Once the king of admixtures, calcium chloride was dethroned decades ago because it initiated corrosion of steel in concrete. It has never risen to its former heights and today there is a lot of confusion about whether its character is good or bad. Its good character relates to countering the effects of cold weather. It contributes to earlier set and strength development because it forces earlier cement hydration that liberates heat to counter the cold. Its bad character relates to steel corrosion and, to a lesser degree, its potential for increasing drying shrinkage.
Way back in the 1950s, when we worked at the Portland Cement Association (PCA), the use of 1% or 2% calcium chloride in concrete to accelerate strength development was common and permitted by the American Concrete Institute (ACI) without restraints. But there was less history of corrosion then—maybe because less reinforcement was used—the chemistry of cements was different, thinner interior concrete elements were used so concrete could dry out, and there were fewer reinforced-concrete structures containing chloride.
Maybe the higher tri-calcium contents of portland cement in use during the 50s immobilized the chlorides because they got tied up chemically during hydration as calcium chloroaluminates. Maybe steel reinforcement was buried deep enough so oxygen needed for corrosion couldn't get to the steel. That certainly was not the situation in Europe where concrete reinforcement was designed with thin cover and positioned within the carbonated layer of concrete. That prompted the Europeans to unknowingly use principals of cathodic protection to re-alkalize concrete around reinforcing steel, thus raising pH to levels that inhibit corrosion. That process also results in chloride removal, which has been commercially used to reduce chloride contents between the vicinity of reinforcing steel and concrete surfaces.
The maximum concentration level for prevention of corrosion by chloride varies with agency and country. Many specifications limit chloride levels to 0.2% to 1% by weight of cement. Others limit it, regardless of cement content, in relationship to concrete volume, for example to specify a range from 2 to 3 pounds per cubic yard. Most specifications fail to mention that they apply only to uncarbonated concrete. Almost any amount of chloride will enhance and accelerate corrosion of steel in carbonated concrete, but because of the reduced pH that accompanies carbonation, steel corrosion may well occur even without chloride!
And finally, as we've mentioned in another article, virtually all halides, except perhaps fluorine, cause corrosion. The chemical halide family includes not only fluorine and chlorine, but also iodine, bromine, and even astatine! We once diagnosed bromide contamination as the mechanism triggering steel corrosion. The reason for the exclusion of fluorine is somewhat unclear, but perhaps the answer is simply that fluoride in concrete immediately reacts to form very insoluble calcium fluoride that inhibits the release of fluoride ions needed to promote corrosion. Calcium fluoride is the film on teeth that is supposed to prolong its time of decay. Remember, a little fluoride in your drinking water is good for you. Perhaps, if we used fluoridated mixing water and could get it to precipitate a continuous film of calcium fluoride on reinforcing steel, corrosion problems would disappear. Could the dental industry be our corrosion salvation?
Chlorides can be introduced in concrete in peculiar and mysterious ways. We mentioned in a previous column a near-catastrophic event in masonry construction several decades ago where a latex, having a formulation similar to plastic wrap, was used as an admixture in masonry mortar, and even in spun precast concrete elements, like lamp standards. This admixture provided tremendous flexural strengths that overcame the great lateral weakness of brick masonry walls. Unfortunately, the stuff contained vinylidene chloride, a relatively insoluble compound except when it is exposed to the high pH of concrete that caused chemical reactions that led to the release of major amounts of chloride that ensured rampant corrosion of lifting rods, anchors, and any other steel around. Fortunately, one of this admixture's cousins, polyvinyl chloride (PVC), commonly used for pipes, tubing, and wire insulation, that is sometimes embedded in concrete, is not similarly affected.
Nothing has replaced the former king of admixtures. Substitutes, like calcium formate and sodium nitrite, are sometimes used, but none gives the whack that calcium chloride does. The king is still with us, albeit much more modestly than during his heyday.
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 47 years.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.