Leaching is caused by the small amounts of lime being washed out of the concrete. Without proper sealing of the concrete, the lime will begin to "leach" out of the concrete.
Some of the additives we use have small amounts of lime in them. If they are used the concrete must be protected from water or the leaching will occur. It is not harmful but unsightly in some uses.
This is the situation we see with brick jobs. The lime in the mortar leaches. Generally after it is removed once there is no longer a problem. Muratic acid is the old standby for removing this leachate.
Berlex is the only additive that we recommend to prevent leaching.