Freeze-Thaw Resistance and Microstructural Analysis of Recycled Aggregate Concrete Incorporating Ferronickel Slag as Fine Aggregate

Abstract

The recycled fine and coarse aggregates prepared from ferronickel slag (FNS) and construction wastes applied in the preparation of recycled concrete have been recognized as an effective way to solve the crisis of scarce natural resources and reduce the carbon emission during the process of engineering construction.  However, the application of recycled aggregate concrete (RAC) incorporating FNS (abbreviated as FNS-RAC) is still limited due to the poor durability, especially the freeze-thaw resistance. Thus, this paper explored the coupling effects of different FNS and RCA contents to respectively replace fine aggregate (0%, 20%, 40%, and 50%) and coarse aggregate (0%, 50%, and 100%) on the frost resistance of concrete. The behavior of capillary water absorption by FNS-RAC before and after cyclic freeze-thaw (F-T) damage was further investigated, and then the microstructural evolution of FNS-RAC was observed through SEM, MIP, and laser scanning microscope (LSM) tests.  The results indicate that the frost resistance of concrete after F-T damage decreases with the increase of RCA content, while it first decreases and then increase as FNS content increases. Generally, the microscopic testing results show that the addition of RCAs forms some weaker multiple interfaces with loose structures within concrete.  However, the RAC with 40% FNS content as fine aggregate exhibits the better F-T resistance. It illustrates that incorporating an appropriate amount of FNS can produce more C-S-H gels to strengthen the microstructures of ITZs within concrete, partially offsetting the adverse effects caused by the poor physical properties of RCA.

Keywords: ferronickel slag, recycled aggregate concrete, freeze-thaw damage, water absorption, Microstructure