MONTREAL -- Quebec construction sites fell idle Wednesday after a union alliance representing about 175,000 workers launched an unlimited general strike, crippling activity on major projects such as the Champlain Bridge and a Montreal superhospital.

The walkout began after construction companies and labour federations failed to sign collective agreements ahead of a strike deadline of midnight Tuesday.

Work schedules and overtime are believed to be at the heart of the conflict in the industrial sector, while salaries are the main stumbling block in the residential sector.

Michel Trepanier, a spokesman for the union alliance, said walking off the job was the only possible option.

"Employers are asking us to sacrifice time with our families to be available for work," Trepanier said. "There are limits and they've been reached.

"A strike was our only option. We negotiated right up to the last second to try to get an agreement. In fact, we accepted several times to work with a conciliator in order to reach a deal without a labour conflict."

Trepanier said the employers' objective is clear: they want to drag out negotiations to benefit from the fact construction workers are not entitled to retroactive pay. Their collective agreements expired April 30.

On Tuesday, Labour Minister Dominique Vien said the provincial government was looking at bringing in back-to-work legislation in the event of a strike, saying a walkout could mean losses of $45 million a day for the Quebec economy.

Vien told a news conference in Quebec City on Wednesday she was summoning all sides to a meeting in Montreal for later in the day.

"I can only express my deep disappointment and concern," she said. "This will have a major economic impact.

"We are not at the point yet of tabling special legislation but I will be ready to do so if the premier asks me to."

In Israel on an economic mission, Premier Philippe Couillard made it clear the walkout will not last long.

"We will not let the strike drag on," he said Wednesday. "Not because we want to favour the employers or the unions but out of concern for employment and the Quebec economy."

The consortium building the new $4.3-billion Champlain Bridge said activity will resume when more than 600 employees on the bridge and nearby highway approaches return to work.

"It is too early to say what any potential impact might be (but) we are closely monitoring the situation," said spokeswoman Veronique Richard-Charrier.

Consortium partner SNC-Lavalin (TSX:SNC) said it doesn't have any other significant projects in the province involving unionized construction workers.

Ivanhoe Cambridge, the real-estate subsidiary of the Caisse de depot, said work has stopped on its Montreal projects such as a new Manulife tower and overhaul of the Queen Elizabeth Hotel, along with work in Quebec City and Sherbrooke.

The Teamsters union said its members are refusing to cross picket lines, interrupting the delivery of ready-mix cement.

Cement maker Lafarge said it will temporarily lay off workers if the strike lasts long.

"The longer the strike goes on the harder it is to recuperate the volumes lost," said spokeswoman Karine Cousineau. "We have a very short construction window in Quebec so the longer it lasts the more product lines are going to be affected."

This is the second general strike in the Quebec construction industry in four years.

The federation that represents chambers of commerce in Quebec is calling for a rapid settlement to the conflict.

"The adage that says 'when the construction industry is doing well, everything is doing well' is also inversely true -- a labour conflict has a major and direct impact on all economic sectors," said Stephane Forget, head of the Federation des chambres de commerce du Quebec.

Forget noted the 10-day work stoppage in 2013 resulted in a drop of 1.1 per cent in the province's gross domestic product.