Powders of calcium carbonate with strontium carbonate were solidified by mechanical compression under hydrothermal conditions. The effects of hydrothermal conditions such as temperature, pressure, and ratio of Ca/Sr on the reaction process and microstructure of the solidified bodies were examined. Solidification of every powder easily proceeded with shrinkage above 200 °C to 300 °C. The compacts of strontium carbonate were more easily solidified, and compressive strength was 190 MPa and its density was 93.5%. The solidification with shrinkage of compacts consisting of mixed powders of calcium and strontium carbonates was accelerated with an increase in strontium carbonate content. It was revealed that the small amount of strontium carbonate powder acts as a bonding material among large calcium carbonate particles.