O molar ratios under the treatment of vacuum dehydration at 120 °C for 24 h.
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Fig. 5. (a) XRD patterns of Na-0.67-15-GP prepared under different conditions; (b) XRD patterns of NaSi-0.85-15.62-GP prepared under different conditions.
structure does not change, and the difference exist primarily at low frequencies [30]. A new bond appears at 553 cm−1 in Na-0.85-15.62GP, representing the double loop vibration of external linkages [30]. This result shows that the formation of Na-A zeolite involves more than a double four-membered ring, in comparison with geopolymers. The band at 668 cm−1 is most likely due to symmetrical stretching vibration of tetrahedrons. It can be concluded from the FT-IR results that Na-GP and NaSi-GP have similar basic frameworks. Between 900 cm−1 and 1000 cm−1, peaks appear, showing that the materials contain predominantly Si-O and Al-O linkages. It is demonstrated that geopolymers are aluminosilicate materials with a main structure that is hydrophilic to some extent.
of NaSi-0.67-15-GP are presented in Fig. 6(a). In the FT-IR analysis of geopolymers activated by water glass, the band at 456 cm−1 is attributed to Si-O bending in internal tetrahedrons, and the band at 561 cm−1 is found to correspond to Al-O linkages. Many peaks with different intensities are observed from 600 cm−1 to 800 cm−1, including a prominent peak at 724 cm−1. This band represents Si-O-Si symmetrical stretching vibration in the internal tetrahedrons. The broad absorbance at 1024 cm−1 is attributed to Si-O-Al (Si) asymmetric stretching vibrations in the geopolymer. The bond at 3459 cm−1 represents water absorption bands, which are attributed to Si-OH, Al-OH and –OH asymmetric stretching vibration. Another part of the water band is observed at 1646 cm−1, which mainly represents H-O-H bending vibrations. Although some water still remains in this sample, – OH vibrations of water (3459 cm−1 and 1646 cm−1) exhibit clearly detectable changes from the case of vacuum dehydration at 120 °C. The residual water (in the form of OH– groups) continues to lost along with heating up to above 300 °C, aiding in polycondensation into -Si-O-Silinkages and the hydroxylation of OH– groups [19]. In Fig. 6(b), the band detected at 1400 cm−1 is attributed to O-C-O stretching vibrations, which may be due to the carbonization process [29]. This finding implies that geopolymer produced by the NaOH activator easily absorbs carbonates from atmospheric CO2 that subsequently accumulate in the geopolymer. By the comparison of the IR spectra of Na-0.8515.62-GP and NaSi-0.67-15-GP, it is concluded that the locations of the absorption peaks are approximately the same, the main body
4. Conclusions According to the compression strength results, the composition of metakaolin and alkali activator was first determined to produce metakaolin-based geopolymers with different water contents. By conducting vacuum dehydration at 120 °C, the compressive strength of the geopolymer samples after the water loss did not decrease; instead, there were different degrees of increase. Two types of metakaolin-based geopolymers produced by two types of alkali activator achieved water discharge under vacuum. The volume of water removal increased with increasing H2O/Na2O. When the H2O/ Na2O molar ratio reached 15, 8.61% of residual water stayed in NaSi-
Fig. 6. (a) FT-IR patterns of NaSi-0.67-15-GP prepared under different conditions; (b) FT-IR patterns of Na-0.85-15.62-GP prepared under different conditions.
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0.67-GP under the vacuum dehydration treatment at 120 °C. In Na0.85-GP, only 1.08% of the residual water remained when the H2O/ Na2O molar ratio reached 15.62, implying that Na-0.85-GP loses much more water that NaSi-0.67-GP under the same experimental conditions. Geopolymer samples under treatment with different activators showed sample different microscopic structures in SEM. The internal part of Na-0.85-GP presented a clearly porous structure, whereas NaSi0.67-GP presented a denser structure. The vacuum treatment at 120 °C can speed up the process of polycondensation of geopolymers, which induces agglomeration in Na-0.85-GP and significantly suppresses micro cracks in NaSi-0.67-GP. During the process of water loss, the results of XRD and FT-IR of geopolymers under different activators' treatment did not have significant changes. In XRD and FT-IR spectra of Na-0.85-GP, the formation of Na-A zeolite was demonstrated. Acknowledgments This work was supported by the Chinese Natural Science Fund (grants 21566006 and 51561135012) and the Guangxi Natural Science Fund (grant 2016GXNSFGA380003). References [1] J. Davidovits, Geopolymers ̵ inorganic polymeric new materials, J. Therm. Anal. Calorim. 37 (1991) 1633–1656. [2] J. Davidovits, Geopolymers: man-made rock deosynthesis and the resulting development of very early high strength cement, J. Mater. Educ. 16 (1994) 91–139. [3] P.N. Lemougna, K.T. Wang, Q. Tang, U.C. Melo, X.M. Cui, Recent developments on inorganic polymers synthesis and applications, Ceram. Int. 42 (2016) 15142–15159. [4] Q. Tang, G.H. Xue, S.j. Yang, K.T. Wang, X.M. Cui, Study on the preparation of a free-sintered inorganic polymer-based proppant using the suspensions solidification method, J. Clean. Prod. 148 (2017) 276–282. [5] J.L. Provis, J.S.J. van Deventer, Geopolymerisation kinetics. 1. In situ energydispersive X-ray diffractometry, Chem. Eng. Sci. 62 (9) (2007) 2309–2317. [6] J.L. Provis, J.S.J. van Deventer, Geopolymerisation kinetics. 2. Reaction kinetic modelling, Chem. Eng. Sci. 62 (9) (2007) 2318–2329. [7] P. Duxson, J.L. Provis, G.C. Lukey, S.W. Mallicoat, W.M. Kriven, J.S.J. van Deventer, Understanding the relationship between geopolymer composition, microstructure and mechanical properties, Colloid Surf. A. 269 (1–3) (2005) 47–58. [8] M.R. Wang, D.C. Jia, P.G. He, Y. Zhou, Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites, Ceram. Int. 37 (5) (2011) 1661–1666. [9] M. Marcin, M. Sisol, I. Brezani, Effect of slag addition on mechanical properties of fly ash based geopolymers, Procedia Eng. 151 (2016) 191–197. [10] H.K. Tchakouté, C.H. Rüscher, S. Kong, E. Kamseu, C. Leonelli, Geopolymer binders from metakaolin using sodium waterglass from waste glass and rice husk ash as alternative activators: a comparative study, Constr. Build. Mater. 114 (2016) 276–289.
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Effect of vacuum dehydration on gel structure and properties of metakaolin-based geopolymers Wei Li, Patrick N. Lemougna, Kaituo Wang, Yan He, Zhangfa Tong, Xuemin Cui
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School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Geopolymer Metakaolin Water Vacuum dehydration Gel
In aluminosilicate mineral materials, water plays a significant role during the process of geopolymer reaction and is related closely to the development of the strength and structure. In this work, a geopolymer is synthesized via the reaction of metakaolin with alkaline silicate solutions, including water glass and a NaOH solution. This study investigated the effect of water on the gel structure and properties of metakaolin-based geopolymers. Geopolymer samples were found to fast release water under vacuum and allowed the development of low-defect porous materials. After 3 days of curing in suitable conditions, the samples were treated under vacuum, during which the compressive strength did not decrease. Under vacuum at 120 °C, the residual water of the sample produced using water glass was 8.31% and that of the sample produced using NaOH solution was only 1.08%. The results of XRD and FTIR demonstrated that the sample produced using NaOH solution consisted primarily of NaA zeolite. SEM analysis indicated that the pores in sample growed with increasing H2O/Na2O molar ratio. Additionally, vacuum dehydration can suppress the formation of micro cracks and reduce the shrinkage of the samples.
1. Introduction Geopolymers, first introduced by Davidovits [1], are aluminosilicate mineral polymer materials synthesized by geosynthesis or geochemistry. Geopolymers are based on amorphous silicate and aluminosilicate mineral materials in alkali activated aqueous solutions, in which silica oxygen tetrahedral and aluminum oxygen tetrahedral units connect in different ways. Geopolymers can be classified as a family of mineral binders closely related to artificial zeolites [2,3]. Metakaolinbased geopolymers is a type of high-performance binder material that exhibit high strength, long-term durability, and the property of being environmentally friendly [2,4]. The development of geopolymer technology has been driven by the application rather than the technology, meaning that a full understanding of the reaction processes and reaction mechanisms of geopolymers could enable the development of additional applications. In the past decades, researchers have discussed and studied geopolymers extensively; most research has been focused on geopolymerization mechanisms [5] and kinetics [6], microstructure [7,8], mechanical properties [9,10] and different alkali activators [11,12]. Although water is a key factor of the reaction process of geopolymers, few studies have examined its influence on the formation process of geopolymers and their microstructure.
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In the formation process of geopolymers, water, as a reaction medium, contributes to the dissolution of aluminosilicate precursors and promotes various ions transfer and polycondensation of Al and Si monomeric and oligomeric species. At the completion of geopolymerization, water still exists in the pores and the structure of the threedimensional network [13]. In a study on the solidification of geopolymer mortar, Perera et al. [14] established that water exists in geopolymers with different forms during the curing process. Approximately 60% of water is called free water, predominantly inhabiting the geopolymer pores, i.e., the interstitial spaces between the gel particles. There is also the interstitial water, accounting for 35%, which is associated with the active cation, such as Si and Al. The remaining water, which represents 5% of the total, is in the form of hydroxyl groups in geopolymers and exhibits a relatively low specific gravity. These hydroxyl groups provide the chemical bonding and remain in the geopolymers within small frameworks of pores. The additional weight loss of water does not affect the Si-Al framework of geopolymers but affects their porous structure, which can provide a useful way to obtain porous geopolymer materials by dehydration [15,16]. On the other hand, by increasing the amount of water, geopolymer can be porous, which is further beneficial to the removal of water during drying [17]. In previous work, although the
Corresponding author. E-mail address: [email protected] (X. Cui).
http://dx.doi.org/10.1016/j.ceramint.2017.07.190 Received 4 July 2017; Received in revised form 24 July 2017; Accepted 26 July 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.07.190
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influence of water content on the structural properties of geopolymers and the relevant research on dehydration process was addressed to some extent, the context was a relatively high-temperature environment [14,18–20]. Although between 600 and 800 °C，geopolymers perform better than Portland cement, such as the compressive strength of these materials rises. But at around 600–800 °C, there is cracks on the surface bending strength is substantially impacted, the properties of geopolymer materials are seriously affected [21–23]. Especially for metakaolin-based geopolymers，900 °C is an important transition temperature, where the geopolymer partially melts and coagulates locally [24]. Considering the water lost at high temperatures, the weight loss at lower temperatures can ensure that the gel structure of the geopolymer synthesis mixture is not destroyed [25]. It is well known that the silica-alumina gel in the geopolymers can crystallize at high temperature. The crystallization phases, consisting of different structures, are based on leucite and contain a small amount of mullite and silica glass. The experiments of this study were conducted in a lowtemperature vacuum environment. This method, based on the alkali aluminosilicate gel, ensures the integrity of the alkali aluminosilicate gel structure and takes advantage of the vacuum environment to increase the amount of water removal. The effects of water loss are further analyzed in the context of the geopolymerization process and the properties of the resulting metakaolin-based geopolymers.
Table 1 Composition of geopolymer samples. Activator
Na2O/Al2O3
H2O/Na2O
Sample label
Water glass
0.57 0.67 0.77 0.67
11
NaSi-0.57-11 NaSi-0.67-11 NaSi-0.77-11 NaSi-0.67-11 NaSi-0.67-13 NaSi-0.67-15 Na-0.75-10.62 Na-0.85-10.62 Na-0.95-10.62 Na-0.85-10.62 Na-0.85-12.62 Na-0.85-15.62
NaOH solution
0.75 0.85 0.95 0.85
11 13 15 10.62
10.62 12.62 15.62
of H2O/Na2O. The compositions of the geopolymer samples are given in Table 1. In this report, we focus primarily on the influence of water content on the properties of metakaolin-based geopolymers. 2.3. Methods X-ray powder diffraction data were recorded using a Rigaku MiniFlex 600 instrument with Ni-filtered and Cu (Kα) radiation operating at (40 kV, 15 mA) with a dwell time of 3 s, a 2θ range of 5–70°, and a scanning rate of 10°/min. The compressive strength of the samples was examined using a DNS100 compression-testing machine (Changchun, China). Six parallel test samples from each mixture were placed in the DNS100 compression-testing machine, and the average compressive strength (in MPa) was recorded. The standard deviation of strength was less than 5%. BET was measured by a BET surface area (Gemini VII, Micromeritics Instrument Corporation, USA). Geopolymer samples were weighed between 0.1 and 0.15 g and subjected to isothermal nitrogen adsorption measurements. The pore size is in the range of 1.70–300.00 nm. The whole vacuum dehydration process was completed in a vacuum device at constant temperature. The samples were placed in the vacuum device where the temperature was set to 120 °C and the vacuum degree was set to 0.1 kPa. After recording the initial mass, the samples were weighed periodically to observe the change of mass. The whole process lasted for 24 h. What the geopolymers lost was the evaporated water. FT-IR of the geopolymers was performed using a Nicolet iS50 infrared spectrometer (Thermo Scientific Company, America). The samples were mixed with KBr for studying the FT-IR spectra over the range from 400 to 4000 cm−1 with a resolution ratio of 4 cm−1. Scanning electron microscopy (SEM) was performed on the sample surfaces using an S-3400N device (Hitachi Limited Company, Japan). The samples were cleaned with absolute ethyl alcohol, underwent ultrasonic treatment for 5 min, and were polished with SiC paper and then coated with gold as a conductive coating.
2. Experimental procedure 2.1. Raw materials In this experiment, metakaolin was used as the main source material for geopolymer samples. The primary raw material was kaolin, which was produced by Chinese Beihai Yankuang Kaolin Co. Ltd. The kaolin was placed in a muffle furnace, and the heating temperature was increased from room temperature to 800 °C at a rate of 5 °C/min. Then raw material was held at 800 °C for 2 h and then allowed to cool to room temperature. The metakaolin was prepared from kaolin by a synthesis process. The chemical composition of the metakaolin was SiO2, 52.89%; Al2O3, 43.50%; K2O, 1.80%; Fe2O3, 1.38%; and MgO, 0.44%, which was determined by X-ray fluorescence (XRF). All experiments were performed using the same reagents and starting materials. The alkaline activator was water glass (solid content of 37.63%), which was produced in Nanning, Guangxi. Chemical grade NaOH was mixed into water glass to prepare modified water glass (with modulus 1.3), which was used as the activating solution. This solution remained stationary for 24 h at room temperature prior to use. The other alkaline activator was a NaOH solution. The sodium hydroxide was produced by the Xilong Chemical Company. All of the raw materials described above were from the same batches of reagents. 2.2. Geopolymer synthesis A mixture of metakaolin, water glass (with modulus 1.3) and deionized water was used as the alkali aluminosilicate solution, and it was mixed on magnetic stirrers for 7 min in a beaker to obtain a homogenous mixture. Then, the solution was poured into cubic molds (20 mm × 20 mm × 20 mm). The molds were sealed in the curing box and cured at 25 °C for 24 h, and then the sample was removed from the molds. Metakaolin was also mixed with the prepared 11 M NaOH solution in the proportion (n(Na2O/Al2O3) = 0.85). As specified earlier [17], the mixed solution was then poured into a sealed mold, and the sample was cured at 60 °C for 24 h. Then the solidified sample, activated by NaOH solution, was removed from the sealed mold, it was maintained at 25 °C before any test was conducted. In this experiment, the ratio of Na2O/Al2O3 was determined by the Na2O content in the samples. According to the ratio of Na2O/Al2O3, metakaolin-based geopolymers were prepared by adjusting the ratio
3. Results and discussion 3.1. Mechanical properties of geopolymers The compressive strength of alkali-activated geopolymers prepared using water glass is represented in Fig. 1(a). The compressive strength reaches its maximum level when n(H2O/Na2O) = 11 and n(Na2O/ Al2O3) = 0.67, corresponding to NaSi-0.67-11-GP. The strength of the samples aged for 3 days and 7 days is 41.26 MPa and 51.71 MPa, respectively. The origin of this high strength is the alkali aluminosilicate gel that fills metakaolin-based geopolymers, increasing the surface density and compressive strength. Fig. 1(b) shows the compressive 2
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Fig. 1. (a) Compressive strength of NaSi-11-GP cured for 3 days and 7 days with different ratios of Na2O/Al2O3; (b) compressive strength of Na-10.62-GP cured for 3 days and 7 days with different ratios of Na2O/Al2O3; (c) changes in the compressive strength of NaSi-0.67-GP (with different ratios of H2O/Na2O) under different conditions; (d) changes in the compressive strength of Na-0.85-GP (with different ratios of Na2O/Al2O3) under different conditions.
Fig. 2. SEM of NaSi-0.67-GP with different H2O/Na2O molar ratios during curing at room temperature (a. NaSi-0.67-11-GP; b. NaSi-0.67-13-GP; c. NaSi-0.67-15-GP); SEM of NaSi0.67-GP with different H2O/Na2O molar ratios during curing at 120 °C under vacuum (d. NaSi-0.67-11GP; e. NaSi-0.67-13-GP; f. NaSi-0.67-15-GP).
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Fig. 3. SEM of Na-0.85-GP with different H2O/Na2O molar ratios during curing at room temperature (a. Na-0.85-10.62-GP; b. Na-0.85-12.62-GP; c. Na-0.85-15.62-GP); SEM of Na0.85-GP with different H2O/Na2O molar ratios during curing at 120 °C under vacuum (d. Na-0.85-10.62-GP; e. Na-0.85-12.62-GP; f. Na-0.85-15.62-GP).
tion of the monomers into polymeric structures and the different forms of water present at the cured geopolymer stage.
strength of alkali-activated geopolymers prepared using NaOH. The compressive strength reaches its maximum level when n(H2O/Na2O) = 10.62, and n(Na2O/Al2O3) = 0.85, corresponding to Na-0.85-10.62GP. The strength of the samples aged for 3 days and 7 days is 16.16 MPa and 19.33 MPa, respectively, which is lower than that of NaSi-0.67-GP mentioned above. From these results and the SEM results (Figs. 2 and 3), Na-0.85-GP exhibits high porosity and low density, making the compressive strength weaker. NaSi-0.67-GP and Na-0.85-GP were chosen to conduct vacuum dehydration experiments with different H2O/Na2O molar ratios of geopolymers. Fig. 1(c) and (d) indicate that the geopolymers lose a substantial amount of water at 120 °C under vacuum, although the compressive strength does not decrease. After 24 h vacuum treatment at 120 °C, the compressive strength of all the samples increased. Duxson et al. [26] confirmed that lost water is recovered at the end of the geopolymer reaction. That is to say, water acts as the reaction medium in the mechanism of reaction, which does not participate in the chemical reaction directly. Appropriate heating can accelerate the polymerization process and help the slurry solidify in a short time and is conducive to form a geopolymer gel phase. Furthermore, a vacuum treatment at 120 °C can promote the evaporation of water and form a network structure with a relatively high strength. In this experiment, most of the recycled water belongs to free water. This type of free water is physically bonded water that exists in the pores and crystalline grains during geopolymerization, and its short-term loss does not influence the compressive strength. Fig. 1(c) and (d) show the different amounts of water in the NaSi0.67-GP and Na-0.85-GP samples. For further analysis, we can identify the effect of water content on geopolymer concrete compressive strength through horizontal comparative research. From the test results, it is clear that an increase in the H2O/Na2O molar ratio causes the corresponding compressive strength to decline. The total water calculated for geopolymer is increasing; thus, as discussed above, a large quantity of free water is located in the gel particles and pores of the geopolymer, which increases the porosity of the geopolymer samples. As we increase the curing time, free water is released from the polymeric gel material, leaving a large amount of pores and resulting in a decreased density in the geopolymer samples. It can be concluded from the compressive strength results that water has an important influence on the mechanical properties of geopolymers in every phase, which is primarily coupled to the dissolution of Si and Al atoms from the geopolymer precursor, polycondensa-
3.2. SEM and BET of metakaolin-based geopolymers before and after vacuum dehydration In Fig. 2, under the function of water glass, metakaolin-based geopolymers form an amorphous substance with a lamellar structure. Unreacted metakaolin particles exist in the samples, which consists primarily of geopolymeric presoma with a loose structure. The gel phase and shrinking percentage are strongly coupled. Under the same experimental conditions, the generated gel phase shrinks during the curing process because of the high concentration of basic ions. Thus, cracks appear in both Fig. 2(a) and (b). With increases in water content, the alkalinity and concentration of silicate ions decrease, leading to decreased gelation and reduced shrinkage. Fig. 2(c) reveals the reduction of cracks. The micrograph indicates that the micro cracks of the samples disappear under the treatment of vacuum dehydration at 120 °C, associated with densification of the gel structure. These results can be correlated to those under the treatment of vacuum dehydration at 120 °C in Fig. 1(c). As expected, the elevated temperature accelerates the formation of a hard structure, particularly in the early stages of the geopolymerization reaction [27]. In Fig. 3, no crack appears in the samples of metakaolin-based geopolymer under the treatment of NaOH solution, and there is a relatively clear porosity structure. The more water that was added, the larger the pores. Metakaolin transformed its solid phase into the liquid phase of alkali-activated material in NaOH solution, and then alkaliactivated material transformed into sodium aluminosilicate gel. However, the amount of gel of Na-GP was lower than that of the sample prepared using water glass. Part of the gel turned into zeolite crystal, and Na-GP failed to form a hard structure. No cracks can be detected in Fig. 3. During the process of crystallization, although NaGP formed NaA zeolite and part of the water transformed into crystal water, the porous regions of the gel allowed water to move easily between the macro pores. Two types of alkali-activated materials were characterized through N2 adsorption experiments, and the results of BET measurements were shown in Table 2. The surface area of NaSi-0.67-15-GP increase from 25.786 to 29.2621 cm2/g after the treatment of vacuum dehydration at 120 °C. The additional weight loss of water affects their porous structure, which the original position occupied by the water will be 4
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was added into the slurry, suitable slurry viscosity and surface tension contributed to the formation of pores. With increasing water content, the quantity of the pores of the geopolymers increased, and evenly distributed pores became connected, which allows the water to discharge easily under vacuum [15,25]. The water loss rate is rapid in the first eight hours, representing primarily the free water on the surface of the geopolymers. Over time, the intergranular water and bound water combined with cations are lost, but some water remains in the pore that is difficult to discharge.
Table 2 BET results of geopolymer samples with different treatment. Geopolymer samples
NaSi-0.6715-GP
NaSi-0.67-15GP 120 °C vacuum treatment
Na-0.8515.62-GP
Na-0.8515.62-GP 120 °C vacuum treatment
Total BET surface area m2/g
25.7865
29.2621
3.7308
4.0300
free after they remove from geopolymer. This finding can provide a useful way to obtain porous geopolymer materials by dehydration. The same trend result was shown in Table 2 that the BET of Na-0.85-15.62GP increase from 3.7308 to 4.0300 cm2/g. The surface area of Na-0.8515.62-GP dramatically lower than NaSi-0.67-15-GP, likely due to samples were transformed into a NaA zeolite (Fig. 5(a)) which result in loose structure. In the measured range, the proportion of pores less than 200 nm is reduced, resulting in a decrease in BET data.
3.4. XRD and FT-IR of metakaolin-based geopolymers prepared under different conditions From Fig. 5(b), the results of X-ray diffractograms show that geopolymers activated by the NaOH solution changed into NaA zeolite (Na2Al2Si1.85O7.7·5.1H2O) crystals with many sharp peaks. In the course of the reaction, its gel phase forms NaA zeolite after rearrangement. The amorphous hump in the figure disappears, which contrasts with the results of XRD in Fig. 5(a). By analyzing the samples of geopolymers before and after vacuum dehydration, the XRD spectrum does not present significant changes, which indicates that water does not participate in the geopolymer reaction and that vacuum dehydration does not affect the results. In the presence of water glass, the XRD spectrum shows that there are no nanometer-sized zeolitic crystal nucleii at room temperature or at 120 °C under vacuum, which contrasts with the experimental results shown in Fig. 5(b). Through analysis of these diffractograms in Fig. 5(a), no sharp peaks to which NaA zeolite crystals would correspond can be located in the figure, which indicates that the NaSi-0.85-15.62-GP samples do not transform into the zeolite phase. Note that unreacted metakaolin is located at 2θmax = 22° [28]. The reason for the observation of unreacted metakaolin is that geopolycondensation and gelation processes are actually faster in geopolymers with water glass as an alkali activator. At this point, there is no apparent limit in the reaction process; thus, undissolved metakaolin is constrained within the gel in the early stages, which hinders the reaction. A broad peak that represents the amorphous Si structure exists at 2θ = 18–25°, also known as the "amorphous peak". This feature indicates that the gel is primarily amorphous and that the products are aluminosilicate materials with a threedimensional network structure. In the X-ray pattern for NaSi-0.8515.62-GP, the peak intensity appears at 2θ = 26–33°, which indicates that the amorphous structure of metakaolin is involved in the reaction and forms a gel phase after the reaction. The FT-IR results of the alkali-activated cementitious materials are shown in Fig. 6. The FT-IR date collected for two different treatments
3.3. Water loss rate of geopolymers under vacuum In this experiment, NaOH solution and water glass were chosen as different alkali activators to form metakaolin-based geopolymers, referred to as Na-0.85-GP and NaSi-0.67-GP, respectively, and samples were produced in different H2O/Na2O molar ratio conditions. The aim was to investigate the water loss of metakaolin-based geopolymers in a vacuum environment. Perera et al. [14] have shown that free water that primarily inhabits geopolymer pores is lost when the temperature is increased from room temperature to 150 °C. Once the temperature reaches 300 °C, the interstitial water that adheres to the Si-Al framework bands of the geopolymer structure is almost entirely lost. As the geopolymers react longer under vacuum at 120 °C, the water content gradually decreases until the amount of remaining water becomes stable. For the Na-0.85-GP sample with a H2O/Na2O molar ratio of 11, the residual water content of Na-0.85-11-GP at 120 °C under vacuum for 24 h was 24.20%. When H2O/Na2O molar ratio for Na-GP was increased to 15, referred to as Na-0.85-15-GP, the residual water content was 8.31% after aging. Comparing Na-0.85-GP using NaOH as an alkali activator with NaSi-0.67-GP, 18.85% of the residual water decreased to 1.08% when the H2O/Na2O molar ratio of 10.62 for Na0.85-GP was increased to 15.62. This finding is consistent with the SEM experimental results discussed earlier. As shown in Figs. 4 and 5, the residual water of metakaolin-based geopolymers at 120 °C under vacuum decreases with increasing H2O/Na2O molar ratio. After water
Fig. 4. (a) Changes in water content of NaSi-0.67-GP with different H2O/Na2O molar ratios under the treatment of vacuum dehydration at 120 °C for 24 h; (b) changes in water content of Na-0.85-GP with different H2O/Na2O molar ratios under the treatment of vacuum dehydration at 120 °C for 24 h.
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Fig. 5. (a) XRD patterns of Na-0.67-15-GP prepared under different conditions; (b) XRD patterns of NaSi-0.85-15.62-GP prepared under different conditions.
structure does not change, and the difference exist primarily at low frequencies [30]. A new bond appears at 553 cm−1 in Na-0.85-15.62GP, representing the double loop vibration of external linkages [30]. This result shows that the formation of Na-A zeolite involves more than a double four-membered ring, in comparison with geopolymers. The band at 668 cm−1 is most likely due to symmetrical stretching vibration of tetrahedrons. It can be concluded from the FT-IR results that Na-GP and NaSi-GP have similar basic frameworks. Between 900 cm−1 and 1000 cm−1, peaks appear, showing that the materials contain predominantly Si-O and Al-O linkages. It is demonstrated that geopolymers are aluminosilicate materials with a main structure that is hydrophilic to some extent.
of NaSi-0.67-15-GP are presented in Fig. 6(a). In the FT-IR analysis of geopolymers activated by water glass, the band at 456 cm−1 is attributed to Si-O bending in internal tetrahedrons, and the band at 561 cm−1 is found to correspond to Al-O linkages. Many peaks with different intensities are observed from 600 cm−1 to 800 cm−1, including a prominent peak at 724 cm−1. This band represents Si-O-Si symmetrical stretching vibration in the internal tetrahedrons. The broad absorbance at 1024 cm−1 is attributed to Si-O-Al (Si) asymmetric stretching vibrations in the geopolymer. The bond at 3459 cm−1 represents water absorption bands, which are attributed to Si-OH, Al-OH and –OH asymmetric stretching vibration. Another part of the water band is observed at 1646 cm−1, which mainly represents H-O-H bending vibrations. Although some water still remains in this sample, – OH vibrations of water (3459 cm−1 and 1646 cm−1) exhibit clearly detectable changes from the case of vacuum dehydration at 120 °C. The residual water (in the form of OH– groups) continues to lost along with heating up to above 300 °C, aiding in polycondensation into -Si-O-Silinkages and the hydroxylation of OH– groups [19]. In Fig. 6(b), the band detected at 1400 cm−1 is attributed to O-C-O stretching vibrations, which may be due to the carbonization process [29]. This finding implies that geopolymer produced by the NaOH activator easily absorbs carbonates from atmospheric CO2 that subsequently accumulate in the geopolymer. By the comparison of the IR spectra of Na-0.8515.62-GP and NaSi-0.67-15-GP, it is concluded that the locations of the absorption peaks are approximately the same, the main body
4. Conclusions According to the compression strength results, the composition of metakaolin and alkali activator was first determined to produce metakaolin-based geopolymers with different water contents. By conducting vacuum dehydration at 120 °C, the compressive strength of the geopolymer samples after the water loss did not decrease; instead, there were different degrees of increase. Two types of metakaolin-based geopolymers produced by two types of alkali activator achieved water discharge under vacuum. The volume of water removal increased with increasing H2O/Na2O. When the H2O/ Na2O molar ratio reached 15, 8.61% of residual water stayed in NaSi-
Fig. 6. (a) FT-IR patterns of NaSi-0.67-15-GP prepared under different conditions; (b) FT-IR patterns of Na-0.85-15.62-GP prepared under different conditions.
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0.67-GP under the vacuum dehydration treatment at 120 °C. In Na0.85-GP, only 1.08% of the residual water remained when the H2O/ Na2O molar ratio reached 15.62, implying that Na-0.85-GP loses much more water that NaSi-0.67-GP under the same experimental conditions. Geopolymer samples under treatment with different activators showed sample different microscopic structures in SEM. The internal part of Na-0.85-GP presented a clearly porous structure, whereas NaSi0.67-GP presented a denser structure. The vacuum treatment at 120 °C can speed up the process of polycondensation of geopolymers, which induces agglomeration in Na-0.85-GP and significantly suppresses micro cracks in NaSi-0.67-GP. During the process of water loss, the results of XRD and FT-IR of geopolymers under different activators' treatment did not have significant changes. In XRD and FT-IR spectra of Na-0.85-GP, the formation of Na-A zeolite was demonstrated. Acknowledgments This work was supported by the Chinese Natural Science Fund (grants 21566006 and 51561135012) and the Guangxi Natural Science Fund (grant 2016GXNSFGA380003). References [1] J. Davidovits, Geopolymers ̵ inorganic polymeric new materials, J. Therm. Anal. Calorim. 37 (1991) 1633–1656. [2] J. Davidovits, Geopolymers: man-made rock deosynthesis and the resulting development of very early high strength cement, J. Mater. Educ. 16 (1994) 91–139. [3] P.N. Lemougna, K.T. Wang, Q. Tang, U.C. Melo, X.M. Cui, Recent developments on inorganic polymers synthesis and applications, Ceram. Int. 42 (2016) 15142–15159. [4] Q. Tang, G.H. Xue, S.j. Yang, K.T. Wang, X.M. Cui, Study on the preparation of a free-sintered inorganic polymer-based proppant using the suspensions solidification method, J. Clean. Prod. 148 (2017) 276–282. [5] J.L. Provis, J.S.J. van Deventer, Geopolymerisation kinetics. 1. In situ energydispersive X-ray diffractometry, Chem. Eng. Sci. 62 (9) (2007) 2309–2317. [6] J.L. Provis, J.S.J. van Deventer, Geopolymerisation kinetics. 2. Reaction kinetic modelling, Chem. Eng. Sci. 62 (9) (2007) 2318–2329. [7] P. Duxson, J.L. Provis, G.C. Lukey, S.W. Mallicoat, W.M. Kriven, J.S.J. van Deventer, Understanding the relationship between geopolymer composition, microstructure and mechanical properties, Colloid Surf. A. 269 (1–3) (2005) 47–58. [8] M.R. Wang, D.C. Jia, P.G. He, Y. Zhou, Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites, Ceram. Int. 37 (5) (2011) 1661–1666. [9] M. Marcin, M. Sisol, I. Brezani, Effect of slag addition on mechanical properties of fly ash based geopolymers, Procedia Eng. 151 (2016) 191–197. [10] H.K. Tchakouté, C.H. Rüscher, S. Kong, E. Kamseu, C. Leonelli, Geopolymer binders from metakaolin using sodium waterglass from waste glass and rice husk ash as alternative activators: a comparative study, Constr. Build. Mater. 114 (2016) 276–289.
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