quicklime

Researchers found that quicklime is the ingredient in Roman concrete that makes it last.

“‘[A]s soon as tiny cracks start to form within the concrete, they [lime clasts] can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material.’ Here we have a convincing explanation of the reactions that, in ancient Roman concrete, ‘automatically heal the cracks before they spread.’”

The process is called self-healing.
https://www.openculture.com/2023/01/the-mystery-finally-solved-why-has-roman-concrete-been-so-durable.html

Medieval tacticians used quicklime at sea to blind opponents before boarding their ships.
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In the Roman Church, we could use this product for healing cracks or blinding opponents or both.