Microscopic mechanisms underlying strength degradation in Alkali-activated slag-based plugging fluids under high-temperature curing

In this study, the microscopic mechanisms underlying strength degradation during the hydration process of metallurgical waste slag-based plugging fluids cured at high temperatures were investigated. Static gel strength analysis, X-ray diffraction, nuclear magnetic resonance spectroscopy, and scanning electron microscopy were employed to illustrate the effects of phase transformation, microstructure, and morphology evolution during hydration on mechanical performance. The results show that the optimized formulation system using 3% NaOH as the alkali activator exhibited rapid growth in strength during the early curing stage, reaching a peak of 22.70 MPa at 5 h, followed by a continuous strength declining trend as the hydration process progressed. Newly formed hydration products, calcium silicate (aluminate) hydrate, promote early-stage strength development owing to their relatively high crosslinking density and Ca 2+ bridging. However, phase transformations and the conversion of four-coordinated aluminum to six-coordinated aluminum led to a reduction in the crosslinking density between microstructural units and an increase in the pore (crack) size, thereby weakening the macroscopic load-bearing capacity of the hardened body. Furthermore, in the later stages of hydration, the hardened body lacked newly generated hydration products to fill internal pores (cracks). Consequently, the crack network structure became interconnected and deteriorated, which further accelerated strength degradation.