Study on the Modification Effect and Mechanism of a Compound Mineral Additive and Basalt Fiber on Coal Gangue Concrete

Compared with ordinary concrete, coal gangue concrete (CGC) is limited by its poor mechanical properties and frost resistance, which seriously restricts its wide application in cold regions. In order to improve the resource utilization rate of coal gangue, this paper takes advantage of the ‘overlapping effect’, ‘micro-aggregate filling effect’ and ‘volcanic ash effect’ of fly ash (FA) and silica fume (SF) and the anti-cracking effect of basalt fiber (BF) to study their effects on the macro performance of CGC and the micro modification mechanism. Modified CGC was prepared by replacing cement with 20% total mineral additives and adding BF. Taking different fly ash and silica fume incorporation ratios (F/S) and the BF content as variables, the research was carried out from two scales of macro performance and microstructure. The results show that the mechanical properties and frost resistance of CGC can be significantly improved by adding mineral additives and BF, and the modification effect is better with a decrease in F/S. When F/S = 1, the compressive strength, splitting tensile strength and flexural strength of the specimens increased by 13.73%, 8.37% and 4.27%, respectively. After 300 freeze–thaw cycles, the specimen was still not damaged by freezing and thawing. At the same time, keeping F/S = 3 unchanged and changing the BF content, it was found that the optimal content of BF was 0.15 vol% under the combined action of BF, FA and SF. In terms of microstructure, the addition of mineral additives and BF segregates and fills the macropores in the structure, greatly reducing the harmful pores and turning them into harmless and less harmful pores. When F/S = 1, the number of multi-harmful pores decreased by 16.89%, and the number of harmless pores and less harmful pores increased by 9.19%, which greatly optimized the pore structure and pore gradation.