Effects of chloride on the early mechanical properties and microstructure of gangue-cemented paste backfill

Abstract The addition of an early strength agent as an additive in cement-based materials provides several security-related, technical, and economic advantages, such as improving the early strength of the paste and accelerating the cycle frequency of mining and filling. In this paper, we present the results of an experimental investigation of the effects of chloride on the early age (3, 7, and 28 days) strength of gangue-cemented paste backfill (GCPB). The GCPB specimens had an initial chloride concentrations of 0‰, 5‰, 10‰, 20‰, 30‰, and 40‰, and the obtained samples were analyzed by performing uniaxial compressive strength tests, X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results show that chloride significantly affects the early-age strength of GCPB. At early ages, chloride can have positive and negative effects, i.e., the GCPB strength increased with an initial chlorine content of 10‰, while it decreased for GCPB with an initial chlorine content of 40‰. These positive or negative effects depend primarily on the initial concentration of chloride in the GCPB. The key reason for this characteristic is the promotion or suppression of cement hydration by chloride ions. The essential factor that affects the GCPB strength is the relationship between the volume of voids and the amounts of calcium silicate hydrate gel, ettringite, and Friedel's salt formed. These results indicate that it is important to consider the influence of chloride on the early strength of GCPB to improve the self-stabilizing strength of GCPB and the economic benefits of coal mines.