Study on the mechanical behavior and micro-mechanism of concrete with coal gangue fine and coarse aggregate

Abstract

Coal gangue is waste rock generated from coal mining, which results in vast land occupation in mines and severe environmental pollution. Owing to the similarity between coal gangue and natural aggregates, concrete preparation from coal gangue has been considered effective for solving gangue problems. In this study, to further improve the on-site utilisation rate of coal gangue, coal gangue was used to replace fine and coarse aggregates for preparing mortar and concrete. The macro and microscopic characteristics of coal gangue mortar (CGM) and coal gangue concrete (CGC) were investigated while considering the effects of coal gangue fine aggregate (CGFA) replacement ratio, curing time, and water-to-cement ratio. To investigate the macro properties, flexural, compressive, and splitting tensile tests were performed. The results show that CGM and CGC with an appropriate CGFA replacement ratio of 25% exhibited optimal performance. Additionally, by replacing all aggregates with CGFAs and coal gangue coarse aggregates, the 28 d compressive strengths of samples W0.5R100 and W0.4R100 were 44.8 and 52.24 MPa, respectively, which satisfied the C30–C50 concrete strength requirements. Moreover, a model is proposed for predicting the splitting tensile strength of the CGC to address the overestimation of existing national standards. For microscopic analysis, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were conducted to illustrate the hydrated products and micromorphology of CGC under different conditions, as well as to discuss the effect of the mechanism on strength. Based on the micro analysis, the mechanism of CGFAs is summarised as a coal gangue–Ca(OH)2–H2O reaction system: volcanic-activated SiO2 and Al2O3 in CGFAs react with calcium hydroxide (CH) crystals, and more CH is consumed; subsequently, more calcium silicate hydrate gels and ettringite crystals are produced in the secondary hydration reaction, contributing to a denser microstructure and improvement in concrete strength at the late stage. In particular, gismondine is observed as a new hydration product in the reaction of CGC, which differs from that of normal concrete. This study provides a basis for the large-scale utilisation of coal gangue in situ.