Efflorescence in concrete simply means appearance of white colored powdered material on the concrete surface. Efflorescence is the formation of salt deposits, usually white, on or near the surface of concrete after it has been finished and causing a change in the appearance.
Light-coloured concrete shows the deposit much less than darker coloured concrete.
Note: With time, efflorescence becomes less extensive and should eventually cease to occur, unless there is an external source of salt.
Why Efflorescence in Concrete Occur? (Causes)
Efflorescence is usually caused by a combination of the following factors:
One or more of the constituents of concrete may contain salts
A high water-cement ratio resulting in a more porous concrete that allows movement of water and salt solutions
Inadequate curing which may leave un-hydrated products near the surface of the concrete
Exposure to rain or other water sources (moisture allows salts to be transported to the surface where they accumulate as the water evaporates)
Slow rate of evaporation of water allowing time for salts to permeate to the surface (this is why efflorescence tends to be more of a problem during the winter months; in summer, high temperatures may cause evaporation and hence depositing of salts within the concrete rather than on the surface)
Variability of concrete (e.g. from compaction or curing) can result in localised problems where water can permeate more easily through the concrete.
How to Prevent Efflorescence in Concrete?
Measures to prevent/control the occurrence of efflorescence include:
Use ingredients containing as little soluble salt as possible.
Use waterproofing admixtures to reduce permeability of concrete/mortar. Note that as some of these products may cause efflorescence themselves (e.g. water-soluble soaps) always check with the manufacturer.
Use a denser concrete, again to reduce permeability. However, this may increase the shrinkage.
Use cement : lime : sand mortars no stronger than required for the application to minimise possible soluble salt levels.
Lime should be hydrated lime free from calcium sulphate.
Avoid premature drying.
Apply curing compounds or same-day sealers to reduce exposure to wetting.
Protect hardened concrete from exposure to moisture by maintaining surface sealers and site drainage, and from rising groundwater by placing a plastic membrane under slabs.
For masonry, ensure flashings, damp-proof courses and copings are detailed correctly, cover the top course at the end of each day’s work, tool joints with a ‘V’ or concave shaped jointer to compact the mortar at exposed surfaces, provide wide eaves and avoid wetting from sources such as sprinklers.
How to Repair Concrete Surface Affected by Efflorescence?
Prior to removing efflorescence, the things that may be causing the problem should first be corrected so as to limit or reduce the risk of re-occurrence.
Soluble salt deposits can be removed with a stiff‑bristle broom. Note that all brushed-off material should be totally removed by vacuum cleaning or other means. If the result is not satisfactory, scrub with clean water then lightly rinse the surface. Note that adding water may result in further deposits. Repeated dry brushing as the deposits appear is probably the best treatment.
Insoluble salt deposits (hard, white, scaly or crusted) cannot be removed by water washing, although the use of a high-pressure water jet is effective.
Method-2 (Using Dilute Acid Solution)
The application of a dilute acid solution is also effective in most cases, and in some cases, may be the only way (as explained below).
Extreme care is required when handling acids. When diluting hydrochloric acid always add the acid to the water, never the reverse. Ensure good ventilation and avoid contact between the acid and the reinforcement. Use only diluted acid to clean the concrete surface.
The recommended proportions are 1 part hydrochloric acid to 20 parts water. Always saturate the surface with water before applying the dilute acid solution. When applying the solution, ensure that the surface is moist but without any free water being present. The applied solution should be allowed to react on the concrete surface for 10 to 15 minutes. The surface should then be thoroughly rinsed and scrubbed with lots of clean water. Repeat rinsing at least twice or until all traces of the acid solution have been removed. The process may be repeated if necessary to produce the required surface finish.
Note: Washing with acid may cause colour variations and alter the surface texture. For coloured finishes a more dilute acid solution (2% or 1 part acid to 50 parts water) may be required. A small trial area should be done first to assess the results.