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Stability of calcium sulfate base course in a wet environment.
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    Blended Calcium Sulfate (BCS) is fluorogypsum (FG), an industrial by-product, blended with lime or limestone. Approximately 90,000 metric tons (100,000 tons) of FG are generated annually in the United States, posing a serious problem for environmental disposal. The Louisiana Department of Transportation and Development (LA DOTD) has been using BCS in pavement construction over the last 15 years. While this material has performed satisfactorily after construction, its moisture sensitivity has concerned LA DOTD engineers because it has presented construction difficulty in wet environments.

    Therefore, there is a need to better understand the strength deterioration of BCS in a wet environment, and find ways to eliminate or reduce such deterioration by stabilizing BCS with various suitable cementitious agents. This study was divided into two major parts: laboratory and field tests. Laboratory tests were conducted to identify factors that significantly affect the strength development of raw BCS and to seek a suitable stabilization scheme for ameliorating water susceptibility of raw BCS. The effectiveness of each stabilization scheme was evaluated from the perspective of water resistance, strength, and volumetric expansion incurred by stabilization. Laboratory tests also investigated the resilient modulus and permanent deformation characteristics of stabilized BCS. Samples tested in the laboratory included ones both molded in the laboratory and cored at the test section of the Pavement Research Facility (PRF) test site at the Louisiana Transportation Research Center (LTRC).

    The field test program included two parts: (1) building a full-scale test section at the PRF site according to proposed construction specifications; and (2) evaluating the performance of stabilized BCS base courses through in-situ tests, such as DCP, FWD, and DYNAFLECT to characterize their strength and structural properties.

    Ground granulated blast furnace slag (GGBFS) with a grade of 120 was used to stabilize BCS to improve its water resistance in this study. Portland cement, lime, and fly ash were also used as additives to GGBFS with different proportions to improve the properties of GGBFS-stabilized BCS.

    The results from this study indicate that moisture content controlled the strength of raw BCS, although other factors such as dry unit weight also influenced the result. Curing conditions affect the strength of raw BCS through the change of moisture content in the material. The loss and regaining of strength is generally a reversible process and the presence of free water among gypsum crystal particles is the reason for this phenomenon. BCS stabilized by 10 percent 120-grade GGBFS by volume can serve as a good pavement base. It achieved a fairly higher stiffness and a structural layer coefficient of 0.30 can be used for pavement design purpose. The tentative construction specifications used in the study proved to be adequate for the field construction. Therefore, it can be used with minor modification for future projects. Researchers recommend that the LA DOTD consider building several field test sections in different traffic and environmental conditions using the GGBFS-stabilized BCS as pavement base course.

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