Resource recycling through artificial lightweight aggregates from sewage sludge and derived ash using boric acid flux to lower co-melting temperature

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Abstract
This study focuses on artificial lightweight aggregates (ALWAs) formed from sewage sludge and ash at lowered co-melting temperatures using boric acid as the fluxing agent. The weight percentages of boric acid in the conditioned mixtures of sludge and ash were 13% and 22%, respectively. The ALWA derived from sewage sludge was synthesized under the following conditions: preheating at 400 °C 0.5 hr and a sintering temperature of 850 °C 1 hr. The analytical results of water adsorption, bulk density, apparent porosity, and compressive strength were 3.88%, 1.05 g/cm3, 3.93%, and 29.7 MPa, respectively. Scanning electron microscope (SEM) images of the ALWA show that the trends in water adsorption and apparent porosity were opposite to those of bulk density. This was due to the inner pores being sealed off by lower-melting-point material at the aggregates' surface. In the case of ash-derived aggregates, water adsorption, bulk density, apparent porosity, and compressive strength were 0.82%, 0.91 g/cm3, 0.82%, and 28.0 MPa, respectively. Both the sludge- and ash-derived aggregates meet the legal standards for ignition loss and soundness in Taiwan for construction or heat insulation materials.

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