The aim of the present study was to find methodological tools to obtain reasonable results for exchangable Ca2+ of gypsiferous bentonites. Cation exchange capacity (CEC) is an important property of clays. Numerous methods for calculating CEC and exchangeable cations exist; determination of exchangeable Ca2+ fails, however, when gypsiferous clays are examined because gypsum is dissolved throughout the exchange experiment, which in turn increases measureable Ca2+ concentrations. Several new methods (AgTUcalcite, CoHexcalcite, and Cu-trien5×calcite) have been developed to overcome a similar problem occurring with calcite by using exchange solutions saturated with respect to calcite prior to the experiment. In the present study these three solutions were also pre-treated with gypsum and labeled AgTUCcGp, CoHexCcGp, and Cu-trien5×CcGp. The special solutions were applied first to a gypsum- and calcite-free bentonite with known reference values for exchangeable Ca2+. The resulting exchangeable Ca2+ values obtained did not match with reference values. The solutions were then applied to natural calcareous and gypsiferous bentonites but only the proposed AgTUCcGp test method was successful. The performance of AgTUCcGp was relatively poor when applied to calcareous non-gypsiferous bentonites, the third group of test materials. Reasonable values for exchangeable Ca2+ of gypsiferous clays were obtained using a combination of two separate results: (1) calcite saturation of exchange solution (e.g. Cu-trien5×calcite) and (2) quantification of gypsum with suitable mineralogical methods. Result 1 eliminates errors caused by calcite dissolution though it is still incorrect because it contains significant amounts of Ca2+ from gypsum dissolution. After proving that gypsum was completely dissolved during the exchange experiment, result 2 was used to subtract the theoretical Ca2+ portion of gypsum from result 1. The initial concentration of gypsum of the samples studied was <1 wt.%, typical of many commercial bentonites. Using this combined procedure the sum of exchangeable cations is very close to the CEC, though it still exceeds the CEC by, on average, 3%, which is a satisfactory improvement. The resulting exchangeable Ca2+ values can be considered as operationally correct using this approach. Ca2+ saturation (Ca/CEC in %) of seven gypsiferous bentonites ranges from 1 to 69%.

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