Rock-Tenn Co. and Lafarge Corp. have formed a joint venture to produce gypsum paperboard liner for Lafarge's U.S. drywall manufacturing plants. Lafarge will own 51% of the joint venture. Seven Hills will sell all of its production to Lafarge's U.S. drywall plants with full-scale production of gypsum paperboard liner projected to begin in the second half of 2001.