Rheology, setting and hydration of calcined clay blended cements in interaction with PCE-based superplasticisers

Calcined clays represent a promising supplementary cementitious material (SCM) to face the growing demand for binders and to compensate for the decreasing availability of established SCMs worldwide. A fundamental knowledge about the interaction of cementitious systems with polycarboxylate ether (PCE)-based superplasticisers is required for their optimised use as clinker substitute. As clays contain different phyllosilicates, this study focuses on the behaviour of calcined phyllosilicates (metakaolin, metamuscovite, metaillite) and one calcined naturally occurring clay. The impact on the rheology, hydration and setting behaviour of blended cement pastes with two PCE-based superplasticisers is investigated using rotational viscometry, isothermal calorimetry and ultrasound, respectively. The investigations reveal a good dispersion effectiveness of both PCE-based superplasticisers even with challenging calcined phyllosilicates. The demand for superplasticiser can be derived largely from the physical properties of calcined materials, except for metamuscovite. The calcined naturally occurring clay is easily dispersed due to its high quartz content. The physical properties are also key parameters for the hydration and setting: the high specific surface area and pozzolanic properties of calcined phyllosilicates can compensate for retardation effects whereas there can be a significant retardation due to superplasticiser overdosage. Calcined clays with quartz may represent a promising future clinker substitute for the concrete industry in view of hitherto uncritical superplasticiser dosages.