Measurement of Free and Combined Water of Gypsum

Abstract

The existing standard ASTM C471 Standard Test Methods for Chemical Analysis of Gypsum and Gypsum Products describes manual gravimetric analytical methods for evaluating the free and combined water of gypsum materials, including natural gypsum, synthetic gypsum, rehydrated plasters, and gypsum board. The existing method describes a sample condition, sample size, time of exposure, and temperature of exposure for each measurement. It is common current industry practice to use automatic moisture balance machines to perform this measurement to achieve fast results with minimal labor. Although the automated measurement is obviously more efficient, no standard currently exists for the conditions and settings of such systems. Also, different moisture balance equipment have varying features and specifications. Therefore, there exists a possibility for significant variability among what are typically considered equivalent measurements from machine to machine and versus accepted manual methods. In 2011, the ASTM C11 Committee undertook the task of updating the existing standard to include automated moisture balance methods. In a preliminary experiment described here, a few variations in automated methods were compared to the existing manual methods to evaluate differences in average measurement results and standard deviation of these results. As a result of this preliminary work, an updated standard will be submitted in 2015, and a following interlaboratory study is planned for 2016 to specifically define the precision and bias of both methods.