Calcined clays are promising supplementary cementitious materials but can impair initial flowability and workability retention in concrete. This study investigated the influence of five superplasticizers on the early flow behavior of 70/30 vol% cement/calcined clay systems with four calcined 2:1 phyllosilicates with different illite and smectite contents. Over 2 h, rotational rheometry was applied to paste and concrete, and spread-flow tests to paste, mortar, and concrete, focusing on non-linear flow behavior and thixotropy. Across paste, mortar, and concrete, superplasticizer demand was governed by calcined clay type rather than superplasticizer type. The general demand followed the order illite > smectite/illite mixture > smectite, although the ranking changed slightly in concrete and quantitative prediction from paste to concrete was not possible. Diverging rheological impacts of seemingly similar clays may stem from colloidal interactions. Workability-retention challenges appeared to originate in the initial flow state: the flowability improvement from superplasticizers decreased by more than 60% within 2 h. A systematic dosage study for one calcined illite in paste showed that a flocculation-based thixotropic recovery became increasingly dominant at higher dosages, in addition to a suggested dosage-independent platelet-based contribution, whereas shear thickening increased with dosage and was confirmed across all investigated calcined clays. A moderate dosage provided the best workability retention. In concrete, limited data indicated pronounced static yield stress, shear-history dependence, and apparent saturation limits for certain clay–superplasticizer combinations. Overall, high superplasticizer dosages in calcined clay systems may impair workability retention or induce shear thickening, shifting the optimum toward lower dosages.     «

Calcined clays are promising supplementary cementitious materials but can impair initial flowability and workability retention in concrete. This study investigated the influence of five superplasticizers on the early flow behavior of 70/30 vol% cement/calcined clay systems with four calcined 2:1 phyllosilicates with different illite and smectite contents. Over 2 h, rotational rheometry was applied to paste and concrete, and spread-flow tests to paste, mortar, and concrete, focusing on non-linear...     »