What causes efflorescence on concrete surfaces?

Efflorescence on concrete surfaces is caused by the migration of soluble salts to the surface, where they react with carbon dioxide in the air to form insoluble deposits. The process begins with water dissolving soluble salts present within the concrete, typically calcium hydroxide (also known as portlandite) formed during cement hydration. This water can come from the original mixing water, rainwater, groundwater, or other sources. The water, carrying the dissolved salts, migrates to the surface through the concrete's pore structure via capillary action. As the water evaporates from the surface, it leaves behind the dissolved salts. These salts then react with carbon dioxide in the air through a process called carbonation, forming calcium carbonate (CaCO3), which is an insoluble, white, powdery deposit commonly seen on concrete surfaces. While efflorescence is generally an aesthetic issue and does not significantly affect the structural integrity of the concrete, it can be unsightly. It is most common in damp environments or where concrete is exposed to moisture. For example, efflorescence is frequently observed on concrete walls, pavements, and masonry structures. Using pozzolanic materials like fly ash or slag in the concrete mix can reduce efflorescence by consuming calcium hydroxide, thereby reducing the amount of soluble salts available to migrate to the surface. Proper drainage and the application of sealers can also help prevent water from entering the concrete and dissolving the salts.