Enhanced synthesis of fly ash-derived hydrated sodium silicate adsorbents via low-temperature alkaline hydrothermal treatment for advanced environmental applications

Yeqing Zhu; Xiaojun Huang; Tianxiang Yu; Kun Liu; Meng Zhang

doi:10.1515/gps-2024-0172

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Enhanced synthesis of fly ash-derived hydrated sodium silicate adsorbents via low-temperature alkaline hydrothermal treatment for advanced environmental applications

Yeqing Zhu , Xiaojun Huang , Tianxiang Yu , Kun Liu and Meng Zhang

Published/Copyright: February 11, 2025

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From the journal Green Processing and Synthesis Volume 14 Issue 1

Abstract

The fly ash (FA) is one of the large amount of solid wastes stored in China. Because of its low price, large specific surface area, and surface electronegativity, reasonable modification can turn FA into treasure, prepared as an adsorption material with good adsorption performance. In this article, the industrial solid waste FA is used as raw material, and the FA-based sodium aluminosilicate was synthesized by a one-step hydrothermal alkali dissolution method and characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, Fourier transform infrared, Brunauer Emmett Teller, and other test methods. The results show that (1) aluminum silicate sodium hydrate can be synthesized by low-temperature method under atmospheric pressure to form P-type zeolite and calcite structure. (2) Alkali concentration, temperature, and reaction time affect the structure of aluminosilicate sodium hydrate. With the increase of alkali concentration, temperature, and reaction time, the product changes from P-type zeolite to calite structure. (3) The optimal synthesis condition of FA for sodium aluminosilicate hydrate is as follows: the concentration of NaOH solution is 4 mol·L⁻¹, the dissolution temperature is 80°C, the reaction time is 21 h, and the specific surface area is 60.54 m²·g⁻¹.

Keywords: fly ash; hydrothermal synthesis; P-type zeolite; sodium aluminosilicate hydrate; sodalite

1 Introduction

Fly ash (FA) is one of the most massive industrial solid wastes discharged on earth and in China [1]. The chemical composition of FA is mainly composed of inorganic elements, among which Al and Si have the highest content, followed by Ca, Fe, Mg, K, and Na. Compared with other ashes, FA has lower carbon content than that of other coal combustion solid waste, and the existence form of various inorganic components has changed. Due to the superimposed influence of various factors such as the nature of raw materials and formation conditions, the resulting FA has a complex composition structure and contains trace toxic elements, which limits its resource utilization. In the process of exploring the high value-added utilization, the chemical composition of FA (wt%SiO₂ + wt%Al₂O₃ > 70%) was found to be extremely similar to that of Zeolite [2,3]. Geopolymerization occurs naturally in alkaline media, and it produces aluminosilicates [4,5,6,7]. The geopolymerization reaction is to use an alkaline solution to excite raw materials containing active silicon and aluminum to form an AlO₄–SiO₄ tetrahedral structure. Since the aluminum outer layer has only three electrons, a negative charge will be generated when forming a tetrahedron. At this time, the electricity price in the structure is balanced by (Na⁺, K⁺, Ca²⁺, etc.) alkali metal or alkaline earth metal ions [8]. At the same time, the cementation ability, frost resistance, and acid and alkali corrosion resistance of the products formed by the geopolymerization reaction are equivalent to or better than those of Portland cement [9]. Because the synthetic aluminum silicate material has good physical and chemical properties and broad application prospects, the research activities of extracting and synthesizing aluminum silicate from FA have been extensively carried out by scholars.

In the feasibility study of its resource utilization, it was found that the structure of FA rich in Al and Si can form alkaline aluminosilicate gel under the influence of an alkaline medium [10]. The alkali-containing aluminosilicate gel has a three-dimensional, disordered crystal structure. They can be used to synthesize Zeolite-like materials, and the formed gel skeleton is loose and porous, which allows the entry of some ions or molecules. As the main constituent elements, Si and Al occupy the central position of the tetrahedral structure. It is similar to the Al–O tetrahedron and Si–O tetrahedron in the Zeolite material. The negative charge vacancies formed by Al replacing silicon are basically occupied by alkali metal cations (Na⁺); it results a good cation exchange capacity [11,12]. The aluminosilicates prepared from FA have natural Zeolite and synthetic Zeolite [13,14]. The methods of making it are mainly divided into high-temperature and low temperature syntheses [4,15]. High-temperature synthesis method is the most common hydrothermal synthesis method. It usually uses alkaline substances as intermediary and at temperatures of 300–700°C. The aluminosilicate material synthesized by FA using the hydrothermal method has excellent physical and chemical properties, which has attracted intensive research [16,17]. Low-temperature synthesis is less than 100°C and also uses alkaline substances as intermediary, and the synthesized amorphous aluminosilicate material has received extensive attention in industrial applications. The high-temperature synthesis method has high energy consumption, and the low-temperature synthesis method requires a long crystallization time for aluminosilicate crystal growth. Both methods have disadvantages.

In this study, FA was used as the raw material, and the most common NaOH solution was used as an alkali source. Through a relatively simple one-step synthesis method of low-temperature alkali dissolution, the Sodalite-type sodium aluminosilicate hydrate material with good crystal phase and excellent adsorption performance was successfully precipitated. By studying the dissolution process and crystallization process, and using a variety of test methods for characterization, the effects of multiple conditions on the synthesized products were analyzed. FA, a cheap and abundant industrial by-product, was transformed into a high-value-added water treatment material, realizing the high-value utilization of waste. By adjusting the synthesis conditions (such as NaOH concentration, reaction temperature, stirring time, and crystallization time), the adsorption performance of sodium aluminosilicate hydrate was optimized.

2 Materials and methods

2.1 Materials

The raw material used in this experiment was FA from the Huaneng Jinling coal-fired power plant in Nanjing, China, and its chemical composition was analyzed as shown in Table 1. Analytically reagent (AR, ≥99.5% purity) anhydrous KH₂PO4 and guaranteed reagent (GR, ≥99.9% purity) NH₄Cl were supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

Table 1

Chemical compositions of FA/wt%

Fly ash	Al₂O₃	SiO₂	Fe₂O₃	CaO	MgO	Na₂O	Loss
FA	41.31	34.30	7.00	9.41	1.54	0.31	1.16

Sodium hydroxide (AR, ≥96.0% purity) was obtained from Aladdin Industrial Corporation, Shanghai, China. For the experiments, deionized water was used for the configuration of all solutions.

The X-ray diffraction (XRD) pattern in Figure 1 shows the FA’s crystalline phases are mainly mullite and quartz, and the bun peak appears in the range of 20°–30°, indicating that there is a glass body with an amorphous phase composition in it.

Figure 1

XRD pattern of FA.

Figure 2 shows the scanning electron microscope (SEM) image of FA. Figure 2(a) was an image at a magnification of 1,000 times. It can be seen that FA is composed of smooth and regular round particles. When the magnification is 10,000 times (Figure 2(b)), the observation of the specific particles shows that the spherical surface of the actual particles is uneven and attached by other smaller particles.

Figure 2

SEM image of FA: (a) ×1,000 times, and (b) ×10,000 times.

2.2 Experimental method

2.2.1 Synthesis method

The dry FA should be placed aside in a container. The NaOH solution with a specific concentration was prepared. The alkali solution was mixed with 10–20 g of FA by a liquid-to-solid ratio of 5:1 in a polytetrafluoroethylene tank thoroughly. The solid–liquid mixture was transferred to a constant temperature water bath and stirred at 150 rpm·min⁻¹ for a specific duration. The resulting product was obtained through filtration and washed multiple times until the pH level ranged between 7 and 9. It was dried in an oven at 105°C for 24 h to yield hydrated sodium aluminosilicate.

2.2.2 Composition and microstructure of synthetic products

The mineral composition of the samples was analyzed by the XRD equipment from Smart Lab of Rigaku Company in Japan [Cu–K ray (=0.15406 nm), tube pressure 40 kV, tube flow 40 mA, scanning range 10°–70°, scanning rate 10°·min⁻¹]. The Fourier transform infrared (FTIR) spectrometer of Thermo Company Nexus 670 was used to group the samples (powder KBr tablet). The morphology of the samples was characterized by the Ultra 55 field emission SEM of Zeiss company.

3 Results and discussion

3.1 Mineral composition

3.1.1 Effect of NaOH solution concentration on synthesized products

The materials synthesized by hydrothermal reaction under different concentrations of NaOH solution were investigated. The concentration effects on the products’ mineral composition were explored. The synthesis conditions are: the liquid–solid ratio of 5:1, the temperature of 90°C, the reaction time of 12 h, and NaOH solution concentration of 1, 2, 4 and 6 mol·L⁻¹, respectively.

Figure 3 shows XRD of the synthesized products under different NaOH solution concentrations. The results show that the concentration affects the final type of sodium aluminosilicate hydrate, and two types, Zeolite P and Sodalite are formed under the experimental conditions. When the concentration of NaOH was 1 mol·L⁻¹, a small Zeolite P peak appeared at 28.04°, and the crystalline phase was still dominated by Mololite and Quartz. When the concentration was 2 mol·L⁻¹, the peak strength of Zeolite P increases at 28.04°. When the concentration increased to 4 mol·L⁻¹, the peak strength of Zeolite P main peak decreased and the diffraction peak of Sodalite appeared, the small part of Quartz gradually disappeared, and the Mololite did not change significantly. When the concentration was 6 mol·L⁻¹, Zeolite P gradually disappeared, the peak intensity of Sodalite continued increasing, the Quartz phase peak disappeared, and the diffraction peak intensity of Mololite did not change significantly. The results show that the concentrations of NaOH solution have a great influence on the composition of the hydrothermal reaction products of FA under the same conditions. Zeolite P was formed when NaOH solution concentration was lower, and Soalite would appear when the concentration was higher. Increasing the concentration, the diffraction peak of Soalite was enhanced, the peak width became narrower, and the slender and sharp peaks indicated the better crystallinity of the synthesized products.

Figure 3

XRD patterns of synthesized products under different NaOH solution concentration conditions. (liquid–solid ratio, 5:1; reaction temperature, 80°C; stirring time, 12 h).

3.1.2 Effect of reaction temperature on synthesized products

The effect reaction temperature on synthesized products was investigated. The experiment conditions were as follows: the liquid–solid ratio of 5:1, NaOH solution concentration of 4 mol·L⁻¹, reaction time of 12 h, and the synthesis temperature of 70°C, 80°C, 90°C, and 100°C.

Figure 4 shows the XRD patterns of the synthesized products at different temperatures. When the temperature was 70°C, there was no obvious diffraction peak, indicating that aluminum silicate in FA couldn’t be precipitated at this temperature. When the temperature increased to 80°C, a Zeolite P diffraction peak appeared. When it was 90°C, the Sodalite diffraction peak appeared, and the diffraction peak intensity of Zeolite P decreased, but a small amount still existed. When the temperature rose to 100°C, the synthetic products were all Sodalite. The results show that when the synthesis temperature increases, the synthesized products change from aluminosilicate gel to Zeolite P, and then from Zeolite P to Sodalite. The increase in temperature is conducive to the transition of the crystal phase from a metastable state to a stable state and also affects the types of synthesized products.

Figure 4

XRD patterns of synthesized products under different reaction temperature conditions (liquid–solid ratio, 5:1; NaOH concentration, 4 mol·L⁻¹; stirring time, 12 h).

3.1.3 Effect of reaction time on synthesized products

The study shows that the length of hydrothermal reaction time affects the products’ crystallization degree and crystal phase composition. The effect of hydrothermal reaction time on the mineral composition of synthetic products was investigated. The synthesis conditions were as follows: the liquid–solid ratio of 5:1, the concentration of NaOH solution of 4 mol·L⁻¹, the temperature of 90°C, and the reaction time of 3, 6, 9, and 12 h.

Figure 5 shows the XRD patterns of the synthesized products at different reaction times. With the extension of reaction time, the diffraction peaks of a small amount of Phillipsite Na and Zeolite P appeared at the initial 3 h. All Phillipsite Na were converted into Zeolite P after 6 h reaction. When the time increased to 9 h, the Zeolite P was transformed into Sodalite, which was a stable crystal phase. When the reaction time was 12 h, the diffraction peak of Sodalite continued to strengthen, and the peak shape became narrower and sharper. This indicates that the Sodalite crystallinity increases. The reaction time affects the purity of the synthesized product’s crystalline phase, and a more complete crystalline phase can be obtained by prolonging the reaction time.

Figure 5

XRD patterns of synthesized products under different stirring time conditions (liquid–solid ratio 5:1; NaOH concentration, 4 mol·L⁻¹; reaction temperature, 90°C).

3.2 Micro-structure

Figure 6 shows the SEM images of solid substances before and after the reaction of FA with NaOH solution at different concentrations with liquid–solid ratio of 5:1, synthesis temperature of 90°C, and reaction time of 12 h. In the presence of NaOH solution, the FA particles lose the original spherical shape, and the surface began to dissolve and become more rougher. Alkaline aluminosilicate gel was formed, and its growth direction was radial. FA particles were basically wrapped up. When NaOH solution concentration was 1 mol·L⁻¹, a very small number of flake crystals appeared as shown in Figure 6(a), and the product was Zeolite P. When the concentration was 2 mol·L⁻¹, the product further grew as shown in Figure 6(b). Figures 6(c) and (d) show that when the concentration increased to 4 and 6 mol·L⁻¹, most of the synthesized products were spheroids, small but very uniform, attaching to each other and in a dense state, with some irregular particles. Combined with the results of XRD, the product was mainly Sodalite sodium silicate hydrate.

Figure 6

SEM diagram of products produced in NaOH solution concentrations: (a) 1 mol·L⁻¹, (b) 2 mol·L⁻¹, (c) 4 mol·L⁻¹, and (d) 6 mol·L⁻¹.

Figure 7 shows the SEM images of the synthesized adsorption materials at different temperatures under the conditions of liquid–solid ratio of 5:1, NaOH solution concentration of 4 mol·L⁻¹, and a reaction time of 12 h. It can be seen from Figure 7(a) that the appearance was still roughly spherical, and the surface was covered by many new products of network structure. This is due to the formation of network structure silicaluminate in the process of dissolution and the crystallization of zeolite P are obvious diamond shape. As the reaction temperature increased, Figure 7(b) shows FA particles covered by aluminosilicate gel parcel formed aggregate, gradually growing to cylindrical and petal-shaped Sodalite.

Figure 7

SEM images of products at 80°C and 90°C. (a) 80°C. (b) 90°C.

Figure 8 shows the SEM images of the products at different reaction times of 6, 9 and 12 h with the conditions of liquid–solid ratio of 5:1, NaOH solution concentration of 4 mol·L⁻¹ and synthesis temperature of 90°C. A new crystalline phase stacked in layers appeared at 6 h. Combined with the XRD pattern of the product, it can be determined as Zeolite P. When the reaction time was 9 h, the Zeolite P crystal phase disappeared, and the petal-like Sodalite crystal appeared. After 12 h of reaction, the Sodalite crystals continued to grow and formed clear and complete petal-like crystals.

$Figure 8 SEM diffraction images of products after 6, 9, and 12 h: (a) 6 h, (b) 9 h, and (c) 12 h.$

Figure 8

SEM diffraction images of products after 6, 9, and 12 h: (a) 6 h, (b) 9 h, and (c) 12 h.

3.3 FTIR

Figure 9 shows the products’ FTIR diagram at different reaction temperatures with the liquid–solid ratio of 5:1, NaOH solution concentration of 4 mol·L⁻¹, and the reaction time of 12 h. The diagram bands of Zeolite P adsorbent materials in the 900–1,030 cm⁻¹ region are attributed to the asymmetric stretching vibration of Al–O/Si–O. The diagram bands in the 650–720 cm⁻¹ belong to the symmetric stretching vibration of Al–O/Si–O. The diagram bands in the 420–530 cm⁻¹ are due to the bending vibration inside [13]. The FTIR diagram of all the synthesized products was consistent with the infrared spectra of Zeolite-like materials. The absorption peaks were near 971, 652, 582, and 373 cm⁻¹, indicating that the samples had the structural characteristics of Zeolite molecular sieve. The product synthesized under the reaction time of 30 h was compared with the original FA, and the infrared spectrum of the synthesized product showed the characteristic vibration absorption peaks of the silicon-oxygen tetrahedron and the aluminum oxygen tetrahedron. The absorption peak of the synthetic materials at 991 cm⁻¹ belongs to asymmetric stretching vibration, which corresponded to the diffraction peak of FA at 1,090 cm⁻¹, which has a small range of offset. This is related to the formed aluminum oxygen tetrahedron (Figure 10).

Figure 9

FTIR diagram of synthesized products at different reaction temperatures.

Figure 10

FTIR diagram of the product at 30 h.

3.4 Specific surface area (SSA)

By comparing the characterizations of the synthesized materials, the changes in the morphology of the materials before and after the modification can be observed, which means that the specific surface area (SSA) has also changed. The size of the specific surface area is a key indicator for evaluating the adsorption strength. In this article, the synthetic materials’ SSA under various experimental conditions was measured to study the relationship between SSA and the adsorption performance. The SSA of the original FA was 1.58 m²·g⁻¹, which was compared with the synthetic materials’ prepared under different NaOH solution concentrations, reaction temperature, and reaction time.

It can be seen from Table 2 that the specific surface area gradually increases as the reaction proceeds. Based on the characterization analysis, it increases exponentially when the material starts to change from aluminosilicate gel to Zeolite P and Sodalite. The longer the reaction time, the larger the SSA, which means that the material gradually transforms into a more stable petal nano-Sodalite.

Table 2

Comparison of the specific surface area of synthetic materials

No	NaOH solution concentration (mol·L⁻¹)	Reaction temperature (°C)	Stirring time (h)	SSA (m²·g⁻¹)
1	1			6.83
2	2			12.83
3	4	90		36.07
4	6			39.36
5			12	5.1
6		70		22.3
7		80		39.97
8		90		40.78
9	4	100	15	38.97
10			18	51.37
11		90	21	60.54
12			24	48.51

4 Conclusions

In this experiment, FA was used as the raw material to prepare sodium aluminosilicate hydrate. Due to the complexity of the formation conditions of FA, it contains a large number of stable structures such as Mololite. Therefore, the Al and Si sources cannot be completely dissolved by the low-temperature method under normal pressure; as a result, the prepared sodium aluminosilicate hydrate contains Mololite and quartz.
The structure of sodium aluminosilicate hydrate synthesized under different concentrations of NaOH solution is different. When the concentration of NaOH solution is below 4 mol·L⁻¹, the main crystal of the product is Zeolite P. When the concentration is greater than or equal to 4 mol·L⁻¹, the main crystal is Sodalite.
The structure of sodium aluminosilicate hydrate formed at different reaction temperatures is different. In this experiment, the products synthesized by the low temperature-alkali dissolution method showed two isomers: Zeolite P and Sodalite, and Zeolite P would be converted to Sodalite when temperature increases.
The increase in reaction time is beneficial for the synthesized product transforming to a stable state. In the early stage of the reaction, the synthesized product was mainly Zeolite P. With the reaction proceeding, Zeolite P gradually dissolved and Sodalite began to appear. When the reaction time is further increased, the crystal structure of Sodalite has no other changes except for slight growth. It can be determined that the stable hydrated sodium aluminosilicate formed in the low-temperature zone is dominated by Sodalite.
The optimum conditions for synthesizing Sodalite-type sodium aluminosilicate hydrate from FA can be summarized as follows: NaOH solution concentration of 4 mol·L⁻¹, the reaction temperature of 90°C, and the reaction time of 21 h. The specific surface area of the sample synthesized under this condition is 60.54 m²·g⁻¹.

Funding information: This study was self-supported by the State Power Environmental Protection Research Institute.
Author contributions: Yeqing Zhu: writing and project administration; Xiaojun Huang: analysis, writing, and methodology, Tianxiang Yu: writing review and editing, Kun Liu: methodology; Meng Zhang: translation and editing.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-08-08

Accepted: 2024-12-15

Published Online: 2025-02-11

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/gps-2024-0172

Keywords for this article

fly ash; hydrothermal synthesis; P-type zeolite; sodium aluminosilicate hydrate; sodalite

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