Preparation and Properties of Phosphoric Acid-Based Porous Geopolymer with High Magnesium Nickel Slag and Fly Ash

: Phosphoric acid-based porous geopolymers were prepared by two different foaming agents (H 2 O 2 and Al powder) with phosphoric acid as the activator. High-magnesium nickel slag (HMNS) and ﬂy ash (FA) were the precursor combination. The effects of foaming agent types and contents on the properties of HMNS-FA-phosphate-based porous geopolymers were investigated in terms of dry density, pore structure, compressive strength, thermal conductivity, and water absorption. The phase was analyzed by x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). It was found that both foaming agents could successfully prepare porous geopolymers, and the compressive strength and dry density of porous geopolymers gradually decreased and the low-thermal conductivity and water absorption gradually increased with the increase in foaming agent content. The foaming agents formed porous structures inside porous geopolymers but did not affect the phases of geopolymerization reactions. This study demonstrates that both foaming agents can be used to prepare HMNS-FA-phosphate-based porous geopolymers for the application of phosphate-activated geopolymers in the direction of refractory materials.


Introduction
Porous geopolymers are a new type of polymer material, mainly composed of aluminum and silicon, and made of clay, industrial waste, or slag as raw materials.It was developed by French chemist J. Davidovits [1] in the 1970s, mainly through the polymerization of oxygen-containing tetrahedra using appropriate processes at a lower temperature (50~180 • C) through chemical reactions with great compressive strength.This type of polymer is a new material with a wide range of prospects.The total amount of CO 2 emitted by traditional silicate cement is about 0.82 t for every 1 t of clinker produced [2].A geopolymer as a new gel material to partially replace cement has important research implications for a reduction in CO 2 emissions [3][4][5][6].
Porous geopolymer properties have been discussed by many scholars [31,32].Gu et al. [33] measured and analyzed the influence of foam content on the dry density, compressive strength, and thermal conductivity of porous geopolymers.
With the increasing global production of stainless steel in recent years, large amounts of high-magnesium nickel slag (HMNS) have been deposited in landfills, causing soil and groundwater pollution.Some studies have shown that HMNS has good thermal stability and the potential to prepare high-temperature resistant materials [34,35], indicating that porous materials made from HMNS can capitalize on its unique characteristics.
However, there have been few studies on porous geopolymer made from HMNS due to its low reactivity with activators at room temperature.Moreover, most studies on porous geopolymers have been focused on alkali solutions [36][37][38][39][40][41].These two reasons have contributed to the scarcity of research on HMNS porous geopolymer and phosphoric acid excitation.FA raises the alkalinity of the geopolymerization reaction as part of the precursor and not only causes a large waste of land resources but also causes a variety of problems, such as water and soil pollution, land pH imbalance, and affects the living environment of plants and animals [42].
This study is expected to encourage further research on HMNS and reduce pollution caused by HMNS and FA.Porous geopolymers were prepared by utilizing geopolymer technology with HMNS and fly ash as raw materials, H 2 O 2 and Al powder as foaming agents, sodium dodecyl sulfate (SDS) as a foam stabilizer, and phosphoric acid as the activator.The major chemical components of raw materials were SiO 2 and MgO.The content of foaming agents was regulated to prepare the porous geopolymer, and the effect of blowing agents on porous geopolymers was examined.The physical properties of porous geopolymers were discussed, and the reaction mechanism of the porous geopolymer was summarized based on the microscopic analysis results.

Raw Materials
The HMNS and FA, obtained from the Baotou factory (Baotou, China), have a high content of silicon and magnesium.HMNS was grounded and screened (100 mesh).The main chemical components of them are shown in Table 1. Figure 1 is the SEM image of HMNS and FA, and Figure 2 is the XRD pattern of HMNS and FA.The major crystal phases in high-magnesium nickel slag are forsterite, ferrosilite, magnesium silicate, and quartz, while the major crystal phases in FA are quartz and mullite, with the potential to prepare cementitious materials.The phosphate activator is 85% pure and comes from Shanghai, China.The purity of H 2 O 2 (from Shanghai, China) is 30%.SDS and Al powder (99% by mass) are both analytical reagents.All of the industrialized products were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).Deionized water was used throughout the experiment.Allow the mixture to cool for 24 h at room temperature before using.In this study, H 2 O 2 and Al powder were selected as foaming agents, and different contents of H 2 O 2 and Al powder were designed to prepare porous geopolymers.The ratio of HMNS to FA was 7:3, the content of SDS was 0.1%, and the content of phosphoric acid was 20%.The experimental design is shown in Table 2.

Sample Preparation
As shown in Figure 3, the HMNS used in this work were air-cooled after being coarsely ground to particulate matter, and then they were passed through a 200-mesh (75 µm) sieve.In this study, FA was directly used without processing.As shown in

Sample Preparation
As shown in Figure 3, the HMNS used in this work were air-cooled after being coarsely ground to particulate matter, and then they were passed through a 200-mesh (75 μm) sieve.In this study, FA was directly used without processing.As shown in Table 2, Al powder and H2O2 were added in different mass fractions.

Preparation Process When H2O2 Is Used as a Blowing Agent
The acid activator was prepared according to the planned procedure and allowed to stand for one hour before use.HMNS and FA were weighed and stirred in a plastic beaker using a glass rod.In deionized water, H2O2 and SDS were added and mixed to form a slurry.Subsequently, the mixed solids were slowly added into the slurry, and then the acid activator was slowly poured in as well.The resulting foam slurry was mixed thoroughly using an electric stirrer.The slurry was then rapidly injected into a 20 mm × 20 mm × 20 mm mold.

Preparation Process When Al Powder Is Used as a Blowing Agent
The acid activator was first prepared according to the planned procedure and left to stand for a duration of 1 hour before use.Subsequently, the required amounts of HMNS, FA, and Al powder were weighed out and stirred together using a glass rod in a plastic beaker.

Preparation Process When H 2 O 2 Is Used as a Blowing Agent
The acid activator was prepared according to the planned procedure and allowed to stand for one hour before use.HMNS and FA were weighed and stirred in a plastic beaker using a glass rod.In deionized water, H 2 O 2 and SDS were added and mixed to form a slurry.Subsequently, the mixed solids were slowly added into the slurry, and then the acid activator was slowly poured in as well.The resulting foam slurry was mixed thoroughly using an electric stirrer.The slurry was then rapidly injected into a 20 mm × 20 mm × 20 mm mold.

Preparation Process When Al Powder Is Used as a Blowing Agent
The acid activator was first prepared according to the planned procedure and left to stand for a duration of 1 h before use.Subsequently, the required amounts of HMNS, FA, and Al powder were weighed out and stirred together using a glass rod in a plastic beaker.
Next, SDS was added to deionized water to create a solution, into which the mixed solids were poured to form a homogenous slurry.The acid activator was then added to the slurry in a slow, gradual manner while continuously mixing the contents using an electric stirrer.The aim of this step was to achieve a uniformly dispersed foam slurry.
Finally, the resulting slurry was swiftly and accurately injected into a mold with dimensions of 20 mm × 20 mm × 20 mm.
The porous geopolymer was demolded for about one day (in an oven at 40 • C) after casting.The tested specimens were cured for 7 days.Sample fragments collected from compression tests were used for XRD and FTIR.

Analytical Method
The specimens for the dry density test are dried in a drying oven at 60 • C. Dry density is calculated according to Equation (1): ρ 0 = dry density in kilograms per cubic meter (kg/m 3 ); m 0 = the drying mass of the specimen in kilograms (kg); V = the volume of the specimen in cubic millimeters (m 3 ).The size of the specimen of porous geopolymer used for the strength test was 20 mm × 20 mm × 20 mm.The average value of 3 samples was taken for the final property result.The compressive strength test method was carried out according to ASTM C109/C109M-2013.
A DYE-300 microcomputer servo press was used to determine the compressive strength of the porous geopolymer samples.A LFA-467 thermal conductivity meter was used to test the thermal conductivity of the porous geopolymers.DSC was tested by Mettler Toledo TGA/DSC 3 + .The specimens were made into thin sheets and tested in a 25 • C environment.First, the specimens for the dry density test were weighed after drying in a drying oven at 60 • C.Then, the specimens were weighed after being placed in a constant-temperature water bath for 24 h.Water absorption is calculated by Equation (2): W R = water absorption (%), calculated accurately to 0.1; m o = mass of specimen after drying, in kilograms (kg); m g = mass of specimen after water absorption, in kilograms (kg).The scanning range of the X-ray diffractometer (XRD) with CuKα radiation is 10 • ~80 • .The FT-IR spectra were recorded over a range of 4000 cm −1 to 400 cm −1 .

Dry Density
The effect of blowing agent content and type on the dry density of porous geopolymers was investigated.As can be seen from Figure 4, the dry density of porous geopolymers was 822 kg/m 3 at the highest and 664 kg/m 3 at the lowest when H 2 O 2 was used as the foaming agent.When Al powder was used as the foaming agent, the dry densities of the geopolymers were 1130 kg/m 3 and 772 kg/m 3 , respectively.It is easy to see that the Al powder only needs to be added in lesser amounts than H 2 O 2 to achieve a similar dry density.
the curing process of the geopolymers.These foams generate larger, more numerous pores inside the geopolymer as the blowing agent content increases and the reaction products grow around the pores formed by the blowing agent at the beginning of curing.This eventually forms a porous geopolymer containing many pores.The most direct macroscopic manifestation of this phenomenon is the decrease in dry density.However, the dry density does not always decrease as the blowing agent content increases because porous geopolymers are difficult to prepare successfully with the addition of excess blowing agent.

Pore Structure
The pore structure was found to change as the type and content of the blowing agent changed.More pores are produced when the content of the blowing agent increases, regardless of the type of blowing agent, as seen in Figure 5.The bubbles generated by the chemical reaction are rapidly immobilized in the geopolymer due to the rapid acid-base neutralization reaction.The number of pore structures increases due to more bubbles being trapped.
Pores with a diameter of more than 0.5 mm are found in porous geopolymers due to the fusion of small bubbles with each other when there is an increase in the foaming agent.
The number of open pores also increased due to the increase in pore structure, which resulted in smaller spacing between pores.The change in pore structure leads to a change in the physical properties of the porous geopolymers, such as the dry density discussed previously.This trend of change is corroborated with the trend of change in dry density.The higher number of pores and smaller pore spacing resulted in a lower dry density of the specimens prepared by H2O2 foaming.When the two blowing agent content elevation rates are the same, the decreasing trend of the specimens foamed with Al powder is significantly smoother.This may be because the liquid form of the foaming agent can produce foam earlier and faster compared to the solid form.When HMNS, FA, and phosphoric acid are fully mixed to form a slurry, the addition of H 2 O 2 generates oxygen more quickly and is more likely to produce more foam during curing.That is also the reason why the dry density of porous geopolymer prepared by H 2 O 2 is lower.
The dry density of porous geopolymers showed a constant decrease with increasing blowing agent content, whether H 2 O 2 or Al powder was used as the blowing agent.Due to the increasing amount of blowing agent, more foam is continuously generated during the curing process of the geopolymers.These foams generate larger, more numerous pores inside the geopolymer as the blowing agent content increases and the reaction products grow around the pores formed by the blowing agent at the beginning of curing.This eventually forms a porous geopolymer containing many pores.The most direct macroscopic manifestation of this phenomenon is the decrease in dry density.However, the dry density does not always decrease as the blowing agent content increases because porous geopolymers are difficult to prepare successfully with the addition of excess blowing agent.

Pore Structure
The pore structure was found to change as the type and content of the blowing agent changed.More pores are produced when the content of the blowing agent increases, regardless of the type of blowing agent, as seen in Figure 5.The bubbles generated by the chemical reaction are rapidly immobilized in the geopolymer due to the rapid acid-base neutralization reaction.The number of pore structures increases due to more bubbles being trapped.

Compressive Strength
The compressive strength of the specimens was tested.This was carried out to investigate the effect of the type and content of the blowing agent on the porous geopolymer.As can be seen from Figure 6a, the compressive strength of the porous geopolymer prepared by two blowing agents decreased with the increase in the blowing agent content after 3 days of curing.After 3 days of curing, the compressive strength of the specimens prepared with H2O2 as the blowing agent was 2 MPa at the highest and 0.4 MPa at the lowest.The compressive strength of the specimens prepared with Al powder as the blowing agent was 1.2 MPa at the highest and 0.8 MPa at the lowest.
As can be seen from Figure 6b, the compressive strength of HMNS-FA-phosphatebased porous geopolymer prepared by both blowing agents gradually decreased with the increase in the blowing agent content after 7 days of curing.The compressive strength of the specimens prepared with H2O2 was 2.8 MPa at the highest and 0.8 MPa at the lowest.The compressive strength of the specimens prepared with Al powder was 2 MPa at the highest and 1.2 MPa at the lowest.
From Figure 6a,b, it can be seen that when both foaming agents were added in the third gradient, the compressive strength of the specimens prepared with H2O2 was higher than that of the specimens prepared with Al powder after 3 days of curing.However, after 7 days of curing, both of them had the same compressive strength.When the content of the blowing agent added was in the fourth gradient, the compressive strength of the specimens prepared by Al powder was the same as that of the specimens prepared by H2O2 after 3 days of curing.However, after 7 days of curing, the compressive strength of the Pores with a diameter of more than 0.5 mm are found in porous geopolymers due to the fusion of small bubbles with each other when there is an increase in the foaming agent.The number of open pores also increased due to the increase in pore structure, which resulted in smaller spacing between pores.The change in pore structure leads to a change in the physical properties of the porous geopolymers, such as the dry density discussed previously.This trend of change is corroborated with the trend of change in dry density.The higher number of pores and smaller pore spacing resulted in a lower dry density of the specimens prepared by H 2 O 2 foaming.

Compressive Strength
The compressive strength of the specimens was tested.This was carried out to investigate the effect of the type and content of the blowing agent on the porous geopolymer.As can be seen from Figure 6a, the compressive strength of the porous geopolymer prepared by two blowing agents decreased with the increase in the blowing agent content after 3 days of curing.After 3 days of curing, the compressive strength of the specimens prepared with H 2 O 2 as the blowing agent was 2 MPa at the highest and 0.4 MPa at the lowest.The compressive strength of the specimens prepared with Al powder as the blowing agent was 1.2 MPa at the highest and 0.8 MPa at the lowest.
for the preparation of porous materials when a better dry density is desired.
The strength of porous geopolymers is influenced by various factors.The acid-activated geopolymerization process is based on the acid-base neutralization reaction [43], and the acid-base neutralization reaction gives a high early strength to the specimen.The porous structure retards the geopolymerization reaction at an early stage, resulting in a decrease in compressive strength.However, as the curing time increases, more gel phases are generated, which increases the strength of the porous geopolymer.As can be seen from Figure 6b, the compressive strength of HMNS-FA-phosphatebased porous geopolymer prepared by both blowing agents gradually decreased with the increase in the blowing agent content after 7 days of curing.The compressive strength of the specimens prepared with H 2 O 2 was 2.8 MPa at the highest and 0.8 MPa at the lowest.The compressive strength of the specimens prepared with Al powder was 2 MPa at the highest and 1.2 MPa at the lowest.
From Figure 6a,b, it can be seen that when both foaming agents were added in the third gradient, the compressive strength of the specimens prepared with H 2 O 2 was higher than that of the specimens prepared with Al powder after 3 days of curing.However, after 7 days of curing, both of them had the same compressive strength.When the content of the blowing agent added was in the fourth gradient, the compressive strength of the specimens prepared by Al powder was the same as that of the specimens prepared by H 2 O 2 after 3 days of curing.However, after 7 days of curing, the compressive strength of the specimens prepared by Al powder was higher than the specimens prepared by H 2 O 2 .This phenomenon occurs because the Al powder produces a denser porous structure with smaller pores.The compressive strength of porous geopolymers with small pores will have better rising potential.
We can learn from Figure 7 that the compressive strength of the porous geopolymers prepared by both foaming agents increased with an increase in curing time.The best specimen prepared with H 2 O 2 had a compressive strength of up to 2.8 MPa, and the best specimen prepared with Al powder had a compressive strength of up to 2 MPa.During the maintenance period, the incompletely reacted phosphoric acid continues to participate in the geopolymerization reaction.Therefore, the strength of the porous geopolymer increases.A phenomenon that can be concluded from Figure 7 is that a geopolymer without an added foaming agent has a higher compressive strength.This is because the geopolymer without an added blowing agent has no pore structure.This indicates that it contains more unreacted than reacted material.Meanwhile, the unreacted material continues the geopolymerization process.At the same time, the porous structure of the geopolymer has more contact area with air; the phosphoric acid is dissolved by the water in the air, and the reduction in phosphoric acid reduces the degree of late reaction.The reduction in the generated product reduces the compressive strength of the specimen.

Thermal Conductivity DSC Analysis
Thermal conductivity was tested to investigate the effect of blowing agents on porous geopolymers.As shown in Figure 9, the thermal conductivity of HMNS-FA-phosphoric acid-based porous geopolymers keeps decreasing.When no foaming agent was added, the thermal conductivity was 0.87 W/Mk.After adding H2O2 and Al powder, the thermal conductivity of the phosphate porous geopolymers reached a minimum of 0.25 W/Mk and 0.19 W/Mk, respectively.
Additionally, it is easy to see from the graph that HMNS-FA-phosphoric acid geopolymers become a porous material with a low dry density and good low-thermal conductivity even with the addition of a minimum amount of H2O2 or Al powder.This gives From Figure 8, we can see that compressive strength shows a decreasing trend with the increase in blowing agent content.The compressive strength of the porous geopolymers prepared by the two foaming agents is similar when the dry densities are the same.It suggests that the type of blowing agent may not have a negative impact on the strength of porous geopolymers.

Thermal Conductivity DSC Analysis
Thermal conductivity was tested to investigate the effect of blowing agents on porous geopolymers.As shown in Figure 9, the thermal conductivity of HMNS-FA-phosphoric acid-based porous geopolymers keeps decreasing.When no foaming agent was added, the thermal conductivity was 0.87 W/Mk.After adding H2O2 and Al powder, the thermal conductivity of the phosphate porous geopolymers reached a minimum of 0.25 W/Mk and 0.19 W/Mk, respectively.
Additionally, it is easy to see from the graph that HMNS-FA-phosphoric acid geo- With the geopolymerization reaction fully carried out, the compressive strength of the specimens prepared with Al powder was better enhanced after seven days of maintenance.H 2 O 2 can be chosen for the preparation of porous materials when the need for compressive strength is greater than the requirement for dry density.Al powder can be chosen for the preparation of porous materials when a better dry density is desired.
The strength of porous geopolymers is influenced by various factors.The acid-activated geopolymerization process is based on the acid-base neutralization reaction [43], and the acid-base neutralization reaction gives a high early strength to the specimen.The porous structure retards the geopolymerization reaction at an early stage, resulting in a decrease in compressive strength.However, as the curing time increases, more gel phases are generated, which increases the strength of the porous geopolymer.

Thermal Conductivity DSC Analysis
Thermal conductivity was tested to investigate the effect of blowing agents on porous geopolymers.As shown in Figure 9, the thermal conductivity of HMNS-FA-phosphoric acid-based porous geopolymers keeps decreasing.When no foaming agent was added, the thermal conductivity was 0.87 W/Mk.After adding H 2 O 2 and Al powder, the thermal conductivity of the phosphate porous geopolymers reached a minimum of 0.25 W/Mk and 0.19 W/Mk, respectively.
HMNS-FA-phosphate geopolymers great potential in the field of insulation and refractory materials, such as insulation and fireproofing materials for buildings.The thermal conductivity of geopolymers can be significantly reduced by the addition of either blowing agent, and an increase in the content of both blowing agents results in better low-thermal conductivity.This is because the foam is rapidly distributed into the geopolymer with the rapid occurrence of the pre-depolymerization reaction and sufficient mixing when the blowing agent is added.As the blowing agent content increases, more foam is produced, which results in more pores in the geopolymer, and these pores bring about an improvement in low-thermal conductivity.
From Figure 9, it can be seen that when the dry density of porous geopolymers prepared by two different blowing agents is similar, the thermal conductivity of both is also similar.This indicates that the type of blowing agent does not produce a phase that affects the low thermal conductivity of the geopolymer.
Figure 10 illustrates the differential scanning calorimetry (DSC) curves obtained from specimens that were prepared with and without foaming agents.Notably, an exothermic peak was observed at approximately 683 °C, which is attributed to the decomposition and rearrangement of magnesium pyrophosphate (Mg2P2O7)n into magnesium pyrophosphate units (Mg2P2O7) [44].Except for the absorption peaks of free and bound water at around 129 °C, no endothermic peaks were detected prior to 800 °C.The findings illustrate the exceptional thermal stability of the geopolymer with a porous structure, which maintains its structural integrity even at elevated temperatures of up to 800 °C.Additionally, it is easy to see from the graph that HMNS-FA-phosphoric acid geopolymers become a porous material with a low dry density and good low-thermal conductivity even with the addition of a minimum amount of H 2 O 2 or Al powder.This gives HMNS-FAphosphate geopolymers great potential in the field of insulation and refractory materials, such as insulation and fireproofing materials for buildings.The thermal conductivity of geopolymers can be significantly reduced by the addition of either blowing agent, and an increase in the content of both blowing agents results in better low-thermal conductivity.This is because the foam is rapidly distributed into the geopolymer with the rapid occurrence of the pre-depolymerization reaction and sufficient mixing when the blowing agent is added.As the blowing agent content increases, more foam is produced, which results in more pores in the geopolymer, and these pores bring about an improvement in low-thermal conductivity.
From Figure 9, it can be seen that when the dry density of porous geopolymers prepared by two different blowing agents is similar, the thermal conductivity of both is also similar.This indicates that the type of blowing agent does not produce a phase that affects the low thermal conductivity of the geopolymer.
Figure 10 illustrates the differential scanning calorimetry (DSC) curves obtained from specimens that were prepared with and without foaming agents.Notably, an exothermic peak was observed at approximately 683 • C, which is attributed to the decomposition and rearrangement of magnesium pyrophosphate (Mg 2 P 2 O 7 )n into magnesium pyrophosphate units (Mg 2 P 2 O 7 ) [44].Except for the absorption peaks of free and bound water at around 129 • C, no endothermic peaks were detected prior to 800 • C. The findings illustrate the exceptional thermal stability of the geopolymer with a porous structure, which maintains its structural integrity even at elevated temperatures of up to 800 • C.

Water Absorption Performance
The water absorption properties of porous geopolymers were tested.As shown in Figure 11, the water absorption of porous geopolymers showed a rising trend.When the amount of Al powder added is 0.02%, the water absorption rate is 30.6%; the water absorption rate is nearly double that of the specimen without a foaming agent.When the amount of H2O2 added is 0.2%, the water absorption rate increases to 50%, which is more than three times that of the specimen without a foaming agent.This also indicates that a porous structure can be formed inside the geopolymer by adding a small amount of foaming agent.This structure causes the dry density of the geopolymer to decrease.As shown in Figure 5, the porous geopolymer has a pore structure inside.The water absorption of the specimen increases because the porous structure preserves a large amount of water.
The water absorption of the specimen with the addition of Al powder was higher when the porous geopolymer prepared by the two blowing agents had the same dry density.This is because the porous geopolymer has fewer open pores when Al powder is used as the blowing agent.Porous geopolymers can be used as water storage materials, depending on the pore characteristics of the specimens prepared with the two blowing agents.

Water Absorption Performance
The water absorption properties of porous geopolymers were tested.As shown in Figure 11, the water absorption of porous geopolymers showed a rising trend.When the amount of Al powder added is 0.02%, the water absorption rate is 30.6%; the water absorption rate is nearly double that of the specimen without a foaming agent.When the amount of H 2 O 2 added is 0.2%, the water absorption rate increases to 50%, which is more than three times that of the specimen without a foaming agent.This also indicates that a porous structure can be formed inside the geopolymer by adding a small amount of foaming agent.This structure causes the dry density of the geopolymer to decrease.As shown in Figure 5, the porous geopolymer has a pore structure inside.The water absorption of the specimen increases because the porous structure preserves a large amount of water.

XRD Analysis
The phases of porous geopolymers were discussed after testing with XRD.As shown in Figure 12, the phases of porous geopolymers are mainly composed of forsterite, aluminum silicon phosphate, newberyite, forsterite ferron, AlPO4, quartz, and monetite.Among them, forsterite ferron, quartz, and mullite are the crystalline phases in HMNS and FA that are not involved in the reaction.
It can be seen from the graph that there is no difference in the phase composition of all specimens.This indicates that the blowing agent does not affect the end product of the ground polymerization reaction.The phases not involved in the reaction result in peaks that are more pronounced in the specimens with an added blowing agent.This indicates that the raw material is not fully involved in the reaction process in these specimens.From this, we can conclude that the geopolymerization reaction of porous geopolymers is reduced to varying degrees [45].
The XRD data indicate that the main products of the phosphate-based porous geopolymers are magnesite phosphate, aluminum silicon phosphate, and aluminum phosphate.The newberyite and aluminum silicon phosphate crystals as the skeleton and the colloid as the bonding agent allow the density of the geopolymer to increase, bringing about an improvement in the strength of the geopolymers [46].These diffraction peaks become progressively stronger with time, indicating that the geopolymerization reaction continues, which makes the content of these crystals continue to grow.This fact coincides with the increasing strength of the porous geopolymer.The gel phase and crystals produced by the geopolymerization reaction increase the strength.The reaction products (AlPO4) and forsterite in HMNS are both high-temperature-resistant phases [47].This will facilitate the improvement of the high-temperature resistance of phosphate-based porous geopolymers.The water absorption of the specimen with the addition of Al powder was higher when the porous geopolymer prepared by the two blowing agents had the same dry density.This is because the porous geopolymer has fewer open pores when Al powder is used as the blowing agent.Porous geopolymers can be used as water storage materials, depending on the pore characteristics of the specimens prepared with the two blowing agents.

XRD Analysis
The phases of porous geopolymers were discussed after testing with XRD.As shown in Figure 12, the phases of porous geopolymers are mainly composed of forsterite, aluminum silicon phosphate, newberyite, forsterite ferron, AlPO 4 , quartz, and monetite.Among them, forsterite ferron, quartz, and mullite are the crystalline phases in HMNS and FA that are not involved in the reaction.
It can be seen from the graph that there is no difference in the phase composition of all specimens.This indicates that the blowing agent does not affect the end product of the ground polymerization reaction.The phases not involved in the reaction result in peaks that are more pronounced in the specimens with an added blowing agent.This indicates that the raw material is not fully involved in the reaction process in these specimens.From this, we can conclude that the geopolymerization reaction of porous geopolymers is reduced to varying degrees [45].
The XRD data indicate that the main products of the phosphate-based porous geopolymers are magnesite phosphate, aluminum silicon phosphate, and aluminum phosphate.The newberyite and aluminum silicon phosphate crystals as the skeleton and the colloid as the bonding agent allow the density of the geopolymer to increase, bringing about an improvement in the strength of the geopolymers [46].These diffraction peaks become progressively stronger with time, indicating that the geopolymerization reaction continues, which makes the content of these crystals continue to grow.This fact coincides with the increasing strength of the porous geopolymer.The gel phase and crystals produced by the geopolymerization reaction increase the strength.The reaction products (AlPO 4 ) and forsterite in HMNS are both high-temperature-resistant phases [47].This will facilitate the improvement of the high-temperature resistance of phosphate-based porous geopolymers.

FTIR Analysis
FTIR analysis was performed on specimens prepared in different ratios, as shown in Figure 13, to further investigate the microstructural composition of HMNS-FA-phosphate-based porous geopolymers.This reveals the presence of bound water inside the porous geopolymers prepared with phosphoric acid, which can be observed from the vibrational peaks that appear around 1645 cm −1 and 3450 cm −1 in Figure 13 [36,43], which also corresponds to the formation of the phases detected in the XRD pattern (aluminum silicon phosphate and AlPO4).The addition of the foaming agent did not change the geopolymerization reaction products of the phosphate-based geopolymer.This is because the position of the peaks did not change from one specimen to another.This conclusion can be corroborated with the previous data.

FTIR Analysis
FTIR analysis was performed on specimens prepared in different ratios, as shown in Figure 13, to further investigate the microstructural composition of HMNS-FA-phosphatebased porous geopolymers.This reveals the presence of bound water inside the porous geopolymers prepared with phosphoric acid, which can be observed from the vibrational peaks that appear around 1645 cm −1 and 3450 cm −1 in Figure 13 [36,43], which also corresponds to the formation of the phases detected in the XRD pattern (aluminum silicon phosphate and AlPO 4 ).The addition of the foaming agent did not change the geopolymerization reaction products of the phosphate-based geopolymer.This is because the position of the peaks did not change from one specimen to another.This conclusion can be corroborated with the previous data.

Discussion
Consistent with the findings of Gu et al. [33], the factors influencing the density of a porous geopolymer also affect the material's strength.Specifically, for porous geopolymers, increasing the amount of almost all types of foaming agents leads to a decrease in both strength and density.The amount of foaming agent is also an important factor affecting thermal conductivity [52] and water absorption [53,54], as expected due to changes in porosity in the sample.
Based on the test analysis results of this study and a comparison with previous research conducted by others [46,55], we have summarized the geopolymerization mechanism of porous geopolymers.As shown in Figure 14, it mainly consists of the gas generation reaction and the geopolymerization reaction.Both occur simultaneously and interact with each other.This is the reason why the properties of porous geopolymers have changed.
The gas generation reaction: The two foaming agents react directly with the water in the slurry, producing O2(g) and H2(g) [36].The gas generation process does not affect the products of the ground polymerization reaction.This is consistent with the results of XRD and FTIR analyses.The reaction equation is as follows: As the geopolymerization reaction proceeds, the gas bubbles generated by the above reaction are trapped inside the specimen during the curing process.This creates voids or pores in the phosphate-base geopolymer that enhance its performance.
The geopolymerization reaction: After the mixed slurry is agitated sufficiently, phosphoric acid ionizes in the slurry to provide H+ for the depolymerization reaction.The reaction equation is as follows:

Discussion
Consistent with the findings of Gu et al. [33], the factors influencing the density of a porous geopolymer also affect the material's strength.Specifically, for porous geopolymers, increasing the amount of almost all types of foaming agents leads to a decrease in both strength and density.The amount of foaming agent is also an important factor affecting thermal conductivity [52] and water absorption [53,54], as expected due to changes in porosity in the sample.
Based on the test analysis results of this study and a comparison with previous research conducted by others [46,55], we have summarized the geopolymerization mechanism of porous geopolymers.As shown in Figure 14, it mainly consists of the gas generation reaction and the geopolymerization reaction.Both occur simultaneously and interact with each other.This is the reason why the properties of porous geopolymers have changed.
The gas generation reaction: The two foaming agents react directly with the water in the slurry, producing O 2 (g) and H 2 (g) [36].The gas generation process does not affect the products of the ground polymerization reaction.This is consistent with the results of XRD and FTIR analyses.The reaction equation is as follows: As the geopolymerization reaction proceeds, the gas bubbles generated by the above reaction are trapped inside the specimen during the curing process.This creates voids or pores in the phosphate-base geopolymer that enhance its performance.
HMNS did not have a heavily involved reaction, which is a good thing in a way.Forsterite ferron is a high-temperature resistant phase in theory.The geopolymerization reaction generates an aluminum phosphate gelling phase encapsulating forsterite ferron to enhance the high-temperature resistance of porous geopolymers.At the same time, the porous structure generated by the foaming process results in better insulation properties.This allows porous geopolymers to have better insulation potential in high-temperature environments.

Conclusions
The main reaction mechanisms involved in the formation of the HMNS-FA-phosphate-based porous geopolymers were physical encapsulation and chemical gelation.The foaming agent provided the system with a porous structure, which did not affect the The geopolymerization reaction: After the mixed slurry is agitated sufficiently, phosphoric acid ionizes in the slurry to provide H+ for the depolymerization reaction.The reaction equation is as follows: The chemical bonds in the feedstock break in the presence of the H+ in the above reaction.This step is the depolymerization process, which provides many ions and ionophores for the polycondensation process.These ionophores are interconnected to generate long chains of [-P-O-Al-] n and [-P-O-Al-O-Si-] n .The long-chain units form the network structure of the porous geopolymer.As the polycondensation reaction proceeds, the AlPO 4 gel phase and the aluminum-silica phosphate gel phase are generated.
Due to the low activity of HMNS, it is difficult to start the reaction with phosphoric acid at an indoor temperature.The quartz and forsterite ferron in the feedstock did not fully participate in the chemical reaction or dissolve into the slurry.The gel phase wraps the other phases together to form the dense structure of the porous geopolymer.As the geopolymerization reaction continues, more of the gel phase is generated.The increase in the late strength of the porous geopolymer is attributed to the increase in the gel phase.It is important to note that the condensation process occurs when the depolymerization process takes place because the two are in dynamic equilibrium.The foaming agent simply provides a porous structure during the reaction.Al powder provides a few Al 3+ when it is used as a foaming agent.However, its content is small and negligible.
HMNS did not have a heavily involved reaction, which is a good thing in a way.Forsterite ferron is a high-temperature resistant phase in theory.The geopolymerization reaction generates an aluminum phosphate gelling phase encapsulating forsterite ferron to enhance the high-temperature resistance of porous geopolymers.At the same time, the porous structure generated by the foaming process results in better insulation properties.This allows porous geopolymers to have better insulation potential in high-temperature environments.

Figure 4 .
Figure 4. Influence of type and content of the foaming agent on the dry density of porous geopolymers.

Figure 4 .
Figure 4. Influence of type and content of the foaming agent on the dry density of porous geopolymers.

Figure 6 .
Figure 6.Effect of the type and content of the foaming agent on the compressive strength of porous geopolymers.(a) 3 days and (b) 7 days.Figure 6.Effect of the type and content of the foaming agent on the compressive strength of porous geopolymers.(a) 3 days and (b) 7 days.

Figure 6 .
Figure 6.Effect of the type and content of the foaming agent on the compressive strength of porous geopolymers.(a) 3 days and (b) 7 days.Figure 6.Effect of the type and content of the foaming agent on the compressive strength of porous geopolymers.(a) 3 days and (b) 7 days.

Figure 7 .
Figure 7. Effect of curing time on the compressive strength of porous geopolymers.(a) H2O2 foaming and (b) Al foaming.

Figure 7 .
Figure 7. Effect of curing time on the compressive strength of porous geopolymers.(a) H 2 O 2 foaming and (b) Al foaming.

Figure 7 .
Figure 7. Effect of curing time on the compressive strength of porous geopolymers.(a) H2O2 foaming and (b) Al foaming.

Minerals 2023 , 18 Figure 12 .
Figure 12.XRD patterns of the specimens prepared without a blowing agent and those prepared with different blowing agents.
[48].These two vibrational peaks are the absorption bands of the -OH stretching and bending vibrations, respectively.The asymmetric stretching vibrations of Si(Al)-O-Si and Al-O-P-O lead to the absorption band at 1069 cm −1 [43].The absorption peak at 793 cm −1 corresponds to the bending vibration of Al-O-P [49]; the absorption peak at 460 cm −1 should be the planar bending vibration of the Si-O or Al-O bonds [50].The absorption bands at 510 cm −1 and 550 cm −1 are due to the stretching vibration of Mg-O [51].The long chains of [-P-O-Al-]n and [-Si-O-Al-O-P-]n form the main polymeric structure of the phosphate-based porous geopolymers

Figure 12 .
Figure 12.XRD patterns of the specimens prepared without a blowing agent and those prepared with different blowing agents.
[48].These two vibrational peaks are the absorption bands of the -OH stretching and bending vibrations, respectively.The asymmetric stretching vibrations of Si(Al)-O-Si and Al-O-P-O lead to the absorption band at 1069 cm −1 [43].The absorption peak at 793 cm −1 corresponds to the bending vibration of Al-O-P [49]; the absorption peak at 460 cm −1 should be the planar bending vibration of the Si-O or Al-O bonds [50].The absorption bands at 510 cm −1 and 550 cm −1 are due to the stretching vibration of Mg-O [51].The long chains of [-P-O-Al-]n and [-Si-O-Al-O-P-]n form the main polymeric structure of the phosphate-based porous geopolymers

Figure 13 .
Figure 13.FTIR spectra of specimens prepared without a foaming agent and specimens prepared with different foaming agents.

Figure 13 .
Figure 13.FTIR spectra of specimens prepared without a foaming agent and specimens prepared with different foaming agents.
Table 1 shows the major chemical constituents of FA are SiO 2 , Al 2 O 3 , and CaO, while the major chemical constituents of HMNS are SiO 2 , MgO, Fe 2 O 3 , and Al 2 O 3 .

Table 1 .
Chemical composition of HMNS and FA.

Table 1 .
Chemical composition of HMNS and FA.

Table 1 .
Chemical composition of HMNS and FA.

Table 2 ,
Al powder and H 2 O 2 were added in different mass fractions.