Influence Of Salt On The Composition And Structure Of Interfacial Transition Zone In Concrete

Compositions and structures of interface transition zone(ITZ)between mortar and aggregates in cement concrete have an influence on the whole performances of concrete.The contents of sodium-salt such as NaCl and Na₂SO₄ have a tendency of increasing in modern concrete,causing salt corrosion and alkali-aggregate reaction (ASR),and it is necessary to inverstigate the influence of salt on interface transition zone.The Composition and structure of interface between aggregate and cement paste,pore structure,the range of the ITZ,element distribution,the orientation and size of crystals were studied by using scanning electron microscopy,energy dispersive X-ray analysis,back scattered electrons image and X-ray diffraction in concrete with inactive or active aggregate by adding extra NaCl and Na₂SO₄ of various content.Test results show that the content of ettringite(Aft) increase obviously in the ITZ of concret in the concrete adding Na₂SO₄,the crystals are in large size,many cracks exist in the ITZ,elements Na,Ca,Al,S accumulate around the interface.The porosity increase with the increasing concentration of Na₂SO₄ in active aggregate concrete but not in inactive aggregate concrete.The orientation of calcium hydroxide is obvious in the concrete adding Na₂SO₄ and the crystals are in large size in the concret with inactive aggregate,the size of Ca(OH)₂ decrease with the increase in distance to the aggregate or the concentration of Na₂SO₄,the size of Aft increase with the increase of concentration of Na₂SO₄.The Composition and structure of ITZ become complicated when the concentration of NaCl increased,showing the appearance of Friedel's salt,and accumulation of elements such as Na,Ca,Al,S,Cl around the interface.The accumulation intensified when the concentration of NaCl increased,which is prominent in the active aggregate concrete.The porosity is higher in active aggregate concrete.The final factor that affect the porosity is the porosity structure or denseness of aggregate.The orientation of Ca(OH)₂ is not obvious.The size of Ca(OH)₂ and Aft decrease when the distance to the aggregate or the concentration of NaCl increase.Larger size of Ca(OH)₂ of Aft were found in concret with inactive aggregate than in those with active aggregate. The porosity in concrete of adding Na₂SO₄ is higher than those of adding NaCl,and two kind of salts have little affect to the porosity in inactive aggregate concrete.The research results is very useful to reveal the corrosion mechanism of concrete suffered from sulfate attack,chloride salt attack and ASR.