A review of research on gypsum particleboard

Abstract: Gypsum board has excellent fire resistance and is primarily used as fire-resistant cladding for floor and wall panels in lightweight wood structures. By incorporating plant fibers, such as wood shavings, into a gypsum matrix, gypsum particleboard can be prepared to improve its mechanical properties without changing the original production process. The gypsum raw material for gypsum particleboard is primarily derived from industrial gypsum, and the shavings are primarily agricultural or industrial byproducts. This approach offers advantages such as waste-to-resource conversion and manageable material costs, thus offering broad application prospects. This article reviews the board preparation process, including the mechanism of action of different retarders in gypsum particleboard, the reinforcing effects of different plant fiber types, post-treatment methods, and the mechanism by which admixtures improve board properties. The current research status of gypsum particleboard, including its physical, mechanical, and fire resistance properties, is also reviewed. Finally, shortcomings of current gypsum particleboard for civil engineering applications are identified, along with areas for further research. This paper provides a reference for further systematic research and engineering applications of gypsum particleboard.

Keywords: gypsum particleboard, preparation process, fire resistance, mechanical properties

Author: Li Mengyu, Yue Kong, Liu Jian, Liu Weiqing, Song Yongming, Tang Lijuan, and Lü Chenglong

Paper-faced gypsum board is made primarily from building gypsum, mixed with appropriate admixtures. After mixing with water, it is poured between facing papers and firmly bonded to the paper, with the paper providing the primary strength. Gypsum board is widely used as cladding for timber-framed walls and floors, primarily for fire protection and decorative purposes. Due to its poor mechanical properties, gypsum board is prone to localized damage during use, such as shear tearing, corner crushing, splice separation, and extrusion damage. In contrast, gypsum particleboard, which boasts superior mechanical properties, is an inorganic plywood composite of wood particles and gypsum, with the wood particles serving as a reinforcement and the gypsum serving as an adhesive. Compared to other wood-based board materials (such as oriented strand board, medium-density fiberboard, and laminate), gypsum particleboard offers advantages such as environmental friendliness, high fire resistance, and excellent sound insulation. It can also be machined, such as sawing, grinding, and polishing. During the production process, the wood units do not need to be dried, and the slabs are pressed at room temperature, resulting in low energy consumption. Furthermore, gypsum particleboards are often made from industrial waste gypsum, a byproduct of many chemical processes. Besides traditional wood, other wood particle materials can also be used, such as bamboo, kenaf, wheat straw, and sugarcane bagasse. This not only fully utilizes resources but also allows for the production of boards that meet specific requirements, while also contributing to ecological conservation. Consequently, scholars both domestically and internationally have conducted extensive research on the preparation, microscopic characterization, and performance analysis of gypsum particleboards.

In gypsum particleboard, the presence of fibers compensates for the inherent brittleness of gypsum, resulting in excellent mechanical properties, particularly a high elastic modulus. This enhanced mechanical performance contributes to improved physical properties, particularly when internal bond strength increases, reducing the board's water absorption thickness expansion, all else being equal. Furthermore, gypsum itself possesses excellent fire resistance, resulting in gypsum particleboard's superior fire resistance. Domestic and international researchers have conducted a series of studies on factors influencing the performance of gypsum particleboard, focusing on fiber type and morphology, component ratios, post-processing techniques, and compounding additives. In addition to traditional flat wood particles, wood chips can also be used as fiber morphology. The ratios of the components vary depending on the fiber type. When wood is used as the reinforcing fiber, and the wood-to-cement ratio and water-to-cement ratio are 0.3-0.4 and 0.2-0.3, respectively, the resulting gypsum particleboard exhibits excellent physical and mechanical properties. Furthermore, post-heating treatment and the addition of Portland cement compounding 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 technology of gypsum particleboard from the aspects of the action mechanism of different retarders in gypsum particleboard, the reinforcing effect of different plant fiber types, the influence of post-treatment methods, and the mechanism of improvement of board performance by admixtures.

1.1 Mechanism of action of retarder

Gypsum typically sets and hardens very quickly, not allowing sufficient time to meet the requirements of industrial production. Therefore, composite retarders are often added to slow the setting of the gypsum. Retarders mainly fall into four categories: organic acids, phosphates, proteins, and compound systems. Different retarders work differently. Generally speaking, bone glue chemically adsorbs on the surface of newly formed dihydrate gypsum crystal nuclei, covering them to reduce their surface energy and, by inhibiting their growth, prolonging the hydration and setting time of the building gypsum. Citric acid adsorbs on the surface of hemihydrate gypsum particles, hindering their dissolution, or adsorbs on newly formed dihydrate gypsum crystal nuclei, prolonging the time it takes for the nuclei to reach the critical nucleation size, thereby extending the setting time of the gypsum. Sodium hexametaphosphate retarder ions form insoluble compounds with Ca2+ on the surface of hemihydrate gypsum crystals, reducing the dissolution rate of the hemihydrate gypsum and slowing its hydration process, thereby extending the setting time of the gypsum.

Marcos Lanzón et al. studied the retarding effect of different concentrations of citric acid at a constant water-cement ratio. Their results showed that the required manufacturing time could be achieved at citric acid concentrations of 500 to 1000 ppm. When the citric acid concentration was above 1500 ppm, the setting time of the gypsum did not change significantly. However, the addition of citric acid had a negative impact on the gypsum strength, which was particularly pronounced at concentrations above 1000 ppm. This was primarily due to the reduction of the interlocking effect between the gypsum crystal microstructure. Overall, the addition of citric acid reduced the liquid phase supersaturation in the early hydration stage of the cementitious material and altered the crystallization behavior and morphology of dihydrate gypsum. The gypsum crystals became significantly coarser, with the crystal shape shifting from needle-like to short columnar. This significantly weakened the overlap between the crystals, increased the pore size of the hardened body, significantly increased the proportion of macropores, and degraded the pore structure, ultimately leading to a decrease in gypsum strength. The strength loss was generally proportional to the retarding effect: higher citric acid content and longer retarding time resulted in greater strength loss. Research has shown that acidic retarders are superior to alkaline retarders in the production of gypsum particleboard and have minimal impact on its properties. Using appropriate amounts of trisodium citrate or citric acid can achieve better performance in gypsum particleboard, extending the initial setting time of the gypsum to 2 hours. Adding an alkaline retarder shortens the initial setting time of the gypsum, while significantly decreasing mechanical properties such as internal bond strength, elastic modulus, and static flexural strength. Since gypsum cures under weak acid conditions, strong alkalis can reduce the bond strength between the gypsum and the wood particles. However, strong acids can also affect the bond strength of the gypsum and reduce the strength of the wood particles. Thermal analysis shows that gypsum curing is an endothermic reaction, and the high heat absorption allows for better curing of the gypsum, resulting in higher bond strength.

1.2 Reinforcement effect of composite fibers

Dasong Dai et al. studied the effects of wood chemical composition on the properties of gypsum particleboard. Their results showed that components such as tannin, acetic acid, hemicellulose, and lignin had no adverse effects on the mechanical properties of gypsum particleboard. However, the addition of a defoamer significantly improved the mechanical properties of gypsum particleboard. The lower strength of gypsum particleboard is not directly related to the chemical composition of the wood, but rather to the high water absorption of the wood units. Gypsum particleboard prepared with wood particles modified with a water-based epoxy resin exhibits superior mechanical properties due to the improved adhesion of the water-based epoxy resin, which improves the bond between gypsum and wood particles and reduces the water absorption of the wood particles.

Morteza Nazerian et al. prepared gypsum boards using bagasse and wheat straw. Their results showed that, for a given wood-to-paste ratio, a higher proportion of wheat straw resulted in lower water absorption, primarily due to the inherently low water absorption of wheat straw. Density also plays a significant role influencing board water absorption; lower density results in higher water absorption. At the same wood-to-paste ratio, a higher amount of wheat straw resulted in higher thickness expansion. 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, preventing the gypsum paste from penetrating the cell walls and preventing a good mechanical interlock between the mineral matrix and the particles. This is primarily responsible for the additional loss of internal bond strength, which in turn affects the thickness expansion of the board. The study also found that a decrease in the wood-to-paste ratio leads to a decrease in static flexural strength and elastic modulus, attributed to the high brittleness and low elastic modulus of gypsum. Pan Shuqing and others used gypsum and bagasse as the main raw materials to manufacture gypsum particleboard. Tests showed that the board has high strength, flame retardancy, thermal insulation, sound absorption and air permeability. 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 morphology, material-paste ratio, amount of latex glue added and raw material pretreatment method on the performance of wheat straw gypsum composite materials, and used stereo microscope and scanning electron microscope to observe the bonding condition of the composite materials. The results showed that when the material-paste ratio was 10%, the water-paste ratio was 35%, the raw material sieve mesh size was 10-20 mesh, the latex glue added amount 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 materials 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 when the fiber length was 9 mm and the mass fraction was 9%, the internal bonding strength of the gypsum particleboard was the highest; when the fiber length was 12 mm and the mass fraction was 12%, the static bending strength of the board was the highest; 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 bonding strength, static bending strength and elastic modulus of the gypsum particleboard due to fiber agglomeration; in addition, the addition of polypropylene fiber leads to a relative reduction in the proportion of particles, thereby improving the thickness expansion rate and water absorption rate of the gypsum particleboard.

Tiziano et al. conducted an experimental study on the nail connection performance of oriented strand board and gypsum fiberboard. The results showed that the ultimate bearing capacity and stiffness of the nail connection of gypsum fiberboard and oriented strand board were similar, but the ductility and energy dissipation capacity of the nail connection node of oriented strand board were better than those of gypsum fiberboard.

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 heating within the temperature range of 30–40°C helps improve the internal bond strength of gypsum particleboard. This is because gypsum solidification is an endothermic reaction. By applying an appropriate amount of heat to the slab, the heat absorbed during solidification is increased, allowing the gypsum to fully hydrate, thereby improving its internal bond strength. However, when the temperature exceeds 40°C, the internal bond strength of the board decreases. This is due to excessive hydration of the gypsum crystals caused by the high temperature. To shorten the heating time and improve actual production efficiency, a 1-hour pressurization process can be used. The effect of temperature on the static flexural strength and elastic modulus of gypsum particleboard follows the same trend as the internal bond strength. The effect on the water absorption rate and thickness expansion rate of the board is not significant. However, within the test range, the water absorption rate and thickness expansion rate of the gypsum particleboard meet the standard requirements.

1.4 Influence of composite additives

Studies have shown that in a dry state, composite silicate cement can increase the static bending strength and internal bonding strength of gypsum particleboard, and effectively reduce the 2 h and 24 h water absorption rate and water absorption thickness expansion rate of the board.

Rangavar et al. combined different proportions of grapevine and Portland cement with gypsum to improve its properties. Their results showed that increasing the cement content from 0 to 10% of the gypsum mass reduced the water absorption rate and thickness expansion of the board. Increasing the cement content from 0 to 5% increased the board's static flexural strength, elastic modulus, and internal bond strength, but increasing the cement content from 5% to 10% had the opposite effect. This is because excessive cement accelerates the setting of the gypsum, hindering the completion of chemical reactions and thus preventing the formation of strong chemical bonds, ultimately affecting the board's mechanical properties. Rapid curing increases the mixture's temperature and volume, leading to microcracks that propagate throughout the board and reduce its flexural strength. The study also found that the addition of cement improves the dimensional stability of gypsum boards. Halim et al. proposed that an appropriate cement content is necessary 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. The experiments showed that adding an appropriate amount of organosilicon waterproofing agents improved the water absorption thickness expansion rate and water absorption rate of gypsum particleboard, and also achieved better mechanical properties. In particular, when the organosilicon waterproofing agent was added at a 3% dosage, the board's waterproofing properties were optimal, and the board's internal bond strength, static bending strength, and elastic modulus also reached their highest values. However, adding too much waterproofing agent did not significantly improve the waterproofing properties of gypsum particleboard and, in fact, reduced the board's mechanical properties. This is because excessive addition of organosilicon waterproofing agents renders the solidified gypsum alkaline, hindering the hydration reaction of the gypsum, reducing the board's internal bond strength, and increasing the water absorption thickness expansion rate and water absorption rate.

2 Physical and mechanical properties of gypsum particleboard

Domestic and foreign scholars have carried out a large amount of basic research on the preparation process, mechanical properties and fire resistance of gypsum particleboard, mainly focusing on indicators such as flexural strength, flexural elastic modulus, internal bonding strength and 24-hour water absorption thickness expansion rate at the material level. The comparison of its basic properties is shown in Table 1.

3 Fire resistance of gypsum particleboard

The fire resistance of gypsum particleboard stems primarily from the bound water, which accounts for approximately 21% by weight of the gypsum (CaSO₄·2H₂O) crystals. The chemical decomposition (dissociation of chemically bound water) of the gypsum during fire exposure occurs in two stages. In the first stage, calcium sulfate dihydrate (CaSO₄·2H₂O) loses 75% of its water, forming calcium sulfate hemihydrate (CaSO₄·1/2H₂O). Further heating of the gypsum triggers a second reaction, in which the calcium sulfate hemihydrate loses the remaining water to form anhydrous calcium sulfate gypsum (CaSO₄). Both reactions are endothermic and require significant energy to complete. Consequently, heat transfer through the gypsum is effectively hindered until the dehydration process is complete. The fire resistance of gypsum particleboard is primarily attributed to this effect.

In lightweight timber structures, gypsum board, used as wall and floor coverings, primarily serves a fire-resistant purpose. Kolaitis et al. conducted fire tests on full-scale timber structures, examining the differences in fire resistance between gypsum board and conventional wood-based panels. The results showed that gypsum board exhibited superior fire resistance compared to wood-based panels, with the wood components covered by it showing no charring. However, wood-based panels exhibited insufficient fire resistance. While the gypsum board panels did not fail throughout the test, the wall covered with particleboard collapsed after 35 minutes of exposure.

Byoung-Ho Lee et al. used a cone calorimeter to study the fire resistance of gypsum particleboard and compared it with other wood-based boards. The test showed that other wood-based boards have an ignition time, but when gypsum particleboard is exposed to fire, its ignition time cannot be measured. At the same time, the heat release rate and its peak value, smoke generation rate, and carbon monoxide generation rate of gypsum particleboard are significantly lower than those of wood-based boards. From the perspective of material properties, it is proved that gypsum particleboard has higher fire resistance and better thermal stability than other wood-based boards. Using it 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 will release toxic gases in the event of a fire. Gypsum particleboard is relatively safer due to its good fire resistance and simple material composition.

4 Conclusion and Outlook

Gypsum particleboard is primarily made from agricultural or industrial byproducts, which are readily available and widely used. Its production process consumes minimal energy, resulting in low material costs. It also emits no toxic gases during production or use, making it a green building material. Furthermore, gypsum particleboard combines the high physical and mechanical properties of wood with the excellent fire resistance of gypsum, offering promising application prospects.

The research on gypsum particleboard has achieved certain results, but it is still in the laboratory stage of material research. Further research is needed in the preparation process, performance improvement, and mechanism research of gypsum particleboard. Specifically, there are the following aspects:

1) The interfacial bonding between the two phases in gypsum particleboard is crucial to the board's mechanical properties. Existing research indicates that this is primarily a physical interaction, resulting in weak bonding strength and limited ability to significantly enhance the board's mechanical properties. In contrast, research on further enhancing interfacial bonding through methods such as chemical bonding is limited.

2) Civil engineering is the primary application area for gypsum board. Due to its low mechanical properties, the structural performance of components such as walls and floors using gypsum board as cladding is generally not considered. While the mechanical properties of gypsum particleboard have been significantly improved compared to conventional gypsum board, its mechanical properties still need to be further improved compared to load-bearing structural materials such as plywood and oriented strand board. Furthermore, research on the structural performance of components such as walls and floors clad with gypsum particleboard has not yet been conducted. Furthermore, key issues related to its application in structural applications, such as the nail connection performance of gypsum particleboard, stress redistribution in load-bearing structural components covered with gypsum particleboard, and their synergistic effects, require further clarification and targeted, in-depth research.

3) Wood-based panels used as cladding for structural components lack good fire resistance due to their high content of combustible wood units. Therefore, they require surface coating with fireproofing materials to meet fire resistance requirements. Gypsum particleboard has improved fire resistance due to the presence of water in the gypsum crystals, but little research has been conducted on the evolution of its mechanical properties and nailing performance during fire. Further research is needed on the fire resistance limits of gypsum particleboards of different strength grades (with varying wood particle mass fractions) and their fire protection mechanisms for internal materials under fire conditions.

Author profile: Li Mengyu, male, master's student, mainly engaged in the research of wood-based gypsum boards, E-mail: [email protected].

Corresponding author: Yue Kong, male, associate researcher, E-mail: [email protected].