Q. Pulverized fly ash as cement replacement?

Pulverized fly ash is a type of pozzolans. It is analuminous or siliceous material that possesses no binding ability by itself. When it's in finely divided form, they may react with calcium hydroxide in the presence of moisture to form compounds with cementing properties.  

At the time of cement hydration with water, calcium hydroxide is formed and it is non-cementitious in nature. Though when pulverized fly ash is added to calcium hydroxide, they react to yield calcium silicate hydrates that is highly cementitious. This leads to improved concrete strength. This explains how pulverized fly ash may be used as cement replacement.