Effect of metakaolin fineness on performance and microstructure of cement-based materials

This work studied the effect of metakaolin fineness on the performances of cement-based materials through measurements of calorimetric response, mechanical properties, and capillary water absorption. Using multi-technique characterization (X-ray diffraction, SEM, TGA, MIP, and solid state 29 Si NMR), the phase composition and microstructural characteristics of samples were analyzed. Results indicate that the presence of metakaolin accelerates the hydration of cement, and the acceleration effect is more significant with the finer MK. Increased MK fineness enhances its pozzolanic reactivity at early age. Fine metakaolin significantly enhances the mechanical properties of cement mortars compared to coarse metakaolin. Finer metakaolin reduces both initial and secondary water absorption rates, with optimal effects observed at 20% replacement. The incorporation of metakaolin refines the ITZ and the cement matrix, these effects are more pronounced with increasing fineness. However, 29 Si NMR results suggest an optimal fineness below which further benefits do not increase. • Metakaolin improves the cement hydration and cement mortar strength, more pronounced as higher replacement and fineness. • Water absorption rates reduce as the metakaolin fineness increases. • Metakaolin improves the ITZ and refines the pores, more pronounced as the higher metakaolin fineness. • 29 Si MAS NMR indicates an optimum fineness of metakaolin.