Partially fly ash incorporated recycled coarse aggregate based concrete: Microstructure perspectives and critical analysis

Abstract

This paper presents experimental investigation of the microstructure of partial cement substituted fly ash incorporated recycled aggregate concrete (FARAC). Testing techniques such as thermo-gravimetric analysis, scanning electron microscopy, nano-indentation and X-ray tomography were adopted for this study. Effects of parameters like (i) percentage of fly ash (20% and 30% as cement replacement by weight), (ii) particle packing mix design and conventional method, (iii) curing ages (7, 28 and 90 days), and (iv) coarse aggregate types (100% natural or recycled) are examined. Degree of hydration of FARAC, though higher than natural aggregate concrete, did not translate directly into compressive strength due to formation of more loosely connected low density C-S-H confirmed by SEM images. It is observed that micro-cracks pass mostly through the old and fresh bulk paste rather than the old ITZ in 90 days cured concrete. Thickness of the old ITZ as well as heterogeneities of the new ITZ reduced in FARAC. Upto 30% fly ash can compensate higher porosity of RAC through formation of spherical smaller-size pores.