Modification mechanism of flue gas desulfurization gypsum on fly ash and ground granulated blast-furnace slag alkali-activated materials: Promoting green cementitious material

Alkali activation and the combination of fly ash (FA) and ground granulated blast-furnace slag (GGBS) has been used for cementitious material, which is seen as an effective method to dispose of industrial solid waste. The GGBS-FA AAMs have the characteristic of drying shrinkage and a large number of tiny pores distributed in the matrix, resulting in strength deterioration. The paper is aimed to optimize the strength property by incorporating another solid waste of flue gas desulfurization gypsum (DG), which could be regarded as a ternary alkali-activated materials (AAMs). The unconfined compression tests are carried out to study the effects of DG content on strength with different curing times, and the effects from the modulus of the alkali activator are also analyzed. Then X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) are performed to reveal the microscopic mechanism of the effects of DG on the mechanical property. Results show that the incorporation of DG can significantly improve the early strength due to the increase of calcium. The SO42- in DG continuously reacts with Ca2+ and active silicate to form ettringite during the curing period, which can fill micro-pores and tiny cracks to form a denser matrix structure. In addition, the presence of SO42- has the effect of secondary enhanced activation, further promoting the polymerization reaction and enhancing the strength, especially for the low alkaline reaction system. These all contribute to the strength improvement. It should be noted that the DG content must be controlled to prevent the volume-filling effect that leads the AAMs matrix to be loose and porous, and the generation of geopolymer gel would also be impeded in a high-calcium reaction system, further influencing the final strength. The results will promote the application of high-performance, cost-saving, and green low-carbon cementitious material based on the synergistic utilization of GGBS, FA and DG.