Inhibiting pan-alkali in desulfurization gypsum-based materials with trace magnesium oxide: Synergistic interaction of polymerization degree, impair porosity and free water

Abstract

The degree of alkalinity significantly affects the practical application of geopolymers by diminishing its compressive strength and durability, to improve the weathering resistance and durability of desulfurization gypsum (DG) based cementitious materials and broaden the practical application scenarios of the materials, it’s firstly proposed the concept, that magnesium oxide as an admixture to inhibit pan-alkali in desulfurization gypsum-based cementitious materials. The macroscopic mechanical property changes of the materials were evaluated via compressive strength test, and the correlation mechanisms between microstructure and properties were thoroughly investigated using various characterization techniques. Specifically, XRD, SEM, FT-IR and NMR were used to reveal the changes of phase, micro-morphology, chemical bond and molecular structure, respectively. In addition, the pore structure characteristics of the materials were quantitatively characterized through BET and MIP analyses, which elucidates the regulatory mechanisms of porosity and pore size distribution in geopolymers. It’s revealed form the XRD and SEM tools that trace magnesium oxide dosage makes a large number of gel products formed, mainly including hydrated calcium aluminum silicate gels and hydrated magnesium silicate gels (C(M)-S-H). Furthermore, the 29Si MAS NMR results demonstrate that the GDM-9 cementitious material system is more conducive to the conversion of Q2, which enhances the polymerization degree of M-S-H silicone tetrahedron. And the polymerization degree increase inhibits the formation of harmful pores and reduces the content of free water, thus greatly reducing the phenomenon of alkalinity and enhancing the mechanical properties of the cementitious material. Compared with other DG-based geopolymers containing different magnesium oxide dosage, it’s illustrated that GDM-9 cementitious material system exhibits the lowest porosity of 36.18 %, largest compressive strength of 61.2 MPa and the flexural strength of 10.5 MPa. Moreover, visual observation of pan-alkali behavior presents that the surfaces of samples GDM-4.5 and GDM-9 exhibits a fine degree of pan-alkali, while sample GDM-0 presents the most severe pan-alkali, followed by sample GDM-18. In summary, the concept proposal, that the preparation of desulfurization gypsum based cementitious materials with the incorporation of trace magnesium provides promising alternative candidates for the solid waste treatment from the perspective of aquatic ecosystem protection and a novel idea for the inhibition of surface pan-alkali of desulfurization gypsum-based cementitious materials.