Likely partially. Slaves definitely were used on construction projects, but large-scale construction projects, like these aqueducts, would have included a majority-free force of unskilled wage laborers.

Funny enough, you probably would be more likely to see skilled slaves than unskilled slaves on such a project - unskilled slaves were used primarily in the worst and most tedious jobs that no one else wanted to do on the regular (mines, monocrop plantations, mills, bakeries); if you had a slave who you were going to employ full-time in construction, which was less horrid and also only intermittent in demand, it was probably someone with an actual skill, like an engineer.

The past is a strange place!