Mechanism Of Fly Ash In Relation To Alkali-Silica Reaction

According to alkali-aggregate reaction prevention test subject research of GUI GUANG railway, some experimental research about the alkali active characteristics of slate aggregate and the mechanism of fly ash preventing alkali-silica reaction has been done in this thesis.The alkali-silica reaction of slate aggregate is determined by means of petrographic examination, mortars-bar test, accelerated mortars-bar test and X-ray diffraction as well as scanning electron microscopy. It's proved that this slate aggregate has alkali reactivity, and the active mineral is micro-to crypto-crystalline quartz, not dolomite, so it doesn't have alkali-carbonate reactivity. Besides, what's different with traditional alkali-silicon reaction mechanism of inflation is that the alkali-silica reaction is not only happened in the aggregate and cement interface but also in the internal of aggregate, that is because the slate aggregate has the characteristics of bedding structure, and the external alkali will permeate into internal along the bedding and react with the active component, which causes expansion and cracks.The trial about preventing alkali-silicon reaction of slate using fly ash has been done in this paper. In order to show the suppressing ability of fly ash and the suppressing effect to higher alkali-activity of aggregate, the trail has also researched the suppressing effect to the mixture of slate and 5% Pyrex glass. In addition, by referencing the improved ASTM C441 method, preventing studies have also been done to different activity Pyrex glass aggregate. It is proved that the fly ash has different suppressing effect to different aggregate. The minimum content of fly ash required is lower while the activity of aggregates is lower.This paper presents a test method used to evaluate the effectiveness of mineral admixtures preventing alkali-silicon reaction which based on accelerated mortar bar method, the linear relationship between ASR expansion ratio and alkali content is found, and there are some difference in expansion characteristics between slate and 1% Pyrex glass aggregate. According to the preventing test result by the two kinds of aggregate, it is found that the mineral admixtures suppress alkali-silicon reaction ability can be identified through the test methods, the final conclusions of the two methods are similar. It can be used to evaluate the effectiveness of mineral admixtures in preventing the alkali-silica reactionThe influence of Ca(OH)2 to fly ash suppressing alkali-silicon reaction is also being studied, the Ca(OH)2 content is adjusted by additional Ca(OH)2 method, and some experimental researches about the fly ash preventing alkali-silicon reaction in different Ca(OH)2 content have been done. It has been proved that Ca(OH)2 is one of adverse factors in suppressing alkali-silicon reaction, and the inflation is bigger in higher Ca(OH)2 content than lower content in the same situation, and the minimum admixture required of fly ash is bigger in the higher Ca(OH)2 content, the relation between the inhibition rate and Ca(OH)2/SiO2 rate in certain scope is in direct proportion. Combined with microscopic analysis, it is concluded that the mechanism of fly ash suppress alkali-silicon reaction mainly from two reasons, one is the reduction of Ca(OH)2 content by the secondary pozzolanic reaction, the other is the reduction of alkali content by low Ca/Si rate C-S-H gel's 'binding' function to alkali.