The effect of heat treatment and cooling rate on the properties of lightweight aggregates

Lightweight aggregates (LWA) were produced from clay in the laboratory. After firing different heat treatments and cooling rates were applied and the resulting material was investigated with respect to strength and microstructure. Fast cooling led to the formation of micro cracks and weakened the material whereas slow cooling enhanced the strength of LWA. The residence time at temperatures between 700 °C and 900 °C led to differences in average oxidation state of iron in the matrix phase leading to substantial changes in thermal behaviour of the matrix phase. The combination of a highly oxidized shell and a reduced core proved to enhance the strength of LWA. A two hour heat treatment at 800 °C in air combined with a subsequent slow cooling rate (0.7 °C/min) applied to LWA produced in an industrial rotary kiln led to a strength increase of 114% compared to material of the normal production without changing any other property.