In a properly constructed arch that is shaped like an inverted catenary, compressive forces push the stones in the arch together, and cement is not needed.
Work Step by Step
This is discussed on page 234.
You can help us out by revising, improving and updating this answer.Update this answer
After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.