The current C989 specification has 7-day and 28-day slag activity index (SAI) requirements for Grade 100 and grade 120 slag cements. These limits were incorporated into the document over thirty years ago, when there were very few slag cement producers and the limits were based on the properties of slag cements supplied in the marketplace at that time. The original source of granulated blast-furnace slag that was largely the basis for SAI limits no longer exists. Several other companies have since become producers of slag cement, and the amount and number of local and imported granulated blast-furnace slag sources has increased significantly. The SAI requirements set thirty years ago on the very limited number of slag cements do not accurately represent relative 7-day and 28-day slag activity index values for slag cements currently used in the marketplace, nor do they reliably predict the potential strength of a concrete manufactured with these slag cements. While the 28-day SAI limits provide a means of classifying slag cements, the 7-day values typically require a manufacturer to grind slag cement to a higher fineness than required to meet 28-day SAI limits. However, the higher SAI laboratory test values resulting from increased fineness are often more than offset in field concrete mixtures by effects of higher water demand. Therefore, the 7-day SAI limits are often counterproductive. This issue is also relevant because SAI values are only intended as an index for broadly classifying slag cements and to provide the user with assurance that the slag cement has the potential to contribute to strength. And even though Appendix X1 of ASTM C989 reviews factors impacting the contribution of slag cement to concrete strength, many users inappropriately assume that the SAI values are directly predictive of the rate of strength development for their specific job concretes. This proposal would retain the 28-day SAI limits for Grade 80, 100 and 120 slag cements to permit the producer to classify slags and permit the user to evaluate relative strength potential. Such evaluation is enhanced by use of the common reference portland cement now included in C989 for SAI testing. Limits for the 7-day SAI would be removed, although producers would still be required to test and report the 7-day SAI values for Grade 100 and 120 slag cements for informational purposes. As is the case in the current standard, 7-day SAI testing is not required for Grade 80. Proposed additions are underlined and proposed deletions are shown in strikethrough font. Only changes so indicated are being balloted. Other text is provided for information only.
Keywordsblast furnace slag; granulated blast furnace slag; slag activity index; slag cement;