Cuttlebone-inspired magnesium oxychloride cement reinforced by biochar as green adhesive for wood industry
Graphical abstract
Introduction
The global wood adhesive market is expected to reach $6.18 billion by 2025 (Liu et al., 2019), in which more than 90% are still formaldehyde-based and petroleum-derived adhesives, which continuously release hazardous formaldehyde and volatile organic compounds during the production and application, threatening the ecological environment and human health (Xu et al., 2021). In addition, these wood composites are highly combustible and their wide applications in indoor building materials are exacerbating fire safety problems. Consequently, it is urgent to develop an eco-friendly and flame retardant inorganic wood adhesive to replace the hazardous formaldehyde-based adhesives.
Magnesium oxychloride cement (MOC), composed of MgO–MgCl2–H2O ternary system, presents diverse properties superior to Portland cement (PC), such as rapid hardening rate, high mechanical strength, low carbon footprint, better durability and good abrasion resistance (Gu et al., 2021; Q. Huang et al., 2021; Li et al., 2021). Besides, the low alkalinity (pH 8–9.5) of MOC affords better compatibility with wood and plant fibers; thus, alleviating the retarding effect of wood extractives on hydration process (He et al., 2019, 2020; J. Wang et al., 2021; Zhou and Li, 2012). In addition, the development of MOC exhibits two obvious environmental benefits: 1) MOC posses high solidification/stabilization and carbon dioxide (CO2) sequestration potentials, so its applications can mitigate the environmental problems of high CO2 emissions and high energy consumption caused by the traditional cement industry (Gong et al., 2021; Tang et al., 2019); 2) the utilization of magnesium chloride (MgCl2), the primary by-product or waste of the potassium fertilizer industry, is conducive to the recycling of waste magnesium resources and the mitigation of environmental hazards (Han et al., 2020a; Tang et al., 2019).
While, MOC often suffers from poor water-resistance, which severely limits its potential for practical applications because the phase 5 (5Mg(OH)2·MgCl2·8H2O), the primary mechanical strength provider in MOC, can easily convert into Mg(OH)2 under water. Numerous strategies have focused on improving the water stability of MOC by introducing various modifiers, such as acids (-COOH and PO43−), acid salts (-COO-, HPO42−, H2PO4− and SO42−) and active SiO2 (pulverized fuel ash, silica fume and sewage sludge ash) (L. Chen et al., 2019; He et al., 2017b; K. Li et al., 2020; Li et al., 2016; Luo et al., 2020). However, acids and acid salts can induce the formation of microcracks and micropores during the hydration process, and active SiO2 can compete with phase 5 for Mg2+, which therefore usually deliver a destructive effect on the mechanical strength of MOC (Han et al., 2020b; He et al., 2017a; Huang et al., 2019, 2021; X. Huang et al., 2021; Wang et al., 2019; Ye et al., 2021b). In addition, the naturally porous structure of the wood will have a filtering effect on MOC, making the water-soluble MgCl2 more easily to penetrate into the pores of the wood than the insoluble MgO particles; thus, destroying the reasonable composition ratio of MOC at the bonding interface (Fig. 1) (Zhou et al., 2021). This result will hinder the generation of phase 5 at the bonding interface and ultimately affect the adhesion strength of MOC/wood composites. Therefore, it still remains a challenge to design MOC adhesive simultaneously engaging improved water resistance, compressive strength and adhesive strength.
Cuttlefish can sustain water pressure of about 20 atm in the deep-sea environment (100–400 m) (Mao et al., 2021). The excellent mechanical efficiency mainly profited from its unique chambered porous bone structure which is composed of inorganic aragonite and organic component (chitin and protein). Thereinto, the aragonite crystallites serve as a rigid skeleton to support organisms, avoid stress concentration and withstand multiple loadings (hydrostatic pressure and shear stress) during movement; thus, providing excellent strength, toughness and damage tolerance for cuttlefish (Ca Dman et al., 2013; Mao et al., 2021; Xu et al., 2020; H. Yang et al., 2020). The organic content can act as the mineralization template to facilitate the mineralization process and improve the damage resistance (Mao et al., 2021). Inspired by this exquisite design of nature, the construction of cuttlebone-inspired porous structure is expected to enhance the performance of MOC.
Biochar is a carbon-rich product with naturally porous structure formed via the pyrolysis of biomass (such as waste straw, wood bark, wood husks, and livestock and poultry manure) under anaerobic conditions (Ahmad et al., 2020; Pan et al., 2021). Recent studies have found that the incorporation of biochar could promote the hydration process and improve the mechanical properties of cement due to its internal curing effect and high water retention capacity (Chen et al., 2019, 2022a; Gupta and Kua, 2019; Wang et al., 2019). For example, Wang et al. reported that the addition of 1 wt% biochar increased the compressive strength of cement by 8.9% (Wang et al., 2020a, Wang et al., 2020b). Gupta et al. found that the addition of biochar also contributed to reducing the shrinkage and improving the properties of silica fume-cement (Gupta et al., 2020). In addition, Chen et al. also confirmed that the addition of biochar can achieve the compressive strength of concrete to 10.3 MPa, meeting the strength requirement of the partition blocks (7 MPa). More importantly, the life cycle assessment suggested that the biochar blocks with 30 wt% biochar and supplementary cementitious materials could sequester 59–65 kg CO2 tonne−1 and generate the overall profit of 35.4 USD m−3 (Chen et al., 2022b). Therefore, biochar is potentially a promising, clean and sustainable additive in improving the properties of MOC and MOC/wood composites.
The porous structure, composed of amorphous calcite and silicon, not only serves as the stress dissipation unit to withstand loadings and avoid stress concentration but also can form a protective core-shell structure and encapsulation with hydration products to improve the stability of phase 5 and alleviate the filtering effect of wood; thus, endowing the composites with excellent mechanical property, water resistance and adhesive strength. In addition, the abundant active functional groups (e.g., carboxyl, phenolic and hydroxyl groups) in biochar can serve as the mineralization template to undergo multiple interactions (physic absorption, electrostatic attraction and surface complexation) with Mg2+ in MOC, which can provide more nucleation sites for the generation of phase 5 and form an internal network with excellent cohesion strength in MOC (Li et al., 2017; Zhao et al., 2020). More importantly, the efficient utilization of this value-added biochar can avoid the environmental hazards from the incineration of waste straw and promote the sustainable development of MOC (Wang et al., 2020a, Wang et al., 2020b; T. Yang et al., 2020).
Herein, inspired by the porous structure of cuttlebone, an eco-friendly MOC-based adhesive with integrated water-resistant, compressive strength and adhesive strength was developed via introducing sustainable biochar with naturally porous structure and multiple active functional groups. Our strategy exploits the elegant collaboration of macroscopic scale structural design and molecular level multiple interactions. The resultant MOC/Biochar composites are envisioned as a promising candidate in the applications of construction, dams and fences, of particular interest, they could be used as eco-friendly and flame retardant wood adhesive in the wood manufacturing industry to replace the hazardous formaldehyde-based adhesives.
Section snippets
Materials
MgO with 64% activity was obtained from Guangzhou Danlin Trade Co., Ltd. (Guangzhou, China). Magnesium chloride hexahydrate (MgCl2·6H2O, 98% purity) was supplied by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Biochar was provided by Nanjing Zhironglian Technology Co., Ltd. (Jiangsu, China). Basswood (4 mm) was obtained from Baolelai Model Company (Jiangsu, China).
Production of biochar
Biochar was prepared under anaerobic conditions through a patented slow-pyrolysis process (China patent No.
Characterization of biochar
The water absorption rate of biochar was 113.28%, suggesting that it can be applied as internal curing materials due to its remarkable water absorption capacity (Chen et al., 2022a). The particle size distribution of biochar was presented in Fig. S4. The overall size of biochar ranged from 0.96 to 417 μm, with d10, d50 and d90 of 2.90, 10.42 and 79.08 μm, respectively (Gupta and Kua, 2019). The structure of biochar was confirmed by FTIR spectroscopy (Fig. 3a). The band at 3200–3500 cm−1 was
Conclusions
In summary, inspired by the porous structure of cuttlebone, an eco-friendly and flame-retardant MOC wood adhesive with integrated water resistance, compressive strength and adhesion strength was constructed via introducing biochar, which can effectively replace the hazardous formaldehyde-based adhesives. The naturally porous structure of biochar can form a protective core-shell structure with hydration products, which not only contribute to improving the stability of phase 5 and facilitating
CRediT authorship contribution statement
Yufei Han: Conceptualization, Data curation, Investigation, Roles/Writing - original draft. Yantao Xu: Methodology, Visualization. Sheldon Q. Shi: Software, Writing – review & editing. Jianzhang Li: Funding acquisition, Project administration, Resources, Supervision, Validation. Zhen Fang: Formal analysis, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 32071701), the National Key Research & Development Program of China (No. 2017YFD0601205) and Beijing Forestry University Outstanding Young Talent Cultivation Project (No. 2019JQ03004).
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