Design and preparation of metakaolin-based mineral admixture and its effects on the durability of concrete

Abstract

Many attempts have been made to find a highly-effective way to improve concrete durability. Metakaolin-based mineral admixtures were designed and produced for this purpose and their effects on concrete durability were investigated in this study. The design principles of the admixture were based on the match of the potential active ingredients, such as active SiO 2 and Al 2 O 3, among the major active mineral materials including metakaolin (MK), fly ash (FA) and limestone powder (LSP). The proportions of MK, FA and LSP in the admixtures were determined by triangle phase diagram of CaO-SiO 2 -Al 2 O 3. Based on the above principle, three MK-based mineral admixtures were designed and applied in the concrete by substituting ordinary Portland cement, namely C1 (5 wt%MK + 2 wt%FA), C2 (7 wt%MK + 2 wt%FA + 2 wt%LSP) and C3 (9 wt%MK + 2 wt%FA + 2 wt% LSP). Finally, the effects of the admixtures on the durability of concrete were studied by a range of analytical techniques. The chloride permeability resistance of concrete was significantly improved with the addition of the admixtures. The chloride diffusion coefficient decreased by over 50% at 56 days. The admixture significantly reduced the shrinkage strain of concrete, and further decrease occurred with the increase of limestone powder content and metakaolin content in admixtures. Improvement of concrete performance resulted from the increase of amount of hydration products, refinement of pore structure and densification of interfacial transition zone (ITZ) caused by the admixture.