What is the white chalky powder found on surfaces of concrete or brick walls?

The white chalky powder found on surfaces of concrete or brick walls is called efflorescence. This occurs when water-soluble salts migrate to the surface of the wall as water evaporates, leaving behind a residue that appears as a white, powdery substance. Efflorescence is typically seen on concrete, brick, masonry, or stone surfaces, and it can indicate moisture issues within the wall.

This phenomenon often occurs in conditions where there is excess moisture, which can come from rainwater, groundwater, or even from the building's interior. The salts originate from the materials used in the concrete or masonry, and as the moisture travels through the wall, it brings these salts to the surface, where they crystallize when the moisture evaporates.

Understanding efflorescence is important for home inspectors as it can be a sign of potential water intrusion problems, which require investigation and possibly remediation to prevent further damage to the structure. Other options mentioned do not represent this phenomenon accurately, as gypsum is a mineral used in construction, flaking refers to peeling paint or other surfaces, and algae is a biological growth that typically requires different conditions than those that cause efflorescence.