Research On The Activation Of Fluorogypsum Activity And Its Hydration And Hardening Mechanism

Fluorogypsum is a by-product of HF production.Due to its poor hydration activity,it cannot be used directly as a cementitious material,and its massive and long-term stockpiling is likely to cause environmental pollution,for which relevant research has been carried out at home and abroad.In order to activate fluorogypsum,firstly,the properties and hydration hardening characteristics of fluorogypsum were investigated in comparison with natural anhydrite.Secondly,the influence laws of chemical modifiers,combined physical-chemical modification and microwave heating on the activity of fluorogypsum were explored,and its hydration hardening mechanism at different ways were revealed by testing its hydration rate,hydration heat,dissolution properties,as well as XRD,SEM,TG and other tests,combined with relevant mechanisms.Thirdly,the distribution states and leaching laws of fluorine ions in the hardened fluorogypsum were explored.The main research results show that:（1）The main mineral of fluorogypsum was anhydrite[CaSO₄.Ⅱ],pH value was 2.3,poor grindability,F^-ions contained in it make the crystal plane spacing narrow,and the degree of crystallization was low.Although low pH value was beneficial to the dissolution rate of fluorogypsum at the initial stage of hydration and increases the reactant ion concentration,the Ca²⁺and SO₄^2-ion concentrations remain in the low range and the nucleation and growth of dihydrate gypsum crystals were slow.This is the fundamental reason for the lack of crystal contact points,low hydration rate,poor activity,unstable crystal structure network and low strength of fluorogypsum.（2）Both carbonates and bicarbonates excited fluorogypsum activity by participating in hydration reactions.The former accelerates the hydration of fluorogypsum by generating"unstable double salts"（sulphates:sodium sulphate,syngenite,ammonium sulphate）,Na₂CO₃,K₂CO₃ and（NH₄）₂CO₃ all have the best excitation of activity when the content was 1%.The latter accelerates the conversion of fluorogypsum to dihydrate gypsum by"catalysis effect"through the production of unstable bisulphate salts,but the modification of fluorogypsum activity by NH₄HCO₃ was based on the dissolution-crystallization mechanism only.NaHCO₃ at 1%was best for activation excitation,while KHCO₃ and NH₄HCO₃ were dosed at 0.5%.Modifiers increased the solubility of fluorogypsum,inhibited the dissolution of dihydrate gypsum,increased the supersaturation of dihydrate gypsum,and increased the nucleation and growth rate of dihydrate gypsum.At the same time,the growth rate of the different crystalline axes of dihydrate gypsum was changed and the crystal morphology was changed from plate-like to columnar or needle-like to improve the compactness of the hardened fluorogypsum.The bisulfate doesn’t participate in the hydration reaction process,and the modification of the activity was based on the solution-crystallization mechanism.When the best activity was obtained,the content of NaHSO₄ and KHSO₄ was 3%and 0.6%respectively.（3）Fluorogypsum has poor grindability,and quicklime can improve grinding efficiency and solidify free F^-ions.After grinding,the crystal structure of fluorogypsum was passivated,the degree of lattice distortion increased,the specific surface area of particles increased,which was conducive to dissolution and increased the hydration rate.When the quicklime content was 0.75%for grinding with a 15 min,the grinding effect of fluorogypsum was the best.Naphthalene superplasticizer plays a role in grinding and dispersion by adsorbing to the surface of fluorogypsum,and reducing the agglomeration effect in the grinding process.Thus,the solubility was increased,and the nucleation and growth rate of dihydrate gypsum was accelerated.The difference of crystal structure led to the competitive adsorption of fluorogypsum and dihydrate gypsum on superplasticizer,which inhibited the dissolution of dihydrate gypsum and was conducive to the hydration of fluorogypsum.When the content of naphthalene superplasticizer was 0.6～0.9%,the grinding aid modification of fluorogypsum was the best.Neither carbonate nor bicarbonate enhanced the grinding efficiency of fluorogypsum.（4）The microwave heat treatment did not change the mineral composition of the fluorogypsum,but the X-ray diffraction peaks of anhydrite was shifted to the left and the crystal plane spacing increased.After calcination the fluorogypsum morphology was loosely structured and the particles tend to be lamellar.The optimum temperature system for microwave heat treatment of fluorogypsum was 400℃ with a 15 min holding time.The initial and final setting times of the modified fluorogypsum paste were 269 min and 401 min respectively;the compressive strengths at 3 days and 28 days were 6.2 MPa and 21.6 MPa respectively;the 28d hydration rate was 22.3%higher than that of the control sample.The hydration of fluorogypsum follows the"dissolution-crystallization"mechanism,and its solubility increases,which increases the concentration of reactant ions in the liquid environment at the initial stage of hydration,and increases the supersaturation of dihydrate gypsum,which is conducive to the nucleation and crystallization of dihydrate gypsum,and accelerates the hydration of fluorogypsum.The degree of crystallinity increased during the growth of dihydrate gypsum,but the growth rates of the different crystalline axes were not affected and the morphology was plate-like,optimising the compactness of the hardened fluorogypsum.（5）The F^-ions in the hardened structure were mainly in the form of CaF₂,with a reduced distribution density.The micro-structure of hardened fluorogypsum directly affects the leaching and diffusion of free F^-ions.Optimisation of pore size and reduction of porosity block the connectivity of pores and fissures and destroy the leaching channels for F^-ions.In addition,the gradual encapsulation of CaF₂ solidified F^-ions as the dihydrate gypsum crystals grew also inhibited the leaching of F^-ions.The F^-ion concentration in the leachate was significantly reduced and was below 1.0 mg/L at 28 days.The concentration of F^-ion leaching from the modified fluorogypsum was greatly reduced.This dissertation contains 144 figures,30 tables and 160 references.