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Industrial solid waste red mud discharge has caused serious environmental problems. This study utilized red mud as an additive to loessial silt being used for roadway subgrade material. In this study, unconfined compressive test, direct shear test, electrical resistivity test, and hydraulic conductivity test were conducted on red mud stabilized loessial silt (RMLS) with different red mud dosage (DR) to investigate DR effect on mechanical-electrical-hydro properties. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were carried out to reveal the mechanism from micro perspective. The results showed addition of appropriate amount of red mud (30–42%) effectively improved unconfined compressive strength of treated loessial silt but reduced resistivity and hydraulic conductivity. Significant correlation between resistivity and strength performance of RMLS mixture was developed. Microscopic analysis indicates red mud addition will promote generation of hydration products such as calcium silicate hydrations (C-S-H), calcium silicate aluminates hydrations (C-A-S-H), and ettringite (Aft), which will tightly connect surrounding particles of loessial silt and hydrates. Red mud particles adhere to surface of soil particles and fill in pores between them improving a compact and stable structure. This study demonstrated the feasibility of using red mud as a stabilization material for roadway subgrade and proved that resistivity measurement is a nondestructive testing method to evaluate mechanical properties for RMLS mixture. [ABSTRACT FROM AUTHOR]

The precipitation and dissolution processes of CaCO 3 are essential to the marine carbon cycle and are relevant to climate research. Nevertheless, one of the nagging questions is why the precipitation of CaCO 3 rarely occurs without the help of biological activity even in the surface seawater, where the calcite saturation state (Ω, expressed as the ratio of the calcium and carbonate ion concentration product to the stoichiometric solubility constant of calcite) is up to 5, but commonly takes place in hard-water lakes, springs and the low-salinity zone (practical salinity <5) of estuaries. Studies have found that Mg2+ plays a significant role in inhibiting all aspects of CaCO 3 precipitation, namely crystal formation, solubility, and precipitation rate. However, whether the presence of Mg2+ in seawater is sufficient to inhibit CaCO 3 precipitation has not been clearly stated, and the specific inhibitory role of Mg2+ in CaCO 3 precipitation still requires clarification. In this study, we used four different seed materials to induce CaCO 3 precipitation in artificial seawater of four Mg:Ca ratios and examined the crystal morphologies of the resulting precipitates using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). We found that Mg2+ inhibits the precipitation at the first stage of nucleation, which drastically increases the difficulty of calcite nucleation. At a seawater Mg:Ca ratio of 5.2, aragonite preferentially nucleates instead of calcite. Among the tested materials, only calcite and aragonite seeds exhibit marked catalysis of CaCO 3 precipitation, indicating that not all the particles in natural seawater can catalyze CaCO 3 precipitation. Our results demonstrate while the precipitation of CaCO 3 is inhibited by Mg2+ at all stages, the inhibition at the nucleation stage is the main reason massive spontaneous CaCO 3 precipitation does not occur under the surface seawater conditions, supporting the notion that the incorporation of foreign ions such as Mg2+ increases the surface energy and the solubility of the overgrowth or cluster, and thus inhibits CaCO 3 precipitation. • CaCO 3 precipitation in seawater is primarily inhibited in the nucleation stage. • Only particles of CaCO 3 crystal structure promote CaCO 3 precipitation effectively. • Mg2+ inhibits CaCO 3 precipitation by increasing the energy needed for nucleation. [ABSTRACT FROM AUTHOR]