How to Protect Concrete Against Sulphate Attack?

Concrete deterioration due to sulphur reactivity is more common than you may realize

Let’s start at the beginning! What are sulphates?

Firstly, sulphates and sulfates are the same thing, depending on whether you use UK or US spelling, respectively.

Sulphates are reactive ions (part-molecules) consisting of Sulphur and Oxygen (-SO4). You can find out more here, but they are part of our everyday lives, in products as diverse as batteries, fertilizer, and plasterboard. That word ‘reactive’ is key to this discussion – that is why we need to talk about them in the context of concrete.

What is the threat to concrete?

Sulfate attack of concrete is a complex process, which includes physical salt attack due to salt crystallization and chemical sulfate attack by sulfates from soil, groundwater, or seawater.” – Developments in the Formulation and Reinforcement of Concrete

External sulfate attack is a chemical breakdown mechanism where sulfate ions from an external source (underground water, sea water, some earthworks) attack components of the cement paste. Such attack can occur when concrete is in contact with sulfate-containing water, e.g. seawater, swamp water, ground water or sewage water. The often massive formation of gypsum and ettringite formed during the external sulfate attack may cause concrete to crack and scale. For external sulfate attack, the reaction propagates from the surface towards the concrete core.” – Science Direct – Sulfate Attack

You must admit, that covers a broad array of places where concrete might be at risk. Bridges, culverts, wharves, sewerage infrastructure – the list of at-risk structures goes on. The burning question is, how can they be protected?

What is the answer?

There are a number of suggestions for new construction – in particular, reducing the permeability of the concrete in various ways. Then there is this comment: “Proper placement, compaction, finishing, and curing of concrete are essential to minimise the ingress and movement of water, which is the carrier of the aggressive salts.” – ibid

Did you spot that phrase “water, which is the carrier of the aggressive salts“? There’s our clue.

Block the Carrier

Concrete is not perfectly waterproof in and of itself. Despite the common addition of flyash or other fines, bleedwater porosities will occur during curing. Unless you have included a self-healing agent in the mix (that’s another subject), those porosities guarantee the concrete will still accept moisture ingress. That moisture, as notes, is the carrier of the aggressive salts – in this case, sulphates.

And here, for new construction, we submit AQURON 300 waterproofing admixture. AQURON 300 admixture is a pozzolanic derivative material, protecting against sulphate attack by converting the leachable calcium hydroxide into a hydrogel, not allowing it to react with the sulphate ions which normally reacts with the calcium hydroxide and creates a expansive force in the concrete. Removal of calcium hydroxide reduces the risk of attack by magnesium sulphate. Along side this, because the concrete is waterproof, it limits any attack to the top surface – the internal matrix is protected.

Further, we recommend coupling this together with spray-applied AQURON 7000 as a curing and durability hydrogel treatment. This ensures the critical curing phase is taken care of, and proper hydration takes place giving a higher quality concrete, as well as secondary waterproofing as a belts-and-braces approach against sulphate attack.

Project Examples

This approach has been applied in such instances as fertilizer sheds in Balance and Ravensdown, NZ; acid tanks in Tasmania; along with waste water tanks and structures for various councils around Australasia.

Interestingly, a study by UNSW in 2000, which examined a comprehensive number of benefits of AQURON hydrogel treatment, confirmed that the hydrogel stops the reaction of sulphate ions. A copy of this study is available on request.

But that doesn’t help us with existing concrete … or does it?

As a matter of fact …

No, of course an admixture won’t assist for existing concrete (unless you’re doing cementitious repairs).

But AQURON 7000 is a real friend for even aged concrete. The treatment penetrates around 150mm into the substrate, regardless of gravity, and arrests the reactivity occurring deep inside the matrix. By immobilizing the moisture, the reactions are robbed of their food and drink, and cannot extend. This means effective protection for the reinforcing steel.

Real projects, real answers

But you’re wondering how all this applies to your concrete – the infrastructure you’re in charge of – not to mention your schedules and budget. You will be pleased to know these treatments are outstandingly practical.

  • Minimal downtime
  • No need to re-apply
  • Cost effective
  • Ecolabelled, nil VOC, and potable water certified

Interested? We’d love to talk about your concrete challenges – get in touch!

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