Characterisation of calcined waste clays from kaolinite extraction in alkali-activated GGBFS blends

Laura Stefanini (Corresponding Author), Deeba Ansari, Brant Walkley, John L. Provis

Research output: Contribution to journalArticleScientificpeer-review


Metakaolin is a well-studied supplementary cementitious material (SCM) and precursor in alkali-activation. The utilisation of limited kaolinite content clays generated from large-scale industrial activity, denoted as waste clays, in alkali-activation is still largely unexplored. This paper investigates the use of five samples of calcined waste clays from different locations in a kaolinite extraction site as precursors in alkali-activated cements (AACs). Calcined waste clay precursors are blended with ground granulated blast furnace slag (GGBFS) and activated with sodium silicate. The physical and chemical properties of the calcined waste clays are characterised. Both Chapelle and R3 tests for pozzolanic reactivity are examined based on calcined waste clay properties. The R3 test data obtained for sieved particle fractions < 63 µm show marked improvement in the case of C4, elucidating potential calcined waste clay behaviour with prior processing in alkali activated systems. Clay particle size distribution and morphology are identified as key factors to be considered for initial calcination and in mix designs, strongly affecting both the fresh properties and workability of blended pastes. Good workability is highlighted as being crucial for achieving well-behaving calcined waste clay blended binders with resulting dense microstructures and high strength values, comparable with other reported GGBFS systems. Quartz, muscovite, and alkali-feldspar present in the calcined waste clays remain stable throughout alkali-activation. Reaction of the amorphous phase fraction present in the calcined waste clays results in the formation of additional cross-linked gel. A replacement level of up to 20 wt% calcined waste clay is found to exhibit similar compressive strengths to pure GGBFS binders, whilst achieving lower porosity within the bulk. This study provides a methodology for characterising the behaviour of calcined waste clays in alkali-activated systems and a proof of concept in the formulation of novel binders that incorporate waste clays.

Original languageEnglish
Article number107777
JournalMaterials Today Communications
Publication statusPublished - Mar 2024
MoE publication typeA1 Journal article-refereed


This research study was carried out in the framework of the “By-products for sustainable concrete in the urban environment” (URBCON) project, funded by the Interreg North-West Europe Programme ( NWE725 ) under the EU Cohesion Policy and financed by the European Regional Development Fund (ERDF). Furthermore, the authors wish to express their gratitude towards Ecocem for providing the GGBFS, to PQ Silicates for supplying the sodium silicate, and to Imerys for providing the clays used within this study, for the calcination and for performing the modified Chapelle test.


  • Alkali-activation
  • Calcined clays
  • Geopolymers
  • Ground, granulated blast furnace slag (GGBFS)
  • Solid state nuclear magnetic resonance (NMR) spectroscopy
  • Valorisation
  • Waste clays


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