Long-term durability of cast-in-situ UHPC under sulfate attack: The synergistic effect of silica fume and limestone powder

The high cement content of conventional Ultra-high performance concrete (UHPC) results in considerable carbon emissions, while the durability of sustainable ternary-blended UHPC containing silica fume (SF) and limestone powder (LP) under sulfate attack remains insufficiently understood. This study prepared cast-in-situ UHPC specimens by partially replacing cement with SF and LP. The evolution of macroscopic physical properties (apparent morphology, mass, and size change rate) and mechanical properties (flexural strength, compressive strength, corrosion resistance coefficient and flexural-compressive strength ratio) under sulfate environments was analyzed. Changes in mineral phases, microstructural morphology, and pore size distribution were characterized at multiple scales employing X-ray Diffraction (XRD), Thermogravimetry (TG), Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS), and Mercury Intrusion Porosimetry (MIP) techniques. Result indicates that during the initial stages of corrosion, the reaction between sulfate ions and Tricalcium aluminate (C 3 A) leads to ettringite formation, which accelerates the hydration reaction. This leads to significant changes in the physical properties (Maximum mass change rate and size change rate reached 1.89% and 0.07%, respectively.) of the specimens and enhances the strength by 8–15%. In the late stages of corrosion, sulfate attack reduces the Ca/Si ratio in the calcium silicate hydrate (C-S-H) gel, triggering expansive damage. SF addition reduces the content of available C 3 A and free calcium hydroxide (with a content of only 2.46 at 1 day), thereby inhibiting the formation of ettringite and gypsum. The synergistic effect of SF and LP inhibits the formation of corrosion products and enhances microstructural stability, thereby reducing the intrusion of external SO 4 2- and suppressing the early formation of gypsum and ettringite. Compared to distilled water environments, the addition of SF and LP to sulfate solutions resulted in only 7.87% increase in capillary and macropores within the specimens. Furthermore, the use of SF and LP as cement substitutes reduced carbon emissions compared to the control group while maintaining excellent mechanical properties. These findings provide certain theoretical guidance for the infrastructure construction of cast-in-situ UHPC in salt lakes or coastal areas.