Effect of Salt Admixtures on Air Content in Fresh Air-Entrained Concretes

Combined with the foam stability of air-entraining admixtures (AEAs), the effect of salt admixtures (including CaCl2, Ca(NO3)2, Na2SO4 and NaNO2) on morphology of air-voids in cement paste with a couple kinds of air-entraining admixtures (including SJ-2 and DH-9) was examined by means of SEM. In accordance with foam stability and the morphology, air content of fresh concrete was studied and analyzed more deeply. The results pointed out that these salts mitigated the foam stability of SJ-2 solution, and the deteriorated effect of CaCl2 and Ca(NO3)2 was more obvious. By contrast, these salts seemed to make no difference to the foam stability, but CaCl2 and Ca(NO3)2 reduced the foaming power of DH-9 solution. The walls of air voids in cement paste without using AEAs were compact. Differently, it was a distinct shell that formed around most of the entrained air voids with AEAs. This shell connected with capillary pores. For SJ-2 mixed, the air content of fresh air-entrained concretes was decreased by CaCl2 and Ca(NO3)2, but was enhanced by Na2SO4 and NaNO2; for DH-9 mixed, with the increasing usage of CaCl2, Ca(NO3)2, Na2SO4 or NaNO2, the air content was rising gradually.