We examined the addition of quartz to powder compacts consisting of kaolinite and slaked lime to determine the influence of quartz addition on the deposition and porous structure of solidified materials treated by a hydrothermal process. After hydrothermal treatment, hydrogarnet formed in the solidified materials, and hydrogarnet formation was independent of the addition of quartz to the powder compacts. The amount of hydrogarnet in the hydrothermally solidified materials with quartz as a starting material was higher than that in the materials without quartz. The porosity of the hydrothermally solidified materials depended on the amount of hydrogarnet. A different degree of hydration was observed for hydrogarnet in the hydrothermally solidified samples derived from powder compacts with and without quartz. The degree of hydration played an important role in hydrogarnet shape.