Q.: We recently had a driveway pour and the contractor covered the wet concrete with polyethylene sheeting in case it rained. When the poly was taken off the next day, we saw discolored concrete in waves and stripes. Will this go away over time or can it be washed out now?
A.: This color change is probably due to differential curing because of variations in water content on the surface. More specifically, it’s due to the uneven hydration of tetracalcium aluminoferrite hydrate, which is the component of portland cement containing iron and aluminum—raw materials—that gives concrete its gray color. Where plastic is in contact with concrete, more moisture is supplied to the concrete, and a darker color results. Plastic above the concrete reduces the amount of hydration and a lighter color results. It may fade somewhat over time, but the variation will probably always be visible. It does not, however, have any negative effect on the durability of the surface, and it’s much better than having had rain on the surface. In fact, putting the poly down will result in a stronger surface than if it had not been covered because it helps retain water in the surface and helps the cement to gain higher strength.
Immediately after the plastic cover is removed, try wet curing the slab for a couple days and then allowing it to dry out. Repeat this cycle. It should even up the coloration. You can also apply a 20% solution of ammonium citrate dibasic sold by Jost Chemical, St. Louis, 314-428-4300, to even up the coloration of the surface. Apply it to a test area first however.