Considering the total alkalis versus SiO2 diagram (TAS; Ezatian 1998), Shahindej dacite is an acid tuff with respect to its chemical compositions.Despite its high amount of lossonignition (LOI) and higher content of K2O relative to Na2O, which have negative affect on geopolymeric paste strength (Xu and Van Deventer 2000), the extent of dissolution of Si and the molar Si∕Al ratio of the dissolved material in solution for this pozzolan have a significant affect on compressive strength (Table 4). It has a high percentage of zeolitic minerals, and its pozzolanic properties can be changed by heat treatment and elevated-temperature curing. Comparing the XRD results indicates that when this material is calcined at 700 and 800°C, the intensity of the peaks for albite, clinoptilolite, and calcite are changed and these minerals are converted to mordenite and opal, respectively [Fig. 2(a)]. Furthermore, in comparing the alkali solubility results of the untreated and calcined forms at optimum boiling time, solubility increases because of calcining up to 700°C (Figs. 3 and 4).