Hydration and fiber bonding properties of ultra-high-performance concrete supplemented with calcium hydroxide

Abstract This study investigates the impact of calcium hydroxide (CH) on the hydration characteristics of ultra-high-performance concrete (UHPC) and the static and dynamic behavior of steel fibers from the UHPC matrix. Cement was partially replaced with CH at dosages up to 5%, and a series of chemical analyses, compressive strength, and pullout tests were conducted. The findings reveal that the compressive strength of UHPC increases with CH content up to 2%, after which it gradually declines as CH content rises to 5%, attributed to reduced aluminum substitution into C–S–H and increased porosity. The mean chain length of UHPC with 2% CH reached a maximum of 16.7, while porosity reached a minimum. The incorporation of 2% CH in UHPC resulted in the highest static bond strength of 22.48 MPa, 2.8 times greater than that of the plain UHPC. Although bond strength slightly decreased with CH contents above 2%, it remained above 20 MPa up to a 5% CH dosage. The CH-modified UHPC exhibits superior bond strengths under impact loading than the plain UHPC. This suggests that CH incorporation enhances the pullout resistance of steel fibers under both static and dynamic conditions. Additionally, under impact loading, the rapid pullout of steel fibers resulted in greater retention of matrix material on the fiber surfaces compared to static conditions. Lastly, the dynamic increase factor (DIF) for bond strength was more pronounced in the control UHPC (reaching up to 4.4) than in the CH-modified UHPC (ranging between 1.6 and 2.1).