Effect of Fly Ash, Sinking Beads and Metakaolin on the Workability, Strength, Free Shrinkage and Chloride Resistance of Concretes: A Comparative Study

The aim of this study is to compare sinking beads (SB) with fly ash (FA) and metakaolin (MK) with regard to their ability to improve the performance of concretes. The workability, compressive strength, free shrinkage and chloride resistance of concretes with and without the above mineral admixtures are investigated. To explicate the differences of above results, the pore structure and pore solution chemistry were also studied. Besides, the Chapelle test was adopted to compare the pozzolanic activity of the mineral admixtures. The water-to-binder ratio of concretes and pastes was 0.35 by mass. The incorporation level of MK was 5 and 10 wt%, while that of FA and SB was 10, 20 and 30 wt%. A systematic improvement in flowability, compressive strength, chloride resistance, free shrinkage is observed for SB concretes compared with FA concretes due to the smaller particle size, more round particle shape and higher pozzolanic activity of SB. MK shows the overwhelming ability to improve compressive strength and chloride resistance ability of concretes compared with SB and FA for the same replacement level, but comparable compressive strength and chloride resistance for SB concretes can be obtained by increasing the content of SB. In addition, inclusion of 20 wt% SB demonstrates remarkable potential to reduce free shrinkage to 25% less than concretes with 10 wt% MK. It may be ascribed to the dense structure of SB, which brings less volume reduction or even some increase during pozzolanic reaction.