The free water content of surplus soils with high fine content is an effective parameter for evaluating the strength of clay stabilized with additives like cement or lime. Therefore, this study attempted to determine the water absorption rate Wab (the amount of water absorbed and retained by 1 g of stabilizer) of waste-based stabilizers such as paper sludge-based stabilizer (PSAS), coal fly ash (FA), biomass ash (BMA), and hemihydrate gypsum (HHG) by a suction filtration method. It was found that the Wab PSAS, FA, and BMA increased with curing time, while the Wab of HHG did not change significantly. Subsequently, clayey soils with high water contents were treated with each stabilizer by adding various proportions and curing for certain periods. Cone index tests were performed to evaluate the cone index qc to confirm the potential of the stabilized soft soils as a road subgrade, railway embankment, and harbor construction material. Based on the test results, the effects of the Wab on the qc were discussed, introducing a modified water content w*. The w* was calculated from the measured water content, Wab, and the addition ratio of stabilizer, A. As a result, the relationship between qc and w* evolved and showed a more reasonable trend than the relationship between qc and w. This relationship can therefore be utilized to estimate the potential of the stabilized soft soils as road subgrade, railway embankments, and harbor construction materials.