Carbon dioxide absorption material and method of producing carbon dioxide absorption material

Abstract

To provide a carbon dioxide absorption material made from recycled coal ash.SOLUTION: The carbon dioxide absorption material, which absorbs carbon dioxide contained in gases, is produced by mixing coal ash emitted from coal combustion with cement. The carbon dioxide absorption material is a porous body composition with a porosity of 5% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide absorbent material that is made from recycled coal ash such as fly ash and clinker ash and has the ability to absorb carbon dioxide in gas, and a method for producing the same. [Background technology]

[0002] Coal ash, including fly ash and clinker ash, emitted from coal-fired power plants and the like, is disposed of as industrial waste by being landfilled at industrial waste treatment plants, but as the capacity of these treatment plants is approaching its limit, it has become an urgent issue to reuse and effectively utilize coal ash without disposing of it in landfills.Fly ash is a by-product of coal combustion at coal-fired power plants, and the fine ash particles that float in the combustion gas and are collected by an electrostatic precipitator or the like are called fly ash.Clinker ash is coal ash particles that aggregate together and form clumps that accumulate at the bottom of the boiler, and are then crushed into sand-like particles.

[0003] For this reason, in recent years, products have been proposed that can be used in a variety of fields, primarily civil engineering and construction applications, such as cement admixtures, concrete admixtures, soil improvement materials, road subgrade materials, and backfill materials, which are made by recycling coal ash, such as fly ash and clinker ash. Patent Documents 1 and 2 listed below, for example, disclose one method of recycling coal ash, which involves hardening coal ash with cement to produce coal ash granules and coal ash mixtures, as well as recycled concrete products using the same. Patent Document 3 also discloses a technology for recycling coal ash by adding and mixing magnesium oxide, gypsum, dolomite, ferrous sulfate, fused phosphate fertilizer, and the like to coal ash. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-124212 [Patent Document 2] Patent No. 6289704 [Patent Document 3] Japanese Patent Publication No. 2022-86381 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, global climate change has become a major issue in recent years, and in response, one of the SDGs has been set as "taking concrete measures to combat climate change," with over 120 countries and regions aiming to achieve carbon neutrality by 2050. In other words, as is well known, reducing carbon dioxide (CO2) emissions has become a major global goal.

[0006] Japan's domestic CO2 emissions are 1,044 million tons per year (2020, Ministry of the Environment), of which 3.95 million tons per year is generated during construction work (2020, Japan Federation of Construction Contractors), with CO2 emissions from the operation of construction machinery also being a factor.For this reason, reducing carbon dioxide emissions during construction work has become a major challenge for government agencies and local governments that oversee infrastructure development, and the construction industry that undertakes construction work.

[0007] The inventors of the present application have been actively researching and developing methods for recycling coal ash that will contribute to the global goal of reducing carbon dioxide emissions, and have now developed a carbon dioxide absorbent using coal ash that has the ability to absorb carbon dioxide in gases.

[0008] Therefore, an object of the present invention is to provide a carbon dioxide absorbent material made from recycled coal ash and a method for producing the carbon dioxide absorbent material. [Means for solving the problem]

[0009] To achieve the above object, the carbon dioxide absorbent of the present invention is a carbon dioxide absorbent that absorbs carbon dioxide contained in gas, and is characterized in that it is produced by mixing cement with coal ash emitted by coal combustion, and is a porous composition having a porosity of 5% or more.

[0010] To achieve the above object, the present invention provides a method for producing a carbon dioxide absorbent that absorbs carbon dioxide contained in a gas, the method comprising: a mixing step of mixing cement with coal ash emitted by coal combustion; and a molding and solidification step of molding and solidifying the mixture produced in the mixing step to form a porous composition having a porosity of 5% or more. [Effects of the Invention]

[0011] According to the present invention, a carbon dioxide absorbent having a carbon dioxide absorbing effect can be provided and manufactured by recycling coal ash emitted by coal combustion. Furthermore, a carbon dioxide absorbent having a high carbon dioxide absorbing effect can be provided even if the particle size is relatively large. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a flowchart showing a method for producing a carbon dioxide absorbent material according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram showing the measurement results (CO2 concentration) of Experiment A. [Figure 3] FIG. 10 is a diagram showing the measurement results (CO2 concentration) of Experiment B. [Figure 4] FIG. 10 is a diagram showing the measurement results (humidity) of Experiment B. [Figure 5] FIG. 10 is a diagram showing the measurement results (CO2 concentration) of Experiment C. [Figure 6] 6(a) and 6(b) show the measurement results of Experiment D, showing the change in CO2 concentration and particle size of the sample, and the change in CO2 absorption rate and particle size of the sample, respectively. [Figure 7]FIG. 10 is a diagram showing the measurement results of Experiment E, illustrating the relationship between the porosity and water absorption rate of a cylindrical specimen (carbon dioxide absorbent material). [Figure 8] FIG. 10 is a diagram showing the measurement results of Experiment E, illustrating the change in CO 2 concentration in cylindrical specimens (carbon dioxide absorbent materials) with different porosities. [Figure 9] FIG. 10 is a diagram showing the measurement results of Experiment E, illustrating the relationship between the porosity and CO 2 absorption rate of a cylindrical specimen (carbon dioxide absorbent material). [Figure 10] 10(a) and 10(b) show the measurement results of Experiment F, and are graphs showing the change in CO2 concentration in each test specimen (FIG. 10(a)) and the CO2 absorption rate of each test specimen (FIG. 10(b)). DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings. However, the technical scope of the present invention is not limited to these embodiments.

[0014] FIG. 1 is a flowchart showing a method for manufacturing a carbon dioxide absorbent according to an embodiment of the present invention. The carbon dioxide absorbent is prepared by mixing and kneading a predetermined ratio of coal ash, which is a combustion residue emitted by coal combustion, cement, and water into a mixer (mixing device) (S100). The mixture is then uniformly kneaded. The mixture is then transferred to a vibration molding machine and molded by vibration and pressure (pressing) (S102). The molded product is then transferred to a curing facility, where it is cured for a certain period of time under predetermined temperature and humidity control, and solidified (S104). If necessary, the particle size of the molded product may be adjusted (S106) and processed to a desired size. The carbon dioxide absorbent is a three-dimensional solid that can be processed into various sizes and shapes depending on its intended use. It can be used as a material for various structures in an environment exposed to the atmosphere, or it can be placed and installed by itself.

[0015] The production of the carbon dioxide absorbent is substantially the same as the production of recycled coal ash products, particularly recycled concrete products, which are produced by hardening coal ash with cement as materials used in civil engineering and construction work. For example, the production methods for recycled concrete products disclosed in Patent Documents 1 and 2 above can be adopted.

[0016] (Example) To examine the carbon dioxide (CO2) absorption properties of the carbon dioxide absorbent produced by the above method, Experiments A, B, and C described below were carried out.

[0017] (Experiment A) The test specimens were carbon dioxide absorbents (CO2 absorbents) with particle sizes between 40 and 5 mm and crushed road stone with particle sizes between 40 and 0 mm (less than 40 mm). Each specimen weighed 1,000 g in an air-dried state and placed in a glass desiccator with a capacity of approximately 3 liters. The desiccators were sealed to prevent air from entering or leaving the desiccator. A CO2 meter was used to measure the change in CO2 concentration within the desiccator over time after sealing. The results of Experiment A are shown in Figure 2. While CO2 concentrations are generally considered to be around 400 ppm in the outdoor atmosphere (Japan Meteorological Agency: Changes in Carbon Dioxide Concentration and Annual Increase at Japan Meteorological Agency Observation Points, updated December 19, 2022), the CO2 concentration immediately after sealing the specimens (0 minutes later) was around 870 ppm. This indicates that the CO2 concentration in the indoor test chamber where the experiment was conducted was higher than that outdoors.

[0018] As shown in Figure 2, the CO2 concentration in the desiccator sealed with crushed road stone was almost constant at about 870 ppm, whereas the concentration in the desiccator sealed with the carbon dioxide absorbent of the present invention started at about 870 ppm and decreased over time to about 70 ppm after 10 minutes. This confirmed that the carbon dioxide absorbent of the present invention has the ability to absorb CO2 in a relatively short time.

[0019] (Experiment B) Since Experiment A above confirmed that the carbon dioxide absorbent of the present invention has the effect of absorbing CO2, Experiment B was conducted to compare it with recycled crushed stone produced by crushing waste concrete. In Experiment A, it was thought that the capacity of the desiccator might have been too small for the amount of test specimen. Therefore, in Experiment B, the amount of test specimen was halved and the capacity of the desiccator was quadrupled. Specifically, 500 g of air-dried carbon dioxide absorbent of the present invention with a particle size of 40 to 5 mm and recycled crushed stone with a particle size of 40 to 0 mm (particle size less than 40 mm) were used as test specimens. The test specimens were placed in a glass desiccator with a capacity of approximately 12 liters, sealed to prevent air from entering or leaving the desiccator, and the change in CO2 concentration within the desiccator over time after sealing was measured using a CO2 concentration meter. The results of Experiment B are shown in Figure 3.

[0020] As shown in Figure 3, the CO2 concentration decreased over time for both the carbon dioxide absorbent of the present invention and the recycled crushed stone, but the decrease was faster for the carbon dioxide absorbent of the present invention. Because both the carbon dioxide absorbent of the present invention and the recycled crushed stone are hardened cement bodies, it was predicted that the reduction rates of CO2 concentration would be about the same, but the carbon dioxide absorbent of the present invention showed a greater reduction effect.

[0021] The change in humidity measured along with the CO2 concentration is shown in Figure 4. As shown in Figure 4, in the humidity measurements in Experiment B, the humidity increased over time for both the carbon dioxide absorbent material of the present invention and the recycled crushed stone. This suggests that Ca(OH)2 in the hardened cement paste absorbs CO2 and generates water, as shown in the chemical formula (1) below. Ca(OH)2+ CO2 = CaCO3+ H2O (1)

[0022] (Experiment C) Since the carbon dioxide absorbent of the present invention was found to be more effective at reducing CO2 concentration than recycled crushed stone in Experiment B, Experiment C was conducted to measure changes in the CO2 concentration of coal ash. 500 g of coal ash with a moisture content of approximately 15% was placed in a glass desiccator with a capacity of approximately 12 liters, sealed to prevent air from entering or leaving, and the change in CO2 concentration inside the desiccator over time after sealing was measured using a CO2 concentration meter. The results of Experiment C are shown in Figure 5.

[0023] As shown in Figure 5, coal ash was found to have a CO2-reducing effect. In other words, the carbon dioxide absorbent of the present invention exhibited a greater CO2-reducing effect than recycled crushed stone. While recycled crushed stone, which is a hardened cement body, itself has some carbon dioxide absorption properties, the carbon dioxide absorbent of the present invention not only absorbs CO2 as a hardened cement body, but also retains the CO2-absorbing ability of coal ash even after hardening with cement. This is thought to be due to the synergistic effect of these two properties. Furthermore, since carbonation of concrete, which is one of the hardened cement bodies, is one of the major causes of reduced durability, recent technology has focused on how to suppress the progression of carbonation. Meanwhile, carbonation is a reaction between CO2 and alkali ("Mechanism of Carbonation of Concrete," I. Kobayashi et al., Journal of Concrete Engineering), and cement hydrates are expected to absorb CO2. Therefore, the experimental results in Figure 3 suggest that hardened cement bodies that do not contain coal ash also have carbon dioxide absorption properties.

[0024] As described above, in the embodiment of the present invention, cement that is readily available throughout Japan is used to recycle coal ash emitted from coal-fired power plants and other facilities that are struggling to process it, and the carbon dioxide (CO2) absorption capacity of the hardened cement and coal ash is utilized to reduce CO2 concentrations that are higher than those in the general atmosphere to levels below those in the general atmosphere in a relatively short period of time. This provides a carbon dioxide absorbent that can reduce CO2 concentrations and contribute to curbing global warming.

[0025] While indoors, the building environmental hygiene standard requires CO2 concentrations to be below 1000 ppm, there are no outdoor standards, but it is estimated that localized high concentrations can occur near heavy machinery that emits exhaust gases.In addition, because it has the ability to reduce the concentration of CO2 generated by construction work in a relatively short period of time, it is expected to improve the air environment at construction sites and reduce health problems such as respiratory problems among construction workers.

[0026] (Additional Embodiments) The carbon dioxide absorbent of the present invention may be adjusted in particle size and used as a construction material such as a roadbed material or backfill material. In this case, the material is loaded onto a transport vehicle, unloaded from the transport vehicle, and transported to the construction site by a shovel or the like, and construction work proceeds by steps such as leveling with a motor grader or the like and compaction with a tire roller or the like. Normally, as a result of these operations, particles with smaller particle sizes sink to the bottom (lower layer) and particles with larger particle sizes remain on the top (surface layer), so that the lower layer is dominated by particles with smaller particle sizes and the surface layer is dominated by particles with larger particle sizes.

[0027] On the other hand, the carbon dioxide absorption effect depends on the surface area of ​​the carbon dioxide absorbent, and it is presumed that larger particle sizes have a smaller surface area and therefore a smaller carbon dioxide absorption effect. In other words, if the number of large particle sizes increases in the surface layer as a result of the series of operations, there is a concern that the carbon dioxide absorption effect will decrease.

[0028] Therefore, in order to verify the relationship between the particle size (particle size) of the carbon dioxide absorbent and the carbon dioxide (CO2) absorption effect, Experiments D, E, and F, which will be described below, were carried out.

[0029] (Experiment D) In Experiment D, the CO2 absorption rate of carbon dioxide absorbents with different particle sizes was measured to investigate the effect of the particle size of the carbon dioxide absorbent on the carbon dioxide absorption effect. Table 1 shows the particle sizes of the carbon dioxide absorbent samples used in Experiment D.

[0030] [Table 1]

[0031] As shown in Table 1, the particle size of current carbon dioxide absorbents was adjusted to three different particle sizes: the standard particle size of 4005 (particle size 40-5 mm), the larger particle size of 4020 (particle size 40-20 mm), and the smaller particle size of 1005 (particle size 10-5 mm). In Experiment D, 500 g of each was placed in a desiccator with a capacity of approximately 10 L, sealed, and the change in CO2 concentration was measured while blocking airflow. The results of this experiment are shown in Figure 6. Specifically, Figure 6(a) shows the change in CO2 concentration versus sample particle size, and Figure 6(b) shows the change in CO2 absorption rate versus sample particle size.

[0032] As shown in Figure 6(a) regarding the change in particle size and CO2 concentration, the CO2 concentration of samples 4005 and 1005 decreased over time, whereas the CO2 concentration of sample 4020 increased until 15 minutes later and then decreased after 20 minutes.

[0033] The CO2 absorption rate is given by the following formula: CO2 absorption rate (%) = (CO2 concentration at 0 minutes - CO2 concentration after n minutes) / (CO2 concentration at 0 minutes) x 100

[0034] In the relationship between particle size and CO2 absorption rate shown in Figure 6(b), sample 4020 did not absorb carbon dioxide until 20 minutes later, and after 30 minutes its CO2 absorption rate was about half that of sample 4005. Conversely, sample 1005 had a CO2 absorption rate about two to three times that of sample 4005. This shows that the CO2 absorption rate is greatly affected by particle size, and sample 4020 with a larger particle size takes longer to start absorbing carbon dioxide than sample 4005 with a normal particle size, and its effect is halved.

[0035] In other words, it was confirmed that as a result of the series of steps involved in laying the carbon dioxide absorbent material on road surfaces and other surfaces as construction materials, the carbon dioxide absorbent material with large particle sizes accumulates in the surface layer of the road surface, etc., and as a result, it takes time for the material to start absorbing carbon dioxide, and its effectiveness is almost halved.

[0036] As mentioned above, sample 4020 takes longer to start absorbing carbon dioxide than sample 4005, and the CO2 absorption rate is reduced to about half, so if there are more particles with large particle sizes in the surface layer, the time until carbon dioxide absorption begins is delayed and the CO2 absorption rate is halved. Therefore, in order to prevent the time until carbon dioxide absorption begins and the effect from being halved even when there are more particles with large particle sizes in the surface layer, it is necessary to double the CO2 absorption rate of the carbon dioxide absorbent.

[0037] (Experiment E) As the particle size increases, the apparent surface area decreases and the carbon dioxide absorption effect decreases, but it is presumed that if the porosity is increased, the apparent surface area can be increased and the decrease in the carbon dioxide absorption effect can be suppressed, and to verify this, the following Experiment E was carried out using test specimens (samples) of carbon dioxide absorbents with different porosities. As shown in Figure 1, carbon dioxide absorbents are produced by adding water to cement and coal ash, which is the combustion residue emitted by coal combustion, and mixing it, molding it in a vibration molding machine, and solidifying it by curing, and this manufacturing process forms numerous pores inside the carbon dioxide absorbent of the present invention, and it is produced as a porous composition with a predetermined porosity.

[0038] Because it is extremely difficult to achieve a uniform grain size for all specimens, we molded cylinders measuring φ10 × 20 cm to eliminate the influence of grain size. Specifically, in the carbon dioxide absorbent manufacturing process shown in Figure 1, when a mixture of coal ash and cement was molded by vibration and pressure (press) using a vibration molding machine (S102), the vibration and pressure conditions were appropriately adjusted to create carbon dioxide absorbents with identical cylindrical shapes. By adjusting the magnitude, time, and period of vibration, as well as the magnitude and time of pressure, carbon dioxide absorbents with different porosities can be created. The porosity can be measured using the calculation formula described below. Ten cylindrical specimens with porosities ranging from 4.3% to 23.9% were prepared, and these were used as samples for Experiment E.

[0039] In Experiment E, 10 cylindrical specimens with porosities between 4.3% and 23.9% were placed in a desiccator with a capacity of approximately 10 L, sealed, and airtight, and changes in CO2 concentration were measured using a CO2 concentration meter. The results are shown in Figures 7 to 9. In Figure 7, the relationship between the porosity and water absorption rate of the carbon dioxide absorbent was measured before measuring the CO2 concentration. Furthermore, the porosity of materials currently manufactured for use as construction materials using the manufacturing process shown in Figure 1 (hereinafter sometimes referred to as "materials for construction use") is approximately 3 to 4%.

[0040] Figure 7 shows the relationship between the porosity and water absorption rate of a cylindrical specimen (carbon dioxide absorbent). Note that the porosity and water absorption rate are calculated using the following formulas.

[0041] Porosity (%) = (unit volume mass at 0% porosity - unit volume mass of specimen) / (unit volume mass at 0% porosity) x 100 (calculated from the mass immediately after molding) Water absorption rate (%) = (mass in surface-dry state - mass in bone-dry state) / (mass in surface-dry state) x 100 (Surface dry state = surface dry and saturated with water, bone dry state = state after drying the test piece at 105°C for 24 hours)

[0042] As shown in Figure 7, the porosity and water absorption rate of carbon dioxide absorbents are proportional, with the water absorption rate increasing as the porosity increases. Generally, when immersed in water, the internal voids fill with water, so it is believed that the higher the porosity, the higher the water absorption rate. In addition, as the porosity increases, the apparent surface area increases, increasing the contact area with water, which is thought to make it easier to absorb water. The same is true for gases; as the porosity increases and the apparent surface area increases, the contact area with gas increases, which is thought to make it easier to absorb gas.

[0043] Figure 8 shows the change in CO2 concentration for cylindrical specimens (carbon dioxide absorbent) with different porosities. The porosities of the 10 cylindrical specimens, ranging from 4.3% to 23.9%, were 4.3, 5.5, 6.7, 7.9, 9.1, 10.2, 12.7, 16.8, 21.3, and 23.8%. As shown in Figure 8, the decrease in CO2 concentration was smaller for specimens with a porosity of 4.3%, similar to that of current construction materials. The CO2 concentration actually increased after 20 minutes, but began to decrease after 25 minutes, similar to the behavior of Sample 4020 with a large grain size in Experiment D. Furthermore, the decrease in CO2 concentration tended to increase with increasing porosity. However, the decrease in CO2 concentration was particularly significant for specimens with a porosity of 16.8% or higher.

[0044] Figure 9 shows the relationship between the porosity and CO2 absorption rate of the cylindrical specimen (carbon dioxide absorbent). Specifically, Figure 9(a) shows the CO2 absorption rate 10 minutes after the start of measurement, Figure 9(b) shows the CO2 absorption rate 20 minutes after the start of measurement, and Figure 9(c) shows the CO2 absorption rate 30 minutes after the start of measurement. As shown in Figure 9, there is a high correlation between porosity and CO2 absorption rate, with CO2 absorption rate increasing with increasing porosity. After 10 and 20 minutes, when the porosity was approximately 3-4%, similar to current construction materials, the CO2 absorption rate was approximately 0%, meaning no carbon dioxide reduction effect was observed. However, when the porosity was 5% or higher, the CO2 absorption rate was 10% or higher, indicating a carbon dioxide reduction effect. Furthermore, after 30 minutes, when the porosity was approximately 3-4%, similar to current construction materials, the CO2 absorption rate was approximately 10%, but when the porosity was 5%, the CO2 absorption rate doubled to approximately 20%. From this, it was discovered that by increasing the porosity of the carbon dioxide absorbent from approximately 3 to 4%, which is the same as that of current materials used in construction materials, to at least approximately 5%, it is possible to double the CO2 absorption rate without delaying the time it takes to start absorbing carbon dioxide. In other words, if the carbon dioxide absorbent is made of a porous composition with a porosity increased to 5% or more, it is possible to provide a carbon dioxide absorbent that has a rapid and sufficient carbon dioxide absorption effect, even when the surface layer of the road surface contains a large number of particles with large particle sizes when laid on the road surface, etc.

[0045] From the above, it was found that the CO2 absorption rate can be controlled by adjusting the porosity of the carbon dioxide absorbent. In other words, even if there are many large particle sizes in the surface layer of a road or the like on which the carbon dioxide absorbent is laid, by making the carbon dioxide absorbent a porous composition with a porosity of 5% or more, a rapid and sufficient carbon dioxide absorption effect can be achieved. As mentioned above, the porosity of the carbon dioxide absorbent material can be adjusted by the magnitude (amplitude strength), time, period, etc. of the vibration applied in the molding step of the manufacturing process, as well as the magnitude and time of the pressure applied.

[0046] (Experiment F) Experiment F was conducted to verify that even if large particle sizes predominate in the surface layer, sufficient carbon dioxide absorption effects can be achieved by increasing the porosity of the carbon dioxide absorbent to around 5%. Table 2 shows the porosity and particle size of the carbon dioxide absorbent sample used in Experiment F.

[0047] [Table 2]

[0048] As shown in Table 2, two types of carbon dioxide absorbent materials were used as test specimens (samples): 5.2%-4020 (particle size 40-20mm) with a porosity of 5.2% and a relatively large particle size, and 4.3%-4005 (particle size 40-5mm) with a porosity of 4.3% and a normal particle size. In Experiment F, 500g of each material was placed in a desiccator with a capacity of approximately 10L, sealed, and the change in CO2 concentration was measured while preventing air from entering or leaving. The results are shown in Figure 10. Specifically, Figure 10(a) shows the change in CO2 concentration in each test specimen, and Figure 10(b) shows the CO2 absorption rate of each specimen.

[0049] As shown in Figure 10, the 5.2%-4020, which has a porosity of 5.2% and a large particle size, and the 4.3%-4005, which has a porosity of 4.3% and a normal particle size, show equivalent carbon dioxide absorption performance. This demonstrates that even if the surface layer is dominated by large particle sizes, a carbon dioxide absorbent made of a porous composition with a porosity increased to at least about 5% can achieve rapid and sufficient carbon dioxide absorption effects.

[0050] The present invention is not limited to the above-described embodiments, and it goes without saying that the present invention also includes design changes that do not deviate from the gist of the invention, including various modifications and alterations that would occur to a person with ordinary knowledge in the field of the present invention.

Claims

1. A carbon dioxide absorbent that absorbs carbon dioxide contained in a gas, A carbon dioxide absorbent characterized by being a porous composition produced by mixing cement with coal ash discharged from coal combustion, and having a porosity of 5% or more.

2. A method for producing a carbon dioxide absorbent that absorbs carbon dioxide contained in a gas, comprising: a mixing step of mixing coal ash discharged by coal combustion with cement; a molding and solidification step of molding and solidifying the mixture produced in the mixing step into a porous composition having a porosity of 5% or more.

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