Question: When I look at the mill test report for my Type I portland cement, the chemical composition section listed a value of 21% silicon dioxide. Does this mean there dangerous levels of respirable silica in the cement?
Answer: No. The oxide analysis in a mill report tells what percentages of elements, expressed as oxides are in the cement. Limestone, clay and sand are commonly used in making portland cement. In their raw form, these materials contain varying amounts of silicon dioxide. But when they're blended and heated to high temperatures in the cement kiln, the resulting clinker contains new compounds. What was originally crystalline silica has reacted to become part of the hydraulic calcium silicates. According to Howard Kanare, group manager of chemical services at Construction Technology Laboratory (subsidiary of the Portland Cement Association), Skokie, Ill., most ASTM C 150 Type I portland cements contain less than 0.1 % crystalline silica by weight. "Gypsum is added to the clinker before final grinding. We believe impurities in the gypsum are the source of the very small amount of crystalline silica," says Kanare. Producers should review recently revised Material Safety Data Sheets from their cement suppliers. The OSHA has required manufacturers to clearly list any potential hazard from crystalline silica.