Controlling hydration and setting of UHPC incorporating waterglass at different times of addition

Tailoring fresh-state properties and hardening behavior becomes critical when utilizing ultra-high-performance concrete (UHPC) in new construction applications. In this paper, waterglass, a liquid form of sodium silicate, was incorporated at different times to modify the hydration, fluidity, and hardening behavior of UHPC. The effect of waterglass on early-age hydration and microstructure development was monitored by isothermal calorimetry and ultrasound measurement. Additionally, the mechanical performance and mineralogical characteristics were investigated, considering the side effects of waterglass. When the addition of waterglass was delayed, the loss of fluidity and hydration acceleration was significant. In particular, when a moderate content of waterglass was incorporated later, the main hydration immediately activated, and the setting time significantly reduced. In contrast, directly added waterglass merely diminishes the retardation of hydration due to the superplasticizer, and the initial setting occurred before the onset of acceleration due to the formation of additional hydrates. The result indicated that the structural build-up due to direct addition of waterglass can be reversible by shearing; therefore, direct addition of waterglass is applicable in ready-mix concrete for precast/cast-in-field application. Moreover, in shotcrete/digital fabrication applications, the delayed addition of waterglass would be favorable when activating the hydration reaction, resulting in rapid strength development.