Study On Modification Of ?-Hemihydrate Phosphogypsum And Preparation Of Self-leveling Screeds

Gypsum based self-leveling screeds is one kind functional building motar which owes the self leveling capacibility by gravity and high fluidity.Compared with cementitious self-leveling floor mortar,it has advantages of good volume stability,quick hydration and hardening,and is normally made with ?-hemihydrate gypsum.However,the preparation of ?-hemihydrate gypsum consums large amount of energy and leads to high cost.It would be possible to prepare gypsum based self-leveling screeds with ?-hemihydrate phosphogypsum(PG)at lower cost,which could also broaden the approaches of utilization of PG as building materials.On the other hand,the properties of ?-hemihydrate PG are largerly influenced by its impurities,leading to the characteristics of acidity,inadaptability to chemical admixtures and low strength.For the purpose of displacing ?-hemihydrate with ?-hemihydrate PG to prepare self-leveling screeds,on this basis,the ?-hemihydrate PG was modified with cements and admixtures to improve its strength and adpaptability with admixtures,and gypsum based self-leveling screeds was finally prepared.Ordinary Portland cement(OPC)and the calcium aluminate cement(CAC)were chosen to modify ?-hemihydrate PG.It was found that both of the two cements could effectively improve the mechanical properties and water resistacne of hemihydrate.The crystals of dihydrate were wrapped by gels produced by the cements hydration,leading to reduce the contact points between water and crytals,as well as the length to radius ratio of gypsum crystals.Besides,it was also found that the hydration process of ?-hemihydrate PG would be affected after incorporating cement.Precicely,both of the cements would prolong the decelaration stage,and OPC would advance the accacelaration stage.The inhibition phenomenon of ?-hemihydrate PG hydration in early 2h was observed with incorporation of cements.Comparatively,CAC showed less impediment on early hydration of hemihydrate,and lead to less negative impact on 2h mechanical properties.The dispersion effect to ?-hemihydrate PG and the improvement effect of mechanical properties were SMF>PC>FDN orderly.Due to the decrease of water to gypsum ratio,the denser structure with lower porosity was obtained with the incorporation of superplasticizers,which contributed to larger extent to the increase of compressive strength compared with flexural strength.Generally,there was a tiny impact on ?-hemihydrate PG hydration process with superplasticizers,howvever,the crystal morphology was changed due to its adsorption effect,mainly resulting the inhibition of crystal growth towards c axis and making crystals plates.On the other hand,retards were studied to improve the operatibility of ?-hemihydrate PG.It was showed that proteins baesd retarders PE performed desireablely and showed lower negative effect to mechanical properties of hardend gypsum compared with other retarders.The retarders played its role mainly by extend induction period of hemihydrate hydration process,which would result in shortening and platening dihydrate crystals,and lead to reduce the contact points of crystals.As a result,a looser structure was obtained with decrese of strength.Based on the previous study,CAC was chosen as the modifier at the dosage of 20 %,and the composite was used as the cementious material to prepare self-leveling screeds.The adapabilities between admixtures and composite were studied.Results showed that composites were more sensitive to the admixtures compared with ?-hemihydrate PG,requiring fewer demand of admixtures to reach the same properties.In addition,greater improvement was observed when admixtures added to composite.It was found that superplasticiezers could more enhance the compressive strength of hardnend composites to 100.11 %,and PE retarder did less harm to mechanical properties as well.On the basis of the selection of applicable cementious materials,the self-leveling screeds with high early strength and desirable workability was prepared.The fluidity loss of SLS after 30 min only reached 2mm,which could be appropriately used as the underlayer SLS.