Research On Fly Ash Properities Of Long-term Wetting

The long-term wetting fly ash of low-calcium is a kind of spreading industrial solid waste of coal power plants that is packed in stacks both at home and abroad, and they are slightly used. Researchs on low-calcium wetting fly ash were aboundent, which were generally focused on road pavement, building products and application after activation. But there is a bad shortage of investigation on its basic properties after long-term wetting. A well investigation on its property change will be helpful both for the sustanible development of power plants and the environment protection.To avoid the gererally-existed research threshold in determining the property changes--- obtaining the same batch of dry fly ash that has the same composition with the long-term wetting one, a simulative wetting process was carried out in lab. The rules of wetting age on its properties were expermentally performed. A five-year long time wettingg fly ash was analysed instrumentally and its performance when used as concrete mixture was also carried out. The results showed that:1) Due to the self-cementing process to some degree, low-calcium fly ash particles were coated with hydration production after a long-term contacting with water. Meanwhile, the particle size became coarser and its distribution became broader. Even though a 10% reduction in water requirement was occured due to its deduction in water-reducing effect after the early period of wetting, it could still fullfill the water requirement when used as a concrete mixture. Fly ash became much more sound after wetting, while its loss on ignition grew up which was attributed to its hydration process; 2) The distinction of the minerals and crystallizing degree of low-calcium fly ash before and after wetting was neglectable. After a long time wetting process( 3 years at most), major part of the low-calcium fly ash kept their original morphology while some others were erodened and their inner crystal particles became seen while the out part of regular crystals became smaller, all of which was directly influenced by its CaO content.3) A more than 5% but less than 10% activity loss occurred due to a first five-month of wetting, while the later period of activity loss became smaller. After a wetting process, the early hydration process was decelerated while the later hydration could be accelerated. The alkalinity of wetting fly ash was higher than 7, which surged up swiftly during the first serval days of wetting and declined slowly afterwards. It also showed that the higer content of CaO, the much swift of the declination of its alkalinity. The alkalinity peak of low-calcium was postponed when compared with high-calcium ash. 4) A high wetting temperature(95℃) quickened the self-cementing process, during which the fly ash was also activated, but the activity of it was lowered. The chemical composition, water acidity, static or dynamic state of wetting water effected the activity slightly. The finer of the wetting fly ash, the lower of its later-period of activity, and its fineness had little function on its early activity changes. The declination speed and span of activity became wider with the content of CaO became higher. 5) Though there was a noticable declination of water reducing effect of low-calcium fly ash due to long-term wetting, but the slump loss lowered down while the bleeding and adaptation with admixture were quite the same with dry fly ash. 6) The early strength of long-term wetting fly ash concrete decreased rapidly but the later strength declined gentaly: The specific compressive strength of concrete with fly ash wetting for 5 months decreased by 5% while concrete with 30% fly ash wetting for 36 monthes still reached 75%, and there was no visible strength discrepancy of fly ash concrete wetting for 3 and 5 years at 28d. The durability of long-term wetting fly ash concrete was quite the same with dry ash ones.Though most concrete performance with long-term low-calcium wetting fly ash are declined, it can still be used as an admixture when applied in concrete supplimentarily.