The Prevention of Efflorescence
An explanation of the prevention of efflorescence on concrete pavers
As yet, there is no concrete additive known to prevent the calcium hydroxide from migrating to the surface of concrete units. Thus, as there is no direct cure, the only action that can be taken, is to consider the direct causes of primary and secondary efflorescence, and act accordingly to limit the effect.The more excess water there is in the mix that may be allowed to migrate to the surface after the initial concrete set, the greater the risk of primary efflorescence.
This makes it necessary to have the mix as dry as possible at the time of pouring. Naturally, this may conflict with demands placed on the workability of the concrete but attempts should be made under all circumstances to keep to the lower limits when adding water.
A second method of preventing primary efflorescence is to try and make the chemical reaction of carbonisation, as described earlier, take place below the surface of the concrete unit in question.
In this way, the calcium hydroxide is converted into an insoluble state on the inside of the unit, and thus forms a barrier, preventing or limiting further deposition of the calcium hydroxide on the unit's surface. In practice, this can only be achieved by slowing down the curing or drying process, such that it is made easier for the carbon dioxide in the air to penetrate into the concrete unit where it can react with the dissolved calcium hydroxide into a insoluble carbonate before it reaches the surface.
A second preventive measure is to ensure that the concrete units in question dry out as slowly as possible. This is best achieved by making sure that the humidity of the surrounding air is as high as possible.
Closed curing chambers are particularly suitable for this. However, it is often sufficient enough simply to cover the concrete elements with plastic sheeting allowing the humidity that is generated during the hydration / curing process to be captured and increased rapidly during the first few days of curing.
It is however, important when adopting this process, to avoid moisture condensation on the paving units. Contamination of the concrete in question by this condensate can lead to severe secondary efflorescence. Efflorescence of this kind also frequently occurs when freshly made concrete units are stacked too close together and evaporating excess water condenses on the face of the units.
For this reason, wherever possible, concrete paving units should be stacked with a space between them so that evaporating water can escape freely between the slabs. Should it not be possible to slow down the drying in this way, the products should be kept inside the building or under protection until completely dry.
In other words, place them outdoors as late as possible after the initial curing process has occurred. Freshly manufactured concrete units can dry out very quickly in the open air due to direct sunshine and/or wind passing over their surfaces, and this may also lead to severe efflorescence on the unit surfaces.
Even when stored in a building, draughts should therefore be prevented in an effort to minimise the likelihood of this occurring. There is very little that can be done to prevent secondary efflorescence which occurs long after the material has been made.
It is important therefore to choose a method of manufacture, such as wet-cast pouring to achieve good compaction of the material utilising selected and suitably graded aggregates with good consistency. In this way a denser concrete unit is guaranteed which will prove more difficult to penetrate by moisture and thus limit the potential to develop efflorescence long term.