Effects of corchorus olitorius derived binder on functional properties and thermal shock resistance of a Kaolinite refractory.

Abstract

The effect of corchorus olitorius derived binder on the functional properties (fracture strength- modulus of rupture, thermal conductivity and Young's modulus or elasticity) of a kaolinite-based refractory was investigated. The three-bend test was used to investigate modulus of rupture; while scanning electron microscope (SEM) micrographs were used to analyze the microstructure of the fractured surfaces. The flexure strengths of green samples rose linearly from 18.39 MPa" (plain water plasticized samples) to 24.51 MPa (samples plasticized with a formerly thickened binder), thus giving flexure strength improvement of 32.28% above binder free samples. Fired samples' strength reached a maximum of 106.61 MPa (at binder's concentration of 0.683) from strength of 37.5 MPa (for plain water plasticized samples) corresponding to strength improvement of 184.29% compared to binder free samples. SEM micrographs revealed an increase in pore closure (evident by the increased coalescence of kaolinite plates) with increasing binder concentration. Fracture paths were also observed to change from one dominated by intergranular fractures (in plain water plasticized samples) to mainly intragranular fracture mode in samples plasticized with higher binder concentrations. Ultrasonic Nondestructive Technique (NOT) was used to investigate the elastic properties namely: ultrasonic longitudinal velocity (VL) and Young's modulus of elasticity and the results compared with some empirical models. A strong linear relationship was observed between the samples' porosity and the measured ultrasonic longitudinal velocity (VL). Young's modulus rose gradually from 17.39 GPa (plain water plasticized samples) to reach a plateau at a binder concentration of 0.683 (after attaining a peak value of33.07GPa). Transient hot wire method was used to measure the thermal conductivity in the temperature range of 20 to 600De. Values of thermal conductivities were noted to be strongly dependent on binder concentrations (at all temperatures). Samples plasticized with higher binder concentrations had slightly higher values of thermal conductivity compared to those plasticized with binders of lower concentration and binder free samples. Effective medium IV approximation was observed to agree well with the experimental data whereas geometric mean model gave values, which were slightly higher. Thermal shock test employing water quenching was done for temperature differences ranging between 80 to 580°C; damage parameters (figures of merit) . were also evaluated. The results showed that samples plasticized in the higher binder concentration, which initially had very high strengths; experienced the largest strength loss (over 60% of their initial strengths) at quench temperature difference ("..T, ) exceeding 345°C; the critical temperature difference ("..T, c) of the samples tested was noted to be lying between 270 - 345°C. SEM micrographs of samples quenched at temperature difference of 580°C showed that samples plasticized with the optimal binder concentration (0.683) experienced severest cracking of the matrix compared to their counterparts plasticized with plain water, whose microstructures also showed an inhibited crack propagation.