Carbonation Pretreatment of Slag for Improving the Rheological Behavior and Sustained Grouting Performance of Alkali-Activated Grouting Materials

Alkali-activated slag (AAS) is a promising low-carbon alternative to ordinary Portland cement owing to its low CO2 emissions and excellent mechanical performance. However, its rapid structural build-up, fast loss of flowability, and short setting time restrict its application in grouting engineering. In this study, slag was subjected to controlled carbonation pretreatment to regulate its physicochemical properties, and the effects of carbonated slag (CS) on the fresh and hardened properties of AAS grouting materials were systematically investigated. The physicochemical characteristics of CS were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), while rheological behavior, setting time, compressive strength, grouting performance, and hydration products were further evaluated using NaOH- and waterglass-activated systems. The results showed increased carbonate formation and discrete Ca-rich deposits on the examined slag surfaces, while no major change in the predominantly amorphous aluminosilicate structure was detected. The time-dependent thermogravimetric and fresh-state results were consistent with slower early reaction-product accumulation and structural build-up in the CS systems. After 6 months of carbonation, the initial setting times of NaOH- and waterglass-activated grouts increased from 16 to 80 min and from 28 to 286 min, respectively, while their corresponding final setting times increased from approximately 34 to 92 min and from 40 to 300 min. Furthermore, carbonation effectively suppressed yield stress development during cyclic shearing, with the yield stress remaining below 1 Pa in the waterglass-activated CS systems. The improved workability significantly enhanced sustained grouting performance, with grouting saturation after 20 min increasing from approximately 33% to 90% in the waterglass system. Meanwhile, carbonation had little influence on the 28-day compressive strength, which remained within 60.5–62.8 MPa and 89.9–96.2 MPa for NaOH- and waterglass-activated systems, respectively. These findings indicate that slag carbonation can improve workability and sustained groutability while maintaining comparable 28-day strength. The results are consistent with carbonation-induced changes in surface chemistry and early reaction behavior.