Improving strength of recycled fine aggregate concrete using metakaolin
  Krishnamurthy Pandurangan and Rajendiran Arunpandian
  In India, traditionally river sand is used as fine aggregate in concrete production. However, due to continuous exploitation of river sand, its availability has become scarce. Construction industry has started using alternate materials as fine aggregate. Out of which manufactured sand is one of the popularly used material as fine aggregate. Materials such as quarry dust and recycled concrete aggregate have issues of dumping and environmental pollution. These materials are viable alternatives for replacement of natural river sand. Recycled concrete, if used as coarse aggregate has problems of water absorption and reduction in strength. Hence, an attempt has been made in this research to use recycled concrete aggregate (RCA) as fine aggregate in concrete. Further, use of RCA as fine aggregate in concrete may reduce the compressive strength of concrete. Hence, it is proposed to use metakaolin as a supplementary cementitious material in concrete with Recycled Fine Aggregate (RFA) as fine aggregate. This study investigates the effect of metakaolin and RFA on the mechanical characteristics of concrete mixtures. Mechanical properties such as compressive strength, split tensile strength, flexural strength at 7 and 28 days are determined. Metakaolin is added to cement in the range of 7.5% to 20% by weight of cement. Natural sand is replaced with RFA in the range of 0-100% by weight of sand. From the study it is found that RFA could be used as alternative material for natural sand. However, as complete replacement of RFA reduces the strength of concrete, metakaolin addition is mandatory. A minimum of 7.5% metakaolin is required to prevent strength loss in concrete. However, if RFA alone need to be used without addition of metakaolin, only 50% replacement of sand with RFA is recommended.