Comparative study on modifications of pH-adjusted fluorogypsum by potassium carbonate and potassium bicarbonate

In view of the poor hydration activity and difficult utilization of industrial by-product fluorogypsum, the effects of potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) on the hydration mechanisms of fluo-rogypsum (pH-adjusted to 7.3) were investigated and compared through hydration rate, hydration heat, solu-bility and microscopic tests. The results show that both K2CO3 and KHCO3 could significantly increase the hydration rate, hydration heat release rate, and accelerate the hydration process of fluorogypsum. The optimum contents of K2CO3 and KHCO3 were found to be 1% and 0.5%, respectively, and the compressive strength of the fluorogypsum increased from 3.5 MPa (control group) to 20.3 MPa (increase by 480%) and 18.5 MPa (increase by 429%) after curing for 3 days. Both of the two increased the solubility of fluorogypsum, supersaturation of dihydrate gypsum, the nucleation and growth rate of dihydrate gypsum, and inhibited the dissolution of dihy-drate gypsum. The compactness of the hardened fluorogypsum was improved and the porosity was reduced. K2CO3 was found to be more effective than KHCO3, resulting in a higher early strength. The acceleration mechanisms of the two were also found to be different: K2CO3 increased the hydration of fluorogypsum by forming an unstable double salt syngenite; while KHCO3 mainly promoted the formation of dihydrate gypsum through a 'catalysis effect' by unstable potassium bisulfate. With the addition of any of these two modifiers, the crystal morphology of dihydrate gypsum was changed from plate to column or needle, which was beneficial for the strength development.