Ageing of Concrete
Concrete is an natural product consisting of sand, stone, cement and water. Even the cement is made by burning natural quarried limestone. Cast concrete not only incorporates the natural variations present in the ingredients, it will also have subtle local moisture differences and microscopic surface texture variances. All of these contribute to the soft and often warm colour nuances so natural to cast concrete. Pigmenting or colouring enhances this natural effect, which brings the surface to life as opposed to a typical bland painted surface.
Concrete undergoes a natural ageing process over the first few years. This is caused by the very slow loss of moisture via the tiny pores not visible to the naked eye. Along with the water migration will be some dissolved salts, which in turn will be laid down on or near the surface with evaporation. Very limited and local shrinkage of the concrete still happens over time. Upon close inspection, hairline cracks will be visible on the surface of mature concrete. By capillary action the cracks will attract some moisture from the outside, which makes the cracks appear more prominent. There is a water-based interplay of evaporation, re-absorption, bleanching and leaching. All of these are surface phenomena and is not simtomatic of something sinister going on deeper into the concrete. In fact, concrete gains some more strength after the initial 28 day period and this may go on for more than two years.
All and all, the naturalness of concrete becomes more apparent as it gets older. Many prefer the charm of aged concrete to any form of paint or covering over it. Architects and concrete artists make use of rough textures to enhance it further.
Efflorescence in Concrete
Efflorescence is visible as powdery white spots and stains on the suface of newly laid concrete paving or bricks. Coloured (pigmented) concrete will accentuate the stains. The appearance of the white stains raises the suspision that a production error is to blame. This is fortunately not the case as efflorescence is the result of a natural and temporary process.
Cement is the essential constituent of all concrete. It is procuced by burning alumina and limestone. When newly cast concrete products are exposed to the elements like rain, condensation or dew, water penetrates the pores of the concrete. Here it partially dissolves the residual lime still present there. On drying out, the solution diffuses to the surface and the water evaporates, laying down a white lime haze which is not quick to dissolve. However with further exposure the lime slowely degrades and will wash away after a time. Only the lime which is not firmly bonded in the concrete like the rest of the constituents, can rise to the surface. Once it has been removed, this effect will not occur again. Depending on various conditions, efflorescence can last up to two years.
The cleaning up of efforescence may be expidited by washing it with a mild acid or other products available. This has the added advantage of dissolving some of the lime just under the surface.
Source document: Concrete Manufacturers Association (CMA) Publication on Efforescence