Calcined Clays: Why Feedstock and Process Define Performance

Calcined clays (commonly referred to as metakaolin when derived from kaolinite-rich feedstocks) are rapidly emerging as a key pathway for reducing the embodied carbon of cement and concrete systems.

Lower activation temperatures, the absence of limestone decarbonation, and strong performance as supplementary cementitious materials (SCMs) have positioned calcined clays as a promising alternative to conventional clinker.

However, translating this potential into consistent, commercial products is not straightforward...

Figure: SEM by Hendrik Gildenhuys at Reformix Materials Group of kaolin, different from metakaolin, different materials with different properties… but tuned just right for a project we were involved in.

Not All Clays Are Equal

A common misconception in the industry is that calcined clay is a uniform material.

In practice, the performance of calcined clays is highly dependent on the characteristics of the original feedstock.

Even materials sourced from the same deposit can exhibit significant variation in:

• Particle size distribution

• Molar Si/Al ratios

• Mineralogical composition and crystallinity

• Surface area and reactivity, qXRD, NMR etc.

• Different activation temperatures, enthalpic/‘energy’ requirements and ultimately;

• Different embodied activation costs.

  
  

These differences directly influence both the technical performance of the resulting binder and the economic feasibility of processing.

In applied projects, it is not uncommon for materials of similar origin to require entirely different processing pathways to achieve consistent cementitious performance.

Figure: Sample metakaolin-based binders from an assortment of clays.

From Kaolin to Metakaolin: A Process, Not a Product

The transformation of kaolin to metakaolin is an enthalpic driven activation process that fundamentally alters the material’s structure, transitioning from crystalline kaolinite to an amorphous, reactive aluminosilicate.

While activation temperatures for calcined clays are significantly lower than those required for Portland cement clinker (~1300 °C), they are not uniform across all feedstocks.

Variability in activation temperature (e.g. ~650-1000 °C) and energy demand directly impacts:

• Operational cost

• Energy consumption

• Final product reactivity

In some cases, materials may function effectively as reactive binders, while others may contribute primarily as fillers unless further processed or optimised.

The Importance of Characterisation and Process Control

Successful deployment of calcined clay systems therefore depends on more than identifying a suitable raw material.

It requires a process-driven approach, including:

• Detailed feedstock characterisation (SEM, XRD, particle sizing, chemical analysis)

• Controlled and optimised thermal activation

• Tailored mix design for specific applications

• Quality assurance and control (QA/QC) to ensure consistent performance

  
  

Without these steps, variability in raw materials can translate directly into variability in final products - limiting scalability and commercial reliability.

Scaling Calcined Clays for Industry

As highlighted through recent industry discussions and events such as Concrete 2025, calcined clays and high-SCM systems are gaining significant global traction.

However, their successful adoption at scale will depend less on material availability and more on the ability to standardise processing pathways.

Calcined clay is not simply a material substitution, it is a process engineering challenge.

Industrial adoption will therefore rely on:

• Defining consistent processing parameters

• Managing feedstock variability

• Establishing reproducible performance outcomes

  Figure: Our metakaolin, available upon request.

Work With Reformix

Calcined clays represent a compelling pathway toward lower-carbon cement systems.

However, real-world implementation requires a shift in thinking:

From selecting a material…to designing and controlling a process.

At Reformix, this approach underpins our work in developing commercially viable cementitious materials from both natural feedstocks and industrial by-products.

Reformix works with industry partners to evaluate feedstocks, develop cementitious materials, and design processes for the commercialisation of alternative binders and SCM systems.

If you are exploring calcined clays, metakaolin, or other supplementary cementitious materials, or looking to assess the feasibility of your material streams, feel free to get in touch.

Figure: More SEM by Hendrik Gildenhuys at Reformix Materials Group of kaolin.