Investigation of the Properties and Microstructure of EVA-Modified Underwater Repair Mortar

Ethylene–vinyl acetate (EVA) as a redispersible polymer can improve the workability, mechanical properties, and interfacial bonding of repair mortar, but the regulatory effect of different EVA dosages on underwater performance remains unclear. This study investigates EVA–polyacrylamide (PAM) composite-modified repair mortar and systematically evaluates the influence of varying EVA content on mortar performance. Multi-scale methods including setting time measurement, flowability test, flexural and compressive strength tests, underwater flexural bonding strength, water-to-air strength ratio, cement loss rate, porosity, and scanning electron microscopy (SEM) characterization were employed. The results indicate that mortar with 8% EVA exhibits the optimal overall performance. The initial setting time of the cement paste increased from 35 to 47 min, and the final setting time from 55 to 72 min, improving workability. In terms of mechanical properties, the 28-day flexural strength increased from 7.5 to 9.2 MPa, and the compressive strength from 44.4 to 50.6 MPa. Regarding underwater performance, the 28-day underwater flexural bonding strength rose from 5.1 to 6.5 MPa; the water-to-air strength ratio increased from 79.1% to 95.2%, and cement loss rate decreased from 3.18% to 0.32%; pH changes were maintained within a moderate range. Microstructural analysis shows that the total porosity at 28 d decreased from 16.8% to 13.5% for mortar containing 8% EVA, and SEM observations revealed that hydration products uniformly filled pores forming a dense network, consistent with the trends of mechanical and interfacial bonding performance enhancement.