Adsorption Of Na～+,SO₄～(2-) And Cl～- On The Surface Of C-S-H And Its Influence On Durability Of Concrete

The adsorption characteristics of Na⁺,SO₄^2- and Cl^-ions on C-S-H surface werestudied and thereby some ideas how to improve durability of concrete were provided.C-S-H samples were prepared by means of solution synthesis. On the basis of surfacephysical chemistry, experimental methods to investigate the adsorption ability of C-S-Hsurface for Na⁺,SO₄^2- and Cl^-ions were studied. Experimental results show that it isfeasible to research the adsorption ability of C-S-H surface by means of chemical analysisand the measurement of ζpotential. Using these methods, the adsorption ability of C-S-Hsurface for Na⁺,SO₄^2- and Cl^-ions was studied, and the effects of Al³⁺ ions that weresolid dissolved in C-S-H on adsorption ability of C-S-H surface for Na⁺,SO₄^2- and Cl^-ions were probed into. The investigation indicates that alkalis may be adsorbed on the C-S-H surface as the counter ions, and the proportion of alkalis among counter ions isinterrelated with the solution composition closely; C-S-H surface negatively charged canadsorb SO₄^2- ions or Cl^-ions that bear homo-charges with the surface of solid; theadsorption ability of C-S-H surface for Cl^-ions is larger than that for SO₄^2- ions. Theamount of Cl^-ions adsorbed by C-S-H surface includes both that in Stern layer and that inthe diffusion layer. SO₄^2- ions can hardly enter into Stern layer and SO₄^2- ions adsorbed byC-S-H surface distribute in the diffusion layer for the main. Adsorption ability of C-S-H inwhich Al³⁺ ions are solid dissolved for Na⁺ ions increases, and that for SO₄^2- ions and Cl-ions decreases.The possibility of producing zeolite in C-S-H that sucks Al³⁺ ions and Na⁺ ions wasexamined. XRD indicates that after autoclave zeolite can be produced in C-S-H samplesthat possess appropriate composition, but in the C-S-H samples in which Al³⁺ ions aresolid dissolved no zeolite is discovered. Adsorption ability of C-S-H in which zeolite hasbeen produced for Na⁺ ions increases, and that for SO₄^2- ions and Cl-ions decreases.The increase of Na⁺ ions adsorbed on C-S-H surface decreases the free Na⁺ ions in thesolution, which is helpful to weaken ASR. SO₄^2- ions adsorbed on C-S-H surface canbecome sulphate source of delayed ettringite in concrete, the ettringite crystal may be alsoformed in C-S-H gel. SEM shows that Al³⁺ ions that are solid dissolved in C-S-H influencethe crystallization status of ettringite. C-S-H surface adsorbs Cl-ions fast. If the amount ofC-S-H gel increase in concrete the corrosion of reinforcing steel bar will decrease.