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Mitigating calcium oxychloride formation
Concrete sidewalks, driveways, and pavements in the northern half of North America are increasingly showing signs of a specific form of damage that forms primarily along the joints.1-3 This damage is troubling, as it can emerge during the first decade of service2 and can be both difficult and costly to repair.4,5 For example, repair of joints in damaged concrete pavements has been estimated to cost in excess of a million dollars a mile.2 Examples of the pattern of damage created in pavement are shown in Fig. 1. Although the actual cause of the damage is complex, it can be attributed to two primary factors:
* The fluid saturation of the concrete leading to conventional
* A reaction (phase change) between the cementitious matrix.6-8
While the first factor has received extensive study,9,10 less has been reported about the second factor. Recent work6,8 has suggested that the reaction between deicing salts and the cementitious matrix in concrete can be a dominating deterioration mechanism that occurs in addition to the more classic form of freezing-and-thawing damage, particularly when the deicing salt concentrations are rather high. This damage can even occur at temperatures above freezing. This article aims to inform concrete designers, specifiers, and inspectors about this lesser known deleterious reaction between cement paste and deicing salts in the hope that steps will be taken to minimize damage.
Deleterious Reaction between Cement Paste and Chloride
Deicing salts are generally used to melt ice that forms on concrete. Deicing salts used in the United States are typically chloride-based, although formate- and acetate-based formulations also exist. In this article, we focus on chloride-based deicing salts. While chloride-based salts can reduce the amount of ice on the pavement surface, making it safer for pedestrians and cars, saltwater solutions can be absorbed or diffuse into the concrete. As deicing salt solutions are transported into concrete, it is known that a portion of the chlorides in the salt can react with the hydrated aluminate phases of the concrete, forming Friedel's and Kuzel's salts.11 In addition, the chlorides in the deicing salt can react with the calcium hydroxide (Ca(OH)2) in the cementitious matrix and form an expansive product called calcium oxychloride.8,12
Calcium hydroxide is found in all portland cement concretes,...