Investigating the performance of silica fume as a supplementary cementitious material in the production of pozzolanic cement with the aim of achieving the maximum compressive strength

In this work, microsilica or silica fume (SF) waste material was used as supplementary cementitious materials (SCM) in cement production. The filling and pozzolanic properties of microsilica significantly affect the strength and durability of concrete. By replacing 10% of microsilica in cement, the compressive strength increased to 569 MPa at the age of 28 days. Chemical analysis of cement and microsilica samples used is effective in achieving this high resistance. In addition to speeding up the pozzolanic reaction, the very small size of microsilica particles increases the percentage of water and the setting time of the cement paste compared to the control mortar. So that the initial setting time increased more than twice compared to the control mortar. Therefore, after optimizing the percentage of microsilica replacement in cement, calcium chloride additive was used in mortar production as a setting accelerator and reducing the percentage of water absorbed by the cement paste. Since calcium chloride also affects the hydration reactions of cement, the central composite design method (CCD) with Design Expert software was used to find the best percentage of calcium chloride and microsilica replacement in cement with the least number of tests. The optimal mixture was calculated by replacing approximately 6% microsilica and 1% calcium chloride in cement with a 28-day compressive strength equal to 603 MPa. Also, the initial setting time for the optimal mixture was equal to 135 min, which is very similar to the control mortar.