Simultaneous thermal analysis of different bentonite-sodium carbonate systems: an attempt to distinguish alkali-activated bentonites from raw materials

Alkali activation with sodium carbonates is a traditional method to improve bentonite properties for a variety of applications. In some applications, natural sodium-rich bentonites are preferred, and custom regulations require proper declaration of Na-rich bentonites, with respect to activation. Consequently, there is need for a method that can unambiguously distinguish between natural and activated Na-rich bentonites. The paper deals with the preparation of several alkali-activated sets, specifically (a) anhydrous Na2CO3 with trace amounts of thermonatrite (Na2CO3 center dot H2O) and trona (Na-3(CO3)(HCO3)center dot 2H(2)O), hereafter called ASC, (b) mixtures of ASC with CaCO3, and (c) mixtures of ASC with CaCO3 and a Ca2+-rich bentonite at different moisture contents, to distinguish natural and alkali-activated bentonites by simultaneous thermal analysis (STA) linked with a mass spectrometer for the analysis of evolved gases. STA linked with MS revealed alkali activation of bentonites, even in the presence of CaCO3 The moisture content during activation and storage of activated samples, however, has a strong influence on the detection of activated samples by STA-MS. Uncertainties remain with respect to unknown foreign phase contents of technical ASC used for alkali activation in practice and the influence of carbonates like dolomite or siderite, which are often present in natural bentonites.