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Abstract: Gypsum board possesses excellent fire resistance and is primarily used as a fire-resistant cladding for floors and walls in lightweight wood structures. By incorporating plant fibers such as wood shavings into a gypsum matrix to prepare gypsum particleboard, its mechanical properties can be improved without altering the original manufacturing process. The gypsum raw material for producing gypsum particleboard mainly comes from industrial gypsum, while the wood shavings are primarily agricultural or industrial byproducts, offering advantages such as waste recycling and controllable material costs, thus possessing broad application prospects. This paper reviews the mechanisms of action of different retarders in gypsum particleboard, the reinforcing effects of different plant fiber types, post-treatment methods, and the mechanisms by which admixtures improve board performance, as well as the current research status on the physical, mechanical, and fire-resistant properties of gypsum particleboard. Finally, it points out the current shortcomings of gypsum particleboard for civil engineering and directions for further research, providing a reference for further systematic research and engineering applications of gypsum particleboard.
Keywords: gypsum particleboard, preparation process, fire resistance, mechanical properties
Authors: Li Mengyu, Yue Kong, Liu Jian, Liu Weiqing, Song Yongming, Tang Lijuan, Lü Chenglong
Paper-faced gypsum board uses building gypsum as the main raw material, with appropriate additives added. After being mixed with water, it is poured between the facing paper and firmly bonded to the paper. Its strength is mainly provided by the facing paper. Gypsum board is widely used as a cladding panel for wood-framed walls and floors, primarily for fireproofing and decoration. Due to its relatively poor mechanical properties, gypsum board is prone to localized damage during use, such as shear tearing, corner crushing, splice detachment, and compression damage. In contrast, gypsum particleboard, which has better mechanical properties, is an inorganic plywood composed of wood shavings and gypsum, with wood shavings acting as reinforcement and gypsum as an adhesive. Compared to other wood-based panels (oriented strand board, medium-density fiberboard, laminate, etc.), gypsum particleboard has advantages such as environmental friendliness, high fire resistance, and good sound insulation. It can also be machined by sawing, grinding, and polishing. In the preparation of gypsum particleboard, the wood units do not need to be dried, and the board blanks are pressed at room temperature, resulting in low production energy consumption. Furthermore, the gypsum particleboard produced is mostly made from industrial waste gypsum, a byproduct of many chemical processes. In terms of particle raw materials, besides traditional wood, bamboo, flax stalks, wheat straw, and sugarcane bagasse can also be used. This not only achieves full utilization of resources but also produces boards that meet specific requirements, and is more conducive to environmental protection. Therefore, scholars both domestically and internationally have conducted extensive research on the preparation, microscopic characterization, and performance analysis of gypsum particleboard.
In gypsum particleboard, the presence of fibers compensates for the inherent brittleness of gypsum, resulting in excellent mechanical properties, particularly a high modulus of elasticity. On one hand, improved mechanical properties contribute to better physical properties, especially under otherwise constant conditions, where increased internal bond strength leads to a decrease in the water absorption thickness expansion rate. On the other hand, gypsum itself possesses excellent fire resistance, thus gypsum particleboard exhibits good fire resistance. Domestic and international scholars have conducted a series of studies on factors influencing the performance of gypsum particleboard, primarily including fiber type and morphology, component ratios, post-treatment processes, and composite additives. Among these, fiber morphology can utilize not only traditional flat particleboard but also wood chips. The proportions of each component vary depending on the fiber type; when wood is used as reinforcing fiber, with a wood-to-gypsum ratio of 0.3–0.4 and a water-to-gypsum ratio of 0.2–0.3, the prepared gypsum particleboard exhibits good physical and mechanical properties. Furthermore, heat treatment and the addition of silicate cement composite additives are effective measures to improve the physical and mechanical properties of gypsum particleboard.
1. Gypsum Particleboard Preparation Process
The following is a review of the research on the production and preparation process of gypsum particleboard, focusing on the mechanisms of action of different retarders, the reinforcing effects of different plant fiber types, the influence of post-treatment methods, and the mechanisms by which admixtures improve the performance of the board.
1.1 Mechanism of action of retarder
Gypsum typically sets and hardens very quickly, failing to provide sufficient time for industrial production. Therefore, composite retarders are often added to slow down the setting speed. Retarders mainly fall into four categories: organic acids, phosphates, proteins, and compound systems. Different retarders have different mechanisms of action. Generally speaking, bone glue chemically adsorbs onto the surface of newly formed dihydrate gypsum crystal nuclei, covering the nuclei and reducing their surface energy. This inhibits crystal growth and prolongs the hydration and setting time of the gypsum. Citric acid adsorbs onto the surface of hemihydrate gypsum particles, hindering their dissolution, or adsorbs onto newly formed dihydrate gypsum crystal nuclei, extending the time it takes for the nuclei to reach the critical nucleation size, thus prolonging the setting time. Sodium hexametaphosphate ions form insoluble compounds with Ca2+ on the surface of hemihydrate gypsum crystals, reducing the dissolution rate of hemihydrate gypsum and slowing its hydration process, thereby prolonging the setting time.
Marcos Lanzón et al. studied the retarding effect of different concentrations of citric acid using a constant water-to-gypsum ratio. The results showed that when the citric acid concentration was between 500 and 1000 ppm, the required setting time could be achieved. When the citric acid concentration was higher than 1500 ppm, the setting time of gypsum did not change significantly. However, the addition of citric acid had a negative impact on the strength of gypsum, especially when the dosage was higher than 1000 ppm. This was mainly because citric acid reduced the interlocking effect between the microstructures of gypsum crystals. Overall, the addition of the retarder reduced the supersaturation of the liquid phase in the early stage of cementitious material hydration, altered the crystallization habit and morphology of dihydrate gypsum, significantly coarsened the gypsum crystals, and changed the crystal shape from needle-like to short columnar, greatly weakening the overlap between crystals. The pore size of the hardened body increased, the proportion of macropores increased significantly, the pore structure deteriorated, and ultimately, the strength of the gypsum decreased. The strength loss was basically proportional to its retarding effect; the higher the dosage and the longer the retarding time, the greater the strength loss. Studies have shown that acidic retarders are superior to alkaline retarders in the production of gypsum particleboard, and have less impact on its performance. Using appropriate amounts of trisodium citrate or citric acid, gypsum particleboard can achieve better performance, and the initial setting time of gypsum can be extended to 2 hours. When alkaline retarders are added, the initial setting time of gypsum is shorter, and the mechanical properties of the gypsum particleboard, such as internal bond strength, modulus of elasticity, and static bending strength, significantly decrease. Since gypsum curing occurs under weakly acidic conditions, strong alkalis reduce the bond strength between gypsum and particleboard. However, strong acids also affect the bond strength of gypsum and reduce the strength of particleboard. Thermal analysis indicates that gypsum curing is an endothermic reaction; high heat absorption allows gypsum to cure better, resulting in higher bond strength.
1.2 Reinforcing effect of composite fibers
Dasong Dai et al. studied the effects of wood chemical composition on the properties of gypsum particleboard. The results showed that tannins, acetic acid, hemicellulose, and lignin had no adverse effects on the mechanical properties of gypsum particleboard. However, the mechanical properties of gypsum particleboard were significantly improved after the addition of a defoamer. The low strength of gypsum particleboard was not directly related to the chemical composition of the wood, but rather to the high water absorption rate of the wood particles. Gypsum particleboard prepared by modifying wood shavings with water-based epoxy resin exhibited higher mechanical properties because the water-based epoxy resin had better adhesion, improving the bonding between gypsum and wood shavings and reducing the water absorption rate of the wood shavings.
Morteza Nazerian et al. prepared gypsum boards using sugarcane bagasse and wheat straw. Their results showed that, at a constant wood-to-gypsum ratio, a higher proportion of wheat straw resulted in lower water absorption of the boards, primarily due to the inherently low water absorption of wheat straw. In addition, density was also a significant factor affecting the water absorption rate; lower density led to higher water absorption. Under the same wood-to-gypsum ratio, a higher amount of wheat straw resulted in a higher thickness expansion rate. This is because the epidermal cells of wheat straw are the outermost surface cells and are covered by a hydrophobic wax layer. This layer reduces the wettability of the straw with water, thus preventing the gypsum paste from penetrating into the cell walls. Furthermore, the lack of good mechanical interlocking between the mineral matrix and particles is a major cause of the additional loss of internal bond strength, thereby affecting the thickness expansion rate of the boards. The study also found that a lower wood-to-gypsum ratio leads to a decrease in static bending strength and modulus of elasticity, which is caused by the high brittleness and low modulus of elasticity of gypsum. Pan Shuqing et al. manufactured gypsum particleboard using gypsum and bagasse as the main raw materials. Tests showed that the board has high strength, fire resistance, heat insulation, sound absorption and breathability. Its surface is white and does not affect subsequent surface spraying and veneering, so it can be used as a decoration material.
Zhang Xianquan et al. conducted experimental research on the effects of raw material form, material-to-solid ratio, amount of white glue added, and raw material pretreatment method on the properties of wheat straw/gypsum composite materials. They also used stereomicroscope and scanning electron microscope to observe the bonding status of the composite materials. The results showed that when the material-to-solid ratio was 10%, the water-to-solid ratio was 35%, the raw material sieve mesh size was 10-20 mesh, the amount of white glue added was 12.5%, and the wheat straw was treated with hot water for 3 hours, the physical and mechanical properties of the prepared wheat straw/gypsum composite material reached the optimal level and met the standard requirements.
Deng et al. studied the effects of polypropylene fibers of different lengths and contents on the properties of gypsum particleboard. The results showed that the internal bond strength of gypsum particleboard was the highest when the fiber length was 9 mm and the mass fraction was 9%; the static bending strength of the board was the highest when the fiber length was 12 mm and the mass fraction was 12%. An appropriate amount of polypropylene fiber helps to improve the mechanical properties of the board, but adding too much polypropylene fiber will lead to a decrease in the internal bond strength, static bending strength and elastic modulus of gypsum particleboard due to fiber agglomeration. In addition, the addition of polypropylene fiber leads to a relative reduction in the proportion of wood chips, thus improving the thickness expansion rate and water absorption rate of gypsum particleboard.
Tiziano et al. conducted an experimental study on the nail connection performance of oriented strand board (OSB) and gypsum fiberboard (GFB). The results showed that the ultimate bearing capacity and stiffness of nail connections between GFB and OSB were similar, but the ductility and energy dissipation capacity of OSB nail connections were better than those of GFB.
1.3 Impact of Post-processing Methods
Deng Yuhe et al. studied the relationship between heat and the properties of gypsum particleboard using a heating method. The results showed that within the temperature range of 30–40 °C, heating helps improve the internal bond strength of gypsum particleboard. This is because gypsum setting and solidification is an endothermic reaction; adding appropriate heat to the board increases the heat absorption during the gypsum setting and solidification process, allowing the gypsum to hydrate completely, thereby improving its internal bond strength. However, when the temperature exceeds 40 °C, the internal bond strength of the board decreases due to excessive gypsum crystallization caused by the high temperature. To shorten the heating time and improve actual production efficiency, a 1-hour pressurization process can be adopted. The effect of temperature on the static bending strength and modulus of elasticity of gypsum particleboard shows the same trend as that on the internal bond strength. The effect on the water absorption rate and thickness expansion rate of the board is not significant, but within the experimental range, the water absorption rate and thickness expansion rate of the gypsum particleboard meet the standard requirements.
1.4 Effects of Composite Additives
Studies have shown that, under dry conditions, composite silicate cement can increase the static bending strength and internal bond strength of gypsum particleboard, and effectively reduce the 2-hour and 24-hour water absorption rate and the water absorption thickness expansion rate of the board.
Rangavar et al. combined different proportions of gypsum board and silicate cement to improve its properties. The results showed that when the cement content increased from 0% to 10% of the gypsum content, the water absorption rate and thickness expansion rate of the board decreased. When the cement content increased from 0% to 5%, the static bending strength, elastic modulus, and internal bond strength of the board increased, but the opposite was true when the cement content increased from 5% to 10%. This is because excessive cement accelerates gypsum setting, hinders the completion of chemical reactions, and thus prevents the formation of strong chemical bonds, ultimately affecting the mechanical properties of the board. Specifically, during rapid curing, the mixture temperature rises, the volume increases significantly, and microcracks are generated. These microcracks propagate within the board and lead to a decrease in flexural strength. The study also found that the addition of cement helps improve the dimensional stability of gypsum board. Halim et al. proposed that the proportion of cement added must be appropriate to improve the physical and mechanical properties of gypsum particleboard.
Xuan Ling et al. studied the effects of organosilicon waterproofing agents on the physical and mechanical properties of gypsum particleboard. Experiments showed that adding an appropriate amount of organosilicon waterproofing agent could improve the water absorption thickness swelling rate and water absorption rate of gypsum particleboard, resulting in better mechanical properties. In particular, when the amount of organosilicon waterproofing agent added was 3%, the waterproof performance of the board was optimal, and the internal bond strength, static bending strength, and modulus of elasticity of the board also reached their highest values. Adding too much waterproofing agent did not significantly improve the waterproof performance of gypsum particleboard; instead, it reduced the mechanical properties of the board. This is because excessive organosilicon waterproofing agent makes the gypsum curd alkaline, hindering the hydration reaction of gypsum, reducing the internal bond strength of the board, and increasing the water absorption thickness swelling rate and water absorption rate.
2. Physical and mechanical properties of gypsum particleboard
Domestic and foreign scholars have carried out a lot of basic research on the preparation process, mechanical properties and fire resistance of gypsum particleboard. The research mainly focuses on the material level, such as bending strength, bending elastic modulus, internal bond strength and 24-hour water absorption thickness expansion rate. The basic performance comparison is shown in Table 1.
3. Fire resistance of gypsum particleboard
The fire resistance of gypsum particleboard primarily stems from the approximately 21% bound water in the gypsum (CaSO4·2H2O) crystals. During firing, the chemical decomposition of gypsum (dissociation of the bound water) occurs in two stages. In the first stage, calcium sulfate dihydrate (CaSO4·2H2O) loses 75% of its water, forming calcium sulfate hemihydrate (CaSO4·1/2 H2O). Further heating of the gypsum triggers the second stage reaction, where the hemihydrate loses the remaining water to form anhydrous calcium sulfate gypsum (CaSO4). Both reactions are endothermic and require significant energy to complete. Therefore, heat transfer through the gypsum is effectively hindered until the dehydration process is complete; this effect is the main reason for the fire resistance of gypsum particleboard.
In lightweight timber structures, gypsum board is primarily used as a cladding for walls and floors, serving a fire-resistant function. Kolaitis et al. conducted a full-scale fire test on timber structures, investigating the differences in fire resistance between gypsum board and conventional wood-based panels. The results showed that gypsum board exhibited better fire resistance than wood-based panels, with no charring observed in the gypsum-clad timber components, while wood-based panels showed insufficient fire resistance. Throughout the test, gypsum board did not fail, but walls clad with particleboard collapsed after 35 minutes of exposure to fire.
Byoung-Ho Lee et al. used a cone calorimeter to study the fire resistance of gypsum particleboard and compared it with other wood-based panels. The experiment showed that while other wood-based panels had an ignition time, the ignition time of gypsum particleboard could not be measured when exposed to fire. Furthermore, the heat release rate and its peak value, smoke generation rate, and carbon monoxide generation rate of gypsum particleboard were significantly lower than those of wood-based panels. From a material property perspective, this demonstrates that gypsum particleboard has higher fire resistance and better thermal stability than other wood-based panels, and its use as an indoor material can effectively improve the fire resistance of buildings. Studies have shown that the emissions of formaldehyde and total volatile organic compounds from particleboard increase with increasing temperature. Therefore, particleboard releases toxic gases during a fire, while gypsum particleboard, due to its better fire resistance and simpler material composition, is relatively safer.
4. Conclusions and Outlook
The raw materials used in gypsum particleboard are mostly agricultural or industrial by-products, which are abundant and widely available. Furthermore, its production process consumes little energy, resulting in low material costs. It does not release toxic gases during production or use, making it a green building material. At the same time, gypsum particleboard possesses the high physical and mechanical properties of wood fibers and the excellent fire resistance of gypsum fibers, thus showing promising application prospects.
While some progress has been made in the research of gypsum particleboard, it remains in the laboratory stage of materials research. Further research is needed on the preparation process, performance improvement, and mechanism of action of gypsum particleboard, specifically in the following aspects:
1) The bonding between the two phase interfaces in gypsum particleboard is particularly important for the mechanical properties of the board. Existing research shows that it is mostly a physical process, with relatively weak bonding strength, making it difficult to significantly improve the mechanical properties of gypsum particleboard. In contrast, there are fewer studies on further improving the interfacial interaction through chemical bonding and other methods.
2) Civil engineering is the primary application area for gypsum board. Due to the low mechanical properties of gypsum board, its contribution to the structural performance of walls and floors using it as a cladding material is generally not considered. Compared to conventional gypsum board, the mechanical properties of gypsum particleboard have been significantly improved, but compared to load-bearing structural plywood and oriented strand board (OSB), its mechanical properties still need further improvement. Furthermore, current research on the structural performance of walls and floors using gypsum particleboard as a cladding material has not yet been conducted. In addition, key issues in the application of gypsum particleboard in structural engineering, such as the nail connection performance of gypsum particleboard, stress redistribution in load-bearing structural components covered with gypsum particleboard, and their synergistic effects, need further clarification and targeted in-depth research.
3) Wood-based panels used as cladding for structural components, due to their high content of combustible wood units, lack good fire resistance and require surface coating with fire-retardant materials to meet fire resistance requirements. Gypsum particleboard exhibits improved fire resistance due to the presence of water of crystallization in gypsum crystals; however, research on the evolution of its mechanical properties and nail-connection performance under fire conditions is limited. Further in-depth research is needed on the fire resistance limits of gypsum particleboard of different strength grades (different wood shaving mass fractions) and its fire-resistant mechanism for internal materials under fire conditions.
About the author: Li Mengyu, male, master's student, mainly engaged in the research of wood-based gypsum board, E-mail: dayujs@163.com.
Corresponding author: Kong Yue, male, associate researcher, E-mail: yuekong@njtech.edu.cn.