Free superplasticizer concentration is a key influencer of the instantaneous thixotropy rates of cementitious pastes

Thixotropy, the property of a fluid to decrease in apparent viscosity under stress and recover its viscosity upon stress relief with observable kinetics, is a crucial characteristic in the behavior of cement pastes in addition to plasticity - the presence of a yield stress, or shear-thinning. This phenomenon has significant implications for the workability, stability, and overall performance of cement-based materials. Understanding thixotropy in cement pastes is particularly important for optimizing the placement and finishing processes in concrete construction, as well as for the development of advanced materials with tailored rheological properties. From concrete placement to additive manufacturing, it is an influential feature of cementitious pastes that scales up to the full-scale material. While numerous studies linked the admixture adsorption kinetics to the slump evolution, this paper aims at illustrating how thixotropy and structural buildup kinetics are affected. A first result of the present work on adsorption is that full superplasticizer consumption is observed at low dosages and beyond five minutes of observation, which does neither correspond to polymer capture by early hydrates nor comply to Langmuir-like mechanisms. Simultaneously to adsorption measurements, rheology flow curves were acquired and modeled according to a simple framework based on a time-evolving structural parameter. The fitted model parameters were then compared to the adsorbed amounts over time. The observations show that after the moment of full polymer consumption, structural buildup strongly accelerates while the ultimate dispersion state of the suspensions drops sharply. On the other hand, as long as enough polymer remains in solution to fuel further adsorption, the ultimate dispersion degree is high and structural buildup remains slow. This work sheds a different light on the role of superplasticizers, which behave as thixotropy-mitigators as much as pure dispersants through their adsorption kinetics.