What Is A Sulphate Attack and How It Could Affect Your Future Home

Sulphate attack is a serious problem that can affect concrete floors in homes built between 1945 and 1970.

It occurs when sulphate-bearing materials in the hardcore beneath the floor react with components of the concrete, causing chemical changes and physical damage.

Sulphate attack can lead to cracking, heaving, and deterioration of concrete floors and adjacent walls, potentially compromising the structural integrity of a property.

This issue is particularly prevalent in certain regions of the UK where industrial by-products like colliery spoil were commonly used as hardcore material. Homes in these areas are at higher risk of sulphate damage, which can develop slowly over many years.

Prospective homebuyers should be aware of this potential problem and consider having a property surveyed for signs of sulphate attack before making a purchase.

Recognising the signs of sulphate attack is crucial for homeowners and buyers alike. Telltale indicators include lifting or cracking of floor slabs, damage to skirting boards, and deterioration of wall plaster near floor level.

If left unchecked, sulphate attack can worsen over time, leading to costly repairs and potential safety hazards.

Understanding Sulphate Attack

Sulphate attack is a complex chemical process that can severely damage concrete structures. It occurs when sulphate ions react with components of cement paste, leading to expansion, cracking, and loss of strength.

Chemistry of Sulphate Attack

Sulphate attack involves reactions between sulphate ions and cement hydration products. The main culprits are calcium hydroxide and calcium aluminate hydrates in the cement paste.

Sulphate ions combine with calcium hydroxide to form gypsum:

Ca(OH)2 + SO4^2- + 2H2O → CaSO4·2H2O + 2OH^-

Gypsum then reacts with calcium aluminate hydrates to produce ettringite:

3CaO·Al2O3·6H2O + 3(CaSO4·2H2O) + 26H2O → 3CaO·Al2O3·3CaSO4·32H2O

Ettringite formation causes significant expansion, leading to internal stresses and cracking.

Different sulphate sources vary in aggressiveness:

• Sodium and potassium sulphates primarily form ettringite
• Magnesium sulphate is more destructive, also attacking calcium silicate hydrates

Physical Manifestations of Sulphate Attack

Sulphate attack causes visible signs of concrete deterioration. Early symptoms include:

• Surface scaling
• Map cracking (interconnected cracks forming a pattern)
• Spalling (fragments breaking off the surface)

As the attack progresses, more severe damage occurs:

• Expansion and swelling
• Loss of concrete mass
• Reduced compressive strength
• Increased porosity

In extreme cases, complete structural failure may result. The rate and severity of damage depend on factors such as:

• Sulphate concentration in the environment
• Concrete permeability
• Cement composition
• Presence of supplementary cementitious materials

Regular inspections can help detect sulphate attack early, allowing for timely intervention and repairs.

Identifying Sulphate Attack in Concrete

Sulphate attack can cause significant damage to concrete structures, particularly floor slabs. Recognising the signs early is crucial for preventing extensive structural issues. Visual inspection and testing are key methods for identifying this problem.

Signs in Floor Slabs

Concrete floor slabs affected by sulphate attack often exhibit visible symptoms. Look for surface cracking, which may start as fine lines but can develop into wider fissures over time. The edges of the slab may curl upwards, a phenomenon known as ‘doming’.

White, powdery deposits on the surface or within cracks can indicate sulphate efflorescence. This occurs when sulphates crystallise as moisture evaporates. In severe cases, the concrete may become soft and crumbly, particularly at the edges or corners of the slab.

Discolouration of the concrete surface, often appearing as dark patches, can suggest moisture ingress associated with sulphate attack.

Impact on Structural Integrity

Sulphate attack can severely compromise the structural integrity of concrete floors. As the reaction progresses, it leads to expansion within the concrete matrix. This expansion causes internal stresses, resulting in cracking and loss of strength.

In advanced stages, the floor slab may exhibit significant heave or uplift. This movement can affect adjacent walls and structures, causing further damage. The concrete’s compressive strength diminishes over time, potentially leading to localised failures or collapse under load.

Reinforcing steel within the concrete may become exposed due to spalling, accelerating corrosion and further weakening the structure.

Distinguishing from Other Types of Damage

Sulphate attack can be mistaken for other forms of concrete deterioration. Unlike freeze-thaw damage, sulphate attack typically affects the entire slab rather than just the surface. The presence of white, crystalline deposits differentiates it from carbonation.

Thermal cracking usually produces a more uniform pattern, whilst sulphate attack causes irregular, map-like cracking. Alkali-silica reaction (ASR) can produce similar expansion, but often results in a gel exudate rather than the powdery sulphate deposits.

Chemical testing of concrete core samples can confirm the presence of sulphates. Petrographic analysis may reveal ettringite formation, a telltale sign of sulphate attack.

Sulphate Attack Risk Factors

Sulphate attack on concrete is influenced by several key factors. The materials used in construction and the geographical location of a building play crucial roles in determining the risk level.

Construction Materials and Contamination

Portland cement, a common building material, can be vulnerable to sulphate attack. The addition of fly ash, slag, or other supplementary cementitious materials can increase resistance.

However, contaminated infill materials like building rubble or ash from coal mines, steelworks, or power stations may introduce sulphates into the concrete mix.

Water-soluble sulphates in the soil or groundwater can penetrate concrete structures over time. The type and quantity of cement, as well as the concrete’s permeability, affect its susceptibility to sulphate attack.

Higher cement content and lower water-to-cement ratios generally improve resistance.

Geographical Prone Areas

Certain regions are more prone to sulphate attack due to their soil composition or proximity to sulphate-rich environments.

Areas near coal mines, industrial sites, or coastal regions exposed to seawater may have higher sulphate concentrations in the soil or water.

In the UK, some regions have historically used sulphate-bearing hardcore in construction, increasing the risk for domestic properties.

The Ministry of Housing, Communities & Local Government has identified specific areas where this practice was more prevalent.

Temperature and pH levels also influence sulphate attack rates. Warmer climates and acidic environments can accelerate the deterioration process.

Buildings in these conditions may require additional protective measures or specialised concrete mixes to mitigate the risk of sulphate attack.

Preventing Sulphate Attack in New Homes

Effective prevention of sulphate attack in new homes requires a multi-faceted approach. Key strategies include careful material selection, thoughtful building design, and adherence to regulatory standards.

Material Selection and Treatment

Selecting appropriate materials is crucial for preventing sulphate attack. Type II or Type V cement, known for their resistance to sulphates, should be used in high-risk areas. Concrete mixes with low water-to-cement ratios enhance durability.

Incorporating pozzolanic materials like fly ash or ground granulated blast-furnace slag can improve concrete’s resistance to sulphate attack.

These additives reduce the amount of calcium hydroxide in the cement paste, making it less vulnerable to sulphate ions.

For the ground floor slab, using sulphate-resistant concrete is essential. This specially formulated concrete contains cement with low tricalcium aluminate content, which is less reactive with sulphates.

Building Design Considerations

Proper building design plays a vital role in preventing sulphate attack. Installing a damp proof membrane beneath the ground floor slab creates a barrier against moisture and sulphates from the soil.

Ensuring adequate drainage around the foundation helps minimise water accumulation, reducing the risk of sulphate ingress. Proper grading of the surrounding landscape directs water away from the building.

Designing concrete elements with minimal exposure to sulphate-rich environments is crucial. Where exposure is unavoidable, protective coatings or sealants can be applied to concrete surfaces to provide an additional layer of defence.

Regulatory Compliance

Adhering to building regulations is essential for preventing sulphate attack. UK building codes specify requirements for sulphate resistance based on the site’s sulphate classification.

Site investigation reports should be obtained to determine the sulphate levels in soil and groundwater. This information guides the selection of appropriate concrete mixes and protective measures.

Building control officers may require proof of compliance with sulphate resistance standards. This can include concrete mix designs, material certificates, and test results demonstrating the concrete’s sulphate resistance.

Regular inspections during construction ensure that specified materials and design elements are correctly implemented. This includes verifying the proper installation of damp proof membranes and the use of sulphate-resistant concrete in critical areas.

Detecting Sulphate Attack in Established Homes

Identifying sulphate attack in existing properties requires careful examination and professional expertise. Early detection is crucial to prevent extensive structural damage and costly repairs.

Professional Assessment and Surveys

Qualified surveyors and structural engineers play a vital role in detecting sulphate attack. They conduct thorough inspections of the property, focusing on areas prone to sulphate exposure.

A damp survey is often performed to assess moisture levels and potential sulphate sources.

Specialists examine concrete floors, foundations, and walls for telltale signs such as:

• Cracking or spalling of concrete surfaces
• White, powdery deposits on concrete
• Softening or flaking of concrete
• Expansion and heaving of floors

In regions like the North West of England, where sulphate-bearing soils are more common, surveyors may take samples of concrete and underlying fill material for laboratory analysis.

Evaluating the Extent of Damage

Once sulphate attack is suspected, professionals evaluate its severity and potential for further deterioration. This assessment involves:

1. Core sampling of affected concrete
2. Chemical analysis of concrete and surrounding soil
3. Measuring the depth of sulphate penetration
4. Assessing the structural integrity of affected elements

Engineers may use non-destructive testing methods such as ground-penetrating radar to map the extent of damage without causing further harm to the structure.

The results of these evaluations help determine appropriate remediation strategies, which may range from localised repairs to complete replacement of affected concrete elements.

Remediation and Repair of Sulphate Attack

Addressing sulphate attack in concrete requires targeted measures to halt damage progression and restore structural integrity. Effective solutions involve a combination of immediate repairs and long-term management strategies.

Strategies for Mitigating Damage

Concrete replacement is often necessary for severely damaged areas. This involves removing affected concrete and replacing it with sulphate-resistant materials.

Waterproofing measures, such as applying sealants or membranes, can prevent further water ingress and sulphate penetration.

For less severe cases, surface treatments like silane or siloxane coatings may suffice. These create a hydrophobic barrier, reducing moisture uptake. Crack injection with epoxy resins can seal fissures, limiting sulphate access to internal concrete.

Strengthening techniques may be required if structural problems have developed. This could involve installing steel reinforcement or carbon fibre wraps to bolster weakened elements.

Long-term Management and Maintenance

Regular inspections are crucial for early detection of recurring issues. These should focus on identifying signs of renewed sulphate attack, such as cracking or spalling.

Ongoing moisture control is vital. Proper drainage systems and guttering can divert water away from foundations and internal walls. Dehumidification in basements or crawl spaces can reduce ambient moisture levels.

Periodic testing of surrounding soil and groundwater helps monitor sulphate concentrations. If levels remain high, additional protective measures may be needed.

Maintaining concrete strength through appropriate mix designs and curing practices is essential for new construction or repairs in sulphate-rich environments. Using sulphate-resistant cement types can provide added protection against future attacks.