As a concrete admixture, sodium nitrite can play three roles:
1) As an early strength agent, improve the early strength of concrete;
2) As a rust inhibitor to prevent corrosion of steel bars;
3) As an antifreeze agent, prevent the construction concrete from being damaged by freezing in winter.
However, sodium nitrite is a sodium salt, which will bring a large amount of sodium ions into the concrete, and may cause the alkali aggregate reaction. Therefore, it should not be used in concrete of important structures. In addition, sodium nitrite is more toxic, so you need to be careful when using it.