OHSI – Onset Heat Size Index: A new parameter to predict rheology of cement-calcined clay blends

Calcined clays (CCs) are promising supplementary cementitious materials given their potential for improving concrete durability and sustainability. However, they often cause high water demand and associated workability challenges. In this study, the rotational rheology of 16 cement and largely kaolinitic-CC blends was analyzed, revealing a wide range of dynamic yield stress (DYS) values (25–300 Pa). These DYS values strongly correlate with the ASTM C1437 paste flow, ranging from ∼30 % to 140 % (R 2 > 0.9). Interestingly, calorimetry results indicated that the rheological properties are linked to the onset heat (OH). By combining 3 unique parameters: OH, and particle size distribution (PSD) outputs ( D 10 and D 50 ) a new parameter has been introduced, the onset heat size index (OHSI = OH/ D 10 ·D 50 ). This OHSI can reliably predict the DYS (R 2 = 0.90, RMSE = 26 Pa), opening a pathway for employing widely adopted calorimetry and PSD measurements to simultaneously predict reactivity and rheology of cement-CC blended systems. • Both the physical and chemical composition influence the water demand of calcined clays. • Commercially available calcined clays display a wide range of rheological properties (DYS ∼25–300 Pa). • Binary and ternary blends are effective options for optimizing rheology and strength. • Onset Heat Size Index can predict the rheology of cement-calcined clay blends (R 2 = 0.9, RMSE = 26 Pa).