In-situ polymerized cementitious materials (iPMCM) exhibit exceptional fluidity, interfacial adhesion, and toughness, making them promising candidates for rapid repair engineering and 3D printing applications. However, the delayed cement hydration caused by polymer significantly impedes early strength development, limiting their practical applications. Nano-silica (NS) has been proven to considerably enhance the early-age mechanical properties of cementitious materials due to its filler effect, nucleation effect, and pozzolanic effect, offering a solution for improving the strength development of iPMCM at the early age. However, the synergistic effects of NS and in-situ polymerization on the mechanical development remain unclear. This study systematically investigates the interaction of NS and in-situ polymerization on the hydration kinetics, microstructure, and mechanical properties of iPMCM. And the enhancement mechanism of NS on both the cement microstructure and the polymer network was in-depth discussed. The findings from this study provide an effective strategy for improving the early-age mechanical properties of iPMCM, broadening their real-world applications.

Orthogonal experiments L₂₅ (5⁶) were designed to optimize acrylamide (AM) monomer (1%-5% of cement mass), potassium persulfate (KPS) initiator (1%-5% of monomer mass), and NS (0-0.4% of cement mass) dosages based on 6 h compressive strength. P.I 42.5 cement, AM, KPS, and 15 nm NS were used with a fixed water-to-cement ratio of 0.4. NS was pre-dried at 60 ℃ for 24 h before dispersing in water with AM and KPS, and then the resulting solution was mixed with cement to prepare pastes. Samples were cast into 40 mm × 40 mm × 40 mm molds. After demolding curing for 1 d, the samples were cured at 20 ℃ and 95% RH for specific curing ages prior to mechanical property measurement. The hydration kinetic of cement pastes was monitored by a TAM Air calorimeter. The hydration products of hardened cement pastes were characterized by XRD and TGA. SEM and MIP were employed to observe the microstructure and to analyze the pore structure of iPMCM at different ages, respectively

The orthogonal experiment identified 4% AM, 0.3% NS, and 4% KPS as the optimal formulation, achieving a 284% increase in 6 h compressive strength (from 1.38 MPa to 5.3 MPa) and 381% increase in 6 h flexural strength (form 0.36 MPa to 1.73 MPa) compared to the control group. In addition, the optimal group exhibited varying degrees of improvement in compressive and flexural strengths than the control and in-situ polymerization groups after curing for 7 d and 28 d. The evolution of heat flow analyses during the cement hydration indicates that NS reduced the nucleation barrier for C-S-H formation and thus advancing silicate hydration. It resulted in an increase of CH content by 17.8%. Concurrently, NS promoted the earlier AM polymerization releasing a more significant heat, which contributed to accelerating cement hydration, and thereby improving the early-age compressive strength. MIP results suggest that the incorporation of NS into iPMCM reduced harmful pore (>50 nm) volume by 71.1% and increased less-harmful pore (20-50 nm) content, compared to the control group, significantly refining the pore structure of the hardened cement pastes. SEM observations demonstrate a denser organic-inorganic network in NS modified PAM hydrogel where NS is likely to act as a physical crosslinker via hydrogen bonding with polyacrylamide strengthening the polymer network.

This study demonstrates that modification using nano-silica effectively enhances the early strength of iPMCM with the optimal formulation of 4% AM, 0.3% NS, 4% KPS, which achieved a 284% improvement in 6 h compressive strength while maintaining superior 28 d flexural strength. The comprehensive experiment analyses conclude that the roles of NS in the early-age mechanical development of iPMCM includes: (i) Improving cement hydration and microstructure. Owing to its nucleation effect, NS accelerates the silicate hydration compensating for the delaying effect of in-situ polymerization on cement hydration. And the presence of NS refines the pore structure of iPMCM by reducing the amount of harmful pores, thus further improving the early strength of iPMCM. (ii) Strengthening the polymerization network. NS advances the occurrence of polymerization with a significant heat released. In iPMCM, NS may interact with polyacrylamide via hydrogen bonding, as a physical crosslinker strengthening the polymer network. These further contribute to the early-age strength development of iPMCM. The findings from this study provide a scientific foundation for the applications of iPMCM in rapid repair engineering, 3D printing structures, etc. The long-term durability and scalability for industrial applications will be further explored in future studies.