Solidified materials were prepared by subjecting powder compacts consisting of allophane and slaked lime to hydrothermal reaction at various temperatures. Hydrogarnet was formed after the hydrothermal reaction because of the reaction between allophane and slaked lime. The bending strength of the materials solidified by hydrothermal reaction at 80 degrees C was greater than 6 MPa. The improvement in the strength of materials by hydrothermal reaction at 80 degrees C was due to the formation of fine deposits at the surface of allophane aggregates; these deposits filled the spaces between particles of the starting materials. Hydrothermal reaction at temperatures greater than 120 degrees C resulted in the collapse of the allophane structure, leading to a decrease in the strength of the materials. (C) 2009 Elsevier B.V. All rights reserved.