An investigation was carried out on pastes of Portland cement partially substituted by a geothermal silica waste (GSW), which was obtained as a byproduct from a power plant that uses geothermal underground resources. The waste is predominantly nanometric amorphous silica with sodium and potassium chlorides. Pastes containing 0, 10, and 20% GSW and 0, 10, and 20% chlorides by weight of GSW were cured at 20 and 60 A degrees C in order to evaluate the phases formed in the presence of chlorides by means of scanning electron microscopy. The calcium hydroxide consumption was followed using X-ray diffraction. Pore solution tests were performed utilizing a leaching method, which is an alternative technique that could provide an easier and more efficient analysis method compared to the conventional pore expression methodology requiring high pressures and special equipment. The results indicated that the blended cements showed an intense consumption of calcium hydroxide by the pozzolanic reaction and a more compact matrix of hydration products. Furthermore, the high concentration of chlorides provided by the GSW affected the pore solution, reducing the pH, and increasing the concentration of sodium and potassium ions. These were related to the formation of Friedel's salt, which could be correlated with the reduction in chloride content in the aqueous phase.