The aim of the present study is to test Greek amorphous silica-rich rocks used as partial replacements of cement [25 and 50%] in cement mortars. The raw materials studied were diatomite rocks occurred in Zakynthos, Milos and Samos islands, and tuffite located in Milos Island. Cement substitutes participated in screed materials in percentages of 5% and 10% of the total dry mass. The raw materials were characterised chemically, mineralogically and technically. Microstructural analysis of raw materials and screed pastes was performed in order to specify reactions of silica phases in hydrated systems. The relationship between reactive silica of raw materials and compressive and flexural strength of the final products was also investigated. Properties of final products were examined and compared with commercial one. The water demand of pastes was increased by the addition of the siliceous raw materials, whereas their compressive and flexural strength was decreased. Conclusively, the siliceous rocks studied can be used as partial substitutes of cement in mortars only in ratios raw materials/cement lower than 1/2.