Sieve-retained coarse fraction as a practical descriptor of permeability and fast-drying in extruded red clay ceramics

This study evaluated the sieve-retained coarse fraction above 44 µm ( R 44 ) as a practical descriptor of permeability and fast-drying performance in vacuum-extruded red clay ceramics. Ten formulations containing kaolinite-based, muscovite-based, and montmorillonite-based clays were prepared according to a simplex-centroid mixture design and subjected to a 60 min drying cycle at 160 °C. The Sauter mean diameter of the complete particle-size distribution ( d vs ), the Sauter mean diameter of the retained coarse fraction ( d vs ,>44 ), the Darcian permeability coefficient ( k 1 ), residual water after drying ( RW ), and dry compressive strength ( DCS ) were determined. Permeability was strongly formulation-dependent, with k 1 ranging from (1.07 ± 0.40) × 10⁻¹ ⁷ to (21.60 ± 2.55) × 10⁻¹ ⁷ m². Neither d vs nor d vs ,> 44 showed a consistent relationship with permeability, whereas R 44 exhibited a clear positive association with k 1 . Increased permeability enhanced moisture removal, and only the montmorillonite-based formulation failed to reach the target residual water content of 2 wt% after 60 min. Within the investigated formulation domain, all compositions with R 44 > 15 wt% reached RW < 2 wt%. Muscovite-rich binary and ternary mixtures provided the most balanced combination of permeability, residual water, dry strength, and post-firing performance. The most promising ternary composition, containing 66.7 wt% muscovite-based clay and 16.7 wt% of each of the other clays, achieved RW = 0.64 ± 0.02 wt% and DCS = 0.96 ± 0.09 MPa. After firing at 900 °C, water absorption ranged from 8% to 16%, all specimens exceeded the minimum compressive-strength requirement, and acceptable dimensional stability was obtained for all formulations except the montmorillonite-based composition. These results demonstrate that R 44 is a simple and industrially accessible parameter for screening red clay formulations intended for short-cycle drying.