Gas sorption surface area measurements have been important for developing microstructural models for the structure of calciumsilicate-hydrate (C-S-H), the major hydration product of portland cement. However, surface area measurements using the nitrogen BET method have historically had high variability and have not always been reproducible. In this study, the variability of the nitrogen BET surface area of hardened cement paste (HCP) was investigated by varying the D-drying time and by storing the dried cement for various times before the BET measurement. The surface area of the samples was found to decrease with increasing storage time, and the surface area values became independent of the initial D-drying time after about 2 weeks. The storage time accounted for most of the observed variation in the data. ADVANCED CEMENT BASED MATERIALS 1996, 3, 72--75 KEY WORDS: BET, Cement, Surface area ~ ardened portland cement paste (HCP) has a large specific surface area, which is primarily associated with the calcium-silicate-hydrate (C-S-H) E gel phase. The specific surface area of HCP is usually measured using the gas sorption technique developed by Brunauer, Emmett, and Teller, commonly known as the BET method [1]. A good general discussion of this technique can be found in the book by Gregg and Sing [2].
When measured with water vapor at room temperature using the gravimetric BET technique, HCP has a specific surface area of about 200 m 2 per gram of dried