Keywords

1 Introduction

One of the most crucial characteristics to consider in structures is their durability. It is critical that structures maintain the attributes for which they were designed for in all the environmental situations where they are employed.

Structures may be subjected to different degradation agents throughout their life cycle, posing a hazard to their long-term sustainability. These agents could be external, because of the structure's environmental influence, or internal, because of the matrix of constituents that make up the structure (Findik & Findik, 2021).

Structural environment plays a vital role in the service life of concrete structures, especially in the areas that have sever climate conditions such as the Arab gulf. These countries are known for their hot-humid or hot-dry weather conditions combined with salt air-contaminated particles that -if not considered- will dramatically affect the durability of the structure (Al-Gahtani & Maslehuddin, 2002).

Mechanism of concrete deterioration might be physical; for example, caused by the discrepancy between the cement paste and aggregate thermal properties. Mechanical, usually related to abrasion, and chemical, caused by the attack of acids, saltwater, sulphates, and other elements (Findik & Findik, 2021).

Sulphate attack is one of the damaging agents for structures. The by-products of this reaction begin to chip away at the paste that maintains the concrete together. New chemicals, such as ettringite, form as sulphate dries. These new crystals fill the gaps in the paste, causing it to break and further damage the concrete.

Sulphate attack causes billions of dollars in damage to concrete such as wastewater collection and treatment facilities all over the world (Pan et al., 2017), thus this phenomenon’s mechanism, mitigation, protection of structures during their life time is critical.

2 Sulphate Attack Overview

2.1 Sulphate Attack Definition

Sulphate attack is a degradation process in which sulphate ions strike the constituents of cement paste. Water-soluble sulphate-containing salts such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulphates, which are chemically reactive with concrete components, cause sulphate attack. Sulphate attack on concrete can take several forms, depending on the chemical type of sulphate and the exposure of the concrete to the environment. (See Fig. 1)

Fig. 1
A photograph of sulphate attack on concrete walls. Two parallel walls are disintegrated at the bottom with the loss of concrete plaster. There is stagnation of sulphate mixed water at the bottom.

Structure affected by sulphate attack. (Suryakanta, 2015)

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Sulphate sources might be internal or external. Internal sources are less common, but they appear with materials used in concrete, such as hydraulic cements, fly ash, aggregate, and admixtures. External sources are more common, typically soils and ground water have high-sulphate content, as well as pollution from the atmosphere or industry (Zhao et al., 2020).

2.2 Forms and Mechanism of Sulphate Attack

There are two main forms of sulphate attack: chemical or classical form and physical or salt attack. The creation of ettringite and the formation of gypsum are the two principal processes of chemical sulphate attack, according to ACI's Guide to Durable Concrete (201.2R). However, more forms can be identified because of sulphate chemical interactions with cement hydration products. See Fig. 2

Fig. 2
An illustration of 2 types of sulphate attack. A. classical forms are subdivided into 2 categories ettringite formation with primary and secondary ettringite, and gypsum with higher volume causing a crack. B. Physical forms are subdivided into 2 categories thaumasite which disrupts the binding properties of cementitious components and physical attack in which repeated cycles of sulphate salts crystallization and hydration with volume expansion deteriorate concrete.

Mechanism of sulphate attack

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Classical (chemical) Form of Sulphate Attack

This mechanism of sulphate attack result in products that have expansive volume which will cause internal tensile stresses on the concrete and cause cracks, spalling, and/or disintegration.

Ettringite Formation

There are two types of Ettringite, the first one is labelled as primary ettringite, formed after few hours of blending water and cement in a termed process of Early Ettringite Formation (EEF), this type doesn’t cause any remarkable damage despite of its big volume, it is consistently and individually distributed at a sub-microscopic level across the cement paste, which functions as a covering over the surfaces of cement grains shortly after mixing, regulate concrete setting. Because of its pore filling function, primary ettringite can raise strength, lower permeability, porosity, and provide dimensional stability.

The other ettringite type is called secondary ettringite and formed in Delayed Ettringite Formation (DEF) process. DEF starts when the primary ettringite generated dissolves at high curing temperatures over 65–70 °C when the temperature of concrete dips below 70 degrees Celsius, the ettringite re-forms to secondary ettringite, causing the hardened concrete to expand and crack.

This type developed in concrete that has been cured at high temperatures (steam curing) and in large pours where the heat of hydration has caused high temperatures within the core area. DEF causes the paste to expand while the aggregate does not, resulting in fractures (or gaps) surrounding the aggregate, with the larger the aggregate, the larger the gap. (See Fig. 3).

Fig. 3
A micrograph of cement disintegration due to ettringite contains a small patch of cement work surrounded by dark pore-like patches.

Secondary ettringite causing a gap around aggregate.(Sulphate attack in concrete 2017)

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Gypsum Formation

Gypsum, often known as calcium sulphate dihydrate, is a soft sulphate mineral having a hardness of 2 on the Mohs scale. (See Fig. 4).

Fig. 4
A photograph of gypsum formation of concrete. A thick layer of gypsum is formed on the concrete due to sulphate attack.

Gypsum Mineral (Vasavan, 2017)

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The hydration of Portland cement's silicate phases releases lime. Gypsum is formed when sulphate ions react with calcium hydroxide. This reaction result has a higher solid volume than the original ingredients, which may contribute to concrete degradation in some situations.

Thaumasite form of Sulphate Attack

The Thaumasite form of sulphate attack necessitates the presence of both carbonate and sulphate ions in solution. Thaumasite is formed when calcium silicate hydrate (C-S–H) and calcium hydroxide are degraded by reactions with sulphate attack and carbonate.

For TSA to occur, a low temperature (4–10) oC is required, and severe damage occurs only when the ground is quite damp. TSA is a rather uncommon type of sulphate attack, with little probability of occurrence unless the environment is a combination of cold weather, presence of sulphate ions, carbonates, and mobile groundwater. Concrete becomes a friable material when it hardens. TSA-affected concrete can be easily fractured with fingers, and the coarse aggregate can be removed. (See Fig. 5).

Fig. 5
A photograph with a cylindrical specimen of T S A affected concrete held in a person's hand. The top layer of concrete is absent with the dark coarse aggregates inside exposed, and it is friable.

TSA affected concrete. (Thaumasite-attack. 2022)

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Physical form of sulphate attack

Chemical sulphate attack on concrete structures has long been thought to be the most common cause of concrete deterioration in sulphate-rich environments. However, it was shown that under specific environmental conditions, concrete is primarily affected by physical sulphate damage. (Suleiman, 2014).

Bloom (presence of sodium sulphate salts) at exposed concrete surfaces is a common sign of physical sulphate attack. It's not only an aesthetic issue; it's also a visual sign of chemical and microstructural destruction in the concrete.

Capillary action and diffusion allow sulphate salts to enter the pore spaces of concrete in solution. The wick's motion draws the sulphate solution to the exposed surface, where it evaporates, gradually increasing the sulphate ion concentration until it crystallizes. The sulphate salts go through cycles of crystallization and dissolution, or hydration and dehydration, as the ambient temperature and relative humidity change. Repeated cycles of crystallization and hydration with volumetric expansion can induce concrete deterioration comparable to that caused by freezing-and-thawing cycles. (See figure 6).

Fig. 6
A pair of photographs of physical sulphate attacks on concrete. The salts on the walls are crystallized and a bulge is formed on the wall.

Physical sulphate attack on concrete (Suleiman, 2014)

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2.3 Sulphates are Not Equal

Sulphates can be rather very aggressive according to which cation they are coupled with, moderately aggressive, or low aggressive.

MgSO4 (magnesium sulphate) is the most aggressive (Rachel Detwiler, 2021) whereas calcium sulphate (CaSO4) is the least destructive since it is the least soluble. Even so, due to the production of ettringite, it will cause expansion and cracking. Sodium sulphate, often known as Na2SO4, is a moderately aggressive substance. Because of the sodium ions, there is enough NaOH in solution to keep the calcium silicate hydrate gel, the cement paste's main strength-producing component, stable. Because it does not contain calcium, unlike CaSO4, it will attack calcium hydroxide. Due to the development of both ettringite and gypsum, it promotes expansion and cracking.

Scholars from early time investigated the factors that affect this detrimental process, for example, Dhole et al. (2019), Qiang Yuan et al. (2021) and Liu et al. (2020) stated that Sulphate attack on concrete is influenced by a number of elements, including the type and quantity of sulphate solution, temperature, pH value, cement composition, admixtures, and erosion form. The most essential elements in sulphate attack are the temperature, concentration, and kind of sulphate solution.

3 Structure Deterioration in the Arab Gulf Due to Sulphate Attack

3.1 Sulphate Sources and Distribution in UAE and Arab Gulf

The existence of sabkha soil in the Arab Gulf is largely responsible for the deterioration, notably of columns and footings.

The geology of the Arabian Gulf coast, including the UAE and several neighboring countries, is dominated by Sabkha, a gypsum-rich deposit formed inland by tidal sea water evaporation. Sabkha's ground water contains gypsum, anhydrite, calcium carbonate, and sodium chloride. Concrete structures constructed in Sabkha are particularly vulnerable to sulphate and chloride attack, increasing the risk of deterioration. (Khan, 2018)

Other than that, Saleh (2014) investigated the amount and distribution of different sulphate types in two areas: Al Khatem and Remah in Abu Dhabi, UAE. The results are shown in the map below. (See Fig. 7). The main source of sulphate here is the volcanic and sedimental rocks.

Fig. 7
A map of Al Khatem and Remah in Abu Dhabi with 6 classifications of the presence of sulphates. Al Khatem has large sulphate deposits in the soil and groundwater ranging from 2827.2 to 5,456.5 milligrams and Remah with fewer deposits of 1142.6 to 2,194 milligrams.

Distribution map for sulphate distribution

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3.2 Current Situation Structure Deterioration due to Sulphate Attack in the Arab Gulf

Even though the weather conditions are harsh and fluctuating and sulphate concentration in the soil and ground water is excessive, no severe concrete deterioration has been reported because of this phenomenon. The use of Type V cement with a low C3A component was most likely to attribute. Additionally, the presence of chloride ions, which protects against sulphate attack limit structure deterioration.

The Yas Island Water project in Sabkha is representative of most projects in the Arabian Peninsula that are exposed to a sulphate and chloride-laden environment. Ground water chemical testing revealed the existence of an aggressive chemical environment due to the inclusion of gypsum layers beneath, accounting for roughly 49% of the oxide element, according to the Geotechnical interpretive report. (Saleh, 2014)

4 Sulphate Attack Requirements in the International Codes

International codes, such as the American code ACI 318, the British standards BS 8500–1, and the European code EC2, provide provisions for the structural design of durability against sulphate attack.

ACI 318–19 classifies concrete in contact with soil or water that contains concentrations of water-soluble sulphate ions as exposure category S. This category is further divided up into four classes, with S0 indicating low levels of water-soluble sulphate. S1, S2, and S3 exposure classes are designated to structural concrete members that have direct contact with soluble sulphates in soil or water. From Exposure Class S1 to S3, the severity of exposure increases. Table 19.3.2.1 shows the maximum allowable w/c ratio, the minimum compressive strength, the cement type and cementitious material, and the permissible use of calcium chloride admixtures (ACI Committee 318 2019).

Similarly, when concrete is exposed to sulphate-containing soil or natural water, the British code exposures include XA1 for a fractionally aggressive environment, XA2 for a moderately aggressive environment, and XA3 for a highly aggressive environment. The maximum allowable w/c ratio, minimum cement or combination content, and indicative strength are also specified in Tables A.4 and A.5. Furthermore, the standard considered the source of sulphate attack from a concrete constituent: aggregate. Sulphate in aggregate (A.7.5) and Alkali-aggregate reaction (A.8.2) (British Standards Institution, 2019).

The Euro code is based on EN 206-1, Table 2. XA1 Chemical environment that is fairly aggressive XA2 chemical environment is moderately aggressive, and XA3 chemical environment is highly aggressive. (EN1992-1–1 2004).

Overall, these codes provide categories for exposure conditions based on sulphate levels, and for each exposure class, critical value limitations are stated. Numerous detailed studies have examined these values and proposed other performance effective factors in resisting sulphate impact such as (Bentivegna et al. 2020) and (Obla et al., 2017).

5 Case study for Deterioration of Structure due to Sulphate Attack - Cheng-Kun Railway Tunnels

Liu et al. (2017) investigated the deterioration of Cheng-Kun Railway tunnels in China. The structure experienced Concrete failure due to the action of sulphates.

5.1 Visual and Field Inspection

The field investigations resulted in the following: (See Fig. 8)

Fig. 8
A set of 4 photographs of disintegrated concrete in the tunnel. a and b. Layers of concrete are removed using a chisel hammer. c and d. Crystal-like formations are present.

Deteriorated concrete in the tunnel

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  • A 5 mm thick surface layer was removed from the concrete lining. (a, b).

  • After the surface layer was peeled, the aggregates of the concrete were seen, and a significant number of white crystals emerged on the concrete surface. (c)

  • Exposed and rusted Steel bars. (d)

5.2 Test Results

The findings revealed that:

  1. 1.

    Large amounts of Sodium Sulphate were generated causing layer-by-layer detachment.

  2. 2.

    Ettringite and gypsum, were also detected in the neutralized concrete lining.

5.3 Analysis of Findings

Although Sodium Sulphate crystallization caused the concrete to detach, the main product of the detached concrete pieces was Calcium Carbonate. Ettringite and gypsum in the concrete lining indicate that the chemical sulphate attack occurred within the concrete. The concrete lining contained Sodium Sulphate crystals, but it was not detached and was not carbonated with Calcium Carbonate. As a result, it is concluded that chemical sulphate attack occurred in the inner part of the concrete lining, whereas physical sulphate attack occurred on the concrete lining's neutralized surface layer.

5.4 Lessons Learnt and Repair Measures

The study didn’t mention any repair plan for the tunnel or the percentage of damage in the tunnel, however, there are some lessons to be learnt and approaches for repair might be considered.

To begin with, this tunnel lining suffered from sulphate ingress from the soil due to the use of normal Portland cement, high w/c ratio, high permeability, and minimal protection, in such cases, where structures are exposed to soil and ground water, sulphate resistance cement together with the lowest possible w/c ratio is preferable.

Depending on the size of the impacted area, two methods can be used for repair: grouting and/or Electrokinetic Nanoparticle Treatment. Grouting is a common practice followed in repairing tunnels.

Sulphate attack is a long process, hence, periodic inspection for any cracks are signs of damage must be rectified. Access points for maintenance and inspection must be part of the design, water proofing membranes would also be beneficial.

6 Discussion and Conclusion

Sulphate attack is harmful to reinforced structures, it has two mechanisms that result in products with high volume: ettringite, gypsum, Thaumasite, and salt crystals. This enlargement will cause tensile internal stresses in the concrete, causing it to spall, crack, or disintegrate. Furthermore, these cracks may cause steel corrosion by allowing chlorides and other harmful elements to enter. The severity of the damage is determined by the type of sulphate; testing for such elements is required to implement proper mitigation strategies.

Sulphate sources can be internal or external, internal sources coming from concrete composition and external sources from the soil or ground.

International codes, such as ACI, British, and Eurocode, included provisions for durability design to design structures for sulphate ingress resistance. Primarily, they classify sulphate concentrations according to severity and recommend values for strength, concrete cover, water cement ratio, cement type, and SCM.

Sulphate attack can be identified by the following observations: salt crystals on the surface, cracks, and when lightly touching the surface, it quickly scales or flakes away.

Several repair methods can be applied to affected structures depending on the outcome of the attack; for cracks, based on their type, repairing methods can include epoxy injection, routing and sealing, stitching, drilling, and plugging, gravity filling, grouting, and crack overlay and surface treatment. If spalling occurs, new binding materials of the same quality as the old ones be used. Electrokinetic Nanoparticle Treatment is a microstructural technique that combines sulphate extraction with nanoscale pozzolan injection.

Based on this overview, we can conclude that sulphate attack is harmful to concrete structures in all of their forms. Multiple international codes included provisions to guide the design engineer with preventive measures to consider designing against it.

Multiple approaches to limiting SA are available; however, if sulphate attack caused damage, repair methods can be used to restore its effect.

Sulphates are abundant in the Arab Gulf region. Particularly in the Sabkha region, of which the UAE is a part. The awareness of its effect in the UAE, as well as the implementation of design precautions and proper curing, aided in confining its impact, particularly considering the region's harsh environmental conditions.

7 Recommendation

Overall, the following is recommended:

  1. 1.

    Environmental inspection of any sulphate and its concentration must be completed prior to the executive phase of any project and design for durability as per the codes should be considered.

  2. 2.

    A sulphate attack risk mitigation plan must be developed according to code provisions.

  3. 3.

    Regular inspection and correction of any signs of sulphate assault are required.