Study on activation of fluorogypsum by sodium sulfate and sodium nitrite

Given the issues related to poor hydration activity, long setting time and low early strength of industrial by-product fluorogypsum (FG), the composite modifiers (Na₂SO₄ and NaNO₂) were utilized to enhance its reactivity. The investigation of the mechanism involved the utilization of contemporary analytical methods, including X-ray diffraction (XRD), ¹H low-field nuclear magnetic resonance (NMR), and Scanning electron microscope and Energy Dispersive Spectrometer (SEM-EDS). The results demonstrated that the incorporation of modifiers significantly enhanced both the hydration rate and activity of fluorogypsum. The optimum concentration of the composite modifier was found to be 1.5 wt% Na₂SO₄ and 0.5 wt% NaNO₂. The addition of modifiers (1.5 wt% Na₂SO₄ and 0.5 wt% NaNO₂) significantly shortens the setting time of FG paste, reducing it by approximately 500 min compared to the control sample. After 28 days of curing, the flexural strength and compressive strength of the fluorogypsum sample containing modifiers (1.5 wt% Na₂SO₄ and 0.5 wt% NaNO₂) increased by 55.5 % (reaching 4.2 MPa) and 31.5 % (reaching 37.6 MPa), respectively. The modifiers facilitate the transformation from anhydrite (CaSO₄, AH) to dihydrate gypsum (CaSO₄·2H₂O, DH). Both NaNO₂ and Na₂SO₄ alter the growth rates of different crystal axes during DH crystal growth, transforming them into prismatic and needle-shaped DH. The prismatic and needle-shaped DH crystals were arranged in layers, resulting in a compact structure with low hole content and few pores, which led to increased density of the hardened paste and higher strength. The current study provides evidence that the inclusion of composite modifiers greatly improves the activity of FG, making it more efficient in the field of building materials.