**1. Introduction**

Mine-filling technology processes waste such as coal gangue and fly ash into slurry, which not only reduces the accumulation of solid waste and pollution in coal mines, but also enables effective control of the deformation of overlying strata and surface subsidence in mining areas [1]. However, the application of this technology in coal mines is limited owing to insufficient availability of backfill materials and high costs [2,3]. Therefore, cheap, efficient, and widely available backfill materials are required for mine-filling in the future.

China is rich in kaolin resources and has suitable conditions for development and utilization of kaolin. Kaolin (Al2O3·2SiO2·2H2O) is a type of fine and soft clay mineral having a "single net layer" structure [4–7]. At present, scholars in China and abroad are studying the influences of kaolin admixtures on mechanical properties and the microstructure of concrete materials. Due to the low activity of kaolin, Chun et al. [8] stimulated the pozzolanic activity of kaolin using high-temperature calcination and found that adding an appropriate amount of kaolin into concrete can greatly improve its compressive strength. Lingyan et al. [9] found that calcined activated kaolin has the greatest effect on the early

**Citation:** Li, F.; Yin, D.; Zhu, C.; Wang, F.; Jiang, N.; Zhang, Z. Effects of Kaolin Addition on Mechanical Properties for Cemented Coal Gangue-Fly Ash Backfill under Uniaxial Loading. *Energies* **2021**, *14*, 3693. https://doi.org/10.3390/ en14123693

Academic Editor: Sarma V. Pisupati

Received: 14 March 2021 Accepted: 17 June 2021 Published: 21 June 2021

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strength of concrete admixtures, which increases with the addition of kaolin. When the amount of kaolin is 10%, the strength of the concrete admixture is the highest [10–13]. Meng et al. [14] found that the early strength of cement can be improved by mixing calcined activated kaolin and slag. However, the kaolin production via calcination is a high-temperature process that is cumbersome, consumes high energy, and is expensive. Hao [15] and Yuanyuan et al. [16] found that kaolin can react with Ca(OH)2 slowly at room temperature to form hydration products of cementitious ability, and the hydration reaction with cement can create an alkaline environment and enhance this process. Wei et al. [17] found that kaolin and other mineral admixtures can improve the early strength of cement mortar in an alkaline environment. Annan et al. [18] confirmed the plate morphology of kaolin. The particle size of kaolin is mainly in the range of 0–5 μm, which can be filled into smaller pores of cement paste and be dispersed more evenly, which is conducive to the complete occurrence of chemical reactions. Mengna et al. [19] found that the reaction of kaolin and Ca(OH)2 can produce flocculent substances and platelike crystals. The addition of slag and fly ash is conducive to the diffusion of crystals and destroys the structure of Ca(OH)2, thus reducing the porosity of the cement mortar and improving the density of the slurry and filler–matrix interface. Das et al. [20] found that kaolin contains more SiO2 and Al2O3 under alkaline conditions, leading to a higher pozzolanic activity. Its internal structure contains more chemical bonds, which can weaken the secondary hydration of Ca(OH)2 in cement mortar.

The abovementioned research results are of great significance for understanding the effect of kaolin addition on the properties of cement-based materials such as concrete and mortar. The cost of backfilling can be reduced if cement can be replaced with kaolin in cemented coal gangue-fly ash backfill (CGFB) samples. Moreover, it can provide a new path for resource utilization of kaolin. Therefore, based on the test methods of the loading system, acoustic emission (AE), digital video camera (DVC), scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), we studied the effects of kaolin partially replacing cement on the mechanical properties of CGFB samples, and the feasibility of using kaolin as paste admixture was discussed. The results can provide a theoretical basis for application of kaolin admixture in fill mining.
