The differences among the roles of ground fly ash in the paste, mortar and concrete

•W/B has little influence on its reaction degree of ground fly ash.•At late ages, the strength activity index of ground fly ash in concrete is the highest, and that in paste is the lowest.•W/B has a very small influence on the strength activity index of ground fly ash in mortar or concrete. The differences among the roles of ground fly ash in the paste, mortar and concrete were investigated by determining the reaction degrees of ordinary fly ash and ground fly ash in the pastes and measuring the compressive strengths of pastes, mortars and concretes containing 20% and 40% ordinary fly ash or ground fly ash with water-to-binder ratios (W/B) of 0.4 and 0.3. The results show that increasing the fineness of fly ash by grinding significantly promotes its reaction degree at each age, but W/B has little influence on its reaction degree. Ground fly ash makes much more contributions to the strength of hardened paste than ordinary fly ash, but the increase of fineness and reaction degree of fly ash makes smaller contribution to the strength enhancement at lower W/B. At early ages, the effect of ground fly ash on the improvement of the interfacial transition zone is weak, but at late ages, ground fly ash makes significant contributions to the improvement of the interfacial transition zone especially for the hardened system of concrete. At late ages, the strength activity index of ground fly ash in concrete is the highest, and that in paste is the lowest. W/B has a very small influence on the strength activity index of ground fly ash in mortar or concrete, but the strength activity index of ground fly ash at higher W/B is obviously larger in paste.