Effects of Curing Temperature and Water Glass Modulus on the Preparation of Hierarchical Zeolite Precursors

The hierarchical design of zeolite materials from alkali-activated solid waste is a controversial topic. Deactivated ZSM-5 can serve as a potential foaming material for the preparation of porous zeolite precursors. This article evaluated the suitability of deactivated ZSM-5 for the preparation of porous zeolite material precursors. Curing temperature and water glass modulus were selected to regulate the properties of alkali-activated cementitious material. Two different moduli of water glass (1.4 M and 2.0 M) were applied to determine the effect of samples cured at three temperatures (25 ℃, 50 ℃ and 75 ℃). Characterization analyses such as XRD, FTIR, TG/DTA, SEM/EDS and MIP were performed to analyse the final products and pore structure of the precursors. A higher curing temperature and alkalinity have a substantial influence on the mechanical properties and yield a higher degree of polymerization of the reaction products. The main reaction product in the precursors was the N-A-S–H gel, whose degree of polymerization is strongly influenced by the Si/Al ratio. The purpose of this article is to design porous structures for zeolite precursors. The research results provide promising guidance for the preparation of zeolites with more active sites, high strength and porous structures that are self-supported.


Introduction
Zeolite is a type of crystalline aluminosilicate that is a threedimensional network composed of tetrahedral [SiO 4 ] 4− and [AlO 4 ] 5− [1]. Recently, zeolite has been extensively employed in environmental mitigation, including radioactive waste removal, gas capture, toxic metal ion removal and wastewater treatment, due to its ion-exchange properties and porous structure [2][3][4][5]. Since pioneering work revealed that aluminosilicate gels can be used to synthesize a broad range of zeolites [6,7], extensive research on zeolites from a variety of alumina and silica sources has been carried out. However, the utilization of chemical sources such as silica and alumina for the synthesis of zeolite is relatively expensive [8,9]. Numerous studies have been carried out to identify economical raw materials that are suitable for the synthesis of zeolites [10,11]. Industrial waste such as fly ash can be utilized to produce zeolite through hydrothermal methods and has been widely reported [12][13][14]. The synthesis route is divided into two main steps: (1) synthesis of zeolite precursors (geopolymer) and (2) treatment of zeolite precursors in an autoclave. The beneficial properties (i.e., bulk density, strength and pore structure) inherited from geopolymers are the basis for the synthesis of zeolites [15]. However, precursors with high density and fewer pores will limit the contact between the solution and the adsorbent, resulting in significantly reduced adsorption efficiency [16]. Therefore, foamed precursors with porous structures might be a better option.
Currently, foamed precursors have attracted increasing attention due to their higher surface area, large pore volume and excellent cation exchange capacity [17][18][19][20]. However, many factors affect the properties of foamed precursors, including curing temperature, activator type/concentration, Si/Al ratio, and curing time [21][22][23][24][25], which directly impact the performance of the final zeolite products. Moreover, the pore size distribution of foamed precursors produced by common physical foaming and chemical foaming is uneven and uncontrollable [26]. Al powder, zinc powder and H 2 O 2 violently react under strong alkali conditions, and gas rapidly escapes from the paste, resulting in the collapse of the pore structure [27]. A large volume of air bubbles introduced by the organic foaming agent causes the pores to tend towards macropores, which makes it difficult to regulate and optimize the pore structure [20,27].
Geopolymers are considered potential zeolite precursor materials to synthesize zeolite [28]. The synthesis of geopolymer, a chemical process in which a strong alkali silicate solution is necessary to dissolve the active surface groups of silica-and-alumina materials and to promote surface hydrolysis and potential hydraulic performance of precursor materials, is known as the geopolymerisation reaction or alkali activation. The alkali activator solution has an important role in the synthesis of the final geopolymeric product in geopolymerisation reactions [29]. In addition, the dissolution of silica and alumina in the geopolymerisation reaction was also affected by the curing temperature [30,31].
The commonly utilized alkali activator solutions can be a mixture of one or several media, which includes sodium hydroxide, sodium silicate, potassium hydroxide, sodium carbonate, potassium silicate and sodium carbonate mixed with sodium hydroxide [32][33][34]. NaOH with water glass is the most extensively utilized alkali activator. Compared with using only water glass, the compressive strength significantly increased when a mixture of NaOH and water glass was employed during the polymerization, as reported in previous studies [34][35][36]. A higher Si content in the water glass promotes the development of strength [37]. Furthermore, the raw materials dissolved in a strong alkaline activator generate precursor ions (Al 3+ and Si 4+ ) that are affected by the curing temperature. Fly ash very slowly reacts at ambient temperatures as a raw material for the production of geopolymers [38]. However, when the temperature increases between 45 and 95 ℃, it can serve as a reaction accelerator to promote geopolymer reactions and leads to a gain of compressive strength [39][40][41].
With the high pH of geopolymers, many common materials (Al powder and H 2 O 2 ) selected as foaming materials in geopolymerization processes have difficulty controlling the rate of reaction. Different methods of foaming should be employed. Zeolite Socony Mobil-Number 5 (ZSM-5) is widely applied in refining and petrochemical applications due to its high specific surface area and pore volume, as well as its unique pore structure, which gives it excellent catalytic shape selection properties [42,43]. However, its repeated use will cause deactivation of ZSM-5. The main reasons are presented as follows: (1) pore blockage: coke molecules are adsorbed in the pore wall, cage or channel, which will increase the diffusional resistance and reduce the concentration of reactant in the mobile phase at the active point; (2) site coverage: the adsorption of coke molecules on the active point will lead to the termination of the motion cycle of the active point [42]. Deactivated ZSM-5 from hightemperature incineration was dumped in the pond, resulting in a substantial waste of resources. In recent years, deactivated ZSM-5 has been investigated as a raw material for the production of geopolymers at a given curing temperature and water glass modulus [17,20,44]. As deactivated ZSM-5 contains a large amount of active silica, it can be used as a siliceous raw material to participate in polymerization. Moreover, numerous micropores and mesopores are present in deactivated ZSM-5, which contain a large amount of gas that will be released under certain conditions. For this reason, deactivated ZSM-5 can be used as a foaming agent and source material. However, the effects of temperature and water glass modulus on the application of deactivated ZSM-5 in the production of pore structures are not distinct, which limits the further application of ZSM-5 in the production of zeolite precursors. In addition, the applicability of deactivated ZSM-5 to replace common foaming agents and siliceous raw materials needs to be evaluated in terms of various water glass moduli and curing temperatures.
In this paper, hierarchical zeolite precursors were fabricated by fly ash, water glass and deactivated ZSM-5. A peculiar focus was placed on evaluating the suitability of deactivated ZSM-5 for the preparation of porous zeolite material precursors with different curing temperatures and water glass moduli, which were used to regulate the properties of precursors. The results will contribute to an understanding of porous and self-supported zeolites synthesized from precursors and provide a reference for future improvements and applications of deactivated ZSM-5.

Materials
Industry wastes, deactivated ZSM-5 (Xi'an Qiyue Biotechnology Co., China) and fly ash (Hebei Weiran Building Materials Technology Co., China), were used to synthesize the geopolymer. Figure 1 shows the particle size distribution of two dry powders, deactivated ZSM-5 and fly ash; details on the particle size are shown in Table 1. Table 2 reports the chemical composition of deactivated ZSM-5 and fly ash. The nature of NaOH (Jiangsu Qinghe Chemical Co., China) and water glass (Jinan Mingchuan Chemical Co., China) are listed in Table 3.

Preparation of Alkali Solution
The modulus (M) of water glass is the molecular ratio of silicon oxide (SiO 2 ) to alkali metal oxides (Na 2 O) in water glass, that is, M = SiO 2 /Na 2 O. The initial modulus of the water glass utilized in this experiment was 3.3. To obtain the modulus of the water glass required for the experiment. The NaOH pellets were added to water glass to adjust the modulus of water glass to 1.4 and 2.0. The prepared solution was placed at room temperature for 24 h.

Preparation of Geopolymer Paste
The mix proportions of the raw materials for geopolymer sample preparation are listed in Table 4, and the samples under different preparation conditions are shown in Table 5. First, the raw material powder (deactivated ZSM-5 and fly ash) was added to a mixing pot and then mixed for approximately 3 min. Second, the prepared 1.4 M or 2.0 M water glass was slowly added to the mixing pot, followed by mixing at a low speed for approximately 4 min and then at high speed for approximately  4 min to ensure adequate reaction between the raw material and the water glass, achieving a uniform mixture. Last, the geopolymer mixture was poured into a cube moulds of 100 mm, followed by curing at a given temperature (25, 50 and 75 ℃) for 24 h. To ensure the same shape of the geopolymer samples employed in the mechanical testing, the excess part that was outside the mould size was cut off. At the end of 24 h of initial curing, the geopolymer pastes were demoulded and then stored at room temperature for 6 days.

Raw Materials
The chemical components of deactivated ZSM-5 and fly ash were determined by X-ray fluorescence spectrometry (XRF). X-ray powder diffraction was employed to analyse the composition of the raw materials. Moreover, the particle size distribution of the two dry raw powder materials was determined by a Malvern Mastersizer 3000.

Mechanical Property
The compressive strength of geopolymer pastes at the age of 7 days was determined by a pressure testing machine based on Chinese standard GB/T 50,081-2016 with a loading rate of 0.5 ± 0.1 MPa/s. Each geopolymer paste was retested three times, the average of the three test results was taken and the errors of compressive strength did not exceed 0.8 MPa.
The bulk density of the geopolymer was determined according to ASTM C29 [45].
The viscosity of geopolymer pastes was measured by an NDJ-8S instrument. The slurry was poured into a beaker with a minimum diameter of 60 mm, and then the lift knob was turned to slowly submerge the rotor into the paste in a counterclockwise direction until the mark is at the same height as the liquid level. Three geopolymer slurry samples were simultaneously determined for each mix, and the average viscosity was selected as the result.

XRD, FTIR and TG-DTG of Geopolymer Paste
All geopolymer crushed samples that were collected from the compressive strength test were put into absolute ethanol to stop hydration before testing and then ground to a fine powder.
X-ray diffraction (XRD) using a Bede D1 X-ray powder diffractometer was employed to study the mineralogical composition of the cured geopolymer. All XRD scans were performed using Cu-Kα radiation and a step width of 2° with a scan rate of 2° per min and 2θ ranging from 5° to 80°.
Fourier transformed infrared spectroscopy (FTIR) of cured geopolymers was performed using a Nicolet Nexus670 in absorbance mode with a wavenumber range from 4000 to 400 cm −1 at a resolution of 4 cm −1 .
Thermogravimetric analysis and differential thermal analysis (TG-DTG) of geopolymer samples placed in an open Al 2 O 3 vessel were examined using an STA 449C3/G thermal analyser with a nitrogen atmosphere, performed in the range of 25-1000 ℃ and heated at a rate of 10 ℃ per min.

SEM and MIP
The small samples for each type of geopolymer employed in SEM and MIP testing were collected by compression testing. Thus, samples of the outer surface of each geopolymer were not necessary as the external surfaces of the geopolymer were relatively smooth, and contact with air during curing may produce varying microstructures, so their pore structure and microstructure are not representative.
The microstructure and chemical elemental analysis of geopolymers were observed using EVO MA18 40XVP scanning electron microscopy (SEM) with an accelerating voltage of 10 kV equipped with an energy-dispersive X-ray spectroscopy (EDXS) device. All samples were coated with gold to increase their electrical conductivity before SEM testing.
The pore size distribution was determined by mercury intrusion porosity using AMP 60 K A1, which was calculated by the Washburn equation, and the crushed sample prepared for testing was an approximately 5 mm cube, which was pressed into additional crushed sample cubes under

Phase Identification and Quantification
The crystalline solid phase was determined by mineralogical analysis; and the XRD patterns of deactivated ZSM-5 and fly ash are illustrated in Fig. 2. The existence of a broad diffuse peak between 15-30° in the fly ash pattern shows that the amorphous phase, calcined silica and alumina are the main components with some crystalline phases, such as mullite and quartz. Note that chemical analysis (Table 2) shows that deactivated ZSM-5 contains a small amount of alumina (Al 2 O 3 ); hence, a broad diffuse peak between 20 and 30° suggests that the amorphous phase was mainly derived from silica and small amount of alumina. Moreover, the predominant crystalline phases of deactivated ZSM-5 are quartz, ZSM-5 and cristobalite. The phase quantitative analysis of deactivated ZSM-5 and fly ash was carried out by using X'pert HighScore Plus software. The amorphous content and absolute content of the crystalline phases were calculated by an external standard method. The quantitative analysis of deactivated ZSM-5 and fly ash are listed in Tables 6 and 7. The calculation results show that the fly ash contains 10.43% mullite, 4.16% quartz and 85.41% amorphous. Additionally, deactivated ZSM-5 consists of 59.63% ZSM-5, 5.12% cristobalite, 5.36% quartz and 29.89% amorphous. Figure 3 presents the FTIR pattern of the raw materials. Absorption bands could be observed at 3439 and 3436 cm −1 , which are associated with the stretching vibrations of O-H groups. The bands located near 1629 cm −1 and 1630 cm −1 are assigned to the existence of interlayer adsorbed water, which may be caused by the leftover in the samples or the higher content of water in the raw materials, which is depicted as the molecular water bending vibrations. The

XRD
The XRD patterns of the geopolymer pastes are shown in Fig. 4. It is clearly observed that the sharp peaks in M1.4T25, M1.4T50, M1.4T75, M2.0T25, M2.0T50 and M2.0T75 did not significantly change. A broad hump at 18-30° 2θ was present in all geopolymer pastes, which was the characteristic reflection of amorphous geopolymers. The superposition of amorphous or semicrystalline phases may be responsible for the formation of a broad amorphous hump: (1) the insufficiency reaction and dissolution of the amorphous phase that ranges from 18-30° 2θ, which is derived from fly ash and deactivated ZSM-5, may occur in the geopolymerization reaction [46]. (2) The 2θ of the newly formed amorphous geopolymer generally ranges from 18 to 30° [47]. This finding was consistent with the current view that geopolymers are amorphous to semicrystalline aluminosilicate materials [47][48][49][50]. Moreover, the existence of crystalline minerals of mullite, quartz and cristobalite still existed in the XRD results of geopolymer pastes, which were derived from fly ash and deactivated ZSM-5, respectively, indicating that crystalline phases essentially did not participate or were only partially involved in the geopolymerization reaction but filled the geopolymer samples in the form of inactive fillers. In addition, crystalline components such as ZSM-5, which were confirmed in the XRD pattern of deactivated ZSM-5 (Fig. 2), were not detected in the final geopolymer products. One possible reason was that the concentration or content of crystalline minerals ranges from extremely low to not detectable. Another possible explanation was that the desilication of deactivated ZSM-5 under strong alkaline conditions causes the dissolution and collapse of the zeolite framework, which eliminates the crystallinity of ZSM-5 [51]. The peaks of crystalline zeolite phases of gismondine were observed in geopolymer pastes. In a geopolymer cementitious matrix, geopolymer-like gels, through nucleation and growth, were considered to be a dominant means of forming gismondine [52].  Figure 5 shows the effect of curing temperature and modulus of water glass on the compressive strength of the geopolymer. Notably, varying the modulus of water glass caused diverse mechanical performance; thus, this modulus is an important variable influencing the performance of geopolymers. For the two groups of geopolymers, a modulus of water glass of 2.0 and 1.4 was utilized to fabricate geopolymers. When the polymers were thermally cured at 25 ℃; the compressive strengths were 2.5 and 3.2 MPa, respectively, when they were thermally cured at 50 ℃, the compressive strengths were 4.4 and 5.2 MPa, respectively. This finding indicates that the water glass modulus did not significantly affect the compressive strength of the geopolymer when cured at 25 ℃ and 50 ℃. Although the low-modulus solutions produced more reaction products, the geopolymers did not form hardened structures in a short period due to the retarded setting of geopolymer slurry. However, the compressive strength of the geopolymer reached 6.3 MPa with a water glass modulus of 2.0. The compressive strength was 8.5 MPa with a water glass modulus of 1.4 and increased nearly one-third when cured thermally at 75 ℃. The results show that the compressive strength of the geopolymer increases with a decrease in water glass modulus at three different temperatures. In general, low-modulus water glass solutions lead to extensive dissolution of raw materials and promote the activation of silica, alumina and amorphous materials so that silicon and aluminium leach more in the water glass solution, increasing the extent of the geopolymerization reaction and resulting in the generation of larger  [30,53,54]. Hence, low-modulus alkali solutions enhanced the compressive strength of the final geopolymer pastes.

Compressive Strength
Based on these data (Fig. 5), the curing temperature is more significant than the modulus of water glass in dictating the compressive strength. Moreover, the different compressive strengths caused by various curing temperatures can be confirmed, that is, the compressive strength increased with an increase in curing temperature. It is commonly agreed that the curing temperature has a significant impact on the properties of geopolymers [29,30,53,55,56]. In this study, increasing the curing temperature yields a geopolymer with higher strength, irrespective of the water glass modulus. Note that the compressive strength that is thermally cured at 25 ℃ is significantly lower than that of the geopolymer at 50 and 75 ℃. The main reasons for this finding are presented as follows: (a) at low temperatures, the geopolymerization rate is low with gradual development of the compressive strength. (b) As confirmed by the XRD finding, a large quantity of crystalline phases and an unreactive amorphous phase derived from raw materials are present in the geopolymer, which may affect the rate of geopolymerization. In summary, increasing the heat curing temperature from 25 ℃ to 75 ℃ imparts a higher degree of geopolymerization reaction and facilitates the production of aluminosilicate gels, which is the main binding phase in particle bonding that accelerates the development of compressive strength [30,57]. Moreover, a higher curing temperature also facilitates the formation of a hard structure in a short time. Figure 6 shows the effect of curing temperature and modulus of water glass on the bulk density of the geopolymer. The viscosities of the two geopolymer pastes are shown in Table 8. The bulk densities of samples M1.4T25, M1.4T50, M1.4T75, M2.0T25, M2.0T50 and M2.0T75 were 910, 859, 703, 866, 712 and 627 kg/m 3 . Notably, increases in both curing temperature and water glass modulus reduced the bulk density of the geopolymer. The following critical reasons explain the influence of water glass modulus on the bulk density of geopolymers: (1) the higher viscosity of low-modulus water glass was pronounced in hindering the foaming effect of deactivated ZSM-5; and (2) as previously discussed, more abundant aluminosilicate gels were produced by a lower modulus water glass, which converted the free water to bond water and hence increased the viscosity of geopolymers (Table 8) [58], causing a detrimental effect on the bulk density. During the geopolymerization reaction, strong alkaline solutions dissolved the structure of deactivated ZSM-5, and therefore, the air was released from the zeolite cavity. However, higher viscosity limits the foaming process; hence, the lower modulus of water glass yields geopolymer samples with higher bulk density at a given temperature. In addition, it was determined that the bulk density significantly decreased with an increase in curing temperature, indicating that a more porous hardened structure was formed. An elevated curing temperature promotes the thermal expansion of deactivated ZSM-5 and accelerates the release rate of air from the zeolite cavity. Therefore, the higher the curing temperature is, the lower the bulk density, irrespective of the water glass modulus. In this study, however, there was variation in the relationship between density and compressive strength, which contradicts a finding of previous studies: geopolymers with a higher bulk density exhibit a higher compressive strength. This trend is mainly attributed to the delayed coagulation and hardening of geopolymers at low temperatures; thus, compressive strength is negatively correlated with density. In general, it is preferable to set the curing temperature above 50 ℃.

FTIR
The FTIR spectra of geopolymer pastes with various curing temperatures and water glass moduli are shown in Fig. 7. The bands located near 3450 cm −1 and 1640 cm −1 are assigned to the existence of interlayer adsorbed water, which may be caused by the leftover in the samples or the higher content of water in the raw materials, which is depicted as molecular water bending   [59]. Furthermore, the traces of quartz were identified in spectra located near 790 cm −1 which derived from incompletely reacted fly ash particles and are related to the symmetrical stretching vibration of Si-O (Q1) bonds [60]. The band near 580 cm −1 are associated with vibration of Si-O bands. The absorption bands near 460 cm −1 can be assigned to the vibration of Si-O-Si bands. The broad humps between 800 and 1300 cm −1 were associated with the superposition of reaction products and precursors. As reported in previous studies [61][62][63][64][65], this band was regarded as the T-O asymmetric stretching band, which is widely employed to investigate the amorphous gel structure in geopolymers. Due to the low amount of Ca 2+ contained in the raw material, in low-calcium environments, the broad and asymmetrical stretching band of T-O, where T is Si or Al between 1030 and 1043 cm −1 , is typical of tridimensional frameworks of N-A-S-H-type gels from geopolymers. The investigated reaction products should be notably consistent with the SEM-EDS analysis.
To analyse the main band in question at varying water glass moduli and curing temperatures, the position shifts of the T-O band were identified, as shown in Fig. 8. The T-O band moves from lower to higher wavenumbers with low water glass moduli. On the other hand, the curing temperature also significantly changed the T-O band position. The main reason for the decrease in wavenumber with an increase in modulus is that the binding energy of Al-O is lower than that of Si-O bands, which causes Al to more readily dissolve from solid precursor structures [61]. Therefore, the substitution of Si by Al in the reaction products led to the T-O band position located at lower wavenumbers. Furthermore, higher curing temperatures and lower water glass moduli significantly affected the dissolution of the solid precursor, desilication process of deactivated ZSM-5 and polymerization of the main reaction product N-A-S-H gels. In particular, the quantity of available silica (especially monomers) significantly changes with different moduli [63,64]. This change is reflected in the shift of the main T-O band in the infrared spectrum to higher wavenumbers, suggesting that a larger amount of Si was introduced to the gel network and that the high Si content in cementitious systems enables a Si-rich structure of the reaction products. The Si is obtained from two main sources: (a) desilication process of deactivated ZSM-5 and (2) Si-O groups

TG/DTA Analysis
The thermogravimetric and differential thermogravimetric curves of the geopolymer samples are presented in Fig. 9. The DTA curve of all geopolymer samples changed mainly in the range of room temperature to 200 ℃ and tended to be stable at temperatures above 560 ℃. The mass loss from room temperature to 80 ℃ on the TG curves was ascribed to physically bound water. The main mass loss fell within the range of 80 to 160 ℃ after decomposition of the reaction product in all geopolymer samples. As described in the EDS analysis in Sect. 3.7, the main reaction products N-A-S-H in all geopolymer binders were associated with mass loss in this range. Additionally, this range was consistent with N-A-S-H gels in cementitious systems [61,62]. The negligible decline curves observed between 160 and 500 ℃ were relevant to the polymerized/condensed water evaporated from Si-OH and Al-OH groups [66]. Moreover, the obvious DTA peak range of 500-560 ℃ is caused by the oxidation and combustion of unburned coal in the residual fly ash particles [67]. The weak peak at 800 ℃ was attributed to the carbonated phase.
It is assumed that the main binding phase in geopolymer samples comprised N-A-S-H gels and that the quantity can be calculated by the mass loss percentage of the thermogravimetric (TG) curve ( Fig. 9) in the range of 80-160 ℃. As a result, the mass loss from 80-160 ℃ of all geopolymer samples is given in Fig. 10. Notably, the curing temperature and water glass modulus have a significant effect on the formation of gel products. Samples with a lower water glass modulus have obviously higher mass loss at the same curing temperature. On the other hand, the curing temperature was an important factor that determined the quantity of gel product formation. The trends in the mass loss of gel products showed agreement with the compressive strength of geopolymers.

SEM/EDS Analysis
The microstructure of geopolymer samples at different curing temperatures and water glass moduli and selected representative EDS points are given in Figs. 11 and 12. Figure 11 shows the microstructure of representative samples at low magnification. Notably, the curing temperature and water glass modulus have a substantial influence on the pore structure of the geopolymer samples. The relationship between pore structure and curing temperature and modulus will be discussed in detail in Sect. 3.8. Figure 12 shows the microstructure of all samples at high magnification. It was evidently observed that microcracks existed in the porous and nonhomogeneous microstructure. The cracks may be generated in two ways: (I) shrinkage of geopolymers due to evaporation of water during curing and (II) cracks induced by loading in the compressive strength test [47]. These cracks may have a negative impact on the mechanical properties of the samples. Figure 12(a) and (b) show micrographs of samples M2.0T25 and M1.4T25. It was clearly seen that the morphology of the paste was loose and had less density. Figure 12(c), (d), (e) and (f) display a uniform and denser microstructure, indicating a relatively higher degree of geopolymerization with an increase of curing temperature.
The EDS spectra were employed to analyse the chemical compositions of the reaction products in geopolymers. The EDS points within the binder region were located far from the unreacted particles [68]. This was clearly shown by all EDS data, which mainly consists of Si, Al, Na and O, with trace Fe, Ti and Ca. Considering the typical chemical bonds observed in the FTIR analysis, the main reaction product was the N-A-S-H gel. The structure of N-A-S-H gel is often considered associated with "zeolites" and is closely related to the formation of crystalline zeolites in the system [69,70]. In addition, N-A-S-H gel is a key product in geopolymers, which are widely employed in the synthesis of zeolites. Impurities such as Fe, Ti and Ca in the raw materials, which have a certain impact on the geopolymerization process, were shown in the EDS spectrum [49].
In this study, the effects of curing temperature and water glass modulus on the N-A-S-H gel were reflected by its Si/ Al ratio. With the same curing temperature, the sample with a low water glass modulus has a higher Si/Al ratio due to abundant Si induced by a decrease in water glass modulus, suggesting that low-modulus water glass solutions lead to extensive dissolution of raw materials. Additionally, Si was also incorporated into the structure with an increase in curing temperature. The Si/Al ratio increases with an increase in curing temperature for a given water glass modulus, indicating that a higher curing temperature promotes the dissolution of Si species from raw materials. The results showed that a low water glass modulus and high curing temperature yielded a higher degree of polymerization of the reaction products, which showed agreement with the FTIR analysis (Sect. 3.5).

MIP
Single pore micro, meso-and macropore structures exhibit excellent properties. However, single pore structure materials also have intrinsic deficiencies. For example, certain adsorbent materials that have a single pore channel always possess a limited adsorption capacity due to the small internal pores, so the large molecules formed in the pore channel cannot quickly escape. By comparison, the porous structure retains the adsorption and exchange properties of the original pore structure while creating new pore structures that can increase the pore volume, surface area, porosity, and active adsorption sites of the system and thus enable it to have a wide range of adsorption and exchange capabilities. This section focuses on the pore structure analysis of porous zeolite precursors.
The pore size range of 0.002 to 0.1 μm was very fine and hardly visible in the foamed geopolymer systems. The nitrogen adsorption method is therefore not suitable for the determination of the pore structure of foamed geopolymer systems. Mercury intrusion porosimetry (MIP) in this experiment has been widely applied to determine pore size distributions larger than 0.1 μm. However, when applying this technique in the analysis of geopolymer pore structures, the following issues should be considered. The pressure exerted by intruding mercury will destroy the microstructure of the reaction products and produce high pore volumes. In addition, MIP does not measure the total pore volume but measures the percolated pore volume as MIP assumes that all pores are cylindrical. Hence, the technique was a comparative measurement tool that was employed in the comparison of various geopolymer systems. causes an increase in the porosity of the samples. Many studies have reported on the relationship between the porosity and the compressive strength of cementitious materials [71][72][73]. In this study, however, the results contradict the previous research finding that geopolymer with low porosity yields higher compressive strength. Although geopolymer samples with low porosity exhibit a denser pore structure, as mentioned in Sect. 3.3, longer curing times are required to develop compressive strength due to delayed coagulation and hardening of geopolymers at low temperatures.
The pore volume and pore size distribution of the geopolymer determined by mercury intrusion porosimetry are given in Fig. 13. For the given 1.4 and 2.0 moduli of the water glass series, the geopolymer with a higher curing temperature displays a higher cumulative intrusion, indicating the formation of a porous structure. On the other hand, the cumulative intrusion also increases with an increase in water glass modulus from 1.4 to 2.0. Additionally, as shown in Fig. 13(b), with an increase in curing temperature and water glass modulus, the pore structure becomes porous, which is  expressed as a shift in the critical pore size towards larger values on the pore size distribution curve. As shown in Fig. 13(c), with an increase in water glass modulus and curing temperature, the ratio of pores (< 5 μm) decreased, the ratio of pores (5-30 μm) increased, and the ratio of pores (> 30 μm) decreased. It can be concluded that a higher water glass modulus and curing temperature yield a more uniform distribution of pores and that the redistribution of pore sizes between 5 µm and 30 µm may explain the change in compressive strength. During the geopolymerization reaction, the desilication of deactivated ZSM-5 induced more mesopores on the surface of zeolite, which enabled the alkali solution to enter the microporous inlet. As the alkali solution continued to damage the micropores and dissolve the framework of zeolite (as determined by the XRD result), the air was released from the collapse of the zeolite cavity. Furthermore, the thermal expansion of deactivated ZSM-5 is improved by the higher curing temperature. These are the two main reasons for the change in pore structure caused by the incorporation of deactivated ZSM-5. In addition, the different moduli of water glass and curing temperature have a significant effect on the porous structure of geopolymers and can regulate and optimize the pore structure.

Conclusion
This study presents an experimental investigation of the mechanical performance and microstructure formation of geopolymers, which were prepared with different curing temperatures and silicate moduli of alkaline solutions. The following conclusions were obtained from the experimental results and observations of this study: (1) Industrial waste deactivated ZSM-5 has been successfully applied to prepare hierarchical zeolite precursors. Various curing temperatures and alkali activator moduli have obvious effects on the mechanical properties of geopolymers, including compressive strength, bulk density and pore structure.  (4) Deactivated ZSM-5 can serve as a potential foaming material for the preparation of porous zeolite precursors. Increasing the modulus of the alkaline activator and elevating the curing temperature promoted the transformation of geopolymers to porous and uniform structures.