Ultra-high-performance concrete (UHPC) often faces challenges due to its low water-to-binder ratio. In this case, cement particles will be unable to fully hydrate, resulting in a high degree of initial dimensional instability. As a promising solution, superabsorbent polymers (SAP) and natural zeolite have potential in mitigating shrinkage and achieving self-stressing properties. Despite prior research on strength, shrinkage, and hydration of UHPC, no systematic study has examined the effects of SAP and natural zeolite on porosity and pore structure. To address this gap, a holistic testing program was established in this study to investigate how these materials affect the microstructural properties of UHPC, especially porosity and pore distribution. This included compressive strength, drying shrinkage, and autogenous shrinkage. The hardened mixtures were also characterized using mercury intrusion porosimetry, while nuclear magnetic resonance (NMR) technology was used to evaluate the pore parameters. The results show that UHPC incorporated with zeolite experiences less autogenous shrinkage than SAP due to the internal curing effect of zeolite particles. The compressive strength, however, is reduced attributed to a more porous microstructure. The outcome of this study provided valuable insights into optimizing the balance between durability and mechanical performance, paving the way for more sustainable and cost-effective applications of UHPC in modern construction practices.