Decay of concrete is mainly due to the following reasons:

• Concrete cement contains some free lime(CaO). In acidic water it dissolves making the concrete weak. As lime is more soluble in soft water than hard water, deterioration of concrete is quicker in contact with soft water.
• Sugar also causes concrete failure.Even if 10% sugar is added to cement, the setting time is delayed and its strengh is greatly reduced.
• The metal sulphates present in hard water combines with tricalcium aluminate to form sulpho aluminates which occupies more volume.Hydrolysis of silicates and aluminates also lead to decay of cement.

​Decay of cement can be prevented by:

• Coating surface with epoxy resin paint which makes the surface impermeable to acidic water.
• Treating the surface with sodium fluoride.