Reactions of calcium aluminate concrete with external agents

11 Reactions of calcium aluminate concrete with external agents 

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

In contrast, resistance to attack by alkali hydroxide solutions is low, probably because the hydrous alumina is dissolved. Attack is accentuated if CO2 is also present, as can occur if the calcium aluminate concrete is in contact with Portland cement concrete and exposed to atmospheric CO2 (R48). For CAHjq, the following reactions occur  

The OH ion thus acts as a carrier for atmospheric COj and is effectively regenerated the CAH,o is decomposed giving CaCO, and hydrous alumina. This effect is known as alkaline hydrolysis. A similar process occurs with CjAH(i or calcium aluminate concretes in which conversion has occurred (B132). 

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