The investigation of the combined effect of nano-silica, steel, ‎and polypropylene ‎microfibers on the mechanical ‎characteristics, permeability, and chloride attack ‎resistance ‎of cement composite

The objective of this study was to investigate the combined impact of nano-silica, steel microfibers, and ‎‎polypropylene microfibers on the mechanical properties, permeability, and resistance to chloride attack ‎of ‎cement composite. To achieve this goal, a 2% weight ratio of nano-silica was used as a cement substitute, ‎‎while ‎1.0% steel and 0.2% polypropylene microfibers, respectively, by volume of the binders‎ were ‎separately and simultaneously‎ ‎employed as additives in the cement composite. Experimental analyses, ‎including compressive, flexural, and ‎tensile strength tests, were conducted to evaluate the mechanical ‎properties. Additionally, the ultrasonic ‎pulse velocity (UPV) and sorptivity tests were employed to assess ‎permeability, and the durability ‎against chloride attack was examined using the Rapid Chloride Migration ‎Test (RCMT). The results ‎demonstrate that the simultaneous incorporation of nano-silica, steel microfibers, ‎and polypropylene ‎microfibers in the cement composite mixture resulted in a significant enhancement in ‎compressive strength, ‎flexural strength, flexural toughness, and tensile strength by 59.3%, 32.3%, 67.2%, ‎and 25.9%, ‎respectively, compared to the control sample after a curing period of 90 days. Moreover, ‎significant ‎decreases were observed in terms of the initial and secondary water absorption rates. Furthermore, ‎the ‎penetration depth of chloride ions was notably reduced from 33.6 mm (in the control composite) to 14.2 ‎‎mm (in the composite containing the combined effects of nano-silica, steel microfibers, and polypropylene ‎‎microfibers) after 90 days. The enhancement of mechanical properties, permeability, and durability ‎against ‎chloride attack in cement composite can be attributed to the synergistic mechanisms promoted by ‎the ‎utilization of nano-silica, steel microfibers, and polypropylene microfibers. The filling effect, ‎nucleation ‎sites, and pozzolanic activity of silica nanoparticles significantly contribute to the reduction of ‎porosity and ‎refinement of the cementitious matrix's microstructure. Simultaneously, the inclusion of ‎steel microfibers ‎and polypropylene microfibers reinforces the cement matrix and effectively controls ‎existing microcracks, ‎thereby impeding the propagation of macrocracks and brittle failure in the cement ‎composite. Furthermore, ‎the bridging effect of steel and polypropylene fibers aids in the control of cracks ‎caused by plastic shrinkage ‎during the early stages and secondary or thermal cracks, thereby further ‎improving the properties of cement ‎composite.‎