Cause of White Streaks
White streaks in concrete are most likely to be seen on sealed driveways, but they can also occur in other masonry surfaces. A chemical reaction between salts in the concrete and water, which is triggered by a physical event leads to a phenomenon known as efflorescence. (See Reference 1.)
Factors Leading to Efflorescence
The type of material most likely at fault in a concrete driveway is the presence of calcium hydroxide. This salt is a natural result of the hardening process of mixing and laying out the concrete. Having calcium hydroxide in the cement increases the likelihood of streaking. (See Reference 1.)
The Physical Trigger
Water sitting on the surface of the concrete driveway for a long time may dissolve the calcium hydroxide in the concrete. When it has been dissolved, it can combine with the carbon dioxide in the air. As the water evaporates, what is left behind is calcium carbonate. (See Reference 1.) This appears as white streaks on the concrete. It is not soluble in water, and removing it can be difficult, but the process of weathering will change the calcium carbonate to water soluble calcium hydrogencarbonate.
White Streaks Caused by Sealants and Water Repellants
Water sealants can trap water just under their surface during application, if there is water on the surface or too much sealant is used. That water can dissolve the calcium hydroxide in the concrete and convert it to calcium carbonate through evaporation. If this happens, dissolve the sealant and clean the surface of the concrete before reapplying the appropriate amount of sealant. (See Reference 2.)