Study on the mechanical properties and strength formation mechanism of high-volume graphite tailings concrete

Graphite tailings and coal gangue (CG) are solid wastes produced by mines. The large accumulation of surface materials poses great harm globally, and resource utilization is an urgent problem to be solved. In this paper, steel slag (SS), granulated blast furnace slag (GGBS), and fly ash (FA) are used as binder materials. Graphite tailings sand (GTS) is used as a fine aggregate. Using coal gangue and natural gravel as coarse aggregates, high-volume tailings aggregate concrete was prepared by the alkali excitation method. The strength formation mechanism of concrete was analyzed by combining scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanning calorimetry (TG-DSC). The results showed that with increasing tailings replacement rate, the strength of concrete containing natural gravel and coal gangue as coarse aggregates gradually decreased. The strength of coal gangue aggregate concrete is 10-15 MPa lower than that of natural gravel aggregate concrete. The uniaxial compressive failure of GTS concrete is divided into three stages: quasielasticity, stable expansion, and critical stress. A small amount of steel scoria (less than 30 %) can provide a good calcium ion (Ca2+) environment for the hydration reaction. The concrete hydration products are mainly calcium silicate hydrate (C-S-H) gel, sodium aluminosilicate hydrate (N-(A)-S-H) gel, ettringite (AFt), etc., and the formation of calcium silicate hydrate C-S-H gel and N-(A)-S-H gel can inhibit the cracks that proceed by shrinkage deformation and improve the durability of concrete. GTS concrete can reduce carbon emissions and solidify some of the harmful substances of raw materials.