Evaluation of zeta potential of calcined clays and time-dependent flowability of blended cements with customized polycarboxylate-based superplasticizers

Abstract Workability of cementitious systems is often limited when blending them with calcined clays. The first part of the study compares for the first time results of zeta potential measurements on both, raw/calcined clays in water and synthetic cement pore solution as well as on a Portland limestone cement replaced by 30 wt% of calcined clay. This helps to identify critical parameters that reduce the workability, also over time. The second part evaluates the time-related flow behavior via mini slump test and rotational viscometer and, by using isothermal calorimetry, early hydration kinetics of different calcined common clays with two polycarboxylate-based (PCE) superplasticizers. The results reveal a high negative zeta potential resulting from high kaolinite content. A large specific surface area enhances flocculation and nucleation, while an increasing amount of phyllosilicates and soluble aluminum ions accelerate ettringite formation also with the addition of PCE. Tailor-made PCE, however, can compensate rapid loss in flowability and enable a sufficient workability period without affecting early strength development significantly for most calcined clay blended systems.