A thermodynamic framework for modelling thixotropic yield stress fluids: Application to cement pastes

•A Helmholtz-potential based thermodynamic approach developed for modelling fluids that exhibit both yielding and thixotropic behavior.•The yielding behaviour was captured using an approach of multiple natural configurations.•Thixotropic behaviour was modeled using a structural parameter and its evolution with deformation.•A model developed using the approach corroborates well with experimental data of flow of cement paste. In this paper, a Helmholtz-potential-based thermodynamic approach was developed for modelling fluids that exhibit both yielding and thixotropic behavior. The yielding behavior was captured using a framework of multiple natural configurations. The thixotropy was described using a structural parameter. The Helmholtz potential is made a function of a structural parameter and a stretch quantity, and the rate of dissipation was made to depend on the rate of change of the natural configuration and the time derivative of the structural parameter. The final constitutive equations were obtained by maximizing the rate of dissipation with the reduced dissipation equation as a constraint. For experimental corroboration of the model, steady-shear and step-shear experiments were performed using cement paste. The predictions from the model were found to match well with the experimental data. Finally, the different model variants that can also be obtained using the presented framework are discussed.