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粉煤灰又称做人造火山灰。所谓火山灰,是一种天然的或人造的高活性硅、铝化合物,在水的存在下,它可以和生石灰发生化学反应(火山灰反应),生成不溶性的坚硬物质。粉煤灰(特别是飞灰)由于含有大量玻璃态的硅、铝成分,所以具有很强的火山灰反应活性。再加上在粉煤灰的组成中,通常含有百分之几的钙,而且其中一部份又是以氧化钙而存在的,所以粉煤灰只要和水混合,大多数都显示出自硬化现象。也就是说,粗粒灰或细粒飞灰一旦和水接触,其微量的石灰(CaO)、硅石(SiO_2)、铝土(Al_2O_3)、石膏(CaSO_4)等便发生化学反应,并在粉煤灰的表面上,形成水化硅酸钙凝胶(CSH)以及钙矾石等水和物,可以认为,这种现象和水泥的硬化在本质上是相同的。利用电子显微镜可以观察到这种火山灰反应进行的情况。
Fly ash is also called artificial volcanic ash. The so-called pozzolan is a natural or man-made highly active silicon and aluminum compound that, in the presence of water, can react chemically with quick lime (volcanic ash reaction) to form insoluble hard substances. Fly ash (especially fly ash) has a strong pozzolanic reactivity because it contains a large amount of glassy silicon and aluminum components. In addition, in the composition of fly ash, usually containing a few percent of calcium, and some of which are present in the presence of calcium oxide, so as long as the fly ash is mixed with water, most show self-hardening phenomenon . In other words, once the coarse-grain ash or fine-grained fly ash comes into contact with water, traces of lime (CaO), silica (SiO 2 ), bauxite (Al 2 O 3 ), and gypsum (CaSO 4 ) react chemically and occur in the pulverized coal. On the surface of the ash, water hydration materials such as hydrated calcium silicate gel (CSH) and ettringite are formed, and it is considered that this phenomenon is essentially the same as hardening of the cement. The use of an electron microscope can be used to observe the progress of this pozzolanic reaction.