It was found that the used siliceous earths containing adsorbed and occluded organic substances are partially decomposed in the hydrothermal process like fresh earths to form alkaline silicates. The introduction of oxidizing agent to the hydrothermal treatment of used oiled siliceous earths resulted in partial oxidation of hydrocarbons to high molecular weight acids. As a result, a mixture composed of sodium silicates, sodium salts of organic acids (soaps) and non-reactive hydrocarbons was formed. It was shown that this product obtained from oxidative hydrothermal decomposition of used siliceous earths can be used as an additive to cement mortars to reduce their setting time and water absorbability.