Several key construction projects — including the World Trade Center’s flagship tower — could be stuck in cement following a walkout yesterday by city concrete workers.

Angered over stalled talks and proposed pay cuts, hundreds of union workers walked off the job at 6:55 a.m.

Unaffected by the labor strife is work at the 9/11 memorial and museum, slated to be open for next month’s 10th-anniversary commemoration.

But other WTC projects could be stalled by the stoppage, as well as the Barclays Center in Brooklyn, the Madison Square Garden renovation and the Mercedes Tower at West 54th Street.