Stability of calcined clays during storage

Usage of calcined clay as a cement main constituent is spotlighted as an important part of the decarbonisation of the cement industry. In particular, little knowledge is available on the storage stability of calcined clays. Therefore, the influences of fineness, temperature and humidity for different timescales of storing were determined. Two samples of a calcined kaolinitic‐illitic clay of different fineness (crushed and ground) were stored sealed in a desiccator, as well as at 20 °C and 60 % rel. humidity, and at 40 °C and 100 % rel. humidity. The clay's mineralogical composition, moisture and chemically bound water content were analysed at 7, 28, and 91 days of storage respectively. Additionally, their reactivity was determined acc. ASTM C 1897‐20 and acc. EN197‐1.

The calcined clays which were stored in a desiccator showed only slight alteration over the period of 91 days. Samples stored at 20 °C and 60 % rel. humidity, contained small amounts of moisture and chemically bound water, regardless of their fineness. Storage at 40 °C and 100 % rel. humidity led to a huge increase in moisture and chemically bound water, especially at high fineness. The reactive silicon of the fine samples was diminished independent of storage conditions over time, whereas the coarse samples showed no changes. Samples stored at 40/100 showed a strong decrease in their heat of hydration over 91 days.

This study implicates, that the analysed calcined clay could be stored as coarsegrained fraction at normal conditions.