New composite material for storage and transformation of heat of a temperature range of 250-350 degrees C has been synthesized by precipitation of magnesium hydroxide Mg(OH)(2) in the pores of expanded vermiculite. The hydroxide is stabilized inside the pores as irregular sticks of 1-3 mu m in size across flats of the base and about 30 gm in length. The temperature of decomposition of the confined hydroxide is found to be by some 50 degrees C lower than that of the bulk one as revealed by both TG and DSC techniques. One possible reason of this finding is a significant acceleration of the decomposition reaction of the confined hydroxide as revealed from comparative kinetic tests in the temperature range of 260-300 degrees C. The maximal heat storage capacity is 540 kJ per kg of the composite with the hydroxide content of 67.4 mass. % that is larger than the heat of melting for promising phase change materials. Further study of the new composite material for storage of middle temperature heat could be of high interest to find out reasons of the decomposition temperature reduction in order to use this effect for harmonizing the reaction temperature with the temperature level of heat to be stored. (C) 2012 Elsevier Ltd. All rights reserved.