Abstract: With excellent mechanical properties and durability, alkali-activated cementitious material prepared by industrial solid waste is a kind of green inorganic cementitious material. Fly ash is essentially different from other solid waste because of its unique spherical microstructure, therefore, the reaction mechanism of fly ash in alkali-activated cementitious material needs to be studied. In this paper, slag and fly ash were used as raw materials to prepare cementitious materials by alkali activator. The compressive strength of the material was tested, and the hydration mechanism of alkali-activated slag/fly ash system was explored by XRD, FTIR, and SEM. The detailed effect of fly ash on alkali-activated slag/fly ash system and related mechanisms were investigated. The results show that, when 3% (mass fraction) NaOH is added as alkali activator, and the water-solid ratio is 0.4, with the decrease of fly ash content, the compressive strength first increases and then decreases. When m(slag):m(fly ash) is 4:1, 28 d compressive strength reaches the peak value of 37.1 MPa. Embedded microstructure of fly ash particles has different reaction degrees and sizes at different ages, which has a negative effect on the mechanical properties of the material. However, the activation degree of fly ash gradually increases with the extension of age, which has a continuous contribution to the later strength development. The hydration products of alkali-activated slag/fly ash system contain Friedel's salt, tobermorite, ettringite, C-S-H/C-A-S-H gel, and residual alpha quartz of fly ash. With the increase of fly ash content, the amount of tobermorite decreases, ettringite transforms into Friedel's salt, the production of ettringite decreases, and the production of Friedel's salt increases.

Key words: alkali-activation, fly ash, compressive strength, microstructure, Friedel's salt, tobermorite