The preparation, characterization, preheating mechanism and bloating mechanism of ultra-lightweight ceramics (ULWC) manufactured by dehydrated sewage sludge (DSS) and clay were studied. Three experiments were designed to investigate the addition of DSS, the effect of preheating treatment and sintering treatment. respectively, and then the optimum conditions for preparing ULWC were determined. Chemical components, especially ratios of carbon content to iron oxide content (C/Fe-ratios), were used to explain the preheating mechanism; physical forces (surface tension and bloating force) combined with C/Fe-ratios were used to explain the bloating mechanism. The characterizations (physical properties, microstructure properties and toxic metal leaching properties) of ULWC that were prepared under the optimum conditions were tested. The results showed that the optimum addition of DSS was 20-30 wt.%, and the pellets which preheated at 400 degrees C for 20 min and sintered at 1150 degrees C for 10 min were beneficial to produce ULWC. Property tests of ULWC showed that ULWC was light (with a bulk density of 330.80 kg m(-3)), waterproof (with a water absorption of 5.30 wt.%), nontoxic (contents of toxic metal leaching test were all below the detection limit) and suitable for practical civil engineering. (C) 2009 Elsevier B.V. All rights reserved.