Alkali fusion of bentonite to synthesize one-part geopolymeric cements cured at elevated temperature by comparison with two-part ones

Abstract

One-part geopolymer precursors are synthesized and preliminarily optimized by calcining bentonite with NaOH or Na2CO3 to compare with the two-part ones. The ground powders of the former are hydrated by only adding water at 80°C for 3days and 20°C for subsequent time. Some one-part geopolymers gain satisfying compressive strengths and are quite water-resistant similar to the two-part ones. It is found that alkali alters and deepens the thermal decomposition of the bentonite and transforms quartz impurity into X-ray amorphous cementing-active glassy phases. Nepheline is usually generated in the alkali-fused bentonite, and gehlenite also occurs when calcined at 1000°C. Different kinds of zeolites appear in the one-part geopolymers, while the calcium ferrites, only found in the precursors exclusively activated by Na2CO3, lead to produce additional hydrous gehlenite, so Na2CO3 seems facilitates hydration more effective than NaOH given the pastes from the precursors calcined with Na2CO3 possess higher compressive strengths. Moreover, the ferric oxide and calcium oxide have potential to enhance the cementing properties according to the mechanisms with calcination and hydration. The low hydrolysis rate might lead to poor water-resistance and the softening coefficient of the pastes increases with alkali content and calcination temperature for one-part geopolymers.