Investigating the thixotropy of fresh struvite cement-based composite: Insights on mechanisms of the pastes’ thixotropic behavior

Abstract

Struvite (ST) recovered during wastewater treatment has been sparsely applied in cement-based composites. This study systematically evaluated the thixotropic behavior of cement-struvite (CST) pastes, a mix of 5–20 % ST by mass of cement, and proposed the interaction mechanisms leading to the pastes' thixotropy. The hysteresis loop area was used to establish the pastes' thixotropy and investigate its relationship with increased ST content, temperature, and yield stress. In the thixotropic behavior model used, the equilibrium shear stress was employed to examine the CST pastes' irreversible change; the characteristic time of deflocculation was used to study the flocs destruction process; and the incipient structure parameter and flocs structuration rate were used to evaluate the structure-rebuilding process quantitatively. The results revealed an enhanced CST pastes' initial structural parameter (early re-flocculation), above 1.8 times higher than the control. On the other hand, their long-term structuration rates (0.002–0.004 s⁻¹) were lower than that of the control paste (0.006 s⁻¹), depicting the retardation effect at the hydration products' growth stage and improved workability. The CST pastes' yield stress, plastic viscosity, and thixotropic loop area were 1.2–1.7, 1.8–8.1, and 1.1–3.6 times above the control, respectively. The results suggest the mechanism of CST pastes’ thixotropy arising from the filler effects and attractive interaction that results in early flocculation and subsequent growth of hydration products at the bridge between cement and struvite particles. The increased short-term thixotropy of CST pastes suggests enhanced buildability, essential for 3D concrete printing.