Concrete structure crumbling due to thaumasite attack, with crystalline formations visible.

Concrete Catastrophe: Is Your Limestone Filler Cement Under Attack?

Reese Marlowe in Science & Nature August 2025 • 4 min read.

"Thaumasite Formation: Uncover the hidden dangers lurking within limestone filler cements and how sulphate attacks can compromise structural integrity."

For years, thaumasite formation was considered a rare type of sulphate attack on concrete. Since the 1960s, experts have recognized that it can be a cause of deterioration in concrete made with Portland cement. Thaumasite is a complex salt formed in nature that contains sulphate, silica, and carbonate. It is well-crystallized, with hexagonal crystals typically shaped like needles.

Thaumasite’s structure is similar to ettringite, making it hard to identify using X-ray diffraction (XRD). In thaumasite crystals, silica ions replace aluminum ions found in ettringite. Because of the differences in valence between silica (Si4+) and aluminum (Al3+), and the presence of sulphate ions (SO2-), carbonate ions (CO₂2-) are needed to neutralize the electrical charges within the crystal.

According to existing research, there are two processes that explain how thaumasite forms in cement paste. In the first, ettringite evolves and incorporates Si+4 into its structure, replacing aluminum between the columns, while 3(SO42-) is replaced by [(SO2), (CO32-)2]. The other process involves the interaction of sulphates and carbonates in a solution with CSH (calcium silicate hydrate). This reaction does not involve ettringite and can occur in concretes designed to resist sulphate attacks.

What Makes Limestone Filler Cements Vulnerable to Sulphate Attack?

Concrete structure crumbling due to thaumasite attack, with crystalline formations visible.

The increased use of cements containing finely divided limestone, which acts as an internal source of CO32-, has increased interest in this type of attack. The most serious issue is that thaumasite can form in concretes made with cements designed to resist sulphate attacks. Three cases of concrete made with sulphate-resistant cements that experienced severe deterioration due to thaumasite formation were described by Crammond and Halliwell.

Research on the sulphate resistance of cements containing limestone filler has produced conflicting results. Some researchers have concluded that adding limestone filler can increase the cement's resistance to sulphates, while others believe that it weakens the resistance, depending on the characteristics of the clinker and the amount added.

Material Composition: Cements with limestone filler (CPF) are susceptible to sulphate attacks due to the formation of thaumasite.
Testing: Mortars with 0% and 20% calcareous material in a Na₂SO₄ solution, and cement pastes with 0% and 18% lime in a combined solution of Na₂SO₄ and MgSO₄, were tested at 5°C and 20°C.
Methodology: DRX (X-ray diffraction) technique was used to study the profiles to predict conditions leading to thaumasite formation.

Results indicate that thaumasite forms after attack induces ettringite cracking and gypsum deposition, leading to CSH instability. The presence of calcareous material accelerates this process, increasing aggressive ion penetration due to a higher effective water/cement ratio in the paste or mortar. Low temperatures also speed up the attack.

Protecting Concrete Structures from Thaumasite Formation

Thaumasite formation, accelerated by low temperatures, leads to increased cracking, gypsum deposition, and CH consumption, which causes CSH instability and promotes aragonite formation. To protect concrete structures, it's essential to use appropriate cement types, control sulphate exposure, and maintain adequate temperatures during construction and service. Regular inspections and early intervention can also help mitigate the risks associated with thaumasite formation.

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Everything You Need To Know

What is thaumasite, and why is it a concern in concrete structures?

Thaumasite is a complex salt containing sulphate, silica, and carbonate that can form in concrete, particularly in limestone filler cements. Its structure is similar to ettringite, making it difficult to identify with X-ray diffraction (XRD). The presence of thaumasite is a concern because it leads to the deterioration of concrete, compromising structural integrity due to sulphate attacks. This occurs as thaumasite crystals form, leading to cracking, gypsum deposition, and consumption of calcium hydroxide (CH), causing calcium silicate hydrate (CSH) instability. Since limestone acts as an internal source of CO32-, it increases the risk of thaumasite formation and subsequent damage, even in concretes designed to resist sulphate attacks.

How does thaumasite formation differ from typical sulphate attacks in concrete, and why is limestone filler a key factor?

Thaumasite formation is a specific type of sulphate attack that involves the formation of a complex salt containing sulphate, silica, and carbonate. Unlike typical sulphate attacks, thaumasite formation can occur even in concretes designed to resist sulphates. Limestone filler is a key factor because it acts as an internal source of carbonate ions (CO32-), one of the necessary components for thaumasite to form. This increases the susceptibility of cements with limestone filler to thaumasite formation, as the availability of CO32- accelerates the deterioration process. While ettringite might be present initially, the reaction ultimately leads to the formation of thaumasite, which further weakens the concrete structure.

What are the two main processes by which thaumasite forms in cement paste, and how do they affect concrete?

The two main processes for thaumasite formation involve either the evolution of ettringite incorporating silica (Si+4) while replacing aluminum, or the direct interaction of sulphates and carbonates with calcium silicate hydrate (CSH). The first process involves ettringite evolving and incorporating Si+4 into its structure, replacing aluminum between the columns. The other process involves the interaction of sulphates and carbonates in a solution with CSH. Both processes lead to the deterioration of the cement matrix. The formation of thaumasite results in increased cracking, gypsum deposition, and the consumption of calcium hydroxide (CH), causing CSH instability, ultimately weakening the concrete and compromising its structural integrity.

How do temperature and material composition influence the formation of thaumasite in concrete?

Low temperatures accelerate thaumasite formation in concrete, leading to increased cracking and faster deterioration. Material composition plays a significant role, particularly the presence of limestone filler in cements, which provides an internal source of carbonate ions (CO32-), a key component in thaumasite. Cements with limestone filler (CPF) are therefore more susceptible to sulphate attacks and the formation of thaumasite. Testing has shown that mortars with calcareous material in a Na₂SO₄ solution, and cement pastes with lime in a combined solution of Na₂SO₄ and MgSO₄, promote thaumasite formation, especially at lower temperatures.

What measures can be taken to protect concrete structures from thaumasite formation, and what role do regular inspections play?

To protect concrete structures from thaumasite formation, it is essential to use appropriate cement types, control sulphate exposure, and maintain adequate temperatures during construction and service. Regular inspections are crucial for early detection of deterioration signs, such as increased cracking or gypsum deposition. Early intervention can help mitigate the risks associated with thaumasite formation by addressing the underlying causes and preventing further damage. Monitoring and maintaining the concrete's condition can help ensure its long-term durability and structural integrity, especially in environments conducive to sulphate attacks.