Crystal Morphology Control And Hydration Hardening Properties Of High Strength ?-hemihydrate Phosphogypsum

Phosphogypsum?PG?is an industrial solid waste discharged from the production of phosphoric acid by wet-process.The emissions of PG have reached 75 million tons in 2017 in China,and the utilization was 38%.The main component of PG is dihydrate gypsum?DH?,but PG contains various impurities such as phosphorus,fluorine and organic matter.The utilization of PG is strictly limited by containing harmful impurities,resulting in a low comprehensive utilization rate.The State Environmental Protection Administration classified PG as hazardous solid waste in 2006.In2018,Guizhou Province proposed to achieve a balance between production and consumption of PG.Therefore,how to speed up the progress of resource utilization of PG is the focus of current research.Because of the mild reaction conditions,the preparation of high strength?-hemihydrate phosphogypsum??-HH?by atmospheric salt solution has good application prospects.The control of morphology is crucial to preparation of?-HH.Based on the optimizing reaction conditions,the effects of molecular structure of organic acids on the morphology and mechanical properties of?-HH were discussed.The control mechanism of?-HH morphology was revealed by first-principle calculation combined with fourier transform infrared spectroscope and X-ray photoelectron spectroscope analysis.The microstructural characteristics of hardened paste were analyzed by computed tomography?CT?scanning and scanning electron microscope.Thus,the coupling relationship between the morphology of?-HH,the microstructure and mechanical properties of hardened paste is constructed.The main research achievements are as following:?1?Thermodynamic calculation shows that DH can't spontaneously transformed into?-HH in atmospheric water solution.Adding salt electrolyte in aqueous solution can reduce the water activity and realize the reaction under atmospheric pressure.With the increase of salt concentration,the conversion rate of PG to?-HH is accelerated,and the induction time and growth time are shortened.The morphology of?-HH presents needle-like whisker,but the aspect ratio varies greatly.The conversion of PG to?-HH in Na₂SO₄ solution has the advantages of fast speed and low concentration,the aspect ratio of?-HH is about 14:1,The aspect ratio of?-HH is reduced to 5:1?6:1in CaCl₂ solution,and it does not introduce other cations.The reaction temperature,solid-liquid ratio and dosage of pretreatment agent CaO were 95?,1:3 and 0.6%,respectively.?2?In Na₂SO₄ solution,maleic acid can effectively regulate the morphology of?-HH,and make it evolve from long column to short column,the aspect ratio is close to 1:1.Meanwhile it can inhibit the conversion of PG to?-HH by prolonging the growth time.The poor mechanical strength of?-HH prepared in Na₂SO₄ solution mainly attributes to the frost of hardened paste.Therefore,Na₂SO₄ can't be used as salt medium.In CaCl₂ solution,maleic acid,L-Aspartic acid,succinic acid,phthalic acid and citric acid can regulate the morphology of?-HH,while organic acids such as isophthalic acid,terephthalic acid,L-Glutamic acid and L-Asparagine can't regulate the morphology.The conversion rate of PG to?-HH decreases with the increase of crystal modifier.Among them,maleic acid,succinic acid,phthalic acid and citric acid decrease the conversion rate by prolonging the induction time,it is the rate-determining step of reaction.While L-Aspartic acid inhibits the conversion of PG by prolonging the growth time,and the reaction is controlled by growth.It is found that Boltzmann function can predict the change of crystal water content with reaction time,and it has universality for the conversion process under different concentration of crystal modifier.With increasing crystal modifier concentration,the aspect ratio of?-HH changes from long columnar to short columnar and closes to 3:1-1:1,the normal consistency decreases and the flexural strength and compressive strength increase.?3?The molecular structure characteristics of organic acid crystal modifier are containing two or more COOH and the distance is two C atoms.That is to say,when the distance of adjacent COOH matches with the space of Ca atoms?4.187??on the?111?surface of?-HH,organic acids have the ability to regulate the morphology of?-HH.Besides,the double bond or NH₂ in the molecule structure make the crystal modifier show stronger regulating effect.The results have certain guiding significance for the screening or designing and synthesizing of organic acid crystal modifier.The morphology of?-HH is regulated by crystal modifier mainly through the complexation of surface Ca atoms of?-HH by COOH.Maleic acid reacts with two Ca atoms on?111?surface of?-HH through two COOH,respectively,to form a cyclic compound with the adsorption energy of-201.65kJ/mol,showing the strongest adsorption effect.Only one COOH reacts with Ca on?110?surface,which is relatively weak.The interaction between the?0 1 0?surface and maleic acid is weak.Thus,the growth of?-HH along the c axis was inhibited.?4?The decrease of pH is a characteristic for preparation of?-HH from PG by atmospheric salt solution.The faster the conversion rate,the faster decrease the slurry pH.The reason is that the dissolution of eutectic phosphorus releases phosphate ions,which results in the change of pH from weak alkalinity to acidity.Therefore,the mechanical strength of?-HH decreases rapidly when CaCl₂circulating fluid is directly reused or the pH of CaCl₂ circulating fluid is adjusted to 8.60.The soluble phosphorus can be effectively removed from the solution when the pH was adjusted to 10.0.With the increase of cycles of CaCl₂ solution,the morphology and mechanical strength of?-HH have less effect,and the strength grade reaches?25 JC/T 2038-2010.It can also effectively reduce the concentration of L-Aspartic acid.?5?The conversion of PG to?-HH in salt solution and hydration of?-HH to DH follow the dissolution-crystallization principle.The reaction process can be divided into dissolution stage,dissolution-crystallization stage and equilibrium stage.In the dissolution stage,PG or?-HH dissolve into Ca²⁺and SO₄^2-,and the concentration increases gradually,while the crystal water content of products remains unchanged.During the dissolution-crystallization stage,it will form?-HH or DH nuclei and grow when reaching supersaturation,the crystallization rate is faster than the dissolution rate,the crystal water content of corresponding products decreases or increases and the particle size of products decreases rapidly.After entering the equilibrium stage,the concentration of Ca²⁺and SO₄^2-,the crystal water content and particle size of the product remain basically unchanged,the dissolution and crystallization reach equilibrium.In addition,the relative supersaturation and the aspect ratio of DH decreased as well as hydration time extending for?-HH prepared in the presence of crystal modifier.?6?The hardened paste of?-HH prepared without crystal modifier presents a porous structure,and the hydration product DH is long columnar with large grains and uneven distribution,and the overlap area between crystals decreases,resulting in a low mechanical strength.In the presence of crystal modifier,the hardened paste is compact,and DH in the hardened paste is mainly diamond-shaped.The crystal size decreases with a uniform size,and the accumulation between crystals is more compact,so improvement of mechanical strength.Meanwhile,CT scanning shows that compared with blank test,the specific surface area and porosity in hardened paste of?-HH prepared by adding L-Aspartic acid decreases from 17.28 to 0.99 cm²/g and from 6.07 to 0.80%,respectively,the compressive strength increases from 9.7 to 28.8 MPa.In summary,adding crystal modifier can reduce the aspect ratio of?-HH and the normal consistency,and they maintain a good linear relationship,reduce the number and volume of holes as well as specific surface area of defects in hardened paste,so the hardened paste shows better mechanical properties.