Preparation Technology And Mechanism Of High Performance Gypsum Cementitious Material

China is rich in gypsum mineral resources,with proven gypsum reserves of more than 70 billion tons,however,the high-grade natural gypsum that reaches the first and special grades only accounts for 8%.In addition to natural gypsum,the annual emissions of industrial by-product gypsum represented by flue gas desulfurization gypsum and phosphogypsum have approached 200 million tons in recent years.How to use technical means to process China‘s abundant natural gypsum and large-volume industrial by-product gypsum into high-performance,high value added products is a meaningful research subject.This work not only provides solid technical support for the sustainable development of enterprises emitting industrial by-product gypsum,but also has significant economic,social and environmental benefits.In view of this,this thesis conducts research in the dehydration path of gypsum dihydrate(Ca SO₄·2H₂O,CSD),the phenomenon of soluble anhydrite(?-Ca SO₄)adsorbing the thin water vapor in the air,the crystal transformation mechanism of gypsum into?-type calcium sulfate hemihydrate(?-Ca SO₄·0.5H₂O,?-CSH),the preparation of gypsum cementitious materials by desulfurization ash and industrial sulfuric acid wastewater and light gypsum thermal insulation mortar are studied in this thesis.These works have important practical and theoretical significance for the research and development of key technology and core equipment for the preparation of high-performance gypsum cementitious materials.In this study,the effects of temperature and water vapor pressure on the dehydration reaction of gypsum are investigated through strict thermodynamic calculations.The results show that the dehydration processes of gypsum are related to temperature and water vapor partial pressure.When the temperature is high and the water vapor partial pressure is low,gypsum directly dehydrates to form soluble anhydrite;with the increase of the partial pressure of water vapor,gypsum dehydrates to soluble anhydrite first,then the soluble anhydrite absorbs the water vapor to hydrate to form calcium sulfate hemihydrate again,which can be expressed as Ca SO₄·2H₂O??-Ca SO₄??-Ca SO₄·0.5H₂O;when the temperature is low and the water vapor pressure is high,the gypsum is directly dehydrated to form calcium sulfate hemihydrate(Ca SO₄·2H₂O??-Ca SO₄·0.5H₂O),which is stable in this area;when the temperature is low and the partial pressure of water vapor is further increased,gypsum is stable and no dehydration reaction occurs.?-Ca SO₄ lattice has strong water vapor adsorption capacity because of its honeycomb structure.With the increase of humidity,the crystal water content of Ca SO₄·n H₂O increases from 0 to0.67,that is to say,Ca SO₄·0.67H₂O phase is formed.The crystal habit modifier is a very important technology for the preparation of?-CSH.In this study,molecular dynamics simulation,infrared spectroscopy,selected area electron diffraction and other characterization methods were used to reveal the mechanism of crystal habit modifier regulating the morphology of?-CSH.The carbon chain lengths of sodium succinate and sodium glutarate used as crystal habit modifiers are 4 and 5 carbon atoms respectively.In the aqueous solution,these dicarboxylic acids can adjust their conformation,so as to ensure that the oxygen atoms on the two carboxylic groups of succinic acid or glutaric acid are complexed with the same calcium ion exposed on{114}surface of?-CSH and its symmetrical equivalent crystal surface,thereby hindering the rapid growth of?-CSH{114}.In addition,the effect of disodium phthalate on the morphology of?-CSH was studied.The mechanism of the effect of two carboxyl groups and their relative positions on the morphology of?-CSH was analyzed by experimental method and molecular dynamic simulations.The results show that benzene-1,2-dicarboxylic acid has two adjacent pincerlike carboxyl groups,which can effectively regulate the morphology of?-CSH crystal;benzene-1,3-dicarboxylic acid and benzene-1,4-dicarboxylic acid have little effect on the morphology of?-CSH prepared by hydrothermal method,because the carboxylic acid group has directionality on the benzene ring,it is not easy to adjust its own conformation and difficult to form chelate,so that the two different carboxylic acid groups cannot simultaneously form a complex with the same calcium atom on the end face of the?-CSH crystal,and eventually the?-type calcium sulfate hemihydrate,which originally developed into needle shape grow into?-CSH crystal with short column shape.In this thesis,based on the mechanism study,the hydrothermal process and orthogonal test were used to optimize the process parameters.The?-CSH with properties meeting the highest grade-?50 specified in the building materials industry standard JC/T 2038-2010??-type high strength gypsum plaster?was successfully prepared.According to the research results of gypsum calcination and transformation mechanism of?-CSH,the preparation technology of gypsum cementitious material is applied to the resource utilization of industrial sulfuric acid wastewater and desulfurized ash with calcium sulfite hemihydrate as the main component.High strength plaster of pairs and?-CSH are successfully prepared from desulfurization ash and industrial sulfuric acid wastewater.Calcium sulfate whiskers are acicular?-calcium sulfate hemihydrate,which not only has the characteristics of low bulk density but also has cementitious properties.In chapter 6 of this thesis,?-calcium sulfate hemihydrate whiskers are used to prepare light-weight gypsum insulation materials.Through the mixture experiment,the type II building insulation mortar with performance required by GB/T 20473-2006"Dry-mixed thermal insulating composition for buildings"standard was successfully prepared.