The Composition And Structure Of Interfacial Transition Zone In Harden Concrete Exposed To Salt Environment

Interfacial Transition Zone (ITZ) has great influence to the performance of concrete. In the salt environments, the salt ions invade concrete by a lot of ways, causing changes of ITZ and erosion of concrete. It is nessary to inverstigate the composition and structure of ITZ in harden concrete suffered from salt attacks.In this paper, concrete samples casted with selected active aggregate or non-active aggregate are firstly cured in normal environment to 28 days then dipped in solutions with different sodium sulfate or sodium chloride contents up to 90 days. SEM was used to observe the phase morphology in ITZ. SEM-BSE was used to analyze the element distribution on the direction perpendicular to ITZ. XRD was used to analyze the composition and structure of crystal in ITZ. The theories of pore structure and surface physical chemistry were used to explain the mechanism of element distribution.Resarch results indicate that the element in concrete with active or non-active aggregate which are dipped in solutions of sodium sulfate or sodium chloride, enriched with various degrees along the direction perpendicular to ITZ. The first floor is C-S-H with low Ca/Si, and that draw on ion coming close. The second floor is C-S-H fixed some Al³⁺, which turn out to be C-A-S-H.Then,C-A-S-H adsorb a great deal of Na⁺(or other ion), so that is to format the third. The morphologies of ITZ are obviously influenced by concentration of sodium sulfate solution. Ettringite crystal grain is bigger in solutions of high concentrationof sodium sulfate than in low one. And it is diffult to find Ca(OH)₂ in ITZ. Faujasite and epistibite are found in ITZ of samples exposed to sodium sulfate solutions. Friedel's salt are found in ITZ of samples exposed to sodium chloride solutions.The number or crystal grain of Friedel's salt decreased with the decrease of concentrationof sodium sulfate as well as the number of Ca(OH)₂. Adding NaCl into concrete has same effect as adding NaOH.The changes of ITZ in salt environment help explaining the changes of concrete performances, especially those suffered to salt attack.