Influence of calcined coal-series kaolin fineness on properties of cement paste and mortar

Three calcined coal-series kaolin (CCK) samples, with same chemical and mineral composition but different fineness (Blaine specific surface of 760, 1369 and 1856 m2/kg), were used as pozzolan in cement paste and mortar and the influence of their fineness on materials properties was systematically studied by several methods and analyses. The pozzolanic activity of CCK is evaluated by Frattini test and confirmed by mechanical properties of blended mortars. TG-DSC analysis, complemented with XRD analysis, determines the type and the amount of hydrated phases at different ages to evaluate the influence of fineness on hydration properties of blended paste. Finally the microstructure of blended paste containing finer particles is observed by FESEM analysis. The results show that the finer particles have higher pozzolanic activity, leading to the more consumption of Ca(OH)2 and the more production of additional C-S-H, C-A-H and C-A-S-H, which compact the paste and mortar texture and remarkably enhance the mechanical strength at later ages (after 7 days). The finer particles also have higher physical effect to accelerate the cement hydration and slightly increase mechanical strength, Ca(OH)2 content and combined water at earlier ages (before 7 days).