What is Internal Sulphate Attack?

Internal Sulphate Attack (ISA) is a complex a reaction between sulphates present in the concrete mix and the calcium aluminate of hydrating cement that significantly compromises the integrity and durability of concrete structures.

Internal Sulphate Attack occurs when a source of sulphate is incorporated into the concrete mix, either from the materials used or from external sources such as on site mixing water, and, when curing temperatures greater than 60°C are reached.

Characterized by a distinct pattern of multidirectional cracks and wide meshes, Internal Sulphate Attack comes in two main forms: Delayed Ettringite Formation (DEF), or Thaumasite Sulphate Attack (TSA) if the concrete is carbonated.

TSA is normally more aggressive than DEF, since in TSA the cement hydration products are decomposed as a result of both sulphate attack and of carbonation.

Figure 1. Exposed ISA in a split concrete core

What causes Internal Sulphate Attack of concrete structures?

DEF, the most common type of Internal Sulphate Attack (ISA) typically occurs in concrete cured at elevated temperatures, above approximately 60°C. DEF is found in both heat-cured, precast concrete elements and large-volume in-situ pours, where the heat generated during the cement hydration process can significantly increase the internal temperature.

DEF is a direct result of high curing temperatures, which foster the formation of a meta-stable mono-sulphate compound. Upon subsequent exposure of this concrete to moisture, the compound undergoes a conversion to ettringite, triggering disruptive expansion from within the concrete.

The damage mechanism is primarily physical, stemming from the internal stress and pressure generated by ettringite expansion in hardened concrete, manifesting as a distinctive network of multidirectional cracks and wide meshes. These cracks are not merely superficial but signal deep structural issues, undermining the concrete's overall strength and resilience.

Figure 2. SEM images of concrete with sulphate containing sand undergoing ISA at (a) 120 and (b) 240 days

Unlike DEF, TSA involves the chemical decomposition of hydration products, like calcium silicate hydrates (CSH), leading to the formation of thaumasite. This expansive process not only reduces the concrete's strength and durability but also causes extensive, widespread cracking, significant softening, and loss of cohesion, often accompanied by a distinctive whitish or greenish discoloration.

What are the signs of Internal Sulphate Attack of concrete structures?

Element translation or rotation

Volumetric expansion

Disintegration

Figure 3. Discolouration from thaumasite sulphate attack

How can I identify Internal Sulphate Attack in concrete structures?

Visual survey	Non-destructive testing	Destructive testing
Visual examination Crack index Cracks pattern similar to that of AAR	Ultrasonic velocity transmission Surface waves Impact echo Cover depth (Covermeter) Water permeability GPR	Sampling Compressive strength Modulus of elasticity Microscopic examination Sulphate content determination Residual reactivity to internal sulphates Water porosity Capillary absorption

How can I prevent Internal Sulphate Attack in concrete structures?

Preventing Internal Sulphate Attack (ISA) in concrete is crucial for maintaining the structural integrity of concrete structures and hinges on careful selection of materials and control over the concrete mix design and curing processes.

The key to mitigating ISA lies in choosing materials that inherently have low sulphate content or low reactivity to sulphates and controlling the temperatures reached during their hydration. The use of sulphate-resistant cement and aggregates resistant to sulphate attack is paramount.

Thin-section photomicrographs illustrating thaumasite surrounding aggregate particles and locally replacing paste. A few residual cement particles were present in the thaumasite halos. A dark band presented at the outer edge of the halo (arrows). Upper photo: plane-polarized light; lower photo: cross-polarized light.

Figure 1. Thin-section photomicrographs illustrating thaumasite surrounding aggregate particles and locally replacing paste. A few residual cement particles were present in the thaumasite halos. A dark band presented at the outer edge of the halo (arrows). Upper photo: plane-polarized light; lower photo: cross-polarized light.

High temperatures create ISA, so monitoring and managing the curing temperature to keep it within a safe range is essential. This might involve adjusting the concrete mix to generate less heat or employing external cooling methods.

Adjusting the concrete mix design to replace Portland cement with alternatives like Fly Ash or GGBS which can effectively bind internal sulphates, thereby reducing their reactivity and exhibit significantly lower heat of hydration is also extremely effective. These materials not only contribute to reducing the risk of ISA but also enhance the overall durability and strength of the concrete.

Ensuring a thorough mix and proper curing practices are fundamental to preventing ISA. A well-mixed concrete ensures uniform distribution of materials, reducing the likelihood of localized high concentrations of sulphates.

How can I repair the damage from Internal Sulphate Attack on concrete structures?

Concrete replacement.

Stress release.

Jacketing.