Complex sentence: Part of the reason for the slowness of the calcium--water reaction is a result of the metal being partly protected by insoluble white calcium hydroxide; water solutions of acids, where this salt is soluble, calcium reacts vigorously.
Split into simpler sentences: Part of the reason for the slowness of the calcium--water reaction is a result of the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids, where this salt is soluble, calcium reacts vigorously.