The company announced the lockout early Sunday morning, saying plans were in place to ensure its aluminium operations continue to run safely and efficiently. Management hoped that would limit the impact on its customers.
Rio Tinto says it will shut down about one-third of its production at the smelter.
"Rio Tinto Alcan will begin an orderly shutdown of approximately one-third of its production at its 438,000 tonne Alma smelter in Saguenay-Lac-Saint-Jean, Quebec after initiating a lockout at the facility earlier today (Sunday)," the company said in a statement.
"Rio Tinto Alcan staff will execute the orderly shutdown of 144 out of 432 reduction cells and will continue to operate the remaining capacity until further notice.
"Plans are in place to ensure that our aluminium operations throughout the region continue to run safely and efficiently, and we will work to limit the labour disruption's impact (on) Rio Tinto Alcan's customers."
On Friday, about 800 workers from three negotiating units rejected what the company said would be its final offer.
Union officials had a strike mandate before the talks collapsed and had warned that a work stoppage could be imminent.
The Alma facility hosts one of Rio Tinto Alcan's (NYSE:RIO) most important North American aluminum smelters.