Comparison of the effect of silica fume and fly ash on the mechanical properties of masonry recycled concretes

The present study is devoted to the study of the effect of replacing silica-fume (SF) and fly ash (FA) pozzolans on the mechanical properties of recycled concrete (RC) made from recycled masonry coarse aggregates (RMCA).  In manufactured concretes, natural aggregates were replaced by RMCA at different percentages. In order to improve the mechanical properties of these concretes, different percentages of SF and FA were substituted for part of the cement. Slump and specific gravity tests were performed on fresh concrete and compressive strength, splitting tensile strength, modulus of elasticity, and ultrasonic pulse velocity tests were performed on the hardened concrete. The results indicate that replacement of SF and FA increased the slump and decreased the specific weight of the concretes. Also, the results revealed that replacing 5% and 10% SF improved some mechanical properties of recycled concrete containing 50% of RMCA. Replacement of 15% SF, in concrete containing 25% of RMCA resulted in the close mechanical properties of RC compared with CC without pozzolans. Also, the test results demonstrated that replacing different percentages of Fly Ash, could not improve the mechanical properties of RC.