Analysis of Hardening Characteristics of Aged Concrete Prepared with Highly Mineralized Mine Water—A Mine in the Ordos Mining Area Is Taken as an Example

In order to study the hardening characteristics and formation mechanism of concrete prepared with highly mineralized mine water (which is called C(MW) for short), four mineralized mine water mixtures with different dosages (25%, 50%, 75%, and 100%) were prepared, and concrete specimens were made using coal-based solid waste (gangue and fly ash) as the aggregate and aged for a 70 d long-age test. Strength tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) measurements were performed to determine the relationship between the hardening strength and aging time. The hardening mechanism was studied based on the changes in the characteristic composition and microstructure. The results showed that, compared with the two-stage hardening in σ(C) seen in conventional concrete prepared with ground purified water, drinking water, or surface water (which is called C(N-MW) for short), σ(C) in our experiments had three-stages. The stages included a growth period (0~28 d), in which σ(C) of the 28 d concrete samples prepared with mine water dosages of 25%, 50%, 75%, and 100% increased by 18.0%, 36.4%, 57.2%, and 72.7%, respectively, compared with that of C(N-MW); a rapid decline period (28~56 d), in which σ(C) at 56 d decreased by 47.7%, 43.2%, 36.0%, and 30.5%, respectively, and finally, the stable period (56~70 d~long-age), in which the strength σ(C) remained stable. The mechanisms of the hardening characteristics were different from those of C(N-MW) in the three stages. In the first stage (0~28 d), Friedel’s salt and more ettringite were generated by the secondary hydration reaction, which filled the internal pores of the specimens and thus improved the compactness and σ(C). In the second stage (28~56 d), the amount of Friedel’s salt and ettringite further increased, the crystals inside the specimens expanded, and macroscopic cracks appeared on the specimen surface, thus leading to the decrease in σ(C). In the third stage (56~70 d~long-age), the amount of Friedel’s salt and ettringite plateaued, and σ(C) entered a stable stage, decreasing by 52.5%, 47.8%, 40.4%, and 36.8%, respectively, compared with that of the specimens prepared without mine water. The hardening time of C(MW) was 42 d longer than that of conventional C(N-MW), the hardening strength decreased significantly, and the σ(C) at the final setting time was much lower than that of C(N-MW). Our research results provide a reference for the filling strength design of coal mine rock stratum control.