The effect of the polycarboxylate ether and the complex hydrophobic-plasticizing additive on the structure, phase and mineral composition of cement based on polymineral clay with content of relict minerals more than 85 % (52.49 % of quartz) was studied. As research methods chosen X-ray diffraction pattern, differential scanning calorimetry and scanning electron microscopy. The regularities of the effect of the complex hydrophobic-plasticizing additive based on polycarboxylate ether and octyltriethoxysilane on the formation of cement hydration products in a strengthened clayey soil are established. Changes in structure and composition include the following: the formation of a strong skeleton of the crystallization phase due to the increase in the number of hydrated new formation in the form of low-basic hydrated calcium silicate and ettringite, decrease in portlandite content, as well as an increase in the amount of amorphous phase (up to 16 %) in the form of tobermorite gel filling intercrystalline spaces. In soil-cement hydrated silicate calcium are formed in an amount of more than 4 % and ettringite in an amount of more than 7 %, the amount of portlandite reduced by 46 %. The result of modification of the complex hydrophobic-plasticizing admixture of soil-cement, a denser and homogeneous structure with a pronounced "overgrowth" of the pores formed by gel-like hydrated calcium silicate is formed, which are deposited in a large amount on the minerals of the strengthened soil.