Concrete Containing Recycled Concrete Aggregate with Modified Surface

Abstract

This paper is focused to the recycling of one kind of construction & demolition waste (C&DW), namely concrete. After crushing and sorting, such recycled concrete aggregate (RCA) can be used for production of new concrete. The use of RCA may help to save natural aggregate sources and thus contributes to environmental protection.The most negative properties of such aggregate, comparing the natural one are roughness of its surface and high water absorptivity as well. Those features cause complications during concrete production technology mainly they have negative impact on the amounts of cement and mixing water and also on the final properties of concrete. Using laboratory techniques, the 4/8 fraction of RCA was prepared for experiments aimed on the enhancement of surface quality and performance of such aggregate. To modify the surface properties of RCA, geopolymer material based on fly ash was used in different ways. The first one is the coating of RCA by geopolymer slurry, then drying under ambient conditions and finally application into concrete mixture. The second one is coating directly during mixing of concrete, using so called “triple stage mixing”.In the paper, properties of both sets of concretes will be compared and analyzed in terms of workability, real density, total water absorption and compressive strength. Slight differences in those properties were found, comparing the reference samples with no any surface treatment of RCA, as well as comparing the ways of RCA modification each other; that is why the modification conducted directly during mixing of concrete can be recognized to be preferable due to better simplicity.