The Collaborative International Dictionary of English v.0.48:

Portland cement \Port"land ce*ment"\
   A cement having the color of the Portland stone of England,
   made by calcining an artificial mixture of carbonate of lime
   and clay, or sometimes certain natural limestones or chalky
   clays. It contains a large proportion of clay, and hardens
   under water.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Portland cement
    n 1: a cement that hardens under water; made by heating
         limestone and clay in a kiln and pulverizing the result
         [syn: hydraulic cement, Portland cement]