The high absorption is the primary factor contributing to the performance degradation of foam concrete, and 3D hydrophobic modification represents an effective approach for addressing this issue. This study presents the utilization of polydimethylsiloxane (PDMS) for conferring three-dimensional hydrophobicity to foam concrete and incorporates supplementary cementitious materials such as fly ash and slag powder to further curtail the cost of foam concrete. The results showed that when the dosage of PDMS, slag powder, and fly ash in foam concrete were 1.5%, 10%, and 35% respectively, the surface contact angle reached 132.8°, the water absorption rate decreased by 65.6% and the 28d compressive strength reaches 1.38MPa. The reason is that PDMS forms a low surface energy film on the surface of hydration products, giving the foam concrete as a whole hydrophobicity. The slag powder can enhance the thixotropy of the slurry, thereby improving the stability of the foam and improving the mechanical properties. In addition, the introduction of fly ash and slag powder reduces the cost of hydrophobic foam concrete by 18.2% compared with that of commercially available foam concrete.