Mineral Admixture Governs the Synergy of Polymer and Fibers in Ultra-Low Temperature Concrete

The development of all-concrete liquefied natural gas (LNG) storage tanks is hindered by the susceptibility of conventional concrete to ultra-low temperature (ULT) cycling down to −70 °C. While redispersible polymer powder (RPP) and polypropylene (PP) fibers individually enhance performance, their combined effect in various mineral admixture systems remains unclear. This study investigates the synergy and selective compatibility in hybrid-modified concrete containing fly ash (FA), silica fume (SF), or slag (SG). Comprehensive assessments after 50 ULT cycles reveal that the efficacy of hybrid modification is intrinsically governed by the mineral admixture. Among all systems, the silica fume-based hybrid system (EPSF) exhibits the highest residual compressive strength (57.5 MPa), the lowest strength loss (6.7%), and the most balanced durability. Microstructural analysis reveals that this synergy arises from a dense matrix, continuous polymer network, and effective fiber bridging—achieved only when the mineral admixture enables uniform RPP distribution. In contrast, the FA system exhibits a strength–durability trade-off, with RPP localized at interfaces, while the SG system shows a polymer-activated hydration mechanism. Microstructural and nano-mechanical analyses confirm that RPP acts as a pore filler in cement, an interfacial modifier in FA, a cohesive network former in SF, and a hydration activator in SG. This work establishes that superior ULT resilience requires not merely additive modifications but a matrix-enabled synergy, providing a scientific basis for the rational design of cryogenic concrete.