Influence of Metakaolin and Silica Fume on the Mechanical and Durability Performance of High-Strength Concrete Made with 100% Coarse Recycled Aggregate

Abstract

There is a strong need for a sustainable and ecofriendly approach from the construction industry toward mitigating climate change, as it is one of the critical sectors accountable for exploiting natural reserves and generating greenhouse gases. In order to resolve this issue, various supplementary cementitious materials (SCMs) and artificial aggregates are now in use. The use of recycled aggregate (RA) also seems to be a possible alternative to natural aggregate. However, the practical utilization of RA is rare. The amount of porous attached mortar and the source of the RA determines the quality of RA concrete (RAC). In this study, efforts were made to improve the quality of high-strength RAC, made with 100% RA, by incorporating metakaolin (MK) and silica fume (SF) using the well-known two-stage mixing approach (TSMA). In addition, DIN standards were followed to obtain the maximum particle packing of the aggregate. In total, 10 types of RAC were mixed––two comprising RACs at water-to-binder (w:b) ratios of 0.25 and 0.35, and eight mixes for obtaining a high-strength RAC. Both MK and SF were incorporated into those eight mixes at 10% and 15% replacement levels and w:b ratios of 0.25 and 0.35. The synergistic effects on fresh and hardened RACs, and their durability performance, considering DIN standards, the TSMA, and SCMs, were addressed. In this context, a comparative analysis showed positive results for using SF and MK in RAC, bearing in mind its mechanical and durability performance.