Natural gypsum is an important inorganic building and decorative material because it offers fire resistance, thermal insulation, acoustic comfort, low energy consumption during processing, and broad resource availability. Nevertheless, the practical upgrading of gypsum products is still constrained by their intrinsic brittleness, low load-bearing capacity, weak surface hardness, and poor resistance to abrasion or surface pulverization. These shortcomings are particularly unfavorable for high-value interior panels, decorative components, and other applications requiring both dimensional stability and long service life. Fiber reinforcement has been widely used to toughen gypsum, but fiber dispersion, reduced slurry workability, increased porosity, and cost or sustainability concerns can limit its application. Therefore, a simple, water-based polymer modification route is necessary to simultaneously improve workability, strength, surface integrity, and durability. In this study, natural gypsum was modified with macromolecular polymers, with the aim of screening an effective modifier and clarifying the relationship between polymer dosage, hydration behavior, microstructure, and macroscopic performance.

A series of polymer modifiers were first compared, including acrylic emulsion, water-based epoxy AB adhesive, styrene-acrylic emulsion, waterborne polyurethane (WPU), polyvinyl alcohol (PVA), polyacrylamide (PAM), and polyvinylpyrrolidone (PVP). Gypsum slurry was prepared at a gypsum-to-water ratio of 1.0: 0.6 according to the standard consistency requirement. For solid polymers, aqueous polymer solutions were prepared before mixing; for liquid emulsions or adhesives, the solid content was considered when preparing solutions of the required mass fraction. The polymer solution was added into the mixing vessel, gypsum powder was introduced, and the slurry was mixed, cast, vibrated, demolded after final setting, and cured under laboratory conditions. On the basis of preliminary screening, PVA was selected for dosage optimization, and gypsum composites containing 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% (mass fraction) PVA were prepared. Workability was evaluated by spread diameter, initial setting time, and final setting time. Mechanical properties were characterized by flexural strength, compressive strength, and surface hardness, including wet-state tests after water immersion where applicable. Abrasion resistance was assessed using the pulverization rate after rubbing a 30 mm × 30 mm × 30 mm sample against 320-mesh abrasive paper under a constant load. The phase composition, hydration degree, and microstructure were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), respectively.

The screening results showed clear differences among polymer types. PAM, acrylic emulsion, and styrene-acrylic emulsion caused severe loss of spreadability, probably because ionic interaction, flocculation, demulsification, and rapid viscosity growth produced agglomerated slurry systems. PVA, PVP, WPU, and water-based epoxy AB adhesive allowed the slurry to spread, but their effects on workability and setting behavior differed. Among them, PVA produced the most favorable mechanical improvement; at 0.5%, the flexural strength reached 2.3 MPa, 52.9% higher than that of the control sample. PVA was therefore selected as the main modifier.

With increasing PVA dosage, the spread diameter first increased and then decreased. A low PVA content improved particle dispersion through adsorption, electrostatic repulsion, and steric hindrance, whereas excessive PVA formed a three-dimensional network and bound free water through hydrogen bonding, reducing flowability. The initial setting time exhibited a similar first-increase and then decrease trend, while the final setting time decreased slightly overall. This behavior indicates that hydroxyl groups in PVA can interact with calcium ions and delay early nucleation at low dosage, but the water-binding and thickening effects become more pronounced at higher dosage.

The dry mechanical properties reached an optimum at 1.5% PVA. The flexural strength, compressive strength, and surface hardness were 5.2 MPa, 3.4 MPa, and 16.7 N/mm², respectively, representing increases of 344.4%, 84.9%, and 383.0% compared with the blank gypsum. The pulverization rate decreased sharply from 0.7979% to 0.0545%, demonstrating a remarkable enhancement in abrasion resistance. These improvements are attributed to the combined effects of polymer film formation, pore filling, interfacial bridging, and crystal regularization. PVA molecular chains contain abundant hydroxyl groups that can coordinate with calcium ions and form hydrogen bonds with calcium sulfate dihydrate (CaSO₄·2H₂O), thereby strengthening the intercrystalline interface and promoting efficient stress transfer. SEM images confirmed that appropriate PVA addition reduced pores, tightened crystal overlap, and made the fracture surface more compact. XRD patterns showed sharper characteristic peaks of CaSO₄·2H₂O, especially near 31.2°, indicating improved crystal regularity and orientation. TGA further showed that the mass loss associated with dehydration increased from 13.69% in the control sample to 15.89% at 1.5% PVA, suggesting a higher hydration degree and a larger amount of CaSO₄·2H₂O formed. However, when the PVA dosage increased to 2.0%, local polymer-rich agglomerates appeared, free-water diffusion was restricted, hydration and crystal interlocking were weakened, and the mechanical enhancement declined. Wet-state flexural strength, wet compressive strength, and water-immersed hardness changed only slightly, indicating that PVA mainly improves the dry strength and surface wear resistance, while its positive effect on wet-state performance is limited.

PVA is an effective water-soluble polymer modifier for natural gypsum. Compared with other polymer systems, it provides better compatibility between workability and reinforcement. The optimum dosage is 1.5%, at which flexural strength, compressive strength, surface hardness, and abrasion resistance are improved simultaneously. The strengthening mechanism can be summarized as a synergistic film-forming-filling, interfacial-bridging, and crystal-regularizing effect. Appropriate PVA promotes gypsum hydration, improves CaSO₄·2H₂O crystallization, reduces matrix porosity, and enhances crystal bonding. Excessive PVA, however, causes agglomeration, water-binding, increased porosity, and a diluted load-bearing skeleton, resulting in performance deterioration. This simple modification strategy provides a practical route and theoretical basis for preparing high-strength and wear-resistant gypsum-based composites.