In the world of concrete engineering, the term “Delayed Ettringite Formation” might not be a household phrase, but it plays a significant role in the structural integrity of concrete structures.
This article will delve into the intricacies of Delayed Ettringite Formation (DEF), exploring its causes, effects, preventive measures, and the chemical reactions behind it. Let’s discuss the mystery behind this phenomenon.
What is Delayed Ettringite Formation?
Delayed Ettringite Formation, commonly abbreviated as DEF, is a chemical reaction that occurs in hardened concrete. It involves the formation of ettringite, a mineral compound consisting of sulfate, aluminate, and water, within the concrete matrix.
Unlike other types of ettringite formation, DEF occurs at a delayed stage, long after the concrete has set and hardened.
Mostly the DEF can be observed in the concrete core and the rise of the concrete core temperature is the main consern.
Why does DEF is Formed
Delayed Ettringite Formation (DEF) is a chemical reaction that can occur in concrete, leading to structural damage and reduced durability. It happens when certain conditions are met within the concrete mix. Here are the key factors contributing to the occurrence of DEF:
High Temperature and Moisture:
DEF typically occurs in concrete that has been exposed to high temperatures (e.g., during curing) and moisture. The combination of these conditions can trigger the DEF reaction.
In construction, usually in thick concrete, concrete core temperatures are maintained at curtail levels to avoid the internal cracking and avoiding the delayed ettringite.
The common practice is to maintain the concrete core temperature at 70 degrees of Celsius in order to the avoid the DEF. Those this is a typical value, this may be increase based on the cement combinations used in the mix design.
A high alkali content in the concrete mix can increase the likelihood of DEF. Alkalis, such as sodium and potassium, can react with certain minerals in the concrete, promoting the formation of ettringite.
Rapid cooling of concrete after a high-temperature phase can help prevent DEF. However, if the concrete is not cooled down quickly enough, it provides more time for the DEF reaction to occur.
In larger concretes, there rise of the concrete heat started from first day and it would come to peak in 2-4 days. During this period, curing methods which used to extract concrete heat may be used.
One such method is placing the pipes inside the concrete and circulate the water. This curing methods is more useful in maintain the concrete temperature acceptable level.
DEF is related to the presence of sulfates, which can come from various sources. External sources of sulfates, such as groundwater or soil, can infiltrate the concrete and contribute to DEF. Additionally, sulfates can be present in the cement used in the concrete mix.
The aluminum content in the concrete can also influence DEF. Higher aluminum content can lead to the formation of ettringite, especially when it reacts with the sulfate ions present.
Improper curing practices, such as letting the rise of the concrete temperature unnecessarily would cause the issues.
Therefore, as mentioned previously as well the rises of the concrete core temperature would be controlled by an acceptable method.
Carrying out mockup test is one method to identify the core temperature, temperature gradient and temperature difference between the core and surface.
If we know these values, necessary modifications to the mix design and curing method can be done.
The specific mix design of the concrete, including the type and proportions of materials used, can influence the potential for DEF. Some concrete mixes are more susceptible to DEF than others.
As discussed under the curing of concrete, carrying out a trial to know the behavior of the concrete would enable to modify the concrete mix to reduce the rise in the concrete temperature.
Age of Concrete:
DEF typically occurs within the first few months to a couple of years after concrete placement. It is a delayed reaction and often becomes noticeable as cracking and expansion within the concrete.
To mitigate the risk of Delayed Ettringite Formation, it’s crucial to carefully control the mix design, curing conditions, and the environment in which the concrete is placed. Additionally, using sulfate-resistant cements and taking measures to minimize the ingress of external sulfates can help prevent DEF in concrete structures.
Understanding the Chemical Reactions in DEF
The chemical reactions that drive Delayed Ettringite Formation are intricate and play a pivotal role in its development. The primary reactions involved include:
1. Sulfate Attack on Aluminate Compounds
The initial step in DEF involves the attack of sulfate ions (SO4²⁻) on aluminate compounds in the cement matrix. In Portland cement, one of the primary sources of aluminate is tricalcium aluminate (Ca₃Al₂O₆). When sulfate ions come into contact with tricalcium aluminate, the following reaction takes place:
3Ca₃Al₂O₆ + 3CaSO₄ + 32H₂O → 3Ca₃Al₂(SO₄)₃·32H₂O
In this equation, Ca₃Al₂O₆ represents the aluminate compounds, and CaSO₄ represents the sulfate ions. This reaction generates calcium sulfoaluminate hydrate, a precursor to ettringite.
2. Formation of Ettringite
Once sulfate ions react with aluminate compounds, ettringite begins to form. Ettringite has a characteristic needle-like crystal structure and is a primary component of DEF. The chemical equation for this reaction is:
3Ca₃Al₂(SO₄)₃·32H₂O + 2Ca₃Si₃O₉ → 3Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O
In this equation, the calcium sulfoaluminate hydrate from the previous reaction combines with calcium silicate hydrate, another common component of cement, to produce ettringite.
This formation of ettringite crystals is what contributes to the expansion and cracking observed in DEF-affected concrete.
3. Delayed Reaction
As the name suggests, the formation of ettringite in DEF is delayed. This delay occurs because the reaction requires time to reach completion.
Initially, the concrete appears sound, but ettringite crystals gradually grow and exert pressure on the surrounding structure. The delay can range from weeks to months, making DEF a challenging issue to detect and address.
Understanding these chemical reactions is essential for concrete engineers and builders because it allows them to develop strategies to prevent or mitigate DEF in their projects.
Effects of Delayed Ettringite Formation
Understanding the effects of Delayed Ettringite Formation (DEF) is vital, as it can have a substantial impact on concrete structures:
1. Compromised Structural Integrity
DEF can significantly reduce the compressive strength of concrete. The formation of expansive ettringite crystals weakens the material, potentially leading to structural failure.
This effect is especially concerning in critical infrastructure like bridges and high-rise buildings.
2. Cracking and Expansion
One of the most noticeable consequences of DEF is the development of cracks and expansion within the concrete.
The growth of ettringite crystals exerts pressure on the surrounding matrix, causing visible damage. These cracks not only diminish the aesthetic appeal of structures but also compromise their durability.
3. Long-Term Durability Challenges
Concrete structures affected by DEF often face long-term durability issues.
The presence of expansive ettringite crystals can accelerate concrete deterioration, increasing maintenance costs and reducing the service life of the structure. This poses a significant concern in terms of sustainability and cost-effectiveness.
How to Identify DEF