When the cement in the plaster cures, it creates calcium hydroxide. Carbonates in the water replace the hydroxide to form more durable calcium carbonate. This is what the bicarb startup tries to accomplish. This is sometimes referred to as carbonation.
However, what you're describing sounds like calcium carbonate scaling. This can happen when some of the calcium hydroxide gets in the water and becomes carbonated to calcium carbonate. This is sometimes referred to as plaster dust. An essential part of new plaster startup is controlling plaster dust. This includes daily brushing, filtration and careful control of the chemistry
Note: There is something called the Carbonaro effect. But that's a TV show.
