Impact of the mineralogy of low–purity clays on their suitability as supplementary cementitious materials

The lack of rapid screening and selection criteria limits the use of low–purity and common clays as supplementary cementitious materials (SCMs). Here the potential suitability as SCMs of 73 clays from 27 different geological formations, was investigated. Neither chemical composition nor kaolinite content were appropriate criteria to infer the calcined clays pozzolanic reactivity, which is the recommendation in existing guidelines. Clays exhibiting a total clay mineral content of ≥40 wt%, and a mica content below 60 wt% in the clay mineral fraction present moderate to high pozzolanic reactivity when calcined at 800 °C. Low–purity clays derived from kaolinitic Carboniferous formations, such as the Etruria and Pennine Coal Measures, consistently exhibited higher pozzolanic reactivity, compared with younger or marine–derived clays, particularly those from Jurassic, Cretaceous, and Quaternary aged-units. This new knowledge provides a novel guideline for the exploration and sourcing of promising clays for SCMs production globally.