Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic outer problem only (primary efflorescence causing staining), but can sometimes indicate internal structural weakness (migration/degradation of component materials). Efflorescence occurs when dissolved salts migrate within a porous material to its surface, where the water evaporates and salt precipitate, leaving white spots. Typical efflorescence in materials based on Portland cement (OPC) is caused by calcium carbonate.
Primary efflorescence
This occurs days or weeks after application, during the setting and curing process. Either excess water from the mortar matrix or severe climatic conditions (low temperature, high humidity) extend the setting time and increase the amount of moisture at the surface. The moisture at the surface then reacts with the free lime in the mortar.
For controlling primary efflorescence, formulations containing liquid fatty acid mixtures (e.g., oleic acid and linoleic acid) have been commonly used.
Secondary efflorescence
This can occur years after application due to contact with moisture or when a substrate is subjected to cycles of re-wetting and drying. Moisture penetrates into the matrix and/or leaches substances from it. Calcium hydroxide (a by-product of portland cement) can partly dissolve, or salts (from the substrate) can migrate to the surface.
For controlling secondary efflorescence, admixtures containing aqueous-based calcium stearatedispersion (CSD) are often added at a later stage of the batching process with the mix water.
To prevent (both primary and secondary) efflorescence in cementitious materials is by using special admixtures that chemically react with and bind the salt-based impurities in the concrete.