| It is one of the main research directions in the field of materials to control the crystal growth by designing and synthesis the functional organic molecules.Numerous high-quality crystal materials were obtained in the presence of these functional organic molecules with structural character of biomolecules.?-Calcium sulfate hemihydrate(CaSO4·0.5H2O,denoted as ?-HH)is a very important class of cementitious materials and has multiple applications.In general,the strength of ?-HH was very low because of acicular crystal,the propertics of ?-HH are closely associated with the crystal structure and morphology,the results of experiments showed that the mechanical strength of short column ?-HH with high regularity and good dispersion was high.Therefore,it is very important to explore an approach to control the morphology of high-quality ?-HH for application in a more widely and advanced field.In addition,the study found tha the existence of some special groups such as carboxyl group,hydroxyl group and amino group is the key to control the crystal growth,thus,we use dopamine and DPA-PEI containing hydroxyl and amino groups as the organic additives to control?-HH crystal growth.The content mainly contained the following aspects:In chapter one,the structure characteristics and types of functional organic molecules are introduced briefly.the species and developing history of gypsum are given.The properties,preparation and various applications of ?-HH are progressed.And the significance and the purpose of the dissertation are elucidated.In chapter two,the effect of dopamine concentration and solution pH on crystallization of ?-HH are investigated in salt solution.The ?-HH particles are characterized by scanning electron microscopy(SEM),power X-ray diffraction(XRD)patterns,Fourier transform infrared(FTIR)spectroscopic and thermal gravimetric differential scanning calorimetry(TG-DSC).The results indicate that in salt solution(pH=7.0),dopamine paly a key role on the crystal morphology of ?-HH,it could be selectively adsorbed on {111} crystal surface of ?-HH.With the increase of the concentration of dopamine,the dehydration rate and the length/width(l/w)ratio of ?-HH both decreased markedly.The products were all ?-HH with the concentration of dopamine below 6.0 g·L-1,while the final products were mainly ?-HH,co-existed with a little amount of gypsum and anhydrite in the case of 6.0 g·L-1 dopamine.In the dehydration proeess,pH had little effcet on the dehydration process and crystal morphology of ?-HH.The dry compression strength and dry bending strength of the hydration molding were enhanced,with the decrease aspect ratio of ?-HH,and the maximum value is 42.5 MPa and 15.3 MPa respectively.In chapter three,hexagonal prism ?-HH were sueessfully prepared through hydrothermal dehydration in the presence of 3-(3,4-dihydroxyphenyl)propionic acid grafted poly(ethylenimine)(DPA-PEI0.3).The effect of DPA-PEI concentration on crystal morphology of ?-HH and the dehydration rate were investigated.The ?-HH particles were characterized by SEM,XRD,FTIR and TG-DSC.The possible growth mechanism,crystal structure,crystalline water content of the hexagonal prism ?-HH was discussed.The results showed the crystals were further uniform,each tip was terminated with the {001} facet,length/width(l/w)ratio decreased markedly,with increase of the concentration of DPA-PEI,at pH 5.5.We obtained well regular ?-HH crystals large enough and suitable for the single crystal X-ray diffraction,the refinement gives a trigonal P3221 space group with cell parameters(a=6.9344A,c=6.3362A),0.5 H2O per formula is mostly reasonable in the refinement as reflected by the least R and the rational hydrogen bonding distance between H2O molecules in crystal.The dry compression strength and dry bending strength of the hydration molding were measured to be 73.6 MPa and 31.2 MPa,respectively,when the aspect ratio of ?-HH is down to 0.1.In chapter four,the ?-HH multiple-pore hollow spheres were successfully synthesized through DPA-PEI-mediated self-assembly of small primary building blocks(hexagonal prism ?-HH)in salt solution.The products were characterized by SEM,XRD,FTIR and TG-DSC.Study on morphology evolution of ?-HH multiple-pore hollow spheres reveals that controlled synergy of concentration of DPA-PEI0.6 and pH is crucial for the development of a-HH into multiple-pore hollow spheres.The research shows that ideal ?-HH hollow spheres can only be prepared in the case of 2.0 g·L-1 DPA-PEI0.6 and solution pH 5.5.The possible growth mechanism was discussed in this work.In chapter five,?-HH is a metastable mineral and can only maintain its phase unchanged in salt solution for a period,and then transit into the thermodynamically stable phase of gypsum in low temperature or anhydrite in high temperature due to the 0.45 nm water channels in crystal structure of?-HH.The results showed the concentration of DPA-PEI and temperature influence the metastable lifetime of ?-HH.The metastable lifetime were found to increase with increasing DPA-PEI concentration or along with temperature from 20 to 65? or from 95 to 65?.In chapter six,several common surface modifiers(sodium citrate,sodium stearate,sodium oleate)with carboxylic ion end-groups is used to modify ?-HH particles,the ?-HH/PVC composites were prepared by mechanically mixing ?-HH with PVC materials.The micro-structure and mechanical properties of ?-HH/PVC composites were studied by using SEM and universal mechanical testing machine.The results show that the notched impact strength and the tensile strength of ?-HH/PVC composite incorporated with optimum amount of ?-HH particles were 58.6 MPa and 5.79kJ/m2,respectively.which are much higher than that of neat PVC materials. |
PDF Full Text Request