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    Why NATCEM® is right for Carbonation Resistant Concrete

    Carbonation resistant concrete is a priority in any environment where degradation and breakdown can occur. Concrete breakdown from carbonation is one of the most common causes of deterioration in modern infrastructure. In reinforced concrete, carbonation reduces alkalinity, strips steel of its protective passivation layer, and ultimately causes corrosion, cracking, and loss of strength. This problem is particularly severe in  environments where aggressive chemical and biological conditions accelerate damage. For a detailed explanation of carbonation in reinforced concrete, see the Concrete Society’s guide here.

    Portland cement–based concretes are especially vulnerable to carbonation. NATCEM® products, manufactured using a natural cement binder, behave very differently. Their unique chemistry and dense microstructure make them inherently resistant to carbonation. This gives them exceptional durability in some of the harshest environments.

    Why NATCEM® is Different

    NATCEM® products, based on natural cement manufactured from argillaceous limestone, have a very different chemistry and structure.

    1. Little Free Calcium Hydroxide
      Most lime in NATCEM® reacts with clays during burning and hydration, forming stable calcium silicate and aluminate hydrates. With very little free Ca(OH)₂ available, carbonation has far less impact on alkalinity.

    2. Stable Alkalinity
      NATCEM® concrete repair mortars maintain alkalinity through their hydrates, not through large reserves of free lime. Even if carbonation occurs, pH remains high enough to keep reinforcement passivated.

    3. Dense Microstructure
      NATCEM® mortars are naturally dense and fine-pored. This slows carbon dioxide diffusion and reduces carbonation depth, a crucial factor for long-term durability.

    4. Historical Performance
      Nineteenth and early twentieth century bridges, sewers, and waterworks built with natural cements remain in service today with little or no carbonation damage. 

    Why Carbonation Damages Portland Cement

    Carbonation occurs when carbon dioxide (CO₂) penetrates porous concrete and reacts with calcium hydroxide (Ca(OH)₂), a by-product of Portland cement hydration. The reaction forms calcium carbonate (CaCO₃), which lowers the pH from around 12.5–13.5 to below 9.

    This loss of alkalinity breaks down the protective film around steel reinforcement. Once passivation is lost, moisture and oxygen trigger corrosion, expansion, and cracking.

    Portland cement-based concretes are at risk because they generate large amounts of free calcium hydroxide during hydration. Their alkalinity relies heavily on this Ca(OH)₂, which is easily consumed. They also tend to have porous microstructures that allow carbon dioxide to diffuse quickly.

    The result is that structures in high-moisture, high-CO₂ environments, such as sewers, often degrade long before their intended service life.

    The NATCEM® Advantage for Carbonation Resistant Concrete

    Where Portland cement structures often need costly coatings or linings to protect against carbonation, NATCEM® products provide inherent resistance. They deliver:

    • Long-lasting protection against carbonation.

    • High durability in chemically aggressive environments.

    • Reliable performance backed by historical case studies and modern approvals.

    Carbonation is one of the most damaging processes in reinforced concrete, and Portland cement is particularly vulnerable. NATCEM® products are different. With little free lime, stable hydrates, and dense microstructures, they resist carbonation naturally.

    That is why NATCEM® is the material of choice for environments where concrete breakdown is common. With NATCEM®, structures achieve the durability that Portland cement struggles to deliver, even after more than 100 years of exposure.

    If carbonation resistance is a MUST for you and your work – get in touch:

    t: 01226 38133

    e: james@naturalcement.co.uk

    Date added: 27/08/2025

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