Dozens of workers set up picket lines outside the St. Marys Cement plant in this eastern Ontario town on Sunday over proposed changes to their pension plan, local press reported.
The walkout follows two months of intensive negotiations between the Canadian Auto Workers (CAW) and the Toronto-based company.
Currently, the workers have a defined-benefit plan, meaning the amount of their pensions is fixed.
CAW national representative Keith Osborne said the company wants to change the plan structure to a defined-contribution plan, which could mean lower benefits on retirement.
St. Marys Cement, which has six plants including the one in Bowmanville, Ont., has not commented on the dispute.