Georgia-Pacific Corp., a paper products company that also supplies products to the flooring industry, has agree to be acquired by Koch Industries, Inc. for about $21 billion. Koch, based in Wichita, Kan., manufactures pulp, paper and fibers and operates a range of business including refineries and pipelines. When finalized, the deal stands to create the nation's biggest private company. It will also give Georgia-Pacific a greater level of vertical integration and a steady source of raw material. The proposed acquisition comes less than two years after Koch acquired Invista BV, a nylon fibers business that makes Lycra and Stainmaster, from DuPont Co. In addition to building materials, Georgia-Pacific's consumer brands include Dixie paper cups and Quilted Northern bath tissue.