The raw materials used for the experiment in this study include recycled concrete powder, cellulose, lignin, hemicellulose, chitosan, and oyster shell, gathered from multiple sources. Figures 1(a)-(g) shows photographs of the materials used in the experiment.

The mix proportion of the waste concrete used for production is presented in Table 1 .

To obtain a dried mass of the concrete powder, the powder was dried in an oven at 120˚C for 24 hours. Then, the dried concrete was kept in a zip-locked plastic bag until usage for the green concrete production.

For wood powder, Japanese cedar sawdust was utilized, which was dried in an oven at 96˚C for 24 hours in the laboratory and then sieved to obtain a powder with particles less than 0.6 mm. To unveil the individual impact of wood major components, hemicellulose, cellulose, and lignin were used. Oyster shell, a byproduct of oyster, and chitosan, a biopolymer derived from chitin found in the exoskeletons of crustaceans such as shrimp and crabs, were also used in the experiment.

All the materials used in the experiment were zip-locked and stored before the experiment.

The samples were prepared through press molding using a hot-pressing machine. The pressing apparatus could simultaneously heat and apply pressure to the specimens from both sides attached to pressing discs.

The bending strength of the samples were measured through three-point loading.

The production of the green concrete was conducted using the hot-pressing technique of the Botanical Concrete [4] . All the experimental parameters and ranges are shown in Table 2 . As shown in the table, the experiment cases are divided into three groups, each with several cases. Each case used a different material as a binder and was pressed at three different temperature degrees. The first group used binders such as wood, cellulose, hemicellulose, lignin, chitosan, and oyster shell, individually with the recycled concrete powder.

RCP: recycled concrete powder, W: waste wood powder, L: lignin, C: cellulose, H: hemicellulose, Ch: chitosan, O, oyster shell, H: pressing heat, P: pressing pressure, t: pressing time.

The moisture content of the recycled concrete powder was controlled by adding water based on the dry weight of the powder, which was acquired by drying in an oven at 120˚C for 24 hours. Furthermore, the proportion of the concrete powder and the binder was based on the dried weight of the concrete and the dry weight of the targeted binder. The weights were measured using an electrical scale with 0.1g precision. The mix proportion was maintained at 2:1 (concrete powder:binder) along all the cases. For mixture, the concrete and water, with a quantity of 12.5% of the dried mass of the concrete, were mixed for 1 minute to obtain concrete with 12.5% moisture content. Then, the moist concrete powder and individual binder were mixed for another 1 minute. The mix proportion and mixing method was determined as the most effective after several trials. After that, the mix was placed in a mold and pressed at 25 MPa for 10 minutes. The experiment was conducted at three different temperatures: cold press (24˚C), warm press (60˚C), and hot press (160˚C) individually, to unveil the effect of temperature.

In the second and third groups, all the weighing and mixing procedures of the first group were followed. However, it is worth mentioning that, since these groups used two types of binders, first, the dry mass of both binders was mixed and then added to the moist concrete powder at once and mixed for 1 minute manually. The rest of the test followed the first group experimental procedure. Figure 2 illustrates the concrete molding procedure.

Upon the end of pressing, the specimens were demolded, and the curing period started. Although the specimens were strong enough right after demolding, to obtain further strength, all the specimens were kept at room temperature (24˚C) for a 72-hour curing period prior to the bending strength test.

After 72 hours of curing, all the specimens’ dimensions were measured and prepared for the bending test. The bending strength of the hardened body was evaluated by a three-point bending test with a 40 mm support distance. Additionally, a cement mortar specimen was made and cured for 72 hours to compare its bending strength with that of the green concrete. The dimensions of the green concrete specimens are 50 mm × 50 mm × 5 mm, and as a sample, the photographs of the specimens pressed at 60˚C are shown in Figures 3(a)-(e) .

Figure 4 shows the bending strength of the green concrete made of recycled concrete powder and different bio-based additions, such as wood, hemicellulose, lignin, cellulose, chitosan, and oyster shell, which were measured after 72 hours of curing. As can be seen in the figure, the green concrete bending strength varies based on different binder materials. Additionally, the strength of each concrete with the same mixture and materials differs when exposed to different pressing temperatures. Since this study’s experimental method and the green concrete hardening method follow the binderless particle boards production and solidification mechanism, in which pressing temperature is one of the most important manufacturing parameters to create internal bonds and increase bending strength [21] , the concrete in case RCP-W pressed at room temperature did not harden, and therefore its strength was not measurable. A higher pressing temperature of 60˚C formed a hardened body with a 0.5 MPa bending strength, indicating the bonds created by the partial activation of lignin and hemicellulose because wood lignin and hemicellulose soften in a wide range of temperatures from 20˚C to 180˚C based on wet and dry conditions and different native wood [22] - [25] . At 160˚C press, a stronger hardened body with 2.7 MPa was obtained because at such a high temperature both the lignin and hemicellulose soften even in dry conditions [25] , thus increasing the strength.

As shown in Figure 4 , the RCP-L solidified at all the pressing temperatures. At room temperature and 60˚C, it showed a strength higher than that of the RCP-W, but at 160˚C, its bending strength decreased due to a slight delamination phenomenon. However, the RCP-L bending strength is similar at all the pressing temperatures, 2.4 MPa, 2.3 MPa, and 1.9 MPa, respectively. The lower bending strength and delamination at a high temperature of 160˚C could be linked to the evaporation that may have occurred so quickly that the lignin dried before flowing and developing the strength [9] . It may indicate that the most strength of the RCP-W enhanced at high temperature is not due to lignin. Furthermore, lignin needs more time to soften than hemicellulose [9] , and therefore at 10 minutes pressing, it could be inferred that mostly it was hemicellulose which softened and thus provided strength to the concrete in case RCP-W.

The green concrete containing cellulose (RCP-C) hardened at all the pressing temperatures. As demonstrated in Figure 4 , at room temperature (24˚C), the bending strength is 1.8 MPa and further increased to 6 MPa when molded at 60˚C. However, further temperature elevation to 160˚C led to delamination and a reduction in strength to 1.9 MPa, comparable to the strength achieved at the room temperature pressing. In the same way, the concrete made of hemicellulose and the waste concrete (RCP-H) successfully hardened at all the pressing temperatures. As shown in Figure 4 , the concrete pressed at room temperature exhibits 6.8 MPa, the highest bending strength among others, even higher than that of conventional cement mortar. It is approximately three times more than the bending strength of the RCP-L, because the bond strength provided by hemicellulose is stronger than that of lignin [9] . The bonding strength of the RCP-H increased as the temperature increased, and therefore the specimen molded at 60˚C showed 8.4 MPa bending strength, indicating a sudden increase in bending strength at this temperature. However, at 160˚C, the mix readily flowed during the hot press so that it was difficult to maintain it under 25 MPa pressure. At the end, the sudden downloading, neglecting a cooling period, caused severe delamination and as a result, the strength dropped to 1.4 MPa. It is very interesting to obtain high strength at lower temperatures to not only decrease environmental impacts but also relieve economic and manufacturing challenges.

To unveil the synergistic effect of the binders from wood components, in the case of RCP-L-C-H, an equal combination of hemicellulose, lignin, cellulose nanofiber, and waste concrete was used. As shown in Figure 4 , the concrete hardened at all the pressing temperatures, but the highest strength was achieved at room temperature and 60˚C, respectively. Similar to the RCP-H and RCP-C cases, its bending strength increased from 3.1 MPa at room temperature to 7.2 MPa at 60˚C, but at 160˚C, delamination occurred, and the strength dropped to 2.1 MPa. It follows the bending strength pattern observed in the RCP-H case, which may imply the integration of hemicellulose with the other binders and its significant role in enhancing the concrete strength.

The green concrete containing chitosan and waste concrete (RCP-Ch) had a bending strength of 0.7 MPa at room temperature pressing, but it increased to 3.3 MPa at 60˚C, representing a significant enhancement. However, pressing at 160˚C led to concrete delamination, thus reducing the bending strength to 2.8 MPa.

The oyster shell and waste concrete (RCP-O) hardened at room temperature and 60˚C, but at 160˚C, severe delamination occurred, rendering its bending strength unmeasurable. As shown in Figure 4 , at room temperature pressing, its bending strength was 0.9 MPa, slightly increasing to 1.1 MPa when the pressing temperature was raised to 60˚C.

Figures 5(a)-(e) depict the bending strength of the green concrete, incorporating chitosan and oyster shells as substitutes. As demonstrated in Figure 5(a) , when 50% of the wood was replaced with chitosan in case RCP-W-Ch, the bending strength enhanced with a similar trend at all the pressing temperatures, indicating the creation of more and stronger cellulose-chitosan and hemicellulose-chitosan bonds. This enhancement aligns with previous reports indicating that chitosan improves the strength of wood-pulp webs [26] [27] .

Oyster shells mainly consist of CaCO₃ [28] , with a small amount of chitin [29] . Therefore, the effect of oyster shell powder addition could be expected to differ from that of sole chitosan. As shown in Figure 5(a) , in case RCP-W-O, when oyster shell was substituted, a decline in strength was observed against all pressing temperatures with a similar trend, which may indicate the formation of fewer bonds due to the limited amount of chitin and chitosan present in the oyster shell.

Figure 5(b) demonstrates that the substitution of 50% lignin with chitosan in case RCP-L-Ch considerably improved the bending strength only at room temperature, indicating interactions due to hydrogen bonds between chitosan and lignin [30] . However, the strength was negatively affected at 60˚C and 160˚C. The highest strength was achieved at room temperature, 3.6 MPa. However, at 60˚C the strength reduced to 1.8 MPa, and at 160˚C the specimen slightly delaminated and thus the strength further reduced to 1.6 MPa. However, oyster shell replacement with 50% of the lignin mass in case RCP-L-O improved the strength of the concrete only at 60˚C press. At room temperature, it was 2.2 MPa and increased to 3.2 MPa at 60˚C, but at 160˚C due to delamination, the strength decreased to 1.7 MPa.

Substitution of cellulose with chitosan in case RCP-C-Ch slightly improved the concrete strength at room temperature and 160˚C but adversely affected at 60˚C pressing temperature, as illustrated in Figure 5(c) . This improvement indicates cellulose-chitosan interactions, which enhance mechanical strength . In addition, the strength development could be related to the particle size of the cellulose, as it has been reported to increase specific surface area, ester and hydrogen bonds, and increase cross-linking degree between lignocellulose and chitosan . However, replacement of cellulose with oyster shell in case RCP-C-O reduced the concrete strength across all pressing temperatures, same as the results of wood and lignin substitution with the oyster shell.

Figure 5(d) shows that in both room temperature and 60˚C pressing, in case RCP-H-Ch, the bending strength of the concrete increased when 50% hemicellulose was replaced with chitosan, indicating the formation of covalent imine bonds , and thus yielding a higher strength compared to sole hemicellulose . However, neither the strength nor the delamination changed considerably at 160˚C. At room temperature, the concrete strength reached 7.5 MPa while it was 6.8 MPa in case RCP-H, when pure hemicellulose and the concrete powder were used. In the same way, at 60˚C, the bending strength reached 9.3 MPa while it was 8.4 MPa in case RCP-H, indicating a significant strength enhancement at 60˚C.

As with the other materials, in case of RCP-H-O, the substitution of oyster shell with hemicellulose slightly decreased the concrete bending strength across all pressing temperatures, indicating a negative effect of oyster shell replacement in terms of bending strength. However, further studies are needed to understand the wider impact of oyster shell on green concrete properties.

Figure 5(e) shows the bending strength of the green concretes in cases RCP-L-C-H-Ch and RCP-L-C-H-O, which used waste concrete and a combination of hemicellulose, cellulose, and lignin as 50% of the binder mixture, and 50% chitosan in one group and in another group 50% replacement with oyster shell, respectively. The figure shows that both chitosan and oyster shell substitutions enhanced the bending strength at room temperature molding but had a negative impact at 60˚C and 160˚C. Interestingly, oyster shell addition yielded a higher concrete strength than chitosan in this case, indicating an inverse result in terms of combined binder partial substitution compared to single binder partial substitution. However, delamination still occurred at 160˚C.

Among all the cases, those containing hemicellulose showed the highest bending strength, even higher than conventional cement mortar and the RCP-W case. This is because in case of raw wood approximately 60% of wood is lignin and hemicellulose [34] [35] , which may not be evenly heated during hot pressing due to wood’s low thermal conductivity, potentially limiting their softening and bonding potential. Therefore, the strength of the RCP-W is lower than that of the hemicellulose-based concrete. Additionally, the bonding strength of hemicellulose is more than two times higher than that of lignin, contributing to its superior bending strength [9] , and therefore it yielded the highest bending strength among the other components of wood such as lignin and cellulose.

Producing hemicellulose-based concrete with high bending strength does not require heat, high pressure, or extended time, and it is producible at room temperature, showing sufficient strength right after demolding, though further strength increase requires curing.

Moreover, hemicellulose, abundantly available around the world, is currently underutilized for construction material production. If utilized, it can contribute to reducing a significant amount of biomass waste, including lignin and hemicellulose in black liquor from the pulp industry, currently incinerated for energy production, thus further emitting CO₂.

Furthermore, for marine biomass utilization, the study showed promise in using chitosan and oyster shells in green concrete. These environmentally friendly materials can be used without adverse effects. When used alone, considerable strength was not attained, but when combined with botanical components, comparable or even superior results were achieved.

Hemicellulose-based concrete shows promise for developing green concrete, not only reusing demolished concrete and its fine powder generated during concrete recycling but also effectively utilizing abundant biomass waste to contribute to resource conservation, waste management and recycling, and climate change mitigation.

It is worth mentioning that a comprehensive study of the concrete mechanical properties, durability, chemical interaction, microstructural changes, and its scalability and economic viability are still elusive and need further investigations.