What causes efflorescence on block paving?
Cement, which is used in the manufacture of concrete block paving contains lime or calcium dioxide, which is soluble in water. When the surface of your block paving driveway gets wet some of this calcium dioxide dissolves to forms calcium hydroxide, which when suspended in water is referred to as the milk of lime.
When the block paving dries the suspended particles of calcium hydroxide rise to the surface of the blocks. Here it reacts with carbon dioxide in the air to form calcium carbonate, the dusty white powder.
Eventually this chemical process slows down and stops as all of the lime near to the surface of the blocks has been used up, or the tiny pores in the paving become blocked up. Efflorescence will then stop forming and any residue will be washed away.
Cleaning agents are available to remove efflorescence from the surface of block paving but this tends only to be a temporary solution. The best way to remove efflorescence from block paving is by allowing time for the chemical process to run its course.