The influence of NaOH incorporation on the hydration process and properties of type II fluorgypsum

Fluorogypsum, an industrial byproduct classified as Type II anhydrite, faces limitations in its resource utilization due to its poor hydration activity and low early strength. Theoretically, alkali, which has been proven to possess potential activation properties in cement systems, can be applied to Type II anhydrite systems. This study aims to investigate the effects of different NaOH dosages on the compressive strength, hydration process, and microstructure of hydration products of fluoropgypsum, while explaining the enhancement mechanism from the perspective of hydration kinetics. The results demonstrate that the addition of NaOH significantly improves the compressive strength of fluorogypsum. When the NaOH dosage is 1.2 %, the 7-day compressive strength of modified fluorogypsum reaches 23.7 MPa, representing a 270.3 % increase compared to pure fluorogypsum samples. The addition of NaOH to fluorogypsum increases its early hydration degree and rate constant K. This accelerates the reaction between fluorogypsum and water, forming more hydration products. NaOH also participates in the growth process of hydration products. It increases the N value and affects the growth rates of dihydrate gypsum along different crystal axes. This promotes the transformation of dihydrate gypsum from plate-like to prismatic shapes. The hydration products become more tightly interconnected, creating a denser structure. The increased amount of hydration products and the denser structure contribute to the improved compressive strength of fluorogypsum hardened bodies.