Massive quantities of flue gas desulfurization scrubber sludges, both suffate- and sulfite-rich, are generated by coal burning power plants. Environmentally friendly but economical disposal of this sludge is of great importance to the coal industry. To achieve the goal of effective utilization of the scrubber sludges, we have begun to explore developing value-added materials from these sludges. In this paper, we report how water-to-scrubber sludge (w/s) ratio and the fabrication temperature affected the crystal growth behavior of the sludge under pressure. This was accomplished by conducting scanning electron microscopy, X-ray diffraction, Fourier transform infrared, differential scanning calorimetry, and thermomechanical analyzer measurements on the samples formed at 24.2 MPa. Our results suggest that the hydration of hemihydrate, generated from the sludge, resulted in the formation of two phases of calcium sulfate in our materials formed at four different temperatures using two different w/s ratio settings. At T < 130 degreesC, parallelogram-shaped (columnar) gypsum (CaSO4.2H(2)O) and rectangular-shaped (acicular) beta-hemihydrate (CaSO4.0.5H(2)O) crystals dominated our materials for w/s = 0.2. However, for w/s = 0.6, needle-shaped but orientated crystallites formed in the samples. At T > 130 degreesC, the samples mainly exhibited hemihydrate phase in the form of fibrous growths formed by the splitting of gypsum crystallites. The samples fabricated at 90degreesC exhibited a higher degree of interlocking of the crystallites, which imparted better mechanical property to our materials as depicted by the fracture strength. (C) 2002 Elsevier Science Ltd. All rights reserved.