Investigation on hydration behavior and microscopic pore structure of early-age cement-based grouting material

The present paper reports an experimental investigation into the hydration behavior and microscopic pore structure evolution of early-age cement-based grouting material (CGM) with the same compressive strength as concrete (C60) at various ages (18 h, 1 d, 3 d, 7 d, and 28 d). The hydration process, hydration products, microstructure, and pore structure changes of early-age CGM were analyzed using isothermal calorimetry, thermogravimetry (TG), X-ray diffraction (XRD), backscattered electron imaging (BSEI), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). The results show that the hydration process of early-age CGM can be classified into five stages: pre-induction, induction, acceleration, deceleration, and stabilization. Compared to C60, early-age CGM displays a higher initial heat flow but lower subsequent heat flow, with a cumulative heat release (72 h) only 38 % of that of C60, effectively avoiding temperature shrinkage and ettringite decomposition. During the hydration process, CGM continuously generates portlandite along with substantial quantities of ettringite and calcite. The addition of calcium sulphoaluminate expansive admixture (CSA) is advantageous in facilitating CGM's hydration process, effectively promoting the formation of ettringite and C-S-H gel, thus leading to an enhancement in the microstructure of CGM. At 28 d, the microstructure of CGM displays a relatively compact morphology. The porosity of CGM at 28 d was 13.74 %, with a higher proportion of harmless and less-harmful pores. These research findings contribute to a better understanding of the hydration process and microscopic pore structure evolution of earlyage CGM, providing a reliable theoretical basis for the rational application of CGM in engineering and guidance for the improvement and optimization of CGM.