Research On Corrosion Resistance And Its Mechanism Of Persulfur Phosphogypsum-slag Cement Concrete Under Sea Salt Environment

Phosphogypsum is currently one of by products of phosphorus industrialproduction in China, the emissions is not only large but also difficult to useeffectively. Stacking of phosphogypsum for a long time caused serious pollution tothe environment, so accelerating the utilization of phosphogypsum has become moreurgent. Slag is produced by the blast furnace ironmaking industrial waste iron andsteel enterprises. To promote the utilization of phosphogypsum and slag, this paperuse45%phosphogypsum and49%slag as the main raw material, supplemented by2%steel slag and4%clinker compound, developed a low carbon cement with goodperformance called persulfur phosphogypsum-slag cement. Through the proportionand preparation process of concrete, developed persulfur phosphogypsum-slagcement concrete. By means of comparative testing of ordinary Portland cementconcrete, ordinary slag cement concrete, using XRD, SEM, EDS, TG-DSC, MIP, heatof hydration analysis, systematic studyed the mechanical properties, micro-structuresand mechanism of hydration reaction process of persulfur phosphogypsum-slagcement concrete. Through simulating environment of the artificial seawater to studyanti-corrosion properties of sea salt of persulfur phosphogypsum-slag cementconcrete, and its resistance to chloride corrosion, magnesium sulphate resistance andanti-corrosion properties and mechanism of action is analyzed respectively. Based onthis study, the silica fume and metakaolin can improve the mechanical behavior ofconcrete, microstructure and sea salt erosion resistance and mechanism of persulfurphosphogypsum-slag cement concrete are also studyed. Experimental studies showthat:Under the condition of preparation of Level C30,3d strength of persulfurphosphogypsum-slag cement concrete is lower than ordinary Portland cementconcrete and slag cement concrete, but the7d strength growed rapidly,28d strengthcan reach40.8MPa, close to ordinary Portland cement concrete. Reducephosphogypsum content, increase specific surface area of GGBFS can enhance themechanical properties of persulfur phosphogypsum-slag cement concrete. The hydration products of persulfur phosphogypsum-slag cement concrete aremainly AFt (ettringite) and CSH gel, AFt and the rest of CaSO4·2H2O was tightlywrapped in a gel, form a homogeneous dense structure, and significant differencebetween Portland cement hydration products are not containing Ca(OH)2. Under thesynergistic hydration reaction of slag cement clinker/Steel slag (base excitation) andgypsum (sulphate excitation) in persulfur phosphogypsum-slag cement concrete,more low Ca/Si of CSH gel and gel pores smaller than10nm are formed thanordinary Portland cement concrete. And interfacial transition zone is more dense andsmooth than ordinary Portland cement concrete and slag cement concrete. Due tocement clinker content of persulfur phosphogypsum-slag cement is low, alkalinity issmall and phosphogypsum retarding effect, leading to the rapid dissolution of slagcan not be involved in the release of the active substance hydration, early hydrationheat quantity is low.The compressive strength of persulfur phosphogypsum-slag cement concrete,ordinary portland cement concrete and ordinary slag cement concrete by artificialseawater erosion cycle decreased. With the increase of the number of cycles of seaerosion, more concrete strength decreased significantly. The anti-corrosionperformance of persulfur phosphogypsum-slag cement concrete is stronger thanordinary Portland cement concrete and slag cement concrete. Hydration products ofpersulfur phosphogypsum-slag cement concrete changed after Sea salt erosion,internal ettringite content of concrete increased, gypsum board lamellar contentdecreased with the dissolution reaction, concrete interfacial transition zonemicrocracks generated.Through using electric flux method, rapid chloride ion penetration coefficientmethod and unsteady natural diffusion method, measured resistance to chloride ionpenetration performance of persulfur phosphogypsum-slag cement concrete issuperior to ordinary Portland cement concrete and slag cement concrete, due to itsthin pore structure and the ability of Cl-adsorption to form Friedal salt. Sulfate attackresistance of persulfur phosphogypsum-slag cement concrete is superior to ordinaryPortland cement concrete and slag cement concrete duo to persulfurphosphogypsum-slag cement concrete is rich of SO42-, the compressive strength doesnot reduce but increase in sulfate environment. Magnesium erosion damage had a greater impact on compressive strength of persulfur phosphogypsum-slag cementconcrete, but the damaging effects of erosion is less than ordinary portland cementconcrete duo to its less clinker content. The unique structure of persulfurphosphogypsum-slag cement concrete makes it also have excellent resistance tochloride ion permeability, sulfate resistance, and resistance to corrosion performanceof magnesium, making it superior to ordinary portland cement with sea salt corrosionresistant properties of concrete.Silica fume and metakaolin can significantly enhance the mechanical propertiesof persulfur phosphogypsum-slag cement concrete. Silica fume and metakaolin canimprove the anti-corrosion properties of sea salt of persulfur phosphogypsum-slagcement concrete, make compressive strength of persulfur phosphogypsum-slagcement concrete corroded by sea salt loss rate reduced. Silica fume and metakaolincan improve the hydration products’ morphology and microstructure of persulfurphosphogypsum-slag cement concrete, improve the pore structure of concrete, thusimproving the performance of persulfur phosphogypsum-slag cement concrete.Ultrasonic method and impact echo method are two new kinds ofnon-destructive testing technology, can be used to evaluate the compressive strengthof concrete by measuring the concrete wave velocity, then can be used to determinethe erosion damage of concrete under sea salt environment and of sulfateenvironment.