The article focusses on the effects of waste Si-based material on the mechanical properties of the resulting cement pastes. In this study we use Portland cement CEM I 42.5R and Si-based waste materials from the packaging industry from Recifa, a.s. The difference between individual samples was in the concentration of Si-based materials. We investigate the influence of five different amounts of SiO2-based materials, namely, 10 wt. %, 20 wt. %, 30 wt. %, 40 wt. % and 50 wt. %. The determined mechanical properties were compressive and flexural strengths. Mechanical properties were investigated in 7 and 28 day samples with dimensions equal to 40 × 40 × 160 mm. The results showed a negative effect of Si-based materials on mechanical properties.