Durability of concrete containing fly ash and silica fume against combined freezing-thawing and sulfate attack

- Sulfate resistance of concrete with fly ash (FA) & silica fume (SF) is investigated.
- The effects of FA and SF replacement level under the combined freezing-thawing and sulfate attack are analyzed.
- Sodium sulfate concentration under the combined freezing-thawing and sulfate attack are analyzed.
- Interaction between freezing-thawing and sulfate attack to concrete with fly ash (FA) & silica fume (SF) is discussed.
- SEM analysis of concrete subjected to freezing-thawing cycles in sulfate solutions were conducted.

Durability of concrete containing fly ash (FA) and silica fume (SF) against combined freezing-thawing and sulfate attack was studied in this paper. Concretes with w/b of 0.38 and 0.33 containing FA (i.e. of 10%, 15% and 25% by weight) and SF (i.e. of 5%, 8% and 11% also by weight) as partial replacement of Portland cement (PC) were exposed to 5% and 10% sodium sulfate solution under freezing-thawing cycles. The performance, including deterioration resistant coefficient of compressive strength, relative dynamic elastic modulus (RDEM) and microstructure, of concretes were evaluated after being subjected to certain freezing-thawing cycles in sodium sulfate solution. It was found that when exposed to 5% sodium sulfate solution, both FA and SF can improve concrete's resistance to sulfate attack and in comparison SF performed better than FA. Concrete deterioration was attributed to the interaction between freezing-thawing and sulfate attack. As for concrete without any admixture, its resistance against combined freezing-thawing and sulfate attack increased up to 125 freezing-thawing cycles and then decreased. The replacement level of 25% FA and 5-8% SF both by weight led to significant improvements in the resistance of concrete against combined freezing-thawing and sulfate attack. 10% sodium sulfate solutions more obviously improved freezing-thawing resistance of concrete with 25% by weight FA replacing OPC than 5% sodium sulfate solutions, while 5% and 10% sodium sulfate solution had the similar improvements in freezing-thawing resistance of concrete with 8% by weight SF replacing OPC.