Prevention against Alkali Aggregate Reaction by Using Pozzolan

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Repairing and reinforcing the existing concrete structures damaged by alkali aggregate reaction, in particular, alkali silica reaction (ASR) is usually very difficult, and therefore it is now required to establish the effective methods for preventing the ASR by means of utilization of appropriate materials. Some results of previous tests showed that the addition of pozzolans, such as blast-furnace slag, fly ash and silica fume, reduced or prevented the ASR expansion of concrete, and then prevention method of ASR like this had been recommended in some foreign countries. However, the prevention mechanism of ASR due to addition of pozzolans is not yet made clear, and their use has an adverse effects on ASR expantion in some cases, depending upon an amount of added pozzolan.In this study, utilization of three kinds of pozzolans for preventing the ASR expansion was tested by using the mortar and concrete prism specimens. The results of tests showed that <i> ) the ASR expansion of mortar could not fully be prevented even when 60% of ordinary portland cement was replaced by blast-furnace slag (called as, for example, "60% addition of slag" here-after), although reduced with increasing slag content, <ii> ) the ASR expansion of mortar could be prevented by 25-30% addition of fly ash, but it should be noted that a little amount, for instance, 5% addition of fly ash rather increased the expansion, <iii> ) the silica fume retarted the age at which ASR expansion commenced to occur, and 25-30% addition of silica fume was very effective for preventing the expansion although considerably expanded on the contrary when its added amount is small, and <iv> ) both of the blast-furnace slag cement of B type (slag content: 45%) and the fly-ash cement of B type (fly ash content: 18%) could effectively reduce the ASR expansion of concrete.