Last month, in Part 1 of this article, we explained how water is used in making concrete, and how it later passes through it—or, better said, through cracks in it. This article will talk about impure waters and their effects on concrete.
First, we note that many of the scientific papers on the subject are misleading, at best. Most studies of laboratory concrete are made just “days” after casting—before it carbonates significantly or “matures” adequately to simulate field concrete.
Water dissolves the calcium hydroxide (portlandite, Ca(OH)2 component of portland cement paste. But if the calcium hydroxide has carbonated (due to reactions with atmospheric carbon dioxide) to calcium carbonate (CaCO3 calcite), at least that in the carbonated surface region of concrete, no attack by water is likely because calcite is far less leachable than calcium hydroxide. Experiments that show that concrete may be seriously weakened or destroyed by immersion in water would likely show no attack at all if the tests had been made later after some carbonation had occurred.
An interesting factoid is that calcium carbonate is more soluble in cold water than in hot water. We once worked on a dam where light surface leaching had occurred because it was exposed to borderline lime-deficient water. After petrographic examinations of the concrete in the laboratory, and near exhaustive chemical analyses and evaluations of the water, we concluded that a potential lime-leaching situation existed—but only during winter months when water temperatures were low.
Second, concrete is “basic.” It has a high pH. It is attacked by any acid, usually in proportion to the acid concentration, as measured by pH. As an example, acid having a pH of 4 is ten times as aggressive as acid at pH 5. The attack does depend upon whether the acid is a “weak” or a “strong” one (a subject the reader will be happy we don't delve into) or what the anion is in solution. Strong sulfuric acid, for example, may be less aggressive than weaker sulfuric acid because calcium sulfate (gypsum, CaSO4.2H2O), the major product of attack by that acid on concrete, is insoluble in strong sulfuric acid, and the reactant product, gypsum, precipitates on surfaces and hinders continued attack.
The situation can get very complicated in the case of other salts. For example, magnesium salts, such as magnesium sulfate (MgSO4) in solution, react with the calcium components of the paste and eventually destroy them. Among the products that result is magnesium hydroxide (brucite, Mg(OH)2). But because magnesium hydroxide is very insoluble, it provides a coating that hinders further reactions.
Because there are so many exceptions to any “rule” on the aggressiveness of chemicals in solution on concrete, laboratory identifications of the reactant products and chemical solution exposures are needed before interperating ACI 201 and 515, PCA, or other publications on the subject. Better yet, you can make your life easier and happier by consulting with your favorite concrete chemist and petrographer.
Always remember the complications introduced by concrete because of its chemical nature, and that carbonated concrete may be a lot less affected by many chemicals than the literature indicates.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, PA, and has been involved with all aspects of concrete for over 47 years.