Clay from the Nagrotta area of Jammu (J & K State, India) was calcined at various temperatures. The products were analysed by X-ray diffraction. The mode of loss of hydroxyl group from the structure with respect to temperature is discussed. The results were supplemented by differential thermal analysis. The calcined products were subjected to lime reactivity tests and the data obtained were correlated with those of the above study to investigate the pozzolanic activity at various temperatures.