When concrete is exposed to sulphate, the penetration of aggressive agents and it’s reaction with the cement matrix can cause concrete deterioration. The three main reactions involved are:
- Ettringite formation - conversion of hydrated calcium aluminate to calcium sulphoaluminate
- Gypsum formation - conversion of the calcium hydroxide to calcium sulphate
- Decalcification - decomposition of the hydrated calcium silicates
Each of these chemical reactions can lead to expansion, cracking and the loss of strength and elastic properties of concrete. The form and extent of the damage to concrete will vary depending on the:
- Sulphate concentration
- Type of cations (sodium or magnesium) in the sulphate solution
- pH of the solution
- Microstructure of the hardened cement
NZS 3101:2006 has prescriptive requirements in the durability section for concrete to be used in this environment for a 50 year design life. The exposure class is determined by the severity of the application. These are shown in the table below:
Additional protective measures are required for environments more severe than those listed in NZS 3101:2006, or in more highly acidic and permeable soils with a pH below 3.5 – in order to isolate the concrete from direct contact with the aggressive ground conditions.