Because of space limitations, some things were not included in 'Lesser Known Evils of the Concrete World'; (March 2008 issue). This column includes sources of some of the culprits and more on some of their behavioral problems. Part of our goal has been to make you comfortable and familiar with chemistry and the formulas used in concrete parlance.
We mentioned that the zeolites laumontite and leonhardite (in Group 1) have similar compositions but vary in the amount of water they are married to—for which they have a cyclic jealousy activated by changes in temperature and humidity. When leonhardite grabs some extra water it gets bigger, changing to laumontite, and expands into its neighbors' yards. This new laumontite, in subsequent responses, loses the extra water and contracts to leonhardite, but never returns to its original location causing a slight expansion (a phenomenon known as temperature-moisture hysteresis) because of the “jacking” or “ratcheting” effects. These zeolites enter concrete via aggregates, for example altered granite.
The sodium sulfate salts, thenardite and mirabilite, are like zeolites—they have similar compositions except thenardite is dried up and has no water of constitution unlike its partner mirabilite that contains 10 molecules of water. They cyclically fight over their anhydrous and hydrous states due to varying temperature and humidity. When mirabilite wins, the volume increases because the added water exerts stress upon its neighbors during each cyclic foray.
The zeolite operates within the concrete but in thin zones at the surface, and cause progressive deterioration similar to damage due to cyclic freezing—cracking and scaling. The sodium sulfates operate at the immediate surface and cause progressive loss of very fine paste particles that give surfaces sandpaper textures due to aggregate particles that become exposed.
The anhydrous compounds found in Group 2—free lime, periclase, and magnesium wustite—are man-made and occur in refractory brick, steelmaking and specialty slags, pebble lime, spent sand-clay-sand mixtures used for iron casting molds, and over-limed or improperly burned portland cement. They roughly double in solid volume when they imbibe water and form their hydroxide counterparts.
Potentially expansive slags have been improperly used as concrete aggregate or fill material, and can be dangerous even after stored outside to “age.” Along with other materials, they have been inadvertently introduced as contaminates. They can cause popouts, localized expansion, and, if present in sufficient numbers, bulk concrete expansion.
A triple whammy results from oxidation of the iron sulfides in Group 3(pyrite, marcasite, pyrrhotite). Concrete is not their only prey because they disintegrate when stored as museum specimens and are the expansive component in black shales. Their oxidation is the source of iron oxide (hematite) and gypsum in many natural bedded sedimentary deposits.
Sulfuric acid formed during the oxidation reacts with whatever base materials are available. Hydraulic cement pastes and many aggregates (e.g., limestone, dolomite, and calcareous shale) are sources of calcium—the reaction forms gypsum. In concrete, the acid reacts with all paste compounds and completely destroys the paste—Whammy No. 1. The iron reacts with the sulfuric acid to form iron sulfates that, in turn, oxidize, resulting in iron oxide (hematite) and more sulfuric acid. Some of the compounds that form in concrete cause expansions (Whammy No. 2) while others create unsightly iron stains (Whammy No. 3).
As always, we wish you smooth sailing, par golfing, and good fortune.