Optimising calcination temperature for high reactivity metakaolin: Influence on amorphous content, mineralogy and microstructure

Abstract

The controlled calcination of raw kaolinite clay transforms its natural crystalline phases into reactive anhydrous aluminosilicate. This process facilitates a chemical reaction with calcium hydroxide present in hydrated cement, resulting in the formation of additional calcium silicate hydrate and calcium aluminate silicate hydrate, which improve both the mechanical properties and durability of concrete. This study aims to tackle the challenges associated with optimising the calcination temperature by quantifying the amorphous content, mineralogical transformations, and microstructural evolution of metakaolin at various temperatures. Two relatively similar kaolinite clays, obtained from different sources, i.e., CCC and ADU, were calcined at temperatures ranging from 700°C to 900°C. The CCC clay achieved an amorphous content of 92 % at 750°C, while the ADU clay reached 94 % at 800°C, indicating differences in thermal reactivity. Calcination temperatures exceeding 800°C led to the formation of inert phases such as cristobalite, anatase, moganite, mica, and rutile, which diminished reactivity. Specifically, from 750°C to 900°C, CCC exhibited an inert content that ranged from 6 % to 10 %, while ADU presented 6–9 % of non-reactive crystals from 800°C to 900°C. Scanning Electron Microscopy (SEM) analysis confirmed that 800°C is the optimal calcination temperature for producing highly reactive metakaolin characterised by a well-developed, porous, and fragmented microstructure. Concrete mixtures containing 15 % metakaolin calcined at 800°C demonstrated a compressive strength increase of over 65 % at 7 days and 59 % at 28 days compared to the control mix, thereby confirming its high reactivity. Nonetheless, the increased brittleness observed in the metakaolin-based concrete highlights the necessity for fibre reinforcement to enhance ductility.

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