Abstract
In this work, the formation mechanism, the thermal stability (280–1000 °C) and the surface area properties of calcium silicate hydrates substituted with Cr3+ ions under hydrothermal conditions were investigated. In this experiment, CaO, amorphous SiO2·nH2O raw materials and Cr(NO3)3·9H2O (5 or 15 g Cr3+/dm3 (accordingly, Cr:Ca:Si:H2O ratios were equal to 0.002:1.5:1:1.098 and 0.006:1.5:1:1.094)) solutions were used. Hydrothermal synthesis was performed in unstirred suspensions, in an autoclave, under saturated steam pressure (∼ 10 bar) at a temperature of 200 °C for 1 h. The thermal stability of the synthesized samples was investigated at 280, 500 and 1000 °C with a temperature increase rate of 8.33 °C/h. It was determined that Cr3+ ions changed the formation mechanism of high basicity calcium silicate hydrates and yielded different synthesized compounds: a lower amount of Cr3+ ions accelerated the reaction of the raw material and promoted the crystallization of α-C2SH and xonotlite, while a higher concentration of Cr3+ ions effected only the formation of semicrystalline type compounds (C-S-H (I) and C-S-H (II)). Calcium silicate hydrates substituted with Cr3+ ions remained stable up to 250 °C temperature because, at a higher temperature, calcium chromate started to form. Also, when the calcination temperature was equal to 1000 °C, the Cr3+ ions additive accelerated the crystallinity of calcium silicates (wollastonite and belite). The results of the SBET measurements showed that the Cr3+ ions additive positively influenced the values of the specific surface area (total of 76.17 m2/g). However, these values decreased after the samples were calcinated at a temperature of 500 °C (the value dropped to 48.02 m2/g).
Published Version
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