Influence of red mud calcined at different temperatures on shrinkage deformation of one-part alkali-activated fly ash-slag mortar

The significant shrinkage of alkali-activated materials is recognized as a major drawback since it can induce micro or macro cracking. In this paper, the solid sodium silicate and sodium hydroxide was used as activator to prepare one-part alkali-activated fly ash/slag (AAFS), and the feasibility of using calcined red mud (RM) to improve the autogenous and drying shrinkage of the AAFS was investigated. Moreover, the effect of calcined RM on the alkali-alkali reaction process and microstructure of the AAFS paste were characterized by the X-ray diffraction (XRD), the scanning electron microscope with energy spectrum analyzer (SEM-EDS), and reaction heat. The result illustrated that the reaction activity of RM can be enhanced due to high-temperature calcination. The addition of calcined RM can improve the compressive strength of AAFS mortar through its acceleration on reaction rate and the pore-refining effect, while the RM calcined at 650 degrees C inhibited the development of mechanical strength. In addition, when 15% RM calcined at 600 degrees C was used, the autogenous and drying shrinkage of the AAFS mortar decreased by 84.5% and 38.9%. The improvement effect of calcined RM on the autogenous and drying shrinkage was attributed to the presence of crystal and reduction of mesopore (1.25-25 nm). Furthermore, the absorptive moisture by the RM can be released to maintain the relative humidity inside the matrix, which further reduced the shrinkage. This study demonstrates a sustainable approach for the cement industry, offering a viable solution for waste management and carbon reduction.