Comparative Evaluation of Chemical and Physical Expansive Agents in Cemented Paste Backfill: Expansion Behavior, Mechanical Response, and Microstructural Mechanisms

ABSTRACT This study presents an assessment of four representative expansive additives—calcium sulphoaluminate–calcium oxide, magnesium oxide, sodium lauryl sulfate, and sodium abietate—and their effects on the expansion behavior, mechanical development, and microstructural characteristics of cemented paste backfill (CPB). A series of backfill mixtures containing different expansive agent types and dosages were prepared to examine their volumetric response during mixing and early hydration, uniaxial compressive strength (UCS) evolution at various curing ages, and corresponding microstructural features. The results reveal that the two physical foaming agents generated substantial instantaneous expansion during mixing, with sodium lauryl sulfate and sodium abietate reaching total relative expansion ratios of 24.38% and 10.73% at 3 days, respectively. In contrast, magnesium oxide and expansive cement produced delayed but stable expansion associated with hydration reactions. Strength testing showed strong type‐dependent behavior: mixtures containing magnesium oxide or sodium abietate consistently achieved higher UCS values across all curing ages, whereas those with sodium lauryl sulfate or expansive cement exhibited significant strength reduction. Microstructural analysis further demonstrated that sodium abietate promoted a dense and continuous hydration network without the formation of interconnected pores, whereas expansive cement produced a dispersed and more porous matrix. Integrating the expansion, strength, and microstructural results shows that sodium abietate offers the most favorable balance between controlled expansion and mechanical performance, making it a promising additive for improving roof‐contact efficiency and long‐term structural stability in underground backfilling. The findings provide scientific support for the rational selection and optimized application of expansive agents in CPB.