Compressive behavior and microstructural analysis of low and high volume fly ash concrete containing recycled concrete aggregate

This work studied the compressive behavior and microstructure of cement-based recycled concrete (CE-RC), low-volume fly ash (FA) recycled concrete (LFA-RC), and high-volume FA recycled concrete (HFA-RC) under different curing ages. The compressive properties, energy absorption properties, and microstructure of CE-RC, LFA-RC, and HFA-RC were measured and compared with cement-based normal concrete (CE-NC). The addition of coarse recycled concrete aggregate (CRCA) leads to more macropores and micro cracks in the concrete. As the result, the mechanical property of RC is worse than normal concrete. FA improved the energy absorption performance of recycled concrete in the early stage. With the continuous curing, pozzolanic reaction occurred between FA and the cement hydration product. The internal microstructure of LFA-RC and HFA-RC was improved. However, part of the FA in HFA-RC can only act as a filler and cannot fully participate in the secondary hydration reaction. Therefore, in the mature age, the internal pore distribution of LFA-RC was the most uniform. And the improvement of strength is more obvious.