The main objective of this work is to use low-value SiO2 and CaO-rich wastes like rice husk ash and chicken eggshell powders, respectively for producing semi-crystalline calcium silicate with molar ratio CaO/SiO2 equal to 0.4. The prepared valuable semi-crystalline calcium silicate was used as an additive for a replacement level of metakaolin (0, 1, 3, 5, 7 and 10% by mass). The resulting powders were used to synthesize geopolymer cements using hardener obtained by mixing commercial sodium silicate, sodium hydroxide and distilled water. The final products were characterized by compressive strengths, apparent density, X-ray diffractometry, infrared spectroscopy, scanning electron and optical microscope observations. The results show that the compressive strengths of geopolymer cements were between 33.25 and 51.38 MPa. They increase when metakaolin was replaced by 5, 7 and 10% of semi-crystalline calcium silicate, but no significant change was observed on the strengths of geopolymer cements content 0, 1, 3 and 5% of semi-crystalline calcium silicate. The micrograph images of the selected geopolymer cements were homogeneous and compact and the one from 10% of semi- crystalline calcium silicate shows a coarse microstructure. It was found that the low-value silica and calcium-rich wastes such as rice husk ash and eggshell can be used to prepare semi-crystalline calcium silicate. The obtained product could be used as a low-cost additive to enhance the microstructural and mechanical properties of poly (sialate-siloxo) network.