Concretes incorporating recycled geopolymer aggregate - Implications and properties correlations

Despite extensive research of geopolymers, the potential of recycling after the end of service-life has barely been investigated. The present paper is part of a study investigating recycling of fly ash-slag geopolymer concrete as aggregate in Portland cement concrete mixes. Three types of geopolymer concretes with varying Na2O% and Na: Si ratio were recycled. Properties of concretes such as compressive strength and water absorption were tested. After curing for a period of three months, they were crushed to produce coarse recycled aggregate. The recycled aggregates were subjected to examination of physical properties such as density and water absorption.
Concretes with 20% replacement of coarse natural by recycled aggregate were produced. The compressive strength and water absorption were tested at various ages. The results were evaluated against mixes with natural limestone and recycled Portland cement concrete produced in a similar manner. The results revealed that physical characteristics of recycled aggregates, such as sharpness and shape, affected workability and subsequently strength. Aggregate water absorption was taken into account at mix design, while it appeared to be directly related to concrete water absorption. The effect of geopolymer incorporation on strength seemed to be more dependent on the aggregate type and particularly on Na2O% of original concrete. Overall not significant decreases of strength were observed, while 90 days testing revealed a continuous strength development.