3.1. Particle Characteristics and Chemical Composition of Fillers

The particle characteristics of limestone filler and PPWC filler were analyzed by scanning electron microscopy (SEM, Zeiss Gemini 300, Carl Zeiss AG, Jena, Germany) and laser particle size analyzer (Mastersizer 3000, Malvern Panalytical, Herrenberg, Germany). The chemical composition and phase distribution of the two fillers were analyzed by X-ray fluorescence (XRF) test and X-ray diffraction (XRD) test. According to the study of Gautam et al. [50], a layer of gold was plated on the surface of the sample before the SEM test to improve its conductivity. During the XRD test, the sample must be crushed into powder with a particle size of less than 90 μm.