The Mechanism Of Reactive Mineral Admixture In Suppressing Alkali-silica Reaction

Studies show that the reactive mineral admixture has a good inhibitory effect on Alkali-Silica Reaction (ASR), but more studies are needed on the mechanism of inhibition ASR occurrs in the interfacial transilation zone (ITZ) between cement matrix and aggregate, and the ITZ is the weak link in concrete. This thesis studied the mechanism of reactive mineral admixture in suppressing ASR from the changes of composition and the structure of the ITZ.Cement concrete specimens were prepared for the experiment. Active zeolite-based perlite was selected as aggregate, ultra-fine fly ash, silica fume and ultra-fine slag, and various concentrations of KOH and NaCl were mixed in specimens. The specimens were cured for 28d, the changes of composition and, pore structure in ITZ and the elements distribution across the interface were observed by SEM and SEM-BSE. And the expansion of the corresponding mortar bar and changes in pore solution composition wre measured.The results show that the composition and structure of ITZ of the specimens mixed with ultra-fine fly ash and silica fume improved appearently, and the Ca/Si of C-S-H in the ITZ decreased significantly. And The Ca/Si ratio, structure and morphology of the specimens with high content of reactive mineral admixture were better than those with low content. The ITZ of the specimen mixed with 20% silica fume performed outstanding. The Ca/Si of C-S-H in the ITZ of the specimen mixed with slag also decreased, but not as good as theose mixed with silica fume and fly ash. Lots of CH were found in the ITZ of the specimen mixed with slag, even with 40% slag addition. The concentrations of Ca²⁺, K⁺ and Na⁺ in pore solution of the specimens mixed with fly ash and silica fume decreased significantly, while those mixed with slag increased. Compared with the control sample, the expansion of the mortar bar was much smaller.In view of the results, the mechanism of reactive mineral admixture is proposed. The incorporating of reactive mineral admixture reduces the pore size and porosity of the ITZ. The micro pore with large surface area binds more alkali to prevent it from contacting aggregate. The secondary pozzolanic reaction consumes a large amount of CH in the ITZ, and produces a large number of C-S-H gel with low Ca/Si ratio, reducing the available alkali for ASR.