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• Expandable foam grout (EFG) is a material to repair underground cavities. • EFG specimens with five admixture-cement ratios (AD/Cs) are used for strength test. • Strength and stiffness of the specimens are evaluated at each curing time. • Regression models for EFG properties are established from curing time and AD/C. Underground cavities induced by damaged slurry walls or soil erosion in urban areas require suitable repair materials and methods. The objective of this study is to evaluate the strength and stiffness of expandable foam grout (EFG) as a repair material. EFG consists of water, ordinary Portland cement, and admixtures. EFG was prepared at a fixed water-cement ratio of 100% and different admixturecement ratios (AD/Cs), ranging from 0 to 8%. The unit weights of EFGs with different AD/Cs at the slurry and expanded states are measured, and the air contents for both states and the theoretical expansion ratio (ER) are subsequently estimated. Stressstrain curves are obtained using unconfined compressive strength tests to evaluate the unconfined compressive strength and modulus of elasticity. The experimental results show that the air content in the slurry state gradually increases with the AD/C, while the air content in the expanded state and the ER rapidly increased and then converges. The strength and modulus of elasticity increased with curing time, whereas they decreased with increasing AD/C. Based on logarithmic and exponential regression models, the relationships between the strength/modulus, curing time, and AD/C are established with high values of the coefficients of determination. Thus, the relationships proposed in this study can be effectively used to predict the strength and modulus of EFG with respect to curing time and AD/C. [ABSTRACT FROM AUTHOR]