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.