Roles of packing density and slurry film thickness in synergistic effects of metakaolin and silica fume

Abstract Metakaolin (MK) and silica fume (SF) have been added either individually or in combination to improve concrete performance. Recent studies proved that the combined addition of MK and SF offers synergistic effects, but the mechanism behind is still not unclear. Herein, it is postulated that the synergistic effects arise mainly from the successive filling effects of the MK and SF into voids between larger size particles, which increase the wet packing density (WPD). The present work indicated that the slurry formed of the water and ultrafine particles plays an important role. Good correlation of the workability measures to the slurry film thickness (SFT) revealed the governing factor of SFT. Moreover, the synergistic effects of MK and SF on the compressive and flexural strengths have been studied based on cementing efficiency and synergistic factors. Correlation of the synergistic factor to the WPD revealed that the WPD plays an important role.