Effects of sulfate on the mechanical performances and hydration characteristics of red mud based non-burnt brick

In this work, red mud based non-burnt brick (RMNB) was prepared by using red mud (RM), fly ash (FA) and desulfurized gypsum (DG). The effects of sulfate on the mechanical performances, hydration characteristics, internal structure and micrographs of RMNB were studied. The hydration characteristics were tested by using XRD and other microanalysis methods. The environmental performance was tested by ICP-MS. The results show that the RMNB has the best mechanical performances when the dosage of DG is 6 wt%. The hydration products of the matrix consist of AFt and small number of C-S-H gel, and the amount of AFt in the matrix gradually increases with the increase of sulfate dosage. The mechanical performances of the matrix are mainly associated with the amount and morphology of AFt. The AFt can effectively fill the internal structure of the matrix and improve the mechanical performances when the sulfate dosage is optimum. However, the excessive SO42- continues to react with [Al(OH)(6)](3-) and Ca2+, which makes the AFt will continue to grow. The coarser and wider AFt enlarges the internal pore size, and declines the mechanical performances when the sulfate dosage is excessive. The leaching results indicate that the leaching concentrations of Na+ and heavy metals meet the Chinese national standard. This research provides a significant way for the utilization of RM and other solid wastes. (C) 2020 Elsevier Ltd. All rights reserved.