Lightweight Aggregates from Industrial Sludge–Marine Clay Mixes

Abstract

Waste disposal problems are often the result of rapid industrial and economic developments in land-scarce municipalities. Waste utilization could be a viable choice to alleviate the problems by transforming wastes into useful products while conserving natural resources. Previous studies by the writers indicated that in the process of thermal conversion of industrial sludge and dredged marine clay mixes into concrete aggregates, pyro-expansion and bloating were often exhibited by the aggregates. The expansion and bloating mechanisms, however, have not been looked into in greater detail. This research examines the petrology of sintered industrial sludge–marine clay aggregates in order to better understand the mechanisms leading to pyro-expansion and bloating of the aggregates. The study would enable development of an efficient and cost-effective bloating methodology for lightweight aggregate production. Main factors investigated were sludge–clay mix proportions, firing temperatures, and salt additives. The investigation revealed that firing at temperatures of 1,200 and 1,300°C produced aggregates of lower specific gravity in the range of 0.5–1.6. The highest aggregate porosity can be achieved at 1,200°C with a clay content of 50%. Scanning electron microscope images indicated that the intrinsic lightweight characteristic of the aggregates could be attributed to large pores within a dense surrounding matrix. The effect of lightweight structures may be ascribed to the release of volatile substances at each respective peak endothermic transitions during firing. The release of substances in vapor form required a large amount of energy, which resulted in the peak endothermic transitions that correspond to the formation of the porous lightweight structures. The results suggested that it is possible to promote pores formation in the production of aggregates possessing lightweight characteristics through careful control of temperature and the selection of sludge–clay proportion.