Abstract
Ultra-high performance concrete (UHPC) is not an ideal material for rapid repairs of bridge expansion joints due to its slow early strength development. To mitigate this early strength limitation, this study investigated the use of calcium formate (CF) and anhydrous sodium sulfate (SS) as early strength agents to enhance the early strength of UHPC. The findings indicate that a composite addition of 0.25 % SS and 0.25 % CF significantly improves both strength and workability, resulting in a 28-day compressive strength of 141.3 MPa, a 28-day flexural strength of 12.6 MPa, and a flowability of 227 mm. SS facilitates the formation of ettringite (AFt) and calcium hydroxide (C-H), while CF promotes the generation of smaller gismondine particles. This substantial increase in hydration products reduces the porosity of UHPC, thereby densifying the matrix and enhancing its mechanical strength. The synergistic effect of both additives leads to a higher concentration of calcium ions (Ca2+) around the smaller gismondine particles, which generates additional hydration products, optimizes the pore structure of UHPC, and narrows the width of the interfacial transition zone (ITZ). Consequently, this combination significantly improves the early strength of UHPC, achieving a 45.6 % enhancement in the early hydration rate with the use of 0.25 % CF and 0.25 % SS.
Published Version
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