Resistance of recycled aggregate concrete containing low- and high-volume fly ash against the combined action of freeze-thaw cycles and sulfate attack

The present study investigated the effect of the combined action of freeze-thaw (F-T) cycles and sulfate attack on the resistance of concrete containing low-volume fly ash (LVFA) and high-volume fly ash (HVFA) made with coarse recycled concrete aggregates (CRCAs). Concretes with a water-binder ratio of 0.50 containing fly ash (FA; LVFA and HVFA) and CRCA (i.e., 0%, 20%, 50% and 100% also by weight) as a replacement for coarse natural aggregates (CNAs) were exposed to water, 5% sodium sulfate solution and 5% magnesium sulfate solution under F-T cycles. The performance, including residual compressive strength, relative dynamic modulus of elasticity and concrete microstructure, was evaluated after being subjected to certain F-T cycles in sulfate solutions. Results indicated that the resistance of the concrete mixtures to the combined F-T cycles and sulfate attack increased with the increase in CRCA content as CNA replacement. Compared with the concrete without FA, the LVFA-based concrete showed excellent improvement in the resistance to the combined action of F-T cycles and sulfate attack; however, the HVFA-based concrete had an adverse effect on the resistance. Concrete deterioration was attributed to the interaction between F-T and sulfate attack. Moreover, the resistance of LVFA- and HVFA-based concretes against the combined F-T and sulfate attack increased during the entire test when the concretes were subjected to F-T cycles in 5% sodium sulfate solution. The sulfate attack exerted more positive effects than negative on the F-T cycles. However, the resistance of LVFA- and HVFA-based concretes against the combined F-T and sulfate attack increased during the initial F-T cycles and then decreased in the 5% magnesium sulfate solution. The 5% sodium sulfate solution produced similar improvements in the F-T resistance of the LVFA- and HVFA-based concretes, whereas the 5% magnesium sulfate solution evidently reduced the F-T resistance of the concrete with HVFA than that with LVFA.