In this paper, the authors studied the physical properties of sewage sludge melt-solidified slag to determine their effects on the properties of concrete for use in PCa concrete. Tests were conducted to determine the physical properties of slag aggregate, properties of fresh concrete, strength of concrete, drying shrinkage of concrete, and leaching of trace elements. We tested seven air-cooled slag aggregates obtained from the same sewage works at different times. Most of the slag was of a low quality, but the resultant slag concrete was not badly affected by the slag aggregate according to the results of the tests for bleeding, concrete strength, drying shrinkage, and leaching of trace elements. In the test for leaching of trace elements, it was particularly interesting to note that there were no notable releases of trace elements from hardened concrete containing slag. In the estimation of the relation between the quality of slag and the strength of concrete, the aggregate crushing value and the abrasion loss of slag were the best indexes for estimating the compressive strength of concrete. However, our findings showed that the relation between the indexes and the split tensile strength and the flexural strength of concrete did not exhibit the same tendency as in the case of the compressive strength of concrete.