Search

[Display omitted] • The conventional minimum objective function with non-negative constraints is converted into an unconstrained maximization. • The novel inversion method is reliable to invert the TD-NMR signals with short relaxation time in low SNR. • The novel inversion method is verified by the measured data of fluid migrations in shales and cement hydration. The inversion of time-domain nuclear magnetic resonance (TD-NMR) signals is an ill-posed problem, which presents enormous challenges for the inversion algorithm. We propose a novel inversion method that converts conventional minimum objective function with non-negative constraints into an unconstrained maximization problem in the inversion of TD-NMR signals. Hence, the objective function becomes a differentiable concave function that can be solved more easily. The validity of the proposed method was verified by the uncertainty estimation of NMR inversion spectra with different signal-to-noise ratios (SNR). Through the inversion of simulated 2D D - T 2 and T 1 - T 2 signals under different SNR, the proposed method was proved to be less sensitive to noise than the conventional inversion method. We use the proposed method to study the migrations of oil and water in shales, the components change in shale could be identified and quantified according to the 2D T 1 - T 2 inversion spectra. The proposed method was also used to analyze the hydration process of cement. The 2D T 1 - T 2 inversion spectra could distinctly present the component of tiny volume with short relaxation time, and the migration regularity of capillary water, gel water, and bound water could also be found. In conclusion, the proposed method could be a reliable method to invert TD-NMR signals, especially the identification of the 2D NMR signals with a short relaxation time in low SNR. [ABSTRACT FROM AUTHOR]

The proper treatment of lollingite is of great significance due to its rapid oxidation leading to release of arsenic into the environment. Herein, a green multi-solid waste geopolymer, consisting of red mud, metakaolin, blast furnace slag, and flue gas desulfurization gypsum, was developed. The obtained red mud-metakaolin-based (RMM) geopolymer demonstrated good arsenic retention capability. The results showed that the replacement of SO 4 2− in ettringite with AsO 4 2− via ion exchange, formation of Ca-As and Fe-As precipitates, and physical encapsulation with aluminosilicate gel were the main mechanisms that prevented the release of arsenic. Further dissolution of ettringite in RMM was alleviated by adding a suitable amount of Ca(OH) 2 and controlling the pH of the leachate. TCLP results verified that RMM materials possessed an outstanding ability to stabilize arsenic, with a leaching rate below the permitted value of 5 mg/L for safe disposal. The low leachability of the RMM geopolymers (<0.50 mg/L) is potentially related to the pH buffering capacity of the hydration products at a pH range of 2–5. RMM geopolymers showed a high compressive strength (>15 MPa) and low arsenic leaching concentration (<2.66 mg/L) after 28 days of curing. These results demonstrate the potential of RMM geopolymers to be utilized as an environmentally friendly backfilling cementitious material for sustainable remediation of arsenic pollution. • The evolution pathways of arsenic from lollingite to RMM samples was investigated. • The dissolution of ettringite is correlated with the increase of As leachability. • Arsenic-bearing phases in RMM samples were demonstrated to be stable at pH 2–5. • RMM can be used as environmentally multi-solid waste cementitious material. [ABSTRACT FROM AUTHOR]