1 of 13 Journal of Applied Polymer Science, 2025; 0:e57501 https://doi.org/10.1002/app.57501 Journal of Applied Polymer Science RESEARCH ARTICLE Gypsum-Based Self-Leveling Mortars Modified With an Emulsified Waterproofing Agent: Physical and Mechanical Properties and Microstructure Yuan Gao 1 | Chang Chen 1 | Shaowu Jiu 1 | Yanxin Chen 1 | Yan Liu 2 1 College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, China | 2 State Key Laboratory of Green Building, Department of Architecture, Xi'an University of Architecture and Technology, Xi'an, China Correspondence: Chang Chen (changchen420@xauat.edu.cn) | Yanxin Chen (chen_yanxin@xauat.edu.cn) Received: 25 November 2024 | Revised: 19 May 2025 | Accepted: 8 June 2025 Funding: This work was supported by “the 14th Five-Year” National Science and Technology Major Project of China (2022YFC3801401), Anhui Provincial Science and Technology Major Project (2021e03020003), and Shaanxi Provincial Innovation Capability Support Program (2021TD-5). Keywords: applications | coatings | mechanical properties | microscopy | surfaces and interfaces ABSTRACT Hydrogen-containing silicone oil (HCSO) is widely used as a waterproofing agent in building materials, yet its tendency to ag- glomerate leads to uneven distribution and poor performance. In this study, we developed a controllable emulsification strategy— using Span 80, Tween 80, and OP-10 at a mass ratio of 8:1.6:6.4 (HLB = 9.05)—to produce a stable emulsified HCSO (EHCSO) with a stability value of 1. By incorporating 0.8 wt% EHCSO into gypsum-based self-leveling mortars (GSLMs), we achieved an optimal balance of properties: apparent density of 1.26 × 10 3 kg/m 3 , softening coefficient of 0.85, flexural strength of 6.81 MPa, compressive strength of 20.90 MPa, and contact angle of 126.39°. X-ray diffraction and SEM analyses confirmed that EHCSO does not alter the hydration phases but promotes the in situ formation of short columnar dihydrate crystals that fill pore spaces, refining the microstructure and enhancing mechanical integrity. This work contributes to the field by (1) introducing a scalable surfactant formulation for uniform HCSO dispersion in gypsum systems; (2) quantifying the link between surfactant HLB, emul- sion stability, and waterproofing performance; and (3) demonstrating a dual improvement in both durability and strength, with potential applicability to a broad range of cementitious materials. 1 | Introduction Gypsum-based self-leveling mortars (GSLMs) are functional construction materials with the advantages of excellent fluid- ity and stability, good fire resistance, light weight, low cost, and high construction efficiency [1–3]. GSLMs are used in schools, hospitals, supermarkets, etc. [4], and can improve living condi- tions by providing good sound insulation and humidity regula- tion [5–7]. However, GSLMs are susceptible to dissolution upon exposure to water, resulting in a decrease in mechanical prop- erties of > 60%. These poor waterproofing properties mean that they cannot be used in humid environments [8–11]. Therefore, the development of emulsified water repellents that can work in wet environments has become necessary. Some scholars have used mineral admixtures to improve the water resistance of GSLMs, such as cement, finely ground blast furnace slag [12], mineral powder [13], and fly ash [14, 15] to address the poor water resistance of gypsum-based composites [16–18]. The addition of mineral admixtures leads to the forma- tion of insoluble substances such as calcium silicate and ettring- ite, which are wrapped on the surface of gypsum (CaSO 4 ·2H 2 O) © 2025 Wiley Periodicals LLC.