Spark Plasma Sintering of Industrial Microsilica and Commercial Nanosilica: Phase Formation, Density, and Mechanical Response

Microsilica recovered from silicon-production off-gases offers a secondary feedstock for silica-based ceramics. This study evaluates its consolidation by spark plasma sintering (SPS), with purchased commercial nanosilica serving as a reference. Both feedstocks were processed at a monitored die temperature of 1300 °C, 50 MPa, and a 10 min hold. Characterization combined powder microscopy, qualitative X-ray diffraction, hydrostatic weighing, ultrasonic contact impedance (UCI), and axial compression. Mean hydrostatic densities were 2.246 ± 0.007 and 2.254 ± 0.007 g·cm−3 for microsilica and nanosilica, respectively. Mean maximum nominal stresses were 208 ± 13 and 188 ± 6 MPa for Ø14 × 2 mm disks; values are mean ± sample standard deviation (n = 3 per feedstock). The microsilica specimens exhibited a 10.3% higher mean maximum nominal stress under this loading configuration and higher comparative UCI indications. Both cooled ceramics showed reflections consistent with α-cristobalite. The study extends earlier waste-processing work through a comparison with commercial nanosilica that relates powder characteristics and phase identity to replicate property measurements under a common SPS schedule. The results demonstrate the material feasibility of consolidating microsilica waste into coherent ceramic bodies and support further development of this utilization route.