It's all around us, every moment of the day and night. We ingest it without knowing. We see it without knowing. We walk on its products without being aware. It is ubiquitous—almost—and never ending in its coverage above and below ground and around the world. It enters during our everyday activities from eating and digesting to building bodies or man-made structures. And it keeps concrete in and out of tune. It is in tune when concrete performs normally, but hits a sour note when in excess or gets out of control for a variety of reasons.
What is it? It is sulfate! The chemical radical SO4, an anion that can wed with a variety of cations such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), and a host of others to form compounds that we are familiar and unfamiliar with.
Let's take the most used of them all—the mineral gypsum. Gypsum is calcium sulfate dihydrate (CaSO4·2H2O), also known as selenite and alabaster. Remember, “thine alabaster cities glean” from America the Beautiful? In ancient days, marble was sometimes also called alabaster. From gypsum is made plaster of Paris (named because it was quarried from a hill in the city of Paris decades ago) that is heated a few hundred degrees, which drives off some of the combined water, leaving behind one-half molecule of water so its new formula is CaSO4·½H2O, also known as hemihydrate and bassinite. Add water to it and it chemically reacts to become the gypsum of molds, body splints and casts, plaster walls, drywall, and varieties of other uses. Among its uses is enrichment of bread and touted as a source of calcium (Ca) for bone building—the calcium is from added gypsum or anhydrite (CaSO4. Just look at the list of ingredients on the packaging and you will find CaSO4).
In our corner of the world of portland cement and concrete, some sulfates are common. Aphthitalite (sodium-potassium sulfate) is present in small amounts in all portland cement. Gypsum is interground with portland cement clinker to make portland cement and is needed to control flash setting. During grinding it may decompose to hemihydrate and sometimes to soluble anhydrite where it has lost essentially all of its water of constitution. These dehydrated compounds may give rise to false set that usually is not recognized because it is easily worked through during mixing. Some proprietary patching materials are mixtures of gypsum and portland cement, useful, but only when used in water-free or low relative humidity environments. Otherwise ettringite (3CaO·Al2O3·3CaSO4·32H2O), the cement “bacillus,” will form and cause their destruction.
In portland cement-based products, the sulfate from gypsum reacts with calcium aluminates from the portland cement early in their lives and forms needle-like micro-crystals of ettringite. An exception is delayed ettringite formation where suppression of its early formation by high-curing temperatures years later results in its late formation and attendant concrete deterioration.
The resiliency of the cement bacillus is advantageously used in shrinkage compensating concrete—a materialized dream of its inventor, Alexander Klein, who controlled its potential for expansion.
Concrete exposed to external sulfate solutions undergoes sulfate attack that may or may not be destructive. A main byproduct of that attack usually is et-tringite and, under some circumstances, also thaumasite (CaSiO3·CaCO3·CaSO4·14½H2O). Sometimes it can be gypsum itself, a marauder wearing two masks.
Sodium sulfate (thenardite) and its hydrated version mirabilite (Glauber's salt) are common efflorescence products. The cycling of thenardite to mirabilite may cause surface distress.
Sulfate is so universal and is used so extensively in its many forms that it comes into our lives daily, for good or for worse. It can nourish. But then, it also can destroy. Whatever the case, its multiperformance always enhances our amazement of Mother Nature and the wily ways in which she works. This sulfate of our lifetime provides a panorama of uses along, of course, with some misuses. Whichever the case, we should be thankful that it is in our lives.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 48 years.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago.