Continuous high-rate shearing as a tool to control the stiffness development of cementitious pastes

Controlling the development of stiffness in fresh cementitious composites is crucial for 3D printing. This study revealed that continuous shearing can be an effective tool for controlling stiffness in fresh cementitious pastes. The tool's sensitivity to the water‑to‑binder ratio and Class C fly ash (FAC) was also examined. Using a rheometer, the stiffness evolution was obtained by measuring the static yield stress (τ _sy ) at 10-minute intervals until the paste reached a rigid stiffness threshold. Two binder systems were tested at w/b of 0.40 and 0.48: i) 100% Limestone Portland Cement (PLC) as the binder, and ii) a blended system with PLC (80%) + FAC (20%). Each mixture was assessed under static (no external agitation) and dynamic (continuous shearing at rest) conditions. Results indicate that, regardless of the mixture, it is possible to significantly alter the stiffening process and the physical setting time based on the rheological behavior. For both binder systems, increasing the w/b ratio made the paste more responsive to dynamic conditions, resulting in further delayed stiffening. Additionally, the use of fly ash enhances the ability to control stiffening and setting time through external continuous shearing. Therefore, increasing the water-to-binder ratio and incorporating fly ash extends the options for manipulating the stiffening process and setting time via external shearing. These findings emphasize the potential of pre-placement external continuous shearing as an active tool to regulate stiffening in real time, especially for 3D printing, as it alters stiffness development without altering the mix design.