Effect of water-based nanoclay and ambient temperature on rheological properties of UHPC pastes

Abstract

Rheological properties of ultra-high-performance concrete (UHPC) pastes significantly impact the flowability and segregation resistance of UHPC, thus are of great importance to investigate. This study investigated the coupled effect of the water-based nanoclay suspension and ambient temperatures on rheological properties of UHPC pastes. Firstly, the water-based nanoclay was proved to show good compatibility with used PCE-based HRWR. Besides, the nanoclay content varied from 0 to 0.20% and the evaluated ambient temperatures were 10 °C, 25 °C, and 35 °C. Results showed that the addition of nanoclay and elevation of ambient temperatures significantly increased plastic viscosity and dynamic yield stress of UHPC pastes. The thixotropy of UHPC pastes was also enhanced, which was quantified by a bi-linear model including initial fluctuations of static yield stress (τfloc) and thixotropy index (IA). The enhancement of rheological properties are closely related to the evolution of internal microstructure, including physical and chemical effects. The physical effects refer to the flocculation between particles: (1) the addition of nanoclay accelerated the flocculation by the electrostatic attraction forces; (2) the elevation of ambient temperatures accelerated the flocculation by promoting Brownian motions of particles. The chemical effects refer to the increased hydration reaction: (1) the nanoclay addition increased aluminates contents, thus promoting the hydration of C3A to generate ettringite; (2) the elevation of ambient temperatures increased the dissolution of C3A then promoted the hydration. More importantly, ettringite is the main hydration product to affect the rheological properties of UHPC pastes. This study revealed the coupled effect of water-based nanoclay and ambient temperature on rheology of UHPC pastes and will advance the large-scale application of the water-based nanoclay suspension.