(Old, to salvage) What is Realkalization?

 What is Realkalization? 

Often understood only in general terms, realkalization is a method with the potential to stop and prevent one of the leading causes of concrete failure, being carbonation-induced corrosion.

In this article, we provide an overview of the causes of the issue, as well as explaining both passive and electrochemical realkalization.

Carbonation-Induced Corrosion

Carbonation-induced corrosion is caused by a natural reaction where atmospheric CO2 reacts with concrete, resulting in changes to the concrete chemistry and fall in internal pH. Usually, a high internal pH, approximately between 10 and 12, is required to protect the steel reinforcement from expansive corrosion.

In non-carbonated concrete, reinforcing steel maintains an oxide coating that prevents expansive corrosion, the steel being considered passive. This is caused by the steel being in a highly alkaline environment of ~pH 10 – 12, maintained by non-carbonated concrete.

However, atmospheric CO2 reacts with concrete, gradually changing its chemistry so to cause the pH to fall below 10. This reaction commences from the surface, gradually moving deeper inside (though at a slowing rate) and eventually reaching the reinforcement.

Once the carbonation front reaches the steel reinforcement and depletes the surrounding pH to below 10, the passive oxide layer protecting the steel is lost, and expansive corrosion begins to occur. This corrosive product then swells to cause cracking and delamination of protective coating, and gradual failure of the structure.

Realkalization

Realkalization addresses this issue through restoring the internal alkalinity of carbonated concrete, preventing the initiation or progression of carbonation-induced corrosion.

While this principle of realkalization is widely recognized, the distinction between electrochemical and passive approaches is not.

Electrochemical Realkalization

Within electrochemical realkalization, an electrical current is used to both induce the transport of surface-applied alkali materials towards the reinforcement, along with the production of hydroxides surrounding the reinforcement.

The method typically involves accessing reinforcement inside the concrete, laying a mesh with alkaline paste outside the concrete, and running a current across both over an extended period of time.

This method has seen periodic use within Australia and overseas projects, though adoption is significantly constrained by the high cost, high complexity, and limited experience using the method.

Passive Realkalization

Passive realkalization, by contrast, uses non-electrochemical methods of transport such as capillary action, diffusion, and more to transport alkalis into carbonated concrete. It’s typically far simpler in its application method, resulting in far lower costs and operational complexity compared to the electrochemical method.

The methodology of application can be quite broad and based on specific products, but generally involves a simple application of a product to the surface of carbonated concrete, then letting it migrate inside.

This method has long been established in laboratory studies and is referenced in overseas standards (See EN 1504-9, Method 7.4), though has lacked many commercial offerings demonstrating viability in field applications, or long-term durability.

Penkal, Our Passive Realkalization Solution

At Alkyon, we’ve spent years researching, developing, and verifying a field-viable passive realkalization solution, identifying both the formulation which is able to achieve key performance metrics for passive realkalization, as well as the application methodology to guarantee proper application.

This solution is Penkal, our proprietary passive realkalization formulation and service.

The method is simple. It’s applied as a temporary surface coating to carbonated concrete in 2 parts spaced approximately 1 week apart, and then migrates to carbonation front over approximately 2 months. Low operational complexity, low cost, and limited disruption to the regular operation of most assets.

FAQs:

Can you provide a depth in mm which Penkal will migrate to?

Our experience has shown that there’s no such thing as a universally applicable penetration depths for field-relevant applications for any penetrating coatings.

Hypothetically, we can demonstrate penetration of Penkal or nearly any other coating to beyond 100mm depth using very pourous concrete in laboratory conditions, giving results not representative of real-world use.

Penetration depth of both coatings and CO2 is determined by a wide variety of properties including that of the concrete, the environment, and coating formulation. As such, the carbonation front depth indicates the depth to which Penkal is likely to reach.

Instead, a comparative approach needs to be used between coating products to determine whether the formulation and approach of one outcompetes another on key metrics of penetration depth and durability. Previously, Penkal was tested against several other passive realkalization solutions as part of Transport for New South Wales trials. The only product that could demonstrate penetration to carbonation front across all trials was Penkal.

Is the realkalization coating sold separately to the application service

No, we only provide realkalization as a service, rather than a coating.

The reasons for this are multiple, but mainly revolve around the necessity to ensure quality of application, and technical aspects of the manufacture of the coating formulation.

Supplying our product independent of the service currently provides a risk in that there may be issues during application, resulting in the failure of the passive realkalization work. We’ll prioritize consistency and quality in application until the practice of passive realkalization is standardized to ensure success.

Does Penkal coating have to remain on concrete? How long do I have to wait between coatings?

Once any coating is applied, we generally recommend at least 3 days for the product to migrate inside prior to being washed off if required.

We recommend at least 1 week between the first and second coating to ensure that the first coating has cured.

The coating can be left on the structure for extended periods of time, though is not recommended if the asset is to be accessed by people or other animals due to the residue potentially being alkaline and irritating to skin.