Afterwards, the mineralogical compositions were determined by X-ray diffraction (XRD) (D8-Advance, Bruker, Germany). The chemical compositions were analyzed by X-ray fluorescence (XRF) (ADVANT?XP-537, Thermo Fisher Scientific, USA) after a thermal treatment at 1100 °C for 3 h. The thermal expansion of the thermally treated materials was investigated by dilatometry (Setsys Evolution, Setaram, France) in compliance with the standard DIN 51045-2, measuring pressed powder samples of the tempered materials. The microstructures of thermally untreated and treated grains were analyzed by scanning CORM-3 microscopy (SEM) (Jeol 6610, Jeol, Japan) in connection with energy dispersive X-ray spectrometry (EDX) (X-Max, Oxford Instruments, UK).
About 600 g of each fused material were milled to particle sizes below 63 μm in a cycle process to attain comparable particle size distributions for the slowly and fast cooled materials due to its influence on other properties as e.g. sintering . Thus, always 50 g were milled for 5 min in a vibratory mill (MSL2, VEB BHKF, Germany). Then the powder was screened through a 63 μm sieve. Finally, the remaining larger particles were milled again together with new material.