Interface Transition Zone in Coal Gangue Aggregate Concrete Reinforced by Phosphorus Slag: Macroscopic Properties and Microstructure

This study used phosphorus slag (PS) from yellow phosphorus production to strengthen the structure and performance of the interface transition zone (ITZ) of coal gangue aggregate concrete (CGAC) based on the surface properties of the coal gangue (CG) in western Guizhou, China. The compressive strength, flexural strength, and ITZ microhardness were used to characterize the mechanical properties of the ITZ, and the chemically bound water was identified. X-ray diffraction, contact angle tests, and scanning electron microscopy (SEM) were used to reveal the reinforcement mechanism. The results showed that in the presence of 10% by weight PS, the PS enhanced the mechanical properties of the ITZ in the CGAC, and had a greater effect on the flexural strength than on the compressive strength of the CGAC. Further research showed that a large amount of active SiO2 in the PS reacted with the cement hydration product Ca(OH)2 (CH), which resulted in additional hydration of the main minerals, such as tricalcium silicate (C3S) and dicalcium silicate (C2S), in the cement. Moreover, the cement particles on the aggregate surface and the micropores on the near-surface of CG underwent competitive adsorption or reacted with water, thereby decreasing the ITZ width. In addition, the contact angle of the composite paste with 10% by weight cement replaced by PS on the CG aggregate surface was 16.60% smaller than that of the pure cement paste, resulting in a spread of the composite paste on the surface of the CG aggregate to form a dense structure in the ITZ of the CGAC.