Effects of Heterogeneous Fly Ash on the Hydration Characteristics of Modified Magnesium Slag-Based Cementitious...

The modified magnesium slag (MMS)-based cementitious material (M·C) is a green backfill cementitious material developed using the synergistic effect of regional solid wastes (MMS and fly ash (FA)). Precise selection of FA directly governs the cementitious quality and application performance of M·C. This study investigated the effects of basic characteristics of FA emitted from different power plants in Yulin city on the mechanical and hydration properties of M·C. The results indicate: (1) FA varies in chemical composition, particle size, and hydration activity, while its activity mainly derives from f-CaO and amorphous phases. (2) FA reduces M·C early strength, but the 28 d strength reaches 12.75-19.35 MPa, higher than the control (10.31 MPa). M·C_GH (75-20-5) showed the highest strength among all groups. The FA activity index method in GB 1596 is not fully applicable to the M·C system, and MMS standard samples should be used as the reference specimens. (3) FA reduces M·C heat release rate and cumulative heat. The early hydration heat of M·C activated by various FAs was mainly governed by the coupled effects of particle size and f-CaO content. (4) All hydration products of M·C contain CH, AFt, and C-S(A)-H. Early hydration is controlled by FA particle size and f-CaO content, while late hydration by its pozzolanic activity. FA and MMS jointly provide an active Al source for the formation of AFt. Additionally, heterogeneous FA affects the hydration degree, not the mechanism or product type. (5) In the cases of M·C_MX, M·C_GG, and M·C_GL, the hydration product layers are thin and relatively loose, resulting in lower strengths compared to the other samples. Thus, FA_MX, FA_GG, and FA_GL are not recommended for the M·C system, while FA_GH is preferred. The research results provide a basis for precise FA selection in M·C, as well as theoretical and technical support for its standardized application.