Research On The Performance Of Desulfurized Gypsum-based Composite Cementitious Materials Based On Response Surface Methodolog

Under the background of vigorously preventing and controlling the air pollutant SO₂,desulfurization gypsum（FGD）came into being,which was an industrial by-product gypsum produced from flue gas wet desulfurization.In recent years,the annual production of desulphurized gypsum in China had reached nearly 100 million tons,which brought great challenges to the ecological environment.When the undisturbed desulfurized gypsum（DFGD）was used alone,it was difficult to generate strength.Although its strength could be improved when it was calcined at high temperature into semi-hydrated desulfurized gypsum（HFGD）or anhydrous desulfurized gypsum,its high energy consumption and secondary pollution were not conducive to the large-scale application of desulfurized gypsum in construction industry.At the same time,desulfurization gypsum had poor water resistance and bending resistance,and the current research methods were mostly focused on single factor test.The research method was simplex.Therefore,in view of the above-mentioned problems,in this paper,admixtures and fibers were added and the desulfurized gypsum was modified,and the fiber-desulfurized gypsum-based composite cementitious material（FRFGDCM）was prepared for the purpose of multi-objective optimization.The following studies were carried out:（1）In order to utilize desulphurized gypsum with large dosage and low energy consumption,the single factor test was designed to explore the effects of the ratio of semi-hydrated desulphurized gypsum to unmodified desulphurized gypsum（HFGD:DFGD）,sulphoaluminate cement（SAC）,mineral powder and quicklime on the mechanical properties,dry shrinkage properties,water resistance and dry and wet resistance of desulphurized gypsum based composite cementing materials（FGDCM）.The mechanism of actions were revealed by microscopic analysis of X-ray diffraction（XRD）and scanning electron microscopy（SEM）.The results showed that the fast hydration and crystallization properties of hemihydrate desulfurization gypsum could be used to achieve the solidification of the original desulfurized gypsum.The shrinkage of the hardened body due to water evaporation could be compensated by the micro-expansion effect of the sulfoaluminate cement.The mineral powder could significantly enhance the various properties of the desulfurization gypsum.The quicklime mainly played the role of providing an alkaline environment for the substrate.（2）The Box-Behnken test of the response surface method（RSM）was used to establish the regressed model of mechanical properties,water resistance and dry and wet resistance of FGDCM.The regressed models were evaluated,and the effect of the interactions of the admixtures on the properties of FGDCM were explored through the three-dimensional response surface.Finally,the optimal mixing ratio obtained by using the Numerial function of Design-Expert was:The contents of sulfoaluminate cement,mineral powder and quicklime were 7.82%,21%and 5.22%,respectively.（3）Based on the optimal mixture ratio of admixtures obtained by RSM test,the effects of basalt fiber（BF）,polyvinyl alcohol（PVA）fiber and hybrid fiber（BF:PVA fiber=1:1）on mechanical properties,water resistance and dry and wet resistance of FGDCM were investigated by single factor test.The effects of different fibers and hybrid fibers were evaluated,and the mechanism and failure mode of fibers were revealed by SEM analysis.The results showed that the bridging effect of fibers could significantly improve the properties of FGDCM.The failure mode of specimens were changed from brittle failure to ductile failure.As far as fiber modification effects were concerned,hybrid fiber showed positive hybrid effect on the whole.