Effect of nano-calcium carbonate on mechanical properties of interfacial transition zone between fiber surface and concrete

This study investigated the effect of nano-CaCO3 content on the hardness (H) and Young’s modulus (E) at the interfacial transition zone (ITZ) surrounding fibers and ultra-high performance concrete (UHPC). Nano-CaCO3 was incorporated at varying contents from 1% to 4% by weight of cement. The nanoindentation (NI) test and scanning electron microscopy (SEM) to examine the mechanical properties and microstructures at the ITZ. The results of the study indicated that UHPC incorporating nano-CaCO3 exhibited an improvement in both H and E at the ITZ compared to UHPC without nano-CaCO3. Specifically, the H and E values for UHPC containing 3% nano-CaCO3 were measured at 3.49 ± 0.15 and 51.47 ± 1.23 GPa, respectively, whereas those values of UHPC without any nano-CaCO3 were 3.10 ± 0.12 and 49.44 ± 1.22 GPa, respectively. This enhancement in H and E at the ITZ is attributed to the toughening effects at the interface caused by the nano-CaCO3, along with improved hydration. Besides, SEM images revealed that UHPC containing nano-CaCO3 displayed a denser and more homogeneous microstructure compared to its counterpart without nano-CaCO3. Furthermore, the addition of nano-CaCO3 increased the compressive strength of UHPC by 1.47% to 9.49% as the content rose from 1% to 4%, attributed to improved particle packing and a more compact microstructure. In contrast, UHPC flowability declined as nano-CaCO3 content increased, as indicated by a reduction in slump flow from 185 ± 15 mm to 160 ± 5 mm, which is associated with increased water absorption by the extensive surface area of the material.