Concrete Catastrophe: Is Your Limestone Filler Cement Under Attack?
"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?
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.
- 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.
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.