Rheological study of the interactions between MHEC and superplasticizers in aqueous solutions for applications in cement industry

Innovative cement-based materials are characterized today by rather complex formulations. In particular, when cement pastes are considered, their formulation generally includes two different types of polymers: a rheology modifier (RM) (typically a cellulose derivative), which is used to control the viscosity of the system, and a superplasticizer (SP), which is used as an anti-flocculant for the solid phase suspension. Owing to the polar nature of both polymer and solvent (water), chemico-physical interactions between the two polymers are expected. In this paper, the rheology of aqueous solutions containing a RM and different types of SPs is studied. Solutions of the two components are prepared by respecting their relative proportions in the final cement formulations. Their behavior is studied by using rotational rheometry and performing viscosity and viscoelasticity measurements. The results show that, when the SP is added to the RM solution at room temperature, the system undergoes a dramatic, sharp transition from a high viscosity plateau to a low viscosity one. Such a viscosity drop, which can be of even four orders of magnitude, takes place at a well-defined shear stress threshold, and is not present in the pure RM solution. Additionally, when the viscoelastic moduli of the solution are measured as a function of temperature, a similar sharp transition in the rheological properties takes place when a threshold temperature (typically, in the 40-60°C range) is reached. The observed behavior, which is a clear proof of the thermodynamic interactions between the two polymers, is discussed and possible explanations are given.