Abstract
To resolve the limited large-scale methods of disposal of low calcium fly ash with poor activity, based on the double excitation principle, clinker and desulfurized gypsum are used as alkali/salt activators to activate fly ash and slag, avoiding the inconvenience of strong alkali activating fly ash in industry. Firstly, the strength test of a filling body with multiple ratio composite cementing material is carried out, and the weight coefficient of each material to strength is analyzed by grey correlation degree. The composition of the hydration products, microstructure, and pore structure of the filling body was analyzed by X-ray diffractometer, scanning electron microscope, thermogravimetric test, and mercury compression test. The strength mechanism of the cemented body was confirmed. The results show that cemented backfill prepared by composite cementitious material, which contained high content and low-quality fly ash, can meet the strength requirements of subsequent backfill in a mine. The degree of composite cementitious material influence on 7 d strength is slag > desulfurized gypsum > fly ash > clinker; the degree of influence on 28 d strength is: fly ash > slag > desulfurized gypsum > clinker. The main hydration products of the composite cementable material with high content low-quality fly ash are C–S–H gel and ettringite, and the unreacted fly ash particles can still be seen at 28 d. As the curing age grows, the difference in the number of hydration products under different proportioning conditions has a weaker effect on the strength, while the influence of raw materials and product morphology on the pore structure determines the development trend of the strength. Therefore, the threshold pore size can be used to characterize the strength advantages and disadvantages reasonably.
Highlights
Fly ash is a byproduct of coal-fired power plants
The 7-d and 28-d strengths varied prominently with the fly ash content, which decreased with the increasing fly ash content overall
The grey correlation analysis revealed that the components with the greatest influence on 7 d and 28 d strength were slag and fly ash, respectively, and it was conjectured that the effect of fly ash on strength was mainly from the hydration reaction and micro-aggregate effect
Summary
Fly ash is a byproduct of coal-fired power plants. The fly ash from most power plants inChina has low quality, which is generally grade III or off grade, with low calcium content [1]. Fly ash is a byproduct of coal-fired power plants. The fly ash from most power plants in. The main source of activity of poor quality fly ash is vitreous, which requires large internal energy to be excited and presents a low activity state at room temperature. The rate and degree of early hydration reaction are extremely low, so there are few ways to utilize fly ash, which is substantially accumulated and can cause serious environmental issues [2,3]. Analysis of its chemical composition finds that various components enable its usage as a geopolymer to replace cement in concretes [4]. The cementitious properties of fly ash are attributable to a combination of three effects: pozzolanic reaction effect, microaggregate effect, and particle morphology effect. The pozzolanic reaction is chemical, while the other two effects are physical [5–7]
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