The Formation of Efflorescence
An explanation of the formation of efflorescence on concrete pavers
Analysis of the white deposits on the surface of concrete indicates that in the majority of cases, it
consists predominantly of calcium carbonate (CaCO3), which is insoluble in water.
Only very rarely will the deposits be of other salts, as these are usually soluble in water, and are
therefore washed off very rapidly by rain, etc.
The presence of this calcium carbonate can be explained as follows:
- Whilst the concrete is hardening, (that is, during the reaction of the cement with the mixing water)
not only are familiar calcium hydrosilicates formed but, at the same time, considerable amounts of
calcium hydroxide (Ca (OH)2 ) are emitted.
- This unavoidable by-product of the hydration process of cement, being a relatively soluble substance,
dissolves in the excess water that is always present in wet-cast concrete.
- During the drying or curing phase of concrete, this excess water mixture, which is actually a calcium
hydroxide solution, migrates to the surface of the paving or concrete unit, where it evaporates.
However, since only the water can evaporate, the entrained calcium hydroxide remains behind on the
surface of the concrete unit.
- Here, there is a very rapid carbonation of the hydroxide in which the latter is converted through the
absorption of carbon dioxide (CO2) from the atmosphere into water-insoluble calcium carbonate, which we
find again in the chemical analysis of this efflorescence: Ca (OH)2 + CO2 = CaCO3 + H20
- This kind of efflorescence, which is actually formed during the curing of concrete, is known as “primary
efflorescence” and usually covers the concrete unit in a uniform and closed layer. In extreme cases it
may appear to alter the colouring of the affected concrete unit.
- As we know, the curing of concrete occurs over a long period extending over several weeks and longer.
As a result of this curing process, fresh calcium hydroxide is constantly being released inside the concrete
unit during this period.
Hence, even if primary efflorescence can be avoided in the initial concrete mix, renewed penetration of water,
(ie a wet / dry cycle) into the concrete unit can and will dissolve this calcium hydroxide, and again transport
it to the surface in the same way as described above.
- This type of efflorescence, which is frequently caused by rain or condensation on the concrete unit, is known
as “secondary efflorescence”. It usually differs from primary efflorescence in that it occurs very locally, and
does not manifest itself evenly over the concrete unit.