Alkali Burning / Discoloration

DESCRIPTION

Color loss and overall deterioration of paint film on fresh masonry, concrete, or stucco.

POSSIBLE CAUSE

Stucco and mortar joints contain hydrated or "hot" lime, a dry compound added to improve the workability of these materials. Different mortars are compounded with different amounts of "hot" lime, depending on the end use of the mortar. This problem is then made worse by more "hot" lime being added at the job site.
The alkalinity of new mortar, cement mixes, and concrete is expressed numerically by pH readings. The pH of fresh materials is 13 to 14. It takes approximately 30 days for fresh materials to drop to an approximate pH of 9. A pH of 9 is less alkaline than a pH of 13, and a coating has a greater assurance of success when applied to a less alkaline surface.
Once these surfaces are cured, the introduction of moisture into these walls may reactivate the high pH alkalinity, resulting in efflorescence, blistering, and peeling of the paint film.
The alkalinity of "hot" surfaces will destroy an alkyd or oil product within 6 months. A vinyl acrylic coating will perform slightly better; however, it will ultimately fail. An acrylic coating will give the best service performance.

SOLUTION

Remove all surface contamination (peeling paint, heavy chalk, efflorescence, laitance, concrete dust, etc.) by hand- or power-tool cleaning or washing with an appropriate cleaner; rinse thoroughly and allow to dry. All new surfaces must be cured according to the supplier's recommendations - usually about 30 days. Remove all form release and curing agents.

If painting cannot wait 30 days, allow the surface to cure 7 days and a pH of less than 13, then prime the surface with Loxon^® Acrylic Masonry Primer. Cracks, voids, and other holes which allow moisture into the surface should be repaired with an elastomeric patch or sealant or appropriate caulk. Rough surfaces can be filled using Loxon^® Block Surfacer to provide a smooth surface.