A revised proposal to permit interground limestone in portland cement has some concrete producers wondering how the changes will affect their operations. Originally submitted in December 1996, the ASTM C01.10 subcommittee ballot to revise ASTM C 150, Standard Specification for Portland Cement, was withdrawn and revised to more clearly define limestone and require cement manufacturers to report the quantity of limestone in the cement. The Portland Cement Association's proposal to allow limestone additions cites two benefits: reduced energy consumption and reduced emissions, solid wastes (cement kiln dust) and raw material usage. These benefits, say supporters of the revision, would be achieved without a harmful effect on cement performance. Although the National Ready Mixed Concrete Association hasn't taken a position on limestone additions, some individual producers have. Producers' major concern can be summarized in one statement: They will obtain no benefit from the revised cement specification, but will bear the brunt of customer complaints if the cements don't perform as claimed. The biggest worry is that strengths of cements containing limestone additions won't be equivalent to those of currently available C 150 cements. Although a state-of-the-art review of limestone use in portland cement concludes that strength equivalence is possible, critics say the range of strength values in the review's data show that sometimes strength does suffer. Cement manufacturers acknowledge that manufacturing cement with limestone additions must be done properly to produce strength equivalence but say that the steps needed to optimize the process are achievable. The article also includes proposed ASTM C 150 revisions. [keywords: limestone, ASTM C 150]