Design of Ultraearly Strength of Shotcrete via Microstructure Optimization: Synergistic Effects of Chelating Acids and Nano C–S–H Seeds

The ultraearly strength of Portland cement-based shotcrete used in tunnels is crucial for engineering safety and construction efficiency. To address the critical demand for ultrarapid structural stability in shotcrete applications, this study proposes a dual-strategy approach to achieve ultraearly strength by leveraging the synergistic effects of chelating acids and nano C-S-H seeds. The hydration process, pore structure characterization, and mechanical properties of accelerated cementitious materials were systematically investigated. The results demonstrate that the optimized cement mortar achieved a compressive strength exceeding 10 MPa at 6 h, 10-fold higher than the strength attained with accelerators alone. During early hydration, nano C-S-H seeds did not significantly promote the relative hydration degree. However, the agents played a crucial role in refining the pore size distribution and reducing the total porosity of the hardened cement specimens. This improvement is attributed to the formation of a densified microstructure through interlocking AFt-C-(A)-S-H networks, thereby enhancing the overall mechanical properties of the shotcrete. Compared with the conventional chelating acid (fluorosilicic acid), the AFt crystals formed by the synergy of the sustained-release effect of l-glutamic acid and accelerated nucleation by nano C-S-H seeds exhibited finer morphology and higher quantity. This synergistic enhancement effect further facilitates the microstructure construction of the hardened cement paste and provides a design basis for optimizing ultraearly strength shotcrete preparation.