Effect of Acrylic Emulsion on Drying Shrinkage, Mechanical Properties, and Hydration Behavior of UHPC Matrix

Significant drying shrinkage and early-age cracking have remained critical constraints to the broader application of ultra-high performance concrete (UHPC). This study attempted to address these limitations through the addition of acrylic emulsion (AE), and the influence of its content (0–10 wt.%) on the setting time, mechanical properties, drying shrinkage, hydration behavior, and microstructure of UHPC matrix was systematically investigated. The results show that AE addition delayed setting and early hydration, substantially reducing cumulative heat release and thereby lowering the risk of thermal cracking. AE incorporation reduced compressive strength, and this adverse effect became more pronounced at higher dosages. In contrast, the formation of intertwining polymer films within the hydration products provided effective pore-filling and “micro-fibre” effects, which enhanced flexural strength development and alleviated drying shrinkage that originates principally from moisture loss within the C-S-H gel. At an optimal dosage of 5 wt.%, AE reduced the 14-day and 90-day drying shrinkage of UHPC matrix by 28% and 25%, respectively, while increasing the 90-day flexural strength by approximately 3%. However, a further increase in the dosage deteriorated overall performance, primarily because excessive polymer promoted agglomeration and introduced more air bubbles. These findings provide a theoretical foundation for designing low-shrinkage, low-heat-release UHPC tailored for large-volume engineering structures.