Heated blends of clay and phosphate sludge: Microstructure and physical properties

•The sintering process of clay-phosphate sludge blends was monitored by impedance spectroscopy.•Fluorapatite was not involved in the neoformation processes of the crystalline phase (gehlenite).•The relations between the experimental factors and some ceramic properties were determined.•The weights of the experimental factors followed the order: temperature > clay content > soaking time.

Blends of a naturally occurring clay (0–30 wt.%) and phosphate sludge were heated at different temperatures and times and their microstructures were investigated using impedance spectroscopy, dilatometry, X-ray diffraction and scanning electron microscope. The weights of the effects of the change of temperature, soaking time and clay addition on some physical ceramic properties (shrinkage, water absorption and compressive strength) were assessed. For the latter purpose, the response surface methodology was used. The results showed that the sintering process was effective between 750 and 1000 °C and occurred by melt flow. It was accompanied with low activation energy for ionic conduction (0.20–0.35 eV). Due to the quantitative formation of gehlenite (the unique neoformed phase), the ionic conduction regressed and the melt formation was limited. Also, it was shown that the effects of the experimental factors on physical properties of the blends were well described with the adopted polynomial models, and the weights of the effects of the factors followed the order: temperature > clay content > soaking time. The effects of the interactions between the factors on the properties studied were evaluated and discussed in relation to the microstructure change.