Facile preparation and hardened properties of porous geopolymer-supported zeolite based on swelled bentonite

Construction and Building Materials 228 (2019) 117040

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Facile preparation and hardened properties of porous geopolymer-supported zeolite based on swelled bentonite Jun Jiang a,b, Ying Yang b, Li Hou a,⇑, Zhongyuan Lu a,⇑, Jun Li a, Yunhui Niu a a b

State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China

h i g h l i g h t s  Bentonite causes the formation of submicron pores, decreasing thermal conductivity.  Zeolite formation in geopolymer decreases thermal conductivity.  Porous zeolite was prepared by curing porous geopolymer for 24 h at 90 °C.  Compressive strength of 2.3–8.7 MPa at pore volume of 0.47–0.80 cc/g was achieved.

a r t i c l e

i n f o

Article history: Received 12 June 2019 Received in revised form 19 August 2019 Accepted 17 September 2019

Keywords: Geopolymer-supported zeolite Pore structure Porous geopolymer Thermal conductivity Compressive strength

a b s t r a c t Porous geopolymer-supported zeolites (PGZs) were successfully prepared by crystallizing bentoniteamended porous geopolymers, to study their synthesis, pore-forming mechanism, pore structure, and hardened properties. The results revealed that geopolymer gel and montmorillonite layers in porous geopolymers could subdivide and refine capillary voids, thereby forming submicron pores. Further, hydrothermal curing resulted in the formation of a zeolite-based matrix. The pores and the zeolitebased matrix caused a considerable reduction in the thermal conductivity of the PGZ. The PGZ pore volume rose steadily from 0.47 cc/g to 0.80 cc/g with increasing bentonite slurry from 40% to 60%. The contents of the bentonite slurry were added to the geopolymer paste and then crystalized for 24 h at 90 °C, yielding PGZs with a thermal conductivity, compressive strength, and dry density of 0.121–0.167 W/m K, 2.3–8.7 MPa, and 774–930 kg/m3, respectively. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Geopolymer foams have been widely investigated for use as thermal insulation in urban buildings, due to their lightweight, low cost, safety, fire resistance, environmental friendly nature [1–6]. Usually, the thermal conductivity of geopolymer foam fabricated by foaming technologies is 0.15–0.59 W/m K with a dry density of 556–1600 kg/m3 [7–9], precursors instead of foam can be used to further decrease the thermal conductivity, for example, geopolymer foam with a thermal conductivity of 0.10 W/m K and dry density of 524 kg/m3 was obtained using resin precursor [6]. However, the relatively high thermal conductivity of these materials compared to widespread polymer foams has greatly hindered their implementation as thermal insulation materials [10,11].

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Jiang), [email protected] (L. Hou), [email protected] (Z. Lu). https://doi.org/10.1016/j.conbuildmat.2019.117040 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Several studies have conducted to address this problem. Specifically, optimizing air-void structure, such as increasing the percentage of 0.03 mm voids and generating hexagon-shape voids, can reduce the value of thermal conductivity [12,13]. Besides, closed cells also can decrease convective heat transfer to improve the thermal insulation performance of the materials. Instead, improving the content of closed pores only slightly decreases the thermal conductivity of porous materials, because of the low percentage of convective heat transfer in heat transmission [14,15]. Decreasing the density or improving the porosity can considerably reduce the value of thermal conductivity of geopolymer foam, for example, a porosity of 79.9% can generate a thermal conductivity of 0.074 W/m K [16]. Further, Wu et al. added metakaolin into this geopolymer, finally resulting in foam with a thermal conductivity of 0.06–0.09 W/m K and dry density of 150–300 kg/m3 [17]. Hence, improving porosity is regarded as the most effective approach for enhancing thermal insulation performance. However, due to an excess of unstable air bubbles, it is very difficult to control their air-void structure [1,18,19]. Although stabilizers can improve the

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stability of air bubbles in a matrix to some degree by increasing viscosity or reducing surface tension of liquid film [18,20], the nature of high energy corresponding to the gas–liquid interface and of a thermodynamically unstable state cannot change [21]. Lightweight aggregates can be used to replace air bubbles in a fresh matrix to solve this drawback caused by air bubbles. Instead, this solution doesn’t result in porous geopolymer with low thermal conductivity, however, owing to difficulties for achieving low density [22–25]. Thermal processes can slightly decrease density, while such processes typically imply a high environmental impact (production of CO2) [6]. Hence, researchers have sought to develop other approaches to further decrease thermal conductivity and thus enhance the thermal insulation performance. Thermal conductivity of air can be decreased by reducing pore size [26], contributing to a reduction in the thermal conductivity of materials. Additionally, the extended and complex heat transfer path caused by decreased pore size also reduces the thermal conductivity of porous materials [27]. However, according to the structure and constitution of porous materials, reducing the thermal conductivity of air contributes to improved thermal insulation performance. Thus, thermal-conductivity reduction of the matrix can be another approach to further reducing the value of thermal conductivity and enhancing the thermal insulation performance of porous materials. As such, a low thermal conductivity matrix based on geopolymers with air voids replaced by smaller pores can be used to obtain higher-performing thermal insulation materials than geopolymer foam. Swelled bentonite consists of excess water and several montmorillonite layers [28]. When pre-swelled bentonite is introduced into geopolymer paste, excessive water cannot be consumed by geopolymerization in geopolymer systems, resulting in a large number of capillary pores. Further, montmorillonite layers in the voids can cause void refinement and segmentation, ultimately increasing the number of small voids [27]. Hence, pre-swelled bentonite is promising for the construction of small voids in hardened geopolymers. More importantly, pores formed by pre-swelled bentonite are different from those formed by traditional foaming technologies: they are not only smaller for enhancing the thermal insulation performance, they also can avoid the drawbacks caused by the instability of air bubble in the matrix. Moreover, geopolymer gels usually exhibit a zeolite-like structure at the atomic scale, which can be regarded as the precursor of zeolite [29,30]. Based on this characteristic, Zhang et al. and Duan et al. used geopolymer gel to obtain self-supporting zeolites under hydrothermal conditions [31,32]. In addition, because the cage framework of zeolite and mean free path of phonon are in similar scale and zeolite contains rich micropores [26], the heat transfer in zeolite is highly limited, resulting in lower thermal conductivity than a geopolymer matrix [33–35]. Hence, the transformation from geopolymer gel into zeolite and the formation of tiny pores from introducing swelled bentonite can together decrease the thermal conductivity and improve the thermal insulation performance of porous geopolymer. Furthermore, montmorillonite can be also used as Al or Si source to prepare zeolite in an alkali and facile environment (<100 °C), thus montmorillonite in geopolymer system may transform into zeolite, further contributing to the thermal-conductivity reduction of the matrix [36,37].

In this study, we attempted to enhance the thermal insulation performance of porous geopolymers by forming small pores and crystallizing the geopolymers into zeolite. Porous geopolymersupported zeolites (PGZs) were fabricated by introducing the bentonite slurry to a fresh geopolymer and then crystallizing the geopolymer gel into zeolite. Synthesis conditions, pore-forming mechanism, pore structure, and hardened properties of the PGZs were also studied. We expect that the results of this investigation will contribute to obtaining high-performance materials for thermal insulation. 2. Materials and methods 2.1. Materials The raw materials included metakaolin, water glass, NaOH, and Na-bentonite. Metakaolin was supplied by Inner Mongolia Super Building Material Technology Co., Ltd. (Hohhot, China). Nabentonite was supplied from Junhui Bentonite Development Co., Ltd. (Sichuan, China). The chemical compositions, particle size distributions (PSDs), and mineral phases of the bentonite and metakaolin are respectively shown in Table 1 and Figs. 1–2. The bentonite comprised montmorillonite, calcite, saponite, quartz, and illite, and the metakaolin mainly consisted of amorphous silicon, alumina, and quartz. The microstructure of the raw materials is presented in Fig. 3. The activators used included water glass (SiO2/Na2O = 2.71, Xinjie Chemical Co., Ltd., Mianyang, China) and NaOH (Chengdu Cologne Chemical Co., Ltd., Chengdu, China, purity > 97%). 2.2. Mix proportions The PGZs were prepared by crystallizing porous geopolymer, which was designed using a volumetric method (Table 2). Fresh porous geopolymer was formed with the bentonite slurry and

Fig. 1. PSDs of metakaolin and bentonite.

Table 1 Chemical composition of raw materials. Compositions (%)

Na2O

MgO

Al2O3

SiO2

SO3

K2O

CaO

Metakaolin Bentonite

0.18 2.57

0.17 4.08

43.85 15.79

52.92 68.28

0.17 0.02

0.18 0.87

0.28 5.50

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J. Jiang et al. / Construction and Building Materials 228 (2019) 117040 Table 2 Mix parameters of PGZs. Geopolymer paste (m3) 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 1.0 0.9 0.8 0.7 0.5 0.3

Fig. 2. XRD curves of metakaolin and bentonite.

geopolymer paste. As described in previous research [31,32], mole ratios of Na/Al = 1/1 and Si/Al = 3/2 can be used to fabricate geopolymer for zeolite preparation. In our study, these ratios were used in all mix proportions. Except for synthesis conditions (temperature and time), the water content and the content of bentonite slurry were also important for zeolite formation. Thus, the waterto-powder ratio (w/p) of the geopolymer paste was varied at fixed volumetric percentages of the bentonite slurry (40% and 60%) to determine a suitable water content. Then, the volume content of the bentonite slurry was varied at selected w/p to obtain a suitable content for geopolymer transformation. 2.3. Preparation and curing Swelled bentonite was first produced by mixing tap water with bentonite and stirring for 1 h. The slurry was then left undisturbed for 24 h to ensure swelling. The water-to-bentonite ratio was 5:1, which had been optimized in our previous study [27]. During the preparation of the geopolymer paste, water and NaOH were placed into a water glass and stirred until the pellets were totally dissolved to prepare the composite alkali activator. After the activator was cooled to 20 ± 2 °C, it was poured into a mixer. Then, metakaolin was put into the mixer and mixed with the activator for 1.5 min at 7 r/s. For porous geopolymer slurries, swelled bentonite (Table 2) was added to fresh geopolymer and stirred for 1 min at 7 r/s. Subsequently, these uniform pastes were immediately poured into 40 mm  40 mm  40 mm molds and placed in a curing chamber for 24 h (temperature: 20 ± 2 °C, relative humid-

(water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder (water-to-powder

ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio: ratio:

Bentonite slurry (m3) 0.45) 0.50) 0.55) 0.60) 0.45) 0.50) 0.55) 0.60) 0.55) 0.55) 0.55) 0.55) 0.55) 0.55)

0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0 0.1 0.2 0.3 0.5 0.7

ity: 90%). Afterward, the samples were demolded and sealed with plastic film to create hydrothermal conditions and cured at different temperatures for several hours.

2.4. Test procedures Three samples (40 mm  40 mm  40 mm) of each composition and synthesis condition were dried at a curing temperature, and then placed in a TYE300 mechanical testing machine (China Jianyi Machinery Plant) to measure the compressive strength at a loading rate of 2.4 kN/s and currency of ±1% based on ISO 679. Three samples of each preparation condition and mix proportion the same as the above samples were tested using DRE 2C equipment (China Xiangtan Xiangyi Instrument Co., Ltd.) operating in transient mode for obtaining thermal conductivity, according to ISO 22007. After dried at 45 °C, the bentonite and metakaolin were tested using a Mastersizer 3000 (Malvern, UK) under ultrasonic leaching to derive the PSD curves. The dispersion medium was alcohol. Anhydrous alcohol was used to replace water in the PGZs, which were then dried at 45 °C. After grinding the PGZs, raw materials and geopolymers, their mineral phases were detected with an X-ray diffractometer (XRD, RIGAKU Dmax-RB, Cu target, scan speed: 8°/min). The sample and raw materials were coated in Au and then observed using a MAIA3LMU scanning electron microscope (SEM; Tescan, Czech Republic) in high vacuum mode under acceleration voltage of 8 kV. Furthermore, the pore structure of PGZs was tested with AutoPore 9500IV mercury intrusion porosimetry (MIP, Micromeritics, USA). Mercury entered the pores with continuous measurements to draw the pore size distribution curves, as intrusion pressure of 0 to 207 MPa was applied. Other

Fig. 3. SEM images of bentonite and metakaolin.

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pore parameters were obtained at the same time. The reactions of bentonite in the alkali systems were determined via SPECTRUM ONE AUTOIMA Fourier transform infrared spectroscopy (FIIR, PerkinElmer, USA), and the spectra were traced from 400 to 4000 cm 1. 3. Results and discussion 3.1. Crystallization and mechanical strength Synthesis conditions and mixture proportions influence the dissolution of Si- or Al-baring materials, polycondensation, and recrystallization of geopolymer, because the alkalinity and the aluminum or silicon content in the systems are related to mixture parameters, further, nanocrystal formation and growth are affected by synthesis temperature and time [38]. Therefore, suitable parameters including synthesis and mixture parameters should be identified for the formation of zeolite. Moreover, considering applications in the future, the compressive strength was simultaneously used to optimize the synthesis parameters and mixture proportions of the PGZs. 3.1.1. Water-to-powder ratio As described in previous researches [29,38–40], the dissolution of metakaolin in the activation solution and the polycondensation happen rapidly and early, such that 24-h curing is sufficient to prepare the geopolymer gel for zeolite formation. Thus, to reduce the curing age, porous geopolymers with different w/p ratios of geopolymer pastes and bentonite contents (40% and 60%) were prepared after 24-h curing. Then, they were crystallized for 24 h at 90 °C to find a suitable w/p ratio. Fig. 4 presents the XRD curves of samples, the intensity of zeolite X increased sharply with a 40% bentonite slurry when increasing the w/p ratio from 0.45 to 0.60, indicating that the crystallinity of zeolite X improved, and that well-crystallized zeolite X could be obtained at high w/p ratios. Unlike with PGZs that had a 40% bentonite slurry, zeolite A and X existed in PGZs with a 60% bentonite slurry at high w/p ratios (w/p 0.55 and w/p 0.60); only zeolite X occurred in PGZs with low w/p values (w/p 0.45 and w/p 0.50). Similarly, the crystallinity of zeolite increased as the w/p increased, and better crystalized zeolite X and hybrid zeolites (zeolite X and zeolite A) were prepared at higher w/p values than 0.45 and 0.50. The formation of zeolite is related to dissolution, diffusion, geopolymerization, and recrystallization. Although we maintained the SiO2/Al2O3 ratio, the transient ratio of SiO2/Al2O3 in the dissolved species changed significantly during the geopolymerization process. Usually, the dissolution of aluminum in metakaolin is fas-

Fig. 5. Dry density and compressive strength of PGZs with various w/p ratios.

ter than silicon, causing a relatively low SiO2/Al2O3 ratio. Under this situation, zeolite A from the geopolymer gel is easy to obtain [41,42]. In the study, relatively low content of water generates high alkalinity, accelerating the dissolution of silicon, which increases the ratio of SiO2/Al2O3, this easily promotes zeolite X formation [43]. However, when the w/p and the content of the bentonite slurry are relatively high, there is more water stemming from the pre-swelled bentonite slurry and the increasing w/p, ultimately decreasing the alkalinity of the system. This decreases the content of transient SiO2 and forms a ratio such that the geopolymer cannot completely transform into zeolite X and partly exists as zeolite A [38]. Moreover, zeolite A forms first, before the formation of zeolite X [38]. The low alkalinity of such systems cannot provide enough activation energy for zeolite A to transform into zeolite X because the latter needs a higher activation energy, owing to the complexity of the repeated unit of zeolite X [38]. Hence, the combined effects lead to hybrid zeolites in PGZs with high bentonite content (60%) and w/p ratios (0.55 and 0.60). Moreover, due to the high content of water caused by increasing w/p, ion diffusion and migration accelerate in low-concentration solutions. This enhances the crystallization of the existing zeolite. Thus, the crystallinity of zeolite improves with increasing w/p ratios in PGZ systems. As presented in Fig. 5, the compressive strength and dry density of PGZs fell when increasing the w/p from 0.45 to 0.60. Specifically, the strength and dry density of PGZs varied from 21.4 MPa to 1.5 MPa between 1022 kg/m3 and 868 kg/m3 (40% bentonite slurry) and 7.3 MPa to 0 between 907 kg/m3 and 740 kg/m3, respectively. With a higher w/p (0.60), there was a sharp reduction

Fig. 4. XRD curves of PGZs with various w/p values.

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Fig. 6. XRD curves of PGZs under different curing ages.

the w/p results in more water introduced to the PGZ systems. After drying, the excess water leads to the formation of coarse voids and decreased density, contributing to a decrease in strength [27]. Although the montmorillonite layers or zeolites transformed from the layers in these voids may reduce the void size, there is a limit to how small they can become, owing to the limited content of montmorillonite layers or zeolites.

Fig. 7. Dry density and compressive strength of PGZs under different curing ages.

in strength, such that it could not be used to prepare PGZ practically. However, with a 40% and 60% bentonite slurry, the compressive strength of the samples was 8.7 MPa (930 kg/m3) and 2.3 MPa (774 kg/m3) at a w/p of 0.55, respectively. Considering the zeolite crystallinity and strength of PGZs prepared by 40% and 60% bentonite slurries, a 0.55 w/p gives high crystallinity and relatively suitable strength. Thus, we used these PGZs in subsequent experiments for parameter optimization. Strength reduction is related to density variations and pores caused by high w/p ratios. Increasing

3.1.2. Curing time Based on the study above, a w/p of 0.55 was chosen to prepare PGZs with 40% and 60% bentonite slurries to find suitable crystallization time. As shown in Fig. 6, due to the time extension of crystallization, the crystallinity of zeolite X in PGZs with a 40% bentonite slurry and hybrid zeolites (zeolite X and zeolite A) in PGZs with a 60% bentonite slurry was enhanced significantly with time extension from 12 h to 36 h. The compressive strength and dry density of PGZs under different curing ages were used to evaluate the parameters for PGZs preparation. The dry density of the PGZs varied slightly from 930 kg/m3 to 970 kg/m3 and 749 kg/m3 to 790 kg/m3, when the bentonite slurry content was 40% and 60%, respectively (Fig. 7). However, the compressive strength rose from 7.5 MPa to 8.7 MPa (40% bentonite slurry) and 1.2 MPa to 2.3 MPa (60% bentonite slurry) between 12 h and 24 h (Fig. 7), further extending curing age caused slight change of compressive strength. This enhancement in strength is caused by the fact that increasing the zeolite size from better crystallization may contribute to a decrease in the size of the voids in the PGZ systems due to more void-space occupation by large zeolite particles. This,

Fig. 8. XRD curves of PGZs under different curing temperatures.

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zeolite [35,38]. These crystals occupy the void space formed by water loss, ultimately decreasing the size of the voids and contributing to an increase in strength. However, the considerable reduction in geopolymer gel caused by zeolite formation weakens the system. As such, these two effects only slightly change the strength of the PGZ with 60% bentonite slurry when the temperature rises from 70 °C to 90 °C. Due to more geopolymer gel caused by a low substitution of bentonite slurry, there is more remaining geopolymer gel in PGZ with 40% bentonite slurry than that in PGZ with 60% bentonite slurry. This results in a continuous increase in strength of PGZ with 40% bentonite, when the temperature changes from 70 °C to 90 °C.

3.1.3. Curing temperature PGZs with 40% and 60% bentonite slurries and a w/p of 0.55 were prepared by curing for 24 h at different curing temperatures to seek the proper preparation temperature, as shown in Fig. 8. The XRD curves of PGZs indicate that improving the curing temperature can contribute to enhancing the crystallinity of zeolite in 40% and 60% bentonite slurry systems. This may be attributed to the enhanced rate of diffusion, migration, and reorganization of ions, silicon and aluminum caused by increased temperature [35,38]. Fig. 9 shows that the dry density of PGZs changes slightly after curing at 50 °C–90 °C. The compressive strength of the PGZs rose from 5.7 MPa to 7.4 MPa (40% bentonite slurry) and 2.0 MPa to 2.4 MPa (60% bentonite slurry) as the temperature rose from 50 °C to 70 °C. It then further increased to 8.7 MPa and changed only slightly thereafter. This is mainly because further increasing the curing temperature enhances the crystallinity and size of the

3.1.4. Content of the bentonite slurry Owing to the better crystallinity of zeolite and the acceptable strength, 90 °C was used to study the effect of bentonite slurry content on PGZ preparation. As presented in Fig. 10, after increasing the substitution of the bentonite slurry (0%–30%), there was no crystalline phase after hydrothermal curing; After further increasing the bentonite slurry to 50%, zeolite X occurred, and the crystallinity of the zeolite enhanced; When increasing the bentonite slurry to 70%, hybrid zeolites (zeolite A and zeolite X) occurred and the intensity of the zeolites enhanced. All indicate that wellcrystallized zeolite can be formed in PGZs with a high bentonite slurry, with a minimum of 40% bentonite slurry for PGZ formation. Crystal formation and their differences are affected by the alkalinity of system and the transient SiO2/Al2O3 ratio, as described in Section 3.1.1. Pre-swelled bentonite consists of excess water and montmorillonite layers. When the content of the bentonite slurry increases, the alkalinity of the PGZ system decreases. As mentioned in Section 3.1.1, the alkalinity can affect the formation of zeolite. When the bentonite slurry is high (60% and 70%), the alkalinity of system is low, which results in hybrid zeolites formation. Decreasing the bentonite slurry from 50% to 40%, and the corresponding increase in the system, promotes the formation of zeolite X in PGZs. However, the low content of water caused by a low introduction of bentonite slurry (10%, 20%, and 30%) hinders the dissolution and migration of ions, silicon, and aluminum. This limits the formation of zeolite, such that no crystals form in these pastes. Moreover, due to more water from an increase in bentonite slurry (40%–70%), the crystallinity of zeolite enhances, for the reasons given above. Fig. 11 indicates that the compressive strength and the dry density of samples decrease with an increasing bentonite slurry. Specifically, the dry density of samples decreased from 1481 kg/

Fig. 10. XRD curves of PGZs with different contents of bentonite slurry.

Fig. 11. Dry density and compressive strength of PGZs with various contents of bentonite slurry.

Fig. 9. Dry density and compressive strength of PGZs under various curing temperatures.

in turn, increases the compressive strength. However, the crystallization of geopolymer gel decreases the binder amount, weakening the strength of PGZs [35]. Together, these two effects result in only a slight change in the PGZs when the curing time is prolonged (24 h–36 h). Given the slight change in strength and relatively good crystallinity, it is unnecessary to extend the curing time. Thus, we used a 24-h curing time with the subsequent experiments for parameter optimization.

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was 40%, the dry density and compressive strength of PGZ were respectively 930 kg/m3 and 8.7 MPa. The reduction in the strength and dry density of PGZs is due to more water in the PGZ systems from increasing the bentonite slurry, insofar as water increases the number and size of the voids. 3.2. Pore structure and pore-forming mechanism

m3 to 602 kg/m3, and the related strength of samples fell between 60.1 MPa and 0 MPa when increasing the bentonite slurry from 0 to 70%. Thus, PGZ with a 70% bentonite slurry cannot be used in practice because it is too weak. The dry density and compressive strength of PGZ were 774 kg/m3 and 2.3 MPa, respectively, when the bentonite slurry was 60%. This indicates that the maximum dosage of bentonite slurry is 60%. When the bentonite slurry

As previously mentioned, a 40–60% bentonite slurry at a w/p of 0.55 can be used to prepare PGZs with crystalized zeolites and apt strength under a curing temperature 90 °C for 24 h. Thus, PGZs with different bentonite slurry contents (40–60%) were prepared under optimized parameters to investigate the pore structure. Fig. 12 shows that the cumulative pore volume rises from 0.47 cc/g to 0.80 cc/g, which reveals that bentonite slurry leads to pore formation when the bentonite slurry increases from 40% to 60%. Fig. 12 shows a slight change in the dominated pore size of PGZs with 50% and 60% bentonite slurry—namely, 550 nm—despite a relatively large dominated pore size of 660 nm in PGZs with 40% bentonite slurry. A similar phenomenon is observed in the variation of the mean pore size of PGZs, which reduced from 573 nm to 513 nm when increasing the bentonite slurry from 40% to 60%, and the porosity increased from 48.0% to 62.5% (Fig. 13). Additionally, PGZs with hybrid zeolites (60% bentonite slurry) contained another concentrated distribution of pores, whose dominant pore size was about 130 nm. In general, the main pores of all PGZs were in the range of submicron. Pore formation is related to the pore forming agent (pre-swelled bentonite). The change of montmorillonite in geopolymer and PGZs was studied through XRD, as shown in Fig. 14. The sharp 001 peak related to montmorillonite in bentonite disappeared in porous geopolymer and PGZ systems, indicating that montmorillonite survives as single layers or transforms into geopolymer gel or zeolite, because montmorillonite platelets or layers can be dissolved in situ into silicon and aluminum species and transform into geopolymer gel and zeolite [36,37,44]. These in voids refine the capillary space into smaller one and contribute to submicron space formation. As shown in Fig. 15, peaks at 1039 cm 1 and 797 cm 1(vibration of Si-O-Si), 3443 cm 1 and 1645 cm 1 (bending vibration of H-OH) and 467 cm 1 (coupling vibration of OH) corresponding to bentonite still existed in porous geopolymers and PGZs. Additionally, the disappeared peaks of Al-OH (3623 cm 1 and 916 cm 1) in PGZs and geopolymers reveal that montmorillonite layers react with the activator, confirming that the remained layers and newly generated geopolymer gel or zeolite from the layers exist and divide

Fig. 14. XRD curves of PGZ, porous geopolymer and bentonite.

Fig. 15. FIIR of PGZs and porous geopolymers.

Fig. 12. Pore size distributions of PGZs.

Fig. 13. Porosity and mean pore size of PGZs.

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Fig. 16. SEM images of PGZs.

Fig. 17. Thermal conductivities of PGZs and porous geopolymers.

the capillary space into smaller one. Moreover, the peak of Si-OH at 520 cm 1 disappeared in geopolymers [45], which also indicates that montmorillonite layers may react in alkali systems. After crystallization and drying, the excess water in the refined capillary spaces was removed. This caused the formation of refined pores (submicron pores). However, as shown in Fig. 16, the crystals in PGZs with higher bentonite slurry were larger due to better crystallization. These bigger crystals occupy more pore spaces, ultimately leading to smaller pores. This may reduce the pore size of PGZs with higher bentonite slurry. However, the increased water in PGZ with a 60% bentonite slurry causes larger pores due to more capillary water and limited montmorillonite layers or zeolites in the pores. These two opposing effects result in only a slight change

to the pore size in PGZs with 50% and 60% bentonite slurries. Moreover, due to hybrid zeolites in PGZ with 60% bentonite and the differences in their formation, zeolite X may be smaller than zeolite A. Zeolite X fills part of pore space, partially refining the pores, which forms a part pores at small pore size range. This causes two dominant peaks in the pore size distribution of PGZ with a 60% bentonite slurry. 3.3. Thermal insulation performance A bentonite slurry content of 40–60% with a w/p of 0.55 under optimized conditions was used to prepare PGZs. Fig. 17 shows the thermal conductivity increased between 0.121 W/m K and

J. Jiang et al. / Construction and Building Materials 228 (2019) 117040

0.167 W/m K when dry density of the PGZ rose from 774 kg/m3 to 930 kg/m3. A 60% bentonite slurry resulted in a dry density of 774 kg/m3 and a thermal conductivity of 0.121 W/m K. The thermal conductivity of the precursor of PGZs (porous geopolymers) is also presented in Fig. 17(a). These samples had a thermal conductivity of 0.125–0.173 W/m K and a dry density of 731–920 kg/m3. This reveals that the transformation of geopolymer into zeolite can further decrease the thermal conductivity. Compared to porous geopolymers and other common porous cement-based materials, PGZs show excellent thermal insulation performance, insofar as they have the lowest thermal conductivity (Fig. 17(b)) [8,9,27,46]. The promising thermal conductivity of PGZs at a fixed density can be attributed to their submicron-pore structure, because this structure reduces the thermal conductivity of the air [26]. In addition, smaller pores yield a longer and more complex heat-transfer path than typical air voids at the same porosity [27]. These effects lead to the low thermal conductivity of porous geopolymer (the precursor to PGZ) with a given density. Moreover, the thermal conductivity of zeolite is smaller than that of a geopolymer matrix, further decreasing the thermal conductivity of porous geopolymer and resulting in better thermal insulation performance. According to their strength and thermal conductivity, as mentioned above, high-performance thermal insulation materials can be prepared using our approach. 4. Conclusion PGZs with submicron-size pores were successfully prepared with a crystallized porous geopolymer using swelled bentonite slurry. (1). Dissolution difference of aluminum and silicon in raw materials led to low SiO2/Al2O3 ratio, promoting the formation of zeolite A during the hydrothermal process, while increased alkalinity, due to the decreased water, can promote silicon dissolution and provide enough activation energy for zeolite X generation from zeolite A. After introduced more water, the alkalinity of PGZ system decreased, finally leading to the formation of hybrid zeolites. (2). Remained montmorillonite layers and newly generated geopolymer gel or zeolite from these layers divided the capillary space into smaller one, contributing to the formation of submicron pores. When the bentonite slurry increased from 40% to 60%, the mean pore size of PGZ fell from 573 nm to 513 nm, an increasing total pore volume (0.47– 0.80 cc/g) and porosity (48.0–62.5%) were obtained. (3). Because of the submicron-porous structure and zeolitebased matrix of the PGZs, the thermal conductivity of air and matrix decreased, and the heat-transfer path increased significantly, resulting in lower conductivity and better thermal insulation performance in PGZs, respectively, compared to other inorganic porous materials. (4). PGZs with a thermal conductivity, compressive strength, and dry density of 0.121–0.167 W/m K, 2.3–8.7 MPa, and 774–930 kg/m3 respectively, were obtained by introducing swelled bentonite to geopolymer and then crystallizing it, demonstrating the potential of PGZs for thermal insulation applications.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement The first author is grateful for funding from Southwest University of Science and Technology for his research in the UK. Further, the authors are grateful for funding from the Science and Technology Project of Sichuan Province (No. 2018GZ0152, No. 18YYJC0904, No. 2019ZDZX0024), the National Key Research and Development Plan of China (No. 2016YFC0701004).

References [1] Y. Cui, D. Wang, J. Zhao, D. Li, S. Ng, Y. Rui, Effect of calcium stearate based foam stabilizer on pore characteristics and thermal conductivity of geopolymer foam material, J. Build. Eng. 20 (2018) 21–29. [2] K. Komnitsas, D. Zaharaki, Geopolymerisation: a review and prospects for the minerals industry, Miner. Eng. 20 (2007) 1261–1277. [3] J.S.J. Deventer, J.L. Provis, P. Duxson, G.C. Lukey, Reaction mechanisms in the geopolymeric conversion of inorganic waste to useful products, J. Hazard. Mater. 139 (2007) 506–513. [4] J.L. Provis, S.A. Bernal, Geopolymers and related Alkali-activated material, Annu. Rev. Mat. Res. 44 (2014) 299–1232. [5] A.M. Rashad, Alkali-activated metakaolin: a short guide for civil engineer-an overview, Constr. Build. Mater. 41 (2013) 751–765. [6] G. Roviello, C. Menna, O. Tarallo, L. Ricciotti, F. Messina, C. Ferone, D. Asprone, R. Cioffi, Lightweight geopolymer-based hybrid materials, Compos. Part B: Eng. 128 (2017) 225–237. [7] E. Kamseu, B. Ceron, H. Tobias, E. Leonelli, M.C. Bignozzi, A. Muscio, A. Libbra, Insulating behavior of metakaolin-based geopolymer materials assess with heat flux meter and laser flash techniques, J. Therm. Anal. Calorim. 108 (2012) 1189–1199. [8] Z.H. Zhang, J.L. Provis, A. Reid, H. Wang, Mechanical, thermal insulation, thermal resistance and acoustic absorption properties of geopolymer foam concrete, Cem. Concr. Compos. 62 (2015) 97–105. [9] F. Xu, G. Gu, W. Zhang, H. Wang, X. Huang, J. Zhu, Pore structure analysis and properties evaluations of fly ash-based geopolymer foams by chemical foaming method, Ceram. Int. 44 (2018) 19989–19997. [10] J. Jiang, Z.Y. Lu, Y.H. Niu, J. Li, Y.P. Zhang, Investigation of the properties of highporosity cement foams based on ternary Portland cement–metakaolin–silica fume blends, Constr. Build. Mater. 107 (2016) 181–190. [11] J. Jiang, Z.Y. Lu, Y.H. Niu, J. Li, Y.P. Zhang, Study on the preparation and properties of high-porosity foamed concretes based on ordinary Portland cement, Mater. Des. 92 (2016) 949–959. [12] W. She, G. Zhao, D. Cai, J. Jiang, X. Cao, Numerical study on the effect of pore shapes on the thermal behaviors of cellular concrete, Constr. Build. Mater. 163 (2018) 113–121. [13] F. Batool, V. Bindiganavile, Air-void size distribution of cement based foam and its effect on thermal conductivity, Constr. Build. Mater. 149 (2017) 17–28. [14] F.K. Akthar, J.R.G. Evans, High porosity (>90%) cementitious foams, Cem. Concr. Res. 40 (2010) 352–358. [15] P.G. Collishaw, J.R.G. Evans, An assessment of expressions for calculating the apparent thermal conductivity of cellular materials, J. Mater. Sci. 29 (1994) 2261–2273. [16] J. Feng, R. Zhang, L. Gong, Y. Li, W. Cao, X. Chenget, Development of porous fly ash-based geopolymer with low thermal conductivity, Mater. Des. 65 (2015) 529–533. [17] J. Wu, Z. Zhang, Y. Zhang, D. Li, Preparation and characterization of ultralightweight foamed geopolymer (UFG) based on fly ash-metakaolin blends, Constr. Build. Mater. 168 (2018) 771–779. [18] W. She, Y. Du, G. Zhao, P. Feng, Y. Zhang, X. Cao, Influence of coarse fly ash on the performance of foam concrete and its application in high-speed railway roadbeds, Constr. Build. Mater. 170 (2018) 153–166. [19] Z. Huang, T. Zhang, Z. Wen, Proportioning and characterization of Portland cement-based ultra-lightweight foam concretes, Constr. Build. Mater. 79 (2015) 390–396. [20] W. She, Y. Du, C. Miao, J. Liu, G. Zhao, J. Jiang, Y. Zhang, Application of organicand nanoparticle-modified foams in foamed concrete: reinforcement and stabilization mechanisms, Cem. Concr. Res. 106 (2018) 12–22. [21] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Ultra stable particle stabilized foams, Angew. Chem. 118 (2006) 3606–3610. [22] M.Y.J. Liu, U.J. Alengaram, M.Z. Jumaat, K.H. Mo, Evaluation of thermal conductivity, mechanical and transport properties of lightweight aggregate foamed geopolymer concrete, Energy Build. 72 (2014) 238–245. [23] Y. Zaetang, A. Wongsa, V. Sata, P. Chindaprasirt, Use of coal ash as geopolymer binder and coarse aggregate in pervious concrete, Constr. Build. Mater. 96 (2015) 289–295. [24] V. Medri, E. Papa, M. Mazzocchi, L. Laghi, M. Morganti, J. Francisconi, E. Landi, Production and characterization of lightweight vermiculite/geopolymer-based panels, Mater. Des. 85 (2015) 266–274. [25] P. Posi, C. Ridtirud, C. Ekvong, D. Chammanee, K. Janthowong, P. Chindaprasirt, Properties of lightweight high calcium fly ash geopolymer concretes containing recycled packaging foam, Constr. Build. Mater. 94 (2015) 408–413.

10

J. Jiang et al. / Construction and Building Materials 228 (2019) 117040

[26] J.J. Zhao, Y.Y. Duan, X.D. Wang, B.X. Wang, Effects of solid–gas coupling and pore and particle microstructures on the effective gaseous thermal conductivity in aerogels, J. Nano Res. 14 (2012) 1024. [27] J. Jiang, Z. Lu, J. Li, Y. Xie, K. Luo, Y. Niu, Preparation and properties of nanopore-rich lightweight cement paste based on swelled bentonite, Constr. Build. Mater. 199 (2019) 72–81. [28] M. Benna, N. Kbir-Ariguib, C. Clinard, F. Bergaya, Card-house microstructure of purified sodium montmorillonite gels evidenced by filtration properties at different pH, Progr. Colloid. Polym. Sci. 117 (2001) 204–210. [29] H. Yan, X. Cui, L. Liu, X. Liu, J. Chen, The hydrothermal transformation of solid geopolymers into zeolites, Micropor. Mesopor. Mater. 161 (2012) 187– 192. [30] E. Papa, V. Medri, S. Amari, J. Manaud, P. Benito, A. Vaccari, E. Landi, Zeolitegeopolymer composite materials: production and characterization, J. Clean. Prod. 171 (2018) 76–84. [31] J. Zhang, Y. He, Y. Wang, J. Mao, X. Cui, Synthesis of a self-supporting faujasite zeolite membrane using geopolymer gel for separation of alcohol/water mixture, Mater. Lett. 116 (2014) 167–170. [32] J.X. Duan, J. Li, Z.Y. Lu, One-step facile synthesis of bulk zeolite: a through metakaolin-based geopolymer gels, J. Porous Mater. 22 (2015) 1519–1526. [33] V.V. Murashov, M.A. White, Thermal properties of zeolites: effective thermal conductivity of dehydrated powdered zeolite 4A, Mater. Chem. Phys. 75 (2002) 178–180. [34] M.B. Jakubinek, B.Z. Zhan, M.A. White, Temperature-dependent thermal conductivity of powdered zeolite NaX, Micropor. Mesopor. Mater. 103 (2007) 108–112. [35] J. Henon, F. Pennec, A. Alzina, J. Absi, D.S. Smith, S. Rossignol, Analytical and numerical identification of the skeleton thermal conductivity of a geopolymer foam using a multi-scale analysis, Comp. Mater. Sci. 82 (2014) 264–273.

[36] E. Prud’Homme, P. Michaud, E. Joussein, C. Peyratout, A. Smith, S. Rossignol, In situ inorganic foams prepared from various clays at low temperature, Appl. Clay Sci. 51 (2011) 15–22. [37] I.D.R. Mackinnon, G.J. Millar, W. Stolz, Low temperature synthesis of zeolite N from kaolinites and montmorillonites, Appl. Clay Sci. 48 (2010) 622–630. [38] E. Ofer-Rozovsky, M.A. Haddad, G.B. Nes, A. Katz, The formation of crystalline phases in metakaolin-based geopolymers in the presence of sodium nitrate, J. Mater. Sci. 51 (2016) 4795–4814. [39] Z. Li, S. Zhang, Y. Zuo, W. Chen, G. Ye, Chemical deformation of metakaolin based geopolymer, Cem. Concr. Res. 120 (2019) 108–118. [40] Q. Tang, H. Yan, Y. Wang, K. Wang, X. Cui, Study on synthesis and characterization of ZSM-20 zeolites from metakaolin-based geopolymers, Appl. Clay Sci. 129 (2016) 102–107. [41] A. Molina, C. Poole, A comparative study using two methods to produce zeolites from fly ash, Miner. Eng. 17 (2004) 167–173. [42] C.W. Purnomo, S. Chris, H. Hinode, Synthesis of pure Na–X and Na–A zeolite from bagasse fly ash, Micropor. Mesopor. Mater. 162 (2012) 6–13. [43] G.G. Hollman, G. Steenbruggen, M. Janssen-Jurkovicˇová, A two-step process for the synthesis of zeolites from coal fly ash, Fuel 78 (1999) 1225–1230. [44] G. Lagaly, S. Ziesmer, Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions, Adv. Colloid. Interf. Sci. 100 (2003) 105–128. [45] N.R. Yang, W.H. Yue, The Handbook of Inorganic Matalloid Materials Atlas, Wuhan University of Technology Press, Wuhan, 2000. [46] F. Colangelo, G. Roviello, L. Ricciotti, V. Ferrandiz-Mas, F. Messina, C. Ferone, O. Tarallo, R. Cioffi, C.R. Cheeseman, Mechanical and thermal properties of lightweight geopolymer composites, Cem. Concr. Compos. 86 (2018) 266–272.