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

Interfacial Transition Zone (ITZ) plays an important role in concrete. High-alkaline environment may cause changes in ITZ, thus affect the performance of concrete. It is necessary to investigate the changes of ITZ exposed to alkaline environment for the purpose of understanding the mechanism of alkali-silica reaction(ASR).In this paper, concrete respectively prepared with one non-reactive limestone aggregate and two active aggregates of zeolite and crystal glass were exposed to various concentrations of alkali solutions. The composition and structure of ITZ in the conctete were investigated and compare with each other. SEM / EDS was used to observe and analyze the morphology and composition of hydrates in ITZ. SEM-BSE was used to study the effects caused by different aggregate and alkali concentration on the pore structure of ITZ. X-ray tomography was used to analyze the orientation of the calcium hydroxide and the average size of the ettringite (Aft). The re-distribution of the material base on distribution of the elements of ITZ were also inverstigated.The results show that there is no significant difference among morphology of hydrate in the ITZ of ordinary concrete at various concentrations of alkali solution, while the morphology of calcium silicate hydrate in alkaline-active concrete is various, especially in the high alkaline environment. The calcium hydroxide (CH) which is hexagonal prism morphology was difficult to find and so did needle like ettringite. The orientation of the CH in ITZ is obvious. The grains of CH and AFt were refined obviously. The porosity of ITZ in alkaline-activity concrete decreased compare with that in ordinary concrete, but the number of more hazardous holes significantly improved. The ITZ in alkaline-activity concrete with high Ca/Si, high sodium concentration and low aluminum content is prone to induse ASR.These characteristics of ITZ in alkaline environment provide theoretical and experimental bases for explaining various changes of properties of concrete, especially for analyzing the mechanism of ASR and finding methods to prevent ASR.