Efflorescence is a deposit of soluble salts, usually white, that forms on the surface of masonry or concrete as moisture evaporates. On its own it does not affect structural performance. The decisive question for anyone assessing a façade is not whether salts are present, but where the crystallisation happens: on the surface, where it is cosmetic, or within the pore structure, where it can break the material apart from the inside. The hidden version, known as cryptoflorescence, occurs below the masonry surface and is the more damaging mechanism because expanding crystals in fine pores eventually fracture the surrounding material.

What is the difference between efflorescence and cryptoflorescence?

Soluble salts travel through the pore network of sandstone, brick and mortar dissolved in water. As the water evaporates at the masonry face, the salts crystallise out of solution and form the fluffy white crystals recognised as efflorescence, which can project 10–20mm from a surface. Cryptoflorescence, by contrast, crystallises within the pores just below the surface. Fine pores cannot accommodate the growing volume of salt, so the crystal growth generates expansive forces that break the pore walls apart, causing progressive surface decay.

Industry guidance distinguishes two practical categories of efflorescence based on appearance and timing:

  • Early-age or "new building bloom": uniform deposits appearing shortly after construction, caused by normal moisture loss from new materials. These are typically washed away by rain if left unattended.
  • Late-age or recurrent efflorescence: heavy deposits, often white streaks below mortar joints or localised patches, indicating that water is entering the wall at a higher point. These so-called leachates — also called lime spots or lime runs — point to an ongoing moisture ingress problem requiring corrective action.

Which salts cause the most damage?

Laboratory analysis of contaminated masonry typically reveals sulphates, nitrates, chlorides, sodium, potassium, magnesium and calcium ions in varying concentrations. The most frequently cited compounds are sodium sulphate, gypsum (calcium sulphate) and sodium chloride — common table salt. Each salt has distinct solubility and hydration behaviour: sodium sulphate, for instance, can exist as thenardite or mirabilite depending on its hydration state, and a metastable heptahydrate form has also been observed in research. These structural changes during hydration and dehydration are part of why salt damage can be unpredictable, remaining dormant for long periods before becoming active.

Where do the salts come from?

  • Rising damp carrying salts from soil and groundwater, often visible as a "tide mark" above ground level
  • Sea salt in coastal environments
  • De-icing road salt and fertilisers
  • Acid gases from atmospheric pollution, which convert carbonates in limestone and lime-based materials into more soluble sulphates
  • Residues from cleaning compounds if not thoroughly rinsed from façades
  • Salts already present within mortars or building materials themselves

Limestone and other calcium-carbonate-bearing materials, including lime-based mortars and calcareous sandstones, are considered particularly susceptible because acidic pollutants readily convert their carbonates into sulphates. cement">Portland cement, while harder and less porous than stone, is noted as another potential sulphate source, especially where it has been used in past repair grouting on historic stonework.

How is salt-related durability tested?

A recognised method is the Building Research Establishment standard durability test for building limestone, which exposes samples to repeated cycles of immersion in sodium sulphate solution followed by oven drying, typically across 15 cycles. Weight loss after the cycling indicates the material's resistance to salt-driven breakdown, offering a standardised way to compare how different stone types will perform under real wetting-and-drying conditions on a building façade.

What determines whether masonry is vulnerable?

Susceptibility depends on both the aggressiveness of the environment — relative humidity, temperature, and how often these fluctuate — and the physical character of the material itself. Building stones with a high proportion of very fine pores, generally below 0.5 microns, combined with some larger pores, have been identified as particularly vulnerable to salt crystallisation damage, because the geometry concentrates crystallisation pressure exactly where it does the most harm.

For façade specification and conservation planning, this means moisture management is inseparable from material selection. Reducing water ingress — through sound detailing and damp-proofing at ground level, maintaining breathable renders, and avoiding cement-based repairs on historic lime substrates — addresses the root cause more reliably than treating surface bloom after the fact.