Research PaperClayey wastes-based lightweight aggregates: Heating transformations and physical/mechanical properties

- The neoformation processes in the heated blends of clayey wastes were ruled by calcium.
- Intensive melt derived from the clay breakdowns, fluorapatite and mullite.
- The physical/mechanical properties were essentially controlled by melt flow.
- The heated pellets are suitable products for structural/insulating LWAs concrete.

Heating transformations of pellets of selected binary blends of raw clay, cement kiln dust and phosphate sludge were investigated by using different techniques (XRD, DSC, FT-IR and SEM), and their main technical properties were followed as a function of temperature. It was found that calc-aluminosilicate phase (gehlenite/labradorite) neoformed at T < 900 °C from the breakdowns of kaolinite and chlorite, and lime of decomposed carbonates. Moreover, fluorapatite (a component of phosphate waste-containing blends) gave rise to calcium phosphate (Ca3(PO4)2) and melt, which played a chief role in the sintering process of the pellets. The melt abundance induced the dissolution of mullite, which was the major constituent of the cement kiln dust. The results also showed that the compressive strength of the pellets increased and water absorption together with density (except for the phosphate waste-clay blend) decreased with increasing temperature. These evolutions were discussed in relation to the microstructure changes. The suitability of the blends in LWAs manufacturing was evaluated basing on the magnitudes of the measured properties.