Use of Interground Limestone
The limestone addition strategy consists of replacing cement with interground limestone. Since interground limestone is added at the end of the cement production line, the cement-related greenhouse gas (GHG) emissions will be reduced proportionally to the amount of limestone added. The GHG savings arise from avoiding GHG emissions associated with cement production during fuel combustion and calcination in the kiln.
The maximum GHG savings generated by this strategy is 5 percent, which is the maximum limestone allowance per American Society of Testing Material (ASTM) C 150, a major nationwide cement specification. Since the effect of limestone addition to cement had not been studied in detail, Caltrans sponsored a comprehensive study, Use of Raw Limestone in Portland Cement (January, 2008) (PDF, 907KB), designed to evaluate the three primary indicators of concrete performance: strength, drying shrinkage, and permeability. It was found that limestone improved strength and permeability (at early ages).
Since 2007 Caltrans has been accepting 2.5 percent limestone addition. After concluding the limestone study, Caltrans will accept the full 5 percent but implement a performance-based specification to control shrinkage. Although 5 percent of limestone is allowed per ASTM C 150, it is estimated that the statewide limestone addition may not exceed 3.5 percent based on manufacturing limitations. According to reports from the Portland Cement Association (PCA), the estimated average nationwide is only 2.5 percent.
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