Effect of sodium sulfate and sodium nitrite on air-void system in air-entrained concrete

The purpose of this work was to explore the effect of sodium salts on the air-void structure in air-entrained concrete, and to observe the morphology of the air voids in the presence of sodium sulfate (Na₂SO₄) and sodium nitrite (NaNO₂) as well as the use of sodium abietate. The foam index test was used in analysing the foam stability of sodium abietate, microscopic determinations of the Powers spacing factor in hardened air-entrained concrete and the air-void distribution were characterised, and a microscopic evaluation was carried out using scanning electron microscopy for the morphology of air voids in cement paste. The foam index test indicated that the introduction of either sodium sulfate or sodium nitrite results in a drop in foaming power, but has no effect on foam stability. The addition of sodium salt was found to lead to a decrease in the pore size of air voids in air-entrained concrete. Adding sodium sulfate in conjunction with sodium abietate was found to gradually increase the Powers spacing factor, while the corresponding results for sodium nitrite were the reverse. Regardless of the sodium salt used, a distinct shell was observed to form around the entrained air voids. However, with the addition of sodium sulfate, more needle-like crystals were formed on the surface.