Design and microstructural analysis of the mixture proportion of alkali-activated fly ash-slag composite cementitious material

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Abstract To explore the resource utilization of fly ash, slag, and coal gangue, the composition of hydration products and strength characteristics of fly ash-slag composite cementitious material (FSGF) were studied with NaOH as an alkali activator. First, response surface analysis was used to determine factors influencing the optimal NaOH content, basalt fiber dosage, and length to obtain the complete mix ratio of the composite cementitious material. Microscopic techniques such as XRD, FTIR, TG-DSC, and SEM were employed to analyze the crystal structure, thermal properties, and micro-morphology of the composite cementitious material, and to investigate the mechanism of NaOH-activated fly ash-slag cementitious material. The results indicated that the sensitivity of each factor affecting the mechanical properties of the composite cementitious material followed this sequence: NaOH content > basalt fiber length > basalt fiber dosage, with varying degrees of interaction among them. When the mass ratio of fly ash, slag, and coal gangue was 5:1:4, with 3% NaOH by weight, 2% basalt fiber dosage, and a fiber length of 3 mm, the optimal mix was achieved. The composite material achieved a compressive strength of 8.97 MPa after 28 days of standard curing at room temperature. NaOH, as an alkali activator, provided the strong alkaline environment required for the initial hydration of fly ash-slag composite cementitious materials, promoting the hydration of slag and fly ash. The hydration products in the fly ash-slag composite system were unevenly distributed, primarily consisting of gels like C-S-H, C-A-H, and C-A-S-H. NaOH was highly effective as an alkali activator in the fly ash-slag system.