If your soil report mentions sulphates or clay, you will likely need FND2 concrete for your building foundation. But what exactly is this specialist concrete mix, and when is it essential?
What Is FND2 Concrete?
FND2 is a type of ready-mixed concrete designed to resist chemical attacks from sulphates in the ground. The name comes from British Standard BS 8500, where FND means foundation and 2 refers to Design Chemical Class 2 (DC-2).
This concrete mix contains special cement types and has a minimum cement content of 330kg per cubic metre. This creates a denser mix with better chemical resistance than standard concrete, providing long-term durability for foundations in aggressive soils.
You might also see this called FND2/Z2 concrete or sulphate-resistant concrete. These names all mean the same thing.
Why Do Sulphates Damage Normal Concrete?
Sulphates are chemicals found naturally in some soils, particularly clay. When groundwater carries sulphates into contact with ordinary concrete, a reaction occurs that creates expanding minerals inside the mix.
Over time, this expansion causes cracking and weakening. The damage happens slowly over the years, often going unnoticed until serious structural problems develop. Using the correct FND2 concrete mix prevents this from happening.
When Do You Need FND2 Concrete?
Your structural engineer or building control officer will confirm if FND2 is required based on soil testing. Common situations include:
Clay and sulphate-rich soils naturally contain chemicals that attack standard concrete. Soil testing determines whether your site falls into this category.
Brownfield sites with previous industrial use may have contamination from old factories or chemical works, creating aggressive soils that require specialist foundations.
Large-scale projects on marginal ground often specify FND2 for both unreinforced foundations and reinforced concrete base applications to ensure the structure lasts.
Commercial buildings frequently require sulphate-resistant mixes because engineers specify higher durability standards for business premises.
| Design Chemical Class | Ground Conditions | Concrete Required |
|---|---|---|
| DC-1 | No sulphate risk | Standard GEN mixes |
| DC-2 | Moderate sulphates | FND2 |
| DC-3 | High sulphates | FND3 |
| DC-4 | Very high sulphates | FND4 |
Building control requires evidence that the correct specification was used, as outlined in the government’s Approved Document A covering structural requirements.
How Is FND2 Different?
FND2 has a compressive strength of around 30N/mm² at 28 days, similar to standard mixes. The key difference is durability, not strength. The higher cement content and specialist cement types provide chemical resistance that standard concrete lacks.
This makes FND2 concrete slightly more expensive, but the additional cost is essential protection for any concrete base on sulphate-bearing ground.
Get the Right Mix Concrete for Your Project
At Base Concrete, we have supplied specialist foundation mixes across North London and Hertfordshire since 2002. Our volumetric mixers produce fresh FND2 concrete on site, meaning you only pay for what you use with no waste.
Our QSRMC-accredited ready mixed concrete meets BS 8500-2 standards, giving you and building control confidence that the specification is correct.
Whether you need a small domestic foundation or concrete for larger commercial buildings, contact our team today. We will ensure you get exactly the right mix of concrete for your ground conditions.