A novel composite for the removal of color in waste water was prepared by subjecting slurries consisting diatomaceous earth and slaked lime to a hydrothermal reaction at 180 degrees C. Subsequently, calcium silicate hydrate gels covered the surface of diatomaceous earth due to the reaction between the amorphous silica of diatomaceous earth and slaked lime. The formation of calcium silicate hydrate gels led to an increase in the specific surface area. The composites showed higher methylene blue adsorption capacity compared with diatomaceous earth. The improved adsorption capacity of the composites depended on the amount of the calcium silicate hydrate gels and their silicate anion chain-lengths. (C) 2010 Elsevier B.V. All rights reserved.