Workability challenges imposed by calcined illitic and smectitic clays: Practical insights into their behavior as SCM in low-carbon cements

The use of calcined clays (CCs) as supplementary cementitious materials is gaining increasing attention as a strategy to lower the carbon footprint of cement production. To date, research has largely concentrated on kaolinite-rich (1:1) clays, which are relatively scare in Europe, where illitic and smectitic clays are more prevalent. This work investigates the interaction between polycarboxylate (PCE) superplasticizers and calcined illitic, smectitic, mixed-layered clays, and meta kaolin in binary OPC/CC blended systems. Meta kaolin and calcined illitic clays exhibited notably increased water demand. A series of methallyl ether (HPEG) PCEs were synthesized and applied, demonstrating that dispersing efficiency is strongly influenced by the molecular structure. PCEs with medium-long side chains and high anionicity (e.g. 52HPEG10) exhibited optimal fluidizing effectiveness. Notably, calcined illitic clays required substantially higher PCE dosages, one sample even could not be adequately dispersed despite the excessive dosage (2 wt% of binder). Interestingly, delayed addition of PCE could reduce PCE dosage demand significantly, especially at low water-to-binder ratios (0.4–0.5), indicating a simple and effective strategy to mitigate the adverse impact of CCs on workability. Mortar tests further revealed severe slump retention challenges posed by illite- and smectite-rich CCs. Adsorption and XRD analyses indicate that PCEs interact with CCs only via physisorption, with calcined illitic clays showing linear adsorption isotherms, which could explain their poor workability. Overall, the results highlight the critical role of PCE molecular architecture and clay mineralogy, emphasizing the need for tailored superplasticizers in calcined clay based low-carbon cements.