Multi-Scale Modification of Sodium Polyacrylate-Modified Cement Grouts: Rheology, Microstructure, and Mechanical Properties

The rehabilitation of underground infrastructure requires cement grouts that combine high injectability into micro-cracks with superior mechanical strength and durability. Conventional grouts, however, are limited by excessive yield stress and the formation of weak crystalline phases. This study investigated sodium polyacrylate (PAAS) as a multi-functional modifier to address these limitations. Through a multi-scale approach combining rheological measurements, spectroscopic analysis (FTIR, LF-NMR), diffraction (XRD), and electron microscopy (SEM), we elucidated the synergistic modification mechanisms of PAAS. The results demonstrated that PAAS operated via two pathways: (i) chemically, its carboxyl groups chelated Ca2+ ions, suppressing Ca(OH)2 crystallization and refining C-S-H gel; (ii) physically, it provided electrostatic and steric dispersion, dismantling flocculated networks to reduce yield stress by 80.3% and enhance fluidity by 30.7%. This drastically improved injectability was complemented by micro-structural optimization, where PAAS eliminated percolation pores (>1 μm) and promoted a homogeneous, dense matrix. Consequently, the mechanical properties were significantly enhanced, with a 0.04% PAAS dosage maximizing compressive strength (15.56 MPa, +26.2%) and a 0.06% dosage elevating flexural strength (5.74 MPa, +29.3%). This work establishes that low-dosage PAAS modification enables a unique combination of high fluidity, strength, and durability by leveraging synergistic chemical and physical mechanisms, providing a tailored, cost-effective solution for advanced grouting applications.