Preparation And Modification Of α Hemihydrate Gypsum

Gypsum has properties of light weight,heat insulation and sound absorption.Moreover,gypsum product is a really green material.On one side,gypsum products will not produce any pollution during the preparing process;on the other side,gypsum products can be infinitely recycled theoretically.Hemihydrate gypsum is the main raw material of producing gypsum products,and it has two kinds of crystalline phase,one of a hemihydrate gypsum and the other of P hemihydrate gypsum.Compared with β hemihydrate gypsum,a hemihydrate gypsum has the advantages of excellent mechanical performance and low energy consumption.The scale production of a hemihydrate gypsum is of great significance to the development of gypsum industry and the realization of energy conservation.Normal pressure hydrothermal method has become the main method of producing a hemihydrate gypsum for its short production cycle,mild process conditions,low demand for equipment and controllable product quality.However,the study of producing a hemihydrate gypsum with normal pressure hydrothermal method is still limited so far.In this paper,phase transformation kinetics and the crystal transformation mechanism of a hemihydrate gypsum are studied.TG and scanning electron microscope(SEM)are used as characterizing and testing methods,in order to consistantly optimize the technogical parameter and formula ratio,and ultimately produce a hemihydrate gypsum in the shape of six short prismatic.Generally speaking,gypsum material strength is a little low,even the pure a hemihydrate gypsum,its compressive strength is also a little lower than that of cement.One of the important reasons is the actual mixing water the gypsum needs is much larger than the theoretical hydration water,a themihydrate gypsum actual standard consistency water demand between 0.3 to 0.4,(3 hemihydrate gypsum actual standard consistency water demand is as high as 0.6,the high content of water will directly lead to increase of hardening body porosity,and strength decrease.Adding water reducing agent can guarantee a good compressive strength of hardening body and keep a good slurry rheology,this is currently the most practical and effective way of gypsum modification.Just as water reducing agent can increase the property of concrete,the development and the application of gypsum water reducing agent must make a deep influence on the gypsum material and product.The basic theory and key technology in the application of gypsum water reducing agent were studied in this research.TG,SEM and compression testing machine were used to research the effects of polycarboxylic superplasticizer(PS)and naphthalene-based superplasticizer(NBS)on a hemihydrate gypsum from the points of the hydration process,microstructure of the hydration products,the water requirement at standard consistency and compressive strength.Results showed that PS could decelerate the whole hydration process of a hemihydrate gypsum,while NBS could accelerate the early hydration,but decelerate the late hydration.Both of them could increase the highest hydration heat and shift the hydration heat peak.Both of PS and NBS could help the formation of "board crystals" and give a more compact structure of hydration products.PS could coarsen the crystals,while NBS could cause the formation of"broken crystals".Its properties would be the best with 0.5 wt%NBS or 0.4 wt%PS.PS had better performance in improving the properties of gypsum gel material under the same conditions,compared to NBS.