Efflorescence

Interlocking concrete pavers are rapidly becoming more popular among manufacturers and consumers alike. As acceptance and use of these products grow, end-users will naturally become more and more concerned with the quality, appearance, and cost of your product. Raw materials, gradations of sand and aggregate, concrete mix design, and chemical admixtures are all important elements to the success of this product in your market. However, a common and very undesirable characteristic of concrete pavers is efflorescence, which appears as a white powder on the surface of the finished product. Efflorescence can in some cases disrupt the surface by expansion within the surface pores. Fortunately, efflorescence can be reduced to accep­table levels by various means, some of which provide more consistent and more efficient results than others.

Here is all that you need to know about efflorescence and as to how you can eliminate it and make sure that the quality of your paver structure is maintained.

What is Efflorescence?

Efflorescence is the dry residue of soluble salts contained within the finished paver. These salts continue to be dissolved by moisture and are transported towards the surface by evaporation or hydrostatic pressure. Evaporation will continue to take place, leaving salt as an unwanted visible deposit.

In theory, efflorescence would not occur if salt or moisture could be eliminated from the process. In practice, however, this is difficult to achieve because the salts that cause efflores­cence are found not only in the sand and the aggregate but also in the cement and water.

However, much care must be taken to select the proper sand and aggregate components, the cement may still contain enough salts to ruin the appear­ance of your finished pavers. The problem of efflorescence can be con­trolled by militarizing the volume at cement and water used in the mixture.

It has been demonstrated that low water absorption is the best assurance against efflorescence. When made with properly graded aggregates, an ade­quate cement content, a low water/ cement ratio, and thorough curing, the concrete paver will have maximum water permeability. Consequently, the upward capillary movement that brings these salts to the surface will be severely restricted.

What this means is that efflorescence can be greatly reduced by compacting the surface, which serves to constrict the capillary tubes, and this can be achieved by proper mix design, including chem­ical admixtures specifically designed for semi-dry concrete.

The Importance of Proper Mix Design in Eliminating Efflorescence

The goal of proper mix design is to achieve required compressive strength and durability in the hardened state while retaining adequate worka­bility and cohesiveness in the plastic state. Proper mix design can also help to assure a consistent, visually appealing end product which is more profitable to produce. To design such a mix, however, a natural conflict between strength and workability must be resolved.

Compressive strength and durability are directly related to the water/cement ratio. While strength can be increased by reducing water content, decreasing the water content will also reduce the workability of the mix. It also should be noted that increasing the cement con­tent will not always result in an increase of the compressive strength.

Durability also depends on a fully compacted concrete, while workability, again, depends on the water/cement ratio as well as the shape, texture, max­imum and minimum particle size, and particle distribution of the aggregate.

Finally, cohesiveness is required to prevent the mix from falling apart and to reduce the possibility of segregation of fine and coarse particles during handling. Cohesiveness is adversely affected by inadequate water content and the presence of rough or angular aggregate particles.

Optimum levels of cohesiveness, compressive strength, durability, and workability can be achieved through the use of specialty chemical admixtures, such as PAVERMIX and PAVERMIX EXTRA which are designed specifi­cally for semi-dry concretes.

Water content in the mix design is usually insufficient to fully hydrate the cement. Consequently, it is generally inappropriate to add cement for two reasons. Additional cement has the effect of lowering the workability and, because of the lack of water in the paver manufacturing process, the cement acts as a filler after a certain point and can actually reduce strength. An admixture designed specifically for semi-dry con­crete will enable the producer to reduce cement and still achieve the desired workability characteristics as well as the necessary strength.

The effects of the specialty paver admixture can easily be optimized by inspecting the fine and coarse aggregate in terms of sieve analysis since curves are available to select the best mix de­signs related to these aggregate gradations.

In summary, experience has proven that the addition of certain specialty admixtures will produce a higher quality paver while also eliminating the problem of efflorescence. At the same time that it reduces overall production costs as well. So, make sure of preparing the perfect paver mix, because it is all in the mix design at the end.