Multi-scale mechanical property correlation of sustainable ultra-high strength mortar: from rheology to interfacial transition zone

Sustainable ultra-high strength mortar promotes efficient waste utilization while offering excellent durability and minimal maintenance requirements due to its superior mechanical performance. The effect of w/b and silica fume dosage on the workability, microstructure, and pore structure was systematically examined using multi-scale research methods, aiming to identify the key factors contributing to the strength development of sustainable ultra-high strength mortar. With SF dosage not exceeding 30 % or w/b not below 0.17, the mechanical properties of the specimens were elevated with an increase in SF dosage or a decrease in the w/b. The maximum compressive strength reached 134.7 MPa. On a macroscopic scale, this improvement can stem from an increase in the plastic viscosity of the fresh mortar with a reduction in the water film thickness on the surface of solid particles and an enhancement in matrix packing density. The optimal plastic viscosity range is 15–20 Pa s. Microscale and mesoscale investigations reveal that the filling effect, nucleation effect, and pozzolanic reaction of SF, along with the reduction of free water, effectively refined the pores and ITZ of the specimens. Although a lower w/b lowered the degree of hydration in these specimens, the mechanical properties of sustainable ultra-high strength mortar were primarily influenced by porosity and gel pore volume. Therefore, this paper systematically explains the multi-scale mechanism by which coordinated regulation of rheological optimization, pore structure control, and ITZ strengthening collectively enhance the mechanical properties of sustainable ultra-high strength mortar. Furthermore, it offers a multi-scale regulatory approach for the precise design of sustainable ultra-high strength mortar.