From Thermochemistry to Process Control: Standardising Geopolymer Activator Synthesis for reliable manufacturing*

Geopolymer technology continues to advance as a low-carbon alternative to Ordinary Portland Cement. Yet one major limitation remains:

There is no widely accepted standard method for preparing stable activator solutions.

In a recent case study turned peer-reviewed study published by Skane et. al.* in Ceramics International, we addressed this challenge by integrating:

• Quantitative 29Si NMR silica speciation

• Thermodynamic modelling

• Solubility surface mapping

• Process sequencing analysis

The goal was simple:

Move activator preparation from informal laboratory practice toward controlled, reproducible process engineering.

Challenging the “24-Hour Equilibration” Assumption

Many published geopolymer studies reference overnight or ≥24-hour activator equilibration periods.

Our findings show this is unnecessary.

Silica speciation equilibrates in seconds to minutes, while alkali thermodynamic stabilisation occurs in approximately 1-1.5 minutes.

Extended waiting periods introduce risk:

As activators cool, they can transition from metastable to unstable regions, leading to irreversible precipitation and gel formation.

In other words: Longer waiting does not increase stability, it can reduce it.

Defining a “Time-Stability Window”

Using temperature-dependent solubility modelling of the H₂O-NaOH-SiO₂ system, we constructed three-dimensional stability maps that define:

• Minimum stabilisation temperature

• Maximum cooling threshold before instability

• A quantifiable “time-stability window”

  
  

These hypersurfaces allow prediction of when a solution will cross from stable to metastable or unstable domains, providing:

• Process control guidance

• QA/QC boundaries

• Reduced batch variability

• Safer handling of concentrated alkali systems

The Overlooked Variable: Mixing Sequence

A key practical finding was that feedstock sequencing directly determines stability and shouldn't be mixed randomly as is sometimes done (by being unstandardised).

The optimal order:

1. Water

2. Alkali hydroxide

3. Soluble alkali silicate

Reversing this sequence can push identical compositions into unstable regions, forming gelatinous precipitates even under identical ambient conditions

This highlights an important principle:

Composition alone does not define performance, process history does.

Toward Standard Operating Procedures

By combining silica speciation data with thermochemical and solubility modelling, this research establishes a framework for:

• Rapid activator preparation

• Defined stability thresholds

• Reproducible geopolymer synthesis

• Industrial process scaling

Geopolymer adoption depends not only on chemistry, but on engineering control.

Standardisation begins at the solution phase.

"Modelled cooling time data from Skane et al. [16] of [an] activator solution with labelled thermodynamic stability milestones (with respect to differential temperature stability) and “metastable” and “unstable” solubility stability regions."*

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*Citation: R. Skane, F. Jones, A. van Riessen, E. Jamieson, X. Sun and W. D. A. Rickard, “Optimisation of activator solutions for geopolymer synthesis: Thermochemical stability, sequencing and standardisation,” Ceramics International, vol. 51, no. 28, pp. 59419-59429, 2025.