We're making precast panels using integral color in the concrete. The panels are covered with a tarp at night to hold in the heat and make them cure faster. Our problem is that moisture condenses on the underside of the tarp and drips on the panel. When the drops evaporate they leave a white powder that stains the panel surface. What can we do to prevent the stains or to remove them afterwards?
The stain is probably an alkaline salt that has leached out of the concrete. When water evaporates, it doesn't carry the salts with it. So I'd guess that drops of pure water on the surface cause salts from within the concrete to migrate toward the pure water. Some cements cause greater amounts of efflorescence than others, so you might solve the problem by changing cement source. Try a low-alkali cement if one is available. Changing cements, however, isn't a good idea if you've already cast a number of panels. The new ones probably won't match the color of the ones already cast. Another option is removing the efflorescence by washing and brushing with a stiff-bristle brush. Do this as soon as possible after form stripping when the salts are possibly still water soluble. An acid may work if plain water doesn't dissolve the salts. Try diluted vinegar first. If that doesn't work, a muriatic acid may be needed, followed by a neutralizing solution. All of these cleaning methods are labor intensive and will drive up your cost per panel. Also, it's important to standardize the procedure so washing itself doesn't cause color changes. A third option is using the panels as is. Discuss the situation with your client. Stains of this type may become less noticeable with the passage of time. If all panels look nearly the same, the condition may be tolerable.