Disclosing the mechanism behind rheological challenges in calcined clay-based cements

Calcined clay is gaining recognition as a viable cement substitute for producing low-carbon binders like limestone calcined clay cement (LC3). However, its impact on workability remains unclear. This study investigates the mechanisms causing rheological issues in calcined clay-based mixes. LC3 was formulated with two calcined clays of similar chemistry but differing specific surface areas. Differential Scanning Calorimetry (DSC) and Time-Domain Nuclear Magnetic Resonance ( 1 H TD‐NMR) measured free water at varying water-to-solid ratios, while paste and mortar rheology were evaluated and modeled. Results reveal significant differences in internal specific pore volume (0.108 cm³/g vs. 0.055 cm³/g), corresponding to varying water absorption (30 % vs. 15 % of mixing water), which increased yield stress from 106 Pa to 163 Pa and superplasticizer demand from 1.2 % to 1.3 %. A model to estimate the apparent yield tress (YODEL) clarified the role of water absorption in drastically increasing the yield stress of paste. • Calcined clay's internal porosity varies with geological origin of raw material. • Combined DSC and 1 H‐TD‐NMR quantify free water and capillary water in pastes. • Doubling calcined clay's porosity doubles its absorbed mixing water. • Doubling water retention raises dynamic yield stress by nearly 50 %. • YODEL was used to correlate free water reduction with yield stress increase.