A few weeks ago I participated in a webinar about mass concrete and thermal control plans. I was dismayed to hear the presenter cite delayed ettringite formation as the reason for limiting the temperature of the concrete, even referring to it as “the cancer of concrete”. In fact, delayed ettringite formation is quite rare.
How does delayed ettringite formation occur?
Ettringite is a product of portland cement hydration. Tricalcium aluminate, or C3A in cement chemistry notation, hydrates to form monosulfate. Cement contains about 5% gypsum to control setting. The sulfates in the gypsum react with the monosulfate to form ettringite. This is an expansive reaction, but it normally takes place before the concrete sets, so it doesn’t cause problems.
At high enough temperatures ettringite is not stable, so monosulfate forms instead. When the concrete cools, ettringite is the stable form, so the monosulfate and additional sulfates in solution will combine to form ettringite. This expansive reaction, known as delayed ettringite formation, can cause cracking, but it happens only under certain conditions. The monosulfate and gypsum must be in solution. That is, the concrete has to be saturated. For there to be enough water to saturate the concrete it must be at least partially under water at least some of the time. In addition, some other mechanism must cause the concrete to crack to let the water in.
Delayed ettringite formation came to the attention of the concrete industry in the 1990s. Heat curing of precast concrete railroad ties had caused monosulfate to form in preference to ettringite. The ties cracked initially due to alkali silica reaction. Because the rail bed didn’t drain properly, the ties were sitting in water. The cracks provided channels for the water to penetrate the concrete, and ettringite formed, forcing the cracks apart.
Naturally there was a flurry of research activity to determine why and how this happened. Some in the industry began to refer to “the cancer of concrete” to describe the deterioration—or just the presence of ettringite. There was some disagreement as to the temperature at which monosulfate forms, but eventually 160˚ F (about 70˚ C) emerged as a safe maximum curing temperature.
Ettringite everywhere
Because ettringite is present in all portland cement concrete, it appears in forensic investigations of concrete. What many people don’t recognize is that it would also appear in healthy concrete if we looked for it. It’s just that we don’t bother to core and examine concrete unless we observe signs of distress. By then, petrographic examination will show the damage itself, along with normal signs of aging that may or may not be related. Large ettringite crystals may be signs of sulfate attack or delayed ettringite formation. Or they may simply have recrystallized in the available space from smaller crystals that were there from the beginning.
In the 1990s, pavements in the upper Midwest began to deteriorate at the joints. Cores of the distressed concrete showed signs of freeze-thaw deterioration, and ettringite crystals had formed in the cracks and air voids. Some attributed the damage to the clogging of air voids with the ettringite, while others maintained that the ettringite had simply migrated to cracks where it had room to grow.
Unfortunately, some people promulgated their ideas before they had any solid facts to go on. Instead of admitting that they didn’t know and seeking out better information, they raised concerns about “the cancer of concrete”. That is, they assumed that ettringite was the cause of the deterioration. This is an example of the cum hoc ergo propter hoc (“with this, therefore because of this”) fallacy.
By recreating the deterioration in the lab and observing the process, not just the end result, a colleague and I were able to establish that large crystals of ettringite formed only after the cracks. That is, the ettringite formation was a consequence, not a cause, of the deterioration.
Conditions in mass concrete
While mass concrete could easily reach temperatures above 160˚ F without appropriate control measures, the other necessary conditions are unlikely to occur. Mass concrete normally contains a large percentage of supplementary cementitious materials. These not only reduce the rate and amount of heat generation, but also alter the chemistry of the cement paste. That provides several kinds of protection against delayed ettringite formation:
- It raises the temperature at which monosulfate forms in preference to ettringite.
- It produces a less permeable microstructure, so water doesn’t get in as easily.
- With aggregates susceptible to alkali silica reaction, it controls the expansions and limits the cracking that would result.
Remember, for the monosulfates to react and form ettringite, they have to be in solution. In mass concrete, that means enough water has to migrate from the environment all the way in to the core of the member, where the curing temperature was highest. Some mass concrete members are partially or completely under water. Wharf substructures and bridge piers, for example, have this kind of exposure. But for the water to get in, the concrete would have to experience extensive cracking—the kind of cracking a thermal control plan is designed to prevent.
Although delayed ettringite formation is highly unlikely in mass concrete, there is still good reason to avoid high curing temperatures. When cement hydrates at elevated temperatures, it develops a more open pore structure. That is, it will be more permeable to water and other harmful materials in the environment. Supplementary cementitious materials mitigate this effect, but don’t fully compensate for it.
In mass concrete there’s enough to pay attention to. There’s no need to worry about “the cancer of concrete”.