Abstract
Industrial waste is on the increase, resulting in severe environmental contamination. Against this background, this study sought to investigate the possibility of recycling used phosphate-bonded investments. Commercial phosphate-bonded investment was mixed and heated according to manufacturer's instructions, then powdered with a ball mill machine to be used as recycled investment (Code: R). Mono-ammonium phosphate (NH4H2PO4) of 2, 5, 8, 11, and 14g were added to 100g of R because of residual MgO, and coded as Ra, Rb, Rc, Rd, and Re respectively. Investment properties were then examined. Particle size of R was 50.66μm and close to the original investment. It was found that more NH4H2PO4 led to larger setting expansion, smaller thermal expansion, and lower fired strength. The data of Re in these three aspects were 1.42±0.09%, 0.78±0.21%, and 8.85±0.88MPa respectively, and differences against the original investment were significant. Ra showed significantly longer setting time than other investments. Based on the results of this study, we concluded that it was possible to set used phosphate-bonded investments by adding NH4H2PO4; in particular, Rd showed suitable expansion and acceptable mechanical properties.
