The Romans had pozzolans, in the form of volcanic ash, long before anyone else. The Greeks had it but didn't use it like the Romans until much later. The Belgians had finely ground volcanic pumice and called it trass. And now popular is a kissing cousin, although truly not a pozzolan—ground granulated blast-furnace slag cement (GGBFSC).
In a prior issue, we talked about pozzolans and related matters. We now get more detailed, if not more complicated, first by providing a pozzolan definition. The most common one we found is, “A siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.” To put it more simply, it is usually a form of silica that chemically reacts with lime (calcium hydroxide) to form cementitious products. Its name comes from the Italian locality where the Romans found the best available to them—volcanic ash near the town of Pozzuoli in the shadows of Mount Vesuvius, now a suburb of Naples. The higher the silica content, the more prevalent the glassy phase, and the finer the ash particles, the more effective the ash.
Among natural pozzolans are volcanic ash, diatomaceous earth (opaline marine microfossils), ground opaline shale, ground pumice, ground volcanic tuffs, and Santorins earth (volcanic ash). Among man-made pozzolans are burnt clay, burnt shale, burnt rice husks, silica-glass, silica fume, and the daddy of them all, fly ash.
Most pozzolans do more than one thing well for concrete. Their best functions are fivefold: they reduce costs, water-cementitious materials ratios, porosity and permeability, and the severity of sulfate attack, and help in the “green sustainability” battle. We'll discuss (or rediscuss) these five in more detail.
We consider fly ash the most popular pozzolan in most areas of our country. This byproduct of the electrical power industry used to be thrown away, with some concern by the Isaac Walton League about where to dump it. Now, this waste is sold in increasing amounts and at ever-higher prices because demand has exceeded supply, to concrete suppliers. It chemically reacts with the very weak product of portland cement hydration—calcium hydroxide—to make “good” and stronger cementitious stuff; mainly calcium silicate hydrates, which replace the weaker calcium hydroxide and plugs capillary voids reducing porosity and permeability and increasing strength. When used as a portland cement replacement, it reduces the cost of concrete, pound for pound, and increases long-term strength. Because it decreases permeability, it improves concrete's susceptibility to sulfate and other forms of chemical attack from aggressive environmental solutions.
The use of fly ash has allowed the formulation of very high strength concrete, in part because it uses up calcium hydroxide and replaces it with higher strength-producing hydrates.
GGBFSC, although not a true pozzolan, is the second most widely used supplementary cementitious material, and its use has increased enormously in the last decade, mostly in steel producing areas. But because of its limited availability, imports are now available. Its effects in concrete are somewhat similar to those of fly ash, but it has been studied a lot less in the U.S. Because it has some cementitious properties when used alone, it is considered hydraulic cement. When used as a partial portland cement replacement, it increases early strengths during warm weather and decreases early strength during cold weather. Fly ash acts similarly, but to a lesser degree.
Silica fume, the last man-made pozzolan used with any frequency, almost falls into a separate category. It is expensive, but often a little bit goes a long way because it consists of extremely fine particles—so fine that it is difficult to handle. Unlike fly ash and slag, it comes close to being a 100% effective in converting calcium hydroxide to strength-producing, porosity-reducing, calcium silicate hydrates. Unfortunately, sometimes when the pelletized form is used, the pellets may not disperse into individual particles during concrete mixing so that nodules remain, which undergo alkali silica reaction (ASR). The lumps confuse the concrete into thinking they are one particle and not billions of individual particles. Their nodularization alters the effective water-cementitious materials ratio, the water-cementitious materials ratio becomes higher, and concrete strengths lower.
So, the next time you hear or read abut pozzolans, fly ash, GG-BFSC, silica fume, trass, and the like, think about their great role in our industry and how we are contributing more and more to the betterment of our world by their use.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago. Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa, and has been involved with all aspects of concrete for over 48 years.