Comparison of methods for distinguishing sodium carbonate activated from natural sodium bentonites

A lot of natural Ca/Mg-bentonites are turned into Na-bentonites. By adding sodium carbonate to a Ca/Mg-bentonite in the presence of some water Na+ enters the interlayer and Ca/Mg-carbonates precipitate outside. Natural Na-bentonites and activated Na-bentonites are rather similar. Therefore, in the present study methods are tested to distinguish both. This is relevant not only for customs but also for research and development. For activation different amounts of sodium carbonate are added. The dosage ranges from a few % of the CEC to slightly above the CEC corresponding to 1–5 mass% Na2CO3. Also the water content may vary from the dried state at which the actual activation (cation exchange) does not take place up to the presence of excess water leading to a complete reaction. Altogether four cases had to be considered separately (Na2CO3 above CEC + excess water or dry and Na2CO3 much below the CEC + excess water or dry). If water was absent (cation exchange was not complete) the sodium carbonate phases could be detected by XRD, IR, or with STA–MS measurements. This result was expected but surprisingly, STA–MS–CO2 measurements were found to be applicable even in the most difficult case (sodium carbonate addition below CEC and excess of water = reaction complete). In the case of some samples activated with 2 mass% sodium carbonate only, a weak STA–MS–CO2-peak was observed at about 100 °C. Unprocessed materials are free of any carbonates which decompose around 100 °C. Therefore, this 100 °C peak indicates alkaline activation. This method was applied to five real products with unknown activation and two of which were found to be activated. The pH of the activated materials was only slightly higher than that of a natural Na-bentonite. The measured difference of 0.3 pH units is not considered to be sufficient to unambiguously conclude alkaline activation.