Research On The Mesoscopic Hydration Reaction And Nucleation Mechanism Of Phosphogypsum-Cement Cementitious Materials

The utilization of phosphogypsum is a systematic project that accelerates the effective utilization of phosphogypsum,and finally achieves zero emissions and consumes inventory.It is an inevitable requirement for the sustainable development of phosphogypsum and the ecological protection of the Yangtze River Basin.Through the use of phosphogypsum and fly ash auxiliary glue,the"Ecological Ternary Cement"（Phosphogypsum-cement Composite Cementitious Materials,PBM）is prepared after a suitable ratio of coagulant and cement,which meets the strategic needs of"Peak carbon dioxide emissions",according to the statement released afte the annual Central Economic Work Conference.This work takes the PBM as the research object,and focuses on the hydration reaction mechanism of C₃S,C-S-H and other minerals.The microscopic hydration mechanism was established,the functional relationship between sub-particles and macroscopic properties was established,and the microscopic growth mechanism of silicon chains was studied by means of numerical simulation,thereby characterizing the nucleation mechanism of hydration products of PBM.It provides theoretical basis and scientific support for the large-scale application of phosphogypsum to new"Ecological Ternary Cements".The main work and conclusions of the thesis are as follows:1.Ecological Ternary Cements were perpared with powders of phosphogypsum（PG）,fly ash（FA）and portland cement（PC）.In this thesis,a new type of"Ecological Ternary Cements"with high strength and performance were prepared by mixing phosphogypsum and fly ash as Supplementary Cementitious Materials（SCMs）with cement through thousands of proportioning experiments.2.The hydration reaction mechanism of PBM was characterized by means of XRD,SEM,TG-DSA and FTIR,while the variation of hydration products and concentration of the specified ions were acquired during the hydration process.I.The formation amount of ettringite（AFt）crystals increased at first then decreased,and increased with the increasing of phosphogypsum contents,which the ions of[SO₄]^2-produced by its dissolution could stimulate the activity of fly ash.II.Phosphogypsum and fly ash showed a synergistic effect,the formation of hydration products was slowed in the early stage.During the later stage of the hydration process,the quantity of hydration products increased rapidly,forming a composite of C-S-H gels as matrix,within which there existed randomly dispersed needle-like AFt crystallites as strengthening phase.III.The evolution of the structure of hydration products is mainly manifested in the ionization of phosphogypsum to generate[Si O₄]^2-ions,and the covalent bonds such as Ca-O-Ca and Si-O-Si were broken when exposed to alkali,and the Si-O-Si,Al-O-Al,Al-O,Si-O were combined with covalent bonds,and at the same time recombinant polymerization with acid ions such as[Si O₄]^2-and[Al O₄]^5-generated by fly ash ionization,aggregate to form stable C-S-H and C-A-S-H hexahedral networks.3.The thermodynamic characteristics of solid phase,solid solution phase and aqueous solution in process of hydration about PBM were studied based on the Gibbs-free-energy C-S-H thermodynamic model and GEM-Selektor software,and compared with experimental results,which contributes to constructing the hydration thermodynamic model of PBM.I.Thermodynamic experiments confirmed that the early compressive strength increased with the increasing of the amount of hydration products,and the rate of hydration products was inversely proportional to the amounts of hydration products.II.The thermodynamic properties were simulated and calculated by GEM-Selektor,and the evolution rules of mineral admixture and cement hydration product C₃S dissolution and C-S-H gel formation were quantitatively explained.III.Based on the Gibbs free energy C-S-H thermodynamic model analysis,it is concluded that the values of single-phase Ca O,Si O₂^H and H₂O_hyd in the hydration products gradually increased with the increase of the Ca/Si ratio,while the values of Ca O_ext and H₂O_OH showed a positive proportional relationship,and the values of Si O₂^Hand Si O₂ showed an inverse proportional relationship.4.By comparing the effects of pretreated phosphogypsum and untreated phosphogypsum on the hydration of tricalcium silicate,the hydration reaction synergistic effect of phosphogypsum and tricalcium silicate in the phosphogypsum-cement composite gelling system were studied.I.The pretreatment of phosphogypsum and pure tricalcium silicate（C₃S）were studied respectively,and the 3D atomic visualization simulation of the microstructure of phosphogypsum was carried out using VESTA software.Based on the DFT theory,the Ca OH⁺,H₂Si O₄^2-,Al（OH）₄^-and Ca²⁺plasma interactions in the hydration reaction of C₃S solution was obtained on the nucleation and growth of hydration products.II.The analysis by XRD,SEM,TG-DTG and ²⁹Si MAS-NMR showed that the addition of phosphogypsum in the early stage promoted the formation of AFt,the dissolution of C₃S,the polymerization of C-S-H gels and the morphological transformation.Some of them could make C-S-H gels were converted to a denser C-A-S-H gels.III.Rievlted-XRD was used to quantitatively analyze the crystal structure and composition of the mineral phase.Rievlted-XRD and ICP-AES were used to quantitatively determine the crystal structures,composition and surface ions leaching amount of the mineral phase of the hydration product of the hardened slurry.A schematic diagram of the accelerated nucleation and growth process of C-S-H and C₃S on the surface of solid particles were constructed,revealing the synergistic effect of PG and C₃S hydration.5.By analyzing and summarizing the NMR spectra of ²⁹Si and ²⁷Al in the existing composite cementitious materials,the application of ²⁹Si and ²⁷Al NMR in SCMs-cement-based composite cementitious materials were studied,and the ²⁹Si and²⁷Al NMR spectra of PBM were constructed,analyzing the nucleation mechanism of Ca/Si and Al/Si alignment silicon chains.I.The spectra of ²⁹Si and ²⁷Al NMR were analyzed by Magic Angle Spinning（MAS）,Cross Polarization（CP）and other methods,and the fitting spectra of ²⁹Si and²⁷Al NMR in SCMs-cement-based composite cementitious materials were constructed,and the results of ²⁹Si and ²⁷Al NMR in SCMs-cement Characteristic parameters of matrix composite cementitious materials.II.By combining Dynamic Nuclear Polarization（DNP）,Density Functional Theory（DFT）and NMR experiments,the effects of different Ca/Si and Al/Si ratios on silicon chains were analyzed,and the atomic-scale model of C-（A）-S-H structural unit was builted.The characteristic parameters of ²⁹Si and ²⁷Al NMR in PBM were determined by relative coordination energy,XRD and NMR analysis.III.By analyzing the relationship between the characteristic parameters of ²⁹Si and²⁷Al NMR and C-（A）-S-H,the molecular dynamics equation of the C-S-H structure based on DFT theory is given,and the two-dimensional spectrum of ²⁹Si and ²⁷Al NMR is constructed.Through Materials Studio software drawed the C-A-S-H molecular dynamics model and summarized the nucleation mechanism of Ca/Si and Al/Si alignment of silicon chains.