An index rebuild is a fully logged operation.

Your clustering key is 62 bytes wide and you have 1.2B rows. That's just the data that gets distributed across the non-leaf levels.

The leaf level of the CI contains 286 bytes per row, including 2x the max length because of the Unicode and the 2 extra bytes per column to store the length of the variable-length string. Multiple that by 1.2B rows. It's not a trivial table.

To see how much space your table is consuming, you can use sp_spaceused:

execute sp_spaceused 'dbo.DemoTable';

Is it really any wonder why rebuilding the CI consumes log space?