Research on the Hydration Mechanism of Active Roof-Contact Backfill Materials: Effect of Expansive Agent Types and Dosages

Failure to fully backfill the goaf may result in increased exposure of roof strata, significantly raising the risk of roof collapses in mining zones and potentially causing surface subsidence, thereby endangering the safety of mining personnel. To address this issue, expansive agents are utilized to produce active roof-contact backfill (ARCB) materials, which promote localized self-compaction of backfill materials in unroof-contact areas through hydration reactions. In this study, an isothermal calorimeter was employed to measure the ARCB hydration heat release rate curves of three types of expansive agents, CaO-based, MgO-based, and ettringite-based, at dosages ranging from 6% to 12%. Hydration kinetic parameters were calculated based on the Krstulovic-Dabic model. The influence of expansive agent type and dosage on these parameters was analyzed, and the hydration mechanism of ARCB materials was investigated. The results indicate that the hydration process of grouting materials using all three expansive agents follows five distinct stages: rapid reaction, induction, acceleration, deceleration, and decay. However, increasing the dosage of the CaO-based expansive agent will enhance heat release and prolong the duration of the acceleration stage. When the dosage is 12%, the total heat release reaches 327.4 J·g^-1. At the same dose, the sample doped with MgO-based expansive agent was only 254.3 J·g^-1, which was 22.3% lower than that of CaO-based, and the occurrence time of the second heat release peak was earlier. In contrast, the ettringite-based expansive agent shows a decreasing trend in heat release with increasing dosage. Furthermore, the use of CaO-based and MgO-based expansive agents allows the hydration process to bypass the phase boundary reaction (I) stage and directly enter the diffusion (D) stage. Ettringite-based expansive agents still undergo three stages, but exhibit a shortened nucleation and growth (NG) stage and an extended induction stage. Additionally, different expansive agents have varying effects on the crystal growth index (n), reaction rate constant, and degree of hydration.