2015-9-10 · cement plants with wet process kilns is 3-05-006, and the six-digit SCC for plants with dry process kilns is 3-05-007. 2.1 CHARACTERIZATION OF THE INDUSTRY1-4 As of December 1990, there were 112 operating portland cement plants in the United States, with 109 of these plants producing gray cement and the other 3 producing white cement. These 112 ...