Discuss the reasons for ammonium nitrate’s corrosion of concrete
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When ammonium nitrate is in humid air, especially during the transition period between spring and summer, it readily absorbs moisture and dissolves into an aqueous solution or forms water mist; this has a severe corrosive effect on concrete, leading to the formation of new substances. This type of corrosion is mainly chemical corrosion damage. The damage caused by ammonium salts to cement stone primarily affects the hardened cement mass; it induces a chemical reaction with the free lime in the cement stone. The reason is: 2NH4NO3+Ca(OH)2 = Ca(NO3)2+2H2O+2NO3↑; it then reacts with calcium silicate hydrate or acts directly on calcium silicate, resulting in its decomposition. 6 NH4NO3 + 3CaO•SiO2 → 3 Ca(NO3)2 + SiO2 + 3 H2O + 6 NO3↑4 NH4NO3 + 2 CaO•SiO2 → 2 Ca(NO3)2•SiO2 + 2 H2O + 4 NO3↑
After the concrete disintegrates, Ca(NO3)2 and Ca(NO3)2•SiO2 are formed; ammonia volatilizes, and the cement’s setting and hardening properties are completely lost. The resulting material becomes a loose, flocculent substance or a muddy, disintegrated mass.
Ammonium nitrate reacts with the cement components in concrete, particularly with calcium hydroxide (Ca(OH)2) and calcium silicate (CaO·SiO2). These reactions produce calcium nitrate (Ca(NO3)2) and silicon dioxide (SiO2), while releasing ammonia and water. This chemical change causes the cementitious properties of the cemented mass to be lost, the structure becomes loose, and ultimately leads to the corrosion and disintegration of the concrete. .