We manufacture concrete pavers and are experiencing a severe efflorescence problem. Can we reduce efflorescence by using admixtures such as plasticizers or pozzolans? Or should we use silicone-type water repellents or film-forming sealers on the finished product?
Most efflorescence occurs when soluble calcium hydroxide, a byproduct of cement hydration, leaches out of the concrete and reacts with atmospheric carbon dioxide to form insoluble calcium carbonate. A German researcher (see reference) found that plasticizers help to reduce efflorescence by improving the compactibility of concrete used to make pavers. Less water penetrates a well-compacted concrete, and thus less dissolved calcium hydroxide is transported to the surface. Pozzolans react with calcium hydroxide to form insoluble calcium silicate hydrates that also decrease concrete permeability. Because this reaction occurs at a slow rate, however, efflorescence can still occur. Also, an excessive amount of pozzolan would be needed to react with all of the calcium hydroxide in concrete. Silicone-type water repellents increase the surface tension of water, thus preventing liquid water from penetrating the concrete. However, silicones don't prevent the passage of water vapor. Once this vapor condenses and fills capillary voids, calcium hydroxide can still rise to the surface, causing efflorescence. The researcher found that a thin coat of an acrylic or other polymer-type sealer does seal the pores, preventing efflorescence. However, coatings can be applied only to pavers produced on a board (single-layer) machine, and only the top surface can be coated. If a cube of these coated pavers gets wet, the pore water can transport dissolved calcium hydroxide to the uncoated bottoms of the pavers. Calcium carbonate may then form on the tops of the underlying pavers. You didn't mention the effect of cement-sand ratio on efflorescence. This, along with water-cement ratio, has a pronounced effect on paver porosity and efflorescence. We'll discuss this in a future issue of The Concrete Producer. Reference P. Kresse, "Efflorescence and its Prevention," Betonwerk + Fertigteil-Technik, October 1991, pp. 73-87.