Vegetable-based concrete and method for manufacturing vegetable-based concrete

Abstract

To provide vegetable concrete having excellent water resistance and bending strength.SOLUTION: Vegetable concrete of the present invention comprises concrete powder and kraft lignin.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to vegetable-based concrete and a method for producing vegetable-based concrete. [Background technology]

[0002] The rapid increase in global concrete consumption has led to adverse climate impacts, worsening environmental pollution, and raw material shortages, raising awareness that society needs to increase its use of renewable materials. Recycling and reuse of concrete debris for developing sustainable building materials is considered a promising approach, with the most common approach being the reuse of recycled concrete aggregate in fresh concrete. The traditional approach of replacing natural raw materials in fresh concrete with recycled concrete debris is a promising concept from both environmental and economic perspectives. However, this process has limitations, such as poor sample performance, especially at high raw material replacement rates. Furthermore, the fine solid debris and cement mortar particles generated in the recycling process are difficult to reuse and lead to new pollution. Furthermore, new cement input is still required, and the shortage of concrete raw materials has yet to be fundamentally addressed.

[0003] Attempts have been made to address the shortage of concrete raw materials by eliminating the use of new cement in the concrete recycling process, thereby eliminating waste generation in the concrete recycling process (Non-Patent Document 1). In these attempts, concrete rubble was crushed and ground into powder without separating the gravel and other components, and then compressed under high pressure to create new concrete samples. However, high compression pressures of up to 100 MPa were required during the molding process, making it impractical for use in commercial production.

[0004] On the other hand, more than 8 million tons of waste wood is generated annually, but much of it ends up being incinerated or disposed of in landfills, and recycling is not being achieved.

[0005] In this context, attempts have been made to obtain botanical concrete (BC), which can recycle waste wood and concrete rubble by simultaneously hot-pressing concrete rubble and wood waste powder. It has been discovered that, compared to conventional concrete recycling processes, the addition of waste wood powder significantly reduces the high molding pressure required in the method proposed in Non-Patent Document 1. Furthermore, the resulting samples exhibited excellent bending strength.

[0006] However, like other wood-based composites, plant-based concrete has poor water resistance and dimensional stability due to the hydrophilic nature of the wood particles used as raw materials, and improvements in these properties were needed.

[0007] Lignin is one of the most abundant and renewable resources on Earth and is a by-product of the pulp and paper industry. Attempts have been made to use lignin, which is cost-effective and environmentally friendly, as an adhesive, mainly in the production of wood-based panels, instead of expensive synthetic resin binders. It has been found that wood-based panels obtained in this way have excellent moisture stability and mechanical properties (e.g., Non-Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Non-Patent Document 1] Journal of Advanced Concrete Technology vol.14 2016 47-54 [Non-patent document 2] Biomass and Bioenergy vol.21 2001 211-224 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a plant-based concrete having excellent water resistance and flexural strength, and a method for producing such plant-based concrete. [Means for solving the problem]

[0010] In order to solve the above problems, the inventors first investigated the effects of various molding conditions on the water resistance and dimensional stability of plant-based concrete, and then discovered that the properties of plant-based concrete can be improved by adding kraft lignin, which led to the completion of the present invention.

[0011] The present invention provides the following: (1) Vegetable-based concrete containing concrete powder and kraft lignin. (2) The plant-based concrete according to (1), further comprising wood flour. (3) The plant-based concrete according to (2), wherein the particle size of the wood flour is 100 to 500 μm. (4) The plant-based concrete according to (2) or (3), wherein the wood flour has a moisture content of 1 to 15%. (5) The vegetable-based concrete according to any one of (2) to (4), wherein the blending amount of the kraft lignin is 3 to 30% by weight based on the bone dry weight of the wood flour. (6) A method for producing plant-based concrete, comprising: a mixing step of mixing concrete powder, wood flour, and kraft lignin; and a hot pressing step of hot pressing the mixed powder obtained in the mixing step. (7) The method for producing vegetable-based concrete according to (6), wherein the heating temperature in the hot pressing step is 160 to 250°C. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a plant-based concrete having excellent water resistance and bending strength. Furthermore, according to the present invention, it is possible to provide a method for producing such plant-based concrete. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a photograph of a hot press machine used to produce the plant-based concrete of the present invention. [Figure 2] 1 is a graph showing the effect of wood flour particle size on the expansion rate and water absorption rate for plant-based concrete molded under condition 1. [Figure 3] 10 is a graph showing the effect of the moisture content of wood flour on the expansion rate and water absorption rate for plant-based concrete molded under condition 2. [Figure 4] 1 is a graph showing the effect of compaction temperature on the expansion rate and water absorption rate for plant-based concrete molded under conditions 4 and 5. [Figure 5] 1 is a graph showing the effect of kraft lignin content on the expansion rate and water absorption rate for plant-based concrete molded under condition 6. [Figure 6] 1 is a graph showing the effect of kraft lignin content on density for vegetable-based concrete molded under condition 6. [Figure 7] 1 is a graph showing the effect of kraft lignin content on flexural strength for vegetable-based concrete molded under condition 6. [Figure 8] 10 is a graph showing the effect of compression pressure on the expansion rate and water absorption rate of plant-based concrete molded under condition 3. [Figure 9] 10 is a graph showing the effect of compression pressure on springback for plant-based concrete molded under condition 3. [Figure 10] Photographs showing the color of plant-based concrete under different molding conditions: (a) color of plant-based concrete with different particle size; (b) color of plant-based concrete with different wood flour moisture content; (c) color of plant-based concrete with different compression pressures; (d) color of plant-based concrete with different kraft lignin content; (e) color of plant-based concrete with different compression temperatures. [Figure 11]SEM observations of some plant-based concrete samples: (a) SEM observation of plant-based concrete formed with 10% wood flour moisture content in condition 2; (b) SEM observation of plant-based concrete formed with 15% kraft lignin content in condition 6; (c) SEM observation of plant-based concrete formed with 15% kraft lignin content in condition 6, where the only difference is that the concrete powder was replaced with sand of the same particle size (178 μm). DETAILED DESCRIPTION OF THE INVENTION

[0014] The vegetable-based concrete (BC) of the present invention contains concrete powder and kraft lignin.

[0015] (lignin) Lignin is one of the most abundant and renewable resources on Earth and a by-product of the pulp and paper industry. It is known as one of the main components of "black liquor" (25-35%), which is produced by digesting wood particles in alkaline pulping. Kraft lignin is one of the most common industrial lignins and is only soluble in alkaline environments.

[0016] (Kraft lignin) Kraft lignin that can be used in the present invention is separated from black liquor discharged from a process of kraft cooking wood. Here, wood as a raw material for kraft cooking can be, for example, broad-leaved trees, coniferous trees, miscellaneous trees, bamboo, kenaf, bagasse, or empty palm bunches after palm oil extraction, with broad-leaved trees and coniferous trees being preferred.

[0017] (Kraft lignin manufacturing method) Acid and / or carbon dioxide is added to the black liquor obtained after kraft cooking to adjust the pH of the black liquor to 1 to 11, preferably 2 to 8, thereby precipitating the modified lignin dissolved in the black liquor. This process may be repeated two or more times. The acid used may be an inorganic or organic acid. Examples of inorganic acids include sulfuric acid, sulfurous acid, hydrochloric acid, nitric acid, nitrous acid, phosphoric acid, and carbonic acid, with sulfuric acid being preferred. Examples of organic acids include acetic acid, lactic acid, oxalic acid, citric acid, and formic acid. The black liquor can be concentrated using an evaporator or the like before adjusting the pH, and the solid content is preferably 10% by weight or more, and more preferably 20% by weight or more and 50% by weight or less. The temperature when adjusting the pH of the black liquor to 1 to 11 is preferably room temperature to 100°C.

[0018] Furthermore, prior to the step of adjusting the pH to 1 to 11, it is preferable to add a step of adding carbon dioxide to adjust the pH to 7 to 11. The treatment temperature is not particularly limited, but is preferably about 60°C. The method of adding carbon dioxide is not particularly limited, but includes a method of blowing in carbon dioxide under atmospheric pressure, or a method of blowing carbon dioxide into a sealed container and pressurizing it (0.1 to 1 MPa).

[0019] The precipitate of the modified lignin obtained in the step of adjusting the pH to 1 to 11 by adding an acid and / or carbon dioxide is dehydrated and washed with water. Devices that can be used to dehydrate and wash the precipitate include a filter press, a drum press, a centrifugal dehydrator, and a suction filter. The water used for washing preferably has a pH of 1 to 11 and a temperature of room temperature to 80°C.

[0020] (concrete powder) The concrete powder used in the present invention is powdered concrete. Concrete is generally produced by adding water to cement, sand, and gravel. From the perspective of contributing to the realization of a recycling-oriented and sustainable society, it is preferable to use powdered concrete rubble. The method for powdering concrete is not particularly limited, but examples include crushing the concrete using a jaw crusher and then pulverizing it using a disc mill or the like until it passes through a 300 μm sieve.

[0021] (wood flour) The wood flour used in the present invention is obtained by powdering wood. The particle size of the wood flour is preferably 100 to 600 μm, more preferably 200 to 500 μm, even more preferably 250 to 450 μm, and particularly preferably 250 to 350 μm. By keeping the particle size at or below the upper limit of the above range, it is possible to prevent the wood flour from being difficult to distribute uniformly in the molded body and to prevent heat from being difficult to penetrate to the interior of the wood flour. By keeping the particle size at or above the lower limit of the above range, it is possible to prevent the water absorption capacity from becoming too large due to the large surface area of the wood flour, and it is possible to reduce the energy consumption required to finely grind the wood flour.

[0022] The type of wood flour is not particularly limited, and examples include cedar, cypress, bamboo, broad-leaved trees, etc. A mixture of these types may also be used. Although bamboo is strictly speaking a grass family and not wood, in this specification bamboo is also referred to as wood.

[0023] The moisture content of the wood flour used in the present invention is preferably 1 to 15%, more preferably 5 to 10%. By keeping the moisture content at or below the upper limit of the above range, adverse effects on the bending strength of the molded body can be suppressed, and by keeping the moisture content at or above the lower limit, the alkaline properties of concrete can be exhibited, and the kraft lignin in the molded body is solubilized, resulting in excellent dispersibility and binding properties, and the resulting molded body has excellent water resistance.

[0024] (vegetable-based concrete) The plant-based concrete of the present invention contains concrete powder and kraft lignin as essential raw materials, and preferably further contains wood flour. The amount of wood flour (bone dry) mixed per 100 parts by weight of concrete powder is preferably 20 to 200 parts by weight, more preferably 50 to 150 parts by weight. When the amount of wood flour mixed is equal to or less than the upper limit of the above range, deterioration of the water resistance of the resulting plant-based concrete can be suppressed, and when the amount is equal to or greater than the lower limit, the resulting plant-based concrete has excellent flexural strength.

[0025] In the plant-based concrete of the present invention, the amount of kraft lignin to be blended is preferably 3 to 30% by weight, more preferably 5 to 25% by weight, and even more preferably 8 to 20% by weight, based on the bone dry weight of the wood flour, from the viewpoint of providing excellent water resistance and mechanical properties such as bending strength of the resulting plant-based concrete.

[0026] (Method of manufacturing plant-based concrete) The method for producing the plant-based concrete of the present invention is not particularly limited, but can be produced, for example, by a method including a mixing step of mixing concrete powder, wood flour, and kraft lignin, and a hot pressing step of hot pressing the mixed powder obtained in the mixing step.

[0027] (Mixing process) In the mixing step, concrete powder, wood powder, and kraft lignin are mixed as raw materials.

[0028] (Hot press process) The mixed powder obtained in the mixing step is molded by hot pressing (compression molding while heating) using a hot press machine to obtain a molded body. The heating temperature in the hot pressing step is preferably 160 to 250°C, more preferably 180 to 230°C, and even more preferably 200 to 220°C, from the viewpoint of excellent dimensional stability and bending strength of the resulting plant-based concrete.

[0029] The compression pressure in the hot pressing step is not particularly limited as long as it is within a range that allows the production of plant-based concrete with the desired properties. Although a higher compression pressure is more effective in improving the water resistance of the plant-based concrete, from a practical standpoint, it is preferable that the compression pressure be 50 MPa or less. The lower limit is preferably 20 MPa or more, and more preferably 30 MPa or more.

[0030] The plant-based concrete of the present invention contains kraft lignin and therefore has excellent water resistance and bending strength. [Example]

[0031] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0032] (material) The maximum particle size of the concrete powder used in the experiment was 300 μm. It was crushed with a jaw crusher and then pulverized in a disc mill until it passed through a 300 μm sieve. The concrete used had a water-cement ratio (w / c) of 0.55, and the detailed composition was 173.8 (kg / m 3 ), cement 316.0 (kg / m 3 ), sand 846.8 (kg / m 3 ), gravel 1083.1 (kg / m 3The moisture content of the concrete was 1.1%, measured by the oven method (105°C for 48 hours). Three types of wood (cedar) powder particles (manufactured by Naka Wood Co., Ltd., Tokushima Prefecture) with maximum sizes of 178, 300, and 500 μm were used in the experiment. These were produced as by-products of the lumber milling process. The moisture contents of the three types of wood particles with sizes <178, <300, and <500 μm were 4.38, 5.16, and 3.08%, respectively. The kraft lignin used in the experiment was produced by the method described in Preparation Example 1 below, dried in an oven at 105°C for two days, and then stored in a Ziploc plastic bag. The relative molecular weight (Mr) of this kraft lignin was 2,000 to 3,000, the glass transition temperature (Tg) was 160 to 190°C, the amount of hydroxyl groups was 4.0 to 6.0 mmol / g, the amount of phenolic hydroxyl groups was 1.0 to 2.0 mmol / g, and the sulfur content was 1.0 to 3.0 wt%.

[0033] (Production Example 1) (Kraft lignin production) Black liquor (solid concentration 16.0%) obtained from kraft cooking in the kraft pulp production at a paper mill was charged into a reactor and heated to 60°C. Carbon dioxide gas was then introduced into the reactor until the pH reached 9.8. The precipitated lignin was dehydrated using a filter press (Lab Pressure Filter VPA 04, manufactured by Metso). The filter cloth was a plain-woven polypropylene P28 (manufactured by Yabuta Sangyo, air permeability 1.0 cm). 3 / cm 2 The obtained lignin was 22.9 kg (7.8 kg in dry equivalent).

[0034] Next, 30.5 kg of water was added to the cake-like lignin precipitate obtained above, and the mixture was suspended in a reaction tank while stirring and adjusting the temperature to 50° C. Next, 2.2 kg of sodium carbonate (special grade reagent, Wako Pure Chemical Industries, Ltd.) was added and dissolved.

[0035] Next, sulfuric acid was added to the suspension (solid content concentration 15%) until the pH reached 2.5. The amount of sulfuric acid added was 4.0 kg. Thereafter, stirring was continued for 1 hour to precipitate lignin.

[0036] The resulting lignin cake was then dehydrated using a filter press in the same manner as above. The dehydrated lignin cake was then washed by passing industrial water (pH 7.2) through the filter press. The washing was continued until the electrical conductivity of the washing filtrate reached 0.5 S / m or less. The electrical conductivity and pH were measured using a portable pH / ORP / electrical conductivity meter (D-74, manufactured by HORIBA).

[0037] Next, the filter chamber of the filter press was pressurized to 7.5 bar, and the mixture was squeezed. Subsequently, air was introduced into the filter chamber to remove as much moisture as possible from the lignin, yielding 9.6 kg of kraft lignin (4.6 kg in dry equivalent).

[0038] (Test specimen preparation and experimental procedures) The BCs in the experiments were manufactured using the hot press shown in Figure 1. All experimental parameters and ranges are listed in Table 1. In the experiments, the moisture content of the wood particles (wood flour) was controlled by adding water (based on the dry weight of the wood flour) to the dried wood flour obtained by drying it in an oven at 105 °C for two days. The proportion of kraft lignin added was based on the dry weight of the wood flour. Two types of samples were prepared: 45 mm x 65 mm (for bending tests) and 50 mm x 50 mm (for water resistance tests). For the tests, well-mixed concrete particles and wood particles (with kraft lignin added in some conditions) were placed in a die and compressed under various molding conditions, as shown in Table 2.

[0039] [Table 1]

[0040] [Table 2]

[0041] The weight ratio of concrete powder to wood flour was 1:1 for all specimens in the experiment. The swelling and water absorption rates were tested according to JIS A 5908-2015 after 24 hours of immersion in water, and the flexural strength of the BC was evaluated as specified in ASTM C 78(2015). After the immersion test, springback was measured after oven drying (105°C) and reconditioning at 20°C and 62% relative humidity, in accordance with the requirements of JIS A 5908-2015. Springback is the irreversible thickness loss of an expanded compact upon rehydration and heating, and is primarily due to relaxation of applied stress and the breakdown of interparticle bonds. Three samples were measured for each molding condition, and the average value was calculated. Some samples (dimensions: 8mm x 8mm x 2mm) were embedded in epoxy resin. Sections (8 mm × 2 mm) were polished with an argon ion beam and observed under a scanning electron microscope (SEM, JSM-7000F, JEOL, Tokyo, Japan) at an accelerating voltage of 5 kV.

[0042] Figure 2 shows the effect of wood particle size (particle size of wood flour) on the swelling rate and water absorption rate of BC formed under condition 1, as shown in Table 2. The trends in the swelling rate and water absorption rate of BC were almost the same, increasing with increasing wood particle size. Furthermore, both of these parameters reached their minimum values when the maximum wood particle size was 300 μm. Figure 3 shows the effect of wood flour moisture content on the swelling rate and water absorption rate of BC formed under condition 2 (see Table 2). The results showed that when the moisture content of wood flour was approximately 5 to 10%, the moisture content of wood flour had a significant positive effect on the swelling rate and water absorption rate of BC. These effects gradually decreased with further increases in moisture content (>10%). Furthermore, when the moisture content of wood flour was less than 5%, there was no significant difference in the swelling rate and water absorption rate of the formed bodies.

[0043] Figure 4 shows the expansion and water absorption of BC compressed at temperatures ranging from 4 to 5, with and without kraft lignin, respectively. Similar to the effect of wood flour moisture content, increasing the compression temperature significantly improved the water resistance of BC and its dimensional stability. Furthermore, in experiments, compacts without kraft lignin were heated at 260°C. At this temperature, cracks appeared in the samples, and visible burn marks were observed on the wood. This was not observed in Figure 4. Using kraft lignin significantly reduced the water absorption and expansion of BC compared to the respective compacts without kraft lignin, as shown in Figure 4.

[0044] Consistent with the results above (Figure 4), the swelling and water absorption of BC compressed under condition 6 (Table 2) generally decreased with increasing kraft lignin content, as shown in Figure 5. However, BC with a higher kraft lignin content (>10%) showed higher swelling than BC with 10% kraft lignin, especially for compacts containing 15% kraft lignin. Density analysis (Figure 6) of all compacts in Figure 5 showed a similar trend to the swelling of BC shown in Figure 5. Increasing the kraft lignin content also significantly improved bending strength (Figure 7). Sample-to-sample variability in bending strength gradually decreased with increasing kraft lignin content.

[0045] Figure 8 shows the swelling rate and water absorption rate of BC compressed under the range of compression pressures in Condition 3 (see Table 2). The effect of compression pressure on the swelling rate of BC first increased and then decreased. This trend was similar to the springback trend of BC (Figure 9). The water absorption rate was almost constant when the compression pressure was less than 40 MPa, but decreased as the compression pressure was further increased.

[0046] Figure 10 shows the colors of the BC compacts pressed under various experimental conditions. Figure 10(a) shows the colors of BC compacts with different particle sizes, Figure 10(b) shows the colors of BC compacts with different wood flour moisture contents, Figure 10(c) shows the colors of BC compacts with different compression pressures, Figure 10(d) shows the colors of BC compacts with different kraft lignin contents, and Figure 10(e) shows the colors of BC compacts with different compression temperatures. Essentially, the colors of all compacts can be distinguished by a dark central region and a light peripheral region, but the dark region and boundary vary depending on the molding conditions.

[0047] Figure 11 shows the SEM images of some of the samples used in the experiment. Figure 11(a) shows the BC compression at 10% moisture content under condition 2 (see Table 2). Figure 11(b) shows the BC compression under condition 6 (see Table 2), with 15% kraft lignin added. Figure 11(c) shows the SEM image of a BC produced under almost the same conditions as the sample shown in Figure 11(b). The only difference is that the concrete powder was replaced with sand of the same particle size (178 μm). The differences between the three figures are significant, especially when considering the pore distribution at the wood-concrete interface.

[0048] (Effect of concrete powder on vegetable-based concrete) The main goal of developing BC is to recycle demolished concrete and small concrete powders that cannot be effectively processed using current concrete recycling methods. In this case, concrete powder is an important component of BC. Furthermore, its natural alkaline properties may be beneficial to both the wood particles and kraft lignin within BC. In the case of wood particles, the alkalinity may promote surface roughness and introduce a large proportion of reactive OH groups into cellulose fibers during hot pressing, resulting in better bonding between the different particles of BC. It has also been found that the thermal stability of wood particles tends to decrease after alkaline heat treatment, and as internal chemicals decompose, the particles become more compressible. Therefore, the alkaline environment can help reduce energy consumption during the manufacturing process. In the case of kraft lignin, the alkaline properties of concrete powder may help kraft lignin dissolve, disperse, and fuse within BC.

[0049] Kraft lignin is only water-soluble in alkaline environments, and adhesives must form molecular-level contact with the surface of the material to form a good bond. This suggests that solubilizing kraft lignin may be a promising method for strengthening the bond compared to powdered lignin, which failed to provide sufficient adhesion to the mat. Research has shown that the pH of waste concrete leachate is higher than 10, providing an alkaline environment for dissolving kraft lignin. In conclusion, the alkaline properties of concrete powder not only help achieve strong lignin dispersion by solubilizing the lignin, but also lead to a significant improvement in bonding ability compared to the original powdered lignin. This is supported by the SEM results shown in Figures 11(b) and 11(c). As can be seen, at the same kraft lignin loading (15%), the porosity of the specimens made with waste concrete powder was significantly lower than that of the specimens made with sand. Furthermore, the wood-concrete interface was narrower in the concrete samples. Furthermore, the alkaline properties of concrete powder can hinder fungal attack, preventing the decay of wood particles within the BC and ensuring its durability.

[0050] (The effect of wood flour in vegetable-based concrete) The results above show high water absorption and expansion rates for BC, similar to those obtained with other wood-based composites. Compacts made from smaller wood particles tend to have better water resistance. This is thought to be because water-resistant materials such as lignin are more easily released from smaller fractured particles. Under the same molding conditions, thinner particles are more easily fractured by compression, which makes it easier for chemicals to be released to the particle surface. Furthermore, finer particles have a larger surface area. Therefore, chemical reactions during hot pressing tend to be more intensive.

[0051] This is further evidenced by the bright color of the surrounding area of the compact, as shown in Figure 10(a). This may be related to the tendency for compacts made with finer wood particles to be more yellow during heating, which may be related to the chemical reaction between lignin and hydrolyzed carbohydrates. The decomposition of carbohydrates, such as hemicellulose, within the wood may significantly improve its water resistance. Therefore, the use of finer wood particles in BC exhibits better water resistance. However, as shown in Figure 2, it should be noted that in tests, compacts made with wood particles with a particle size of 300 μm exhibited better water resistance, albeit less obviously, than compacts made with wood particles with a particle size of 178 μm. This may be due to the increased surface area caused by the fine grinding process, resulting in a higher water absorption rate for BC. Given that finer particle sizes require more energy consumption, the particle size of the wood particles should not be too fine (<300 μm).

[0052] Considering the influence of wood flour moisture content, it was clear that the moisture content of wood flour significantly affected the promotion of water resistance of BC and the dispersibility of kraft lignin within BC. The promotion of water resistance of BC may be due to the intense hydrolysis of chemicals within wood cells under humid conditions. As shown in Figure 10(b), the strong chemical activity inside the compact under high moisture content conditions resulted in a more pronounced color change than compacts with low wood flour moisture content. Furthermore, the color difference between the central and peripheral regions of the other compacts shown in Figure 10 was due to the difference in moisture content between these two regions during the molding process. Although moisture is known to evaporate at temperatures above 100 °C, the minimum molding temperature in the test was 180 °C. Because the female mold was not firmly attached to the plug, moisture escaped through the gaps during the heating process. Clearly, moisture evaporated fastest in the outer parts of the specimens, resulting in a different color. It should be noted that the pressure difference between these two regions may be another reason for the color difference. This point will be discussed later.

[0053] Furthermore, because the alkaline properties of concrete are only apparent under certain humidity conditions, the moisture content of wood flour may provide moisture support for the alkaline environment. The increased dispersibility of kraft lignin was caused by the dissolution of kraft lignin in an alkaline environment. This will be discussed later. Therefore, BC with low wood flour moisture content (<5%) had approximately the same expansion rate and water absorption capacity (Figure 3). Although the moisture content of wood flour improved the water resistance of BC, the moisture content of wood flour must be controlled to a certain level. Once the moisture content reaches a certain level (>10%), the increase in water resistance decreases due to the limitation of the hydrolysis reaction, as shown in Figure 3. Furthermore, previous research has shown that when the moisture content of wood flour in BC exceeds 10%, the increase in moisture content of wood particles adversely affects the flexural strength of the compact. Therefore, the optimal wood flour moisture content for BC is in the range of 5–10%.

[0054] Considering the beneficial effects of wood itself, wood flour significantly contributed to the flexural strength of the specimens and functioned as a fiber reinforcement. Under certain pressure and temperature conditions, wood exhibits properties similar to plastics. As shown in Figure 11, the wood cells inside the wood are compressed, deformed, and fully consolidated with the concrete particles. Furthermore, compared to conventional concrete, incorporating wood into BC can significantly reduce the concrete's density. It should be noted that the wood-to-concrete ratio in this study was 1:1. Therefore, although the effect of the wood-to-concrete ratio was not investigated in this study, it can be inferred that appropriately reducing the wood powder content will improve the water resistance of BC. The wood-to-concrete mixture ratio needs to be determined by the end use of the BC (e.g., structural or nonstructural, interior or exterior material, etc.). Research on BC is still in its early stages, and it is clear that the type of wood, wood particle morphology, and other wood-related parameters affect BC performance, requiring further improvement and optimization in the manufacturing process.

[0055] (Effect of Kraft Lignin on Plant-Based Concrete) The experimental results on the addition of kraft lignin, as described above, showed positive effects on water resistance, dimensional stability (Figure 5), and flexural strength (Figure 7), making kraft lignin a highly beneficial additive to BC due to its low price and easy availability. The positive effect of kraft lignin on BC performance can be easily explained by the properties of kraft lignin as a plastic. The strengthening of interparticle bonds caused by the adhesive properties of kraft lignin, similar to the adhesive properties of plastics at high temperatures, may contribute significantly to the improvement of flexural strength. Furthermore, the SEM results shown in Figures 11(a) and 11(b) (with and without 15% kraft addition, respectively) indicate that kraft lignin significantly reduces the porosity of the samples, which may also help improve flexural strength.

[0056] The water resistance of BC increases with increasing kraft lignin content, due to kraft lignin's similar water resistance properties to plastics. Regarding the expansion rate of BC, kraft lignin, like plastics, can reduce the expansion rate due to its thermosetting properties. However, as shown in Figure 5, excessive kraft lignin (>10%) can adversely affect the expansion rate of the compacts. Compared to the sample with 10% kraft lignin, the relatively high expansion rate may be due to a higher compression ratio. This is indicated by the density of the compacts, as shown in Figure 6. A higher kraft lignin content has a lubricating effect, promoting increased sample density, but also resulting in a higher expansion rate due to interparticle cohesion that does not resist the expansion force. It was estimated that 50% less force was used to overcome friction between particles or between particles and skin during the pellet formation process. The lubricating effect of kraft lignin is expected to significantly reduce energy consumption during the formation process. Furthermore, the time and pressure required for hot pressing are expected to be further reduced.

[0057] Considering the reduced energy consumption of BC and its improved water resistance and bending strength, the use of a higher kraft lignin content (>10%) is suggested. However, as shown in Figure 5, adding a high amount of kraft lignin can increase the expansion rate of the sample. Therefore, additional methods must be developed to further reduce the expansion rate of samples with a high kraft lignin content. To further reduce the expansion rate of samples, attention must be paid to the lignin contained within the wood. Lignin is one of the three major components of wood (cellulose, hemicellulose, and lignin) and acts as an adhesive between polysaccharide fibers, shaping, supporting, and structuring wood. In the case of BC without added kraft lignin, lignin was released from the wood particles under high temperature and pressure, contributing to the bonding and water resistance of BC. This is one of the reasons for the self-adhesion mechanism of BC. However, the low compressibility of the large amount of lignin residue within the wood particles prevented further improvement in the strength and dimensional stability of the specimens.

[0058] When excess kraft lignin was added, the self-adhesive benefits of lignin extruded from wood cells were offset by the kraft lignin, resulting in lignin within the wood cells, which adversely affected the compressibility of the wood cells. Therefore, to further improve the performance of BC, further pretreatment of the wood is necessary to remove the partial lignin within the wood cells. However, lignin removal must be rigorous, as excessive lignin removal may be detrimental to the sample's water absorption. Furthermore, in this experiment, a maximum kraft lignin content (20%) was added to BC. Although no additional tests were performed on compacts with much higher kraft lignin ratios (>20%), the performance of BC with higher lignin ratios (>20%) can be estimated. Based on previous research using waste plastics as an adhesive for compressed plastic concrete, it is speculated that when kraft lignin content is added at a certain high level, the flexural strength of the specimens reaches its peak and then remains stable.

[0059] (Effect of external molding conditions) Aside from the above factors, the effects of compression pressure and temperature have not yet been analyzed. The improvement in the water resistance of BC with increasing compression pressure may be due to its denser structure and the release of lignin on the surface of the wood particles. Wood cells become more susceptible to cracking at higher compression pressures (as shown in Figure 11(a)), allowing water-resistant materials such as lignin to be released into the wood. This was also reflected in the surface color of the compacts. As shown in Figure 10(c), as the specimen pressure increased, the dark areas became larger, likely due to lignin release from the wood particles. Regarding the effect of compression pressure on the expansion rate of BC, the results are quite consistent with those of kraft lignin. The plotted results (shown in Figure 8) indicate that the maximum average expansion rate reached approximately 60% at a compression pressure of 40 MPa, but not at compression pressures of 30 or 50 MPa.

[0060] Previous studies have assumed that the dimensional stability of wood-based composites may be affected by the release of stress accumulated in wood microfibrils or the relaxation of interparticle bonds when the tensile forces generated by expansion are greater than the interparticle bonding forces. Three main reasons that may affect the dimensional stability of wood composites have been proposed: 1) cross-linking reactions between the molecules of matrix components, 2) the release of internal stress between the matrix and microfibers, and 3) the improvement of the water resistance of wood particles. The above explanations suggest that there are different possible reasons for the different values of expansion ratios of compacts compressed under various compression pressures. The larger expansion ratio value of the compact compressed at 40 MPa compared to the other two compacts may be due to a combination of weaker adhesion than the compact compressed at 50 MPa due to the higher proportion of kraft lignin, and a greater internal stress release compared to the compact compressed at 30 MPa, which has less internal stress accumulation.

[0061] As shown in Figure 9, the compact compressed at 40 MPa likely had a higher internal stress release compared to that of the compact compressed at 30 MPa. This was further confirmed by the fact that the compact compressed at 40 MPa had the highest springback value (Figure 11). Furthermore, the lower expansion rate of the compact compressed at 50 MPa compared to that of the compact compressed at 40 MPa indicates that interparticle bonding plays a more important role in reducing the expansion rate of BC. Therefore, more attention should be paid to improving interparticle bonding to ensure the dimensional stability of BC in the future.

[0062] As mentioned above, pressure differences may be one of the reasons for the color difference between the center and periphery of the compact, as shown in Figure 10. The female die was not firmly attached, allowing powder to pass through the gaps. The compression force of the wood particles in the center of the die, compressed by the outer portions of the wood particles, reached a maximum. Due to distortion of the wood particles in the outer portions, the compression force gradually decreased with increasing distance from the center. This difference was expected to increase with increasing pressure, as shown in Figure 10(c). It should be noted that this difference could be partially eliminated by increasing the kraft lignin content, as shown in Figure 10(d). Combined with the detrimental effects of uneven stress and the large expansion rate caused by high compression pressure, the compression pressure needs to be further reduced in future studies.

[0063] Among these variables, compression temperature has the most significant effect on the water resistance and dimensional stability of BC. This is consistent with the results of previous studies related to the thermal treatment of wood. This means that temperature primarily affects the wood particles within BC. Research has shown that hemicellulose, an important component that affects the absorption capacity of wood particles, decomposes at temperatures above 160°C. In our experiments, all compression temperatures were higher than 160°C. As shown in Figure 10(e), the color of the samples darkened with increasing temperature.

[0064] As the temperature increases, the chemical reaction accelerates and the water absorption rate decreases. Furthermore, for kraft lignin, 160°C is the initial glass transition temperature, as shown in Table 2. Furthermore, wood is a composite of polymers (lignin, cellulose, and hemicellulose), which exhibits thermosetting properties when heated. As a result, all of these properties improve the performance of BC. However, excessive heating reduces the performance of BC due to carbonization of wood particles. It is important to note that the compression temperature must be maintained at a certain level, and tests have recommended a temperature of 220°C.

[0065] As shown in this study, the various factors investigated can improve the water resistance and dimensional stability of BC. However, the water absorption rate of bricks must be less than 19%. For particleboards, the water absorption rate must be between 5 and 13%. The expansion rate of load-bearing particleboards must be controlled to remain below 14%. However, when BC was exposed to water, adding kraft lignin to the compact did not fully achieve the expected results (e.g., the expansion rate of BC was less than 14%). This indicates that the heat treatment of wood during the hot-pressing process alone was not sufficient to protect BC from water exposure.

[0066] Although it was stated that the water resistance of the specimens could be further improved by reducing the wood flour content, such a method is not optimal because it results in a loss of strength in the specimens. To reduce the high expansion of BC and maintain its dimensional stability, further treatment, such as pre-treating the wood particles before compression, is necessary. It is expected that the large variability in the performance of BC caused by the inconsistent properties of the wood particles may be further reduced after pre-treatment of the wood particles.

[0067] This study investigated the effect of various molding conditions on the properties of BC and the innovative use of kraft lignin in the production of BC to further improve its performance. Kraft lignin combined with waste concrete within BC was found to aid in the dissolution of the kraft lignin due to the alkaline properties of concrete. Furthermore, this allows for the addition of higher percentages of kraft lignin, thereby providing new insights into the utilization of kraft lignin and its added value as a renewable resource. The main results of the experiment are as follows:

[0068] 1. The smaller the particle size of wood particles used in BC, the better their water resistance tends to be. However, if the particles are too fine, the surface area increases and the water absorption capacity increases. Within the scope of this study, wood particles of 300 μm are strongly recommended.

[0069] 2. For the synthesis of BC, the use of wood flour with a moisture content of 5–10% is suggested, which not only provides a moist environment for wood hydrolysis but may also facilitate the dissolution of kraft lignin in an alkaline environment.

[0070] 3. High compaction pressure was beneficial in improving the water resistance of BC. However, it may not be effective in expanding, especially because the accumulated internal stress was released and the bonding between particles was insufficient. The compaction pressure used in the study needs to be further reduced.

[0071] 4. High kraft lignin content is beneficial for improving the dimensional stability and bending strength of BC. However, it may slow the flow of lignin to the wood surface and reduce its compressibility. Further investigation is needed to determine the optimal kraft lignin content for various products, and more attention should be paid to improving the adhesive properties of kraft lignin to improve the performance of BC in the future.

[0072] 5. As a renewable material, BC can potentially be used in humid environments. To achieve better water resistance, it is necessary to pretreat the wood particles before compression.

[0073] The improved water resistance of the BC obtained by the present invention further broadens its applications, allowing it to be used in humid and outdoor environments. The BC of the present invention has high flexural strength, is quick to form, and is an environmentally friendly and sustainable material. Because the water resistance of the BC has been further improved by the present invention, the BC of the present invention can be used as construction materials such as bricks and blocks for non-structural parts of buildings, both inside and outside. The BC of the present invention can also be incorporated with other materials as reinforcement and used as a prefabricated component as part of the structural framework of a building.

Claims

1. The plant-based concrete contains concrete powder, wood powder, and kraft lignin.

2. 2. The vegetable-based concrete according to claim 1, wherein the particle size of the wood flour is 100 to 500 μm.

3. 3. The vegetable-based concrete according to claim 1, wherein the wood flour has a moisture content of 1 to 15%.

4. The vegetable concrete according to any one of claims 1 to 3, wherein the amount of kraft lignin mixed is 3 to 30% by weight based on the bone dry weight of the wood flour.

5. A mixing step of mixing concrete powder, wood flour, and kraft lignin; a hot pressing step of hot pressing the mixed powder obtained in the mixing step; A method for producing plant-based concrete, comprising:

6. The method for producing vegetable-based concrete according to claim 5, wherein the heating temperature in the hot pressing step is 160 to 250°C.

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