Use of rice husk as a source of silica for the production of self-flowing refractory castables

Abstract

The objective of this study was: i) to obtain rice husk ash (RHA) with high purity silica for the production self-flowing refractory castables with different percentages of RHA as a partial substitute for microsilica; ii) analyze the mechanical, physical, thermal, and microstructural properties of the formulations studied in relation to the effect of firing temperatures. The rice husks were washed with acetic acid in autoclave, dried at 105 °C for 24 h and calcined at 600 °C for 4 h. The ashes obtained were analyzed by particle size distribution, X-ray fluorescence spectrometry (FRX), X-ray diffraction (XRD) and scanning electron microscopy with X-ray microanalysis (SEM-EDS). The presence of amorphous silica of high purity was verified with irregular grains and sizes between 40 and 145 μm. Refractory castables were produced by replacing 0%, 5%, 10% and 15% (mass) of the microsilica by rice husk ash. After curing, the specimens were dried at 110 °C for 24 h and a part was sintered at 800 °C for 5 h, while the other was sintered at 1430 °C for 8 h. The samples were submitted to tests of free creep; apparent density; linear variation; resistance to bending, compression and thermal shock. The formulation with the best mechanical performance, modulus of rupture (55.10 MPa) and compressive strength (119.62 MPa), was 10% replaced by rice husk ash and sintered at 1430 °C. This result was due to the formation of higher strength alumino-silicate phases and stronger bonds between the matrix and the aggregates. In addition, all analyzed materials using rice husk ash showed high density and good resistance to thermal shock, which is desirable for self-flowing refractory castables.