WO2023080676A1 - Method for manufacturing coal fly ash-based geopolymer foam by using microwaves - Google Patents

Abstract

The present invention can manufacture a geopolymer foam, which has a low thermal conductivity and is lightweight, through microwave irradiation by using coal fly ash as a material for a geopolymer foam and not using foaming agents. A geopolymer foam according to the present invention can be used, as an eco-friendly inorganic material, for improving the insulation performance and safety of buildings.

Description

Manufacturing method of geopolymer foam based on coal fly ash using microwave

The present invention relates to a method for manufacturing a geopolymer foam based on coal fly ash through microwave irradiation.

As the issue of global warming emerges, greenhouse gas emissions and energy consumption issues are drawing attention. Currently, Portland cement, which is mainly used in the construction industry, accounts for more than 95% of the total cement use, and is widely used because of its abundant raw materials and easy production. However, the Portland cement production process is energy intensive and has a problem in that it emits a large amount of carbon dioxide, which accounts for 8% of global carbon dioxide emissions.

Due to these problems, geopolymer, an environmentally friendly and sustainable material, is attracting attention as an alternative to Portland cement. Geopolymer is an inorganic material with an aluminosilicate structure with a three-dimensional structure, and is made by mixing materials rich in silicon and aluminum in a strong alkaline solution. It is reported that geopolymers have high compressive strength, excellent fire resistance and acid resistance, and can reduce carbon dioxide emissions by about 70% compared to the manufacturing process of Portland cement. Furthermore, geopolymers show advantages in terms of resource circulation through recycling of industrial by-products because various industrial by-products (such as coal ash from thermal power plants or blast furnace slag from blast furnaces) can be used as raw materials.

In addition, the demand for building materials with thermal insulation performance is increasing. Currently, energy used in buildings accounts for 40% of total energy consumption. As one way to reduce this, efforts are being made to increase the efficiency of cooling and heating of buildings. The most widely used insulator in Korea is organic insulator, which has high insulation performance and low price. However, organic insulators are vulnerable to fire and generate toxic gases in the event of a fire. For this reason, inorganic insulators that have high insulation performance, are not vulnerable to fire, and do not generate toxic gases are attracting attention. Accordingly, research on insulation materials using geopolymers is currently being conducted in industry and academia. For example, a method of synthesizing geopolymer foam using an additive that generates a gas through a reaction, such as hydrogen peroxide, aluminum or silica fume, is being studied. V. Ducman and L. Korat used hydrogen peroxide and aluminum powder, and E. Prud'homme et al. Various additives have been utilized, such as synthesizing geopolymer foam using silver silica fume.

However, since the aluminum powder production process is energy-intensive, it is desirable to minimize the amount used from an environmental point of view. In general, the surfactant added together as a bubble stabilizer is an organic material, so there is a problem of generating toxic substances through high-temperature decomposition. In addition, silica fume is an industrial by-product generated in the production process of silicon or silicon alloy, and is mainly used in a densified form for convenience of distribution. For this reason, when preparing a geopolymer foam by adding silica fume, there is a problem in that a somewhat irregular foam structure is formed because the particles are not sufficiently dispersed due to the large particle size. Therefore, there is a need to develop a method for synthesizing geopolymer foam without using additives.

In addition, coal fly ash is a major by-product of coal-fired power plants and is used as a geopolymer raw material because it has small particles and is rich in silica and alumina. Si and Al in the coal fly ash are dissolved by an alkali activator such as sodium hydroxide to act as a binder. Typically, a compound of sodium hydroxide (or calcium hydroxide) and/or sodium silicate (or potassium silicate) is used as an alkali activator, and geopolymer concrete is generally 40 A curing time of 24-72 hours was required at a temperature of ~80°C. Therefore, there is a problem in that energy and time consumption are high.

Korean Patent Registration No. 2279744 discloses a method for manufacturing a coal ash-based geopolymer foam. The manufacturing method has the above-mentioned problem because it includes the step of using silica fume as a foaming agent and curing at 75 ° C for 72 hours and then further exposing to a high temperature of 200 ° C to 500 ° C.

Accordingly, the present inventors have confirmed that when the geopolymer foam is prepared by using coal fly ash as a raw material for the geopolymer foam and foaming through microwave irradiation, the geopolymer foam having low thermal conductivity and light weight can be manufactured in an environmentally friendly manner. The present invention is based on this.

In order to solve the above problems, an object of the present invention is to use coal fly ash as a raw material for geopolymer foam and to provide a method for synthesizing a geopolymer foam that is environmentally friendly and has low thermal conductivity and is lightweight by a foaming method through microwave irradiation. will be.

In order to solve the above problems, the present invention (a) preparing an alkali activator solution by mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH); (b) preparing a dough sample by adding coal fly ash to the alkali activator solution; (c) preparing a molded body by filling the dough sample into a mold and irradiating microwaves; And (d) demolding the molded body; provides a method for manufacturing a geopolymer foam based on coal fly ash using microwaves.

In addition, the present invention provides a coal fly ash-based geopolymer foam prepared by the above manufacturing method.

According to the present invention, a geopolymer foam having low thermal conductivity and light weight can be manufactured by using coal fly ash as a raw material of the geopolymer foam and using microwave irradiation without using a foaming agent.

The geopolymer foam according to the present invention is an environmentally friendly inorganic material and can be used to improve the insulation performance and safety of buildings.

1 is a graph of average temperature and maximum temperature immediately after microwave irradiation of a geopolymer foam according to the present invention.

Figure 2 is a cross-sectional photograph according to the microwave irradiation time of the SS90 geopolymer foam specimen according to the present invention.

Figure 3 is a cross-sectional photograph of SS100, SS90, and SS80 geopolymer foam specimens at 10 minutes of microwave irradiation according to the present invention.

Figure 4 is a graph of the results of the volume density versus the microwave irradiation time of the geopolymer foam according to the present invention.

5 is a graph showing the results of compressive strength versus microwave irradiation time of the geopolymer foam according to the present invention.

Figure 6 is a SEM picture of the SS90 geopolymer foam according to the microwave irradiation time according to the present invention.

7 is a graph of thermal conductivity versus microwave irradiation time of the geopolymer foam according to the present invention.

8 is a graph of thermal conductivity versus porosity of the geopolymer foam according to the present invention.

9 is a graph showing the result of bulk density versus L/S ratio of the geopolymer foam according to the present invention.

10 is a graph of results of compressive strength versus L/S ratio of the geopolymer foam according to the present invention.

11 is a graph of thermal conductivity versus L/S ratio of the geopolomer foam according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail. In addition, in the following description, many specific details such as specific components are shown, which are provided to help a more general understanding of the present invention, and it is common in the art that the present invention can be practiced without these specific details. It will be self-evident to those who have the knowledge of And, in describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted.

One embodiment of the present invention is (a) preparing an alkali activator solution by mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH); (b) preparing a dough sample by adding coal fly ash to the alkali activator solution; (c) preparing a molded body by filling the dough sample into a mold and irradiating microwaves; And (d) demolding the molded body; provides a method for manufacturing a geopolymer foam based on coal fly ash using microwaves.

Step a) may be a step of mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH) at a ratio (weight ratio) of 10:2.5 to 0.

Preferably, step a) may be a step of mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH) at a ratio (weight ratio) of 10:2.5 to 1, but is not limited thereto.

The water glass (Na ₂ SiO ₃ solution) is added to the mixture to form a stable foam and improve the curing speed.

For example, the water glass (Na ₂ SiO ₃ solution) may have an SiO ₂ content of 28-30% and a Na ₂ O content of 9-10%.

The sodium hydroxide (NaOH) may be a 13 to 15 M NaOH aqueous solution.

The composition of the alkaline activator solution determines the viscosity and chemical reactivity of the specimen. The addition of NaOH aqueous solution can lower the viscosity of the geopolymer paste, resulting in large, non-uniform pores and hindering structural development.

As a result of the experiment, the sample with NaOH aqueous solution showed an initial low level of expansion, but since the NaOH aqueous solution serves to improve the geopolymer reactivity, more pores were formed when the microwave irradiation was prolonged, and small pores were formed. was found to have an effect on

The present invention uses coal fly ash as a raw material for geopolymer foam. If coal bottom ash is used as a raw material, somewhat irregular foam may be formed.

The coal fly ash may have an average particle size of 10 to 100 μm, preferably 20 to 70 μm, and more preferably 30 to 50 μm.

In the coal fly ash, the sum of SiO ₂ , Al ₂ O ₃ and Fe ₂ O ₃ may be 70% or more, preferably 80%.

The step b) may be a step of mixing the alkali activator and the coal fly ash at a ratio (weight ratio) of 0.60 to 0.80 (L/S ratio) with respect to 1 (weight ratio) of the alkali activator.

Preferably, the step b) may be a step of mixing the alkali activator solution and the coal fly ash at a ratio (weight ratio) (L/S ratio) of 0.70 to 0.75, but is not limited thereto.

If the L/S ratio is 0.60 or less, the bulk density is high, and thus the thermal conductivity is high, and the curing of the specimen is unstable, resulting in a low compressive strength. voids can be created.

By mixing within the above range, a geopolymer paste in a flowing state can be prepared. This geopolymer paste is called a dough sample in the present invention. In the present invention, the dough sample means that the mixture of the alkali activator solution and the coal fly ash flows without forming a gel.

In step b), the coal fly ash may be added in an amount of 50% to 65% by weight, preferably 55% to 60% by weight based on the total weight of the dough sample.

In one embodiment of the present invention, step b) may be a step of stirring at 200 to 400 rpm for 3 to 10 minutes, but is not limited thereto.

The manufacturing method of the present invention is characterized by comprising the step (step c) of filling the dough sample into a mold and irradiating microwaves.

In the case of conventional geopolymer foam synthesis, a step of curing at a temperature of 40 to 80 ° C for 24 to 72 hours was required, resulting in high energy consumption. Forms can be synthesized.

Curing by microwave is very efficient in terms of energy consumption than using a drying oven, and is more effective because uniform heating is possible over the entire geopolymer specimen. Therefore, the microwave rapidly raises the temperature of the geopolymer paste, and the resulting hardening of the specimen and vapor generated by evaporation of internal moisture enable the synthesis of the geopolymer having a porous structure. This phenomenon occurred faster as the microwave output increased, resulting in greater expansion of the specimen. However, at microwave power above a certain level, the specimen did not expand significantly anymore. In addition, at microwave power below a certain level, the curing time of the specimen is greatly increased and the outer shape of the specimen is deformed due to excessive heat exposure.

Microwave power and microwave irradiation time can be adjusted considering various factors such as coal fly ash raw material, type and concentration of alkali activator, and physical properties of geopolymer products.

In one embodiment of the present invention, step c) may be a step of irradiating the mold with microwaves of 300 to 1000 W for 5 to 15 minutes, preferably 8 to 12 minutes. At this time, the molded body may be cured at an average temperature of 80 to 140 °C or a maximum temperature of 100 to 260 °C.

As a result of the experiment, as the microwave irradiation time increased, the temperature of the geopolymer increased, and the difference in pores formed according to the temperature of the geopolymer specimen was shown. In addition, large-sized pores were observed in the low temperature range where free water or physically bound water evaporated, but small-sized pores were formed in the high temperature range where chemical bound water or chemical reaction water evaporated. . It was confirmed that these small-sized voids replaced large voids in the specimen rather than expanding the specimen, reducing the overall pore size of the specimen and forming a uniform void distribution. In addition, the expansion of the specimens with increasing microwave irradiation time generally reduced the compressive strength, bulk density, and thermal conductivity of the specimens.

In addition, the present invention provides a coal fly ash-based geopolymer foam prepared by the above manufacturing method.

According to one embodiment of the present invention, the coal fly ash-based geopolymer foam may have a bulk density of 0.530 to 0.688 g/cm ³ , thermal conductivity of 0.166 to 0.361 W/mK, and compressive strength of 2.14 to 8.23 MPa.

The advantages and features of the present invention, and how to achieve them, will become clear with reference to the detailed description of the following embodiments. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, and only the present embodiments will complete the disclosure of the present invention and allow common knowledge in the art to which the present invention belongs. It is provided to fully inform the owner of the scope of the invention, and the present invention is only defined by the scope of the claims.

<Example 1> Preparation of geopolymer foam based on coal fly ash

Coal fly ash (CFA) was supplied from Seocheon Thermal Power Plant. The particle size distribution of the coal fly ash was measured using a particle size analyzer (Partica LA-960, HORIBA, Japan). The chemical components of the coal fly ash were subjected to X-ray fluorescence (XRF) analysis, and the results are shown in Table 1 below (ZSK Primus II, Rigaku, Japan). Referring to Table 1 below, since the sum of SiO ₂ , Al ₂ O ₃ , and Fe ₂ O ₃ among the components was 86.09%, more than 70%, it was classified as Class F fly ash.

ingredient	SiO 2	Al 2 O 3	Fe 2 O 3	CaO	MgO	Na 2 O	K2O	TiO 2	etc
weight(%)	55.9	22.2	7.99	4.84	1.17	1.40	1.84	1.23	3.43

Water glass (Sodium silicate solution, SiO ₂ : 28-30%, Na ₂ O : 9-10%) was purchased from Daejeong Chemical Gold. NaOH pellets (Sodium hydroxide pellet, EP grade) were purchased from Duksan Chemical. NaOH pellets were dissolved in tap water to prepare a 14 M NaOH aqueous solution, left for 24 hours, cooled to room temperature, and then used.

An alkaline solution was prepared by mixing the water glass and 14 M NaOH aqueous solution in a mixing ratio shown in Table 2 below.

The coal fly ash and the alkali solution were put in a Teflon beaker at the ratios shown in Table 2 below and mixed at a speed of 300 rpm for 5 minutes using an overhead stirrer. At this time, the mixing ratio (L / S ratio) of the alkali solution and the fly ash is shown in Table 2 below. After pouring 200 g of the mixed sample into a 50Х50Х100 mm ³ Teflon mold, it was cured in a household micro-oven (MW25B, LG electronics, South Korea) at a power of 400 W. After curing, the mold was removed, and the expanded part was cut with a diamond saw to prepare a geopolymer foam specimen.

The names of the geopolymer foam specimens shown in Table 2 below were named as follows.

SS□ : □=Ratio of water glass in alkaline solution (Unit: %).

For example, a geopolymer foam specimen in which the proportion of water glass in an alkaline solution is 80% is named SS80.

The mixing ratio of the raw materials and the corresponding sample group names are summarized in Table 2 below.

psalm name	Coal fly ash (% by weight)	alkaline solution		L/S ratio	Microwave irradiation time (min)
		Water glass (% by weight)	NaOH aqueous solution (% by weight)
SS100	57.14	42.86	0	0.75	8 to 12
SS90(d)		38.57	4.29
SS80		34.29	8.57
SS90a	62.50	33.75	3.75	0.60	9
SS90b	60.61	35.45	3.94	0.65	9
SS90c	58.82	37.06	4.12	0.70	10

For the SS90a, SS90b, SS90c, and SS90d specimens, a, b, and c are additionally described to distinguish that the microwave irradiation time is set differently according to the curing temperature.

<Example 2> Physical property change of coal fly ash-based geopolymer foam by microwave irradiation

Changes in physical properties of the specimens prepared in Example 1 were investigated according to the following measurement method.

The bulk density was calculated by measuring the mass of the specimens sufficiently cooled at room temperature, and then a thermal conductivity analyzer (TPS500S, Hot Disk) using the TPS method (Transient plane source method), Sweden) was used to measure the thermal conductivity of the specimen. The compressive strength of each specimen was measured using a compressive strength tester (PL-9700H, Woojin Precision Co., South Korea) according to ASTM C109. After picking out the inner part of the specimen destroyed by measuring the compressive strength and finely pulverizing it using a mortar and pestle, the No. The sample powder passed through a 100 standard sieve (mesh 150 μm) was used for measuring the powder density. The true density was measured using a Gas pycnometer (AccuPyc Micromeritics, USA) and H ₂ gas, and the porosity was calculated using the following formula from the measured true density and the bulk density of the sample.

In addition, the foam structure was analyzed using SEM (Scanning electron microscopy, SU8010, HITACHI, Japan) and surface photographs.

1. Thermal behavior and pore structure under microwave irradiation

The thermal behavior and pore structure of the specimens prepared in Example 1 according to microwave irradiation were investigated. 1 is a graph showing the temperature of geopolymer foam specimens according to microwave irradiation. As the microwave irradiation time increased, the temperature of the specimen increased rapidly. Due to this temperature rise, vaporization of moisture in the specimen proceeds, and the resulting vapor forms pores in the specimen. The specimens with microwave irradiation times of 8 and 9 minutes had the highest temperature formed between 100 and 130 ° C, and free water vaporization proceeded. Vapor is generated by dihydroxylation or polycondensation reaction by combining T-OH groups.

2 is a cross-sectional photograph of a geopolymer foam specimen prepared through microwave irradiation. As the microwave irradiation time increased, the overall size of the specimen did not change significantly, but the foam structure inside the specimen changed. That is, in the specimen with a short microwave irradiation time, large voids appeared from the center of the specimen. This is due to the characteristics of microwave heating. Heating by microwave penetration heats the entire specimen evenly, but heat loss to the outside occurs, so microwave heating forms a temperature distribution with a high center temperature and a low external temperature, In the center with high , since moisture vaporization occurs actively, large pores are established. This structure changed with increasing microwave irradiation time. That is, the steam generated in the paste of the specimen forms pores, and since these pores replace the previously formed large-sized pores, the effect of reducing the size of the pores as a whole and forming a relatively uniform pore distribution.

2. Changes in specimen structure and bulk density under microwave irradiation

With respect to the specimens prepared in Example 1, changes in specimen structure and bulk density according to microwave irradiation were investigated. 3 is a cross-sectional photograph of SS100, SS90, and SS80 geopolymer foam specimens when irradiated with microwaves for 10 minutes. The SS100 specimen had the greatest expansion, and the SS90 specimen had the least expansion. This is because the viscosity and reactivity of the geopolymer paste vary depending on the alkali solution used. Since aqueous sodium hydroxide solution has a higher water content than water glass solution, it has a low viscosity and can greatly reduce the viscosity of geopolymer paste. The decrease in viscosity of the geopolymer paste facilitates the movement of vapor within the specimen, resulting in the formation of larger pores, or facilitates the release of vapor to the outside of the specimen, hindering specimen expansion. Therefore, the SS100 specimen with high viscosity has a large expansion, but the expansion degree is decreased in SS80 and SSS90 specimens with relatively low viscosity.

On the other hand, the SS80 sample had a relatively higher sodium hydroxide solution content than the SS90 sample, so the viscosity of the geopolymer paste could be lowered, but the expansion rate was higher than that of the SS90 sample. The reactivity of the geopolymer of the SS80 specimen is greater than that of the SS90 specimen because the aqueous sodium hydroxide solution is highly reactive due to its high pH. Because the geopolymer reaction generates moisture, the SS80 specimen with increased geopolymer reaction exhibits greater expansion than the SS90 specimen.

Figure 4 shows the change in bulk density of geopolymer foam specimens according to microwave irradiation. As the microwave irradiation time increased, the bulk density decreased. SS100 specimen showed the lowest bulk density of 0.56~0.53 g/cm ³ and SS90 specimen showed the highest bulk density of 0.69~0.64 g/cm ³ . The SS80 specimen showed a bulk density of 0.62~0.66 g/cm ³ .

3. Compressive strength change according to microwave irradiation

With respect to the specimens prepared in Example 1, the change in compressive strength according to microwave irradiation was investigated. 5 is a graph showing the change in compressive strength of geopolymer foam specimens according to microwave irradiation. As shown in FIG. 5, the compressive strength of the geopolymer foam specimens was highest when the microwave irradiation time was 8 minutes, and the compressive strength of the geopolymer foam specimens decreased as the microwave irradiation time increased. This is because voids are continuously formed in the specimen as the microwave irradiation time increases, and cracks occur in the specimen at a temperature above a certain level. The SS90 specimen showed the highest compressive strength (8.23~4.40 MPa), and the SS100 specimen showed the lowest compressive strength (3.53~2.14 MPa). This is because a difference occurred in the expansion of the specimen depending on the composition of the alkali solution used. The SS90 specimen exhibits high strength due to its high bulk density, whereas the SS100 specimen exhibits low strength due to its low bulk density.

In addition, SEM analysis was performed on the geopolymer foam specimens according to the microwave irradiation time, and the images are shown in FIG. 6 . No significant difference was found in the specimens with microwave irradiation times of 8 and 9 minutes. However, the crack indicated by the white arrow was formed at the time of 10 minutes of microwave irradiation, and the crack gradually got worse as the microwave irradiation time increased. As a result of microwave irradiation, these microcracks result in evaporation of moisture that maintains the geopolymer structure due to an excessively high temperature, resulting in thermal shrinkage inside, or the pressure of steam generated in the geopolymer matrix as it reaches a high temperature. It appears to have risen and caused cracks in the matrix.

4. Changes in thermal conductivity according to microwave irradiation

The specimens prepared in Example 1 were cut vertically and the thermal conductivity of the center portion was measured, and the results are shown in FIG. 7 .

As shown in FIG. 7, as the microwave irradiation time increased, the thermal conductivity of the geopolymer foam specimens decreased. That is, the thermal conductivity decreased to 0.202~0.167 W/mK for SS100 specimen, 0.223~0.166 W/mK for SS90 specimen, and 0.222~0.154 W/mK for SS80 specimen. This decrease in thermal conductivity is due to the increase of voids inside the specimen by microwave irradiation.

The relationship between porosity and thermal conductivity of geopolymer foam specimens is shown in FIG. 8 . As can be seen in FIG. 8, the highest porosity was shown in the SS100 specimen, which had the highest degree of expansion of the geopolymer foam specimen. The SS100 specimen showed high thermal conductivity compared to the porosity. On the other hand, the S80 specimen showed low thermal conductivity compared to the porosity. That is, there does not seem to be a certain correlation between the porosity and the thermal conductivity of the geopolymer foam specimens. It is believed that the difference in the material formed may occur depending on the alkali activator, and the pore structure, size, and distribution affect the thermal conductivity. For example, it has been investigated that as the number of small pores in a specimen increases, the thermal conductivity decreases. Therefore, it is estimated that the S80 specimen with many small pores shows low thermal conductivity even with low porosity.

<Example 3> Physical property change according to L/S ratio of coal fly ash-based geopolymer foam

Changes in bulk density, compressive strength, and thermal conductivity according to the L/S ratio of the geopolymer foam specimens were confirmed.

9 is a graph showing the bulk density according to the L / S ratio of geopolymer foam specimens. The bulk density of the geopolymer foam specimens decreased as the L/S ratio increased, because the increase in the alkalizing agent solution used to fabricate the geopolymer foam specimens contained more moisture that could expand the specimens. In addition, the bulk density decreased by 11.4% as the L/S ratio decreased from 0.60 to 0.65, but the bulk density decreased by 4.7 to 3.5% as the L/S ratio increased. As the L/S ratio increases, the decrease in bulk density is thought to be gradually reduced because the viscosity of the geopolymer foam specimen decreases and the ability to trap vapor decreases.

10 is a graph showing the compressive strength of geopolymer foam specimens according to the L/S ratio. The compressive strength of the geopolymer foam specimen showed the highest value of 8.43 MPa when the L/S ratio was 0.65, and as the L/S ratio increased, the compressive strength gradually decreased to 5.65 MPa when the L/S ratio was 0.75. showed up Contrary to this phenomenon, as the L/S ratio increased from 0.60 to 0.65, the compressive strength increased from 7.74 MPa to 8.43 MPa. This phenomenon is shown to be that when the L/S ratio decreases below a certain level, the temperature rise due to microwaves occurs rapidly, adversely affecting the curing of the specimen and reducing the compressive strength.

11 is a graph showing the thermal conductivity of geopolymer foam specimens according to the L/S ratio. The thermal conductivity of the SS90 geopolymer foam specimens decreased as the L/S ratio increased. The increase in the L/S ratio of the geopolymer foam specimens is due to the increase in the internal pore structure. When the L/S ratio was 0.60, a value of 0.251 W/mK was shown, but when the L/S ratio was 0.75, a value of 0.187 W/mK was shown.

Claims (7)

1. (a) preparing an alkaline activator solution by mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH);

   (b) preparing a dough sample by adding coal fly ash to the alkali activator solution;

   (c) preparing a molded body by filling the dough sample into a mold and irradiating microwaves; and

   (d) demolding the molded body; comprising

   Manufacturing method of geopolymer foam based on coal fly ash using microwave.
2. According to claim 1,

   Step a) is a step of mixing water glass (Na ₂ SiO ₃ solution) and sodium hydroxide (NaOH) in a ratio (weight ratio) of 10: 2.5 to 0, the manufacturing method.
3. According to claim 1,

   The step b) is a step of mixing the alkali activator and the coal fly ash at a ratio (weight ratio) of 0.60 to 0.80 to 1 coal fly ash (weight ratio).
4. According to claim 1,

   In step b), the coal fly ash is added in an amount of 50% to 65% by weight based on the total weight of the dough sample.
5. According to claim 1,

   The step c) is a step of irradiating microwaves with an output of 300 to 1000 W for 5 minutes to 15 minutes, the manufacturing method.
6. Coal fly ash-based geopolymer foam prepared by the manufacturing method of any one of claims 1 to 5.
7. According to claim 6,

   The coal fly ash-based geopolymer foam has a bulk density of 0.530 to 0.688 g/cm ³ , a thermal conductivity of 0.166 to 0.361 W/mK, and a compressive strength of 2.14 to 8.23 MPa.

Images