Various clays, and/or calcium carbonate, are a component of many papers, including cardboard. They are there to make the paper stiff, to adjust the absorption of inks, to keep the paper hard when it is wet (wet strength), and as a filler (Clays are less expensive than wood fiber in many cases, depending upon the quality of the paper.) These minerals are abrasive and dull the scissor blades. (copied from the site below)
http://www.newton.dep.anl.gov/askasc...1/gen01215.htm